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This article was published in Journal of Applied Geophysics, Vol. 57, Page Nos. 155-166,Copyright (2005), and is posted with permission from Elsevier.
Delineation of groundwater-bearing fracture zones in a hard
rock area integrating Very Low Frequency Electromagneticand Resistivity data
S.P. Sharma and V. C. Baranwal
Department of Geology and Geophysics,
Indian Institute of Technology
Kharagpur, 721302, India
Address for Correspondence:Dr S.P. Sharma
Associate Professor
Dept. of Geology and Geophysics
IIT, Kharagpur, 721302, India
Tel: +91-3222-283386
Fax: +91-3222-282268
E-mail: spsharma@gg.iitkgp.ernet.in
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Abstract
Integrated electrical and electromagnetic surveys were carried out in hard rock
areas of Purulia district (West Bengal), India, for delineation of groundwater-bearing
zones that would be suitable for construction of deep tube-wells for large amounts of
water. Groundwater movement that occurs through fractures in hard rocks is suitable to
be delineated by very low frequency (VLF) electromagnetic surveys. A detailed survey of
the area was done using a VLF-WADI instrument and appropriate locations were selected
for further study using Schlumberger resistivity sounding. Hence, the entire area was
surveyed in a relatively short time by the combined use of resistivity and electromagnetic
surveys.
Areas showing VLF anomalies may or may not be appropriate for drilling tube-
wells. In the northern part of the area, fracture zones are shallow, as exhibited by the
small magnitude of VLF anomalies and by shallow conducting structures interpreted
from the resistivity data. A VLF survey and subsequent resistivity sounding at suitable
locations suggest the existence of deep groundwater sources in the southern part of the
area. VLF anomalies have shown larger magnitudes in the southern part of the area than
those in the northern part of the area. Self-potential and resistivity profiling data also
showed correlation with results obtained using VLF and resistivity sounding. A typical
variation in self potential (SP) anomaly, i.e., positive SP anomaly for low resistivity, was
observed near the locations found suitable and could be interpreted as the result of
potential developed due to streaming of fluid within the fractured rocks.
Keywords: Groundwater exploration, Hard rock areas, Integrated interpretation, VLF
electromagnetic method, Resistivity method, Self potential
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1. Introduction
Electrical and electromagnetic geophysical methods have been widely used in
groundwater investigations because of good correlation between electrical properties
(electrical resistivity, etc), geology and fluid content (Flathe, 1955; Zohdy, 1969;
Fitterman and Stewart, 1986; McNeill, 1990). Electrical profiling, i.e. multi-electrode
Wenner profiling, which is used for mapping lateral resistivity variations can be replaced
by EM techniques as the electrical technique is slow and thus is not cost effective relative
to the electromagnetic technique. From various electrical methods, the direct-current
(DC) resistivity method for conducting a vertical electrical sounding (i.e. Schlumberger
sounding) is effectively used for groundwater studies due to the simplicity of the
technique, easy interpretation and rugged nature of the associated instrumentation. The
technique is widely used in soft and hard rock areas(e.g. Van Overmeeren, 1989; Urish
and Frohlich, 1990; Ebraheem et al., 1997).However, groundwater investigations in hard
rock areas are often more difficult, as tube-wells must be located exactly to be successful.
Tube-wells drilled without proper geophysical and hydrogeological study often fail to
produce groundwater.
In hard rock areas, groundwater is found in the cracks and fractures of the localrock. Groundwater yield depends on the size of fractures and their interconnectivity. Use
of Schlumberger sounding is well known for determining the resistivity variation with
depth. However, it is very difficult to perform resistivity soundings everywhere without a
priori information. The VLF method has been applied successfully to map the resistivity
contrast at boundaries of fractured zones having a high degree of connectivity (Parasnis,
1973). Further, the VLF method yields a higher depth of penetration in hard rock areas
because of their high resistivity (McNeill et al., 1991). Therefore, a combined study of
VLF and DC resistivity has potential to be successful (Benson et al., 1997, Bernard and
Valla, 1991). VLF data are also useful in determining the appropriate strike direction to
perform resistivity soundings (i.e. parallel to strike), again improving the likelihood of
success.
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Integrated geophysical studies were performed on the campus of Sainik School,
Purulia, West Bengal, India. Tube-wells have been drilled in the area in the past time,
have failed. Therefore Sainik School is dependent on a direct supply of water from the
riverbed. Thus, due to the failure of the tube-wells, an integrated study was needed. This
study was accomplished by a VLF survey followed by resistivity sounding, an SP survey
and a Wenner profiling to find suitable locations on the school campus.
2. Geology of the area
The area is characterized by gently-dipping metamorphic rocks striking
approximately N200
W to S200
Ewith low land areas on the east as well as on the west
side of the area under study. The topography of the area is such that it forms a ridge type
structure with its axis approximately perpendicular to the strike of the formations. The
rock type is granite gneiss, amphibolite, mica schist, quartzite, quartz vein, calc-silicate
rocks with interbanded crystalline limestone. The upper surface of the study area is
composed of thin soil cover followed by crystalline massive metamorphic rocks of very
high resistivity. Metamorphic rocks are also exposed on the surface at several locations.
The surface exposure shows the strike of the formation to be approximately in the E-W
direction and it is gently dipping. The most common rocks in the Purulia district are
granites and granite gneiss in which metabasics occur as intrusives.
Previous studies carried out in another part of the district by the Central Ground
Water Board, India (CGWB) show the occurrence of ground water is mainly in (1)
fractured zones of hard rock (2) the narrow zones of unconsolidated sediments along
major river valleys and (3) the weathered zone. The interbanded rocks are supposed to be
fractured at depth and groundwater movement occurs through these fractures. The
potential aquifer essentially contains two units: (1) weathered residuum, 8-10 m thick
with porous and uncompacted rocks containing water in the interstices and (2) underlying
fractured hard rocks, which store water within the secondary porosity.
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3. Geophysical Surveys
a) VLF electromagnetic survey
The radio signals transmitted from worldwide transmitters, used for navigation
purposes in the frequency range of 5-30 kHz are used as a source for the primary field in
a VLF survey. Such type of transmitting source makes VLF instrument very light and
portable, and can be useful to survey a large area quite quickly. VLF magnetic field
measurement makes use of E-polarization in which a transmitter is selected in the
direction of strike and measuring profiles are taken perpendicular to the strike direction.
Generally, the horizontal and vertical components of magnetic fields are measured, and
real and imaginary anomalies are computed using the expression given by Smith and
Ward (1974)
tan( / ) cos
( / )2
2
1 2
=
H H
H H
z x
z x
(1)
and
eH H
H
z x=
sin
1
2, (2)
where is dip angle, e is ellipticity, Hz
and Hx
are the amplitudes, the phase difference
= z x
, in which z
is the phase of Hz
and x
is the phase of Hx
and
H H e Hz
i
x1 = + sin cos . The tangent of the tilt angle is a good approximation of
the ratio of the real component of the vertical secondary magnetic field to the horizontal
primary magnetic field. The ellipticity is a good approximation of the ratio of the
quadrature component of the vertical secondary magnetic field to the horizontal primaryfield (Paterson and Ronka, 1971). These quantities are called the real (= tan 100 %)
and imaginary (= e 100 %) anomalies, respectively and they are normally expressed as
percentage.
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VLF data were collected using an ABEM-WADI instrument. Since the strike of the
formation was approximately in the E-W direction, a transmitter in this direction with a
frequency of 19.8 kHz was used. This transmitter was appropriate for E-polarization VLF
surveys. Next, we covered much of the area by making suitable traverses along
hydrogeologically suitable locations. Traverses 0100E, 0090E, 0200E, 0210E, 0300E,
0400E, and 0410E were on the east side of the Campus, while 0500E and 0510E were on
the west side of the campus as shown in Fig. 1.
b) Electrical surveys
Schlumberger resistivity soundings were performed at ten locations using a DC resistivity
meter.Sounding locations were selected by detailed study of the area with a VLF survey
as well as by their hydrogeological suitability. The locations where resistivity soundings
were performed are shown in Fig. 1. Current electrode spacings were gradually increased
up to 800 m for delineation of deeper structures. Electrodes were spread in the east-west
direction, i.e. parallel to strike direction (as determined by the VLF study).
Over layered earth structures (1-D situation) variation in apparent resistivity with current
electrode separations is quite smooth (Koefoed, 1979). Further, this variation is also
smooth when the direction of spread is parallel to the strike and erratic when the direction
of spread is perpendicular to the strike for 2-D situation (Keller and Frischknecht, 1966).
In the present study, a rather smooth variation in apparent resistivity is observed up to
large electrode separations in the strike (east-west) direction. Therefore, we assume that
in such situation 1-D interpretation will yield significant subsurface features for the
recommendation of appropriate drilling locations.
Resistivity data are interpreted using a Very Fast Simulated Annealing (VFSA) 1-D
global inversion scheme (Sharma and Kaikkonen, 1999). Several solutions are derived
for a particular sounding, and the mean model is computed. Here, it is important to
mention that the original algorithm was modified from Sharma and Kaikkonen (1999), so
that the mean model as well as its fitting with the observed data is improved.
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Uncertainties in the mean model parameters are also computed from covariance matrices
obtained from various solutions.
Finally, Wenner profiling and SP survey were carried out on a traverse covering
the most important area (VES-3 to VES-7) and the anomalies were studied for correlation
with VLF and resistivity interpretations.
4. Results
a) VLF data interpretation
The VLF WADI instrument displays the filtered real anomaly on the screen, and this
anomaly can be roughly interpreted on site. This feature of the instrument is used to
select sounding locations for resistivity surveys. For further detailed information of the
subsurface, the measured real and imaginary anomalies were re-discretized at 1 m
interval and filtered using the approach of Karous and Hjelt (1983). This process yields
pseudo-section of relative current density variation with depth. A higher value of relative
current density corresponds to conductive subsurface structures. It is observed that
apparent current density cross-sections using real and imaginary anomalies show almost
similar features. Therefore, for simplicity only the real component results are presented in
Figs. 2 to 8. Apparent current density cross-section also gives a rough idea about the dip
direction; however, exact dip angle can not be estimated due to the vertical axis variable
being a pseudo depth only.
Site VES-1 was selectedfor resistivity sounding at the beginning of VLF profile 0100E
due to suitable hydrogeology of the location. Current density cross-sections obtained
using real anomalies along profile 0100E (Fig. 2) show accumulation of current between
stations 200 and 250 m. On profile 0090E (Fig. 3), which is 100 m east and parallel to
profile 0100E, little accumulation of current density is observed. So we performed a
resistivity sounding VES-2 between these two profiles, 200 m from the starting point of
the profile. Asymmetry in the observed real and imaginary anomalies suggests the
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dipping nature of a subsurface conductive body (Ogilvy and Lee, 1991; Kaikkonen and
Sharma, 1998).
The apparent current density cross-section along VLF profile 0200E (Fig. 4) shows
shallow conductive features near stations 50, 180, 275 and 360 m. Profile 0210E (Fig. 5),
which is 50 m east of profile 0200E and parallel to it, reveals a highly conductive
subsurface near station 100 m. This anomaly is not clearly seen in profile 0200E;
however, there is an indication of the presence of a conducting feature near station 50 m
on profile 0200E. This feature is in accordance with the strike direction of the formations
in the area. There is a good correlation between these two cross-sections (Figs. 4 and 5).
Resistivity soundings VES-3 and VES-4 were performed at stations 50 and 360 m on the
profile 0200E.
The pseudo current density cross-section along profile 0300E (Fig. 6) shows conducting
features near stations 50 and 300 m. This profile has the most significant anomalies in
this study. Hence soundings VES-5 and VES-6 are positioned at stations 50 and 300 m,
respectively. A sudden change in magnitude of the real anomaly of VLF data is observed
near 300 m location (Fig. 6). Further along this profile, an asymmetric anomaly near the
station 100 m reveals a dipping structure. Observed data between stations 100 to150 m
reveal that the profile direction is down dip of the structure, and this dip direction is also
reflected in the pseudo current density section (Fig. 6). A high in the pseudo current
density section is observed at station 300 m. There is neither low-lying land near 50 and
300 m locations, nor any source of moisture in the ground. The top surface is totally dry
and compacted at these locations. Hence this high current density suggeststhe likelihood
of conductive material at depth, and can be interpreted as an indication of the presence of
fractures containing groundwater.
Profile 0400E is 500 m long and is a continuation of 0300E in the same direction. The
magnitude of the anomaly is not large and it is also scattered. The pseudo current density
cross-section along this profile shows conductive features between stations 250 to 350 m.
This feature may be due to a low land area and excess moisture in the ground. The
anomaly also occurs near a pond, so we did not select a location for a resistivity sounding
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on this profile. On profile 0410E (Fig. 7) approximately 150 m east of profile 0400E, a
conductive feature is seen near 40 m. This feature may be a continuation of the fractures
seen in profile 0300E. The location of the feature was selected for sounding VES-7.
Profiles 0500E and 0510E (Fig. 1) are on the west side of the campus. This area is
composed of rocks of very high resistivity (outcropping at several places) covered with
saturated clay. Due to this clay at the surface, the VLF signal could not penetrate deeper
conducting objects. Both profiles show conducting features with less than 20% current
gathering. Two resistivity soundings VES-8 and VES-9 were performed in this area.
VES-8 was selected according to the hydrogeology of the location and VES-9 was
selected on the basis of profile 0510E, which shows a conducting feature at station 250 m
(Fig. 8). Since the west side of the area does not seem suitable, therefore, current density
cross-section for profile 0510E is presented only.
b) Resistivity data interpretation:
Interpretations of all the resistivity soundings are presented in Fig. 9. The resistivities of
various layers of interpreted models are shown numerically on the figure and thicknesses
are marked on the abscissa. Solid symbols represent the observed data while solid lines
represent the corresponding model data. The maximum half-current electrode separation
(AB/2) is limited to only 100 m for VES-1 and VES-2 due to space limitation; however,
it increases up to 400 m for VES-5, VES-6 and VES-7. Fitting between the observed and
model data is very good for all the soundings except soundings VES-8 and VES-9.
Figure 9a shows the interpretation of VES-1 and VES-2. VES-1 was selected on the basis
of hydrogeological suitability of the location, and shows only a two-layer structure. The
observations do not show any signature of the fractured formation at greater depth. The
current flow in the subsurface is very small, indicating that the formation is compact at
depth. A small borehole drilled previously near this location failed to yield water, in
agreement with the interpreted results. The interpreted results for VES-2, which was
selected on the basis of the VLF pseudo current density cross sections (Figs. 2 and 3),
show a six-layered structure. Though the sounding curve looks like a 2-layer curve, it was
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not possible to interpret the data using 2 and 3-layer models. The current flow in the
subsurface has initially an increasing and then a decreasing trend with increase of
electrode separation. This trend shows the presence of a multiple conducting and resistive
structures at depth. The third and fifth layers show the fractured formations; however,
they are very shallow and their thicknesses are too small to justify the construction of a
tube-well. Therefore, the location, which shows a VLF anomaly, is found unsuitable after
interpretation of the resistivity sounding.
Figure 9b shows the interpretation of soundings VES-3, VES-4 and VES-10.
Interpretation of resistivity sounding VES-3 shows a five layer model of alternatively
high and low resistivities. Alternate variation of resistivity is also indicated by variation
of current flow in the formation. The second and fourth layers exhibit low resistivities,
suggesting fractured formations, but due to their small thicknesses, this site can not be
recommended for tube-wells. Generally, this type of location is suitable for large
diameter dug wells. However, there is already a dug well exactly west of this sounding
location which yields a good amount of water. This dug well dries out in the summer
season, because of the shallow source of groundwater. Interpretation of sounding data
VES-3 clearly demonstrates this feature. The finding of the geophysical surveys is also
supported by three deep tube-wells drilled previously about 50 to 100 m north-west of
this location. These tube-wells failed to yield any groundwater. Sounding VES-4 is
located about 300 m south of VES-3. Both soundings look similar, but their
interpretations are different. For VES-4, the bottom layer is interpreted as a conducting
layer. This interpretation is supported by the increase in current flow at larger electrode
separations.
Soundings VES-5, VES-6 and VES-7 are similar to sounding VES-4. For each
sounding, the bottom layer has been interpreted as a conductive layer. A sudden increase
in current flow for the same applied voltage is observed from 200 to 400m AB/2 values at
these sounding locations therefore, assumption for the bottom layer to be conductive is
reasonable. Figure 9c shows the same apparent resistivity curves at larger current
electrode separations for these three soundings. The interpreted resistivity of the bottom
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of the second and fourth layers are promising for ground water availability with layer
thicknesses of approximately 3 and 4 m, respectively. The last layer is conductive
compared to the fifth layer but its resistivity is high, showing that the formation at depth
is not fractured much. Hence, the second and fourth layers are too shallow to be suitable
for a deep tube-well, and the last layer is not likely to yield groundwater. This location is
therefore also not very promising for a deep tube-well.
c) Resistivity profiling and SP Survey
After measuring the VLF responses on various profile lines and performing resistivity
soundings, we conducted resistivity profiling using a Wenner configuration with a 150 m
current electrode separation (i.e. a=50 m) to map the variation of resistivity
approximately at 50 m depth. The traverse was made approximately perpendicular to the
strike at intervals of 10 m. The variation of apparent resistivity with distance is shown in
Fig. 10b.
The SP response was also measured along the same profile line with a 10 m potential
electrode separation. If the conductive zones were due to presence of mineralization, then
there should be high negative SP anomalies corresponding to these bodies, such
anomalies are not observed. The observed positive SP anomaly is interpreted to be caused
by a streaming potential developed due to groundwater flow in the fractured formation
(Pozdnyakova et al., 2001). There is some correlation of low resistivity with a positive SP
response and high resistivity with a negative SP response. The approximate positions of
various soundings are marked on the SP anomaly curve (Fig. 10a).
5. Discussion and conclusions
Fractures are the primary source to store and allow movement of groundwater in hard
rock areas. The size and location of the fractures, interconnection of the fractures, amount
of the material that may be clogging the fractures and recharging sources determine how
much water one can get out of the hard rock. The volume of water stored in fractured
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hard rocks is less compared to the conventional aquifer. When fractures become narrower
at depth, this amount further decreases. The total amount of water storage in the fractures
of hard rock area is small; hence, groundwater levels and the well's yield can decline
dramatically during the summers. Therefore, the location of potential fracture zones in
hard rock area is extremely important to yield large amounts of groundwater and this can
not be done using just one approach. Thus groundwater potential of any location in hard
rock areas must be assessed by several approaches (geophysical as well as
hydrogeological). A location found suitable on the basis of several approaches is less
likely to fail in yielding groundwater.
The efficacy of the combination of VLF electromagnetic, DC resistivity soundings, SP
measurement and Wenner Profiling is presented here to map the fractures in a hard rock
area. The anomaly obtained in VLF measurements is an indication of the presence of
conductive zone, which may or may not be suitable as VLF can not discriminate between
deep and shallow sources. Therefore it is necessary to follow the location of these VLF
anomalies with a technique that investigate the depth of these conductive sources.
Resistivity profiling and SP measurement also add valuable information about the
presence of a conducting fracture and groundwater movement. A positive SP anomaly is
observed over those fractures which contain flowing fluid.
The integrated interpretation undertaken in the hard rock area reveals that the fractures in
the northern part of the area are shallower than those in the southern part. The magnitudes
of VLF anomalies are the largest in southern part of the area (profile 0300E and 0410E).
As the fractures are shallow and show smaller magnitude of VLF anomalies in northern
part, it is likely that fractures will become dry in summer season. However, fractures
showing a large anomaly due to a deeper conductive zone in the southern part of the area
will be more suitable for groundwater exploitation for longer duration and it is unlikely
that they will dry in summer season.
The most important phenomenon observed during the resistivity sounding in southern
part of the area is that the current flow in subsurface regions increases dramatically for
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larger current electrode separations, showing the presence of a conducting formation at
depth. A sudden increase in current flow for the same applied voltage is observed from
200 to 400 m AB/2 values at VES-6 and VES-7 locations. Such increase in current flow
over a large range of AB/2 values may be due to presence of a thick fractured saturated
formation. Isolated 3-D conductive objects may not show such behavior. Hence the
sounding locations VES-6 and VES-7 are the most suitable locations for drilling a deep
tube-well. The sounding locations VES-4 and VES-5 are also suitable; however, the best
locations are VES-6 and VES-7 and most probably these two locations are interconnected
with a common subsurface fracture.
The presence of a recharging source is very important to obtain a continuous supply in a
hard rock area. A recharging source is present near the area which is nearest to the VES-6
and VES-7 locations. If the recharging sources were exhausted in an extreme season, then
the tube-well may go dry. However, recharge exhaustion is unlikely near the locations
VES-6 and VES-7. Drilling of a 100 to 120 m deep tube-well is recommended at these
locations. It is important to note that, if these locations (VES-6 or VES-7) fail to yield the
appropriate amount of groundwater, drilling at other locations would be meaningless.
Further, the VLF survey reveals that there are several shallow and deep fracture zones in
the area. It is unlikely to obtain a large amount of groundwater supply from a single
source in hard rock areas. Therefore, groundwater should be collected from several
sources, such as dug-wells. As the movement of groundwater takes place, the subsurface
will become more and more productive due to an increase in secondary porosity.
Acknowledgements
We would like to thank the Editor Dr A. Hordt; Reviewer Dr M. Hatch and other
anonymous reviewer for their comments and suggestions to improve the quality of
manuscript. The study is a part of the project ESS/23/VES/099/2000, DST, Govt. of
India.
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References:
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Ebraheem, A.M., Sensosy, M.M., Dahab, K.A., 1997. Geoelectrical and Hydro-
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Fitterman, D. V. and Stewart, M. T., 1986. Transient Electromagnetic Sounding forGroundwater. Geophysics, 51, 995-1005.
Flathe, H., 1955. Possibilities and Limitations in Applying Geoelectrical Methods to
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Kaikkonen, P. and Sharma, S.P., 1998. 2-D nonlinear joint inversion of VLF and VLF-Rdata using simulated annealing, J. of Applied Geophysics, 39, 155-176.
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Keller, G.V. and Frischknecht, F.C., 1966. Electrical Methods in Geophysical
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Koefoed, O., 1979, Geosounding Principle-1, 276 pages, Elsevier, Amsterdam.
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McNeill, J. D. and Labson, V. F., 1991. Geological Mapping using VLF Radiofields. InNabighian, M. C. (Ed), Geotechnical and Environmental Geophysics, vol. 1,Review and Tutorial. Tulsa: Society of Exploration Geophysicists, 191-218.
Ogilvy, R. D. and Lee, A.C., 1991. Interpretation of VLF-EM in-phase data using current
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Paterson, N. R. and Ronka, V., 1971. Five years of surveying with the very low
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Pozdnyakova, L., Pozdnyakov, A. and Zhang, R., 2001. Application of geophysical
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Figure 1: Location map of the area
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Figure 2: Pseudo current density cross-section along profile 0100E using Real anomaly of VLF data
Figure 3: Pseudo current density cross-section along profile 0090E using Real anomaly of VLF data
Figure 4: Pseudo current density cross-section along profile 0200E using Real anomaly of VLF data
50 100 150 200 250 300 350
Distance (m)
-60
-40
-20
0
Depth(m)
-
-
-4
4
1
2
[%]
0 50 100 150 200 250 300 350
Distance (m)
-60
-40
-20
0
Depth(m)
[%
50 100 150 200 250 300 350 400
Distance (m)
-70
-50
-30
-10
Depth(m)
-1
-6
2
1
[%]
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Figure 5: Pseudo current density cross-section along profile 0210E using Real anomaly of VLF data
Figure 6: Observed Real anomaly and Pseudo current density cross-section along profile 0300E using Real
anomaly of VLF data.
50 100 150 200 250 300 350 400
Distance (m)
-70
-50
-30
-10
Dep
th(m)
-
-
-
2
[%
50 100 150 200 250 300 350
Distance (m)
-60
-40
-20
0
Depth(m)
-2
-8
4
16
28
[%]0 50 100 150 200 250 300 350
-40
-20
0
20
40
Realanomaly(%)
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Figure 7: Pseudo current density cross-section along profile 0410E using Real anomaly of VLF data
Figure 8: Pseudo current density cross-section along profile 0510E using Real anomaly of VLF data
0 50 100 150 200 250 300 350 400
Distance (m)
-70
-50
-30
-10
Depth(m
)
-2
-1
-1
1
2
[%]
0 50 100 150 200 250
Distance (m)
-50
-30
-10
D
epth(m)
-3
-2
-1
0
1
[%
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Figure9: Fittings between the observed and computed data for VES-1 to VES-10. Solid symbols
(, , +) show observed data and corresponding solid line ( ) shows model data
for a particular sounding. Interpreted resistivities are shown numerically and thicknessesof various layers are shown on AB/2 axis for each sounding.
0.1 1.0 10.0 100.0 1000.0
AB/2 (m)
10
100
1000
10000
App.Res.(Ohm.m)
VES-8
VES-9
1512 50000 Ohm.m33
22547 108
298579 Ohm.mVES-8VES-9
(d)
28
1 10 100AB/2 (m)
10
100
1000
App.Res.(Ohm.m
)
VES-1
VES-2
3113 Ohm.m18 348 92
1132 159
6210 Ohm.mVES-1
VES-2
(a)
0.1 1.0 10.0 100.0 1000.0AB/2 (m)
10
100
1000
10000
App.Res.(Ohm.m
)
VES-3
VES-10
VES-4
199 45
1884 & 80
9995 Ohm.m
479 56
297 42 276
91 & 28894
2877 Ohm.m
0.5 Ohm.m
380 & 108
41914
VES-3
VES-4
VES-10
(b)
0.1 1.0 10.0 100.0 1000.0
AB/2 (m)
10
100
1000
10000
App.Res.
(Ohm.m)
VES-5
VES-6
VES-7
36 10 73 99983 10392 27 3350 466 276632209 55 86
1801 149 245918
1 Ohm.m
VES-5VES-6
VES-7
(c)
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Figure 10: Self-potential anomaly and resistivity profiling from VES-3 to VES-7.
0 100 200 300 400 500
Distance (m)
-40
-20
0
20
40
SPanomaly(m
V)
0 100 200 300 400 500
Distance (m)
200
400
600
App.
Res(Ohm.m
)
VES-10 VES-4
VES-5
VES-7
(a)
(b)
VES-3
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Figure captions
Figure 1: Location map of the area
Figure 2: Pseudo current density cross-section along profile 0100E using Real anomaly ofVLF data.
Figure 3: Pseudo current density cross-section along profile 0090E using Real anomaly ofVLF data.
Figure 4: Pseudo current density cross-section along profile 0200E using Real anomaly of
VLF data.
Figure 5: Pseudo current density cross-section along profile 0210E using Real anomaly of
VLF data.
Figure 6: Observed Real anomaly and Pseudo current density cross-section along profile0300E using Real anomaly of VLF data.
Figure 7: Pseudo current density cross-section along profile 0410E using Real anomaly of
VLF data.
Figure 8: Pseudo current density cross-section along profile 0510E using Real anomaly of
VLF data.
Figure9: Fittings between the observed and computed data for VES-1 to VES-10. Solid
symbols (, , +) show observed data and corresponding solid line ( )
shows model data for a particular sounding. Interpreted resistivities are shown
numerically and thicknesses of various layers are shown on AB/2 axis for eachsounding.
Figure 10: Self-potential anomaly and resistivity profiling from VES-3 to VES-7.
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