electrical techniques

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Electrical Techniques

1D Resistivity Sounding

2D Resisivity Imaging (Subsuface Imaging)

Electrokinetic Sounding (EKS) Time Domain Induced Polarisation (TD-IP)

Spectral Induced Polarisation

Spontaneous (Self) Potential

Geomembrane Leak Location


Outline | Detail | Results |

OUTLINE

Resistivity measurements are made by passing anelectrical current into the ground using a pair of

electrodes and measuring the resulting potentialgradient within the subsurface using a second

electrode pair (normally located between the

current electrodes). Resistivity sounding involvesgradually increasing the spacing between the

current/potential electrodes (or both) in order to

increase the depth of investigation. The data

collected in this way are converted to apparent

resistivity readings that can then be modelled inorder to provide information on the thickness

of individual resistivity units within thesubsurface.

DETAIL

Resistivity measurements are made by passing an

electrical current into the ground using a pair of

electrodes and measuring the resulting potentialgradient within the subsurface with a second

(potential) electrode pair (normally located between

the current electrodes). Resistivity soundingsinvolve gradually increasing the spacing between

the current/potential electrodes (or both) in order to

increase the depth of investigation. The resistancedata collected in this way are converted to apparent

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resistivity readings that can then be modelled toprovide information on the thickness of individual

resistivity layers within the subsurface.

Measurements are taken manually using a ground

resistivity meter that will normally provide a directreadout of resistance. In order to convert the

resistance reading to an apparent ground resistivity,a geometric factor is applied to the data, based on

the type of electrode configuration being used. The

most common electrode array used in soundingwork is the Schlumberger array. In this

configuration the two potential electrodes are

located at the centre of the spread and are closely

spaced compared to the two current electrodes thatare also located symmetrically about the centre

point.

In order to increase the depth of investigation the

current electrode separation is increased whilst the

potential separation remains constant. In this wayonly two electrodes require moving compared to all

four in other configurations such as the Wenner

array illustrated above.

The most common problem encountered in

resistivity sounding work is high contact resistances

at the current electrodes. Whilst this does notdirectly affect the measured value of resistance,high contact resistances (>2kOhms) will reduce the

maximum current that can be applied with theoutput voltage available from the meter (typically

300-400V). In order to overcome high resistances

electrodes can be watered with a saturated saltsolution or placed in hole filled with bentonite

or clay slurry.




RESULTS

1D Resistivity sounding results are presented as

combined plots of measured apparent resistivity

against half the current electrode spacing (AB/2)

and the modelled resistivity versus depth (right-hand image). The dotted lines in the right-hand plot

indicate equivalent resistivity models that alsosatisfy the observed data. Equivalence results from

the inability to uniquely resolve the thickness and

resistivity of a layer from its resistance value

(thickness x resistivity). As long as the resistivityand thickness are changed within limits to give the

same product there will be no appreciable

variation in the apparent resistivity curve.



OUTLINE

2D Resistivity Imaging uses an array of electrodes

(typically 64) connected by multicore cable toprovide a linear depth profile, or pseudosection, of

the variation in resistivity both along the survey

line and with depth. Switching of the current andpotential electrode pairs is done automatically using

a laptop computer and relay box. The computer

initially keeps the spacing between the electrodesfixed and moves the pairs along the line until the

last electrode is reached. The spacing is then

increased and the process repeated in order toprovide an increased depth of investigation.






















DETAIL

Measurement of ground resistivity involves passing

an electrical current into the ground using a pair of

steel or copper electrodes and measuring the

resulting potential difference within the subsurfaceusing a second pair of electrodes. These are

normally placed between the current electrodes.

Unlike conventional resistivity sounding and lateral

profiling surveys, 2D resistivity imaging is a fullyautomated technique that uses a linear array of up

to 64 electrodes connected by multicore cable. The

current and potential electrode pairs are switched

automatically using a laptop computer and controlmodule connected to a ground resistivity meter

(that provides the output current). In this way aprofile of resistivity against depth ('pseudosection')

is built up along the survey line. Data is collectedby automatically profiling along the line at different

electrode separations. The computer initially keeps

the spacing between the electrodes fixed and movesthe pairs along the line until the last electrode is

reached. The spacing is then increased by the

minimum electrode separation (the physical

distance between electrodes which remains fixedthroughout the survey) and the process repeated in

order to provide an increased depth of investigation.

The maximum depth of investigation is determined

by the spacing between the electrodes and thenumber of electrodes in the array. For a 64

electrode array with an electrode spacing of 2m this

depth is approximately 20m. However, as thespacing between the active electrodes is increased,

fewer and fewer points are collected at each 'depth

level', until on the final level only 1 reading is

acquired (see figure). In order to overcome this thearray is 'rolled-along' the line of investigation in

order to build up a longer pseudosection.

The raw data is initially converted to apparent

resistivity values using a geometric factor that is

determined by the type of electrode configurationused. Many 2D resistivity imaging surveys are



carried out using the Wenner Array. In thisconfiguration the spacing between each electrodes

is identical. Once converted the data is modelled

using finite element and least squares inversion

methods in order to calculate a true resistivity

versus depth pseudosection.

RESULTS

The modelled results are displayed as scaled

resistivity-depth pseudosections as illustrated

below. Blues represent areas of low resistivity

whilst reds are relatively higher. The wedge shapeof the plot illustrates the gradual reduction in the

amount of data acquired as the current and potential

electrode spacings are increased. As discussedearlier this is overcome by gradually rolling the

electrode array along the survey line. The

interpretation of the resistivity-depth pseudosection

is normally provided as a separate diagram beneaththe data or overlain directly on top. The results are

calibrated using any available borehole or trial pit

information together with modelled results from 1Dresistivity soundings taken on the 2D

resistivity imaging line.

Electrokinetic Sounding (EKS)

Outline Detail | Results |

OUTLINE

EKS provides a measure of the variation in the

hydraulic conductivity of saturated subsurface

layers with depth by measuring the seismicallyinduced time varying electromagnetic response

from saturated aquifers. Readings are taken using

two pairs of electrodes aligned symmetrically aboutthe seismic source. This normally consists of a

mechanical weight drop or simple hammer and

plate. In order to relate hydraulic conductivities todepth additional information on the seismic

velocity of subsurface layers and their





















conductivities is required.

DETAIL

EKS provides a measure of the variation in thehydraulic conductivity of saturated subsurfacelayers with depth by measuring the seismically

induced time varying electromagnetic response

from saturated aquifers. Readings can be takenusing two pairs of electrodes aligned symmetrically

about the seismic source. This normally consists of

a mechanical weight drop or simple hammer and

plate. The vertical resolution of a particularsounding is a function of the frequency of the

seismic energy that can be generated and

propagated as well as the permeability contrastbetween different regions within a saturated

aquifer.

In most situations a hammer source is sufficient and

azimuthal soundings are carried out at each

measurement station to monitor anisotropy. In orderto relate hydraulic conductivities to depth

additional information on the seismic velocity of

subsurface layers and their conductivities is

required. These properties are independently

derived elsewhere, for example by calibrationagainst boreholes, from TDEM soundings and

shallow seismic refraction surveys.

The EKS method as discussed above is somewhat

controversial with some researchers claimingseismically induced Raleigh-wave signal as the

main source of so-called EKS signal.

RESULTS

The results of EKS surveys are presented as 1D

plots of the measured EK signal in millivolts versus

depth for both the left and right channels (the twochannels represent the electrical dipoles either side

of the source position). The two signals are used to

calculate approximate permeabilities (in

metres/day) for saturated bedrock aquifers. This











information is normally displayed next to the EKprofile.

Where a series of EK soundings have beencollected along a profile or a series of profiles, 2D

sections of permeability versus depth can beconstructed by extrapolating the 1D models (see

figure below). If the data is extensive thenpermeability maps can be produced for individual

depth slices.

2D profile across limestone aquifer illustratinghigh permeability zones (reds) within the rock

beneath the valley and an impermeable zone

(blue)on the adjacent hill.

Time Domain Induced

Polarisation (TD-IP)


OUTLINE

Time domain IP surveys involve measurement of

the magnitude of the polarisation voltage (Vp) thatresults from the injection of pulsed current into the

ground. Polarisation voltages primarily result from

electrochemical action (ionic exchange) within the

pores and pore fluids of the material beingenergised. The current is applied in the form of a

square waveform, with the polarisation voltagebeing measured over a series of time intervals aftereach current cut-off using non-polarising

electrodes. The measured value of Vp is divided by

the steady voltage (observed whilst the current ison) to give the apparent chargeability of the ground.

This provides qualitative information on the

subsurface geology. TD-IP is primarily used in

mineral exploration surveys.






















DETAIL

Time domain IP surveys involve measurement of

the magnitude of the polarisation voltage (Vp) that

results from the injection of pulsed current into the

ground.

Two main mechanisms are known to be responsiblefor the IP effect although the exact causes are still

poorly understood. The main mechanism in rocks

containing metallic conductors is electrodepolarisation (overvoltage effect). This results from

the build up of charge on either side of conductive

grains within the rock matrix as they block the flow

of current. On removal of this current the ionsresponsible for the charge slowly diffuse back into

the electrolyte (groundwater) and the potentialdifference across each grain slowly decays to zero.

The second mechanism, membrane polarisation,results from a constriction of the flow of ions

around narrow pore channels. It may also result

from the excessive build up of positive ions aroundclay particles. This cloud of positive ions similarly

blocks the passage of negative ions through pore

spaces within the rock. On removal of the applied

voltage the concentration of ions slowly returns toits original state resulting in the observed IP

response. In TD-IP the current is usually applied inthe form of a square waveform, with thepolarisation voltage being measured over a series of

short time intervals after each current cut-off,

following a short delay of approximately 0.5s.These readings are integrated to give the area under

the decay curve, which is used to define Vp. The

integral voltage is divided by the observed steady

voltage (the voltage due to the applied current plusthe polarisation voltage) to give the apparent

chargeability (Ma) measured in milliseconds. For a

given charging period and integration time themeasured apparent chargeability providesqualitative information on the subsurface geology.

The polarisation voltage is measured using a pair of

non-polarising electrodes similar to those used in

spontaneous potential measurements and other IPtechniques. Although a variety of current/potential



electrode configurations can be used in IP surveys,as in DC resistivity measurements, the most

popular configuration is the dipole-dipole

array.

RESULTS

The figure, right, illustrates a 2D resistivity-depth

profile across part of the survey area. The dark bluelow resistivity zone highlighted in yellow provided

a coincident high IP chargeability and

represented a known orebody.

Spectral Induced Polarisation


OUTLINE

Spectral IP surveys involve measurement of themagnitude and relative phase of the polarisation

voltage that results from the injection of an

alternating current into the ground. Polarisation

voltages primarily result from electrochemicalaction (ionic exchange) within the pores and pore

fluids of the material being energised.

Measurements of the relative phase shift betweenthe transmitted current and the measured signal and

the magnitude of the polarisation voltage are taken

over a range of different frequencies, typically

between 0.125 and 1000Hz. This results in adistinct IP response spectrum or 'dispersion' at each

measurement position that can be used to determine

various parameters of the subsurface materials such

as relaxation time and chargeability. Spectral IP iscurrently being tested in the detection of

hydrocarbon contamination.





























DETAIL

Spectral IP surveys involve measurement of the

magnitude and relative phase of the polarisation

voltage that results from the injection of an

alternating current into the ground. The polarisationvoltage essentially results from the ability of the

ground to store charge (i.e. to become polarised) ina similar manner to an electrical capacitor.

Two main mechanisms are known to be responsiblefor the IP effect although the exact causes are still

poorly understood. The main mechanism in rocks

containing metallic conductors is electrode

polarisation. This results from the build up of charge on either side of conductive grains within

the rock matrix as they block the flow of current.On removal of this current the ions responsible for

the charge slowly diffuse back into the electrolyte(groundwater) and the potential difference across

each grain slowly decays to zero. The second

mechanism, membrane polarisation, results from aconstriction of the flow of ions around narrow pore

channels. It may also result from the excessive

build up of positive ions around clay particles. This

cloud of positive ions similarly blocks the passageof negative ions through pore spaces within the

rock. On removal of the applied voltage theconcentration of ions slowly returns to its originalstate resulting in the observed IP response.

In spectral IP surveys measurements of the relativephase shift between the transmitted current and the

measured signal, and the magnitude of the

polarisation voltage, are taken over a range of different frequencies, typically between 0.125 and

9000Hz. This results in a distinct IP response

spectrum or 'dispersion' at each measurement

position that can be used to determine variousparameters of the subsurface materials such as

relaxation time and chargeability.

Spectral IP was initially used in trying to determine

the type and texture of mineralisation in exploration

work but is being increasingly applied to thedetection of subsurface contamination. GSI (UK)



Ltd. is currently investigating the use of thetechnique in hydrocarbon mapping in conjunction

with a research project into the use of ground

penetrating radar with Shell Research.

RESULTS

The results of spectral IP surveys can be presentedin a number of different forms. Raw and decoupled

phase and apparent resistivity pseudosections are

used to present an overview of the data for

initial interpretation and quality checking.

Spontaneous (Self) Potential


OUTLINE

The spontaneous potential (SP) method is a passive

electrical technique that involves measurement of

naturally occurring ground potentials. The twomain sources of SP signals important in

environmental and engineering studies are

streaming potentials, due to movement of waterthrough porous subsurface materials, and diffusion

potentials resulting from differing concentrations of

electrolytes within the groundwater. SPmeasurements are made using a pair of non-

polarising electrodes (normally comprising a

copper electrode immersed in a saturated copper

sulphate solution) connected to a highimpedance voltmeter.





























DETAIL

The spontaneous potential (SP) method is a passive

electrical technique that involves measurement of

naturally occurring ground potentials. These can be

generated from a number of different sourcesalthough all require the presence of groundwater to

some degree. The two main sources of interest inenvironmental and engineering studies are

streaming potentials, due to movement of water

through porous subsurface materials, and diffusion

potentials resulting from differing concentrations of electrolytes within the groundwater.

SP measurements are made using a pair of non-polarising electrodes. These normally comprising a

pot containing a copper electrode immersed in asaturated copper sulphate solution. A porous base

to the pot enables the electrolyte to percolate outand make contact with the ground. The potential

difference between the two pots is measured using

a high impedance voltmeter.

Common applications for SP measurements includeassessing seepage from dams and embankments,

fluid migration pathways in landfills, mapping coal

mine fires and for the study of drainage structures,

shafts, tunnels and sinkholes.

RESULTS

The results of SP surveys are generally presented as

colour-coded grids or single profiles depending onthe amount of data collected. Areas of fluid ingress

appear as low voltage anomalies in SP data whilst

zones where fluid is migrating downwards are

generally high. This information can be used to

map migration pathways as in the example abovewhich illustrates the results of a SP survey over a

closed landfill. Areas of low background voltageare identified as blue whilst highs are red. The two

circular blue anomalies near the centre of the site

where later found to be due to ingress from afreshwater spring at the base of the landfill.

Individual profiles can be used to identify potential






leakage zones within features such as embankmentsand cut-off trenches.

Geomembrane Leak Location


OUTLINE

Geomembrane leak location surveys (GLLS)

involve applying an electrical potential either side

of synthetic geomembranes in order to identify

holes and tears. These are commonly introducedduring emplacement of the protective cover but

may also result from later weld failures/puncturing

during loading with waste or fluid. The GLLmethod relies on the extremely high electrical

resistivity of liner materials (such as HDPE and

PVC) which prevents current flow across thegeomembrane except through any holes or tears.

This results in anomalous high potential gradients

in the protective cover material above any holes

which can be mapped over the surface of thegeomembrane using a pair of roving potential

electrodes.

DETAIL

Geomembrane leak location surveys (GLLS)

involve applying an electrical potential either side

of synthetic geomembranes in order to identifyholes and tears. These are commonly introduced

during emplacement of the protective cover but

may also result from later weld failures/puncturing

during loading with waste of fluid.

The GLL method relies on the extremely highelectrical resistivity of liner materials (such as

HDPE and PVC) which prevents current flow

across the geomembrane except through any holes

or tears. This results in anomalously high potentialgradients in the protective cover material above any

holes which can be mapped over the surface of the






















liner using a pair of roving potential electrodes (seefigure).

The high contact resistances (resistance to currentflow) encountered at holes means that a high-

voltage current source is generally required in orderto induce current flow through any holes. The

method has been used to successfully locate smallholes (<5mm) below up to 1m of protective cover.

For cells with a single geomembrane, current is

passed across the liner using two current electrodes;one outside the cell (sink) and the other within the

protective cover material (injector). In the case of

double lined landfills the outer (sink) electrode is

placed in the sandwich between the two liners. Thesensitivity of the survey is primarily dependent on

adequate isolation of the material outside the cellfrom the protective cover material above. Poorisolation, due to tying in of the geomembrane

within anchor trenches, results in high levels of

noise around the edges of the cell. Isolation isgenerally not a problem on double lined cells where

the sandwich is effectively isolated from the

material above.

A typical leak location system for use on soil

covered landfills comprises a high voltage

(+1000V) current regulated power supply and a pairof mobile non-polarising potential electrodes.

Voltages are logged automatically to a digital data

logger, enabling immediate download andprocessing of the data in the field for interpretation

and quality control.

A modified leak location system is available for use

on fluid covered and uncovered liners, such as

leachate lagoons and ornamental lakes and cell side

slopes where there is no soil cover. Due to the low

amplitude of the anomalous signals within waterthe system has a built in amplifier and provides

an audible indication of any anomalies.




RESULTS

The results of geomembrane leak location surveys

over soil-covered membranes, are presented as

scaled colour-coded grids illustrating the variation

in electrical potential across the surface of thecover. These may be overlain by a contour plot of

the data (as illustrated here) in order to indicate thestrength of each anomaly. Voltage anomalies

indicative of holes appear as sub-circular highs (the

red areas on the image at right). These are often

flanked by a weak low.

In addition to the dimensions of the hole in the

membrane, the size and shape of a particularanomaly will depend on a number of other critical

factors including the strength of the applied current,the position of the current injector electrode and the

thickness of the protective cover.

Results from leak location surveys over lagoons, oruncovered membranes are monitored on-site by the

operator who may flag any anomalies during

the survey.

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