Training Basics Gp Ok

8/11/2019 Training Basics Gp Ok

1/29

IRIS INSTRUMENTS, Training course on Electrical methods, 2006 Page 1 of 29

TRAINING COURSE

ON ELECTRICAL METHODS

PRINCIPLES

FIELD OPERATION

INTERPRETATION

Jean BERNARD

January 2 6


2/29


3/29


PRINCIPLES

OF ELECTRICAL METHODS

iris-instruments.com


4/29


APPLICATIONS OF ELECTRICAL METHODS

AIM OF GEOPHYSICAL METHODS:- To make measurements on the surface

and get ideas of what is located underground

- To select the best place where to make a holeand find the target

AIM OF ELECTRICAL METHODS:

- To detect conductive or resistive layers

and bodies at depth

Groundwaterexploration

Geotechnical studies

Environmental surveys

Archaeologicalinvestigation

Mineral

exploration

?


5/29


RESISTIVITY METHODS FOR GROUNDWATER, MINING

ENVIRONMENT, GEOTECHNICAL, ARCHAEOLOGY

Fracture and fault Waste disposal Polluted areadetection site investigation detection

Depth and thickness Fresh / salt water contact Salt water wedgeof aquifer layers determination determination

fresh

salted

__ _ _ _ __ _ _ _ _ _ _ _ _ _

_ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _

_ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _

_ _ _

. . . . . . . . . . . . . .. . . . . . . +

+ + . . . + + ++ + + + + + + +

+

Cave, cavity and karst Buried structure Mineral orebodydetection detection exploration

freshsalted

Sea

+ + + + + + + ++ + + + + + +

+ + + + + + + +

Depth to bedrock Thickness of bedrock Geological contact

measurement alteration detection


6/29


POROSITY AND PERMEABILITY

OF ROCKS

DEFINITIONS

POROSITY: quantity of water, existing in rocks(unit: %) = volume of water / volume of rocks

PERMEABILITY: speed of the water, when pushed by pressure(unit : m/s) = yield per unit of hydraulic pressure gradient

NUMERIC VALUES FOR VARIOUS TYPES OF ROCKS

TYPE OF ROCKS POROSITY(%)

PERMEABILITY(m/s)

gravel 30 10- 2

sand 25 10- 4UNCONSOLIDATED

(Soft sediments)clay 50 10- 12

massive 1 10- 10

fissured / fractured 5 10- 8

CONSOLIDATEDsandstonelimestone

granite, basalt weathered 15 10- 6

NB: These values are only indicative and largely depend on local conditions

Production well

Saturated zone (water only in pores)

GROUNDWATER PUMPING

WELL YIELD= Coeff x permeability x thickness

PUMP POWER= yield x height x g

Vadose zone (water and air in pores)Water level

Non permeable layer

thickness


7/29


ELECTRICAL PROPERTIES OF ROCKS

THE ELECTRIC CURRENTFLOWS INTO THE GROUND4THANKS TO THE IONS OF SALTS DISSOLVED IN THE WATER

Salt ions + + ions: Na, Ca, K, _ - ions: Cl, SO4, NO3, ...

Current

TYPE OF POROSITY TYPE OF WATER

TYPE OF ROCK

matrix free sand, gravel

fracture freelimestone,sandstone

adherence bound clay

THE RESISTIVITY OF ROCKS DEPEND ON:

THE WATER CONTENT (Porosity)

THE RESISTIVITY OF THE WATER

THE CLAY CONTENT

THE CONTENT IN METALLIC MINERALS

VALUES OF RESISTIVITY OF ROCKS:

0.1 ohm.m SALTED WATER1 ohm.m MASSIVE SULPHIDE

10 ohm.m CLAY100 ohm.m SAND, MARL

1 000 ohm.m DRY SAND, LIMESTONE

10 000 ohm.m HARD GRANITE, BASALT


8/29


RESISTIVITY AND POROSITY

OF ROCKS

Relation between the resistivity of and the porosity for non clayey rocks

whole rock

water

sediment

ROCK RESISTIVITY = F x WATER RESISTIVITY

F = Formation Factor = a / (porosity)N

(ARCHIE FORMULA)

10% Porosity ()For example :

Water resistivity = 10 ohm.mPorosity = 20% Rock resistivity = 250 ohm.mFormation factor = 25

FormationFactor (F)

100F 1 / 2 ( a = 1, N = 2)

RESISTIVITY SCALE FOR WATERS AND ROCKS

salt water fresh water

1 10 100 1 000 10 000 ohm m

clay sand gravel

altered rock hard rock


9/29


CONDUCTIVITY AND SALINITY

OF WATERS

CONDUCTIVITY(Siemens) = 1 / resistivity (ohm.m)

USUAL UNITof conductivity = microS / cm

Conductivity (microS / cm) = 104/ resistivity (ohm.m)

SALINITY(mineralization): Total Dissolved Salt (TDS)

TDS (mg / l) = 0.7 x conductivity (microS / cm)

NUMERIC VALUES FOR VARIOUS TYPES OF WATER:

Type ofwater

Resistivityohm.m

ConductivitymicroS / cm

Salinitymg / l

very fresh 200 50 35

fresh 20 500 350

salted 10 1 000 700

very salted(sea water)

0.3 30 000 21 000

Usual rule for drinkable water: resistivity > 10 ohm.m conductivity < 0.7 g/l


10/29


HOW TO DETECT AN AQUIFER

FROM THE VALUE OF THE RESISTIVITY ?

FROM THE ABSOLUTE VALUE OF THE RESISTIVITY

Resistivity (rock)

Resistivity (water) / porosity

FROM THE RELATIVE VALUE OF THE RESISTIVITY

Fresh water resistivity Porosity Aquifer resistivity

10 to 200 ohm.m 1 to 30% 50 to 2000 ohm.m

Geological background Aquifer resistivity

Hard rock (resistant) Lower than background

Clayey or salty (conductive) Higher than background


11/29


MEASUREMENT OF RESISTIVITY

MEASUREMENT OF RESISTIVITY OF A LINEAR CONDUCTOR

RESISTANCE (ohm) = Voltage (V) / Intensity (A) (OHM law: V = R x I)

RESISTANCE (ohm) = Resistivity (ohm.m) x length (m) / section (m)

RESITIVITY = (section / length) x voltage / intensity

Rho = K x V / I

MEASUREMENT OF RESISTIVITY OF A 3-DIMENSION CONDUCTOR

Resistance R

Intensity I

Voltage V

A M Voltage VMN N B

Intensit IABTX

RX

Ground resistance, resulting fromseveral layers with different resistivities

APPARENT RESISTIVITY = (coefficient) x voltage / intensity

Rho = K x VMN/ IAB

K = 2 x Pi / ( 1/AM 1/AN 1/BM + 1/BN )

Units: Rho (ohm.m), K (m), VMN(mV), IAB(mA)

len th

section


12/29


TWO TYPES OF RESISTIVITY

INVESTIGATIONS

VARIATIONS OF RESISTIVITY WITH DEPTH: ELECTRICAL SOUNDING

LATERAL VARIATIONS OF RESISTIVITY: ELECTRICAL PROFILING

The variation of the depth of investigationis obtained by increasingthe length of thecurrent line AB:

small lines: shallow long lines: deep

The depth of investigationvaries fromabout 1/3 to 1/10 of the length of AB

APPLICATIONS: * Thickness an depth of aquifer

Thickness and depth of layers

Depth of bedrock

Thickness of weathered layers

Quality of aquifers (from the value of the resisitivty)

The lateral variations of the resistivityalong a line is obtained by translatingthe current line AB and the potential

line MN at the same time.

The lengths of the lines AB and MNare kept constant, which means thatthe depth of investigationdoes notvary along the profile

APPLICATIONS : * Localization of fractures an weathered zones

Mapping of geological contacts

Delineation of clay lenses

Localization of mineralised dykes


13/29


MAIN ELECTRODE ARRAYS

SCHLUMBERGER SOUNDING AND PROFILING

WENNER SOUNDING AND PROFILING

GRADIENT ARRAY

RECTANGLE ARRAY

DIPOLE DIPOLE ARRAY

A M N B

MN


14/29


PRINCIPLE OF INDUCED POLARIZATION

MEASUREMENTS

The Induced Polarization (IP) is a phenomenon which occurs withsome types of minerals (mainly metallic particles, but also some clayminerals).

It is equivalent to a charge / discharge behaviour of capacitors whencurrents are switched on / off.

When IP effects are present, a decay curve is observed at the receivingelectrodes (MN), when the pulse of current is over.

ON+ OFF

ON - OFF

IABIntensity

VMN

VoltagewithoutIP effect

VMNVoltagewith

IP effect

DEFINITION OF THE INDUCED POLARIZATION PARAMETER :

CHARGEABILITY = v(t) dt / t x VMNUnit of chargeability : mV / V, or per mil

V(t) VMN


15/29


FIELD OPERATION




16/29


RANGE OF RESISTIVITYMETERS:

FROM SHALLOWEST TO DEEPEST

SYSCAL Kid

200V, 25W, 0.5A

SYSCAL Junior

400V, 100W, 1.2A

SYSCAL R1 Plus

600V, 200W, 2.5A

SYSCAL R2

800V, 250W, 2.5A800V, 1 200W, 2.5A


17/29


PACKING LIST

FOR A RESISTIVITY SURVEY

MAIN EQUIPMENT

The equipment itself (resitivitymeter), with charged batteries

The PC computer for data transfer and interpretation

REELS AND ELECTRODES Reels with electric wires for transmission of current (AB line)

Reels with electric wires for measurement of voltage (MN line)

Metallic stakes, with hammers

Cables and clips for wire to stake connection

OTHER ACCESSORIES

Measuring tapes (100m)

Tool kit box (pliers, screwdrivers, voltmeter, isolating tape, ) Field note book and data sheets

An external 12V car battery

Resistivity survey with SYSCAL Junior resistivitymeter


18/29


TRANSMISSION OF THE CURRENT

IN THE RECEIVER CIRCUIT MN:

Basic formula: Apparent RESISTIVITY = K x VMN/ IAB

Hence: VMN = Apparent RESISTIVITY x IAB/ K

When AB is large (deep investigation), K is large, and in case of low values of theapparent resistivity, the voltage VMN can be difficult to measure unless the intensityIAB is large enough (usual values of the intensity: 10, 50, 100, 500 mA, ).

Hence the usefulness of having sometimes an intensity of current as high as possible

IN THE TRANSMITTER CIRCUIT AB:

Ohms Law: IAB = VAB / RAB

To increase the intensity IABof the current: increase VABon the equipment(50, 100, 200, 400, 600, 800V)

or decrease RABon the ground(10k, 1k, 100 ohm )

To decrease the ground resistance of the electrodes A and B:

push the metallic stakes more deeply into the ground

use several stakes per electrode (and connect them in parallel

Pour water onto the stakes (if possible water with salt)

Intensity IABTX

RX

Voltage VAB

A M Voltage VMN N B


19/29


MEASUREMENT OF THE VOLTAGE VMN

NUMBER OF READINGS Noise level

1 READING ONLY Noise(1)

N READINGS (N STACKS) Noise(1) / sqrt (stack number)

IDEAL

CASE

REAL

CASE

VOLTAGE

noiseINTENSITY

IABVOLTAGE

VMN

IAB

+IAB

-IAB

VMN

Telluric currents,

Self Potential,Industrial noise

no noise

+VMN

-VMN


20/29


OPERATING PROCEDURE

OF A SYSCAL RESISTIVITYMETER

PREPARING THE READING

BATTERY Check the capacity of the battery

ARRAY Select the electrode array, ex. Schlumb. Sounding

SPACING Introduce the values of the lengths of the lines

Ex: AB / 2 = 10, MN / 2 = 2CHECK Check the connection of the electrodes

and the values of their ground resistances

PARAMETERS Introduce various parameters such as the pulseduration (ex. 500 ms or 1s)

TAKING THE READING

START Check the various values displayed:

Intensity IAB in mAVoltage VMN in mVResistivity RHO in ohm mStandard deviation Q in %Chargeability M in mV/VNumber of stacks in #

STOP Check the average (stacked) values of

the previous parametersPlot the resistivity value on the diagram

USING THE MEMORY

STORE Save the parameters in the internal memory

READ Recall a previously stored reading

TRANSFER Load the data into the PC computer


21/29


FIELD DATA SHEET

FOR RESISTIVITY MEASUREMENTS

SITE: DATE: VES nb :

OPERATOR : EQUIPMENT:

AB/2m

MN/2m

VABm

VMNm

IABmA

RHOohm.m

M

mV/VMem

3 0.5

4 0.5

4 1

5 0.5

5 1

6 1

8 1

8 2

10 1

10 2

15 2

20 2

20 5


22/29


INTERPRETATION




23/29


INTERPRETATION OF VERTICAL

ELECTRICAL SOUNDINGS (VES)

QUANTITATIVE INTERPRETATION

A bilogarithmic representation in Log(Rho) and Log (AB/2) is used because a 10m thicklayer at 10m depth has the same effect than a 100m thick layer at 100m depth (theresolution decreases with depth).

PRINCIPLE OF EQUIVALENCE

CASE 1:

1 3

2

CASE 2:

2

1 3

Solutions (resistivity-thickness) which give the same sounding curves are equivalent

Apparent resistivity

resistive resistive

100ohm.m

conductiveAB/2

10 100m

3m 1 000 ohm.m

30m 20 ohm.m

00 5 000 ohm.m

FOR THE SECOND LAYER:

the thickness is undetermined

the resistivity is undetermined

BUT THE LONGITUDINAL CONDUCTANCE

S = thickness / resistivity

IS WELL DETERMINED

Example: 30m / 20ohm.m = 1.5 Siemens

15m / 10ohm.m = 1.5 Siemens

FOR THE SECOND LAYER:

the thickness is undetermined

the resistivity is undetermined

BUT THE TRANSVERSE RESISTANCE

T = thickness x resistivity

IS WELL DETERMINED


24/29


TWO LAYER MASTER CURVES

FOR SCHLUMBERGER SOUNDINGS

FIRST LAYER

SECOND LAYER

Resistivity : Rho1

Thickness : E1

Resistivity : Rho2

Rho1 Rho2

E1

Apparentresistivity

AB / 2


25/29


EXAMPLE OF THREE LAYER CURVES

FOR SCHLUMBERGER SOUNDINGS

FIRST LAYER

SECOND LAYER

THIRD LAYER

Resistivity: Rho1

Thickness: E1

Resistivity: Rho2

Thickness: E2

Resistivity: Rho3

E1

Rho1

Rho2

Rho3

Apparentresistivity

AB / 2E2 / E1

With master curves, the interpretation is carried out by placing the experimentalsounding curve (data points) over one of the master curves which best fits the data.

The thickness and resistivitiesof the various layers are then read on the mastercurves according to the mentioned indications.

Due to the equivalence law, several theoretical solutions can be found for onegiven set of experimental data.


26/29


FIELD EXAMPLE : PARIS BASIN

SCHLUMBERGER SOUNDING IN ALLUVIUMS (LOIRERIVER)

The 12 first meters (about 100 ohm.m resistivity) consist in sands, then6m show a marlscomponent with a lower resistivity (less then 20ohm.m).

At 18 m depth, starts the waterwhich overlies the first series ofLimestone layers which at their top are frequently karstified (hence theirlow resistivity, of the order of 150 ohm.m)


27/29


FIELD EXAMPLE : NIGER

Sand Alteration Bedrock

Sand

lteration

Bedrock

ault (F)

LOOKING FOR A FAULTED AREA IN A CRISTALLINE BEDROCKFOR SUPPLYING GROUNDWATER TO A VILLAGE

First, aSchlumbergersoundingtodetermine thedepth of thebedrock

Then, a

Schlumbergerprofilingtolocate laterallythe faulted areaand sit a drillhole


28/29


FIELD EXAMPLE : LA REUNION ISLAND

LOOKING FOR A THICK, NON SALTY, NON CLAYEY AQUIFER

IN VOLCANIC ROCKS (BASALT, ASHES, LAVAS)

FOR AGRICULTURAL PURPOSES

Schlumbergersoundingscarriedout every 300mpermit to identifyvarious geologicalformations

At a few hundredsmetres from the seacoast, the intrusionof salt wateris adanger. Low values

of resistivity (


29/29

FIELD EXAMPLE : BORA BORA ISLAND

LOOKING FOR THE MAXIMUM DEPTH OF A DRILL HOLE IN A

CORAL ISLAND (PACIFIC OCEAN)

In a coral island, the fresh water is in equilibrium above the saltwater coming from the sea and usually forms a kind of lens.

Electrical soundings carried out on the Bora Bora island confirm thisshape of lens, with a maximum thickness of fresh water sandsof 40m in the middle of the island. The drillholes do not have to

overpass this depth for the salt not to contaminate the fresh water.

Training Basics Gp Ok

Documents

Transcript of Training Basics Gp Ok