ground water system

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Multi Disciplinary Edu Global Quest (Quarterly), Volume 1, Issue 1, January-March 2012

www.mdegq.com Page 63 ISSN 2250 - 3048

QUICK ASSESSMENT OF GROUND WATER BY ELECTRICAL

RESISTIVITY SURVEY.

Mr. Aditya N Contractor

Mr. Jignesh B Patel

Dr. Sandip A Vasanwala

INTRODUCTION

Development in science and technology has helped us find new sophisticated, accurate and

reliable methods. These methods can be classified into two heads: Surface methods and Sub-

surface methods.

In surface investigations, ground water can be investigated by working on the earth surface.

Sometimes these methods also provide quantity and quality of water. Latest methods of surface

investigations are:

1) Electrical resistivity method.

2) Seismic Refraction method.

3) Gravity method.

4) Magnetic method.

Amongst these methods, electrical resistivity method is the most economical, versatile, less time

consuming and reliable method. Surface investigation is advisable to adopt before starting any

big project to study ground water depth and characteristics.

Detailed and comprehensive study of ground water and condition under which it occurs can only

be made by sub-surface investigations. Ground water, its location, movement, quality and

quantitative data can be obtained by lowering an instrument inside the earth and operating it

from the surface.

The problem of ineffective water distribution system can be easily and economically solved by

Geophysical approach-Electrical Resistivity method which also is an alternative solution to

drilling and boring the earth.





PRINCIPLE AND CONCEPT

Every substance has physical property of electrical resistivity, depending upon this

property all substance can be classified. In reverse if this property is known corresponding

material can be predicted. Resistivity of soil vary over wide range depending upon the mineral

content, density, porosity, pore size and shape, water content and quality of water, temperature.

Resistivity is defined as the resistance in ohms between opposite faces of a unit cube of material

(Elvis R. Sverko. 1999).

If a material of resistance “R” has a cross sectional area “A” and length “L” then its resistivity

“ ” can be expressed as

= ohms - meter.

Where, R = Resistance of material (ohms-centimeter.), A = Cross-Sectional Area (square-

centimeter.), L = Length (centimeters.)

Actual resistivities are determined from an apparent resistivity which is computed from

measurements of current and potential differences between pair of electrodes placed on the

ground surface.

Procedure involves measuring a potential difference between two electrodes resulting from outer

two current electrodes. If the ground beneath the electrodes were uniform everywhere then

current travelling under the ground is like rays in all direction from electrode point as centre.

These current rays and lines are drawn by joining equi-potential point on that line we get

orthogonal network. By this method we can know the value of the “apparent” or “effective”

resistivity only. The “apparent” resistivity in fact represents the resultant resistivity of underlying

soil strata‟s. Thus, the measured current and potential differences yield an apparent resistivity

over an unspecified depth. If the spacing between two electrodes is increased, a deeper

penetration of the electrical field occurs and different apparent resistivity is obtained. This is

show in Figure 1 as shown below,





The electrical resistivity methods are of the following two types:

a) Electrical resistivity profiling method

b) Electrical sounding method

AIM OF EXPERIMENTAL STUDY

Aim of this experimental study is to observe the water table, movement of water and sea water

intrusion at „Various stations at Costal area of the Surat city at Dumas region ‟.

Surat city is situated near river Tapti. At every 15 days interval, tidal waves from sea attack the

village side. At every high tidal wave, highly saline water spreads over portable fresh water. This

saline water pollutes not only Tapti water but it also pollutes the underground water of Surat city

.Due to frequent coming of tidal waves, normal water table at some of the stations does not

remain at constant level and fluctuate with the tidal waves.

Due to this sea water intrusion on both the sides there is one particular zone which is always

affected because of this phenomenon. Ultimate target of this study is to find this zone and give

information to the concerned authority regarding this zone. This study can be divided into three

parts:

1. Survey of an area and selection of points for taking observations.

2. Experiments at selected points.

3. Verifying the result obtained with electrical resistivity meter.





EXPERIMENTAL SETUP AND OBSERVATION

Instrument used was Soil resistivity meter. Here we have used Wenner‟s arrangement (Rex A

Crouch, 2008). Before starting the experiment, instrument requires calibrations. Procedure of

calibration of instrument is given in the booklet of the instrument. Different observations were

taken by changing the spacing between the electrodes, from 1 meter to 10 meters. Six set of

observations for electrical resistivity and Bore hole were taken at C.K.P.C.E.T., Hanuman

Temple, Gandhighar Bhimpore, Langhar Circle, Kailash well Sultanabad, Ramji Mandir

Kadifadiya areas in Dumas region of Surat district only typical result is illustrated in figure 2.

DATA INTERPRETATION

For the interpretation of results obtained by the resistivity meter, graph of depth versus resistivity

is required. From the change in the nature of graph and from the value of the slope of the graph,

strata beneath the surface can be obtained. Since the value of resistivity changes with the change

in strata and water content graph of depth versus water content is also needed for more precise

results. Beneath the earth there is no strata of soil which has conductivity as water. That is why

water table is easy to interpret with the help of graph only. Due to less resistivity, when water

strata come in effect of electrode spacing then sudden drop of resistivity occurs. Hence the

interpretation of water table is very easy.

As different soils have different values of resistivity and these values also vary with position,

condition, water content and porosity, therefore interpreting the data for soil strata is very

difficult. Resistivity of soil has very wide ranges. This requires any reference of soil strata within

the area surveyed. By any bore hole or by any how if soil strata are available then it gives an idea

that soil available in the area is of this type and has this much range. Bore log is plotted with the

graph of depth versus resistivity.

The data interpretation has been carried out by comparing various Indian Standards ( IS 15736-2007 and IS 1892-2002), literature (A.K.ARORA), manuals (ABEM, SWEDON). On the basis

of comparison approximate average values of soil and different types of water has been shown in

table 1 below.





TABLE 1 APPROXIMATE VALUES OF THE ELECTRICAL RESISTIVITY TAKEN

IN CONSIDERATION FOR THE INTERPRETATION OF SOIL AND WATER.

MATERIALS. RESISTIVITY Ω (Ohm-meters).

CLAY

1-30 (saturated)

10-50 (saturated)

50-150 (partly-saturated)

This increases with the in depth.

>1000 (dry)

SILT

2-100 (saturated)

This increases with the in depth.

20-50 (saturated or partly-saturated )


This increases with the in depth.>10

(dry)

ALLUVIUM SOIL 10-800

SANDY CLAY & WET SILT SAND 100-250

CLAYEY SAND AND SATURATED SAND 200-500

SAND

0.5-500 (saturated)


>10 (dry)

GRAVEL

50-700 (saturated)

700-5000

>10 (dry)

GROUNDWATER(FRESH)

1-100

10-100

100-300

SEA WATER

0.15-0.2

0.2-1

2-100

Note: - Electrical resistivity range is taken in consideration with respect to the depth and

on the basis of the value the range of the soil is taken from the approximate values

available.




www.mdegq.com 8 ISSN 2250 - 3048

Depth from the G.L to 3 m is taken dry, from 3 to 6 m is taken partly saturated and from

6 m and below is taken saturated with the varying range of depth(N.Mohamad Nor, et

al. 2008).

TABLE 2. LANGHAR CIRCLE DUMAS-BHIMPORE

Depth in

meters.

Electrical

Resistivity (Ω) in

ohms-meters.

I.S. Soil

Classification

.

Range of E.R.

of the soils.

Water

content

in %.

Remarks.

0 0 SM-ML 2-100 0 *G.W.T at a

depth = 4.00 m.

* Here due to

water logging

the range does

not increase

and with

respect to soil

available range

is under

approx. Values.

Of E.R.

1 6.97434 SM-ML 2-100 33.13

2 12.94336 CI 1-30 38.383 18.47256 ML-SM 10-50 26.54

4 24.12743 ML-SM 10-50 24.65

5 29.53097 ML-SM 10-50 24.74

6 34.68318 ML-SM 10-50 -

7 39.14424 ML-SM 10-50 27.59

8 43.22831 SP 0.5-500 -

9 47.50088 SP 0.5-500 -

9.45 - SP 0.5-500 18.27

10 50.89380 SP 0.5-500 -

Table showing differing soil strata with varying depth according to which the profiling of soil

has been done. .

RESULT AND DISCUSSION

The graph plotted shows the values of Electrical Resistivity and Natural water content with

respect to depth. The table represents the prediction of ground water table from the values

obtained in graph.





GRAPH AND TABLE FOR DETERMINING THE GROUND WATER TABLE AND ITS

DISCUSSION

FIGURE 2

CONCLUSION

From our discussion and experiment we have concluded following points.

1. Electrical Resistivity Method can be used to derive the water table without drilling.

2. Through this method we can detect that water is usable and it ranges from 10 to 100

ohms-meters ER reading.

3. By comparing the water quality at different stations it can be concluded that the water at

C.K.P.C.E.T. is not drinkable and that the water found at Dumas region and its nearby

Villages is only usable.

4. The flow of underwater can be determined by electrical resistivity for entire area using

regular grid with respect to the mean sea level which can be made used by S.M.C. and

other governing bodies.

5. The underground pipeline provision can consider whether to use steel pipelines or

concrete pipelines on the basis of soil profile and underground water table quality.

TABLE 3 PREDICTION OF THE

GROUND WATER TABLE

FROM THE ABOVE GRAPH.

Actual G.W.T. 4.00 m.

G.W.T. by Electrical

resistivity

8.00 m.

G.W.T. by Natural

Moisture content

4.00 m.

Error in G.W.T by

Electrical resistivity

-4.00 m.





6. It will also help the government bodies to provide the type of electrical lines, pipelines

and sewage lines.

7. Comparison has been carried out with the standard taken into consideration and the actual

range of the soil strata and ground water table in Indian conditions.

RECOMMENDATION

The survey by Electrical Resistivity of Surat District/City by SUDA/SMC will provide water

level contours of ground for recharging and base water supply.

REFERENCES

1. Elvis R. Sverko. (1999). “GROUND MEASURING TECHNIQUES: ELECTRODE

RESISTANCE TO REMOTE EARTH & SOIL RESISTIVITY.”

2. Indian Standard (I.S.:1892:1979). “CODE OF PRACTICE FOR SUBSURFACE

INVESTIGATION FOR FOUNDATION.”

3. Indian Standard (I.S.:15736:2007). “GEOLOGICAL EXPLORATION BY

GEOPHYSICAL METHOD (ELECTRICAL RESISTIVITY) – CODE OF PRACTICE.”

4. Rex A. Crouch, (2008). “ELECTRICAL RESISTIV ITY OF SOIL- SOIL RESISTIVITY

FUNDAMENTALS AND THE SOIL RESISTIVITY MET ER”. Pp 1-20.

5. N.Mohamad Nor, R. Rajab and K. Ramar (2008). “V ALIDATION OF THE C ALCULATION AND

M EASUREMENT T ECHNIQUES OF E ARTH R ESISTANCE V ALUES ”. American Journal of Applied

Sciences 5 (10): Pp. 1313-1317.

6. S. Sreedeep, A. C. Reshma and D. N. Singh., (2004) “GENERALIZED RELATIONSHIP

FOR DETERMINING SOIL ELECTRICAL RESISTIVITY FROM ITS THERMAL

RESISTIVITY.”

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