sharma baranwal personal

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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: [email protected]


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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:

Benson, A. K., Payne, K. L. and Stubben, M. A., 1997. Mapping groundwater

contamination using DC resistivity and VLF geophysical methods - A case study:Geophysics, 62, 80-86.

Bernard, J. and Valla, P., 1991. Groundwater exploration in fissured media with electrical

and VLF methods,Geoexploration, 27, 81-91.

Bhattacharya, P.K. and Patra, H.P., 1968. Direct Current Geoelectric Sounding, Elsevier,

Amsterdam.

Ebraheem, A.M., Sensosy, M.M., Dahab, K.A., 1997. Geoelectrical and Hydro-

geochemical studies for delineating ground-water contamination due to salt-waterintrusion in the northern part of the Nile delta, Egypt. Ground Water, 35, 216222.

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

Hydrogeological Problems in the Coastal Areas of North West Germany.Geophysical Prospecting, 3, 95-110.

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.

Karous, M., and Hjelt, S.E., 1983. Linear-filtering of VLF dip-angle measurements:

Geophys. Prosp., 31, 782894.

Keller, G.V. and Frischknecht, F.C., 1966. Electrical Methods in Geophysical

Prospecting, Pergamon, New York.

Koefoed, O., 1979, Geosounding Principle-1, 276 pages, Elsevier, Amsterdam.

McNeill, J. D., 1990. Use of Electromagnetic Methods for Groundwater Studies. In:

Ward, S. H. (Ed.). Geotechnical and Environmental Geophysics, vol 1: Review

and Tutorial, Society of Exploration Geophysicists Investigations No.5, 107-112.

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

density pseudo-sections, Geophys. Prosp., 39, 567-580.


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Paterson, N. R. and Ronka, V., 1971. Five years of surveying with the very low

frequency electromagnetic method, Geo-exploration, 9, 7-26.

Parasnis, D. S., 1973. Mining Geophysics: Methods in Geochemistry and Geophysics.

Elsevier, Amsterdam.

Pozdnyakova, L., Pozdnyakov, A. and Zhang, R., 2001. Application of geophysical

methods to evaluate hydrology and soil properties in urban areas, Urban Water, 3,205-216.

Sharma, S.P. and Kaikkonen, P., 1999. Appraisal of equivalence and suppressionproblems in 1D EM and DC measurements using global optimization and joint

inversion. Geophys. Prosp., 47, 219-249.

Smith, B.D., and Ward, S.H., 1974. On the computation of polarization ellipseparameters: Geophysics, 39, 867-869.

Urish, D.W., Frohlich, R.K., 1990. Surface electrical resistivity in coastal groundwaterexploration. Geoexploration, 26, 267289.

Van Overmeeren, R.A., 1989. Aquifer boundaries explored by geoelectrical measure-ments in the coastal plain of Yemen: a case of equivalence. Geophysics 54, 3848.

Zohdy, A. A. R. 1969. The Use of Schlumberger and Eqatorial Soundings in Ground-

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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



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 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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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)


22/23

22

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


23/23

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.


VLF data.

Figure 6: Observed Real anomaly and Pseudo current density cross-section along profile0300E using Real anomaly of VLF data.


VLF data.


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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