Geophysical exploration for ground water around Digha in the coastal region of West Bengal, India

- Elsevier Publishing

Geoexploration

Company, Amsterdam

- Printed in The Netherlands

Research Papers GEOPHYSICAL DIGHA H. P.

PATRA

Department

EXPLORATION

FOR

IN THE COASTAL REGION

GROUND

WATER

OF WEST BENGAL,

AROUND

INDIA

AND P. K. BHAT-TACHARYA

of Geologyand

Geophysics,

Indian Institute

of Technology,

Kharagpur

(India)

(Received May 23, 1966)

SUMMARY

Electrical resistivity soundings using the Schlumberger configuration (max. spread = 1,000 m) have been carried out at 24 stations around Digha in the coastal region of West Bengal, India, in connection with ground-water investigations for Digha township. Geophysical surveys were conducted to examine the possibility of saline water invasion and to investigate the existence of saline water pockets reported earlier. The results have been interpreted by the graphical-analytic method and the lithological logs prepared. The interpretation of sounding data has been facilitated by the lithological logs of bore holes drilled at selected points and the resistivity data supplemented by bore-hole data. The probable lateral and vertical extents of the aquifers have been delineated. There has been no indication of saline water invasion probably due to the presence of some impervious layers located in the area. Besides, no saline water-bearing pockets of sand have been detected.

INTRODUCTION

AND SURFACE

GEOLOGY

The present geophysical investigations were carried out as a part of the twin problems of availability of ground water and erosion of the beach around Digha under “Digha Project”, financed by the Government of West Bengal. Formerly a small village, Digha, is being developed as a sea-resort by the Government of West Bengal. Digha is situated on the shore of the Bay of Bengal (shown on the location map: Fig. 1) at a distance of about 120 km from Kharagpur (116 km west of Calcutta). The area covered by detailed soundings include an area of about 30 km’ around Digha (shown by shaded portion in Fig.1) which falls on the topo sheet no.l3-0 (Survey of India) and is bounded by 87”30’ and 87”25’E, and by 21”35’ and 2 l”4O’N. Geoexploration,

4 (1966) 209-218

210

H. P. PATRA AND P. K. BHATTACHARYA

Fig. 1. Location map.

The area under investigation forms a coast of emergence. The ground surface is more or less flat (except for a few sand dunes) with a gentle slope towards the sea. Recently the sea started advancing towards the land, endangering the township of Digha (NIYOGI and CHAKRABORTY, 1966). Here the shoreline is of the “sandy type” and has been formed against the unconsolidated sediments of Upper Tertiary age which dip at low angles to the south-southwest. The area is covered by unconsolidated clays, silt and sand deposited in recent times by marine transgression. The monotony of the coastal plains around Digha is broken by four prominent lines of sand dunes (Fig.1) nearly parallel to the present shoreline. Of Ceoexploration,

4 (1966) 209-218

EXPLORATION

FOR GROUND WATER AROUND DIGHA, INDIA

these, the innermost

line lies 11 km inland,

This dune

5 km from the northernmost

is about

211

rising locally to heights of about part

of the investigated

12 m. area

(Fig.1). The third line of dunes through Deuli and Ramnagar passes through the northernmost edge of the present area. Besides these two, there are coastal dunes and the second line of dunes as shown in Fig.1. The subsurface formations consist of alternate sand and clay layers. The sedimentary section is very thick. The basement has not been struck even down to a depth of 200 m in any of the bore holes. Digha coast forms a part of a coast of emergence and the climate is tropical and humid with an annual rainfall of about 160 cm.

STATEMENT OF THE PROBLEM

Geophysical studies were made to investigate the possibility of salt water invasion, if any, and the problem of ground water for Digha township. SINGHAL (1963) reported the presence of an impervious clay layer (Fig.2) at Jaldha,

about

10 km east of Digha

/ m.s.l

---Y--

Sea

dyke
well

at

along

the coastal

zone where fresh water is

Joldho

‘\

____ _____________-___. _,,,~7===~~~~~~z-z.z = = _ -=_=~~z~~===_

LL== ~--------=---------; 5_-____-_____ __------------=_-

6 Fresh

water

Fig.2. Fresh and salt water relationship near the coast at Jaldha. (From

available

below saline water. The lateral

extent

SINGHAL,

of this clay horizon,

1963.)

however,

is

not known and the probability of its being extended up to Digha to the west or the existence of a similar layer at Digha is not ruled out. Recent geophysical investigations around Jaldha by PATRA (1966) have indicated absence of any invasion from the sea but confirmed the presence of a saline water-bearing pocket of sand at a shallow depth. If this condition, i.e., absence of invasion at Jaldha is found to exist around Digha also, there should not be any cause for alarm regarding ground-water supply for Digha township. Otherwise, it is feared that the aquifers might have been seriously invaded by sea water with considerable effect on the ground-water situation. The problem is much more complicated due to the reported existence of saline water pockets (SINGHAL, 1963), irregularly scattered in this coastal area. Thus for the ground-water supply to the fast developing Digha township, it Geoexploration,

4 (I 966) 209-218

212

H. P. PATRA

was decided before

to study the problem

the scheme

could

be taken

AND P. K. BHATTACHARYA

of saline water invasion in hand.

Accordingly,

from the sea, if any, detailed

geophysical

surveys were carried out (resistivity sounding), supplemented by drill holes at selected points. In brief, the problems were to study the general ground-water conditions, relation between fresh and salt water and the existence of saline water pockets.

RESISTIVITY

SURVEY

AND INTERPRETATION

Preliminary geophysical investigations around Digha indicated a more or less homogeneous geological section from the shoreline towards land without significant lateral variation. Later detailed electrical resistivity soundings have been carried out at a number of stations to investigate the depth, thickness and other

Litholo

ical

Logs

t

Fig.3.

Locations

of sounding

stations

and drilling

points.

Geoexploration,

4 (1966) 209-218

EXPLORATION

213

FOR GROUND WATER AROUND DIGHA, INDIA

properties of water-bearing

sand and the possible bearing on the ground-water

supply. Electrical resistivity soundings using Schlumberger configuration (maximum AB = 1,000 m) have been completed at 24 stations numbered S-O to S-23 as indicated in the diagram (Fig.3). 30. 20.

100

IO Fig.4.

Sounding

curves

representing

section

1000

PQ.

The data collected have been plotted on a double-logarithm graphsheet of modulus 62.5 mm (same as the standard master curves available) and denoted as VES-0 to VES-23 (vertical electrical sounding). All H, A, K and Q types of curves have been obtained including a number of four-layer curves like HA, KH and AA. A number of selected curves at suitable locations have been presented in Fig.4, 5 and 6. The analytic-graphical method has been used for interpretation of field curves. This semi-empirical method or the so-called “Auxiliary Point Method” given by KALENOV (1957) and recently reviewed by ZOHDY (1965) can be used for

I

30

8

Fig.5.

,,,,

Sounding

AB/2

IO

curves

representing

(meter) 100

section

RS. Geoexploration,

4 (1966)

209-218

214

H. P. PATRA AND P. K. BHATTACHARYA

interpretation

of any type of curves and for any number of layersl.

layer problem

is progressively

into equivalent

reduced

to a simple two-layer

Here a multi-

case by conversion

layers.

For interpretation

of field curves, frequent

use has been made of the album

of curves by the COMPAGNIE GBNBRALE DE G~OPHYSIQUE (1963) and the Puletki OGILVI and KSMELEVSKOI (1964). K-Q

auxiliary

point

charts

The Paletki

mentioned

above includes

(also known as “guide curves”)

VES-9

by

H-A and

besides the two- and

(Depth Scale same as abscissa )

VES-IO VES-II

-

AB/g(meters)

, , ,100

, , , , , ,,

200

Fig.6. Some representative type curves.

three-layer auxiliary

master

curves.

Use has also been

points in the interpretation

used in the choice grams for obtaining

of the suitable

made

of these

stage. The principle

of equivalence

set of curves for matching

may be made to the monograph

to get the has been

and Pylaev’s

the limiting values. For detailed procedure

tion of field curves, reference

charts

nomo-

in the interpretaby BHATTACHARYA

and PATRA (1966). The interpretation logs prepared

of the field data has been facilitated

from the drill-hole

1 Note of the Editor: The method 10(l):

data at a number

has also been described

by the lithological

of points.

by Ebert,

1943. Beitr. Angew. Geophys.,

1-17. Geoexploration,

4 (1966) 209-218

EXPLORATION

FOR GROUND

WATER AROUND

DIGHA,

215

INDIA

RESULTS AND DISCUSSIONS

The field curves have been interpreted to prepare the geophysical logs and subsequently the lithological logs. The interpretation of the data and the preparation of the suitable lithological logs have been facilitated by the bore-hole data at suitable locations. Attempts have been made to put some soundings as near to the drill holes as possible. Some drill holes have been made where geophysical sounding could not be taken due to obstacles and inaccessibility. The drilling points have been shown on the map (Fig.3) as H-f to H-5 along with the sounding points S-0 to S-23. The maximum depth reached by drill holes is about 200 m. The lithological logs from these holes are presented within Fig.3. All possible care has been taken to correlate the bore-hole data with resistivity values obtained from sounding.

fR)

f S) Om NE

SW Ikm

0

50m

A

(0)

(P) W

E

B Fig.7. Geological sections, prepared from sounding and bore-hole data. A. Section PQ. B. Section RX (For location of these sections and legend see Fig.3.)

The lithological logs prepared from sounding and bore-hole data (shown in Fig.6 and 7) are more or less homogeneous with the northern and eastern sides of the area studied. In almost all the logs the expected shallow water-bearing sand (resistivity, 20-30 am) is overlaid by an impervious clay layer. At a few locations, however, the clay layer is replaced by a silty sand (Fig.7). The soundings, V&S’-0 Geoexplorafion,

4

(I 966) 209-218

H. I’. PATRA

216 to VES-6 mostly interesting

A- and AA-type

and almost

similar

curves,

features

(at least 200 m) there is no indication

taken

(Fig.4).

AND P. K. BHATTACHARYA

right over the beach have very Within

the depth of investigation

of any saline water-bearing

formation.

This

is possible probably due to the presence of impervious clay layer at the top and the impervious silty sand below. It has been established by comparing the geological logs (from drilling data) with the corresponding sounding data that the low resistivity layer (1-2 Qm) corresponds to a plastic clay and not a saline water or brackish water-bearing formation. A legend (inset, Fig.3) giving details of the resistivity values and the corresponding lithological information has been prepared. The resistivity of the top layer has not been detailed in the legend. It varied from as high as 500 Qm (dry loose sand) to as low as 0.8 firn (fine wet sand at the beach, saturated with sea water, i.e., beach sand). Some representative sounding curves through which sections have been drawn, are given in Fig.4 and 5. Fig.6 represents a few more sounding curves (along with the corresponding lithological logs) passing through the points indicated in the diagram. Two geological sections running approximately in the east-west and southwest-northeast directions respectively (along PQ and RS) have been prepared with the help of the geophysical logs and the lithological logs as presented in Fig.7. The section PQ passes through the sounding stations S-O to S-6 and the section RS passes through stations S-5, S-IO, S-15, S-19 and S-23 and represented by the corresponding VES curves. These sections along PQ, RS, along with the bore-hole data at five points (Fig.3, H-Z to H-5) and the logs represented in Fig.6 are used to study the nature and thickness factors, locations

of the aquifers

and other formations.

use has been made of the other sounding

However,

in evaluating

data (not presented)

these

at different

shown in Fig.3.

Two horizons of interest mentioned hereafter as shallow and deep aquifers at two separate depths have been located in the area and are found to be water bearing. A low resistivity of the aquifers (20-30 am) in general, indicates that the water is either highly mineralized or the formation contains clay in banded or dispersed form. The shallow aquifer consisting of medium- to coarse-grained sand with clay is located within a depth of 50-60 m underlaid by a clay layer and overlaid by silty sand grading into a fine sand towards southern part of the area. This fine sand forms a lenticular body of a very limited lateral extent. Its limits are defined by S-IO to the north and S-2 to the southwest. In both cases, this fine sand grades into silty sand as shown by the two geological sections (Fig.7). The shallow aquifer mentioned earlier formed of medium- to coarse-grained clay sand also grades into medium to fine sand and ultimately to silty sand to the Geoexploration,

4 (1966) 209-218

EXPLORATION

FOR GROUND

WATER

AROUND

DIGHA,

INDIA

217

southwestern and western part of the area. This sand, however, continues to exist even beyond the area studied towards northern and eastern sides of the area. Thus the aquifer seems to be of limited extent, very gently dipping towards the south. The second water-bearing horizon, i.e., the deep aquifer is at a much deeper level which consists of medium-grained sand and seems to have considerable extension both laterally and vertically. This deep aquifer, whose depth varies between 100 and 120 m, thins out slightly towards the southwestern part and grades into silty sand, However, to the north and eastern sides the deep aquifer also extends even beyond the area studied. Besides these two major aquifers a coarse fresh water-bearing sand has been located in S-9, S-IO and S-11 at a very shallow depth, i.e., just below the alluvial cover. The representative YES’ curves for these stations along with the corresponding logs (interpreted) have been presented in Fig.& This aquifer appears to be very thin and has limited lateral extensions. Good potable water from this horizon is now being used by the local people. The water here is derived from percolation of rain water through the dune sand during rainy season. Though these stations are very near to the sea-shore, i.e., just north of the second sand dune, no invasion of saline water has been detected. It may be noted here for information that saline water-bearing pockets of sand (as reported earlier, SINGHAL, 1963) have not been located within the area under study.

CONCLUSION

The content of this paper is the result of resistivity sounding around Digha supplemented by bore-hole data at some selected points. At present, detailed geological studies of the adjacent areas (including Digha) are being made in the geological section of the Department. Attempts are being made to find the lateral extent of the aquifers, locate the possible intake or catchment areas and the possible bearing on the life of the ground-water supply.

ACKNOWLEDGEMENT

The authors are grateful to Dr. D. Niyogi for his helpful criticismand Sri A. Chakraborty for making available the geological logs of the bore holes and for stimulating discussions. The authors record their indebtedness to the Government of West Bengal for financing the Digha Project under which the present investigations were carried out.

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H. P. PATRA AND P. K. BHATTACHARYA

REFERENCES BHATTACHARYA, P. K. and PATRA, H. P., 1966. Geoelectric Sounding. Dept. Geol. Geophys., Indian Inst. Technol., Kharagpur, 174 pp. COMPAGNIEG~~NBRALE DE GI?OPHYSIQUE, 1963. Master Curves .for Electrical Sounding, 2nd ed. European Assoc. Exploration Geophysicists, The Hague, 49 pp. KALENOV,E. N., 1957. Interpretatsia Krivykh Vertikainogo Electricheskogo Zondirovanya. Gostoptekhizdat, Moscow, 472 pp. NIYOGI, D. and CHAKRABORTY, A., 1966. Applied geomorphology along Digha beach, District Midnapur, West Bengal. Proc. Geomorphol. Seminar, Centre Advan. Study Geol., Univ. Sagaur, in press. OGILVI, A. A. and KSMELEVSKOI, V. K., 1964. Paletki Teoreticheskikh Krivykh Electricheskogo Zondirovanya. Univ. Moscow, 56 pp. PATRA, H. P., 1966. Geophysical investigations for ground water around Jaldha coast, District Midnapur, West Bengal, India. Ground water, in press. SINGHAL,B. B. S., 1963. Occurrence and geochemistry of ground water in the coastal region of Midnapur, West Bengal, India. Econ. Geol., 58: 419433. ZOHDY, A. A. R., 1965. The Auxiliary Point Method of electrical sounding interpretation and its relationship to the Dar Zarrouk parameters. Geophysics. 30(4): 644-660.

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