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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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1
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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IS 15897 : 2011
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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IS 15897 : 2011
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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IS 15897 : 2011
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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IS 15897 : 2011
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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IS 15897 : 2011
FIG
. 1 D
IST
RIB
UT
ION
OF C
UR
RE
NT A
ND
EQ
UI-
PO
TE
NT
IAL L
INE
S IN
AN
EL
EC
TR
ICA
LLY
HO
MO
GE
NE
OU
S S
UB
SU
RFA
CE
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IS 15897 : 2011
FIG. 2 DIFFERENT ELECTRODE CONFIGURATION IN USE IN ELECTRICAL
RESISTIVITY SURVEYS
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IS 15897 : 2011
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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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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IS 15897 : 2011
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.
8.2 Principles of Measurement
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.
8.4 Field Procedures
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.
8.5 Processing of Data
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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IS 15897 : 2011
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.
9.2 Principles of Measurement
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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IS 15897 : 2011
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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IS 15897 : 2011
9.4 Field Procedures
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.
9.5 Processing of Data
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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IS 15897 : 2011
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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IS 15897 : 2011
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.
10.2 Principles of Measurement
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.
10.4 Field Procedures
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.
10.5 Processing of Data
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.
11.2 Principles of Measurement
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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IS 15897 : 2011
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.
11.4 Field Procedures
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.
11.5 Processing of Data
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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IS 15897 : 2011
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).
12.2 Principles of Measurement
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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IS 15897 : 2011
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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IS 15897 : 2011
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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IS 15897 : 2011
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