Post on 14-May-2020
Tom Wilson, Department of Geology and Geography
Environmental and Exploration Geophysics I
tom.h.wilson
tom.wilson@mail.wvu.edu
Department of Geology and Geography
West Virginia University
Morgantown, WV
Resistivity III
Stratigraphic correlations from borehole log
response
Tom Wilson, Department of Geology and Geography
SP
Shallow
induction log Density and
neutron
porosityHigh ,
fresh waterLow sp,
fresh water
sandstone
Logging arrangements
SP log – passive log measurement
Tom Wilson, Department of Geology and Geography
Measures natural current
flow produced between
drilling mud, formation
water and shale intervals
of differing resistivity
SP
Shallow
induction log Density and
neutron
porosity
Electrode configurations - logging
Tom Wilson, Department of Geology and Geography
Source
electrode
Sink
electrode
“the ground”
sinkPotential
electrode
Potential
electrode
Source
electrode
d1
d2d3
d4
Think how the electrodes associated with the logging
tool correspond to those of the surface array
Tom Wilson, Department of Geology and Geography
V
A BNM
+ -
d1 d2
d3 d4
Downhole configuration
Tom Wilson, Department of Geology and Geography
Source
electrode
Sink
electrode
d1
d2
1 2 3 4
1 1 1 1
4
iV
d d d d
d1 is the distance from A (the
source electrode) to M (a
potential electrode).
We need to use
the factor 4
How about this arrangement –
lateral log configuration
Tom Wilson, Department of Geology and Geography
d2
d1
1 2 3 4
1 1 1 1
4
iV
d d d d
1 3
1 3
1 10 0
4
1 1
4
iV
d d
i
d d
Source
electrode
Sink
electrode
Normal log arrangement
Tom Wilson, Department of Geology and Geography
d3
Source
electrode
Sink
electrode
d1
d4
1
1
1 4
10 0 0
4
1
4
1 1
4
iV
d
i
d
or
i
d d
The 4 remains since the source
electrode is buried
d2
If d4 is not
too large
The lateral log
Tom Wilson, Department of Geology and Geography
V
A BNM
+ -
d1 d1 d2
Induction log
Tom Wilson, Department of Geology and Geography
Reminiscent of the terrain
conductivity survey, but with
the transmitter and receiver
coils in the borehole.
The secondary current is
induced.
A type of log run in holes drilled
with non-conductive oil-based
muds or air drilled.
Stratigraphic correlations from borehole log
response
Tom Wilson, Department of Geology and Geography
SP
Shallow
induction log Density and
neutron
porosity
Example data from a Wenner sounding?
Tom Wilson, Department of Geology and Geography
See interactive Excel file at
http://www.geo.wvu.edu/~wilson/geol454/ApparentRes.xls
Remember how to compute a?
We went through this briefly last time
Determine k? We have 1from the shortest offset and can
estimate 2 based on the long-offset response
Tom Wilson, Department of Geology and Geography
Doesn’t look like a reasonable
2 for the observed response,
so k is probably wrong.
2 estimated from response,
then k is calculated
Also have a look at this xls file
2LayerReflection.xls
Tom Wilson, Department of Geology and Geography
Linked on the class pages at
http://www.geo.wvu.edu/~wilson/geo252/2LayerReflection.xls
How does the resistivity vary as a function of
depth? Make a guess and see how you do.
Tom Wilson, Department of Geology and Geography
Change this to spacing instead of effective penetration
Consider equivalent solutions
With about 10 % error equivalence suggests
Tom Wilson, Department of Geology and Geography
Depth could vary
from about 6 to 13
meters within 10%
error.
While conductivity
could vary from
about 105 to 310
-m
Ap
pa
rent re
sis
tivity (
-m)
Resistivity (-m)
Inflection
point
4% errorthe computer may give you an “exact” answer but geology
doesn’t behave that way – consider equivalent solutions!
Tom Wilson, Department of Geology and Geography
Depth could vary
from about 7.2 to
9.5 meters within
4% error.
While conductivity
could vary from
about 122 to 185
-m
Ap
pa
rent re
sis
tivity (
-m)
Resistivity (-m)
Inflection
point
½ distance to inflection point
provides an approximate
estimate of depth to the interface
Multi electrode instrumentation provides
sounding and profiling capabilities
Tom Wilson, Department of Geology and Geography
AGI’s Sting and Swift or SuperSting
Finding the sink holesNational Corvette Museum in Bowling Green, Kentucky
Tom Wilson, Department of Geology and Geography
Useful techniques in many areas prone to
sinkhole development and collapse
Tom Wilson, Department of Geology and Geography
Along with terrain conductivity, resistivity is useful for
locating and mapping contaminant plume distribution
Tom Wilson, Department of Geology and Geography
Finding fracture zones to locate water and oil
and gas wells
Tom Wilson, Department of Geology and Geography
Tri-potential resistivity –
a method used to detect fracture zones
Tom Wilson, Department of Geology and Geography
A simple 4-electrode system also
provides multiple observations at
a single spacing - The method is
referred to as the tri-potential
resistivity method
Normal Wenner array configurationCPPC Current-Potential-Potential-Current electrode configuration
Tom Wilson, Department of Geology and Geography
A conductive fracture zone
would likely be one that is
water filled
High conductivity relative
to the country rock = low
resistivity
What is the geometrical factor?
CPCPcurrent potential current potential electrode configuration
Tom Wilson, Department of Geology and Geography
The CPPC and CPCP
electrode configurations
both reveal the presence of
a low resistivity zone
What is the geometrical factor?
CCPP
Tom Wilson, Department of Geology and Geography
The CCPP electrode
arrangement reveals the
opposite response
What is the geometrical factor?
What is the geometrical factor?
Tom Wilson, Department of Geology and Geography
V
a
Potential electrodes
a
+ -
i
i
Current electrodes
C PC
P
a = 15 meters
CCPP
A total of three measurements from one set up. Not
bad for a relatively inexpensive resistivity meter.
Tom Wilson, Department of Geology and Geography
CPPC CPCP CCPP
Geometrical
factors2a 3a -6a
Tom Wilson, Department of Geology and Geography
Tripotential resistivity measurements help establish the association
of a topographic lineament with a possible fracture zone
The work of Dr. Rauch and
some of his students
Tom Wilson, Department of Geology and Geography
Good Devonian shale wells are located near fracture zones
Dr. Rauch and students
Tom Wilson, Department of Geology and Geography
CCPP
CCPP
The fracture zone response
Is this a wet or dry fracture
zone?
Dr. Rauch and students
Looking ahead to the resistivity lab: skim through
Frohlich’s paper (see class page for paper link)
Tom Wilson, Department of Geology and Geography
Some background information about the resistivity lab
Refer to Frohlich
and part 1 of the
resistivity
computer lab
Farmland – gently rolling topography
Tom Wilson, Department of Geology and Geography
S5
S4
S3
S2
S1
Control
well 37
Control
well 16
Tom Wilson, Department of Geology and Geography
Note Drill Hole Locations
along the profile line at left
and below
Glacial outwash overlying pre-glacial channels
that sit on dolomitic limestone bedrock
Tom Wilson, Department of Geology and Geography
Shallow and
deeper aquifers
The bedrock is also a source of water, but has
high dissolved ion concentration
Tom Wilson, Department of Geology and Geography
Bedrock aquifers, may
have increased
concentrations of
dissolved solids and
lower resistivity.
This lowered resistivity
makes it difficult to
“see” the bedrock
interface.
The resistivity contrast
can be minimal.
Tom Wilson, Department of Geology and Geography
Modeling results suggest
this range may be a little
more variable and extend
from ~ 50 to 135 -m
Limestone bedrock generally
appears to have higher
resistivity – up to about 60 -m
This is your interpretation template
Tom Wilson, Department of Geology and Geography
S1 S5S4S3S2
Look over discussion of this
section on pages 347 & 348
of Frohlich’s paper
10m
100m
L/2 where L is the current electrode spacing
Copy over the resistivity data
Tom Wilson, Department of Geology and Geography
Copy over the folder IX1D-Res
Bring up IX1D
Bring up SS1 – Think about what the data
might be telling you about the subsurface
Tom Wilson, Department of Geology and Geography
We will develop a qualitative interpretation of this sounding
using “inflection point” and “extrapolation” rules.
A. Where are the inflection points?
This is a Schlumberger sounding and the x axis
is usually labeled in terms of AB/2 or l OR L/2
Increasing depth
Data collected using
the Schlumberger array
Tom Wilson, Department of Geology and Geography
B. How many layers are there and
C. What are their depths?
Inflection points
Refer to more recent, less noisy, data set S1
The rising and falling apparent resistivity trends provide insights
into relative differences of layer resistivity
Tom Wilson, Department of Geology and Geography
1 23
4 5 6
?
1 = 23-m
2 = 32-m
3 = 23-m
4 = 81-m
5 = 49-m6 = 58-m
Remember AB/2 = l = L/2
depending on text/paper
The result is consistent with the basic
drill hole data
Tom Wilson, Department of Geology and Geography
Limestone bedrock
(~35 m)
thickness depth Interp
25 3.5 3.2
103 8.8 12
40 23 35
75
ft
Constrained
using nearby
drill hole data
Our interpretation is a little more detailed.
We’ll get into details next time
Tom Wilson, Department of Geology and Geography
Grey Clay
gravel
Grey Clay
Near-surface fresh
water gravel
Sandy clay
Bedrock
thickness depth Interp
25 3.2 3.2
103 8.8 12
40 23 35
75
SS1
Questions about Chapter 5 problems?
Have a look at this in-class problem
Tom Wilson, Department of Geology and Geography
5 meters
2=180-m
1=26-m
Current
electrode
Potential
electrode
P
What is the potential measured at P given current of 0.5 amps.
Questions … ?
Tom Wilson, Department of Geology and Geography
Get started on the in-class problem. I
will not pick up, but will check you off
before leaving.
Tom Wilson, Department of Geology and Geography
• Finish up in-class problems (will discuss before leaving)
• Any other questions about Chapter 5 problems? (remember to
measure and calculate angles)
•Don’t forget - problems 5.1-5.3 due next class
• Start looking over Frohlich’s paper and get familiar with the basic
problems he addresses in his paper. We’ll be doing computer modeling
next week.
•Writing section – 1st draft chapter summary due September 22nd.
•Mid term exam September 29th
• No class October 4th and 6th
• Resistivity lab due Thursday October 13th
Looking ahead