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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
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IRIS INSTRUMENTS, Training course on Electrical methods, 2006 Page 3 of 29
PRINCIPLES
OF ELECTRICAL METHODS
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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
?
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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
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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
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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
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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
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IRIS INSTRUMENTS, Training course on Electrical methods, 2006 Page 9 of 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
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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
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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
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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
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MAIN ELECTRODE ARRAYS
SCHLUMBERGER SOUNDING AND PROFILING
WENNER SOUNDING AND PROFILING
GRADIENT ARRAY
RECTANGLE ARRAY
DIPOLE DIPOLE ARRAY
A M N B
MN
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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
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FIELD OPERATION
OF ELECTRICAL METHODS
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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
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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
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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
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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
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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
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IRIS INSTRUMENTS, Training course on Electrical methods, 2006 Page 21 of 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
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INTERPRETATION
OF ELECTRICAL METHODS
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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
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TWO LAYER MASTER CURVES
FOR SCHLUMBERGER SOUNDINGS
FIRST LAYER
SECOND LAYER
Resistivity : Rho1
Thickness : E1
Resistivity : Rho2
Rho1 Rho2
E1
Apparentresistivity
AB / 2
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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.
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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)
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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
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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 (
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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.