IS 15897 2011

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IS 15897 (2011): Surface Geophysical Surveys for HydroGeological studies [WRD 3: Ground Water and RelatedInvestigations]

BIS 2011

B U R E A U O F I N D I A N S T A N D A R D SMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

December 2011 Price Group 12

IS 15897 : 2011

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Indian Standard

SURFACE GEOPHYSICAL SURVEYS FORHYDRO GEOLOGICAL STUDIES

ICS 07.060

Ground Water and Related Investigations Sectional Committee, WRD 03

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the GroundWater and Related Investigations Sectional Committee had been approved by the Water Resources DivisionCouncil.

Groundwater is available almost everywhere. However, its distribution is not uniform due to varyinghydrogeological, topographical and climatic conditions. As a result, groundwater is not always available in therequired quantity and/or quality, particularly in hard rock terrains where the fractures and weathered parts are theonly conduits for groundwater. Therefore, collection of information on prospective groundwater zones, thoughbeing a costly affair, is an essential prerequisite. Surface geophysical methods are currently recognized as costeffective techniques that are useful for collecting this kind of information. Measuring physical properties of theearth and their variation, and then associating finally them with hydrogeological characteristics is the overalldomain of groundwater geophysics.

Of the various geophysical techniques available today, the electrical resistivity method is probably most commonlyused due to its relatively simple and economical field operation, its effective response to ground water conditions,and the relative ease with which interpretations can be made. This type of survey is occasionally supplementedby other techniques such as induced polarization, spontaneous potential, and Mise a la Masse galvanic electricaltechniques. Other geophysical methods in order of preference used for hydrogeological purpose areelectromagnetic, refraction seismic, magnetic, gravity and seismic reflection surveys. More recently developedgeophysical techniques include ground probing radar, electrokinetic sounding, and nuclear magnetic resonance,but these methods are not in widespread use and are not considered further in this report.

Because surface geophysical surveys are carried out at the surface of the earth, the responses received fromdifferent depths often lack unique characteristics. That is, ambiguity exists in interpreted results and the effectiveapplication of these methods often depends on the skill and experience of the investigator, knowledge of thehydrogeological conditions, and the usefulness (and limitations) of the technique(s) themselves. The applicationof two or more geophysical techniques may also be a useful approach to use in some field surveys. Integration ofinformation received from other scientific surveys, such as remote sensing, hydrogeologic characterization,chemical analysis of well water samples, etc, is also useful for interpreting the filed data.

Modern geophysical techniques are highly advanced in terms of instrumentation, field data acquisition, andinterpretation. Field data are digitized to enhance the signal-to-noise ratio, and computers are used to moreaccurately analyze and interpret the data. However, the present-day potential of geophysical techniques hasprobably not been fully realized, not only because such surveys can be expensive, but also because of the inadequateunderstanding of the application of relevant techniques in contrasting hydrogeological conditions.

It has been assumed in the formulation of this standard that the execution of its provisions is entrusted toappropriately qualified and experienced people, for whose guidance it has been prepared.

In reporting the results of a test or analysis made in accordance with this standard, if the final value observed orcalculated, is to be rounded off, it shall be done in accordance with IS 2 : 1960 Rules for rounding off numericalvalues (revised).

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IS 15897 : 2011

Indian Standard

SURFACE GEOPHYSICAL SURVEYS FORHYDRO GEOLOGICAL STUDIES

1 SCOPE

The application of surface geophysical methods is anevolving science that can address a variety of objectivesin groundwater investigations. However, because thesuccessful application of geophysical methods dependson the available technology, logistics, and expertise ofthe investigator, there can be no single set of fieldprocedures or approaches prescribed for all cases.Accordingly, this standard described guidelines thatshould be useful for conducting geophysical surveysfor a variety of objectives (including environmentalaspects), within the limits of modern-dayinstrumentation and interpretive techniques. The morecommonly used field techniques and practices aredescribed, with an emphasis on electrical resistivity,electromagnetic, and seismic refraction techniques asthese are widely used in groundwater exploration.Theoretical aspects and details of interpretationalprocedures are referred to only in a general way.

2 REFERENCES

The following standards contain provisions whichthrough reference in this text, constitute provisions ofthis standard. At the time of publication, the editionsindicated were valid. All standards are subject torevision, and parties to agreements based on thisstandard are encouraged to investigate the possibilityof applying the most recent editions of the standardsindicated below:

IS No. Title

15681 : 2006 Geological exploration bygeophysical method (seismicvibration) Code of practice

15736 : 2006 Geological exploration bygeophysical method (electricalresistivity) Code of practice

3 TERMINOLOGY

For the purposes of this standard, the following termsand definitions shall apply.

3.1 Acoustic Impedance Product of seismicvelocity and density of a layer. Reflection of seismicwave depends on contrast in acoustic impedance.

3.2 Anisotropy Variation in physical property withdirection of measurement is anisotropy. In electricalresistivity method micro-, macro- and pseudo-anisotropy are involved. Anisotropy of a geoelectrical

layer is given as = t L/ where t and L aretransverse and longitudinal resistivities of a layer.

3.3 Apparent Resistivity Ratio of measured voltageto input current multiplied by geometric factor ofelectrode configuration. It would be true resistivity ifthe subsurface is homogeneous (scale of homogeneityreferred to dimension of electrode geometry).

3.4 Aquifer Formation or group of formations or apart of formation that contains sufficient permeablematerial and is saturated to yield significant quantitiesof water to wells and springs.

3.5 Blind Zone Layer having seismic velocity lessthan that in the layer overlying it .

3.6 Bouguer Correction Correction made inobserved gravity data to account for the attraction(gravitational) of the rock between the datum and theplane of measurement. It is 0.041 85 h mgal, where is the density of the rock between the datum and theplane of measurement and h is the difference inelevations between the datum and the plane.

3.7 Bouguer Anomaly Anomaly obtained afterapplying latitude, terrain, and elevation (free air andBouguer) corrections to the observed gravity value andfinally subtracting it from measured value at someparticular station in the survey area.

3.8 Contact Resistance Electrical resistancedeveloped between an electrode planted in the groundand the ground material immediately surrounding it.Contact resistance is reduced by putting water at theelectrodes.

3.9 Convolution Defined as the integral of the productof the two functions after one is reversed and shifted. Indigital signal processing, frequency filtering can besimplified by convolving two functions (data with a filter)in the time domain, which is analogous to multiplyingthe data with a filter in the frequency domain.

3.10 Dar Zarrouk Parameters Longitudinal unitconductance (S) and transverse unit resistance (T) of ageoelectrical layer. These are defined as

S = h/ = (h1/1 + h2/2 + h3/3 + )T = h = ( h11 + h22 + h33 + )

where h1, h2, h3.. are thickness and 1, 2, 3. areelectrical resistivity of different subsurface layers.

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3.11 Deconvolution Process of inverse filtering tonullify the undesired effect of an earlier filter operation.

3.12 Dipole-Dipole Electrode Configuration Configuration in which the spacing between currentelectrode pair and that between potential electrode pairis considerably small in comparison to the distancebetween these two pairs. As the current and potentialelectrode pairs are moved further apart, the depth ofexploration is progressively increased. The dipole-dipole configuration can be azimuthal, equatorial,radial, parallel, axial and perpendicular. Geometricfactors are :

2r3/L l sin azimuthal, 2 r3 /L l equatorial,

r3/L l cos radial, 2r3/L l ( 3 cos2 1) parallel,

r3/L l axial and 2r3/3L l ( sin cos) perpendicular

L is length of current dipole, l is length of potentialdipole, r is distance between centres of current andpotential dipoles, is the angle between the two dipoleaxes.

3.13 Diurnal Correction Correction applied tomagnetic data to compensate for daily fluctuations ofthe geomagnetic field.

3.14 Drift Correction Quantitative adjustment toaccount for a uniform change in the reference valuewith time. Drift in gravity meters is mainly due to creepin the springs of the gravimeter. Correction to measuredvalues are made by repeating readings at 3 h to 4 h at afixed station.

3.15 Eddy Current Current induced in a conductivebody by the primary electromagnetic (EM) field. Thesecondary EM field produced by the eddy currentopposes the primary field.

3.16 Equivalence If target response is a functionof product or ratio of two parameters (say bed thicknessand resistivity), variation in the parameters keeping theratio or product constant can yield almost sameresponse and the various combination of parametersare said to be equivalent. This brings in ambiguity inparameter estimation. It is pronounced if the target isburied and relatively thin. In multi-layer geoelectricalsequence the intermediate layers show equivalenceover a range of parameters.

3.17 Free-Air Correction Correction applied togravity data to account for the fact that gravitymeasurements are made at different distances(elevations) from the center of the earth. The correctionvalue is 0.308 6 h mgal, where h is the differencebetween the elevation of the datum and the plane ofmeasurement. Free-air gravity anomaly is obtained

after applying correction for the latitude and elevation.

3.18 Geoelectrical Layer Layer havingcharacteristic of uniform electrical resistivity.

3.19 Geometric Factor Numerical value dependentupon the arrangement of electrodes which whenmultiplied by the measured voltage-to-current ratiogives the apparent resistivity.

3.20 Geophone Instrument which detects seismicenergy and converts it into electrical voltage. Relativemotion between a suspended coil and a magnet due toseismic wave generates a voltage in the coil whoseamplitude is proportional to the velocity of the excitingseismic disturbance.

3.21 Generalized Reciprocal Method It is atechnique wherein in-line seismic refraction dataconsisting of forward and reverse travel times are usedfor delineating undulated refractors at a depth. Thetravel times at two adjacent geophones are used inrefractor velocity analysis and time-depth calculations.At the optimum inter-geophone spacing, the upwardtraveling segments of the rays to each geophone emergefrom near the same point on the refractor. The depthconversion factor is relatively insensitive to dip anglesup to about 20, because both forward and reverse dataare used. As a result, depth calculations to an undulatingrefractor are particularly convenient even when theoverlying strata have velocity gradients. The GRMprovides a means of recognizing and accommodatingundetected layers, provided an optimum inter-geophone spacing value can be recovered from thetravel-time data, the refractor velocity analysis, and/or the time-depths.

3.22 Gradient Configuration A variation of theSchlumberger configuration where the currentelectrodes (AB) are kept at infinity, that is at a largeseparation and central 1/3rd space is scanned by a smallpotential dipole (MN). The geometric factor is /MN(AB/2)2 {1 x2/(AB/2)2}2 / {1+x2/(AB/2)2} where x isthe distance between the centre of the configurationand the centre of the potential dipole.

3.23 Half-Schlumberger Configuration Configuration in which one of the current electrodesis kept at infinity (large distance) and need not becollinear with the other three electrodes. It can be usedfor soundings along radial lines. The apparentresistivity is given as a = 2a2/I (V/a), where a isthe distance between the active current electrode andcenter of the potential electrode spacing, a is thepotential electrode spacing and V is the potentialdifference.

3.24 Homogeneity Characteristic of a formationwith uniform physical property or properties. It is a

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IS 15897 : 2011

function of scale of measurement in relation to theuniformity in physical property. Inhomogeneity orheterogeneity indicates non-uniformity or dissimilarityin physical property with reference to the scale ofmeasurement.

3.25 In-phase Component of a secondary EM fieldwith the same phase angle as that of the excitingprimary EM field; that is, in-phase component attainsmaxima and minima in step with the primary field.

3.26 Lee-partitioning Configuration A variationof the Wenner array where one additional electrode isplaced at the centre between the potential electrodes.Potential difference between the central electrode andeither of the two other potential electrodes is measured.The geometric factor is 4a. Identical values ofpotential difference at each separation indicate that thesampled ground is homogeneous.

3.27 Longitudinal Conductance Ratio of thethickness of a geoelectric layer to its resistivity[conventionally expressed as S = h/ (mhos)].

3.28 Magnetic Permeability Ratio of magneticinduction (flux density) in a body to the strength ofthe inducing magnetic field.

3.29 Magnetic Susceptibility Ratio of the intensityof magnetization produced in a body to the strength ofthe magnetic field.

3.30 Migration That part of processing of seismicreflection data required to plot the dipping reflectionsat their correct position.

3.31 Non-polarizing Electrode Electrode which isnot affected by electrochemical potential generatedbetween the electrode and ground material in which itis planted. A copper rod placed in a copper sulphatesolution contained in a porous ceramic pot iscommonly used as a non-polarizing electrode.

3.32 Normal Moveout The effect of variation ofshot-geophone distance on time of arrival of seismicreflection.

3.33 Off-set Wenner Configuration A modificationin Wenner configuration to remove or minimize theeffect of lateral inhomogeneities. In this configurationfive equally spaced collinear electrodes are planted inthe ground. Average is taken of consecutive normalWenner measurements taking the left four and rightfour electrodes.

3.34 Overburden That part of the host mediumwhich lies above the target and is usually of no interestin exploration, but has physical properties that affectthe measurements.

3.35 Phasor Diagram Graph obtained by plotting

in-phase and quadrature components of secondary EMfield for different frequencies of primary field. Thevalues of in-phase and quadrature components areplotted along x and y axes, respectively. Theoreticalphasor diagrams are generated for differentconductivity ratios and ratios of layer thickness totransmitter-receiver coil separation, and field data plotis matched.

3.36 Plus-Minus (Hagedoorn) Method Used tointerpret seismic refraction data. The method usesreversed refraction profiles with shots at opposite endsand the addition and subtraction of travel times forvarious locations between the shots to give the depthto the refractor and its velocity.

3.37 Polar Diagram Method of plotting resistivitysounding data. The apparent resistivity values of theradial soundings conducted at a point are plotted forvarious current electrode separations. Results can beused to infer fracture orientations.

3.38 Proton Precession Magnetometer It is alsoknown as nuclear precession magnetometer. Becauseof spin, proton has a magnetic moment. The axes ofprecession are oriented randomly. A magnetic fieldnormal to the earths magnetic field polarizes the nucleifor a short period and a voltage at precession frequencyis induced in a measuring coil which indicates the valueof earths magnetic field at the point of measurement.

3.39 Quadrature Out-of-phase or imaginarycomponent of secondary EM field, it is the componentwhich is 90 out of phase with the inducing primaryEM field. The ratio of the strengths of in-phase andquadrature components of secondary EM fieldsindicate conductivity characteristics of the target.

3.40 Reflector Interface which separates two layersof contrasting acoustic impedance giving rise toreflection.

3.41 Refractor Layer along which the refracted(head wave) wave travels at a velocity that is higherthan that in the overlying layer.

3.42 Remanent Magnetization In-situ residualmagnetization remaining in rock after removal ofinducing field.

3.43 Schlumberger Configuration Collinear four-electrode configuration of current and potentialelectrodes in which potential electrodes are kept closeto the center of the configuration. Conventionally, theseparation between potential electrode (MN) is less than1/5 of the current electrode separation (AB). Thegeometric factor is {(AB/2)2(MN/2)2}/MN.

3.44 Skin Depth Effective depth of penetration ofEM field in a medium. Skin depth is defined as the depth

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where EM field intensity reduces to about 37 percent ofits original value at the surface of the earth. It is dependentupon the conductivity and magnetic permeability of themedium, and the frequency of the EM field. Increasesin these values reduce the skin depth, as does thepresence of conductive overburden. It is expressed as

z = 500 (/f), where is resistivity of the ground andf is the frequency of the EM field generated.

3.45 Snells Law When a seismic wave is incidentat a particular angle (i) on a boundary between twomedia (having different seismic velocities v1 and v2 ,v2 > v1), the wave gets refracted at an angle (r) at theboundary according to Snells law which states thatsin i/v 1 = sin r /v2.

3.46 Stacking Process of compositing data, for thesame parameter, from various data sets for the purposeof eliminating noise.

3.47 Statics Correction applied to seismic data tonullify the effect of elevation differences encounteredalong profiles, as well as the effect of a low velocityweathered layer.

3.48 Suppressed Layer Layer lacking a responsebecause of its small thickness and/or contrast inphysical property with the surrounding environment.

3.49 Terrain Correction Correction applied tomeasured gravity data to nullify the effect of irregulartopographic relief in the immediate vicinity of thestation of measurement. Charts are used to calculatethe required correction. For local surveys in flat areas,this correction may not be required.

3.50 Transition Linear or exponential variation ofa physical property with depth.

3.51 Transverse Resistance Product of thethickness and resistivity of a geoelectrical layer.Conventionally written as T= h (ohm.m2).

3.52 Two-Electrode (Pole-Pole) Configuration Configuration in which one current and one potentialelectrode is kept at infinity (more than 10 times thedistance between active electrodes) and perpendicularto the profile along which the other two activeelectrodes are moved. The geometric factor is 2a,where a is the distance between the active electrodes.

3.53 Vibroseis Seismic survey in which a vibratoris used as a non-destructive source instead of anexplosive to generate controlled frequency seismicwaves in the ground.

3.54 Wenner Configuration Collinear four-electrode configuration of potential and currentelectrodes in which all the electrodes are equidistant,that is, the separation between potential electrodes (a)is 1/3 rd the separation between current electrodes. Thegeometric factor is 2a.

4 UNITS OF MEASUREMENT

Table 1 lists the parameters and units of measurementin common use.

5 PURPOSE OF SURFACE GEOPHYSICALSURVEYS

5.1 Surface geophysical surveys play a vital role ingroundwater exploration. Surveys can be used toconduct either shallow subsurface investigation thatmay be needed for many environmental related projectsor deeper investigations that may be required to identifyproductive aquifers. Also, surveys can be used toestimate the thickness of weathered zones, delineate

Table 1 Commonly Used Geophysical Techniques and Units of Measurement(Clause 4)

Sl No. Method Technique Physical Property Involved Unit for Parameters Measured

(1) (2) (3) (4) (5)

i) Electrical resistivity

Sounding Profiling

Resistivity Ohm-m

ii) Magnetic Mag. Susceptibility Mag. Field intensity

Gammas Nano Tesla Inphase/Quadrature Component ( percent ) do Secondary/Primary Magnetic Field (percent )

iii) Electromagnetic VLF HLEM TEM

Conductivity/Resistivity

Voltage decay, Ohm-m, Sec. Refraction Wave velocity iv) Seismic Reflection (High Resolution)

Acoustic Impedance m/s m/s

v) Induced polarization Chargeability Milli-second vi) Self potential

(Electrokinetic)

Natural Potential mV

vii) Mise-a-la-masse (Charged body)

Development of potential mV

viii) Gravity

Charged body

Density (Lateral variation) Milli-galon

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bed rock topography, demarcate fracture geometry,identify the presence of limestone cavities and/orpaleochannels, and to assess quality of groundwater.Furthermore, surveys can be used to assessgroundwater pollution and the movement of plumes,define vadose zone characteristics required for wastedisposal or artificial recharge projects, demarcate seawater intrusion, differentiate between aquifers andaquitards, monitor the quality and direction ofgroundwater movement etc. Surface geophysicalmeasurements are also used to estimate hydraulicparameters of aquifers. They are increasingly usedbecause they are rapid and cost effective and theysupplement direct methods such as drilling.

Surface geophysical methods can be grouped into twocategories natural field methods and artificial sourcemethods. Commonly used natural field methodsinclude gravity, magnetic and self-potential methodswhich measure variations in earths gravity field,magnetization and natural electric potential of rocks.Microgravity techniques, which detect changes inground water storage, can be used to identify saturatedcavernous limestone features. Artificial source methodsmeasure the response of the subsurface to artificiallyinduced energy like seismic and electromagnetic wavesand electrical currents. These methods includeelectrical resistivity (see IS 15736), inducedpolarization, very low frequency (VLF)electromagnetic, controlled-source electromagnetic,seismic refraction (see IS 15681), and occasionally,seismic reflection.

5.2 One of the well developed method is GroundPenetrating Radar (GPR) which is a high-resolutionsystem for imaging subsurface using electromagnetic(EM) waves in the frequency band of 10Hz-2000 MHz.It is used to detect the anomalous variations in thedielectric properties of the various subsurface materials.

The GPR system consists of the following:

a) A source for transmitting EM waves.b) Receiver for detecting EM waves reflected

from different subsurface features.c) Control and display unit for synchronization

between transmitter and receiver as well asrecording, processing and display of data.

5.2.1 Benefits of GPR

a) Portability

b) Application is non-destructivec) Rapid in data acquisition, andd) High-resolution subsurface imaging.

5.2.2 Applications of GPR

a) Detection of fracture zone,

b) Determination of depth to water table,

c) Location of sinkholes and cavities,

d) Detection of anomalous seepage, and

e) Mapping of archeological remnants.

5.2.3 Limitations of GPR

a) Penetration depth and ability to resolve targetsat a depth is dependent upon the prevailingunderground conditions.

b) Highly conductive soils subsurface materialrender the GPR method ineffective.

c) Sufficient electrical contrast between thetarget and the host materials is necessary.

d) Interpretation of GPR data is subjective.

6 PLANNING

Surface geophysical surveys need to be carefullyplanned in order to meet project objectives. Planningshould include the following aspects.

6.1 General Considerations

a) Effectiveness and accuracy of equipment andpower supply,

b) Easy operation and maintenance,c) Ready to use accessories,

d) Suitability of vehicle for transportation, ande) Safety of equipment.

6.2 Access to the Area

a) Suitable access to the area/site,b) Permission to work in the area,c) Physical constraints in the area,

d) Clearance along profile line(s),e) Noise and cultural disturbances, andf) Overhead power line.

6.3 Equipment

a) Maintenance should be performed asrequired;

b) Should be stored in a stable, dust free, anddry environment;

c) Pre-operation checking should be carried out;

d) Power supply should be checked regularly;e) Precautions given for each equipment are to

be observed; andf) Any deterioration in equipment condition

should be rectified immediately.

6.4 Safety and Precautions in Operation

A safety code or plan should be developed prior tosurveys to account for potential hazards in the field.

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Common hazards include working with high voltagepower lines, in electrical storms, in extremely remoteareas, and with explosives. If possible, surveys shouldbe conducted in dry weather periods to avoid damageto equipment by lightening. Unnecessary use of highvoltage input should be avoided and care should beused when working with systems of 100 V or more, orwith systems having 120 mA or more of current. Inthe event of rain or lightening, the current and potentialcable connections should be removed from theinstrument and no one should be allowed to touch theterminals. Even at a distance of 5 km to 6 km,lightening can damage the circuit.

In seismic surveys, explosives should be handled bytrained personnel stored safely. Overhead power linesshould not be located near the shot hole, which shouldbe dampened by water in the event that vibroseis orweight dropping is not used. Detonators should bealways kept short circuited, even during transportationto the site.

6.5 Planning of Survey

Field crews should be informed of operationalprocedures prior to the survey. Profile lines should bestraight and the distances between transmitter andreceiver should be accurately determined. Spacingsshould be repeatedly checked or confirmed. Otherconsiderations are itemized below:

a) Crew should not touch the electrodes or thecable until instructed to do so by the operator.

b) Movement of the crew near the profile shouldbe restricted and the cable should not bepassed through water or near high voltagepower lines. Also, the crew should not standin water in bare feet.

c) Data should be plotted at the site so that errorscan be removed or readings repeated.

d) Electrodes should not be located near lateralinhomogeneity such as boulders in rockyterrain or buried objects such as pipe lines ortelephone cables.

e) Line should be checked regularly irrespectiveof the applied voltage.

f) The charge (explosive) should not be placedin a highly-weathered zone so as not to overlydissipate the energy.

g) For shallow investigations, the depth ofweathering should be estimated by specialshooting so that charge can be placed belowthe weathered zone.

h) For EM equipment with multiple frequencyselections, frequencies should be changedonly after switching-off the instrument.

j) In magnetic surveys ferrous objects shouldnot be placed near the sensor.

6.6 Quality Control in Field Data Collection

Quality control considerations are a function of theselected equipment and the required level of accuracy.In any case, measurements should be repeated andprofile orientations should be checked.

6.7 Site/Area Details

Investigators should become familiar with the localgeologic and hydrogeologic characteristics of atargeted site prior to conducting a survey.Characteristics may include, but not be limited to,lineament details, lithostratigraphic information, water-level information, and water-quality information. Awell inventory should be conducted to identify sourcesof pertinent data and information.

Depending on the objectives of the survey, candidatesites for field surveys may be selected on the basisof existing information. Final site selection, however,should be based on a more rigorous study ofgeomorphic features and geological structures in thefield. Local representatives may be consulted to helpplan the surveys. Final site selection should be basedon geophysical anomaly positions, accessibility, localconditions, and avoiding physical constraints suchas electrical lines, metallic structures, crossing ofroads, streams, or bridges, and topographicdepressions.

7 ELECTRICAL RESISTIVITY

7.1 Purpose

To identify groundwater-yielding zones (whethergranular or fractured), zone geometry, variations in thechemical quality of groundwater, and the directions ofgroundwater movement (see IS 15736).

7.2 Principles of Measurement

A known amount of electrical current is first sent intothe ground through a pair of electrodes. The potentialsdeveloped within the ground due to this current arethen measured across another pair of electrodes on theground. The distribution of current and equipotentiallines in an electrically homogeneous subsurface isshown in Fig. 1. The potential difference, V, betweenany pair of electrodes at the ground surface, P1P2, asshown in Fig. 2, is then calculated as

1 1 1 1 1

2V

a b c d

= +

where is the electrical resistivity of thehomogeneous ground, I is the electric current with

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FIG

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FIG. 2 DIFFERENT ELECTRODE CONFIGURATION IN USE IN ELECTRICAL RESISTIVITY SURVEYS

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which the ground is energized, and a, b, c and d arethe inter-electrode distances. Usually, both the currentand potential pair of electrodes are placed in a straightline with the potential pair being placed inside thecurrent pair to maintain a symmetry with respect tothe inter-electrode distances. Two electrode arraysbeing used today are the Wenner and Schlumbergerarrays as shown in Fig. 2. In the Wenner array, theelectrodes are equally spaced while, in Schlumbergerarray, the potential electrodes are relatively close toone another as compared to the current electrodes.For the Wenner configuration of electrodes, the aboveequation becomes.

Va KR

I 2

For Schlumberger configuration, it becomes

L VMN KR

MN I

2

1

As shown in the above equations, when the resistanceR is multiplied by K (a constant called the spacing orgeometrical factor which depends upon the spacingbetween current and potential electrodes), it gives thevalue of , the resistivity of the ground. If the groundis homogeneous, the value of gives the true resistivityof the medium or the ground. However, since theearths subsurface is multilayered, the value of aprovides what is called the apparent resistivity value.Along with the electrode spacing, the apparentresistivity value is a function of the thicknesses andtrue resistivities of the individual layers, and deducingthe true resistivity value of any individual layer is adifficult proposition. In practice, as the separation ofthe current electrodes is step-wise increased, the

current penetrates and becomes more focused deeperinto the ground. A plot between the current electrodeseparation and the resultant electrical resistivity valueyields a curve known as vertical electrical soundingcurve (in short VES).

There are two ways of interpreting the VES data. Thefirst involves matching the field curve with mastercurves that have been prepared for multi-layeredsystem with different combinations of resistivity andthickness. The second method is computer aided wherethe VES curve is calculated for an initial best guessmodel of the system and then adjusted by successiveiterations to match observed curves. The matchedmodel curve is assumed representative of a subsurfacewith the same layering and resistivity as indicated inmaster curve.

In resistivity profiling, an electrode array (Wenner arrayis generally preferred) is moved in a line from one pointto another to record variations in resistivity along aprofile. The technique is helpful in locating lateralinhomogeneities owing to the presence of resistive orconductive bodies such as dykes, saline water bodies,etc. Significant resistivity contrasts occur between dryand water-saturated formations, and formations withfresh and brackish or saline water. Sands of variousgrain size, clays, weathered and fractured granites andgneisses, sandstones, cavernous limestones, vesicularbasalts, etc, all have defined but overlapping ranges ofresistivity. The resistivity ranges shown below fordifferent materials are generalized and may varysignificantly based on local hydrogeologicalconditions.

7.3 Instruments

A resistivity survey is carried out using an instrumentknown as a resistivity meter. These meters typically

1 101 102 103 104 m

Clay

Sandy clayClayey sand

Clay shale

Sand, gravelLimestone, gypsum

Sandstone

Crystalline rocksRock salt, anhydrite

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IS 15897 : 2011

employ either a direct current or a very low frequencyalternating current type of suitable wattage and mayalso be equipped with noise filters and digital displaysof current input and measured voltage. Measurementaccuracies for many resistivity meters typically fallwithin the micro-volt range. Meters with multi-selection constant voltage or constant current inputare desired. Required accessories include ruggedwinches/reels of insulated base with 200 m to 500 mof PVC insulated single conductor cable, multi-strandthin wires of low electrical resistance, a rechargeableor non-rechargeable direct-current power source,small diameter stainless steel rods/stakes andhammers, non-polarizing electrodes and connectors,hand-held walkie-talkie sets, and surveyingequipment.

7.4 Field Procedures

There are a variety of electrode configurations used inresistivity surveys. The co-linear, symmetricalquadripole spread of the Schlumberger configurationfor sounding and the Wenner configuration for profilingare the most popular. In the Schlumbergerconfiguration, the practice is to move current electrodesoutward while keeping the closely-spaced potentialelectrodes fixed at the center so long as a measurablepotential difference is obtained. When the potentialdifference becomes so small that it cannot be accuratelymeasured, the potential electrodes are expanded,always with the proviso that their separation does notexceed one-fifth that of the current electrodeseparation. Conventional sounding commences whenthe potential electrode spacing is equal to one-fifth ofthe current electrode spacing. Successive spacing ofelectrodes is usually increased in geometricprogression, with each current electrode spacing being1.414 times the preceding one. As such there shouldbe equal distribution of 6 points to 8 points in each logcycle of double log graph paper used for plotting theapparent resistivity curve. Spacing can be increasedby 2 m to 5 m to study minor changes. In Wennersounding, the potential electrode spacing is set at one-third the current electrode spacing through-out thesurvey (that is, all four electrodes are equidistant andmoved outward for successive measurements). Inhardrock areas, radial soundings (soundings taken at asite along 4 to 8 different directions) may be useful forstudying fracture orientation and for correcting depthestimates.

The Schlumberger and Wenner configurations eachhave advantages. The Schlumberger configurationrequires less manpower and cable and, becauseelectrode movement is relatively small, the effects ofnear surface lateral inhomogenities on the signal isminimized. Also, shifting of the curve with potential

electrode changes are smoothed. The Wennerconfiguration has the advantage of giving higherpotential values because the potential electrodes areequally spaced with the current electrodes.

For sounding curves, apparent resistivity values areplotted against half current electrode separation forthe Schlumberger configuration, against inter-electrode spacing for the Wenner configuration, andagainst the distance between the current and potentialdipoles for the dipole-dipole configuration. For radialsoundings polar diagrams are also prepared. Inprofiling with Wenner/Schlumberger/dipole-dipoleelectrode arrangements, the configuration (of fixedelectrode distance) is moved along a straight-lineprofile taking measurements at fixed spacings (stationintervals). In gradient profiling, current electrodes areplanted well apart, say 800 m to 1 200 m, and thecentral one-third space is scanned by a potential dipoleof 10 m to 20 m in length, at a station spacing of 5 mto 10 m. Gradient measurements can also be madealong closely spaced (50 m apart) parallel profileswithin the central one-third space without changingthe positions of the more distant current electrodes.Groundwater flow and velocity can be measured usinga rectangle configuration of potential electrodesplaced midway between the two current electrodes insuch a way that a uniform electric field exists nearthe potential electrodes.

In profiling, apparent resistivity values are plottedagainst stations on arithmetic graph paper. The centerof the potential electrode spacing is the point ofmeasurement for the Wenner and gradientconfigurations. For the dipole-dipole configuration,the point of measurement is between the current andpotential dipoles. When attempting to trace a fracturezone, because low resistive readings in a single profilemay be erroneous and misleading, profiling shouldbe taken along 2 to 3 parallel profiles located 50 m to100 m apart. Also, profiling should be preceded bytest soundings to select optimum electrode spacings.At least one profile should be conducted with a smallelectrode spacing (5 m to 10 m) to understand theeffects of near-surface resistivity variations on deeperinformation and to reduce ambiguities. In the Wennerconfiguration, the effects of near surfaceinhomogeneity can be reduced by an off-setarrangement of electrodes and by taking averages.

Selecting a site for a survey should serve its purpose.In the event a geophysical anomaly is identified at apoint which is not accessible for drilling, its extensionshould be identified by observing some parallelprofiles. If site is near a concrete structure like road,building or bridge, profile should be laid in such away that potential electrodes do not fall within 10 m

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of the structure and are on homogeneous ground. Rockdebris and building materials lying in vicinity of theelectrodes should be removed.

Locations of the current electrode positions should beidentified before starting the survey. Electrodelocations are accurately measured from the center andsmall pits/holes are made compatible to the size ofpotential electrode base and moisture condition of soil.In dry soil conditions, sufficient water should be putin pits/holes before placing the electrodes in theground. Care should be taken to ensure that theelectrodes are in proper contact with the ground. Incase electrode location falls on dry, compact soil andsand, sufficient water is put in the hole by removingthe electrode and placing again to minimize contactresistance at the electrode. For small electrodespacings, the electrodes should not be driven morethan 40 mm to 50 mm to maintain it as a pointelectrode. For large spacings, the entire length of thecurrent electrodes can be driven into the ground.

The potential values should be higher than 5 mV andin no case below 1 mV. Minor variation in potentialbrings in noise in the apparent resistivity curve. Smallpotential values are generally obtained with largeelectrode spacings for which the geometric factoris quite large and relatively small inaccuracies withthe geometric factor gives relatively large variationsin apparent resistivity.

Ideally, current circuit should not offer path resistanceother than signal resistance. Therefore, in practice itshould be ensured that cable resistance as well ascontact resistance is minimum at both of the currentelectrodes. Contact-resistance can be reduced bydriving the current electrodes deeper, and by puttingsaline water in electrode pits. If necessary, anadditional one or two electrodes could be planted nearthe current electrode, about a meter apart, andconnected in parallel to the main electrode.Alternatively, a sheet of tin foil placed in a wateredpit can be a very effective current electrode.

With the Schlumberger array, when potential electrodepositions are changed, repeat measurements shouldbe made for at least two of the earlier current electrodepositions (with new potential electrode position) foroverlapping curve segments.

It is necessary to plot data during operation, so thattrend of the curve is known and data points with noisecan be repeated and also, the spacing to terminatemeasurements could be properly chosen (for instance,when bedrock is indicated by a steeply ascendingportion of the VES curve). Accuracy of the datadepends on the sensitivity of the instrument to measurepotential differences, to filter out noise by stacking

and displaying the standard deviation of measuredvalues, and to correct measurement of electrodedistances and their alignments.

7.5 Processing of Data

Sounding curves obtained by the Schulmbergerconfiguration are generally discontinuous with upwardor downward shifting of curve segments because ofthe shifting of potential electrodes. Shifting should bein a prescribed manner if there is no lateralinhomogeneity. Sounding curves can be smoothed byshifting the curve-segments up or down, depending onthe type of curve (whether ascending or descending).Conventional shifting of the curve depends on therelative resistivities of the layer sequence. When thepotential electrode spacing is increased, the depth ofinvestigation is somewhat reduced, producing a curvethat ascends upward and not downward. Difficultiesinvolving the shifting of curve segments can beovercome by observing the trends of nearby soundings.Shifting of curve-segments could also be due to surfaceinhomogeneities near the potential electrodes.

Surface inhomogeneities near the current electrodes canalso be recognized by distortion in the sounding curve.A sharp curvature of the maximum value in the soundingcurve is not indicative of a resistive layer of regionalextent, but rather a lateral surface inhomogeneity. Curvesthat suddenly rise or fall with changes in the position ofthe current electrode indicate the presence of alithological contact. In such areas, other nearby soundingcurves can help smooth the distorted curve and identifywhich current electrode has caused the shifting.

7.6 Interpretation

Qualitative interpretation of sounding curves can bemade visually to identify the type of curve and todemarcate areas with similar types of curves (forexample ascending/descending type or H, A, K, or Qtype curves for various combinations of multi-layeredsubsurface resistivity variations (Fig. 3). Quantitativeinterpretation of resistivity sounding data is based onempirical or semi-empirical methods in which the fieldcurves are smoothed and matched with a variety of 2layer and 3 layer theoretical master curves along withthe auxiliary point charts. This graphical techniqueinvolving a sequence of partial curve-matching wheretwo or more homogeneous and isotropic (assumed)layers are combined in a single anisotropic (introduced)layer, which is equivalent to another fictitious singlehomogeneous and isotropic layer. Interpreting resultsfrom soundings made in relatively layers is difficultand to some extent depends on the skill and experienceof the interpreter, and on the availability of localgeological information.

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Development of computer-based inversion techniqueshas greatly aided investigators in interpreting results.With these techniques, parameters values for targetedlayers can be obtained from the iterative adjustmentof estimated guessed values to match the curvesobserved in the field. The equivalence and erroranalysis are done and also some of the layerparameters can be fixed (through boreholeinformation), while inverting. A number of VES froman area can be interpreted simultaneously as batchinterpretation required for a regional consistency inresults. In some of the inversion programmes, a guessmodel is not required. One such programme, whichgives results with deeper layers in increasing order ofthickness, is not very useful. In another, the curve isinverted by the process of peeling-off of layers.Here, the resistivity of the last layer is not correctlyestimated because the process involves extrapolationof the last segment of the curve. Alternatively, it isadvisable to interpret curves by forward modeling aswell as automatic inversion as the former gives scopeof incorporating geological information, while thelatter provides more highly-resolved results, as wellas an estimate of the error in final parameter values.

An empirical approach is used in interpreting soundingcurves from hard rock areas. For some of the currentelectrode spacings, input current becomesautomatically high (when a constant current source isnot used) and the curve shows a descending kink forthat spacing. Statistical analysis has shown that a linearcorrelation exists between kinks observed in the curveand the depths of saturated fractures encountered inthe borehole. The distance of the current electrodeposition for which a kink is observed is almost sameas the depth to the fracture.

Resistivity profiling data are interpreted qualitatively.From gradient profiling data, the ratio of the resistivitylow (indicating saturated fracture zone) to thebackground high is computed and calibrated with theborehole results, if available. That is, similar ratios inthe same hydrogeological environment should indicatesimilar fracture zones. Besides the ratio of a low(anomaly) to background value, actual values as wellas the steepness of the anomaly are also consideredfor an indication of the depth to the anomaly source.Quantitative interpretation should also include essentialaspect of standardization of parameters throughavailable borehole information. The interpretation ismodified with the inflow of drilling data.

7.7 Advantages

The electrical resistivity method is cost-effective andemploys non-destructive field techniques. It is effectivein assessing the quality of ground water and thereforecan be used to locate the saline/fresh ground water

interface, or saline water pockets. Resistivity contrastsassociated with presence or absence of ground watercan be used to delineate the geometry of aquifers andzones favourable for ground water accumulation. Thismethod also provides useful information on lithologiccharacterization, depth to resistive bedrock, directionof ground water flow, orientation of fracture zones,and the locations of faults and paleo-channels, as wellas cavities in limestone. The method also can be usedfor specific environmental applications such asdelineating the area and extent of ground waterpollution, identifying zones suitable for artificialground water recharge, soil salinity mapping, andreclamation of coastal saline aquifers (see IS 15736).

7.8 Disadvantages

Overlapping resistivity ranges and a very wide rangeof resistivity makes it difficult to characterize groundwater targets by their resistivities unless standardizedlocally. Also, the accuracy and resolution of theresponse decreases with increasing depth anddecreasing contrasts in resistivity. Finally, like othermethods based on potential theory, is limited in itspredictive application (see IS 15736).

7.9 Limitations

The presence of very high or very low resistivity surfacesoils can affect interpretation. While the formerincreases the contact resistance, the latter masks thesignals coming from deeper layers. These presence ofsuch soils can be problematic because they canattenuate a considerable percentage of the input signalgoing into the subsurface, as well as the output signalcoming back from deeper zones. The resistivity lowthat may result from the presence of a conductive topsoil/overburden may be mistaken for a suitable target.It is therefore essential that a profile with a very smallelectrode spacing is also conducted to identify the topsoil conductivity effect. Cable resistance and contactresistance affect the ground resistance (measuredsignal) which is generally too low.

Because the response of a resistivity profile isdependant on two parameters, that is, on the geometryand resistivity of the targeted layer, there is no uniquesolution and a number of equivalent models are found.While conducting soundings on a multi-layered earth,it is observed that the parameters of intermediate layerscould be altered to a certain extent, keeping either theratio of thickness-to-resistivity or the product ofthickness and resistivity constant. This would notproduce any appreciable/detectable change (within theaccuracy of the observation) in the shape of theresistivity sounding curves. This phenomenon is knownas equivalence, the effect of which is pronounced ifthe layers are thin. It cannot be resolved by a single

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FIG. 3 FOUR TYPES OF RESISTIVITY SOUNDING CURVES

B

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technique but requires the support of independentinformation for fixing either of the interpretedparameters or by obtaining the same parametersthrough joint interpretation with other techniques. Theresponse is dependent on the depth and resistivitycontrast of the target. Thin layers or layers with lessresistivity contrast with the surrounding aresuppressed.

In a layered sequence, the interfaces betweensuccessive layers having monotonously increasing ordecreasing orders of resistivity cannot be distinguishedaccurately, particularly at greater depths, because ofthe transitions in resistivity.

Dipping layers distort the measurements and produceambiguities. The presence of inhomogeneties, eitherat the potential or current electrodes, produces distortedor shifted curves which can be difficult to interpret.Induction of pseudo-anisotropy due to repetition ofresistive and conductive layers results in an error indepth estimation, requiring corrections by calculatingthe coefficient of anisotropy through transverseresistance and longitudinal conductance.

8 SELF POTENTIAL

8.1 Purpose

To obtain information on the direction of naturalground water movement or seepage, or the movementof ground water induced by pumping of a water well.


Measuring natural self potential is one of the oldestand simplest methods in geophysics. The naturalelectrical potential [also known as Self Potential orSpontaneous Potential (SP)] has two components,namely the electro-kinetic component and theelectrochemical component. The electro-kineticcomponent or streaming potential component of SP islinked to the flow of ground water, making its useeffective in ground water exploration. Streamingpotentials increase in the direction of groundwater flowand the gradient of anomaly is related to the magnitudeof flow. That is, a map of equal SP values would reflectthe direction and magnitude of flow. The magnitudeof streaming potential is generally low, being of theorder of millivolts.

8.3 Instrument

Because the amplitudes of the anomalies produced bythe streaming potential can be quite low, apotentiometer or resistivity meter capable of measuringin the millivolt range is required. Also, a micro-processor based stacking facility would help rejectionof noise.


An SP survey is carried out along 20 m to 50 m spacedparallel profile lines or along radial lines originatingfrom a borehole in 8 to 12 directions. Station intervalscan be kept at 2 m to 10 m, depending on the objectiveof the survey. Water-filled electrode pits areconstructed in advance of the survey so that potentialsare stabilized. If possible, inhomogeneities located nearthe potential electrode should be removed whilemaking the pits, or the pits should be constructed anadequate distance away from inhomogeneities. Whiletaking measurements, the presence of inhomogeneitiesare to be recorded. The electrodes should be firmlyplaced into the pit. If porous pot-type potentialelectrodes are used, they are usually kept connected ina tub containing a copper sulphate solution for 8 h to12 h prior to their use in order to minimize potentialdifferences due to the electrodes themselves.

There are two techniques used in SP fieldmeasurements the total field measurementtechnique and the gradient or leap-frog measurementtechnique. In the total field measurement technique,one of the potential electrodes is kept fixed as a baseor reference electrode at a site geologically suitable(that is without much variation in potential), while theother electrode is moved along the profile lines. If thereference electrode is shifted, a new reference electrodeis tied in with the previous one and measurements areoverlapped. In the gradient measurement technique,both electrodes are moved along profiles lines with afixed separation. The distance between the electrodesis kept very small. The total field measurementtechnique is preferred, as it gives large values ofpotential difference and the error associated withelectrode polarization is less in comparison to that inthe gradient configuration. All measurements shouldbe completed in the minimum time possible to avoiddrift due to polarization.

To study the direction of ground water movementinduced by pumping, measurements are usually takenaround the well before pumping and then repeated afterpumping for a reasonable duration, after switching-off the pump.


In the total field measurement technique, the data arereduced to a common point and corrected for drift.Polarization effects are reduced by linearlyinterpolating it between the measurements. Correcteddata can be plotted as profiles or as iso-potentialcontour maps.

8.6 Interpretation

Interpretation of SP data can be difficult because a

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number of factors, including noise, may distort thestreaming potential anomaly. Also, the order ofmagnitude of the noise may be same as that of theanomaly ( 5 mV to 10 mV). Qualitative interpretationis done by studying the amplitude and wavelength ofthe anomaly and then matching the anomaly withpatterns for known source geometries. Smoothanomalies with longer wavelengths indicate a deepersource mechanism. Shorter wavelengths and higheramplitudes indicate a shallower source. Verticallithologic contacts or structural discontinuities givesteep, asymmetrical anomalies with an amplitudedependent on the resistivity ratio. Streaming potentialsare reduced with increasing clay content. Interpretationof SP profile data is often facilitated by comparisonwith geoelectrical and/or geological sections andtopographic profiles.

8.7 Advantages

Self potential is a relatively inexpensive method forobtaining useful information on the lateral as well asthe vertical movement of ground water flow orseepage. In areas of polluted groundwater, flowinduced by pumping can be traced to plan preventivemeasures. The method is also useful for locatingshallow water-filled cavities in limestones withappreciable ground water flow.

8.8 Disadvantages

Interpretation of results may be difficult because SPanomalies are often complex and of low amplitude.

8.9 Limitations

In long profile lines, SP anomalies are affected bytelluric current variations which may be of much higherorder. Also, SP anomalies are affected by magneticstorms and may yield erroneous anomalies on slopingground. The presence of near surface inhomogeneities,conductive overburden, variations in soil moisture,electrochemical effects, conductive/ resistive bodies inthe subsurface, overhead power lines, and corrodedpipe lines will obscure the anomalies due to thestreaming potential. Measurements are also affectedby the location of the reference electrode and thewatering of electrodes during measurement.

9 FREQUENCY DOMAIN ELECTROMAGNETIC(HORIZONTAL LOOP)

9.1 Purpose

To delineate saturated fracture zones in hard rocks andto estimate the thickness of weathered zones.


In conventional electromagnetic (EM) surveys a

transmitter radiates electromagnetic waves (primaryfield) that penetrate the ground. When the primary fieldencounters a conductor, that is, a body of limitedextensions with an electrical conductivity higher thanits surroundings, eddy currents are produced in theconductor. A secondary electromagnetic field (in adirection opposite to the primary at the conductor) isproduced by the eddy currents and the resultant fieldis measured by a receiver, placed at a given distance,in the form of in-phase and quadrature components[see Fig. 4A]. The receiver also measures the primaryfield. The resultant field is either measured as apercentage of the primary field or its direction relativeto the vertical is recorded. The magnitude, directionand phase angle (which is the time delay of the resultantfield in relation to the primary field) of the resultantfield can be used to locate a conductive body and obtainits parameters. There are several ways to conduct EMsurveys by varying the position and orientation ofreceiver and transmitter loops, namely, vertical loop,horizontal loop, Turam, etc [see Fig. 4B]. Overall, inEM exploration it is generally assumed that there existsa conductivity variation in the subsurface and that theconductive target is located within a non-conducting(resistive) surrounding, or that the conductivity of thetarget is much higher than the surrounds.

The EM method has advantages over the resistivitymethod in that the latter has difficulties in sending acurrent through a highly resistive surface layer, suchas those often found in deserts or in compact rockyterrains. Also, the change in penetration depth can beobtained by changing the frequency of the transmittedelectromagnetic wave, as well as the transmitter-receiver coil separation. Because anomalies ongroundwater targets in hard rocks are caused byconductivity contrasts between the saturated zone andthe surrounding dry medium, a higher contrast wouldprovide a better response. The electromagnetic methodhas been used widely in groundwater exploration,occasionally to compliment the resistivity method andhelp resolve ambiguities in interpretation.

A commonly used technique is the Horizontal LoopElectromagnetic (HLEM) method, also known as theSlingram method. HLEM surveys are controlled-sourcesurveys in which the transmitter can be operated at anumber of frequencies and transmitter-receiver coilseparations. The transmitter and receiver coils areplaced in the same horizontal plane. HLEM profilingwith a number of frequencies and transmitter-receiverseparations gives a depth-wise distribution of electricalconductivity. That is, a reduction in the frequency ofEM waves and/or an increase in the transmitter-receiverseparation would provide deeper information. As inresistivity surveys, a conductive overburden of varyingthickness can create a problem in quantitative

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interpretation and in detecting the target. The primary(incoming) field suffers attenuation etc and the depthto the target is sometimes overestimated as in the caseof the electrical resistivity method.

In-phase and quadrature components of resultantmagnetic field expressed as percentage of primarymagnetic field are measured [see Fig. 5A, 5B].Resultant field is a function of conductivity, frequencyand coil separation. Hence, measured values dependon the response parameter = L2, where ismagnetic permeability, is ground conductivity, is angular frequency and L is transmitter receiver coil

separation. In electromagnetic surveys the termconductivity is preferred as response is generallyproportional to conductivity.

9.3 Instrument

The instrument is comprised of a transmitter, a receiver,and the console. The transmitter can be operated at anumber of high and low frequencies, usually in therange of 100 Hz to 10 000 Hz. The instrument shouldhave repeatability of readings. Teflon-coated cablesof 50 m, 100 m and 200 m lengths are often used forconnecting the transmitter to the receiver.

4A Primary and Secondary EM Field (Horizontal Coils)

FIG. 4 ELECTROMAGNETIC (EM) SURVEYS

4B Vertical Coil (Horizontal Dipole) and Horizontal Coil (Vertical Dipole) Configurations

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HLEM surveys are usually conducted as profiling incombination with electrical resistivity surveys. Evena few resistivity soundings could be conducted in astudy area to define the geoelectrical layering priorto detailed HLEM profiling.

Profile lines are laid across the probable strike of thetarget conductor. Profile lengths are kept much longerthan the expected lateral extent (geometry) of theanomaly.

Station intervals are kept 10 m to 25 m apart,depending on the objective of the investigation andlikely target dimensions. Spacings between thetransmitter and the receiver should be accuratelymeasured. For a spacing of 100 m, a maximum errorof 200 mm is permissible.

Transmitter and receiver coils should be placed onthe ground horizontally and properly oriented towardsthe receiver (that is, in the same plane), correctedthrough inclinometer or tiltmeter. The receiver andtransmitter should be properly aligned. To measurethe response, the transmitter and receiver coils aremoved in unison to successive stations, keeping inter-coil spacing fixed. In-phase and quadraturecomponents of the resultant field can then be measuredat available frequencies. Changes in frequency areindicative of various depths of penetration. Thereforeeach station with in-phase and quadrature datameasured at 4 frequencies to 8 frequencies representsan EM depth sounding. Measured values representthe information obtained from the center of thetransmitter-receiver coil separation. Profiling may beconducted at two or more separations of receiver-transmitter coils.


Data can be plotted for inphase and quadraturecomponents together or separately for differentfrequencies. Noise in the data can be eliminated byvisual inspection. Phasor diagrams can be preparedto estimate the layer parameters.

9.6 Interpretations

Interpretation of the target anomaly can be donequalitatively as well as quantitatively. The width ofthe anomaly is equal to sum of the thickness of theconductor and the coil separation. Quantitativeinterpretation includes curve matching with availableor generated theoretical models that have beenpreviously developed for various subsurfaceconductivity distributions, depth-to-thickness ratios,and conductor altitudes.

The presence of conductive overburden increases theamplitude of the anomaly. At higher frequencies, thequadrature component response produces a base levelshift and may reverse or become negative. Theconductor appears to be buried deeper and moreconductive.

Sounding data can be presented as a phasor diagramand interpreted with available sets of such diagramsthat have been prepared for various layered earthmodels. The presence of a conductive surface layersrotates the phasor diagram clockwise.

Using an initial guess model and certain assumptions,the sounding data can be inverted by software to givelayer models at each point. Interpretation becomesmore useful if some borehole information is availableto identify the character of geologic structuresproducing the response.

9.7 Advantages

EM field operations are fast and cost-effective andcan produce voluminous data. The instrument can beoperated at a number of frequencies and coilseparations for depthwise information. There is noneed of ground (galvanic) contact, so no operatingproblem of current injection or of contact resistancein areas of highly resistive surface layer and also nonoise introduced in the data because of near surfaceinhomogeneities. The EM method provides betterlateral resolution and assessment of rock conductivitythan does the electrical resistivity method.

The method requires less coil separation for deeperinformation than do resistivity soundings. As a ruleof thumb, penetration depths for HLEM are 1.5 timesthe transmitter-receiver coil separation distance,compared with a maximum penetration of about onequarter of the current electrode separation requiredof the Schlumberger resistivity sounding.Consequently, given a favourable subsurfaceconductivity distribution, much deeper informationcan be obtained by covering less ground space. Also,multi-frequency data give deeper information, that is,depth of penetration is not constrained by coil/electrode spacing as in the resistivity method.

9.8 Disadvantages

Success of the method depends on getting ameaningful interpretation of the data, which in turndepends on the conductivity characteristics of theoverburden through which primary field penetratesand returns, introducing two phase lags. That is, theHLEM technique is preferred to detect a conductivetarget through less conductive overburden, which maynot be always available.

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5A HLEM Inphase Response Over a Thin Vertical Conductor

FIG. 5 MEASUREMENT BY HLEM SURVEYS Continued

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5B HLEM Quadrature Response Over a Thin Vertical Conductor

FIG. 5 MEASUREMENT BY HLEM SURVEYS

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For layered earth interpretations, models are highlysimplified which may not be the true condition.Detection of deeper layer is difficult. Skin-depth playsa significant role, as depth of exploration depends onrelative conductivity of deeper layers.

In phase component is affected by topographicvariations and it is always essential to standardize theresponse of the model through known boreholeinformation in the area.

9.9 Limitations

The problem of equivalence exists. Presence ofconductive overburden or surface layer induces phaselag and ambiguity. It is difficult to differentiateanomalies due to overburden variation and those dueto variation within the bedrock.

Interpretation of layer model is not very accurate forhighly conductive and resistive surface layer, that is,for the high contrast in conductivities.

10 TRANSIENT (TIME DOMAIN)ELECTROMAGNETIC

10.1 Purpose

To delineate aquifer zones in a conductive surroundingand delineate conductive saline ground water zones.


The transient electromagnetic (TEM) method isrelated to the frequency domain (continuous wave)electromagnetic methods by the Fourier transform.Instead of making measurement at differentfrequencies, in TEM methods the decay of an inducedvoltage is measured at a number of sampling times.A constant current is passed through the transmitterloop which produces a static primary magnetic field.When the transmitter current is abruptly switched offthe static magnetic field decays and, due to theassociated flux changes, currents are induced inconductors in the ground. This current flowing in ahorizontal closed path below the transmitter loopproduces a secondary magnetic field. The change inamplitude of secondary magnetic field with timeinduces a voltage in the receiver coil. Responsenormalized by the primary field is measured atselected time intervals after switching-off the primaryfield. Because response depends on resistivity of theground, measurements can yield geoelectricalcharacteristics of the ground. Immediately afterswitching off, that is at early time stage inducedcurrent is concentrated near the surface of the earth.Since, the maximum amplitude of induced currentdiffuses downward and outward, deeper geoelectricalinformation can be obtained as time increases, thatis, at later stages. The transient field decays quite fast.

The shape of transient curve (voltage decay versussquare root of time or apparent resistivity versus squareroot of time) does not represent depth-wise resistivityvariations as it could be assessed from conventionaldc apparent resistivity curve. Actually, the depth ofexploration is a function of time (and current flowingin the transmitter loop) and does not depend ontransmitter-receiver separation.

10.3 Instrument

Transient electromagnetic system comprises a receiverand a transmitter loop unit. Transmitter loops ofdifferent sizes are used for exploring different depthranges. TEM instrument uses constant currentwaveform consisting of equal periods of time-on andtime-off. A variety of TEM equipments are availablewith stacking facility. The TEM measurements aremade in a time range of 6 s to 1s after switching offthe primary current. The latest measurement time isdetermined by level of noise. For shallow groundwaterexploration measurement up to 10 millisecond to30 millisecond is done.


The technique can be employed for sounding as wellas profiling. For profiling moving transmitter-receiver configuration is used. Three types oftransmitter-receiver configurations are employed inTEM soundings, namely, grounded line, central loopand loop-loop configurations. The grounded lineconfiguration is used for deep soundings, whilecentral loop and loop-loop configurations are usedin shallower applications. The dimension of thetransmitter loop in central loop configurationdepends on the depth to be explored and is selectedbased on field testing.

The transmitter loop dimensions range between about30 m 30 m to 500 m 500 m to explore shallowzones to depths of about 2 500 m. For better resolutionat early time a small loop size is desired. Large loopsize at later times provides better signal. A peakcurrent of 2 A could be sent through the loop of 30 m 30 m for shallow exploration. Higher amperage(20 A) and large loop size is used for deeperexploration. The receiver measurements can start at6 s after switching-off and therefore shallow zonescan be investigated. The latest time could be up to 10millisecond to 30 millisecond depending on the levelof noise. The minimum detectable signal ranges from10-6 V/A.m2 to 10-12 V/A.m2. A group of four to sixpersons are required as crew.


The voltage decay versus time observed data areconverted to apparent resistivity versus time data.

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10.6 Interpretation

Induced voltage decay curve does not present directpicture of the subsurface geoelectrical condition as incase of electrical resistivity. Data are normalized fortransmitter and receiver parameters and converted toapparent resistivity. Apparent resistivity versus squareroot of time is plotted on double log graph paper.Curves are interpreted either by curve matching or bysoftware packages for inversion and forward modeling. Problem of equivalence exists in this technique also.

10.7 Advantages

Data scattering is not observed in central loopsounding as in case of electrical resistivity sounding.It is least affected by lateral variations in resistivityas the induced current flows in rings around thereceiver and also transmitter loop size is not changedfrequently. Resolution is high in shallow central loopsoundings. It has better resolving capability for S (h/)equivalence and can be used with other techniquesthat respond better to resistive layers (that is direct-current electrical sounding) to help resolve ambiguity.Compared to electrical resistivity sounding, smallerarea/smaller loop size is required for survey to achievesame order of depth. Thus, it can be conducted easilyin confined areas. To probe deeper, transients at latertimes are recorded. It is highly sensitive toconductivity changes, that is, a highly conductive layerunderlying conductive clay overburden is detectedbetter than a resistive layer.

10.8 Disadvantages

TEM equipment is quite expensive. Practically, toovercome noise, transmitter loop size has to beincreased to investigate deeper targets.

A good estimate of first and the last layer may not bepossible, due to equipment constraints. To getinformation for near surface layer very early stage timedata is required.

Target of limited lateral extents may not give a goodmatch in inversion (due to 3-D effects).

Resistive fresh water aquifers underlying thick clayoverburden may not get detected. Also, if the first layeris quite thick and resistive, the deeper relativelyconductive layer may no get detected.

10.9 Limitations

Thin resistive layers can not be detected. Transient EMnoise at later time stage restricts the length of timeduring which transient can be sampled and thus deeperinformation cannot be obtained unless transmitter loopsize or primary current flow is increased. Transientsounding for deeper exploration requires a large area

for loop layout compared with the straight striprequired for co-linear electrical resistivity arrays.However, for similar depths of exploration, dc soundingmethods sample a much larger volume of ground andthe data are therefore more susceptible toinhomogenities and reduced resolution.

Information on ambient noise at the measurementlocation is necessary.

Technique may not be useful if resistivity-thicknesscontrast is comparable with measurement uncertainty.

11 VERY LOW FREQUENCY (VLF) ELECTRO-MAGNETIC

11.1 Purpose

To delineate conductive water bearing fracture zonesin resistive hard rock and to determine approximatethickness of overburden.


VLF method is a type of electromagnetic method inwhich only receiver is in control of the operator.Transmitters are fixed stations located at great distances(up to several thousand kilometre) from the survey area.There are several such transmitting stations around theworld, which are continuously emittingelectromagnetic waves in frequency range of 15 kHzto 30 kHz for navigation purposes. Though the termVLF indicates very low frequency, the technique usesquite a high frequency for geophysical applications.

At large distances from the transmitter, radiated EMwaves travel into the ground as plane wave with ahorizontal magnetic and electric field and a verticalelectric field all mutually perpendicular. These planewaves (primary field) penetrate the earth surface andin case a conductor (relatively conductive saturatedfracture zones) is present eddy currents are created init. A secondary field with arbitrary orientation isgenerated due to the current induced. The resultantmagnetic and electric fields are not in phase with theprimary and so are elliptically polarized. In VLFsurveys, secondary field due to eddy currents ismeasured by a sensitive receiver.

The technique has directional advantages as well aslimitations. Saturated fracture zones in hard rocksoriented in-line with transmitter are picked up withrelative accuracy. VLF has a better resolving powerbecause of higher frequencies, but is effective indetecting shallow fracture zones only. The highfrequency radiations are attenuated fast with depth.Also, in presence of conductive over-burden,attenuation is fast and technique becomes ineffectivein detecting deeper fracture zones. Thus, VLF responsebecomes very susceptible to unwarranted near surface

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inhomogeneities and apparently presents a plethora ofanomalies. VLF receivers can measure tilt of the majoraxis of polarization ellipse and the ratio of minor tomajor axis (known as ellipticity). VLF receivers canmeasure in-phase part, which is approximately equalto tilt and the out-of-phase (quadrature or imaginary)part, which is approximately equal to the ellipticity ofvertical component of secondary field expressed as apercentage of the horizontal primary field. Electric fieldnormal to primary magnetic field is measured tomeasure apparent resistivities.

11.3 Instrument

Several types of instruments are available at present.Besides instrument, conventional surveyingaccessories are required to lay out profiles alongdesired orientations.


VLF method is easy to operate in field. A transmitterof strong and clear transmission is selected. If there isoption of selecting 2 or 3 transmitting stations, theyare to be selected, keeping orientation of fracture zonein mind, to get the maximum response.

Parallel profile lines are laid perpendicular to thedirection of transmitting station that is, along thedirection of primary field.

Profiles are laid 25 m to 100 m apart and station intervalis kept at 10 m. Length of profile is generally kept large,more than a kilometer, to study the regional trend.Selection of station interval becomes quite significantwhere anomalies show high rates of curvature.

Orientation of receiver with respect to transmitter isadjusted by a method given for the instrument selected.

For some instruments, there is no need of keepingreceiver at specific orientation with respect to thetransmitter.

Apparent resistivity of surface layer can also bemeasured by some of the instruments, using two sensorsconnected to instrument and placed on ground about5 m to 10 m apart at each station along the profile.

Operational procedure varies with type of instrument.In some of the instruments data are direct, digitallydisplayed, while in other they are recorded by obtaininga minimum intensity of sound signal adjusting theinstrument in various positions/orientations.

Position of transmitting station with respect to themovement of operator, that is, to his right or to hisleft is to be noted for interpretation of the cross-overs of inphase and quadrature components.

Accuracy of data depends on signal-to-noise ratio andselection of transmitter with reference to the orientationof target.

In field operation, repeatability of readings is to beensured. Instruments in which minimum sound isobserved, accuracy may vary with the operator andaffect the readings.


In-phase and quadrature components are plotted alongprofile line. Noise in the profile is identified andremoved. If data on parallel profiles are availablecontour maps can be prepared for in-phase andquadrature components.

11.6 Interpretation

Data can be interpreted qualitatively as well asquantitatively. Being a reconnaissance survey methodit is mostly used for qualitative assessment. Anomaliesare identified and interpretation of depth and lateralextents of targets and their conductivities are assessed.Mostly, the technique is used to demarcate lateral extentof a target.

Anomalies being affected by the presence of thickconductive overburden, assumption and simplificationsare required in interpretation. Effect of conductiveoverburden and conductive host rock surroundingshould be studied in detail from the available literature,before making any inference.

Quantitative interpretation is also attempted, in whichexperience of interpreter plays a significant role. Inhighly resistive terrain, ratio of in-phase to out-of-phaseresponse is proportional to conductivity of the target.

For quantitative interpretation data can be filtered usingFraser and Karous-Hjelt filters.

Fraser filter is used for in-phase data, which show cross-over response. It turns cross-overs into peaks andtroughs and reduces sharp noise. Filter enhances thoseanomalies, which resemble its shape.

Karous-Hjelt filter is used to determine the subsurfacedistribution of current, which is responsible for themeasured magnetic field. Current density pseudo depthsections are obtained for the purpose.

Quantitative interpretations can also be attempted forlayered earth model.

11.7 Advantages

VLF survey is fast and economical in field operation andused for reconnaissance in delineating saturated fracturezones in hard rocks. Surveys can be made in areas wheresurface layer is highly resistive and high contact resistance

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would be encountered in galvanic resistivity surveys.Lateral disposition of conductive zone is delineatedaccurately. It gives fast estimation of surface soil/overburden resistivity. Use of higher frequency range,enhances resolving power in detecting closely spacedconductivity discontinuities. Detectability of targetincreases in resistive surrounding.

11.8 Disadvantages

The amplitude (or even the continual presence) of theVLF primary field cannot be always guaranteed atthe receiver.

To get proper response and detection, there isrestriction on orientation of the target zones.

Depth of investigation depends on resistivity of thesurrounding media, and is drastically reduced ifsurface layer is highly conductive.

Instrument is expensive and may not deliver as muchinformation of the subsurface as resistivity methodcan, except for reconnaissance.

Because of high frequency used, measurements arehighly susceptible to variation in resistivity andthickness of overburden. Most of the anomalies aregenerated by the variations in overburden alone andcan be mistaken for the underlying fracture zones.So, the data profiles are noisy. Data obtained is afunction of operational procedure and henceambiguities are induced.

11.9 Limitations

Because of high frequency, the fields are attenuatedand phase shifted.

Conductivity resolution is effective over a frequencyrange.

Secondary field attenuates fast and skin-depth is smallin highly conductive formations.

In conductive terrain maximum depth of penetrationis half skin-depth for the medium surrounding thetarget or overlying it.

12 SEISMIC REFRACTION

12.1 General

Seismic refraction technique is quite useful to mapareas suitable for existence of potential aquifers. It isused to determine thickness and differentiatecompactness of sediments, subsurface layering,delineate weathered zone thickness, bedrocktopography, identify fracture zones and palaeo-channels. Sometimes the technique is very effectivein differentiating saturated and un-saturated zones(see IS 15681).


Seismic vibrations are created artificially on the surfaceof the earth either by explosion of dynamite (highenergy), impact at the ground surface either by heavyand accelerated weight drop ( medium energy) or byhammer (low energy). Vibrations thus created spreadto underground spherically in all directions and theirarrival at different distances at the surface of the earthare detected by sensors, planted on the ground, knownas Geophones. The responses of the geophones arerecorded in seismograph with timer circuit so that timesof arrival of these vibrations, called seismic waves,from the shot point, where the vibrations are createdto the detector points, where geophones are planted atdifferent distances, are accurately measured ( in millisecond). The greater the compactness of the medium,the higher the velocity.

In sedimentary or loose alluvial formation, the velocityof seismic wave propagation increases if the mediumgets saturated with water. Similarly seismic wavevelocity in weathered rock will be conspicuously lesscompared to compact rock system. Of the variousseismic waves, generated, Longitudinal wave, alsoknown as Primary wave or, in short P wave, is fastestand first to be detected. Thus, in refraction seismicwork conducted for ground water exploration,propagation of only the P wave through differentsubsurface layers is considered.

The subsurface consists of different layers and is nothomogeneous. The compactness of the layersgenerally increases with depth and as a result, thedeeper layers are expected to have seismic wavevelocity greater than that in the overlying material.This condition, which is necessary for the refractionmethod to be successfully applied, creates refractionof the down moving seismic wave, incident to theinterface of the two layers at a particular angle, calledcritical angle. At the interface, the refracted wave,sometimes called head wave, moves with velocity ofthe lower layer. As a result, after some distancebetween the source and the receiving geophones, therefracted wave takes over the direct wave and is firstto reach the detector.

To explain Snells law here, highly relevant torefraction principle, if incident ray enters the firstmedium with P wave velocity V1 at an angle withvertical and emerges as refracted wave in the secondmedium with P wave velocity as V2 at an angle withthe vertical [Fig. 6A], it may be proved based on simpleprinciple of optics that

1

2

sin

in

V

s V

=

When angle and velocity contrast between two media,

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that is, difference between V1 and V2 becomes such that becomes 90, the above equation simplifies to

= 12

sin

sin 90

V

V

or

1

2

sinV

V =

(Necessary condition for critical refraction is V1

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6C Schematic Presentation of Refraction Seismic Survey and Plot of Travel-time Curve

6B Principle of Head Wave Propagation

FIG. 6 TECHNIQUE MEASUREMENT BY SEISMIC REFRACTION Continued

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authors based on time distance discrete analysistechniques to find out depth of interface at differentdetector points and these are highly relevant toaccurately decipher discontinuity in the refractorinterface, like presence of fractures, faults, etc.Approximate plus-minus technique in refraction

survey, based on wave front techniques are alsoespecially effective in mapping interfacediscontinuities, especially in solv

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IS 15897 2011