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“Step Out From the Old to the New”

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“The Right to Information, The Right to Live”

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“Invent a New India Using Knowledge”

है”ह”ह

IS 15736 (2007): Geological exploration by geophysicalmethod (electrical resistivity) - Code of practice [WRD 5:Gelogical Investigation and Subsurface Exploration]

IS 15736:2007

Indian Standard

GEOLOGICAL EXPLORATION BY GEOPHYSICALMETHOD (ELECTRICAL RESISTIVITY) —

CODE OF PRACTICE

IC’S 93.020

boiizz5 0131S 20070m. BKTREAU OF INDIAN STANDARDS

MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARGNEW DELH1 110002

June 2007 Price Group 7

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the GeologicalInvestigations and Subsurface Exploration Sectional Committee had been approved by the Water Resources Division

Council.

Water resource projects are cost intensive by virtue of their immense magnitude, and considerable amount of resources— financial, human, technical as well as social are utilized for their construction. It is, therefore, imperative that all

aspects that can impact the scope, nature, stability or progress of such projects, are thoroughly examined and exploredin detail.

Subsurface geological exploration are an integral part of pre-planning as well as design stage of any water resource

project. Apart ffom the methods of geological exploration that have been used in the past, there have been developmentsin this field and instrumental methods of geological exploration are coming to the fore. One of the methods beingincreasingly used in geological exploration is the electrical resistivity method. This standard is intended to provide a

Code of practice for application of electrical resistivity method and to provide guidance on the various equipment used

in the same as well as for providing guidance prescribing field procedures and documentation of data.

[t has been assumed in the formulation of this standard that the execution of its provisions is entrusted to appropriately

qualified and experienced people, for whose guidance it has been prepared.

For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observedor calculated, expressing the result of a test or analysis, shall be rounded off in accordance with IS 2: 1960 ‘Rules forrounding off numerical values (?evise~’. The number of significant places retained iri the rounded off value should be

the same as that of the specified value in this standard.

1S 15736:2007

Indian Standard

GEOLOGICAL EXPLORATION BY GEOPHYSICALMETHOD (ELECTRICAL RESISTIVITY) —

CODE OF PRACTICE

1 SCOPE

This standard lays down a summary of the best practicefor geological exploration by electrical resistivity methodincluding equipment, field procedures, and interpretationof data for measurement of the electrical properties of

subsurface materials. Resistivity measurements as

described in this Code are applicable in geological,gGOtGGhniGal, environmental and hydrologicalinvestigations.

2 TERMINOLOGY

2.1 Array — The arrangement of electrodes in resistivityprospecting, also called configuration.

2.2 Anomaly — A deviation from uniformity in physicalproperties.

2.3 Anisotropy — Variation of a physical propertydepending on the direction in which it is measured. Theresist ivity anisotropy coefficient is the square root of theratio of the resistivity measured perpendicular to the

bedding to that measured parallel to the bedding.

2.4 Apparent Resistivity — The ground resistivitycalculated from measured resistance and a geometricfactor derived for the condition where the ground ishomogeneous and isotropic. Apparent resistivity p, is an

Ohm’s law ratio of measured voltage V-toapplied current/, multiplied with a geometric constant K which depends

on the electrode array.

2.5 Apparent Resistivity Curve — A graph of apparentresistivity plotted against electrode separation. [n case ofsoundings, apparent resistivity curves are plotted ondouble logarithmic paper for comparison with normalized

r- theoretica] curves, for the purpose of interpreting thea.. resistivity, thickness and depth of surface layers. In case$?----~ of profiling, the apparent resistivity curve is plotted onm semi log paper.Qm,t-~ 2.6 Electrode — Apiece of metallic material that is used as

an electrical contact with a non-metal. May also refer to agrounding contact used for field surveying, to the metallicminerals in a rock.

2.7 Homogeneous — [Jniforrnity of a physical propertythroughout a material.

2.8 Inversion — The technique for deriving a subsurface

geological model from observed field data that is, solvingfor a spatial distribution of parameters in terms ofthicknesses and true resistivities which could have

produced on observed set of measurements.

2.9 Profiling — A resistivity method whereby an arraywith a fixed electrode spacing is moved progressively

along a traverse to create a horizontal profile of the apparentresistivity.

2.10 Resistivity — The property of a material whichresists the flow of electric current.

2.11 Resistivity Method — Observation of electric fields

caused by current introduced into the ground as a meansfor studying earth resistivity in geophysical exploration.

2.12 Resistivity Imaging — It is an advanced techniquefor gathering continuous subsurface resistivitydistribution in two and three dimensions through an

automatic electrode switching mechanism. [n thistechnique large amount of data is collected, and therefore,it offers more reliable results than the conventionalresist ivity sounding/profiling. It requires specialequipment and software package.

2.13 Sounding —A depth probe or expander. A series of

electrical resistivity readings, with successively greaterelectrode spacing, made while maintaining one point inthe array fixed, thus giving resistivity versus depthinformation (or p,versus depth information).

2.14 True Resistivity — In the idealized condition of aperfectly uniform conducting half space (Homogeneous,isotropic semi infinite), the current flow lines resembles adipole pattern and the resistivity determined with a fourelectrode configuration is the true resistivity of the halfspace.

However, in real situations the resistivity isfor different lithologies and geological(inhomogeneous and anisotropic medium).

1

determinedstructures

IS 15736:2007

3 PARAMETERS MEASURED AND

REPRESENTATIVE VALUES

3.1 The generally accepted unit of resist ivity isohm-meter. ]n most rock materials. the porosity and thechemical content of the water filling the pore spaces ismore important in governing resistivity. The salinity ofthe water in the pores is probably the most critical factordeternlining the resistivity. When pores, particularly thosewith large concentrations of magnetite or graphite, lie above

the water table at shallow depths, or when they occur atsuch great depths that all pore spaces are closed by ambient

pressure, the conduction through them takes place withinthe mineral grains themselves. Under these conditions,

the resistivity of the rock will depend on the resistivity ofthe grains. When the pores are saturated with fluids, theresistivity will be governed by the fluid resistivity as well.

3.2 The range of resistivities among rocks and rock

materials is enormous, extending from 10-5 to 1015ohm-m. Rocks and minerals with resistivities from 10“sto

10’ ohm-m arc considered good conductors: those fromI to 107 ohm-m, intermediate conductors, and those from

108 to 10“ ohnwm poor conductors.

Igneous rocks have the highest resistivity, sedimentarythe lowest, with metamorphic rocks intermediate. However,there is considerable overlapping, as in other physical

properties. In addition, the resistivities of particular rocktypes vary directly with age and Iithology, since porosity

of the rock and salinity of the contained water are affected

by both. The resistivities of some common rocks, soils,waters and minerals are as shown in Table 1.

4 PURPOSE OF ELECTRICAL RESISTIVITY

SURVEY

The purpose of electrical resistivity survey is to determinethe subsurface resistivity distribution by making+neasuremcnts on the ground surface. From these

}n~asurements. the true resistivity of tl]e subsurface canbe estimated. The ground resistivity is related to variousgeological parameters, such as, the mineral and fluidcontent, porosity and degree of water saturation in rock.Electrical resistivity surveys have been used for many

decades in hydrogeological, mining and geotechnicalinvestigations. More recently, it has been used for

envirotlmental surveys. It has the following otherpurposes:

a)

b)

To rapidly explore the subsurface conditions in orderto locate ground water, thickness of overburden,

depth to different rock types and stratigraphicfeatures.

To del ineate weak formations, faults and dykes, ifany, and to identify location of steeply dippingcontacts between different rock types and earthmaterial.

c)

d)

e)

f)

To delineate zones of seepage and identify itssource around various structures of river valleyprojects.

Assessment of groundwater potential, quality anddetermination of aquifer characteristics.

To correlate data from resistivity survey with those

obtained from borehole and trial pit logs.

For earthing of electrical conductors.

Table I Resistivity Values of Some Common Materials

(Clause3.2)

Material ltesistivityohm-m

(1) (2)

Igneous and MetamorphicRocks

Granite

Basalt

Slate

Marble

Qum[?.ite

Sedimentary Rocks

Snndblone

Shale

Limestone

Soils and Waters

Clay

Alluvium

Gmundwater (fresh)

Sea Water

Minerals

Galena

Bauxite

Cuprite

Hematite

Magnetite

Quartz

~Jraninite

Calcite

Rock Salt

Diamond

Mica

5 x I(p I(y’

lo~ – 10”

fjx\~_4.]07

10’–2.s x 10’

10’–2 x 10’

8–4X1O’

20–2.10’

5O–4X1O’

I – 100

10-800

10- 100

0.2

3X1 O-’–3XI(F

zxlo–fjx]()~

10-’-300

3.5 x 10-’– 10’

5 ~ 10-’– 5.7 x 10’

4x }01)-zx lo[~

1 -200

2 x 10”

30– 10”

10– 10”

9 x 10’– 10’”

5 METHODOLOGY

5.0 The measurement of electrical resistivity requires thatfour electrodes be placed in contact with the surface

material as shown in Fig. 1. The geometry, separation ofthe electrode array and spacing are selected on the basisof the application and required depth of investigation.

A direct current, or a very low frequency alternating current,

■✌✌✌✌✌✌✌✌ ,.=.- . . . . .—-.

IS 15736:2007

CURRENT SOURCE CURRENT METER

)

@+-#J-,

NT

-––- VOLTAGE

FIG. 1 EI~IJIIJIY~I-NrIAILINITS.~NIICIJI<II~NrFI.()WI.IN~SIOI/ FCILJREI.~CTROD~ARRAY

is passed into the ground through a pair of currentelectrodes, and the resulting potential drop is measured

across a pair of potential electrodes as shown in Fig.1.The resistance is then derived as the ratio of the voltagemeasured across the potential electrodes and the current

electrodes. Theapparent resistivity ofsubsurface materialsis the resistance multiplied by a geometric factor determined

by the geolmetry and spacing of the electrode array.

where AI’ist hepotential drop and Ithe applied current.

The apparent resistivity depends on resistivity contrastbetween adjacent layers. The apparent resistivity(p~)

depends not only on the nature ofgeoelectric section butalso on geometric configuration of the electrodes used forthe measurement. This apparent resistivity is different from

true resistivity unless the subsurface materials arehomogeneous.

Two rnaiq procedures are employed in resistivity surveys:

a) Elec[riealprojling — It is used to determine lateralvariations ofresistivity. The current and potentialelectrodes are maintained at a fixed separation andprogressively moved along a profile. This methodis employed in mineral prospecting to locate faultsor shear zones and to detect localized bodies ofanomalous conductivity. It is also used in

b)

geotechnical surveys to determine variations in

bedrock depth and the presence of steepdiscontinuities and to evaluate the resistivity oflayers for earthing of electrical conductors.

Vertical electrical sounding (VES) — Also knownas electrical drilling or expanding probe. It isemployed to investigate changes in the resistivityof the earth’s layers in the vertical direction. Thecurrent and potential electrodes are maintained atthe same relative spacing and the whole spread isprogressively expanded about a fixed central point.

Consequently, readings are taken as the currentreaches progressively greater depths. Thetechnique is extensively used in geotechnicalsurveys to determine overburden thickness and alsoin hydrogeology to define horizontal zones of

porous strata.

Generally, the spacing between adjacent sounding or profilemeasurements is determined by the desired degree of lateralresolution. For reconnaissance measurements that do notrequire extensive detailed mapping of the layers of interest,widely spaced measurements are used. For detailedsurveys more closely spaced measurements are required.

5.1 Electrode Array Geometry

Resistivity measurements can be made with anycombination of current and potential electrodes desired.

3

IS 15736:2007

Several standard electrode geometries have beendeveloped for various applications. For engineering,environmental and ground-water studies, the Wenner,Schiumberg,er and dipole-dipole array are the mostcommonly used as shown in Fig. 2. Schlumbergerconfiguration is widely used for vertical exploration ofsubsurface strata, while Wenner configuration is widelyused for lateral exploration. In engineering survey,

dipole-dipole arrangement has found wide usage.Generally, the electrodes are arranged in a line, butdepcndingupon the array, somedivergence isacceptable.

5.1,1 Wenner Arrqv

This arrangement uses four electrodes equally spacedalong a straight line. It is the simplest and the most

symmetrical arrangement. It is designed to measure thepotential difference (Al~ between Mand Nas shown inFig.2. The formula forcalculating apparent resistivityfrom a Wenner measurement is:

()AVp, =27ra ~ . (2)

where ‘a’ is the spacing between adjacent electrodes, A V

is the potential drop and I the applied current.

5.1.1.1 Advanfage.s

a) The Wenner array provides a higher signal to noiseratio than other arrays because its potential

electrodes are always farther apart and locatedbetween the current electrodes. As a result, the

A M N B

WENNER SPREAD

1: 2L ~

21

A g N B

SCHLUMBERGER SPREAD

a——‘- na

vc, c~ PI P2

DIPOLE-DIPOLE SPREAD

CQ

naa

PI P2 c1

POLE-DIPOLE SPREAD

FIG. 2 Sr,mmwm EI.~CTRODEGEOMETRIES

4

b)

c)

5.1.1..

Wenner array measures a larger voltage for a givencurrent than that measured with other arrays.

This array is suitable for high-noise environmentssuch as urban areas.

This array requires lesser current for a given depthcapability. This translates into less severeinstrumentation requirements for a given depthcapability.

2 Disadvantages

The disadvantage of Wenner configuration is that all fourelectrodes need to be moved between successive readings.

5.1.2 Schhmberger Array

This arrangement is symmetric, collinear and uses fourelectrodes. In this arrangement, the current electrodes are

denoted by A and B while the potential electrodes aredenoted by M and N. The interval between Ll and N is

denoted by 21, while the interval between A and B denoted

by 2L as shown in Fig. 2. For this array the currentelectrodes are placed much farther apart than the potentialelectrodes (AB z 5 MN). The formula for calculating

apparent resistivity is:

.TL2 A V——~’= 21 I

.,,. (3)

where,4 V is the potential drop and 1 the applied current.

In depth probing the potential electrodes remain fixed whilethe current electrode spacing is expanded symmetrically

about the centre of the spread. For larger values of L itmay be necessary to increase 1also, in order to maintain ameasurable potential as shown in Fig. 2.

5.1.2.1 Advantages.

a)

b)

5.1.3

This array is less susceptible to contact problemsand the influence of nearby geologic conditions

that may affect readings. The method also providesmeans to recognize the effects of lateral variations

and to partially apply corrections for them.

This method is faster to use in the field as only

current electrodes are moved after each reading.

Dipole-Dipole Arrq

In this array the current electrodes are planted on one sideof the array and the potential electrodes on the other side.

There is always the same distance between the two current

electrodes in the current dipole and the potential electrodesin the potential dip~le as shown in the Fig. 2. The formulafor calculating apparent resistivity is:

AV

()(4)p = zn (H+l)(n+2)a ~3

1S 15736:2007 3

5.1.3.1 Advantages

a)

b)

c)

5.1.4

Requires relatively short cable lengths to explorelarge depths.

Short cable lengths reduce current leakage.

More detailed information on the direction of dip ofelectrical horizons is obtainable.

Pole-Dipole Array

The pole-dipole array is an asymmetrical array and hasthree collinear electrodes: one current electrode on oneend and two potential electrodes on other. The potentialelectrodes are separated by a distance ‘a’ and the secondcurrent electrode is placed at infinity, The distance betweenthe current and the near potential electrode is na, where‘n’ doesn’t have to be an integer (though it commonly is).The total length of the array is (n+l )a excluding the current

electrode at infinity. The formula for calculating apparentresistivity is:

AV

()(5)p =277a(n+l)n —a

1

The geometry of this array is as shown in Fig. 2.Pole-dipole sounding data is plotted as apparent resistivityversus spacing 6n’.

5.1.4.1 Advantages

This array is useful in areas where proper ground stretchis not available. It also has relatively good horizontalcoverage and high signal strength in comparison to dipole-

dipole array.

Evaluation ofresistivity arrays are given in Table 2.

5.2 2-D Resistivity Imaging Surveys

The most severe limitation of resistivity sounding method

is that horizontal (or lateral) changes in the subsurfaceresistivity are commonly found. Lateral changes in the

subsurface resistivity will cause changes in the apparentresistivity values that might be misinterpreted as changeswith depth in the subsurface resistivity. In many

engineering and environmental studies, the subsurfacegeo]ogy is very complex where the resistivity can changerapidly over short distances. The resistivity soundingmethod might not be sufficiently accurate for suchsituations. A more accurate model of the subsurface is atwo-dimensional (2-D) model, where the resistivity changesin the vertical direction as well as in the horizontal direction

along the survey line. In many geological situations, 2-D

electrical imaging surveys can give useful results that arecomplementary to the information obtained by other

geophysical method. 2-D imaging surveys involve about100 to 1000 measurements. 2-D electrical imaging surveysare usually carried out using a large number of electrodes,

25 or more, connected to a multi-core cable. A laptopmicrocomputer together with an electronic switching unit

5

2—120 Bls/ND/07-,

G

Table 2 Resistivity Array Evaluation‘A

2

(Ck7tfse 5.1.4. 1)m. .

zo.s

SI Array SIN Ehf Lateral Resolution Resolution Sensitivity Sensitivity Sensitivity Sensitivity Sensitivity Shielding by -

No. Ratio Coupling Location of Steeply of Horizontal to Depth to Dip to Surface to Surface to Bedrock Uniform

Dipping Layers Inhomo- Inhomo- Topography Conductive

Structures geneous geneous Overburden

Sounding Profiling

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

0 Gradient 3 3 1 1 5 5 1 3* 5 5 1

ii) Dipole-dipole 5 1 2 4 2 2 4 4 2 1 i

iii) Pole-dipole 4 2 3 5 2 3 4 3 1 2 1

iv) Schlumberger 2 4 4 2 1 1 2 1 3 3* 1

v) Wenner 1 5 5 3* 1 1 2 2 3 3* 1m

Code :1 = Best, 2 = Second best, 3 = Third best, 4 = Fourth best, 5 = Worst, (3*) = Uncertainty m estimate

. .

IS 15736:2007

is used to automatically select the relevant four electrodearray for each measurement. Atpresent, field techniquesand equipment to carry out 2-D resistivity surveys arefairly well developed. The electrode layout of 2-D imaging

survey is as shown in Fig. 3A.

To plot the data from a 2-D imaging survey, thepseudosection contouring method is normally used. Inthis case, the horizontal location of the point is placed atthe mid-point of the set of electrodes used to makemeasurement. The vertical location of the plotting point isplaced at a distance which is proportional to the separationbetween electrodes. The pseudosection gives a very

approximate picture of the true subsurface resistivitydistribution. Further this pseudosection is inverted usingavailable standard computer programmed of resistivity

imaging. After inversion, a 2-D image of subsurface trueresistivity is obtained and can be interpreted in terms of

subsurface geology.

5,3 3-P Rwistivity Imaging Surveys

Since all geological structures are three dimensional in

nature, a 3-D resistivity survey using 3-D interpretationmodel gives the most accurate result. With the developmentof multichannel resistivity meters which enables therecording of more than one measurement at a time and theavailability of sophisticated fast computers, the inversionof very large data sets comprising more than 8000 data

points and survey grid of greater than 30 m x 30 m isenabled.

The pole-pole, pole-dipole and dipole-dipole arrays arefrequently used for 3-D surveys because other arrays havepoor data coverage near the edge of the survey grid. The

electrode layout of3-D resistivity imaging is as shown in

Fig. 3B.

6 EQUIPMENT

6.1 The necessary components for making resistivity

measurements include a power source, meters formeasuring current and voltage, electrodes, cable and reels.The power may be either dc or low frequency ac. If dc isused, a set of batteries may be connected in series to give

an output of several hundreds volts. However, due to thelimited current capacity and short life, battery sources have

little advantages except portability. For large-scale work,it is preferable to use a motor-generator having a capacity

of several hundred watts.

6.2 Most modern resistivity meters employ low-frequencyalternating current rather than direct current for two mainreasons. Firstly, if direct current were employed there wouldeventually be a build up of anions around the negativeelectrode and cations around the positive electrode, thatis, electrolytic polarization would occur, and would inhibitthe arrival of further ions at the electrodes. Periodic

reversal of the current prevents such an accumulation ofions and overcomes electrolytic polarization. Secondly,the use of alternating current overcomes the effects oftelluric currents, which are natural electric currents in theground that flow parallel to the Earth’s surface and causeregional potential gradients. The use of alternating currentnullifies their effects, since, at each current reversal thetelluric currents alternately increase or decrease themeasured potential difference by equal amounts. Summingthe results over several cycles removes telluric effects.The frequency of the alternating current used in resistivitysurveying depends upon the required depth of penetration.For penetration of the order of 10 m, a frequency of 100 Hzis suitable. This may be decreased to less than 10 Hz fordepths of investigation of about 100 m. For very deepground penetration, direct current must be used.

6.3 With ac power sources the electrodes may be steel,

aluminum or brass; stainless steel is probably best forcombined strength and resistance to corrosion. If dc poweris used, the potential electrodes, should be porous pots.

Metal electrodes should be at least one foot long so thatthey can be driven several inches into the ground for goodelectrical contact. In very dry ground this contact maybeimproved by watering the electrodes.

6.4 One new development is the multi-core cable in whicha large number of electrodes, 25 or more are connected.A laptop microcomputer together with an electronicswitching unit is used to automatically select the relevantfour electrodes for each measurement.

6.5 Wire, usually wound on portable reels, must be

insulated and should be as light as possible. Plastic

insulation may be more durable than rubber against abrasionand moisture.

7 PLANNING THE SURVEY

7.1 Planning and design of a resistivity survey should be

done with due consideration to the objectives of the surveyand the characteristics of the site. These factors determine

the survey design, the equipment used, the level of effort,the interpretation method selected, and the budgetnecessary to achieve the desired results. Importantconsiderations include site geology, depth ofinvestigation, and the topography. The presence of

noise-generating activities and operational constraintsshould also be considered. It is good practice to obtain as

much relevant information as possible about the site priorto designing a survey and mobilizing to the field.

Before conducting electrical sounding in an area, it is usefulto study the local geology, well sections, depth to watertable, quality of water, yield of water, etc.

The place for conducting a sounding should be carefully

7

IMI ,

1S 15736:2007

selected keeping in view the requirement of plain formation. In hard rock regions, electrode separation

topography for maximum current electrode separation. The should be parallel to strike direction ofjoints and fi-actures.field survey should guard against proximity to houses, This minimize errors caused by random separation.

rivers, ponds, disturbing metallic features like power lines,buried pipes and other objects.

The current electrode separation is chosen in a mannerthat when plotted on a log-log graph, the distance between

In areas of complex geology, where formations dip,neighboring points are approximately equal. This is

electrode separation should always be parallel to strike ofachieved by increasing the current electrode separationby afactorof2 or 1.5.

SPACING 3aI 1c1 P, P2 c~ RESISTMTY

I 3a I 3a I 3a IMETER

SPACING 2aI 1

c1 PI P2 c~LAPToPCOMPUTER

I 2a I 2a I 2a I

SPACINGlaI 1

c, P, P~ C*I al ala I I I I I I I I I I I 1 I I I I

n=loo. .o. ..* O@ O** ● 0.

n=200*O0 ● 00000 ● . .

n=3000090 . . . . .

n=4000000 ● 0

n=50000.

n.6 . .

3A Electrode Layout for 2-D Imaging Survey

RESISTIVITYMETER

1

[LAPTOPCOMPUTER

● ELECTRODE 1 ●

3B Electrode Layout for 3-D Imaging Survey

FIG. 3 ELECTRODELAYOUT FORIMAGINGSURVEY

8

I

7.2 Limitation

Resistivity surveying is an efficient method for delineatingshallow layered sequences or vertical discontinuitiesinvolving changes of resistivity. The method has however,

the following limitations:

Interpretations can be ambiguous. Consequently,independent geophysical and geological controlsare necessary to discriminate between validalternative inte~retations of resistivity data.

Interpretation is limited to simple structuralconfigurations. Deviations from these simplesituations could be difficult to interpret.

Topography and the effects of near-surfaceresistivity variations can mask the effects of deepervariations.

The depth of penetration is limited by the maximum

1S 15736:2007

electrical power that can be introduced into theground and by the practical difficulties of laying

long lengths of cable. The practical depth limit formost surveys is about 1 km.

8 INTERPRETATION

8.1 The interpretation of electrical profiling data is mainlyqtialitative in nature and is useful only for decipheringareas of different resistivities. The anomalies obtained inthe profiling data are interpreted in terms of possiblegeological structures corresponding to the set ofgeological conditions.

8.2 Generally, the profiling data for constant electrodeseparation may be presented as graphs showing resistivityvariation along a traverse as shown in Fig. 4 or as a map

showing isoresistivity contours. Such a map is an

350

350

350

3500 20 40 60

DISTANCE,m

C2 P2 PI c1<) n n n

SURFACESOILSAND

FIG. 4 OBSERVEDRESISTIVITYP~OHLEACROSS RESISTIVE,WENNERARRAY

9

!S 15736:2007

expression of the lateral resistivity variation of the ground~or the arbitrary depth range corresponding to the chosencurrent electrode separation. The value of apparentresistivity, for a given lithology, will depend on the locationand the local geologic setting. The isoresistivity contourmaps are interpreted to locate the zones of low and highresistivity as related to local geological structures. In anumber of cases, the interpretation is confined to locatingtwo dimensional structures such as dykes, faults andcontact zones, provided the spread is run across the strikeof the formations with suitable station interval.

8.3 Sometimes with a view to investigate the lateralinhomogenities in the subsurface up to different depths,

electrical’ profiling is conducted using two or moreelectrode separations. In this case, the fieldwork is suitablymodified to enable the measurements of apparent resistivityalong the stations on the profile, with two or more currentelectrode separations. Such profiling is known as doubledistance or multidistance profiling and is useful not onlyto locate lateral inhomogeneties existing at different depths,

but also in separating useful anomalies from the noise.

%4 Sounding data are normally interpreted quantitativelyas a series of horizontal layers. Sounding data are plottedon log-log paper with apparent resistivities plotted versusAB/2 for Schlumberger soundings as shown in Fig. 5 or

versus AB13 (’a’) for Wenner soundings. In the past,graphical techniques such as curve matching and a related

technique called the auxiliary point method, were used tointerpret the data. The shape of the curve of apparentresistivity versus electrode spacing depends on the

resistivity contrast between the two layers and a family ofcharacteristic curves calculated for different resistivityratios p,/p,. The resistivity contrast is expressed by kfactor defined as:

~=(P2– P,)/(P2 + P,) ... (6)

The k factor ranges between -1 and +1 as the resistivityratio p2/p, varies between Oand co.

Four types of curves normally exist, namely A type

(p, <p, <p, ), Htype(pl >p, <p, ), Ktype(p, <p2>p, )andQ type(p,’>p, > P3) as shown in Fig. 6. Curve matchingis limited to models of three to five layers, whereas partial

curve matching technique, using the auxi Iiary pointmethod, allows the interpreter to model five to eight layersor more, depending on proficiency. It should be noted thata lot of time and practice is needed to attain proficiency inusing these techniques.

8.5 With the advent of high-speed digital computers,

graphical methods are no longer necessary. Sounding

curves can now be computed using convolution.

Computerized interpretation techniques vary from thosefor which the user must make reasonable estimates of thenumber of layers, their thicknesses and resistivties.

The horizontal layer model derived from the interpretationand its corresponding theoretical sounding curve is as

shown in Fig. 7. The theoretical sounding curve is plottedso that discrepancies between it and the field data can bedepicted. Large discrepancies may indicate:

10000 r

~ 1000.x T

; FIELD

F CURVE~g 100.(Y

10‘1 10 100 1000 1000O

ELECTRODE SPACING (AW2) IN METRES

FIG. 5 TYPICAL VERTICALELECTRICALSOUNOINGCURVE

10

IS 15736:2007

A Typep,<p2cp~

/“

AB/2

K Typep,<p2>p~

i-l Typei%>p2<p3

AB/2

Q Typep,>p2>p3

\

AB/2 AEI12

FIG.6 ELECTRICALSOUNDINGCURVESFORA, H, K ANDQ TYPE

10000,_—————

—

1000———

~LAYERING

OBSERVED

CALCULATED

I

q(J--L-l.llHlll I I 1111111 1 I 111111 1 I 111111

1 10 100 1000 10000

ELECTRODE SPACING (AW2) IN METRES

FIG.7 OBSERVEDANDImERPRETEDSOUNDINGCURVE

11

IS 15736:2007 Z-,.. .“

a) Lateral features not amenable to interpretation ashorizontal layers,

b) Cultural effects fi-om fences, pipelines, or othersources,

c) Equipment problems including current leakage, and

d) Operator errors.

Sotnetimes, lateral variations can generate sounding curveswhich are erroneously interpreted as horizontal layers; for

example, a sounding expanded parallel to a nearby faultcan produce a sounding curve which can be interpretedas three-layer model. In practice, the geophysicist should

use all available geological input to make the interpretation,as close as possible to reality.

8.6 Inversion of Resistivity Data

Inversion involves iterative procedures that would be very

efficient with a fast computer. The method assumes theequation for the theoretical response of a multi-layeredground. Each layer is characterized by its thickness and

resistivity, both of which should be determined. A first

estimate of these parameters is made for each layer and

the predicted curve of apparent resistivity versus electrodespacing is computed. The discrepancies between the

observed and theoretical apparent resistivity curves aredetermined at each measured point. The layer parametersused in the governing equations are then adjusted, andthe calculation is repeated with the adjusted values. Thisgives a new predicted apparent resistivity curve which

compared with the tield data. Using modern computersthe procedure can be repeated rapidly until thediscrepancies are less than a pre-determined value. Variousinversion routines are available for resistivity dataprocessing such as least square method, smoothnessconstraint least square method, Gauss-Newton method,Quasi-Newton Method, Robust model inversion, etc. Thechoice of the inversion method is governed by the natureof the known geology of the survey area.

The response of a 1-D vertically layered structure has

analytical solutions.

The response of 2-D/3-D structures is approximated bycomplex numerical solutions based on finite difference orfinite element techniques. The inversion program dividesthe subsurface into a number of small rectangular blocks(2-D case) / Prisms (3-D case), and determines the resistivity

values of the blocks/prisms, so as to minimize thedifference between calculated and observed apparentresistivity values.

9 PRESENTATION OF DATA

The results of a series of profile measurements arepresented as a profile or contour map as shown in Fig. 4.Sounding data are often presented as single geoelectricsection as shown in Fig. 8. An interpreted geoelectricsection showing layer thickness, depths and resistivity isconstructed. Geoelectric cross-sections can be helpful indetermining the depth and lateral extent of layers. Dipole-dipole data are generally presented as resistivity

S9 S8 S7 S5 S4 S3 S2 S1

I 1 I I I 1 I 145 40 ~ 5~5~ 7~ 30 —45

20 LANDFILL20 ohm-m 20 17 20

1Om –9 9

E 90 CONTAMINATED SANDSTONE

$ 20m

F \.

9 10 ‘9 7 7

1-& 200Ltl0 30m 200

200 200 ohm-m 200

SHERWOOD SANDSTONE

40mo 100 200

200

I I J

HORIZONTAL SCALE

FIG.8 G~OtZLIXTRICALSECTION

12

Is 15736:2007

pseudosections, although they can also be presented as information available with dipole-dipole data, twoindividual profiles. The data can be interpreted in a dimensional modeling is required. The pseudosection forqualitative fashion when only the presence or absence of a conductive rectangular body buried in a more resistivean anomaly in a general area is required. In order to fully halfspace is as shown in Fig. 9. This model generates autilize the combination of vertical and horizontal simple apparent resistivity pattern.

DIPOLE-DIPOLE APPARENT RESISTIWTY’ PSEUDO-SECTKN4PROFILE LINE IS INCLINED AT 90.0 DEGREES TO STRIKE

-11 -10 -9 -3 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 91011

I I I 1 I I I I I I I I I I I 1 I 1 I I 1

0000 0000000 00

0000

0000000

0000000

0000 000000

00

P

000

6

0

0

0

0000000000000

000000

2-D RESISTIVI’W MODEL

.11 M1O -9 -8 -7-6-5-4-3-2-101

000000

00

23456789 10 11

I I I I I I I I I I I I 1 I 1 1 1 1 I I o

—

[,

1

— 2

A BIANK = 100 3

A=l4

A ‘- 5

6

FIG.9 PSEUDOFORA CONDUCTIW REC~ANGULARE30DYBURIED INA MORE RESISTIVEHALFSPACE

3—120 BISIND157.

13

GMGIPN—120 BR/ND/07-300

. .

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Amend No, Date of Issue Text Affected

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