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EXPLORING THE SHALLOW FRESH WATER PHERATIC AQUIFERS IN A COASTAL

SEDIMENTARY TERRAIN BY HIGH RESOLUTION GD-10 DC GEOELECTRIC

SYSTEM.

Conference Paper · May 2016

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EXPLORING THE SHALLOW FRESH WATER PHERATIC AQUIFERS IN A COASTAL SEDIMENTARY TERRAIN

BY HIGH RESOLUTION GD-10 DC GEOELECTRIC SYSTEM.

Badrinarayanan T.S 1, Mohamad Abdul Kadhar Prabhu

2 & Sandeep Gupta

3.

1. Geoscientist, B2

geo tech services, Kollidam, Tamilnadu, tsbadri56@gmail.com

2. Country Sales & Marketing Manager-India Chapter,ST Geomative Co Ltd,

Shenzhen,China,abdul.hassan@geomative.com

3. Ground water Enginner, Earth Care Protection Services, Kolkatta,

sandeepguptakol@rediffmail.com

ABSTRACT: Ground water is a prime source for irrigation and tube well irrigation is a common practice in

India. Exploring fresh water aquifers in a coastal sedimentary terrain becomes problematic due to sea

water ingress which is a general scenario in coastal zones. This research paper pertains to a case history

from a coastal sedimentary zone for an agricultural project in Mamakudi village, Sembanarkoil union,

Tarangambadi taluk, Nagai district, Tamilnadu, India. There are 4 shallow tube wells for the farming

activities and is reported to be inadequate to cater the needs especially in summer. To overcome the

problem, a systematic geological, hydrogeological and geoelectrical investigations have been carried out

in the farm with an extent of 25 acres of land. The area comprises Quaternary sedimentary formations

of Recent age. The major geomorphic units are coastal plain under marine influences and flood

plain of fluvial regime with an intermixing section of both fluvial and marine influence. The main

lithological units are soil, alluvium, sand, clay & kankar. The area is covered by sandy soil. Area of

investigation falls under the Cauvery basin and it is in the tail end, south west of the Cauvery estuary.

Sand is the principal & potential aquifer. The shallow fresh water aquifer occurs under unconfined

conditions in alluviums and sand. The deep saline aquifers occur in semi & confined conditions. The top

shallow unconfined freshwater is tapped by coupling 2 or 3 shallow tube wells which is a general

practice. To prospect the shallow fresh unconfined aquifers, two 1D VES have been conducted using

GD-10 DC geoelectrical system, Geomative, ST Geomative Co,Ltd, China employing Schlumberger

configuration with AB/2 separation of 240 meters. The VES data have been processed by Rinvert

software package and both primary and secondary geoelectical parameters have been computed to

delineate the subsurface lithology, water quality and ultimately the shallow unconfined fresh water

aquifer thickness has been detected which is about 10 to 12 meters below ground level. As the shallow

unconfined aquifer only is fresh, it may not be feasible to tap the required quantum of water to cater 25

acres by shallow tube wells, it is recommended to go for a larger diameter ring well. It is suggested to go

for a shallow trial bore to a depth of 12 m engaging hand bore set and finally to go for a ring well of 4

meter diameter up to a depth of 10m. The porous concrete rings have to be erected depending on the

sand aquifer thickness. Thus by this high resolution geoelectric system the shallow fresh pheratic

aquifer could be explored very rapidly and accurately by 1D VES technique.

KEYWORDS: Pheratic aquifer, confined condition, sea water ingress, geoelectric parameters.

AIM OF THE STUDY AND PROBLEMS FACED: The aim of the study is to explore the shallow fresh water

aquifers for an agricultural project where the water tapped from shallow tube wells is insufficient to

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cater the needs during summer period. A tube well earlier constructed to a depth of 70 feet has

discharged saline water. Being a coastal terrain sea water intrusion also is a problem.

AREA OF INVESTIGATION: The farm is about 20 km south east of Sirkali, on ECR NH 45A highway and

the total extent of farm is about 25 acres. The farm is situated on N latitudes 11° 06’ 50.26’’to 11° 07’

00.27’’ & E longitudes 79° 48’ 23.28’’ to 79° 48’ 36.36’’and it is about 5km west of sea coast. The

topography is nearly plain with altitude in the range of 4 to 6 m AMSL [above mean sea level].

Fig-1, area of investigation- ves locations.

METHODOLOGY:

Geological and hydrogeological investigations.

Ground water quality –TDS & EC parameters analysis by digital pens.

Generating TDS & EC contour maps for studying the low anomaly zones.

Geoelectrical investigations – conducting deep VES to delineate the subsurface

lithology.

Interpretation & processing of VES curves- both qualitative & quantitative analysis.

Ascertaining both primary & secondary geoelectrical parameters.

Delineating the subsurface lithology to assess the ground water potential.

Recommending feasible ground water abstracting structures.

GEOLOGICAL AND HYDROGEOLOGICAL SETTINGS: The area of investigation is a coastal sedimentary

terrain comprising Quaternary sedimentary formations of Recent age. The major geomorphic units

are coastal plain under marine influences and flood plain of fluvial regime with an intermixing

section of both fluvial and marine influence [5]. The main lithological units are soil, alluvium, sand,

clay & kankar. The area is covered by sandy soil. Area of investigation falls under the Cauvery basin and

it is in the tail end, south west of the Cauvery estuary. The Sembanarkoil union categorized under over

exploited category [6]. Sand is the principal & potential aquifer. The shallow fresh water aquifer occurs

under unconfined conditions in alluviums and sand. The deep saline aquifers occur in semi & confined

conditions. Rainfall is the chief source of ground water recharge. A small canal is running bordering the

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northern boundaries which may also contribute recharge to the aquifers. The area experiences

maximum rainfall during north east monsoon period i.e. September to December. Being a coastal

aquifer system, sea water intrusion is a common scenario. The top shallow unconfined freshwater is

tapped by coupling 2 or 3 shallow tube wells which is a general practice. Generally shallow pheratic

aquifer is fresh in nature & deep confined aquifers saline [1].

Fig- 2, landsat imagery of Nagai district. Fig-3, hydrogeology map of Nagai district.

HYDROGEOCHEMISTRY: There are four shallow tube wells constructed in the depth range of 7

to 9 meters, being the main ground water resources. Water samples of all the 4 tube wells have

been collected and the electrical conductivity [EC] and total dissolved solids [TDS] ascertained

by digital pens instantly [Table-1]. Iso EC and Iso TDS maps [Figs-of have been generated by

Surfer software version 9.0. to study the distribution of these parameters.

SL

NO

TUBE WELL LOCATION TW NUMBER EC

IN MICRO SIEMENS

TDS

IN PPM.

1 TW in north east side with sub pump. 1 820 403

2 Eastern side tube well 2 724 350

3 Tube well near banyan tree 3 1460 762

4 Southern side tube well 4 1002 486

Table-1, ground water quality of the existing tube wells.

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Fig-4, iso EC map. Fig-5, iso TDS map

GEOELECTRICAL INVESTIGATIONS:

ELECTRICAL RESISTIVITY PROSPECTING: The electrical resistivity method is highly useful to

investigate the nature of subsurface formations by studying the variations in their electrical

properties. Resistivity (denoted by r) is a physical property of a substance. The resistivity is

defined as the resistance offered by a unit length of a substance of a unit area to the flow of

electric current when the voltage is applied at the opposite faces. The resistivity of a geological

formation may be considered as a function of moisture content (porosity and water saturation)

and salinity of the saturating water [8]. As the salt content increases in the water contained in the

Fig-6, four electrode arrangements.

Pores, the resistivity of the formation as a whole decreases substantially [7]. The current (I) is

generally injected into the ground through two outer current electrodes and the potential

difference (V) is measured between the inner potential electrodes. The ratio between the

potential difference and the current sent (V/I), gives the Resistance (R). Apparent resistivity is

calculated by multiplying the value of ‘R’ with geometric factor. If the formation is

inhomogeneous, the potential is dependent on a fictitious resistivity value ‘ra’ known as

‘apparent resistivity’ [12].

1D VERTICAL ELECTRICAL SOUNDING [VES]: Vertical electrical sounding is a technique of

studying the vertical variations in resistivity of subsurface by which thicknesses of various

subsurface layers and their resistivities are estimated. In this approach, the center of the

configuration is kept fixed and the measurements are made by progressively increasing the

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electrode spacing [8]. The apparent resistance values obtained with increasing electrode

separations are used to estimate the thicknesses and resistivities of the subsurface formations.

GEOELECTRICAL PARAMETERS: The fundamental parameters to describe a geoelectric section

are its resistivity p and its thickness h. In multi-layered cases the individual resistivities and

thicknesses of the layers are represented by p1 and h1 respectively, where the subscript stands

for the number of layer. From these fundamental parameters, other secondary geoelectric

parameters namely longitudinal conductance (S), Transverse Resistance (T) are derived. These S

and T are also called ‘Dar Zarrouk’ Parameters [11]. Niwas and Singhal (1981) established an

analytical relationship between aquifer transmissivity and transverse resistance on the one hand and

between aquifer transmissivity and aquifer longitudinal conductance on the other. On a purely empirical

basis it can be admitted that the transmissivity of an aquifer is directly proportional to the transverse

resistance [10].

Fig-7, columnar prism used in describing geoelectric parameters.

1D VES IN AREA OF INVESTIGATION: In the area of investigation two 1D VES has been carried

out using GD-10 series Geomative, ST Geomative Co,Ltd, China employing Schlumberger

configuration with AB/2 separation of 240 meters. GD-10 series is a new electrical geophysical

exploration system with highly engineered & cutting edge technology with a revolutionary

change [4]. GD-10 series supports1D VES, 2D ERI and 3D ERT Resistivity/IP measurement.

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Geomative Studio is the whole data management platform, including device management, project

management and script management.

Fig-8, GD-10 series resistivity system.

The main features of GD-10 series- Can measure the resistivity, time domain induced polarization

method (IP) and spontaneous potential (SP); Maximum current up to 6A, the maximum voltage up to

1200V, the maximum power up to 7200W. Designed with built-in GPS, can accurately determine the

measurement of co-ordinates, Open communication platform for data exchange, data exchange via USB

port.

The SP, V, I & pa parameters saved in the main frame have been transferred to Lap top for further

processing. The VES data have been analyzed, interpreted by software package Rinvert for Windows 2.1

version [9, 11]. Forward modeling, inverse modeling and equivalence analysis have been done [11] and

ultimately primary and secondary geoelectrical parameters like longitudinal conductance [S] and

transverse resistance [T] have been computed.

Fig-9, inverse modeling of VES-1.

The true resistivity, longitudinal conductance [S] and transverse resistance [T] profiles have been

prepared and finally subsurface lithology has been delineated.

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Fig-10, VES-1 true resistivity profile. Fig-11, Longitudinal conductance profile.

Fig-12, VES-1, transverse resistance [T] profile. Fig-13, VES-1, geoelectric section & interpreted lithology

10 M DEPTH

GROUND LEVEL

4 M DIA PRECAST POROUS CONCRETE WELL RINGS

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Fig-14, proposed ring well to be constructed.

RESULT AND DISCUSSION:

ISO EC MAP: To study the salinity of water, the EC contour map of the area of investigation has been

prepared [Fig- 3]. The minimum and maximum EC are 700 and 1500 micro Siemens. The contour interval

is 50 Ms. The low EC anomaly is predominant in north east and south eastern sides indicating the good

quality water.

ISO TDS MAP: To study the dissolved solids nature and quality of water, the iso TDS map has been

generated [Fig-4]. The minimum and maximum TDS are 340 and 780 PPM. The contour interval is 20

PPM. The low TDS anomaly is well exhibited in north east and south eastern sides indicating the good

quality water.

In both the contour maps the same anomaly pattern is exhibited reflecting the more fresh water nature.

VES CURVE AND GEOELECTRIC SECTION: The VES-1 curve is a combination KQ type multi layered VES

curve. [Fig-8].

TRUE RESISTIVITY PROFILE: The minimum and maximum true resistivities are 62 and 1Ωm. The true

resistivity profile exhibits a fluctuating trend [Fig-9]. The minimum resistivity of 1Ωm reflects the salinity

of aquifer and the maximum resistivity of 62 Ωm indicates fresh water aquifers [2].

LONGITUDINAL CONDUCTANCE [S] PROFILE: The significance of saline aquifer system can be very

well predicted by Dar-Zarrouk parameters [11]. The values range of Longitudinal Conductance less

than five Siemens provides the different geology having less saline water intrusion zones [11]. The

minimum and maximum longitudinal conductance are 1 and 11 respectively. The longitudinal

conductance profile [Fig-10] shows an increasing trend with a very ascending type, reflecting that the

salinity increases with depth. At 14 meters depth the conductance reaches 5 which may be the fresh and

saline water inter face. At 120 meters depth it touches the maximum 11 revealing the highly saline

nature of aquifers.

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TRANSVERSE RESISTANCE [T] PROFILE: The minimum and maximum transverse resistance are 11.2 and

480 respectively. The profile [Fig-11] shows a fluctuating trend. Though the transverse resistance

reaches a peak at the depth of 60 meter, it may be a saline aquifer.

CONCLUSION AND SUMUP: As it is evident and reported that a tube well gone up to a depth of 70 feet

in the area is saline in nature, hydrogeologically this area may not be feasible to go for a deep tube well

to extract fresh water. From the predominance of low anomalies of EC and TDS contour maps, it is

evident that the north eastern and south eastern zones are more favorable to tap good quality water at

shallow depth. As the high true resistivity values in the range of 21 to 63 Ωm has been computed up to a

of depth 14 meter below ground level [MBGL] which may be the favorable and feasible saturated sand

aquifers. The longitudinal conductance is low and predominant in the depth range of 1 to 14 mbgl which

crystal clearly reveals that quality of water as well as the saturated sand aquifer thickness are

prospective. As the transverse resistance in the range of 49 to 168 is very predominant up to a depth of

13 mbgl, indicating a high potential nature of aquifers, it may be the potential pocket. Since the S value

increases drastically below 14 mbgl, below 14 meter the quality of water may be saline. Considering the

recharge factor and as the north eastern parts where the least /low EC & TDS anomaly zones are

predominant, it is the more prospective zones of good quality water, It is suggested to go for a shallow

trial bore to a depth of 12 m engaging hand bore set and finally to go for a ring well of 4 meter diameter

up to a depth of 10m. It is recommended to collect soil samples for every 1 meter and the discharge of

the trial bore may be ascertained. Finally a 4 meter dia ring well may be constructed and the porous

concrete rings have to be erected depending on the sand aquifer thickness. Thus by this high resolution

GD-10 series DC geoelectric system good quality accurate geoelectric data could be obtained rapidly and

geoelectric parameters could be computed with ease. The secondary geoelectric parameters also play a

pivotal role in exploring the aquifers.

REFERRENCES:

1. G.K. Anudu, L.N. Onuba and L.S. Uufondu- Geoelectric sounding for groundwater exploration in

the crystalline basement terrain around onipe and adjoining areas, southwestern Nigeria-

Journal of Applied Technology in Environmental Sanitation, 1 (4): 343-354.

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tsunami affected areas of Nagappattinam district, & a report on geoelectrical investigation to

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delineate the fresh and salt water interface and subsurface lithology in the tsunami affected

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civil engineering, Annamalai University, Annamalainagar-2008.

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1997.

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