l) Qu d) ,~J iJ s i,~a Id ;Ji) V.;j U i 0) I' (( (i" & t, 't:,<"

LBL-6325 3/3 c_1

For Reference

Not to be taken from this room

June 1977

... ________ LEGAL NOTICE---------.....

This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Energy Research and Development Administration, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

-

o OJ \ji

111- i

TELLURIC AND D.C. RESISTIVITY TECHNIQUES APPLIED TO THE

GEOPHYSICAL INVESTIGATION OF BASIN AND RANGE GEOTHERMAL SYSTEMS

PART III

THE ANALYSIS OF DATA FROM GRASS VALLEY, NEVADA

J. H. Beyer

() OJ U.J

111- i i

ABSTRACT

This paper contains a detailed interpretation of E-field ratio

telluric, bipo1e-dipole resistivity mapping, and dipole-dipole

resistivity data obtained in the course of geop~ysical exploration of

the Leach Hot Springs area of Grass Valley, Nevada. Several areas are

singled out as being worthy of further investigation of their geothermal

potential. Comparison of the three electrical exploration techniques

indicates that: the bipole-dipole resistivity mapping method is the

least useful; the dipole-dipole.resistivity method can be very useful,

but is~ for practical purposes, exceptlonally expensive and difficult

to interpret; the E-field ratio telluric method can be a highly

successful reconnaissance technique for delineating structures and

relating the resistivities of different regions within the survey

area.

" 1- iii

o OJ d; . U

III-iv

PREFACE

Starting in the Summer of 1973, the Lawrence Berkeley Laboratory

of the University of California has been lnvolved in a geothermal

assessment program with three main goals:

1) To evaluate, on the basis of detailed geological, geochemical and

geophysical data, some geothermal systems in the mid Basin and

Range geologic province.

2) To compare and evaluate geophysical techniques used in the explor­

ation and delineation of geothermal reservoirs.

3) To develop new exploration techniques, and the instrumentation

required, specifically for the deep penetration desired in

geothermal investigations.

This report addresses various aspects of each of these points.

It is well documented that hot water geothermal reservoirs tend to

have lower electrical resistivity than surrounding cold and/or dry rock

by virtue of: (1) increased ion mobility, (2) more dissolved solids,

and (3) increased permeability and porosity of the reservoir rocks as a

result of convection of the geothermal fluids. Vapor dominated geo-

thermal systems are resistive in the steam zone, but display anomalously

conductive halos in intermediate temperature regions·where there is

condensation. Thus, one distinctive feature of geothermal reservoirs is

that they may be electrically conductive targets which, to be of

economic importance, may be a few cubic kilometers in size, but with a

depth of burial of one or more kilometers.

I I I-v

When confronted with the problem of initial exploration of a

several hundred square kilometer region in the vicinity of a hot spring,

a rapid reconnaissance electrical method is important to locate areas of

low resistivity for more intensive investigation. The E-field ratio

telluric method described in Part I of this report appears to satisfy

this need quite adequately.

Subsequent to the location of conductive anomalies by reconnais­

sance techniques an electrical method providing higher resolution and

affording more quantitative interpretation capability is needed. For

this purpose, and for correlation with and evaluation of other electric­

al exploration techniques, d.c. resistivity measurements using the polar

dipole-dipole array were performed as a part of the LBL geothermal

exploration program. A second resistivity electrode configuration, the

Schlumberger method, has been widely used by other investigators. Part

I I of this report is a numerical model study and comparison of these

two resistivity techniques.

An extensive program of geophysical exploration was undertaken by

LBL in the vicinity of Leach Hot Springs in Grass Valley, Nevada. The

detailed interpretation of E-field ratio telluric, dipole-dipole resist­

ivity, and bipole-dipole resistivity mapping data is treated in Part I I I

of this report, along with a description of the implementation of high­

power d.c. resistivity exploration techniques. Several areas in Grass

Valley emerge as being worthy of further investigation for their

geothermal potential, and the interpretation process has provided a

means of evaluating and comparing the exploration techniques.

III-vi

ACKNOWLEDGEMENTS

The major portion of this work has been supported by the U.S.

Energy Research and Development Administration through the University

of California - Lawrence Berkeley Laboratory. Some support at the I

initial stage of the field exploration program was provided by the

National Science Foundation (Grant #GI-3790S).

Special thanks are due Professor H.F. Morrison for numerous

profitable ideas and discussions throughout the course of this work,

and Abhijit Dey for his helpfullness in the use of his computer code

for modeling the resistivity data.

The extensive field geophysical exploration program in which the

author participated required the support of numerous helpful and

capable employees of Lawrence Berkeley Laboratory. Notable among them

are Harold Wollenberg, with whom the author had many discussions

regarding the geology and exploration of Nevada geothermal systems, and

Eugene Binnall, John Saarloos, and Harold Vogel, who developed reliable

instrumentation to meet the demands of field exploration. Particular

gratitude is expressed to Halvard Andersson, Milt Moebus, Ray Solbau,

and the men who worked with them in the field for their conscientious

effort at obtaining excellent field data.·

()' L .. ')' (,.' " . " " "tJ IJ \J ~. ;i ~~ J <.l <0

III-vi i

TABLE OF CONTENTS

Part I II: The Analysis of Data from Grass Valley, Nevada

Introduction I I 1-1

The electrical exploration methods

The E-field ratio telluric method

The bipole-dipole resistivity mapping method

The dipole-dipole resistivity method

The interpretation of telluric and d.c. resistivity

data from Grass Valley, Nevada

The analysis of telluric and dipole-dipole resistivity

data

The analysis of bipole-dipole resistivity mapping

data '

The resistivity structure of Grass Valley

Recommendations

Evaluation of the electrical 'exploration methods

Bipole-dipole resistivity mapping

E-field ratio tellurics

Dipole-dipole resistivity

Conclusions

Recommendations

References

Figures

Appendix II I-A. Geophysical data profile composites,

Grass Valley, Nevada

111-2

111-2

111-4

111-6

111-15

111-17

111-37

111-42

111-46

111-48

111-48

I I 1-49

I I I-50

III-53

III-56

III-57

111-60

111-97

0 () i;J icJ :,} ~ r ,',:-; I:.;~,

~.j

I I 1-1

INTRODUCTION

In the University of California-Lawrence Berkeley Laboratory

program for geophysical investigation of the geothermal potential of

Grass Valley, Nevada, considerable emphasis has been placed upon the

development, interpretation, and evaluation of various electrical

exploration techniques. This report deals with the analysis of data

obtained using two reconnaissance techniques, the E-field ratio telluric

and the bipole-dipole resistivity mapping methods, and the dipole-dipole

resistivity method which, as a result of numerical model ing capability,

offers the possibility of more detailed structural delineation. The

first method makes use of natural electric fields as a source, while the

latter two methods require a high-power controlled source.

The geophysical data which have been obtained in Grass Valley

include gravity, magnetics, self-potential, E-field ratio tellurics,

magnetotellurics, bipole-dipole resistivity, dipole-dipole resistivity,

P-wave delay, microearthquake, seismic ground noise, and active seismic

refraction/reflection. Only the data which are most pertinent to the

interpretation of the E-field ratio telluric and d.c. resistivity data

are presented in this report. Subsequent reports by other authors will

present detailed analyses of other techniques. Regrettably, the magne-

totelluric data has not been fully analyzed at the time of this writing,

and will not be included in the discussion. A final interpretation of

Grass Valley, giving equal weight to all methods of investigation is

forthcoming (Morrison, ~ ~., 1977).

111-2

THE ELECTRICAL EXPLORATION METHODS

The E-Field Ratio Telluric Method

The electric (E) field ratio telluric method is designed for

reconnaissance electrical exploration of survey areas of several

hundred square kilometers. A detailed description of the method,

including a numerical model study of the response to various

two-dimensional structures, is the topic of Part I of this report

(Beyer,1977a).

The E-field ratio telluric method is an abbreviated version of the

conventional telluric current method in which the natural electric field

of the earth is measured. Rather than using orthogonal electrode arrays

to measure the electric field at a roving station with respect to that

at a base station, a collinear, three-electrode array is used to measure

the relative electric field strength between two consecutive dipoles.

The array is leap-frogged along a survey line to obtain a set of ratios

of the component of the electric field in the profile line direction.

The ratios are successively multiplied together so that the value at

each dipole location will be referred to that at the first location.

Instrumentation is simple and data reduction is easily accomplished

in the field. Dipole lengths of 250 or 500 meters are used. The

signals from the two consecutive dipoles, using the central electrode

as common, are narrow bandpass filtered and used as inputs to the x and

y channels of a battery powered x-y plotter. The phase shift between

the two signals is generally negligible so that the plotter will trace

a straight line, the slope of which equals the ratio of the two

observed electric field strengths. In the field a protractor is used

to measure the angle ~ between this line and the horizontal axis of

111-3

the plotter, and a pocket calculator is used to find the slope, tan CP.

Figure I I 1-1 illustrates three successive E-field ratio telluric stations

and the method of data presentation.

As the penetration depth of an electromagnetic field into

conductive material is inversely proportional to the square root of the

frequency, two frequencies have been used as a means of rudimentary

depth discrimination. For the Basin and Range valleys of Nevada, 0.05

and 8 Hz, frequencies at which there is high amplitude in the natural

electromagnetic spectrum, were found to be appropriate.

Only one component of the electric field is measured, and the

associated magnetic field is not measured at all, so calculations of

apparent resistivity cannot be made. The data are interpreted in terms

of relative electric field strength. For example, at a semi-infinite

vertical contact the current density normal to the contact must be

continuous, so the ratio of the normal components of the electric field

at the contact must be proportional to the resistivity ratio, whereas,

away from the contact over a homogeneous half-space the electric field

is proportional to the square root of the resistivity. Such qual ita-

tive interpretation, based upon some numerical modeling results, is quite

adequate for a reconnaissance technique.

In the basin and range valleys of Nevada the LBL Field crew could

generally drive along a straight line from one station to the next. The

wire that was used for telluric surveys was approximately 20 gauge,

containing a few steel strands in addition to several copper strands,

making the wire exceptionally strong. It was found that a 500 meter

length of wire could successfully be tied to the bumper of a jeep and

dragged without stretching, along a straight line so that reeling and

111-4

unreeling of wire was generally unnecessary. Abrasion of the insula­

tion was not severe because the dragged wire would often be suspended

by sage brush or would sl ide easily over soft playa deposits. This

enabled the field crew to limit each data gathering team to two people

and two four-wheel drive vehicles. Under these conditions two people

could obtain high and low frequency data at approximately eight stations

per day along a previously surveyed line. Without steel stranded wire

or over terrain where insulation abrasion would be severe from dragging,

the wire has to be reeled up and laid out again along the survey 1 ine.

This requires two people with a vehicle handling wire, and they will

have difficulty keeping ahead of a third person recording data.

The Bipo1e-Dipo1e Resistivity Mapping Method

The bipo1e-dipo1e resistivity method is an electrical reconnaissance

technique which employs a controlled-source, long-period transmitter.

A thorough treatment of the method is given by Keller, et ~., (1975).

Using a high power motor-generator set, up to 100 amperes of

current is injected into the earth between two shallow grounded

electrodes separated 1.5 to 2.5 kilometers. Numerous receiver stations

are established at various locations. The potential field gradient

resulting from the transmitted current is measured with an orthogonal

array of two grounded dipoles, commonly 100 meters in length. The

bipo1e-dipo1e electrode configuration is depicted in Figure 111-2. A

detailed description of the equipment used is given in the next section.

For each receiver station an apparent resistivity, ~a , is

calculated as the homogeneous half-space resistivity necessary to

produce the observed total electric field amplitude (regardless of

U ~l ~)i .~) ~,:1~ 4$ lJ H)~; ~,j ~j~ ::!)I

jJ ~,,)

111-5

direction) at the centroid of the receiver array as a result of the

transmitter bipole moment. Similarly, the apparent conductance, ~a,

can be calculated as the conductance (conductivity-thickness product) of

a layer over an infinitely resistive half-space required to produce the

observed total electric field amplitude at the receiver array. These

quantities are calculated from the equations

/J.::. /-.!

and

I Tx So:.{. == 1.. e P E

: 1'( 'I \.J. T

where, as illustrated in Figure 111-2,

E - (E 2 + E 2)1/2 - the total electric field amplitude T- /I :.L -

at the receiver array,

I = the transmitted current,

Tx = the transmitter length,

Rl and R2 = the distances from the transmitter electrodes to

the centroid of the receiver array.

Bipole-dipole data are generally presented in the form of a contour

map of apparent resistivity or apparent conductance, with the values

plotted at the receiver station location. The purpose of both

conductance and resistivity maps is the discrimination of layered

models having a resistive basement from more nearly uniform models.

Interpretation is qualitative, based upon the patterns observed

for simple resistivity structures which have been modeled numerically.

Model results for horizontal layers and vertical contacts are

1 11-6

presented by Keller, et~. (1975), and for various two-dimensional

structures by Dey and Morrison (1977). The dipole-dipole apparent

resistivity pseudo-sections in Part 1 I of this report (Beyer, 1977b)

have some applicability to the interpretation of bipole-dipole

resistivity data.

The LBL field crew of four people generally devoted about three

days to setting up the bipole transmitter, which included burying

several aluminum plates at each end of the bipole to obtain low contact

resistance. Once this was accomplished, a team of two people could

occupy six to twelve receiver locations per day, depending upon the

terrain which had to be traversed, and the difficulty in determining

the location of the receiver station. Later in the field program two

mottir-generator se~s were used so that receiver stations would not have

to be reoccupied for a second transmitter location. Furthermore, at

one transmitter position, two orthogonal and a diagonal bipole were

laid out. A two-person receiver crew could then record data for each

transmitter bipole at about four receiver stations per day.

The Dipole-Dipole Resistivity Method

The dipole-dipole resistivity method employs a long-period,

controlled-source transmitter, and is intended to yield the resistivity

structure beneath a straight survey line. As is shown in Figure 111-3

the dipole-dipole method employs constant transmitter and receiver

dipole lengths, ~,with increased depth of penetration being

achieved by increasing the separation between the transmitter and

receiver dipoles by integer (N) multiples of~. The upper limit on N

is determined by the maximum depth of interest or the separation at

0 0 ~J l".J d ('.\ ;; J ~~

~ d ';iI Q .. ,J f )'

111-7

which the signal at the receiver is lost in the telluric or

instrumental noise. Using the current, I, injected into the ground at

the transmitter dipole, and the resulting potential difference,

observed at the receiver dipole, an apparent resistivity, ;UCl ' is

calculated:

A V,

I • ( .) /'. V '1/ d. lV ( N + I) N ·1 ). -"':_y--- (111-1)

The apparent resistivity is defined as the resistivity of a homogeneous

half-space required to produce the observed potential difference at

the receiver electrodes for the known current injected at the transmitter

electrodes and for the geometric configuration of the four electrodes.

The calculated values of ;>d. are conventionally plotted at the

intersection of lines angling down at 450 from the centers of

transmitting and receiving dipoles to produce an apparent resistivity

pseudo-section (Hallof, 1957; Marshal and Madden, 1959). The locations

of point current sources and the positions at which the potential is

calculated are the coordinates on the x-axis.

As has been demonstrated in Part II of this report (Beyer, 1977b),

the interpretation of dipole-dipole apparent resistivity pseudo-section

data obtained over laterally inhomogeneous structures requires, at very

least, a knowledge of modeling results. Dey and Morrison (1976) and

Dey (1976) ha~e described a computer code for calculating the apparent

resistivities over arbitrarily shaped two-dimensional structures. Such

structures are of infinite extent in one lateral direction, which by

definition is the geologic strike direction. The numerical formulation

is ~ finite difference technique which calculates the potential

111-8

distribution at grid nodal points due to point current sources at

desired locations in a half-space consisting of a resistivity model.

This computer program has been used extensively in a rather

successful effort to invert (by trial and error) the dipole-dipole

resistivity data obtained in Grass Valley, Nevada.

Resistivity methods involve forcing d.c. electrical current into

the ground between electrodes at two points and measuring the resulting

potential difference at two other electrodes. While the measurements

are made under d.c. conditions, the transmitting current is a long-

period square wave to separate signal from noise. The amplitude of

the receiver voltage is measured only after transient effects due to

electromagnetic coupling or induced polarization die out.

The dipole-dipole resistivity method has traditionally been used

by the mining industry for mineral exploration. The dipole length

commonly used is 400 feet (122 meters) and occasionally 800 feet

(244 meters), with a maximum separat10n distance of 4 or 5 times the

dipole length. As applied to geothermal exploration, to explore to

the depths of interest in the conductive sediments of Basin and Range

valleys, the Lawrence Berkeley Laboratory field crews commonly used

dipole lengths of 1 kilometer with separations of N = 10 or greater.

The resistivity of the sedimentary basin fill can easily be as low as

5 ohm-meters. Equation (111-1) can be used to estimate the amplitude of

the observed potential difference as a function of the injected

current; for these specifications ..6.. V wi 11 be 1.2 ,L... volt per ampere ;'

at N = 10. Hence, the injected current must be substantial, or the

signal detection system must be sophisticated.

As a means of most quickly getting into the field the brute force

.. ': ;j'\ ':'.' ~ .J' • :i ...... "l • .;.1.,.' il, < t,·t' , ,~, ~.. ,," t'"" ,\.>, ';,<j)

111-9

method was used quite successfully. A 60 kilowatt motor-generator set

with rectifying and switching circuitry was designed, built, and

mounted on a one ton, four-wheel drive truck.

The 60 kw aircraft type generator which was used yields 3-phase

power at 400 Hz with an output voltage which can be continuously

varied from 30 to a maximum of 203 volts. It operates at 6000 rpm, has

self-contained blowers for cooling, and is small and light weight

(60 kg). The generator is powered through a step-up V-belt drive

train by a Hercules 120-horsepower industrial engine equipped with a

governor for maintaining constant speed under varying load conditions.

The bulk of the resistance in the circuit through the ground is at

the current electrodes, which are metal plates or sheets of window

screen buried at shallow depths. This contact resistance, depending

upon the number of plates emplaced and the type and moisture content of

the soil, may range from 5 to over 100 ohms. To get significant current

into the ground the output of the generator goes into a 3-phase step-up

transformer which, when the secondary windings are connected in a

V-configuration, boosts the voltage to 707 volts rms, with full power

(60 kW) taps at 565 volts. The transformer secondary can also be con-

nected in a ~-configuration to give output voltages of 408 and 327

volts rms. With these voltage options available the transmitter can be

approximately impedance matched to the ground.

After the transformer, the high voltage 3-phase 400 Hz signal goes

into a power supply consisting of two full-wave SCR rectifying bridges,

wired with opposite polarity. Control circuitry, which is electrically

isolated from the high voltage system with light emitting diodes,

alternately switches on one and then the other bridge to produce the

output current square wave. The switching rate can be varied between

111-10

1 and 99 seconds. The full-wave rectified signal has, depending upon

the transformer configuration used, maximum D.C. levels of approximately

~ 1000, ~ 800, ~ 577, or ~ 462 volts, with a ripple of less than 10%.

The maximum current capability is 100 amperes.

The transmitter system is controlled and the current amplitude

monitored at a remote control box connected by a 100-foot long

umbilical cord. This enables the operator to be removed from the noise

made by the motor-generator set.

Additional information, including circuit diagrams, regarding the

details of the high power resistivity system (motor-generator set,

remote operator controls and monitoring, safety interlocks) can be

obtained from the Manager, Special Projects Group, Lawrence Berkeley

Laboratory, Berkeley, California 94720.

Number 4 AWG welding cable was used as transmitter wire to handle

the 100 ampere current.

To measure the signals at the receiver dipoles battery--powered

strip a chart recorders were used: The Esterline Angus Tl71B and the

Simpson Model No. 2745 x-y, y-t Recorder. Non-polarizing copper-copper

sulfate porous pot electrodes were used to ground the receiver dipoles.

Signal amplitudes down to 20 microvolts were measured with these

instruments.

Later in the program a smaller transmitter unit was built. The

motor-generator set is a 25 kw, 4000 rpm, 3-phase aircraft type

generator powered by a Volkswagen engine, which is manufactured as a

unit by Geotronics, Inc., Austin, Texas. To this was added rectifying

and switching circuitry similar to that discussed above, with the

exception that mechanical relays rather than LED's were used to isolate

() o J

I I I -11

the high power components.

This lower power system was used in conjunction with clock-

synchronized signal averagers which were designed and built for the

project (Morrison, 1975). Polarity reversals of the

transmitter at a period of 10 seconds are controlled by a crystal clock

oscillator. At the receive~ a signal averager, whose clock is synchro-

nized with that at the transmitter, digitally samples and averages

voltage variations which are synchronous with the transmitter polarity

reversals. A delay time of two seconds is introduced before sampling

begins to eliminate the effects of electromagnetic coupling and

induced polarization. The averaged receiver voltage is displayed

digitally every 100 seconds (10 cycles).

The signal averagers were found to increase the data accuracy

significantly such that larger transmitter-receiver separations were

possible while reducing the power requirements of the transmitter. The

source current could generally be kept below ~ 30 amperes, which

allowed the use of no. 10 gauge wire, and meant that less effort had to

go into the reduction of contact resistance at the current electrodes.

Also, data reduction was simplified and could accurately be performed

in the field during the course of the survey. In addition to the

synchronous detectors the strip chart recorders were still used to

insure that a reasonable signal was being received and to produce a

permanent record.

To perform a dipole-dipole survey at the scale discussed above--l

km length dipoles and separations of 10 km or greater--the logistics

becomes a significant problem. The LBL field crew generally consisted

of four people. Once the dipole-length distances along a profile line

111-12

were surveyed, the crew would devote two or three days to setting up a

site for the motor-generator set from which several successive

transmitter dipoles could be energized. Several current electrodes

were emplaced which would involve burying the sheets of metal or screen

in shallow pits with sprinklings of rock salt and detergent, and then

flooding the filled pits with water. Wire capable of carrying the

transmitter current was attached to each electrode and run to the

generator location. With this accomplished a single operator at the

transmitter could connect the appropriate wires to inject current at

several dipole locations.

To receive the signal efficiently, two people, each with a chart

recorder and a signal averager, occupy stations with receiver dipoles

extending in both directions. With a particular transmitter dipole

being energized each of them can alternately hook up one and then the

second receiver dipole to the measuring equipment. This process is

repeated as the transmitter operator successively energizes a series of

dipoles. The set-up is illustrated in Figure 111-4. During these

measurements the fourth crewperson can be reeling out wire for new

receiver locations.

As a result of the high data density, and the fact that current

electrodes must be emplaced at every interval along the survey line if

a complete pseudo-section is to be obtained, the dipole-dipole method

is very slow. For 1 km length dipoles, and data obtained to a separation

of ten dipole lengths, about five data points were obtained per day.

This average includes the time required for the four-person field crew

to set up the transmitter for several dipole source locations. When

5007meter length dipoles were used the average number of resistivity

0 0 '"\; ~.) r,J ' ) 0 i!

.'t,~ iiJ \.} t{iJ i.l ~ iJ 111-13

values obtained per day increased to about ten.

As a note of warning to those interested in performing large

scale electrical resistivity surveys, a peculiar problem was encountered

as a result of having many long lengths of wire lying paral lel to one

another on the ground. On a particular occasion the amplitude of the

long-period voltage square wave at the receiver dipole was found not to

behave linearly with the amplitude of the transmitter current.

Ultimately the problem was traced back to the high power circuitry at

the transmitter. The 400 Hz 3-phase generator output is full-wave

rectified with silicon controlled rectifier bridges. Each time an SCR

switches on it produces a transient oscillation at several k Hz. The

amplitude of this transient decreases as the voltage across the SCR is

increased. This high frequency signal was found to be transmitted

along any wire lying in the vicinity of the transmitter, whether or

not it was grounded or actually connected to the generator.

Furthermore, this high frequency noise would couple into any wire lying

near and parallel to one emanating from the vicinity of the transmitter.

By this means the noise could couple into a receiver dipole wire,

producing erratic and non-linear behavior of the observed potential

difference. The explanation of this appears to lie in the fact that at

several k Hz the capacitance of a 1 km length of wire will allow for

high current flow. The current polarizes the copper-copper sulfate

electrode at the end of the dipole distant from the recording

instrument, resulting in the erratic behavior.

The solution, of course, is simple. Successive lengths of wire

cannot be allowed to lie parallel to both transmitter and receiver

dipoles which are in use. If a piece of wire which overlaps with

111-14

both is cut at any point between the two, the high frequency noise will

not be coupled through the system.

As is recorded in Figure 111-5, other practical problems have been

found with the use of long wires stretched across Basin and Range

valleys.

() 0 lJ

111-15

THE INTERPRETATION OF TELLURIC AND D.C. RESISTIVITY

DATA FROM GRASS VALLEY, NEVADA

Grass Valley is located in the Basin and Range physiographic

province [n north central Nevada. It lies within an area of higher than

normal heat flow, the Battle Mountain heat flow high (Sass, ~~.,

1971). At the edge of Grass Valley, obout 80 kilometers south of the

city of Winnemucca, is Leach Hot Springs, which has a surface flow of

about 130 liters per minute (Olmsted, et~., 1975), and surface

temperatures which reach 940 , boiling at their 1430 meter altitude.

Grass Valley is bounded to the west by the East Range, and to the

east by the Sonoma and Tobin Ranges. South of the hot springs area the

valley is constricted by the Goldbanks Hills. The distribution of major

lithographic units is illustrated in Figure 111-6, and on a separate map,

Figure I I 1-7, is shown the intricate fault and lineament pattern, based

heavily upon aerial photography interpretation (Noble, 1975). As is

corl1monly true of Basin and Range hydrothermal springs, Leach Hot Springs

is located at the intersection of two fault systems. The hot spring

area (denoted by a small circle near the center of Figure I I 1-7) lies

on a northeast-southwest trending fault (Hot Springs Fault) at its

intersection with the NNW-SSE trending range-front fault system. Opal-

ized sinter deposits are found in the vicinity of the hot springs.

Most of the geophysical data were obtained along the survey lines,

shown with 1 kilometer intervals in Figure I I 1-8. Many of the lines are

oriented northwest-southeast, roughly the principal axis direction of

the telluric field ellipse. Other lines were used as tie lines or to

I 11-16

investigate features of interest as the survey progressed. In Appendix

I I I-A (Figure~ I I I-AI through I I I-AI7) much of the data obtained along

these lines is presented.

As they will be particularly useful for the analysis of the

telluric and resistivity data, the complete Bouguer anomaly gravity map

and P-wave delay map are reproduced in Figures 111-9 and III-10,

respectively. The dominant gravity feature is a NNW-SSE trending low

along the eastern side of Grass Valle~ which, in conjunction with

seismic data, defines the region of greatest sedimentary thickness.

There is a gradual thinning of the sedimentary section to the west. At

the eastern edge of the gravity trough the steep gradient reveals a

high-angle range-front fault. In the vicinity of Leach Hot Springs is

a westward bulge in the range-front fault gravity gradient. This

anomaly correlates with a negative P-wave delay--an advance over bed­

rock travel times--and is interpreted as resulting from extensive

silicification of the sedimentary and a portion of the Paleozoic bed­

rock section in the area of the hot springs. Shallow drilling through

Hot Springs Fault near km E on Line E-E ' showed the fault to be

highly silicified.

During the exploration of Grass Valley it was determined that the

bipole-dipole mapping method was a much less cost-effective reconnais­

sance technique than the E-field ratio telluric method, and that the

bipole-dipole data could in some situations be misleading. (The reasons

will be discussed later.) As a result there is considerably more

coverage using the ratio telluric method, and it was more heavily relied

upon in developing the interpretation. For these reasons there are

presented below a detailed analysis of the telluric and dipole-dipole

; """!I 0 0 ~.; /1 '" :p t..J ' ' <~ q

resistivity data, and a separate discussion of the bipole-dipole

mapping data. It will be useful, however, when viewing the E-field

ratio telluric data in the geophysical data profile composites in

Appendix I I I-A, to note the correspondence between them and the profile

line plot of the bipole-dipole resistivity data. (The notations TXl

through TX5 refer to five bipole transmitter locations, which are shown

in Figures 111-23 through 111-32.)

The Analysis of Telluric and Dipole-Dipole

Resistivity Data

Line E-EI extends across Grass Valley from southeast to northwest,

passing about 1 km NW of Leach Hot Springs. The line is oriented

approximately parallel to the principle axis of the observed telluric

field ellipse, and at 450 to the local basin and range structure. The

general bedrock topography along the line can be inferred from both the

profi Ie gravity and P-wave delay data in Figure 111-A5. In the

vicinity of 3 km E,dense, high velocity, tertiary gravels and Paleozoic

rocks are faulted to the surface. To the NW lies a 2.5 km wide area of

relatively thin sediments. At 0.25 km E a major steeply-dipping fault

markedly increases the sedimentary section thickness. To the NW of 4 km

W the sediments gradually thin, with indication of a horst at 10 km W.

Both gravity and P-wave delay data display a regional gradient increas-

ing to the NW along the line.

Additionally, the gravity contour map in Figure 111-9 suggests

that in the vicinity of the major range-front faulting at 0.25 km E,

I 11-18

Line E-E 1 is perpendicular to the structure. It is at this location

that a fault--the one which apparently controls Leach Hot Springs-­

splays off to the SW from the SSE trending range-front fault system

(see Figure I I 1-7). The gravity bulge at Leach Hot Springs coincides

with a very high P-wave velocity and is interpreted as a region of

extensive silification. The resistivity structure may well be similar

to the density structure.

Figure I I 1-11 presents dipole-dipole apparent resistivity data for

SOD-meter dipoles along the eastern end of Line E-EI. To the west of

2 km W, the section appears to be quite uniformly layered containing a

massive conductive zone overlain by a resistive surface layer. Evidence

of a resistive basement is seen at the largest N-spacings. The eastern

half of the pseudo-section is quite complex, but some distinctive

features can be resolved. (I) The fault at 3.0 km E is made evident by

the high apparent resistivity gradient across the diagonal down-to-the­

left from this location (note the location of the 15 n-m contour). (2)

The major range-front fault at 0.25 km E produces the apparent resist­

ivity gradients across 1 ines extending diagonally down-to-the-left and

down-to-the-right from this location. The pseudo-section is complicated

by the interference of the patterns resulting from the two faults and

from varying resistivity of the sedimentary layer overlying the

resistive bedrock between the two faults. (3) At 2.0 to 2.5 km E this

surface layer is particularly conductive as is demonstrated by the fact

that apparent resistivities down-to-the-left and down-to-the-right from

this location are lower than adjacent values. A dip in both 0.05 and

8 Hz telluric profiles (Figure I I I-AS) supports this interpretation.

(4) The large conductive region enclosed by the 3 ohm-meter contour

o 111-19

extends up closer to the surface in the vicinity of 0.0 to 0.5 km E as

is seen by the low apparent resistivities down-to-the-left from this

location.

Figure I I 1-12 shows a pseudo-section which has been calculated

numerically for the two-dimensional resistivity structure at the bottom

of the figure. The profile 1 ine is at 450 to strike, and the structure

is a projection in this direction. The model contains the points

mentioned above, and the pseudo-section is a good representation of the

field data in Figure I I 1-11.

If the cross-section of the model in the profile line direction is

kept the same (to maintain the identical location of electrodes with

respect to contacts), but the model is rotated to be perpendicular to

the profile line, the pseudo-section will be modified to that shown in

Figure I I 1-13. The contour lines depict anomal ies which are somewhat

closer in form to those seen in the actual data, suggesting that the

survey line may, in fact, be more nearly perpendicular than at 450 to

the local structure. The lowest apparent resistivities in this modeled

pseudo-section are somewhat lower than those seen in the actual data,

but could be adjusted by a slight increase in the resistivities of the

and 4 ohm-meter regions in the model.

The dipole-dipole method is not particularly sensitive to the

resistivity of basement rocks lying beneath a more conductive sediment-

ary section unless one or both dipoles is located on the bedrock, e.g.,

at the edge of the valley (Beyer, 1977b). This is illustrated by

Figure I I 1-14: a model identical to that shown in Figure I I 1-12 is used,

with the exception that the bedrock resistivity is reduced from 150 to

20 ohm-meters. Only the apparent resistivities down-to-the-left from

I I 1-20

3.25 and 3.75 km E are significantly affected. The apparent thickness

of the large conductive region enclosed by the 3 ohm-meter contour is

virtually unchanged. On the other hand, a similar model with the

conductive region extending to a depth of 1.25 km (not shown) rather

than 1.5 km shifts the 3 ohm-meter contour up to N=7. Thus, the dipole­

dipole method, while not being sensitive to the resistivity of a

resistive basement, does show significant response to variations in the

depth to the basement.

Dipole-dipole resistivity data along Line E-E' was also obtained

using I km-Iength dipoles. The pseudo-section in Figure I I 1-15 displays

features which are nearly identical to those observed using 500 meter

dipoles. Comparison should be made for apparent resistivity values

having the same maximum electrode separation, i.e., N = 1, 2, 3, 4, 5

for 1 km dipoles correspond to N = 3, 5, 7, 9, 11 for 500 meter dipoles,

respectively.

The 1 km dipole-dipole data at the eastern end of the pseudo-section

display generally increasing apparent resistivities at very large N­

spacings. The values greater than 10 ohm-meters are obtained for trans­

mitter-receiver arrays straddling the massive conductive region such

that dipoles lie over the thin alluvial layer to the east of 0 km and

over a thin sedimentary section to the west of 7 km. Thinning sediments

to the west of 5 kmW are clearly indicated by the gravity and P-wave

delay data (Figure II-A5). In light of this the apparent resistivities

at large N-spacing in the western portion of the pseudo-section are

exceptionally low. For an extreme case such as a 20 ohm-meter basement

overlain by a 0.5 km thick, 4 ohm-meter alluvial layer, apparent

resistivities exceed 10 ohm-meters for N greater than 2, when 1 km

I 11-21

dipoles are used. The indication, then, is that a massive portion of

the bedrock must be highly conductive. Such an explanation would seem

improbable were it not for the fact that 23 kilometers to the south at

the edge of Buena Vista Valley, Lawrence Berkeley Laboratory geophysical

investigations revealed a massive low resistivity anomaly (modeled at

1 ohm-meter) which is several kilometers in lateral extent. Shallow

drilling encountered graphitic, pyritic Paleozoic Valmy formation and

a graphitic Mesozoic limestone (Goldstein, ~~., 1976). It is

reported (Hohmann, personal communication) that Bear Creek Mining

Company (Kennecott) has also encountered graphitic, pyritic limestones

in Nevada which cause resistivity lows. Similar conductive rock may

lie beneath the Grass Valley sediments at the western end of Line E-EI.

The tightly confined anomaly 'enclosed by the 5 ohm-meter contour at

9 km Wand N = 7 is a feature which, based upon numerical model results

for simple resistivity structures (Beyer, 1977b), should be caused by

two near-surface conductive inhomogeneities at 4-5 km Wand at 13-15 km

W. However, the 8 Hz E-field ratio telluric data (Figure 111-A5) reveal

no such anomalous features in the near-surface material. The 0.05 Hz

tel1urics indicates that a deeper portion of the section is conductive

at these locations. This is an unresolved dilemma: model ing attempts

have fai led to reproduce such a tightly confined anomaly at large N-

spacing for features buried greater than about one-half dipole length,

yet the dipole-dipole data give no indication of a conductive feature

at 4-5 km W (see Figure 111-11).

Figure 111-16 depicts a model which is moderately successful at

reproducing the actual 1 km dipole-dipole pseudo-section. The structure

at the eastern end of the line is similar to that obtained for the

I I 1-22

modeling of the 500 meter dipole data. The 12 ohm-meter block at 4-5

km W is an attempt at slightly reducing the apparent resistivities on

diagonals down from this location without producing an 8 Hz telluric

anomaly. The effect is trivial, however, and is far exceeded by that

resulting from the 3 ohm-meter block at greater depth (beneath 4-6 km W).

This. anomalous feature, and the larger 3 ohm-meter region fa~ther to the

west, are deemed to be masses of graphitic, pyritic bedrock. The shapes

and resistivity used are highly conjectural; the lack of the shallow

apparent resistivities to the west of 9 km W makes more precise modeling

impossible. However, features of this sort are required to obtain low

apparent resistivities at large N-spacing beneath 8 km W (see Figure

I 11-16) and to produce a 0.05 Hz telluric low at 10-14 km W.

As a result of E-field ratio telluric measurements employing only

one component of the electric field and no magnetic field information,

it is not appropriate to perform detailed modeling of this reconnais­

sance method. However, if the E-field ratio telluric response is

calculated over the model used to match the 1 km dipole-dipole data, the

curves in Figure 111-17 are obtained. (A computer code employing the

finite element approach to calculate the response of plane electro­

magnetic waves incident upon two-dimensional resistivity structures,

written by Ryu (1971) with modifications by Lee (personal communication),

has been used.} As the 0.05 Hz total field telluric ellipse was

observed to be roughly parallel to Line E-E ' , with ellipticity less

than 0.3, the transverse magnetic (TM) mode was used for the calculation.

The primary discrepancy between the modeled 0.05 Hz curve and the

observed data is that the former displays anomalous responses with

about twice the amplitude as those seen in the actual data. This effect

i'l f· U J U ~ u ~J J U J <,j

~ I ~J I~ ,~

111-23

could be eliminated by reducing the bedrock resistivity; as has been

discussed above, the dipole-dipole method is very insensitive to this

parameter. Additionally, the dipole-dipole model for a survey line

perpendicular to strike (Figure 1 I 1-13) would allow for some reduction

in both the conductivity and thickness of the 1 ohm-meter region.

Another major discrepancy indicates that resistive basement material

must display less relief between 5 to 11 km W. However, the major

components of the field data are illustrated: a significant increase in

the telluric field at the edges as well as in the central portion of the

valley; a sharp rise over the major fault at 0.25 km E; a dip over

conductive sediments at 2.5 km E.

At 8 Hz telluric ratio data over the same model structure (Figure

111-17) is very similar to the field data. The telluric field does not

significantly penetrate the 0.5 km-thick surface layer at this frequency,

and displays a conquctive anomaly in the vicinity of the Hot Springs,

probably resulting from saturation of a thin layer of alluvium.

For the purpose of detailing in the immediate vicinity of Leach Hot

Springs, a 250 meter dipole-length dipole-dipole survey was run along

a portion of Line A-AI. The southern portion of the resistivity survey

line coincides with Line A-AI as shown in Figure I I 1-8, commencing at 0

km and extending to the northwest. Where Line A-AI bends northward near

Leach Hot Springs, the resistivity line continues straight ahead such

that 5.0 km N falls on Line E-E I at 0.75 km W. Figure 1 I 1-18 displays

an excellent numerical model fit to the dipole-dipole field data.

Significant aspects of the model are (1) the 30 ohm-meter block from

3.0 to 3.5 km N, which represents resistive spring deposits, (2) the

possibility of more extensive silicification to the north and northwest

I 11-24

(120 ohm-meter region), and (3) an increase in both the thickness and

conductivity of sediments to the northwest of the Hot Springs Fault at

3.5 km N. There is an apparent contradiction between the seismic and

gravity data, and the model in Figure I I 1-18, which indicates that the

resistive spring deposits at 3.0 to 3.5 km N do not extend down to

bedrock. A reasonable explanation for this discrepancy is that the

actual resistive structure is roughly circular in plan, whereas, it has

been modeled as a two-dimensional feature with infinite lateral extent.

The model has been devised to provide a conductive path for current

flow beneath the resistive zone, while actually the conductive path is

around the sides of the spring deposits. An attempt at fitting the

Line A-AI data with the identical model having a 100 ohm-meter, rather

than 20 ohm-meter basement, was unsuccessful. As there are Quarternary­

Tertiary gravels outcropping one kilometer to the east of Leach Hot

Springs (Figure 111-6), the resistivity data suggest a moderately

conductive layer of these gravels overlying more resistive Paleozoic

basement between 0.0 and 3.5 km N on Line A-AI.

The 8"Hz telluric data along Line S-SI (Figure III-AI]) which runs

east-west through the hot springs area is not totally consistent with

the resistivity model from Line A-AI (Figure 111-18) in that it suggests

a conductive surface zone at the location of the 30 ohm-meter block

representing spring deposits (at 3.0 to 3.5 km N). However, a thin

conductive surface layer would not be incompatable with the modeled

data. The 0.05 Hz telluric data on Line S-SI display high resistivity

associated with the major range-front fault and spring deposits east of

0.5 km W.

Parallel to Line E-E I and about 2 km to the southwest lies Line 8-BI.

111-25

The ~ontoured gravity data in Figure 111-9 demonstrate that east of 0

km,Line B-B 1 skirts the southwestern edge of the silicified region and

traverses the range-front fault system at an ob1 ique angle. The profile

line geophysical data are displayed in Figure II I-A2. Dipole-dipole

resistivity data suggest a broad conductive feature with a shallow

depth of burial, centered at 0 km, and a surficial resistive layer west

of 2 km W. The telluric ratio data are consistent with such a model,

and in addition, indicate abrupt near-surface resistivity lows east of

1.5 km E, and at 5.25 and 6.75 km W. Comparison of the 0.05 Hz and 8

Hz data indicates that the latter two resistivity discontinuities do

not reach the surface (lest the anomaly amplitude be the same at both

frequencies), yet lie within the surface layer at a depth penetrated

by the 8 Hz signal. The broad low at the center of the E-fie1d ratio

profile is not centered over the dipole-dipole anomaly because the 0.05

Hz telluric field is more responsive than d.c. resistivity methods to

the rising resistive basement and/or spring deposits east of 0 km.

An entirely satisfying two-dimensional model fit to the Line B-BI

dipole-dipole data has proved to be elusive. One of the better of many

attempts is depicted in Figure 111-19. The arcuate shape described by

the 4 ohm-meter contour in the field data is similar to the anomaly

which would be produced by a three dipole-length wide, very thin,

conductive inhomogeneity at a shallow depth in an otherwise homogeneous

half-space (Beyer, 1977b). However, gravity and seismic (Beyer, ~~.,

1976) data dictate a sedimentary section over one kilometer thick at 0

km on Line B-B 1, and it is reasonable to assume that this might be

conductive at depth as appears to be the case along Line E-E 1, two

kilometers to the north. Additionally, the dipole-dipole data indicate

I 11-26

a thick conductive layer ( ~3 ohm-meters) west of about 2 km W. This

thick, laterally extensive conductive model tends to broaden the low

apparent resistivity anomaly pattern and eliminate the limbs which are

observed down-to-the-left and down-to-the-right from 0 km.

A plausable explanation for the mediocre fit of the model data is

that Line B-B' runs past the edge of the densified region around the

Hot Springs. The conductive anomaly seen at N = 2 in the pseudo-section

may be caused by water-saturated sediments surrounding the silicified

zone; these would constitute a thin, steeply-dipping conductive feature

off to the side of the survey line. While two-dimensional modeling is

inappropriate for such a situation, model studies (Beyer, 1977b) do

suggest that the conductive feature is considerably thinner than that

shown in Figure I I 1-19.

A thick conductive section northwest of 2 km W is required by the

dipole-dipole data. This appears to be in contradiction to the concept

of a thinning sedimentary section as is indicated by the gravity data

(after allowing for a regional gradient of about 0.5 milligal per

kilometer, increasing to the northwest). This situation would be

resolved by graphitic, pyritic rocks in the Palezoic basement as was

proposed as a solution to similar observed data and model results along

the western portion of Line E-E', 2.5 kilometers to the north.

Lines M-M' and N-N' have a point of mutual intersection with Line

8-B' at 0 km (Figure I I 1-8) near the center of the conductive anomaly.

Dipole-dipole resistivity data for each of these lines are presented in

Figures I I I-A12 and I I I-A13., respectively. (The letters across the tops

of these figures indicate intersection points with other survey lines.)

In all cases the anomaly is seen to be shallow, and found to have

uO-:u I 11-27

lateral boundaries in all directions except to the north.

The limited depth extent is perplexing as the feature lies at the

center of the gravity "trough" which runs the length of Grass Valley

(Figure I I 1-9). It also coincides with the southwestward extension of

Hot Springs Fault (Figure I I 1-7). Perhaps hydrothermal water is being

injected into the sedimentary section at a relatively shallow depth

along a ridge of silicified sediments overlying Hot Springs Fault across

the deep basement of the valley.

To the north of 3 km S the Line M-M ' dipole-dipole data display the

thick conductive section and resistive overburden layer which, on the

basis of the Line E-E' data have been modeled as a 1 km-thick, 1 ohm-

meter feature overlain by a 0.5 km··thick, 14 ohm-meter layer (Figure

I I 1-12). At the intersection of Lines M-M' and E-E ' ,the former

displays lower apparent resistivities at greater N-spacings because the

survey line is oriented parallel to strike along axis of the gravity low,

the area of greatest sedimentary thickness.

At the northern end of the gravity trough along the eastern edge of

Grass Valley the geologic section displays increased density (note the

closure at the top of Figure I I 1-9)--the sedimentary section either

thins or becomes more dense. To the south of the gravity closure the

Line M-M ' data indicate increased sedimentary resistivity north of 9 km

N. (The deflection of the 4 ohm-meter contours down-to-the-left from

2.5 and 3.5 km N result from low surface resistivity in this location.)

While a shallower depth to basement is quite likely, it also appears

that the sedimentary section may undergo a facies change or a reduction

in the degree of saturation or permeability north of 4 km N on Line

M-M'.

111-28

Modeling of the Line M-M' resistivity data is unnecessary because

an interpretation consistent with other modeled results is quite

straightforward for this simple pseudo-section. Furthermore, the

numerical algorithm used is incapable of modeling data obtained

parallel to strike.

Similarly, the dipole-dipole data along the northeastern half of

Line N-N ' cannot be modeled using two-dimensional methods. An inter­

esting effect was observed for the receiver dipole at 3 to 4 km NE and

transmitters located to the southwest. The received voltage was noisy

and very low in amplitude, resulting in the anomalously low apparent

resistivities down-to-the-left from 3.5 km NE. An explanation appears

to be that the survey line lies adjacent to the resistive depositional

feature around the hot springs, and then traverses the major range

front fault at a very oblique angle. The dipole source field will be

deformed out into the valley sediments resulting in minimal current flow

in the resistive bed rock. The receiver dipole at 3.5 km NE lies in a

pocket of conductive material isolated from the valley sediments, hence,

the surrounding bedrock shields it from high current flow, and the low

material resistivity makes for an exceptionally low observed voltage.

This low resistivity pocket is also observed in the Line F-F ' telluric

data (Figure II I-A6) at 3 km E, and in the gravity data as a small

circular low. At this location there is an outcropp of Tertiary sedi­

mentary rocks among Quarternary-Tertiary gravels (Figure 111-6) .

. Telluric data further to the northwest along Line F-F ' is consis~

tent with that from other methods. The lowest E-field ratios are

observed between 2 and 3 km W where the line traverses the thickest

sedimentary section as seen in the gravity data, and where Line M-M '

111-29

(at 4 km N in Figure I II-A12) indicates a significant conductive anomaly

at depth, and, to a lesser extent, in the surface layer.

At the southern end of Grass Valley Line O-DI extends in a

northeast-southwest direction, approximately perpendicular to the range

fronts. The geophysical data obtained along this line are shown in

Figure I I I-A4. The profi Ie gravity, P-wave delay, and telluric data

indicate a dense, resistive feature at 5.0 to 6.5 km W. On the gravity

map (Figure I I 1-9) this feature appears as a "nose" in the contours 4

kilometers north of the Goldbanks Hills. Dipole-dipole resistivity

data appear not to reveal this anomaly. What is actually the case,

however, is that while the apparent resistivities at N = 1 indicate

that a resistive feature does not lie near the surface, the low appar-

ent resistivity anomaly (enclosed by the Pa = 7 and 8 ohm-meter contours)

is caused by conductive regions to the east and west of a resistive

anomaly located at 5 to 6 km W. Such a structure is clearly suggested

by the telluric data, and as seen in Figure I I 1-20, is consistent with

an excellent two-dimensional numerical model fit to the dipole-dipole

data. The model depicts a resistive surface layer similar to that

observed to the north, underlain by two regions of more conductive

sedimentary material separated by a resistive high in the electrical

basement. Modeling of the dipole-dipole data clearly indicates that

the deep sediments along Line O-DI are at very least twice as resist-

ive as those observed in the gravity trough farther to the north in the

va 11 ey.

The E-field ratio telluric data on Line O-DI display an interesting

effect due to the orientation of the line with respect to the polariza~

tion direction of the observed telluric field. At 2 to 5 km W the

I I 1-30

major [-field axis was measured and found to remain steady (~ 100 ) in a

northwest-southeast direction, which is at 7So to line 0-0 1• Under such

conditions, with the profile line nearly perpendicular to the principal

electric field axis, small deviations in the telluric field direction

can produce large changes in the [-field ratio measured at two success­

ive stations along the profile line, resulting in an amplification of

E-field ratio telluric anomalies (Beyer, 1977a). The anomaly over the

resistive feature near the center of Line 0-0 1 is approximately twice

the amplitude that would be observed for the [-field parallel to the

profile line, and is considerably larger than any telluric anomaly

observed over the sedimentary section along the other telluric lines

in Grass Valley. The others are oriented more nearly parallel to the

principal telluric field axis.

The overshoot, and then drop, in the [-field ratio at both ends of

Line 0-0 1 can be explained by the behavior of the telluric field at a

surface resistivity contrast such as the sediment-bedrock contact at

the edge of the valley. Upon encountering resistive material the

electric field axis rotates perpendicular to the contact, which in this

case is parallel to the traverse line and produces an E-field ratio

high. With greater distance from the contact the axis rotates back

toward that of the incident field, which can then reduce the E-field

ratio anomaly, depending upon the profile line direction with respect to

the telluric ellipse. At the eastern end of the line, however~ the

bipole-dipole resistivity data indicate that a more conductive feature

is encountered in the Quarternary-Tertiary gravels.

Two-dimensional numerical modeling of the amplified E-field ratio

telluric anomalies has been attempted, using the structure derived on

111-31

the basis of dipole-dipole resistivity modeling. However, it is impos-

sible to reproduce the observed telluric field--with a polarization

d~rection of about 450 to strike, an ellipticity less than 0.3, and

either direction of rotation--without putting unrealistic constraints on

the incident ,field. If a two-dimensional model such as that illustrated

in Figure I I 1-20 is used, the observed electric field over the thicker

conductive areas rotates to be essentially parallel to strike regardless

of the polarization direction and ellipticity of the incident field

(except, of course, for a nearly linearly polarized incident electric

field perpendicular to strike). The problem lies in the fact that

Grass Valley does not look two-dimensional to 0.05 Hz incide~t

electromagnetic fields, so two-dimensional modeling is not appropriate

to a quantitative analysis of the telluric response along a line such

as 0-0 1 which is not approximately parallel to the principle electric

field direction. However, with knowledge of the polarization direction,

a qualitative interpretation is consistent with other data and appears

to be quite valid.

At its southeastern end Line G-G 1 commences on Paleozoic rock in

the Sonoma Range and extends to the northwest across valley sediments.

The 0.05 Hz E-field ratio telluric data (Figure III-A7) indicate high

resistivity at this end of the line (5 to 6 km E), with a steady

decrease to the northwest, reaching a low at 2 km W. At 1.4 km E the

line intersects Line 0-0 1 (at 3.5 km W) over the axis of the gravity low.

However, the dipole-dipole model ing for line 0-0 1 (Figure 111-20)

indicates a more resistive sedimentary section than is found farther to

the north. Line G-G 1 continues northwestward over thick sediments with

decreasing resistivity, reaching a low at the intersection with Line

I I 1-32

N-N ' (at 2.9 km SW), at the edge of the shallow thin conductive anomaly

discussed previously. To the northwest of Line N-N ' the gravity data

indicate thinning sediments; the tellurics appropriately indicate

increasing resistivity.

Line K-K ' lies 1 to 3 kilometers south of Line G-G I . The 0.05 and

8 Hz telluric profiles (Figure I I I-AID) are very similar, which implies

that the anomalous resistivity structure is not buried at great depth

beneath conductive material. However, the somewhat greater relief in

the 0.05 Hz data east of 5 km W suggests that the lateral resistivity

contrasts are not entirely surficial. The tellurics east of 1.5 km E

display a very conductive anomaly, coincident with rapidly thickening

sediments south of Panther Canyon (see the gravity ma~ Figure I I 1-9).

The resistive high at 1 km E coincides with that observed at the

intersection with Line A-AI tellurics (4 km 5 in Figure I I I-AI), and

with north-south trending faulting observed in the sediments (Figure

111-7). As was indicated by the tellurics on Line G-G I , the sediments

in the gravity trough (traversed by Line K-K ' at 0 to 1.5 km W) are

not exceptionally conductive. The resistive feature encountered along

Line D-D ' (at 5 to 6 km W) is clearly evidenced in the Line K-K ' data

at 3 to 4 km W. To the west of this the gravity map indicates somewhat

thickening sediments, which the telluric data indicate are particularly

conductive.

Line H-H' is oriented east-west through the Grass Valley saddle

between the Goldbanks Hills and Panther Canyon. The line lies over

thin, relatively resistive sediments north of the Goldbanks Hills, which

result in an E-field ratio telluric high (Figure I II-A8). The decreas­

ing telluric field west of 4 km ~ with an abrupt rise at 6.75 km W, is

o 111-33

similar to the telluric anomaly observed in model results for the elect-

ric field perpendicular to the strike of a semi-infinite, horizontal

conductive slab extending, in this case, to the east of 6.5 km W (Seyer,

1977a). However, it is somewhat unlikely, given the orientation of

Line H-H ' with respect to the geologic structure, that the strictly

transverse magnetic (TM) mode electric field is being observed along

this line. Another interpretation is that the sediments are thicker at

this location (as indicated by gravity data) and that they serve as a

subterranean channel for groundwater flow out of the canyon separating

the East Range and the Goldbanks Hills (Figure 111-7).

At 0 km Line H-H' traverses the gravity saddle point between the

northern and southern parts of Grass Valley, and indicates that the

sedimentary section in this constricted portion of the gravity trough

are very resistive. To the east of 1 km E, however, the conductive

area south of Panther Canyon is encountered.

Telluric data on Line R-R' (Figure 111-AI6) adds confi rmation to

features already mentioned. Commencing with high electric field over

the resistive Palezoic rocks of the Goldbanks Hills (2 to 3 km W), the

line displays increased conductivity over valley sediments (0 to I km W),

a resistive anomaly over the faulted area near the intersection with

Line A-AI (0.5 to 1.5 km E), and a conductive anomaly southwest of

Panther Canyon. East of 4 km E the conductive sedimentary section

appears to thin markedly, and the E-field ratio displays anomalies

which are probably a function of the alluvial thickness in Panther

Canyon.

Lines p-p i and Q_QI traverse the southern Grass Valley goose neck

between the Goldbanks Hills and the Tobin Range. The E-field ratio

111-34

telluric data along both of these li~es (Figures II I-AI4 and II I-AI5)

indicate a highly conductive section. Correlation with the substantial

gravity low in this area suggests that the conductive anomaly is caused

by saturated sediments. The telluric high seen at 1.25 km W on Line

Q_QI occurs at a small topographic high along a north-south trending

ridge at the eastern edge of the Goldbanks Hills. The feature is seen

in the Line p_pl telluric data at 0.25 km W. These anomalies correlate

with the north-south trending faults associated with the resistivity

high observed on Line H-H' at 0.5 km E, R-R ' at 0.75 km E, A-AI at 4.5

km S, K-K' at 1.25 km E, and G-G I at 5 km E.

To further investigate the highly conductive sedimentary valley

south of Panther Canyon dipole-dipole resistivity surveys using'500-

meter length dipoles were performed along the eastern half of Line H-H'

and along Line T-T ' . These lines are perpendicular to one another in a

narrow portion of the valley, so two-dimensional modeling cannot be

expected to yield an exceptionally accurate portrayal of the resistivity

structure. Two-dimensional model results which fit the observed data

should indicate a thinner conductive sedimentary section than actually

exists because, at large N-spacings, the field data will display high

apparent resistivities due to resistive bedrock along edges of the

valley parallel to the survey line. However, modeling results should

be reasonably accurate for small N-spacings and should yield an

estimate as to the bulk resistivity structure at greater depth.

The dipole-dipole apparent resistivity pseudo-sections, along with

modeling results, for Lines H-H' and T-T ' are presented in Figures 111-

21 and 111-22, respectively. The data indicate that near the inter­

section of the two lines (at approximately 2.5 km in both cases) lies

o

the center of a near-surface conductive anomaly. There is no surface

expression of anomalous conditions such as springs, but shallow-drilling

in the area has revealed an abnormally high heat flow (Sass, et ~.,

1976).

The Line H-H' pseudo-section clearly indicates the sediment-

bedrock contact with the high apparent resistivity gradient down-to-the-

left from 4.5 km E. The resistive feature seen in the telluric data

(Figure 1 I I-A8) at 0 to 1 km E is not clearly defined in the pseudo-

section, except for separations of N = 1 at 0.25 km Wand 0.25 km E.

The model which was derived (Figure 111-21) indicates a thin, exception-

ally resistive (100 ohm-meter) layer at the surface in this area, under-

lain by moderately conductive sediments with constant thickness. This

model is not entirely consistent with the topographic indication that

there is bedrock very near the surface in this area, unless the

observed faulting has fractured it sufficiently to reduce the resist-

ivity. A model with resistive material extending up near the surface at

o km on Line H-H' yielded a very poor dipole-dipole fit to the field

data. While the model in Figure 111-21 probably indicates a thicker

conductive section and a more resistive surface layer than actually

exist at 0 km, the mode)ing does suggest that a section of moderately

conductive (~8 ohm-meter) material does lie beneath a more resistive

surface sedimentary layer.

At its northern end Line T-T' transverses resistive Paleozoic

rocks, overlain in places by a thin veneer of dry alluvium. The

gently sloping .20-ohm-meter contour extending down-to-the-right from

o km in the field data pseudo-section (Figure 111-22) suggests a region

of increasing conductivity and slowly increasing thickness to the south.

111-36

The near-surface conductive anomaly at the intersection with Line H-HI

clearly extends to the south beneath a resistive surface layer south of

4 km S. Whether or not the conductive region modeled as 3 ohm-meters is

truncated at about 7 km S is not fully determined by the dipole-dipole

data. There is the suggestion of this in the slightly higher apparent

resistivities down-to-the-left from 7.5 km S, but more data to the south

is required for a definitive answer. Truncation of this conductive

region does tend to increase the apparent resistivi.ties deep in the

pseudo-section for better agreement with the field data, however this

is immaterial because, in fact, it is the truncation of the conductive

anomaly at the edges of the valley parallel to the survey line which is

responsible for the high apparent resistivities at depth.

The break in the 40 ohm-meter contour beneath km S is purely an

artifact of the pseudo-section, as is illustrated by the model results.

The apparent resistivities down-to-the-right from 0 to 1.5 km N are

reduced when the second dipole of the electrode array is located over

the shallow conductive region.

Line A-AI runs NNW-SSE, roughly parallel to the range-front fault

system along the eastern edge of Grass Valley. The 0.05 Hz telluric

data plotted in Figure I I I-AI display many of the features discussed

above, and are well correlated with the gravity data (Figures II I-AI and

I 11-9). The trace of the line can be found in Figure I I 1-8. (1) At

the northernmost end of the line is a telluric low reflecting a thick

sedimentary section. (2) A slight high at 7 to 8 km N corresponds with

somewhat thinner sediments as is also suggested by the gravity data.

(3) At 3.5 km N the line traverses the hot springs area and displays the

resistive sinter deposition at depth. (4) Between 2 km Nand 3 km S

u 111-37

gravity data indicate a relatively uniform depth to basement, but the

tel.lurics increases markedly to the south. This area has been interpret-

ed to have an increasingly resistive sedimentary section to the south.

(5) At 4 km S is the north-south trending resistive anomaly which

separates the northern and southern portions of Grass Valley. (6) South

of 4.5 km S the E-field ratio decreases sharply as the sedimentary

section thickens and becomes highly conductive in the narrow portion of

Grass Valley south of Panther Canyon.

The Analysis of the Bipole-Dipole Resistivity Mapping Data

Bipole-dipole data were obtained for five bipole transmitter

locations in the vicinity of Leach Hot Springs, with a total of 333

receiver stations being occupied along ~everal of the geophysical survey

lines. Both apparent resistivity and apparent conductance have been

calculated; these data are presented as contour maps in Figures 111-23

through 111-32. The bipole transmitters are indicated by a pair of XiS

connected by a double line. The apparent resistivities along a partic-

ular survey line are presented in the geophysical data profile

composites (Appendix I II-A). The designations TXI through TX5 refer to

the five transmitter locations.

The purpose of calculating both apparent resistivity and apparent

conductance is to differentiate between very thick uniform sections and

thin conductive sections overlying a resistive basement. For a uniform

earth uhe apparent resistivity remains constant for all locations,

while the apparent conductance increases as a function of distance from

the transmitter. For the case of a conductive surface layer overlying

111-38

an infinitely resistive basement the apparent conductance is nearly

constant at all locations, while the apparent resistivity increases with

distance from the transmitter. Depending upon the geologic situation,

one mode of calculation could prove more useful than the other in terms

of providing more uniform background values against which to observe

anomal ies.

However, in comparing the apparent resistivity and apparent

conductance maps for a particular transmitter (Figure I 11-23 versus

111-24, Figure 111-25 versus 111-26, etc.) the anomaly patterns are

nearly identical: high resistivity features have low conductance, and

vise versa. There are only a couple of minor exceptions to this. For

transmitter no. 4, the apparent resistivities along Line A-A' (the

string of numbers extending SSE from near the center of this trans­

mitter) are nearly constant, while the corresponding apparent conduct­

ance values display a steady increase to the south. This implies a

thick, uniform section, however, gravity data indicate that only a thin

conductive layer exists under, or a short distance to the east of, these

station locations. Telluric data (Figure I I I-AI) suggest increasing

resistivity to the south.

The data for transmitter no. 5 (Figures I I 1-31 and I I 1-32) display

generally increasing apparent resistivity to the north, and relatively

constant apparent conductance. This implies a layered situation with

a resistive basement, and is in contradiction to the transmitter no. 4

data. Furthermore, the data display this character for many stations

located over the thickest sedimentary section, so the interpretation

seems unreasonable.

If the bipole-dipole apparent resistivity maps (Figures I I 1-23,

111-39

111-25, etc.) are compared it will be seen that some anomalies occur at

the same location for more than one transmitter: the resistive

Paleozoic rocks of the Sonoma Range east of the hot springs; the north-

south trending resistive feature 3 kilometers west of Panther Canyon;

the resistive anomaly southeast of transmitter no. 1. The northeastern

end of this last feature appears in the Line B-B 1 dipole-dipole and

telluric data (Figure II I-A2); it appears to be an highly resistive

portion of the sedimentary surface layer. The location of conductive

areas on the bipole-dipole maps is a function of the transmitter location

and orientation. For a given distance from the transmitter, receivers

located along the transmitter axis do not sense as deeply as those

located equatorially (Keller, ~~, 1975). This is demonstrated in the

bipole resistivity maps for transmitter nos. 1 and 2. Transmitter no.

2, which is roughly perpendicula~ to the range-front fault system,

yields low apparent resistivities over the thick sedimentary section to

the east. The values are comparable to those obtained by the 1 kilometer

dipole length dipole-dipole surveys in this area. (See Figure 111-33).

However, to the east of transmitter no. 1, which approximately parallels

the range-front fault system, the.apparent resistivities are consider-

ably higher. Presumably, for this transmitter orientation resistive

bedrock is being sensed.

An attempt has been made at two-dimensional numerical modeling of

the bipole-dipole data for the resistivity structure east of trans-

mitter nos. 1, 2, and 3. The results are displayed in Figures 111-34,

111-35, and 111-36. The model shown in these figures is similar to that

devised using the dipole-dipole data on Line E-EI (Figure 111-12), and

depicts a thick conductive section west (left) of the major range-front

I I 1-40

fault at 0 km. The conductive (1 ohm-meter) material is overlain by a

more resistive surface layer, and thins to the west. The apparent

resistivity and apparent conductance maps for transmitter nos. 1 and 2

are quite similar to the respective field data maps except in areas

where shallow anomalous features lie. The modeled data for transmitter

no. 3 is not quite as successful in reproducing the field data, but the

problem here may be due to sampling: many fewer receiver stations were

occupied for transmitter no. 3 than for the other transmitters.

It is interesting that these model results yield some of the

characteristics of the field data, yet they are of questionable value to

the interpretation of the Grass Valley structure. In comparing Figure

111-23 with I I 1-34 it can be seen that the model results suggest that

the low resistivity anomaly 1 kilometer southwest of Leach Hot Springs

is simply an artifact of the transmitter orientation, rather than being

caused by a localized anomalous feature. However, a similar anomaly is

present in the field data for transmitter no. 4 (Figure I I 1-29), so it

is unclear whether or not the source of the anomaly is a departure from

the two-dimensional model structure. It appears, then, that two­

dimensional modeling is not particularly helpful in resolving the three­

dimensional data.

In the geophysical data profile composites of Appendix I I I-A the

bipole-dipole apparent resistivity data are plotted along Lines A-AI

through H-HI. In general, there is a high degree of correlation with

the E-field ratio telluric data. Resistive features are, for the most

part, consistently observed regardless of the transmitter location.

(An exception is the sinter deposition at Leach Hot Springs--at 4 to 5

km N on Line A-AI in Figure I I I-AI.) Conductive features, however, are

I 11-41

, j H't/

erratic in their bipole-dipole expression, depending upon transmitter

location or orientation. For example, for the data along Line E-E '

(Figure I I I-A5) west of 0 km, the data for two transmitters (nos. land

4) indicate decreasing resistivity, while for the other three, increas-

ing resistivity is suggested. As has previously been mentioned, this

may to some extent be qualitatively resolved by considering the locatiorr

and orientation of each transmitter.

It is clear from the bipole-dipole profile plots that absolute

values of apparent resistivity are meaningless. At many station

locations there is an order of magnitude range in apparent resistivity,

depending upon the transmitter location. This can to some degree be

attributed to differing structure in the vicinities of the various

t ransm i tte rs .

111-42

THE RESISTIVITY STRUCTURE OF GRASS VALLEY

Based upon the interpretation of the d.c. resistivity and telluric

data, in conjunction with geologic, gravity, and P-wave delay data, ,

there are various regions in Grass Valley which can be delineated.

These areas are outlined in Figure 111-37, and are numbered to

correlate with the text below. Most of the boundaries are not rigidly

defined--they are gradational, and in some cases overlapping.

Before discussing specific areas it should be noted that virtually

all of the Grass Valley sedimentary section north of the Goldbanks Hills

and west of the major range-front fault system includes a resistive

surface layer with a thickness up to 0.5 km and a resistivity of 12 to

18 ohm-meters. East of the range-front fault system is a thinning

layer of alluvium, and Quarternary and Tertiary sediments (Figure

I I 1-6) which have about the same maximum thickness, 0.5 km, and in

some locations a lower resistivity than the sediments to the west.

The exposed bedrock in the surrounding mountain ranges has a

resistivity at least an order of magnitude higher than the sediments.

Beneath the sedimentary section, however, the bedrock resistivity is

somewhat ill-defined by these data. In some areas, particularly east

of the range-front fault in the vicinity of Leach Hot Springs, fracture

permeability may result in low resistivity.

Region __ J.. As outl ined in Figure 111-37, this area is associated

with silica deposition around Leach Hot Springs. It lies at the

juncture of Hot Springs Fault and the range-front fault system, and

displays high density, high P-wave velocity, and, beneath a thin

conductive surface layer, high resistivity.

111-43

Regi?~ ___ ~ .. To the northeast of the hot springs lies,this tightly

confined gravity and resistivity low anomaly at the juncture of the

surface expression of the range-front fault system with a fault para-

llel to, and about 1.5 km north of, Hot Springs Fault (see Figure

111-7). The area lies over outcropping Quarternary-Tertiary gravels

and Tertiary sediments (see Figure 111-6).

Re~!~~.3.. To the southeast of Leach Hot Springs is an area of

thin conductive (3-7"ohm-meter) sediments at the surface, possibly due

to runoff from the springs.

~~_9.!.on ___ ~. To the southwest of the hot springs in the gravity

trough, lying beneath a resistive sedimentary surface layer (250-500

meters thick; 12-15 ohm-meters), is a layer of very conductive sedi-

ments (250-500 meters thick; 1-1.5 ohm-meters). It is unclear why

this conductive material does not extend to greater depth. The source

of the anomaly may be runoff from Leach Hot Springs into a perched

water table. Alternatively, as the area lies along the southwestward

extension of Hot Springs Fault, subsurface spring activity may, as a

result of silicification of the fault zone, inject water into the

valley sediments at a relatively shallow depth.

Region 5. To the north of region 4 is a massive, highly conduc-

tive zone in the gravity trough of the valley. The resistivity is

1-1.5 ohm-meters, and the thickness, beneath the resistive surface

layer, is up to 1 km. To the north of the indicated boundary the

region becomes somewhat more resistive, reflecting a decrease in

porewater conductivity or a facies change. The region is of interest

in that it lies in the direction of hydrologic flow in Grass Valley

and could conceivably be a plume of hot water emanating from the hot

111-44

spring area. However, shallow heat flow drilling over this feature has

revealed no thermal anomaly (Sass, et ~., 1976).

~~gi_~~_§. At the western side of Grass Valley gravity and P-wave

delay data lndicate thinning sediments, while electrical methods suggest

a considerable conductive section. The basement rock in this area may

be the Paleozoic Valmy formation, which can be graphitic and pyritic,

and therefore highly conductive.

~~_~?n7. In the two regions labled 7 the resistive sedimentary

surface layer is somewhat thicker and/or more resistive than in other

portions of the valley.

Gravity data suggest that north of the canyo~ separa­r

ting the Go1dbanks Hills from the East Range lies a subsurface bedrock

canyon which curves to the northeast and merges with the major gravity

trough. At its southern end the sediments in this subterranean canyon

have a resistivity of about 5 ohm-meters, and there is the suggestion

that the conductivity increases to the northeast.

~_~g_.~<:>~._9. This feature appears as a shallow resistive anomaly and

a gravity Iinosell extending north from the Goldbanks Hills. It would

attract little attention were it not for the fact that shallow drilling

has revealed a heat flow high (5. 1 HFU; see Sass, et~., 1976).

~eg i o~.!g_. Th is na r row, nor th- south trend i ng anoma 1 y, mos t

prominently displayed in telluric data, is associated with faulting, and

1 inks topographic highs located at its northern and southern ends. A

more highly conductive section is observed to the east than to the west.

Gravity and resistivity data suggest that at the center the anomalous

response over this region results from highly resistive surface material

rather than extensive bedrock protrusions into the sedimentary section.

() ~:iJ.J ,;0 ,~;:)

111-45

Region 11. Gravity and resistivity lows in this region suggest a

very conductive (3 ohm-meter) sedimentary section about 750 meters

thick, with 1 itt1e or no resistive overburden layer. There is no

surface expression of moisture, but in the northeastern portion of this

region shallow drilling displays high heat flow (Sass, et ~., 1976).

In the Panther Canyon area along the northern edge of this region,

numerous microearthquakes have been recorded.

Region 12. The conductive feature of region 11 continues to the ---,,-_.,," ~--~ ... -.

south beneath a resistive sedimentary surface layer.

111-46

RECOMMENDATIONS

Several of the regions which are delineated in Figure 111-37

warrant further exploration or deep drilling to assess their geothermal

potential.

Re~ i on J... Hot Spr i ngs Fau 1 t and the surface express i on of range­

front faulting intersect at the northeastern edge of this region. This

intersection appears to be responsible for the hot spring activity.

Drilling in the northwestern half of this area could intersect these

faults at depth .

. Region~. This conductive, low-density anomaly is at the junction

of range-front faulting with a fault parallel to Hot Springs Fault. A

shallow heat flow drillhole would be warranted.

~egion~. With the potential of being a plume o~ hot water extend­

ing northwest from Leach Hot Springs, this anomaly is of interest.

However, shallow heat flow drilling to data has been unimpressive (Sass,

et ~., 1976), so it appears that the anomaly must be attributed to a

thick, highly conductive, but cold sedimentary section.

R~gion~. This resistive, high-density feature was found to dis­

play high heat flow. This suggests a basement horst structure, or an

area of hydrothermal deposition if there is hot spring activity. There

is no evidence of surface moisture, however, region 8 to the northeast

of its contact with region 9 demonstrates increasing conductivity, so

there may be subsurface hot spring flow into the sediments in this

region. Heat flow drilling in the vicinity of region 9 and the

northeastern part of region 8 is recommended.

Region.!! .. High heat flow along the northeastern portion of this

region, in conju~ction with a thick, conductive sedimentary section

111-47

(which extends to the south in region 12), and microearthquake activity

in the Panther Canyon area, make region 11 a candidate for additional

hydrologic, geophysical, and shallow heat flow exploration.

111-48

EVALUATION OF THE EXPLORATION METHODS

Bipole-Dipole Resistivity Mapping

Based upon the bipole-dipole mapping data obtained in Grass Valley,

the method appears to have few redeeming qualities.

The comparison of apparent conductance and apparent resistivity data

should allow a discrimination of layered models with a resistive basement ,

from more nearly uniform structures. However, almost without exception

the Grass Valley resistivity and conductance maps for a particular

transmitter yield nearly identical patterns.

For a given transmitter location the bipole-dipole resistivity

mapping method inherently suffers from a lack of ability to discriminate

between shallow and deep anomalies. This has been demonstrated for

receiver locations along the transmitter axis in Part I I of this report

(Beyer, 1977b) and for arbitrary locations on the surface by Dey (1977a).

The fundamental difficulty is the single transmitter location. In an

effort to resolve the ambiguity, mUltiple transmitter locations are

required, but then the method becomes time consuming and expensive, and

can no longer be considered as a reconnaissance technique. Worse yet,

the apparent resistivity (or conductance) maps for two transmitter loca-

tions can be quite different, even contradictory, without there being

sufficient information to resolve the differences. An additional

transmitter location is just as likely to add to the confusion as it is

to resolve the ambiguity. In concept, of course, the more ttansmitter

locations which are employed, the more tightly constrained the problem

becomes. In practice, however, only two or three transmitter locations

are used, and it appears that two-dimensional modeling may be inadequate

OOdU;. u·~ "HJ IJ d;J v'<) {},.) ~)jJ G 111-49

for a method such as this which maps over the surface.

The bipole-dipole data in some locations demonstrate good correlation

with E-field ratio telluric and dipole-dipole data, but in a number of

locations do not. These differences, once again, must be resolved in

terms of transmitter location, or more specifically, the resistivity

structure in the vicinity of the transmitter.

E-Field Ratio Tellurics

The E-field ratio telluric data from the many survey lines in Grass

Valley are very consistent with one another: anomalies tend to correlate

from one line to the next, and similar response is found at the inter-

sections of lines. For example, resistive bedrock and/or sinter deposits

are expressed in the data from Lines A-AI at 4.75 km N, E-E I at 0.75 km

E, and 5-5 1 at 0.25 km E. There is a variation in anomaly amplitude,

however, as the profile line direction changes with respect to the

orientation of the telluric field ellipse. As has been demonstrated by

numerical model studies in Part 1 of this report (Beyer, 1977a) some

caution must be exercised in the orientation of E-field ratio telluric

lines. Yet, Line 0-0 1, which is nearly perpendicular to the observed

principal telluric field axis yields data consistent with the other

telluric profiles and with other types of geophysical data; the

amplitude of anomalies appears to be exaggerated, however.

The telluric data are, for the most part, consistent with a

reasonable interpretation of gravity, seismic and d.c. resistivity data.

Over most of Grass Valley, measurements at only 0.05 Hz were made so that

no depth discrimination was possible. In some areas where it is diffi-

cult to reconcile the 0.05 tellurics with d.ci resistivity results an

III-50

explanation could possible lie in the response of the telluric field to

deeper structure than is observed by the resistivity methods. Along

Line E-E 1 where both 8 and 0.05 Hz telluric data were recorded there is

good agreement between the high frequency tellurics and the dipole-dipole

resistivity data for small N-spacings.

Dipole-Dipole Resistivity

The dipole-dipole resistivity method, when performed with 500 meter

or I kilometer dipoles to separations of N "" 10, is exceptionally slow

and expensive. In normal exploration It would not be used as extensively

as was the case in Grass Valley. The intent, however, was to use the

method as a basis for evaluating other'electrical techniques. If a

geologic strike direction is well defined, two-dimensional numerical

modeling of a highly data-intensive method using a controlled source

affords the possibility of detailed determination of the structure.

The process used to invert the field data was trial and error. At

the present time. inversion of complex two-dimensional data by numerical

estimation schemes is not tractable because there are too many variables.

In the case of the computer code used by the author, the resistivity of

every element in a 113 X 16 grid is a variable. The horizontal

resolution, and the vertical resolution to a depth of two dipole lengths,

is one-fourth of a dipole unit. At greater depths, the nodal separation

increases to one-half~ one, two, and more dipole lengths. The use of

the human mind, rather than a numerical estimation technique, to invert

data such as dipole-dipole resistivity pseudo-sections, has the advan­

tages of incorporating reasonable determinations based upon other types

of data, and allowing for a sophisticated weighting of various subtle

factors observed in the data. The disadvantage, of course, is that

III-51

models based upon preconceived notions or biases of the interpreter may

be developed while other reasonable possibilities are overlooked.

This relates to the problem of uniqueness. To what extent are

models developed by trial and error unique? They are not. However, if

there is a reasonable matching of the field data, and the interpretation

is consistent with geological and other geophysical information, then

the model derived by trial and error is certainly as valid as those

arrived at by other means.

It was demonstrated in Part II of this report (Beyer, 1977b) that

the study of modeled dipole-dipole response to numerous simple structures

was essential to the proper conceptual interpretation of dipole-dipole

pseudo-section data. In deriving the models presented as interpretations

to the Grass Vall~y data is has been learned that modeling (inverting)

dipole-dipole pseudo-sections using detailed two-dimensional models is

an exceptionally difficult, time-consuming, and frustrating task. After

an interpreter has devoted considerable effort to studying pseudo-sections

for simple models and has attempted fitting field data, many character-

istics in the pseudo-section which relate to actual structure will

become evident. Yet, even at this point it may be difficult to obtain

a close fit to the field data pseudo-section. The proper trends may be

present in the model data, but the effects due to a particular portion

of the structure may be undesirably dominant, with the result that con-

tour lines do not resemble those in the field data. For near-surface

model features, extensive juggling of the resistivity, thickness, and

lateral boundary location may be required before a good fit can be

obtained. For example, in the Grass Valley models presented the surface

location of the bedrock-sediment contact is of supreme importance to all

I I I-52

apparent resistivities down-to-the-1eft and down-to-the-right from the

dipole straddling the contact. It was found in modeling the dipo1e­

dipole data on Line D-D' (see Figure I I 1-20) that if the surface contact

at 8.75 W was moved one-quarter dipole length to the right, all the

apparent resistivities down-to-the-right from 8.5 W would be markedly

increased. Furthermore, the apparent resistivity values down-to-the­

right from 9.5 km W would be somewhat reduced.

Some of the models presented yield excellent fits to the observed

dipole-dipole data. One of these, the model for Line A-A', was

arrived at after six attempts. All others, however, required over a

dozen attempts, and in the case of Line E-E', twenty. A satisfactory

model fit'was never obtained for Line B-B', possibly because of

significant conductive structures adjacent to the survey line. On the

other hand, the model for Line A-A', which yields such a good fit, is

not completely consistent with other data in that the resistive mass at

3.25 N (Figure 111-18) does not extend deep enough. The physical

situation is clearly three-dimensional, yet in this case a two-dimen­

sional model produces the desired result.

Clearly, the definition of anomalous structures is not precise.

Certainly the detail displayed in the models at depth cannot be

regarded as exact. However, if considered conceptually, the models

appear to be valid in that they are generally consistent with other

data and can be interpreted in terms of reasonable Basin and Range

geologic structures.

III-53

CONCLUSIONS

No deep drilling and logging has been done in Grass Valley, Nevada,

so ultimately it is impossible to assess either the relative values of

the geophysical methods stressed in this report, or the validity of a

geologic interpretation based on these or other data.

The only tests which remain to evaluate a technique are its internal

consistency, its correlation with geological and other geophysical

methods and its interpretability in terms of developing a reasonable

geologic model.

Using these criteria the analysis of Grass Valley data indicates

several points.

The bipole-dipole resistivity mapping method was the least useful

of the electrical methods e~ployed because of the shallow versus deep

anomaly ambiguity for a single transmitter, and the different but

uninterpretable anomaly patterns for multiple transmitters. The com-

parison of apparent resistivity and apparent conductance maps for Grass

Valley gave ambiguous or contradictory results.

As a reconnaissance electrical exploration technique the E-field

ratio telluric method appeared to be highly successful. The only regret

was that 8 Hz data were not obtained along most of the survey lines.

Every telluric profile was consistent with adjacent or intersecting

profiles, and there was a high degree of correlation with gravity,

P-wave delay, and resistivity data in terms of developing a reasonable

model of Grass Valley. In areas where stations can be occupied along

straight lines whose orientations can range from perpendicular to

strike to parallel to the principle axis of the telluric ellipse, the

method is very useful for estimating the resistivity structure with

· I II-54

respect to another area along the survey line (supplimentary geological

and geophysical information is necessary of course). The greatest

drawback of the method is that absolute values of apparent resistivity

are not obtained; another technique is required to make this assessment.

The E-field ratio telluric method is suggested as a qualitative

technique. A familiarity of the response from simple models is

desirable (Beyer, 1977a), but detailed modeling seems inappropriate

because the magnetic component of the magnetotelluric field is not

measured. Furthermore, however, the two-dimensional modeling which was

attempted indicated that at 0.05 Hz the basin and range structure in

the vicinity of Leach Hot Springs can not be treated as two-dimensional.

This also means that long period m~gnetotelluric data from Grass Valley

cannot be correctly interpreted using two-dimensional modeling.

Two-dimensional modeling of the dipole-dipole resistivity pseudo~

section data was generally successful at delineating geologically

reasonable conceptual models of the Grass Valley resistivity structure,

with, in many areas, a good estimate of the actual resistivity. The

models developed are, for the most part consistent with the gravity and

telluric data, but this may be, because these methods were instrumental

in the designing of the dipole-dipole models. The telluric and dipole­

dipole resistivity data compliment one another particularly well.

In spite of yielding some very useful resistivity models, however,

the dipole-dipole method must be regarded as at best, a very qualified

success. When employed on a large scale (1 kilometer dipoles to a

separation of N = 10) the method requires heavy and sophisticated

equipment, and is very slow. Hence, it is exceptionally expensive. To

probe to great depth in conductive valleys, enormous transmitter-receiver

III-55

separations must be employed. Invariably, major lateral resistivity

contrasts will be traversed which make for difficulty in interpretation.

Indeed, the severest drawback of the dipole-dipole method is that the

pseudo-section data are exceptionally hard to interpret. The point

must be strongly made that a geophysicist unfamiliar with dipole-dipole

pseudo-section model results should neVer attempt to interpret a

resistivity pseudo-section which shows evidence of lateral resistivity

contrasts. Furthermore, even with such knowledge, it can be exception-

ally difficult to obtain a close model fit to complex pseudo-section

data.

I II-56

RECOMMENDATIONS

1) For electrical reconnaissance exploration of several hundred

square kilometer areas, use of the bipole-dipole resistivity mapping

method is not advised.

2) For such exploration, the E-field ratio telluric method,

employing both a high and a low frequency appropriate to the resistivity

structure under investigation, is highly recommended.

3) An electrical method yielding apparent resistivity and which

can be modeled to obtain absolute resistivities, should be used in

conjunction with the E-field ratio telluric method at some locations.

4) For the dipole-dipole method employing large dipole separations

to become useful for the structural delineation of local anomalies of

interest, two-dimensional numerical inversion schemes must be developed.

It may be possible to reduce significantly the number of variables in

a two-dimensional model by first evaluating the significance of

varying the resistivity of various mesh elements as a function of

depth. Larger units~ at depth than have been used in this study could

be assigned a uniform resistivity. Additionally, based upon the

particular field data being inverted, the elements in various near­

surface regions could be constrained to have equal resistivities, or

even a fixed resistivity value. Extensive knowledge of pseudo-section

modeling would be required to make this type of decision, however.

. .

III-57

REFERENCES

Beyer, H., 1977a, Telluric and d.c. resistivity techniques applied to the geophysical investigation of Basin and Range geothermal systems, Part I: the E-field ratio telluric method, LBL-6325-1/3: Lawrence Berkeley Laboratory, University of California, Berkeley.

Beyer, H., 1977b, Telluric and d.c. resistivity techniques applied to the geophysical investigation of Basin and Range geothermal systems, Part I I: A numerical model study of the dipole-dipole and Schlumberger resistivity methods, LBL-6325-2/3: Lawrence Berkeley Laboratory, University of California, Berkeley.

Beyer, H., Dey, A., Liaw, A., McEvilly, T.V., Morrison, H.F., and Wollenberg, H., 1976, Preliminary open file report: geological and geophysical studies in Grass Valley, Nevada, LBL-5262: Lawrence Berkeley Laboratory, University of California, Berkeley.

Dey, A., 1976, Resistivity modeling for arbitrarily shaped two-dimension­al structures, Part I I: User1s guide to the FORTRAN algorithm RESIS2D, LBL-5283: Lawrence Berkeley Laboratory, University of California, Berkeley.

Dey, A., and Morrison, H.F., 1977, An analysis of the bipole-dipole method of resistivity surveying, LBL-6332: Lawrence Berkeley Laboratory, University of Cal ifornia, Berkeley. Also, submitted for publication to Geothermics.

Dey, A., and Morrison, H.F., 1976, Resistivity modeling for arbitrarily shaped two-dimensional structures, Part I: Theoretical formulation, LBL-5223: Lawrence Berkeley Laboratory, University of California, Berkeley.

Goldstein, N.E., Beyer, H., Corwin, R., di Somma, D.E., Majer, E., McEvilly, T.V., Morrison, H.F., Wollenberg, H.A., and Grannell, R., 1976, Open file geoscience studies in Buena Vista Valley, Nevada, LBL-5913: Lawrence Berkeley Laboratory, University of California, Berkeley.

Hallof, P., 1957, On the interpretation of resistivity and induced polarization results: unpublished Ph.D. thesis, Massachusetts Institute of Technology, Cambridge.

Keller, G.V., Furgerson, R., Lee, C.Y., Harthill, N., and Jacobson, J.J., 1975, The dipole mapping method: Geophysics, v. 40, no. 3, p. 451-472.

Marshall, D.J., and Madden, T.R., 1959, Induced polarization, a study of its causes: Geophysics, v. 24, no. 4, p. 790-816.

III-58

Morrison, H.F., 1975, The up-down counter: a synchronous signal averager for earth resistivity measurements, in The study of temporal resistivity variations on the San Andreas Fault: Technical prog­ress report on U.S.G.S. grant # 14-08-0001-G-124, period ending February 1, 1975; Engineering Geoscience, University of California, Berkeley.

Morrison, H.F., McEvilly, T.V., Wollenberg, H.A., and Goldstein, N.E., 1977, (Final report on the geothermal assessment of three sites in north central Nevada. Report in preparation.) Lawrence Berkeley Laboratory, University of California, Berkeley.

Noble, D.C., 1975, Geologic history and geothermal potential of the Leach Hot Springs area, Pershing County, Nevada: a preliminary report to the Lawrence Berkeley Laboratory.

Olmsted, F.H., Glancy, P.A., Harrill, J.R., Rush, F.E., and Vandenburgh, A.S., 1975, Preliminary hydrogeologic appraisal of selected hydrothermal systems in northern and central Nevada: U.S. Geo1. Surv. open file report 75-56.

Ryu, J., 1971, Low frequency electromagnetic sounding: unpublished Ph.D. thesis, University of California, Berkeley.

Sass, J.H., Lachenbruch, A.H., Munroe, R.J., Greene, G.W., and Moses, T.H., 1971, Heat flow in the western United States: JGR, v. 67, no. 26, p. 6376-6413.

Sass, J.H., Olmsted, F.H., Sorey, M.L., Lachenbruch, A.H., Munroe, R.J., Ga1omis, S.P., Jr., and Wollenberg, H.A., 1976, Geothermal data from test wells drilled in Grass Valley and Buffalo Valley, Nevada; LBL-4489: Lawrence Berkeley Laboratory, University of California, Berkeley.

III-59

(for numbering sequence only)

"0 Q) LL:'

10

UQ) '':::: "0 -::3

~:; 1.0 E2 = tan ¢,

E E~ : (tan ¢,) (tan ¢2)

E E~ = (tan ¢,Htan¢2)(tan¢3)

-a.

w E 1____::::::~ __ ~ ________ L_ ____ ~~-------L----~~~~----L-------~-----~«

~ o.d

E,=I (arbitrary)

E,

E' EN Em Ef = tan ¢, ~ = tan ¢2 . Ei = tan ¢3

~¢, ffi]~ ~~ x y 'x y ~x~y,

II E" ___ E:z ____ J I L-----~-----1-11 i E; I Jl ' E2 Jlrumuu-·----E3---m-mmJ lL L---mmmE4m---u---m -Uj

Air

o 0.5 1.0 1.5 2.0 Earth

Distance (kilometers)

X BL 773- 5229

Figure 111-1. Three successive E-field ratio telluric receiver stations and data plotting method.

I

'" o

lll-61

Tx

BIPOLE TRANSMITTER

MOBILE RECEIVER ARRAY

Er

Figure 111-2. Plan view of the bipole-dipoel resistivity mapping array.

J..--o -I- No --I C( 0 ~ TRANSM~ITTE~DIPOLE RECEI~LE

I ~50" I A I A I /7'\ I /' 4tv1'50 7 / ,/,,, / . , x ~ )< y /

'" // "/ " / '" / '" / / N=I 21.7 X X )< -)< /----"/ "'/" / '/ '" / N=2 --28.0 )<. X -)<. ~-----

"-./" / "/ "/ ""-N=3---35.6 X ~ ~ " ." / '" / , /" ""-

N=4---42.8 ;X X ~ ,,--,,/, / '" "-. N=5 --- 47.9 )< ';:'" ~-

'" / "''' " N=6-----Y. ~ , ~ ~

Po = k /).i. WHERE GEOMETRIC FACTOR, k = 17" oN (N + I){ N +2), AND N = 1,2,3, ...

Figure 111-3. The dipole-dipole electrode configuration and the construction of the apparent resistivity pseudo-section. The particular transmitter and receiver dipole locations shown are separated by (N =) 5 dipole lengths, and are used to calculate the apparent resistivitY'~a=47.9.

, ,

I 0"­N

TRANSMITTE R STATION

Motor­Generator

2 RECEIVER STATIONS

Strip Chart Recorder and Signal Averager

:/ "" / / /

/ / / /

/ / / /

'/ / ' / / ''f/ N = 3 / " / " / " / " , / , /

" /

Y N=7

Air

Earth

XBL 776-9367 Figure 111-4. Diagram of multiple dipole-dipole transmitter and receiver locations used by LBL field crew for I km long dipoles. The generator and receiver instruments are connected to dipoles for which apparent resistivity values can be calculated at N = 3 and N = 7 in the pseujo-section.

I

"" w

c r~,

S~'

c::

,\.;;,.-. . ,,-, it; c":

'C. ,;; ~.

0', ~

.r:~~'"

r.'.'" '0,

111-64

, -~

\ 5 ,,~~: (:S ~

~~~ ~ ~5::> '-.J \ ~~

<X\ No matter which direction the time '&tJ'<J flows, this seems like a highly im-probable sequence of events. 4.S

The Observer.

1 ~~

D~ f)~ V J'7 <?

..J-~

~o\' tlOv-l !.-,V"" ''''if,LY v~

(,~~ ()\l~ ... "t:r.",fU~.

) - f:l'\ 7 <ofJ '!:7 c, y... \J

~ ~C,LQ~

\7( ~v~

~\ ltl S ~ I. ~'G Z - ~'(J \)l\'\

-- r}· ~ \,"

\-

~~ ~ /"

~ k... ~\" ~~V'~~ J

~~

-- ."'-1

f t\V" \t.i\~t.

: W ~~ CI .. "V -~~~~ ~

C) ~"'~~ .. - ... '«,!s.. -A\~- il , ~

/--\

~-

Figure 111-5. An encounter between an LBL field crewman and a member of the bovine persuasion over the proper use of a long receiver wire, as recorded on the receiver strip chart record. Additional graffiti appear­ed after the record segment was posted as a warning to others destined for field work in Nevada.

117°45'

i

j

:

LITHOLOGIC MAP LEACH HOT SPRINGS AREA

GRASS VALLEY. NEVADA L! ~·r_O~, ~_~~~ KILOMETERS

! .p .MLES

.'

.

0

~ .

.

,

\~ ~

,;.p t)

111-65

"/

.. , ,.,

\- ..... , ..

\.

.~<'f. _'_' __ :-;-:-_ ~'- t

'\ " "

Figure 111-6. Lithologic map. Leach Hot Springs area. QalxBL7769117 alluvium, Qos: older sinter deposits, Qsg: sinter gravels, ~Tg: Quaternary-Tertiary gravels and fanglomerates, Tb: Tertiary basalt, Tr: Tertiary rhyolite, Tt: tuff, Ts: Tertiary sedimentary rocks, Kqm: quartz monzonite, Kg: granitic rock, md: mafic dike, TRg: Triassic granitic rocks, TR: undifferentiated Triassic sedimentary rocks, P: undifferentiated Paleozoic sedimentary rocks.

FAULT MAP LEACH HOT SPRINGS AREA

GRASS VALLE~ NEVADA ! ";; ? 1 KILOMETERS

• ~ (} ~ MILES

111-66

\ ~ .

Figure 111-7. Fault map of the Leach Hot Springs area. Hachured769116 lines indicate down-faulted sides of scarplets; ball symbol indicates downthrown side of other faults. Leach Hot Springs is denoted by a small circle near the center of the map, and Hot Springs passes through it to the northeast and southwest.

d

0'

® '--":~-f~'~--"

A'

!.... ' -f .f'. : ~.!;~.

~;;~.; .

,,~ ,.'

<<'t,

Figure I I 1-8.

,~~~-j 1,,' '~ J 'I " 1 " ,;, ~ , f) I',,' 'i,}l JP ' .. i \,,J-d

" ,e,l /' 111-67

117" 45'

i~ r

'.:: , .~;:~.-

\@

Geophysical survey

.~- \

, ;'-~j.

~':'i~tJ 'r·~:i·, -"<-1 '

I

I I '-

~.-

.:

:' : ~ ..

.. '-

([) ..

---,-::., .. -:

""p~

,~

-;

I' (-'@

, '<1 ,.'::

,I -"

-'~,; :1- f '"

• -,. ,~ ,0

·,~~:;"'r[~L:~,~::·~ " l 40°30'

:!: .. :-~~r'-~;-7o+7"'.---;--~-1:H@ ). "-- !

i

XBL 75B-3669-A

ines in Grass Valley, Nevada.

Figure 111-9. Nevada.

111-68

XBL 768-3302

Complete Bouguer anomaly gravity map, Grass Valley,

000 u. ...

·100 • Q , ,

1\

P WAVE DELAY MILLISECONDS

111-69

Figure I I 1-10. Normalized P-wave delay, in milliseconds, for southern Grass Valley. Symbol shows location of Leach Hot Springs, XB176B10125

an area of strong negative delays.

west FI~LD DATA -- CRASS VALL~Y. LIN~ ~-~. 500 M~T~R DIP~L~S

DIP~L~-DIPOL~ APPAR~NT R~SISTIVITY PS~UD~-S~CTI~N East -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -oj. 5 -oj. 0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Km

2

I I I I I I I I I I I I I I I I I I I I I I

3

'i

11.1 10.7 10.7 11.5 11.6 11.5 11.7

6.5 6.3 7.9 6.5 6.6 6.1 9.q

10.0 10.3

--7- ~ 5.6 6.0 5.7 5.6 5.6 6.1 -

5

-5 q.6 q.q~

6

-q 3.7 3.7 3.5 3.2

3.6 3.5 3.3 3

3.5 3.2 3.2 ~7~92'72./·5 e 3.3 3.6 3.3 3.3 2.1 2.5

9 3.6 3.6 ~ 3 2.6 2.0 10 3.V~.3 3.7 3.3, 2.2

II q.9 3.9

3

8

9

N

2

XBL 776-9107

Figure 111-11. Field data dipole-dipole apparent resistivity pseudo-section for 500-meter dipoles along Line E-E ' .

I -.....J o

West

M~DEL -- CRASS VALLEY, LINE E-E' (500 M DIP~LES) DIP~LE-DIP~LE APPARENT RESISTIVITY PSEUD~-SECTI~N

PR~FILE LINE IS INCLINED AT ~5.0 DECREES T~ STRIKE East -6.0 -5.5 -5.0 - ... 5 - ... 0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 K m

I I I I I I I I I I I I I I I

N ~

11.11 11.11 11.11 11.11 22.0

2 7.6 ___ 77 1.0 2

3 3

11 .. 5 :r.g-- i.:::-s -:::--::---~----:::-:::-- 5

6 2.6 2.6

7 2.6

s 2.S

9

10

2-D RESISTIVITY M~DEL, ~5. DEC. PR~JECTION--CRASS VALLEY, LINE E-E' 1.0 1.5 2.0 2.5 3.0 3.S Km

~ ~ 6 LfJS! ro. o

I- 0.5

Km 1.0- 150.n..-m f- 1.0

1.5~------------------------------------------------------------------~ f- 1.5

2.0 -

Figure 1 I 1-12. Two~dimensional at 450 to strike) and resultant meter dipoles along Line E-E'.

f- 2.0

XBL 776-9106

resistivity model (projection in the profile line direction dipole-dipole apparent resistivity pseudo-section for 500-Basement resistivity is 150 ohm-meters.

I

'-'

ef t"\

~.::.:

~

1/$;>,

'" ,.,.~

~~"

West

M~DEL -- GRASS VALLEY, LINE E-E' (500 METER DIP~LES) DIP~LE-DIP~LE APPARENT RESISTIVITY PSEUD0-SECTI0N

PR~~ILE LINE IS INCLINED RT 90.0 DEGREES T0 STRIKE East -6.0 -5.5 -5.0 -'1.5 -'1.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 S.S K m

I I I I I I I I I I I I I I I I I I I I

~ N

21/.8

2 2

3 'i.q~t1 q.q 'I.q q.q q.q q.5 3

'I 279-32 .. 6 2.6 2 .. 6 2.6 2 .. & 2.'1 1.9 1.7 2.5 .. 5 2.3 2.3 2.2 2.2 2.1 1.8 1.5 1.7 2.7 5

6 2.2 2.1 2.1 2.0 1.6 1.'1 6

7 2.3 2.2 2.1 1.7 1.'1 7

8 2.5 2.3 1.8 1.5 8

9 2.5 2.0 1.7

10 2.2 1.8

11 2.0

2-D RESISTIVITY MODEL -- GRRSS VRLLEY, LINE E-E' -6.0 -5.5 -5.0 -'1.5 -'1.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 2.5

0.0 J 3.5 Km ! 0.0

3.0 0.5 0.0 1.0 1.5 2.0

0.5

Km 1.0

1.5

2.0

Figure 111-13. line direction pseudo-section

150 .!l-m

Two-dimensional resistivity model (projection in the profile perpendicular to strike) and resultant dipole-dipole apparent for 500-meter dipoles along Line E-E ' .

0.5

1.0

1.5

2.0

XBL 776-9104

resistivity

I -.....J N

West

MODEL -- GRASS VALLEY. LINE E-E' (500 M DIPOLES) DIPOLE-DIPOLE APPARENT RESISTIvITY PSEUDO-SECTION

PROFILE LINE IS INCLINED AT ~s.o DEGREES TO STRIKE East

-£.0 -5.5 -5.0 -\1.5 -1j,.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Km 1 1 1 1 1.1 1 1 1 1 1 1 1 1 1

N N

6.2 S.6 13.3

2 10.7 2

3 3

~ 15.6 '1 --j

5 2.6 2.6 2.5

6 2.3 2.3

16.2 5 /,s-

S.l 13.~ 6

7 2.'1 2.3 6.3 7

6 2.5 7.2 e

S 2.7 S

10 10

11 11

2-0 RESISTIVITY MODEL. ~S. DEC. PROJECTION--QRASS VALLEY. LINE E-E1

0.0 I 3.5 Km I- 0.0

-£.0 -5.5 -5.0 -1j,.5 -1j.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.5 0.5

Km 1.0 1.0

20.n..-m 1.5 1.5

2.0 2.0

XBL 776-9110 Figure 111-14. Two-dimensional resistivity model (projection in the profile line direction at 45 0 to strike) and resultant dipole-dipole apparent resistivity pseudo-section for 500-meter dipoles along Line E-E'. Basement resistivity is 20 ohm-meters.

I

,-' .. ,-,.,' ........ , 0/;..., ... .;>~-

~.' (",",

'i..._

,~

~'';;.

.• 01:;....~

c:

'" "'....,,,' ~" \..r.,,-..'"

"",,",,-

.... (",

(,;~ -.....J, W ·V.

f~':"

CJ

Kilometers

GRASS VALLEY Line E-E '

4 East ~~~~~~-r4-~~~~~rI~1 West 14 12 8 6 4 1 . . . . 4 J45

• 0 5 J ~ 2

4

N 6 8

10

12

14

Dipole-Dipole 8·1 7.7 J.O'" 6.4 6.9 6.2 5.0 5.V)J~2JJ~.1 8.1

A t 1.3 fi·3 . 7.8:==3.5-3.3-3.2-3.2-3.6_ 2:D~ 5.2 P par en 8.8 1.1 .8 \ 4.1~.2;{ A 3.1-- 2.5 2.3 ~3.0 L4.4-4.~

Resistivityo 1· ..... .J.2 8.1 } 6.0 4.9 4.8 ~ '3.2~3.1...1 4.2 4.2/ -5.5~~ 5. 4.1 4.1 4.1--J.6--4.3-4/ 3.9,'5

0_6.0~5.4 5.6 5.4 4.3 5.6 5.2~4.7 ~-"'5.0 .. "'''' 6.6 \ 4.4 t:3." 5. 6.0~6.1 6.8 9.0 6.0

6.0 W/r.-.. ..:J 6.6 1.~10.8-1I.6~9 '0 1 5.4-5. \ t 8. 1.2 ;{19~ 10

o 0"'-1. ...... _6.' 9.~2.4 16.0 13.9 12 10. _----/'14.4 11.3 19.0

10---10.- 12

Contoured in ohm - meters

XBL 776-9109

Figure 111-15. Field data dipole-dipole apparent resistivity pseudo-section for 1 km dipoles along Line E-EI.

I -.....J .::-

Km

West

MODEL -- GRRSS VRLLEY, LINE E-E i (1 KM DIPOLES) DIPOLE-DIPOLE RPPRRENT RESISTIVITY PSEUDO-SECTION

PROFILE LINE IS INCLINED RT qS.O DEGREES TO STRIKE East

-15 -II! -13 -12 -11 -10 -9 -8 -7 --6 -5 -I! -3 -2 -1 0 2 3 K m [ [ [ [ [ [ [ [ [ [ [ [ [ [ [[ [[

2

3

I!

5

6

8

-15 -II! -13 -12

7.

9

10

11

12

6.5 6.8 7.0

2.0 ')~.2

.~ q~.9 5 ".< 5'k7

.8 6.0 710 • iiY'2" "- ".3

3lJ.q9.9

q.O

8 .6.~6. 9. 1<.3 18.3'-"'Z 5.'

7.9' 8·112.8 ' ":s-.....1>' . / 22.q 22 5 8.8 / 11.8 20.0 • 12.6 /" 27.2 25 7 '

// 18.6 2 . 10.1 3.9 30.6 • 11

12

2-D RESISTIVITY MODEL, qS. DEC. PROJECTION -- GRRSS VRLLEY, LINE E-E i

-ll -10 -9 -8 -7 -6 -5 -Ii -3 -2

5

6

7

2

3

I!

I [' [ o I 11'1 I _ I 0

2 2

N

Figure 111-16. Two-dimensional resistivity model (projection in the profile line direction at 450 to strike) and resultant dipole-dipole apparent resistivity section for 1 km dipoles along Line E-EI.

XBL 776-9111

pseudo-

c· 1"""'.,

C· r'" '-'It..._,

~;~,,,,

.. ,.~ .. ~., . .. ~-·C::: , ~~-~

\r;-""

"" (~ .....,

I ,', .. -.......J 'i4~ .. ~ \.i1 ~ ...

100 != :::::a q, ELECTRIC FIELD

Cl)

"'0 ::::3

\ RATIO TELLURICS \

\8 Hz \ b."'D"".a-~-oo-~"~' FIE LD DATA

'b-_~_...o-_.."..._-o-""D-~ __ -a-...o...'"D'"_.o--o--o-"'O""oCl,.,

"b.

:: 10 0-E <!

Cl)

> -o Cl)

0:

16

West 16

• \ \ \ \ \ \ \

\, " .--

14

14

12 10

12 10

MODEL DATA 8 Hz

--o--o--e--o--e_-o--o--o--.--e __ ._-O,

0.05 Hz

8 6 4 2

....

o

: ........ I • I

I I I I I • I

I I

I I ...... - ......... ,1

2 4

Kilometers East

8 6 4 2 0 2 4 O ~ I I I I I 'I I ' t 18 14. ~ I 8 j ~ I _ _ I _ I I 8 •

EI ~

~

2 XBL 776-9140

Figure 1 I 1-17. Modeled E-field ratio telluric response at 0.05 and 8 Hz for the Line E-E' resistivity structure developed on the basis of dipole-dipole resistivity modeling. Telluric profile data can be shifted up or down to any position on the "relative amplitude" axis.

I ""'-J

'"

Q)

"'0 0 ~

111-77

GRASS VALLEY LINE A-AI

North South

0.5

KM 1.0

1.5

50 90 ?O 20 2'0 10 5 10 20 25.1 24.4_'~_1~ .~! 1T.T~.3 2'y//8~3 8.2/3.1 2.a 't.9~'12.9~ 1Il.4 19.0

10- !!J...-IO.o-IO.T~a.o 9.9\ IH-I~1.5 5.9 \! 3.4 3.5 3.1 -3.~.a 8-.0-a.2-1O 1.9 8.3 8.1 ~5.5~4.2\ 1.3 II..LJIO ~ 5.9 6.3 5.0_4.1 4.0 4.2 ~ 6.9 1.2 5_4.9_4.T_~2.T\ 3.4 U 8.5 •• 1.y'U 5~ 6.3 fr:4~~,~5.1~5.4 5.5 5.9

3.8 ;'8 2.4 21 q/ 6.0 ;J:.:~.2 3.T 53 12 8.9 1.1 ~ 5.4 5.2 3)0 2.0 2·1 3.1 ( ~4y'3.8"~ U .1 \. 8.2 9.6 1.6 8~ 5.8

1.8 2.5 16 5.0 ~ .3.4 3! 4.2 3.9 8.6\ 8.6 !([3-....9~ 1:1- 8~ 2.6 3.3 4.9 Y 3.6 3.4 3.8'\ 4.3 4.4 6.6, 9.1 12.8 9.8 8.T 8.5 3 3.4 4.5 4.3 3.4 3.1 4.1 3B 4.8 4. 1.2 11.5 13.1 Ir.l'--,.ll.! \

~ontoured . 4.8 )-9/3.4 3.4 1.2 3.5)U 5.2 4',9 \a.l \".1 18.1 11.5)/

In ohm·meters 4 4 5 7 10

I--

I--

I--

I--

f-

50 50 2020 20105 5 10 '- , I \ /'j \ '-

20~ 33.3 ~~9.0 19.4~_.!y 1.5 \3.8 3.1 3·1 3.2 5{ 10~ 19~ 24.6 23.8 23.7 10- .12.2 .~ . 17.r-:-~8~ 16.1 17.0 '~7.T 7.5\2.8 3.6 3.T 3.9 5~ 7.8~8 20.5 '9.5

6.e-8.! 9.2 !Y.;(2:s:;:-.~.9 II~ 14.3<:,8.~ 1.7 ~9~.3 •• 3.9 4.6 4.4 ~1 1.0 I~ 16.9 5-5.o_5.L~I.~ 2. ;'\68 '1O.5/~}0).5~ 6·0 /1.4~ .• ;a~~ 59 6.1 I~

3.8 3.8 1.6 2.3 /,1.2\2:: 5.3 }.O 2.9"., 4.\ 1.5 8; .• ~ 6.2 6.5 5: 6.2 6.9 10 )'/~.5 2.4 4.0 4.3 5-2.1 3.3 2.2,,5~\ 8.0/"122 1.1~ 6.7 5.3 6.5

3 1.6 2.5 4.5 5.5 ,2~4 2.5 3.1 2.7 2) 5.8, (12.5 IS3 1.9 6.9- 1.0 5.5 2.7 5.0 6.3 ).0 1.6 3.0 ~.6 3~4 3.1 \ 8.4\ 18.8 20.3 8.2 1.1 (3\

3 5.5 6.9 3.4 2.2 2.0 2.5 3.3 3.9 )4 \ 12.5 20.5 ~.I 8.6 1.4 7 7.T 3.7/2.5 2.7 1.7 3.3 3.8/5.8 6.6 \13.4 no 22.0 9.1

3 3 S 11'0 0

30 I 120 I 20 30 I ~ ~ 25

10 4

2

20 fl- M

I I I I I I I I I I I J I J I ~ I I I I 1 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5

KILOMETERS

-

-

-

-

XBL 762-589

Figure 111-18. Dipole dipole apparent resistivity pseudo-section for 250-meter dipoles along Line A-AI: field data, model generated data, and two-dimensional resistivity model.

Kilometers

WestS

2

N4 6

S

111-78

GRASS VALLEY Line 8-81

L-L'

6

c-c' M-M' N-N' A-A'

I 2

0-0'

4 2 o

9.1~6.9 4.3 4.1 5.3 5.8 I~ II. ""}} 5 5~ 8.4 6.8-6.2 5. 3.r 3.4 3.~ 8.3

7% 5.1 ~4 k4.1.........-4.5~, 5 5.8 4.2 ~j~5.2...--6.3 -6.2 ....... 5

44

DIPOLE - DIPOLE 5\ ~:} .2 9.2 10 7

APPARENT 5.4 6.1 . 9.6~

98~43~/3 Contoured in RESISTIVITY 10 ":'27: ~ (' ohm- meters

20"" .5~/5 22.0 ....... 20

MODEL DRTR -- GRRSS VRLLEY. LINE 8-8 1

DIPOLE-DIPOLE RPPRRENT RESISTIVITY PSEUDO-SECTION PROFILE LINE IS INCLINED RT ~5.0 DEGREES TO STRIKE

4 East

-6 -7 -6 -s -q -3 -2 -1 0 2 3 q 5 LI __ -LI __ -LI __ -LI __ -LI __ ~I __ ~I __ ~I __ ~I ____ ~ __ ~I __ ~I __ ~_~

N

.2

3

5

7.7 9.S 7. ~t~:: ~6~ 9,8 7.~ 7.5 5.6 q.8 . 3.3 3i~ .~

11

6

8.7 6.~ 5.3 5. 5. 5 .2

7

7. 6. 1 8.6 ~~ 7.3 8.~5 12.2

6

9

~2.8\s~ 6

~7.3 19.1 7

17.~ 22.0 21.6 9

6

3

s

2-D RESISTIVITY MODEL. ~5. DEG. PROJECTION--GRRSS VRLLEY. LINE 8-8 1

N

2

-6 -7 o 2 5 Km o~--~--~--~--~--~---+--~--~--~--~--~--~~--~o

-6 -q -3 -2 -5 -1

Km 1

2 2

XBL 776-9108

Figure 111-19. Dipole-dipole apparent resistivity pseudo-section for 1 km dipoles along Line B-B 1

: field data, model generated data, and two-dimensional resistivity model.

111-79

GRASS VALLEY Line D-D1

Kilometers L-L' K-K' M-M' G-G' A-A' B-S' West 10 9 I 8 7 $ 5 I 4 I 3 I 2 I I East

~--r-~.---.---;----r~-r~-.~-,~~.---r-~

I 2 3

N 4 5 6 7 8

-10 1

N

-10

0

Km 1

2

DIPOLE- DIPOLE APPARENT RESISTIVITY 20 10 10

39.0 23.9 ~.I )9.4 8 8_7 8.6 8BJ 25.7 scT6fB~5.3 8~O~6~7.5> 9.8/15.0"-z0

74.4) 23. \ ~ I 7.5# ~ 14.3 /45.4 17.6 6.7: 20.9 21.7-""'20

50 32.7 '19.2 1191 21.9 Contoured In 36.1 515 30.9 ohm-meters 50~ ___ ~50

2

10 187.0---/00

MLlDEL DRTR -- GRRSS VRLLEY. LINE 0-0' PRLl~ILE LINE IS INCLINED RT 90.0 DEGREES TLl STRIKE

-9 -8 -7 -6 -5 -l/ -3 -2 -1 I I I I I I I -. I I

38.9

2

3 3

'! '!

5 5

6 39.l/ !.I1·8 2l/.0 6

---- S, 7 87,L lcM'~ 0 7

8 179.1 8

2-D RE:SISTIVITY MLlOEL -- GRRSS VRLLO. LINE 0-0'

-9 -8 -7 -6 -5 -l/ -3 -2 -1

150.n.-m

0 .J

N

0 Km 0

2

XBL 776-9105

Figure I I 1-20. Dipole-dipole apparent resistivity pseudo-section for 1 km dipoles along Line 0-0 1

: field data, model generated data, and two-dimensional resistivity model.

R-R'

111-80

GRASS VALLEY Line H-H'

A -A' T-T' Kilometers I West I 0 2 3 4 5 East

N

r-~--~r--'---'~-'r--'r--'+-~--~--~---'---4

I 2

4

6

8

10

-1.0 I

N

20 15 10 8 r 6 B I~ 15 20~7.-I \/1'5 8·3 5·9 5·3 4'9'~ 8·2 /f(s,.-i0

15 12-5 I H "9'5 7·2 ~ 5·4 5·6 17) 23·6 12·4 "'8) 8·6 6'8 6'2~1i~(.61

12'2 19'9 8'2 10·~7·9./1F9/19·3 }o ... 9·9 9-6~ 13'0'9i~16'5-'lJI 10--- ~ cv

10'5_ BY 15'7'-13)1746'0-50 15~ 16·9 16'7/ 64.(

DIPOLE-DIPOLE 15·4 ~O~ APPARENT 18'2/104'6::':100 RESISTIVITY 2050~2

Contoured in ohm-meters

MODEL DATA -- GRASS VALLEY, LINE H-H' DIPDLE-DIPDLE APPARENT RESISTIVITY PSEUDD-SECTIQN

PRDrILE LINE IS INCLINED AT 90.0 DEGREES TD STRIKE -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 q.o ~.5 S.O

I I I I I I I I I I I I

li:.i-15 ..1St 9

2 12.q

3 12.2

9.Ss::.5 Q.3 ~~6 6 ~/16.~ 7 q.s q.o 5 -";~20.1

7.0 7.9 • ES q. 7.9 3

2

q 17.2 7.9 9.1 6.6 7.9 6. .1 q

5 5

6

7

6

12.9 10.9 12.1 I qO.6

13.1 12.7 1 0>~ 7

16.1 ~6.3 6

17.2 71.2 9

10 92.6 10

6

9

2-D RESISTIVITY MDDEL -- GRASS VALLEY, LINE H-H' ·1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 ~.O q.s 5.0 Km

~o ' ~o 5 0 I---...........,~----'

Km 0.5 0.5

1.0 150.n.-m 1.0

XBL 776-9112

Figure 111-21. Dipole-dipole apparent resistivity pseudo-section for 1 km dipoles along Line H-H': field data, model generated data, and two-dimensional resistivity model.

Kilometers North I

I 2

4

R-R'

111-81

GRASS VALLEY Line. T-T1

H-H'

2 3 4

p-p' I 5

Q-Q'

6 7 South

N 6

Km

8

10

12

M~DEL DRTR -- GRRSS VRLLEY,LINE T-T' DIP~LE-DIP~LE RPPRRENT RESISTIVITY PSEUD~-SECTIDN PR~FILE LINE IS INCLINED RT 90.0 DEGREES T~ STRIKE

-1.5 -1.0 -0.5 ·0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 'l.O tl.s 5.0 5.S 6.0 6.5 7.0 7.5 I I I I I I I I I I I I I I I I I I I

10

11

-\.5 -1.0 -0.5 0.0 O.S 1.0

"~ I I I

I I I

25 7

O.S 150 .1l.-m

1.0

Ll9.0 50.2 12 50.2

13

47. Yo 47. 61.1

75.1 ]8.9 -100-

158.4 lq

10

11

12

13

2~D RESISTIVITY MDDEL -- GRRSS VRLLEY,LINE T-T' I.S 2.0 2.5 3.0 3.5 lI.n IJ..s 5.0 5.5 6.0 6.5 1.0 ?s K m I ¥ ~L----<_; 1--,-----27 I _I I 1----,"'; [:

XBL 776·9113

Figure 111-22. Dipole-dipole apparent resistivity pseudo-section for 1 km d~poles along Line T-T ' : field data, model generated data, and two-dimensional resistivity model.

~12.3 "

011.3

RESISTIVITY TRANSMITTER NO. I

CONTOURED IN OHM-METERS ! ~ I ~ ~ Kllome' ...

111-82

Q : ! . ? Mil .. 4~~~~~~~~~~~~~~~~ll-J __ ~~~~ __ ~~~~~~~~~~

117°45'

Figure 111-23. Bipo1e-dipo1e apparent resistivity map for transmitter no. 1. XBL 768-10090

uouu,

117°45' 40°45'

40"30' 117"45'

"" .

. '"""

;, >

, ".

Figure 111-24. transmitter no.

111-83

Bipole-dipole apparent conductance map for 1.

I

XBL 768-10086

117"45' 40°45'

.' ..

,22.1

1\

9.4

....

'14.5 '14.7

"11.1 '12.2

\", 'B3-·., ;9.1

,>,- --J

- ." .

GRASS VALLEY, NEVADA BIPOLE-DIPOLE APPARENT

RESISTIVITY TRANSMITTER NO.2

CONTOURED IN OHM-METERS

111-84

Z

0

" ,. '.

'",'

:p., '

'10.3-

!

~ " -"

'. 'a~ :. ,

·Oe._

-

I'

I

117°30'

<~:~' . -, ,"c ~<2. 400

45' L-

:~~':J:'l:::,:-'~'~::o~ < • j

>:/

... /.. . ')-<;:':'-t'

11 r

-- -~ l%~'!:r;l~~,.r ,.,

"

:' i ~ ".

;"

l . ~1

! 9 1 ? ~ Kllomete,. .r'" I~~ __ ~::::~Q::~: ::~I::~::~?~M:II:e·:'Jb~l·~:j·'~~'L. ____ ~ ____ ~ ______ :' ____ ~ __ ~ __ ~.J __ ~ __ ~~'~'J~

40"30' ~ 40° 30' 117°45' 117° 30'

Figure 111-25. Bipole-dipole apparent resistivity map for transmitter no. 2.

XBL 768-10087

'J' fl' ; l, i.., UU ,;,

111-85

117 '45' 117'30' !~~--------------~----~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~-r~~~40'45'

40'45'1 <", . ir':~".) i . :~ '.~ ') ,v K .., \ ( "' "

\ )

.I,

CI,,,, ,

/'

, ::'

·248

~~0~~J1" 16, l.""

'~' " i~81' (0

'0 ~I 122 J

" , '117

-- <it4'

·207

<, ,.,;~.

.,."

GRASS VALLEY. NEVADA BIPOLE-DIPOLE APPARENT

CONDUCTANCE TRANSMITTER NO.2

CONTOURED IN MHOS

,.

/ -",

F i ~:t>, . \~':'J

., f ~<,o"

~~~.~:~;~~ .,~

l

.,00'

\

"

'.' J} r \:.,A S''l Ltl ,'\, ( ;,::",,!;~~\ '\-<~{C\ ,~T.'-·<l \' I:~~?~L~( S ~ /"JB(~:-- J " ~i)\ ( ) r '1,"'0.,';

''1 .\

. , r~h'::'h:': :,~" '\

i' "11

!I

! [\

~'I; -. -··l

.- "."", I" " ,'" I

'1 ~<""., "'u

,f,i J ",

,---,-_0 ... , Q"~!'---'I;-'~_---J~ KIt.mote" ,,",

~~~ __ ::~~::-::~,::~,:::::::?~M:II=··~JL~Jt'~~.Ji_' __ ~~ ______ ~ __ .. __ ~ ____ ~ __ ~ __ ~ __ ~ __ ~. __ ~~,~ , ~~ ~w It7'45' 117' 30'

Figure 111-26. transmitter no.

Bipole-dipole apparent conductance map for 2.

XBL 768-10084

111-86

.. -

40"30' 117~45' ~~-Figure 111-27 transmitt . er no.

for XBL 768-10092

117.°45' 40°45

-""'f[

LV

-128

, ;.,\

'-ij

<J'

, ; ;:

'"

-202

t\J

, u\

- /~.

Figure 111-28. transmitter no.

~ , '''i.J at '. ~liJ i~~~) ~>J ',·b Y ~:

111-87

Bipole-dipole apparent conductance map 3.

for XBL 768-10091

, , ----,

:1' i ~ 16

r ----1-

GRASS VALLEY. NEVADA BIPOLE-DIPOLE APPARENT

RESISTIVITY TRANSMITTER NO. 4

CONTOURED IN OHM-METERS I 0 I 2 3 Kllomet."

111-88

, r;;.~ 20

. ·9 : ,. .'? Mil ..

4~~L-~~::==~~~~~::-1~~l-~~-l~ __ i-~J-~-ll~~~~~~ 117°45'

Figure 111-29. transmitter no.

Bipole-dipole apparent resistivity map for 4.

XBL 76B-100BB

J; ~,,:-';! I , ii"Ji V t,f) ~t \-1) 6 \~

111-89

40.dfr, •• ;'::4~5' ____________ ~_~"."I""I~~I'I"'!!I'''''''''''''''''!''I'~~~~'I''''''I'"I''I''_~'1'!'f'''''''!'!''M'''~~_~~~'''1 >

40030' 117"45'

'.~ - ..

CI,,,,."

".~

" ..., ,,\" ~c }"'/o, ,"- 1J ,",-'~ I,,,,.,,,,,~·,r ... :t

-:~--I '~~·':.r:;: , .. !

, .. '

.739

400

./ .

GRASS VALLEY, NEVADA BIPOLE-DIPOLE APPARENT

CONDUCTANCE TRANSMITTER NO.4

CONTOURED IN MHOS ,--,-..:0.." Q~---,:I'-~I..i?:"'-'----l1 Kllom,' ...

_ " ? 10111 ..

[ ;> ->::.:"

1.\. ~:, /;~~; , _. <"it'l,

~~:~,,:~ ':" ,3 \' ': \ i' , ,,,.. ·201'

, }, ~-: \ "

\ 20

;,~" "

,>- ... .. ,

--"

~) ': ,\.;1,'-, , --,:v,,-

"

) " /" '\ c'

" ... ~

I' ,

" I j

" ,i'(

, I , ;' ; 0< , V,,

! " "

Figure 111-30. transmitter no.

Bipole-dipole apparent conductance map for 4.

",

! z

I

XBL 768-10089

\\\-90

lu \

, ,

40"30' L~~~~~~==.IllitU~""':-'.L..:.....J'-:"L"":"-~~~~j7!"I 117°45' t res i st ivi ty map for 31 Bipo1e-dipo1e apparen Figure \\\- •

transmitter no. 5. XBL 768-10085

0(, ,',,' l) l.X;

117°45' 40°45'

/

- . ~ ..

Figure 111-32. transmitter no.

111-91

Bipo1e-dipo1e apparent conductance map for 5.

!

,~ ,

I

XBL 768-10093

117"45' 40°45'

'-.

40"30' 117°45'

111-92

"'~

Figure 111-33. Map of the near-surface apparent resistivity as observed by the dipole-dipole surveys at a separation of N = 1. Grass Valley, Nevada.

XBL 776-9114

Km

111-93

APPARENT RESISTIVITY 7. 0

6. 0

5. 0

1. 0 W 3.0

2. 0 1 1 • 0 \

0 X

- 1 . 0 c:=J - 2.0

-3.0

-1.0

- 5. 0

- 6.0

- 7. 0

- 1 O. 0 -5.0 o 5.0

7. 0

6. 0

5.0

1. 0

3.0

2. 0

1 • 0

APPARENT CONDUCTANCE

Km 0

- 1 • 0

- 2 • 0

- 3.0

- 1 • 0

- 5.0

- 6.0

- 7. 0

- 1 O. 0

West 10

- 5.0

5 I 15

100 1\-m

o Kilometers

o i 1

5. 0

5 I

7.0

6.0

5.0

LO

3.0

2. 0

1. 0

0

- 1 • 0

- 2.0

- 3.0

- i. 0

-5.0

- 6.0

-7.0

10. 0

7.0

6.0

5. 0

LO

3.0

2. 0

1.0

0

- 1.0

-2.0

-3.0

-1.0

-5.0

-6.0

-7.0

10. 0

East 10

~ XBL 776-9139

Figure 111-34. Modeled bipole-dipole apparent resistivity and apparent conductance maps for a two-dimensional structure resembling Grass Valley in the vicinity of Leach Hot Springs. The bipole transmitter is parallel to strike, similar to transmitter no. 1.

Km

Km

111-94

APPARENT RESISTIVITY 7 . 0 7. 0

6. 0 6. 0

5. 0 5. 0

1 • 0 10 40 1. 0

3. 0 3. 0

2. 0 2. 0 / __ ----_7,

" • 0

0

- 1 . 0

- 2. 0

- 3. 0

-1.0

- 5. 0

- 6.0

- 7 . 0

- 1 0 • 0

7. 0

6. 0

5. 0

400

3. 0

2. 0

1 . 0

0

- 1.0

- 2. 0

- 3. 0

-i.0

-5.0

- 6. 0

- 7. 0

- 1 0.0

West 10

Km ~l

1 • 0

0

- 1.0

- 2. 0

- 3.0

. , -i.O

- 5 • 0

- 6. 0

- 7. 0

- 5.0 o 5. 0 1 0 • 0

APPARENT CONDUCTANCE

200

-5.0

5 , , 1$

100 Jl-m

o Kilometers

o i 1

100 mhos

5.0

7. 0

6. 0

5. 0

400

3.0

2. 0

1.0

200 0

- 1.0

-2.0

- 3. 0

-400

-5.0

- 6 • 0

-7.0

1 0 • 0

East 10

t XBL 776-9138

Figure 111-35. Modeled bipole-dipole apparent resistivity and apparent conductance maps for a two-dimensional structure resembling Grass Valley in the vicinity of Leach Hot Springs. The bipole transmitter is perpendicular to strike, similar to transmitter no. 2.

Km

Km

7. 0

6.0

5. 0

1. 0

3.0

2.0

1.0

0

- 1 • 0

-2.0

- 3.0

-1.0

- 5 • 0

-6.0

- 7.0

111-95

APPARENT RESISTIVITY

J7+, (]

7. 0

6. 0

5.0

".0 3. 0

2. 0

1.0

o

- 1.0

- 2.0

- 3.0

-LO

- 5.0

- 6.0

~~~~~~~~~~~~~LL~~~~~Li~LL~~LL~-7.0

- 10.0 -s.O o 5.0 10. 0

APPARENT 7. 0

6. 0

5. 0

1.0

3. 0

Z.O

1.0

0

- 1.0

- 2.0

-3.0

-".0 -5.0

-6.0

CONDUCTANCE

00

100 mhos

7. 0

6.0

5.0

3.0

Z. 0

1.0

o - 1.0

- 2.0

-3.0

-LO

-s. O.

-6.0

-7. 0 ~~~~J-LL~-L~~LL~JL~~~~~LL~~~J-LL~-7.0

- 1 O. 0

West 10

-5.0

5 , os'

100 n-m

o Kilometers

o i f

5.0

5 I

10. 0

East 10

~ XBL 776-9137

Figure 111-36. Modeled bipole-dipole apparent resistivity and apparent conductance maps for a two-dimensional structure resembling Grass Valley in the vicinity of Leach Hot Springs. The bipole transmitter is at 450 to strike, similar to transmitter no. 3.

---\

------}:"-- ---;;;- - - <. .. ,

-G> .. ,0"

c.. ~"

I , i i"

i I"

_____ . ____ 1

1

1

6

1- __ ~.(. ,,'

1 ~ ·~t~9_~--,-~~ KILOMETERS

'r 9 ~ MILES

1170 45'

111-96

"I

u"

117030'

,_ "I , .. ~~-- -". 40 0 45' .. ',. I .:-. .. -':- ~ I

":'\~i';~+-"

.i -I i ~. 1 f

I" ! 1 J

.I :

,,'.

117 0 30'

XBL 776-9149

Figure 111-37. Structural regions in Grass Valley, Nevada, delineated on the basis of the interpretation of telluric and d.c. resistivity data, In conjunction with gravity, seismic, and geological data. The numbered areas are discussed In the section entitled liThe Resistivity Structure of Grass Valley."

111-97

APPENDIX III-A

Geophysical Data Profile Composites

Grass Valley, Nevada

111-98

GRASS VALLEY Line A-A'

£-£' F-F' J-J' e-e' N-N' B-B' 0-0' G-G' K-K' H-H'

.g~ N~~~6E Tbp!olRA~l~! I; I ~ \1 °1 I ~ I Ii '! ~ J ~ ~ ~ 1400L : 'Leach Hot Springs I __ '----'. __ --!. __ ..L. _____ -lj

Y

w~ III 1111 I II

; :::~IT:{ht±Ff~~~ ....... E 53,900 I I I

~ 53,800

~ 54'OOO~. TOTjAL MAGNi EiTIC

c.9 53,700L---L __ --L.....L._--'-___ ....L._.!-._L-__ '--_-'-____ ---....J

U

1.11 -"0 .2:

~

1.11 .... (1)

1i3 E , E L;

o

.-L;

.... --0> U c ~(1) (1) .... -(1)1.11

~"O c-_(1) (1)'+=

0:::

20

0

-20

-40 500

BIPOLE - DIPOLE APPARENT RESISTIVITY

TXI

t::d,(~---~TT--~--r----<>----~-TX4 10~'- ..J~~ . I I "if/I

. I ,I· / 5 TX4 1"'-- j ~ ".- • = TX3

. I .....". I Leach Hot Springs _--'--_-'--__ -'--_-'-_____ --' y

20 1111 I I I

10 ELECTRIC FIELD RATIO TELLURICS

5

I 2

I North 10 8 6 2 0 2 4 6 8 10 South

Kilometers

XBL 7512-9585

Figure I I I-AI. Geophysical data profile composite for Line A-AI.

,

c,.....

.Q ~ -+-Q) o-+->Q)

~E w .......

'~~,y ( {~ ~ . 1) C f' :l~j t,b ~;

'j .-., it;) ,.e)

111-99

GRASS VALLEY Line 8 - 8

1

L-L' c-c' M-M' N-N' A-A' 0-0'

West 1700

1500

I 6 1

8 4 2 0 2 4 Eas I- III I I I 1 I I

1 1 1

-I- TOPOGRAPHY -:... -

I

1300 I I I I -

* ~If 1111 I I

! ~:I:~t~=G=R~t=V=IT=Y~::~==!=====~!=' ====::::==~===: 54,000

in o E

TOTAL MAGNETIC FIELD

§ 53,800 I (9

If) ~ Q)

-+-Q)

E I

E ..c 0

U .;:: ..c -+- -+-Ucr. Q)c -Q) Q)~

-+-~If)

~"O 0_ -Q) Q).-0::-

z

100

10

5 ELECTRIC FIELD RATIO TELLURICS

0.05 1 Hz I ..,.. .... ~ ,""·"'8 Hz

,r' "" 8Hz

0.5

II 11f lit II II I I QallP 10 /0 7 5 5

9j~(9 4.3 4.1 ~.3.' 5.8

2 ~6'8-6.2~. 3.r 3.4 .:::.tt3 7 7.8 5. .4 ~4,~4'~'45

4 5.8 4,2((;i~5.2~6.2""'54 DIPOLE - DIPOLE 5\ %:7 .2 9.2 10 7

6 APPARENT 5.4 6.1 '9.6~ 98/14r13 Contoured in

RESISTIVITY /0 ~ (. ohm· meters 8 20rll.5~/5

L---L---l_..L.----l-_L.--l22.0 ..... 20 --'--~__'__'--_'_--'

West 8 6 4 2 0 2 4 East Kilometers

XBL 7512-9586

Figure II I-A2. Geophysical data profile composite for Line B-B 1

•

111-100

GRASS VALLEY LINE C-C'

E-E' J-J 1

L-L' 8-81 M-M' A-~ F-F' 5:1

We c_ 170

sf 6 5 4 3 2 I J~ I g East Ow

:;: ~ 160 0 +op6 GRhpHY I II I II _ 0

I-

c of- 1500 >(1) ~ E 140 w- 130

r-- -I- -0 ... - -

0

lit I Jli

r 54,00

~ 53S00 TOTAl f- MAGNETI( ~ F IEL[~

-E E 53,800 r--c ~ 53,700

" ~,,.;

I

100

50

i!? 30 ~ 20 Q) ~~r-,....

~ 10 E

.s:::. o 5

Figure 111-A3. for Line C-C I.

~

-

~ .c - :-0 -

XBL 762-542 Geophysical data profile composite

I/) 0 E E 0 (9

I/) -0 c

~

~ (5 ~

::?:

~ 0)

Q; E , E

.<= 0

u c.<= TID o)c

- 0) 0)>-

+-0)1/)

> += "'0 0--0) Q)l+= a::

N

, ,;.) J~;) O.a 111-101

GRASS VALLEY Line 0-0'

L-~ K-K' M-M' G-G' A-~ B-B' 9 8 7 6 5 4 3 2 I East 5:1

001 POol 001 OT901 P

-160 Ptl II ~I I III I

::::lYlUFJ 54000r---.------.--.--.--.--.-----,

53900

53800

53700 0

0.1 0.2 0.3

40

30

20

10

5

3 2

50

30 20

2

~ Odllrr 10 9 8 7 6 5 4 3 2 0

DIPOLE- DIPOLE APPARENT RESISTIVITY I 20 I 10

39.0 239~.1 9.4 8 87 8.6 8~ 25.7 2 5O'ti~~53 -;6"-7S> V 150\.20 3 1\.4)23. \1 7 ~.4 14.3 4 ,454 17.6;". 3).9-217---20 5 50 32.7 "~i:illI 27.9 6 Contoured In 36.1 57~ 30.9 7 ohm-meters ;~ 8iFso 8 1Bi:F,oo 9~~~ __ ~~~ __ ~~ __ L-~~~

Kilometers

XilL 762-5H

Figure II I-A4. Geophysical data profile composite for Line D-D t•

c:~ West 16

-2 Ie 1600~ -Q) 0_ ~Q) -E 1400 lJJ~

54,100 VI E 54,000

~ 53,900 <.9

53,800

53,700

VI

o:~ -0 c: 0 0 Q) (f) 0.4

J!? J "0 ~ ~

100

VI

t Q; E E .c 0

16

2

4

6 Z

8

12

14

West 16

\\\-102

GRASS VALLEY Line E -E'

, ,A-A'

"f-2r"t 2 · E,"

L-L'

14 12

191 8 6 4

=EI I I I I I

TOPOGRAPHY

QaIOTg OTg P

* I IIi I +II • ~

TOTAL MAGNETIC FIELD

SELF· POTENTIAL itif:l BIPOLE - DIPOLE APPARENT RESISTIVITY

\ ELECTRIC FIELD \ RATIO TELLURICS

\8 Hz

\"-v'--'~-'''' I ~-T-'-- ....... ------_..o.."'U'~j-o-""'"i"O"".Q,.,'b.

til I IIi

, 50 JO 1.1 1.1 1.0 6.4 6.9 ~1.11.~~~.1 II

U (~r?V ;:l.I=l.5 ;l.,~ .0---l!i\,\12 1.1 tl fI.11 4.'~l.2 l{ 2.1 2.l Y ).4--i/

,11.2 1.116.0 1.9 4. l.4 l.2 2.4~l.1 d 4.2, -.:J,.RI I '...-c_' , 5.5--6.01': ~ 5.2 4.1 4.1 4.t-J.8_~-4.r 3.!/

6'0p..1.4 44'~ 1.4 ( 4.l /1T"1.2~4.1-.l.!.""'1.0., ,." (i.I~ 1~(l.i"""B.1 U 9.0 1.0,

B.O\ lY/B., ~ B.B 1.y::10.911~"1.9 , ~4~1. \ ~ I. 1.2 /,19~1I.1 "

... , __ l ,9'y':12~ 11.0 Il.9' 11 Contoured in ohm-meters 10. __ --... ',14.4 11.3 18.0

""m~-12.

/1.

6 Kilometers

2 4 Eost

l<IIL 7~2 -9~87A

Figure \ \ I-A5. Geophysical data profile composite for Line E-E I•

c:­oC/) ._ Lo

111-103

GRASS VALLEY LINE F-F'

M-M' A-AI C-C'N-N'

g~ 1500 ~ E L ____ ~---------+-----------_r-w- 1300~

~--~----~------~~-------~ QalTS QTg QTS p

-170 11111 * IJltTT I jl~ 1\t

; ~:~t:r=:kdt J 5~000~--~----~--------r~--------'

C/) L MAGNiTIC FI ELDI E 53,900

E 53,800

~ 53,700 L--_--I.-____ ~I ______ ___I..__.l.. ___ __'

C/) Lo Q) -Q)

E I

E L: o

.2L: Lo_ -0' 0c: ~Q) Q)Lo Q)OO

> .- "0 --.QQ) Q)t;::

0:::

50~--~----~------_.~------_, 30 20

10

5

Txl

5:1

XBL 762-543

Figure 111-A6. Geophysical data profile compos-ite for Line F-F'.

c .--. o (f) -- ~ -+-(1) o-+­>(1)

~E w----

4 West 1700

111-104

GRASS VALLEY Line G-G

1

N-N' M-M' 0-0 1 A-AI

2 0 2 T I I I I

1500 = TOPOGRAPHY I I

1300

4 6 I I

-

---~

111~ 1h ~I~*

! =::~l;;t -!

~ 53,800 I I I (f) 53'900~-N § ~TALMAGNET ~

19 53,700:::-==~====~===:==-~-======::::

U

(f) I.... (I)

-+-(I)

E I

E .c o

-c .c -+--+-uCJ"I ~c (1)(1)

~ (I)+­> (f)

+=-0 o~ -(I) (1)--0::-

400

200 BIPOLE -DIPOLE APPARENT RESISTIVITY

100 I

50r'"""

2g~==~==~~==~====~ 10 ELECTRIC FIELD

RATIO TELLURICS 5

I~-L~ __ ~LL __ LL~-J __ ~-L~

East

West 4 2 0 2 4 6 East Kilometers

XUI. 762-2232

Figure 111-A7. Geophysical data profile composite for Line G-G 1•

to) .))) ",:.;"( ~).~ ~f"'l~ '.

Line L-L'

Wes t 6

fJ ~~~ i<;~ 0 d() ~~~ 111-105

GRASS VALLEY Line H-H'

R-f?/ A-fJ1 'T-T'

4 2 2 4 East ~ 1700 't tOPOGR~PHY I I - I I I I l ~ 1500 - -[

U 0t:

~ (]) ..... (])

E I

E ..c o

..... U..c (]) .....

- Ol (])e (])(])

>~ :.=(/) ..9-0 (])(]) 0:::4=

500~----------------r--r--~-----, BIPOLE -DIPOLE APPARENT RESISTIVITY

TX3 =. Kilometers

West 6 4 2 o 2 4 East 10~~-r--r-~~~r-.--rr~-r.--r~

Dipole length :% 500m

Figure I I I-A8. Geophysical data profile composite for Line H-H'. XBL 7512-9825

c­o(/) -- '--<1> c_ ><1> ~E w .......

00 c

II 1-106

GRASS VALLEY LINE J-J

1

A-A' e-e' N-N'

West 2 b 2 3 4 5 East 5: I

1600 1500 1400

__ ~---~~-+~-n---~---~~~

1300 _________________ ~ _____________ ___

Qal QTg QTpP

~It I Jlt ~Ir T I ~It I ~II 54pOO~---------~~----_r----------------~

E 53,900 TOTAL MAGNETIC FIELD E

~ 53,800

o -i: .c. --00' Q)c: -0) Q)"", Q)­>00

c:!:! -0) Q)--a:: ......

50 __ -----------~~----_r-----------------__,

30 ELECTRIC FIELD RATIO 20 TELLURICS

I. "]

.05 Hz

5 __ ~ ___ ~~~~~ ___ ~~ __ ~ __ West 2 o 2 3 4 5 East

Kilometers XBL 762-571

Figure I II-A9. Geophysical data profile composite for Line J_JI.

."',

c,-... .Q (j) -+- ~ OID

L-E N-N'

111-107

GRASS VALLEY Line K-K'

R-R' T-T'

1700West~ 1 ;1 6 I 1500 ~ TOPOGRA~HY' I I

1 ! ' 1 21 3East

6)ID -E w -- 1300 I I I I

'" 53,900 I I IIU II '. ' ,

j::~~~[ TOt~~ .~..I:: ~ -+­

-+- 01 uc IDID - ~ ID-+-

(j)

.~-o -+-ID 0'­_'i-

&

20 1, --~--~--r-----------~~~

10

5

1.0

ELECTRIC FIELD RATIO TELLURICS

I 0.05 Hz

I ",A"

'I ,/ 'q" A ~n ~ , , .. oJ>',

" '\ / O"'-.o.'''Q, h-~·oO' I ''b ~ ~" '~-~ .. '. ..If I '\>.00'

0.5W S' est 765 4 3 2 o 2 3 East Kilometers

XUL 762-:'56

Figure I II-A10. Geophysical data profile composite for Line K-K'.

\ .., '" '/\ J. Ct f~ fr- f~'i; " ,~/, ffr'l Wi !,}l c-r" t" (' .. ' . ,'<. n<~ ..... ."':', "

(J) ~ 0) -0)

~

North 1700

1500

1300

(J) E53,900

E 0 53,800 (9

53100

U 5 '~...c --UCJ") 3 O)e O)~ gz1n ~-o 0- 1.0 - 0) 0).-O:::'+-

0.5

GRASS VALLEY Line L-L

E-E' 8-8' C-C' K-K' N-N' D-D'H-H' I I

I C( I 2 3 4 5 6 7 8 9 10 II 12 13 14 I~ South 1 I I I I I I : I I I I I I

r TOPOGRAPHY -

I -r -I -

- , ---I -f

11~QTgTpTQTg 11~

TOTAL MAGNETIC FIIELD I-

~

ELECTRIC FIELD RATIO TELLURICS I _.0--':0---

I ,.o. __ ~ .-<1---0---.0-- I

_,0--..0---.0' ~ /,r:r-_o- I

~~ --C:-'-o--

I 0 2 3 4 5 6 7 8 9 10 II 12 13 14 15 Km

Figure III-All. Geophysical data profile composite for Line L-L'. XBL 762-555

o ex>

GRASS VALLEY Line M - M'

F-F' E-E' C-C' N-N' G-G' D-D'

6 North 10 8 6 4 2 0 I 2 I 4 I 6 I 8 South I I I I I I I I I I I I I I I

:= TOPOGRAPHY -

I -t:::::::.- -

I

§<n 1700 .- ~

c~ 1500 >(])

~.5 1300 ~I~ 11~ ~I~ JI~

gjE 53,900 ~ TOTAL MAGNETIC FIEL1 ~ -E1• t ~

53,SOOF' ,. --- I I I § ~i----

<.9 53,700 .

N

I~ 5 6.0 47 !J 2 5A?3~2r 3.7~ 6 4.4 < 3.3 . ,. 2.5 2.Z--24-2.S=::- 6.1 S,j _4.9...... 3.4 8, 4 f- 43 _ 2.7 26 2 2 3.1-.1 3.5- 33 ~'-..!! ;>, 3.5 3.4 3.4~ 22 2.2 H 3 . 3.4~4~r 6

L 4 3.6 ~.7 43 .8 2.3 22 2 .1 3.6 3.9 3.N6 5 I _ . 5. 48 ..8 B- .

3-9 ~~~"-{'-I.!_' 3B-3 27 2 /. )iO~6.1C'5'-4 4/ 5( . 5.7 -5.4-'4 _<".(2 5.3 81- 6

8f--DIPOLE-DIPOLEV' 7~86 ~~4{/63;0i= 8 APPARENT 6 7Jj/ n5 . ~~ 5.~·~,,'/.86 10 I RESISTIVITY 8 n.5 98 lor-8.1 15 ~/,.I C

12.0 ~ 9.0 ontoured in 11.3 ohm· meters

Figure I II-A12. Geophysical data profile composite for Line M-M 1•

XBL 763-619

=~ b'

~;

,.":,,"

&';1:

..t", .. ~F~'"

~.

t&-.~.

;J1"" ,,~ ..

d\ ~~:.'

- .(.-"'h o t.~,,~,

\.0

~j:.

111-11 0

GRASS VALLEY LINE N-N '

L-L' K-K' G-G' B-B' A-A' J-J' F-F' M-M'

21 5:1

z SW7 6 5 4 3 2 () 3 4 5 6 7 NE Ocn - ... ~~ >(1) wE ...1_ w 1300

p aTg Faults II to N-N'

jr H , 54,oooT1l

f 11f Jli JI~ II JlrJlr Jlr III

CJ) TOTAL

~ 53,900 - MAGNETIC FIELD

~ « 53,800 <.!)

53,700

7 6 5 4 2 2 3 4 5 6 7

I 2

3 4

z 5 6 7 8 9

20 10 {i 4 4 5 10 20 29.4 335) II.~ 6.4 __ 4.8 4.7 \ 3.1 ,4.3 4.3J J 17.4'

23.8 (,12.6 8.6 ~C::_~4,I.--4.0~'f' :2712

.O( 63.3 ,19.4 II. 6.3 A~ 4.2 ,-"'5:2 J i§1 7.4 ~2.5

20 17.4 10.S~ 5.6-5.i/ )':J)f;j.7 17\ IS.I 15.7 1I.0~.3 ~.; /12.3 20

20--23.3 ........... 9.0 15.2) 5.9 ,r 7.0/3.1 DIPOLE-DIPOLE 28.2 \.20.4 10.0_9.8/'14.2 Contoured In

APPARENT 34.5 )14.0 18.0 18.0 ohm-meters. RESISTIVITY 20 2~20

10 7 6 5 7

XBL 763-618

Figure I II-A13. Geophysical data profile composite for LineN-N'.

(OJ

c_ 0(1) .~ '-.... CD c .... >0)

.!E w-

0 ".: .c:. ........ 00-CDc -0) CD ... (1)1;; > .-.... "t:J c--0) 0)-

0::: .....

J::)) ':.'.c'ff l~P ,.~.;.~ ~:)' '~J

111-111

GRASS VALLEY LINE p-p'

T-T'

West 3 2 I 0 2 3 4 5:1

5 East ~--~~r-~--~--~--~---r---'

TOPOGRAPHY 1800

1600

1400 TRg Qal Qal P

5.0 IT 11~ JII~t Jlt Jlt'~

3.0

2.0

1.0 0.8 -

0.6 -0;5 0.4 0.3

0.2 ELECTRIC FIELD RATIO TELLURICS

O.J 500 meter dipole spacing .

. 05b-__ ~~ __ ~ __ ~ __ ~ __ ~ __ ~ __ -d

3 2 o I 2 3 4 5 KILOMETERS

Figure 111-A14. Geophysical data profile composite'for Line p-p •. XBL 763-617

c.-.. o II) 0';:: '-o 0> ::--0>0> -E w ........

.c +-0'1 c '0> '--II)

"C

0> ..... () .-'--()

0>

0>

0> ::--0

0> 0:::

West 4

c-

1700 c-

3 2

1 I

111-112

GRASS VALLEY Line Q_QI

T-T'

0 2 3 4 5 6 East I I I I I I I _

TOPOGRAPHY

~ 1500

_./= ----------------~------ -

r-

10.0

5.0 ELECTRIC FIELD 4.0 RATIO TELLURICS 3.0

2.0

0.05 Hz 1.0 (500m dipole) 0.7

0.5 0.4 0.3

0.2

O.I~--~~--~--~--~~~--~--~--~~

West 4 3 2 o 2 3 4 5 6 East

Kilometers Fig u re 1 1 I"; A 1 5 . Geophysical data profile composite for Line Q_QI.

XBL 763-636

\

c:: ---. oen .- '--(I) o+­>(1) (l)E w ........

..c:: +-0'1 c:: (I) '-+-en

'U (I) -()

'-+-() (I) -(I)

(I)

> +-0 (I)

0:::

(OJ 'i.1 ;)) "-~j f;;I -. 'i2J

111-113

GRASS VALLEY Line R-R'

I-llI' A-A' K-K' T-T' I I I I

West 4 3 2 o I 234 5 6 7 8 East

TOPOGRAPHY 1600

""----~-~-1400L-____________ ~--------------------________ ~

P Qal QalQTgQal P P Kg P

I T 11~11~ , 1 l I ! II 5.0

3.0 ELECTRIC FIELD

2.0 RATIO TELLURICS

1.0

0.5 0.4 0.3 (500m dipole)

0,2

0.1 West 4 3 2 0 234 5 6 7 8 East

Kilometers

XBL 763-634

Figure I I I-AI6. Geophysical data profile composite for Line R-R'.

I I 1-114

GRASS VALLEY Line S-51

£-E' 8-8' N-N' A-A' I

M-M' West 3 2\ II 0 2

c_ o CJ)

'';: "- 1600 o Q) TOPOGRAPHY

3 East

> -Q) Q)

W E 1400 1=:±:::±::t:::::::1L_~L __ ..:j Qal

..c -Qal QosQalTb J Qtg Qtg P

I II]~ JIITT Jlt TJII 0' ~ 10 .0 .--;-1-.--.-----.----.-------.

.!: ELECTRIC FIELD CJ) RATIO - TELLURICS

(250m dipole)

() ,-'­-o Q)

Q)

Q)

> of-

8.0 Hz O""q, I I \'o-j-d".a...,o,.,

1.0 0.05 Hz 0,.,

o Q) West 3 2 0

0.:: Kilometers

2 3 East

Figure I II-A17. Geophysical data profile composite for Line 5-5 1,

XBL 763-635

;' U!· .. ,j { -~~~J n, q, i.{J -, 9_ ~ " ~~J ~V' ,jl I I 1-115

If my hypothesis is not the truth itself it is least as naked: For I have not, with some of our learned moderns, disguised my nonsense in Greek, clothed it in algebra, or adorned it with fluxions. You have it in puris naturalibus. And as I now seem to have almost written a book instead of a letter, you will think it high time I should conclude; which I beg leave to do, with assuring you that I am, most sincerely, Dr. Si r, etc.,

B. Franklin

--from a letter to John Perkins, February 4, 1753

. , "

This report was done with support from the United States Energy Re­search and Development Administration. Any conclusions or opinions expressed in this report represent solely those of the author(s) and not necessarily those of The Regents' of the University of California, the Lawrence Berkeley Laboratory or the United States Energy Research and Development Administration.

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f' ~ ''':"

f'1 0 tl ~ t, ~. f

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