Sub Surface Contamination Using Electrical Resistivity Test

Published Date: 02 Nov 2017

With Specialization in

ENVIRONMENTAL ENGINEERING

By

ARUN KUMAR GUPTA

(Roll No. 6004206005)

Under the supervision of

Dr. S. M. Ali Jawaid

Department of civil engineering

madan mohan malaviya engineering college

gorakhpur – 273010 (U. p.)                       (An Autonomous College of G. B. Technical University, Lucknow)

December, 2012

MADAN MOHAN MALAVIYA ENGINEERING

COLLEGE, GORAKHPUR

2010 – 2012

CERTIFICATE

I, hereby, certify that the work which is being presented in the dissertation entitled "Deduction of Sub-Surface Contamination Using Electrical Restivity Test" by Arun Kumar Gupta in partial fulfillment of the requirements for the award of the degree of Master of Technology in Civil Engineering with specialization in Environmental Engineering at Madan Mohan Malaviya Engineering College, Gorakhpur – 273 010 (Uttar Pradesh), India (An Autonomous College of Gautam Buddh Technical University, Lucknow), is an authentic record of my own work carried out during the period from September, 2011 to December, 2012 under the supervision of Dr. S.M. Ali Jawaid, Associate Professor, Department of Civil Engineering, Madan Mohan Malaviya Engineering College, Gorakhpur. The matter presented in this dissertation has not been submitted for the award of any other degree of this or any other University.

Date: (Arun Kumar Gupta)

This is to certify that the above statement made by the candidate is correct to the best of our knowledge and belief.

Date: (Dr. S.M. Ali Jawaid)

The Dissertation viva-voce examination of Arun Kumar Gupta, M. Tech. student has been held on ……........

Signature of Supervisor Signature of H.O.D Signature of External Examiner

Dedicated to

My Brother Dipu....

ACKNOWLEDGMENTS

I would like to express my sincere gratitude to my dissertation supervisor, Associate Professor Dr. S. M. Ali Jawaid, for his constant guidance, constructive suggestions, critical appraisal and regular encouragement throughout this work. It was a great pleasure for me to carry out this study under his supervision.

I would like to thank all the faculty members of the Environmental Engineering who provided constructive comments during the tenure of the course work and dissertation work. I am also thankful to all the technicians and other supporting staff of Environmental Engineering Laboratory and Civil Engineering Department, who gave full cooperation in carrying out the dissertation work.

I feel happy to acknowledge my lab mates for their support and constant encouragement during different stages of my work. In addition, I thank all my friends of MMMEC Gorakhpur and Amit who have always provided an enjoyable and friendly working environment.

Special thanks to my brother Alok for uncountable reasons: like silently helping without being noticed, sweet and short fighting from childhood. I feel shorts of words to express gratitude to my parents, whose unconditional love, affection and encouragement throughout my educational career has made me what I am today.

Directly and indirectly many people have helped me in completing the work and acknowledging the contributions of all may be impossible. There are many more, who have been silently helping without being noticed, I take this opportunity to thank each and every one of them.

Arun Kumar Gupta

ABSTRACT

Geophysical explorations are the study of relatively shallow sub-surface profile in homogeneous soil structures. Its aim is to make specific investigations of economic importance e.g. exploration of oil, gas, ground water and for solving certain civil engineering problems. To ensure safety, success and economy in the construction of major civil engineering structures, it is necessary to be thoroughly aware of the geology of the concern site. In order to acquire the surface details, there two approaches.

1. Direct observations (destructive test)

2. Indirect observations (non-destructive test)

Direct observations (destructive test) can be made by digging, trenching or drilling the ground. These are expansive, laborious and time consuming. But they give exact data of the existing sub-surface condition of the site. Indirect inferences (non-destructive test) are drawn by means of geophysical method of investigation, in which measuring certain physical properties (electrical resistivity, density, magnetic susceptibility etc.) & interpreting them. These are quick, inexpensive, easy and fairly reliable means to get surface details.

Various contaminated soils were selected for this study. Electric resistivity tests were conducted using wenner configuration. The resistivity data was recorded with depth, Auger boring was also carried out to get the soils samples for classification of soil with depth. Resistivity data were compiled with the type of soil. It is found that the resistivity of contaminated soil ranges between 0-5.5 Ω-m where as the resistivity value of virgin soil is above 25 Ω-m. It is also found that resistivity decreases with saturation.

CONTENTS

Chapters Page No.

Certificate ii

Acknowledgements iv

Abstract v

List of Figures viii

List of Tables ix

1. INTRODUCTION 1 2. LITERATURE REVIEW 3

2.1 Subsurface contamination 3

2.1.1 Introduction 3   2.1.2 Methods of site characterization 3

2.1.3 Geophysical / Non-intrusive methods 4

2.2 Electrical Resistivity Method 6

2.2.1 Measurement of Electrical Resistivity of Soil 6

2.2.1.1 Electrode 7

2.2.1.2 Electrode spacing 8

2.2.2 Electrode Arrangement 8

2.2.2.1 Wenner Arrangement 8

2.2.2.2 Schlumberger Arrangement 9

2.2.2.3 Comparison between Different Arrangements 10

2.3 Applied Voltage for Resistivity Test 10

2.4 Types of Resistivity Surveys 10

2.4.1 Sounding 10

2.4.2 Profiling 11

2.5 Depth of current penetration Vs current electrode spacing 11

2.6 Interpretation Theory 13

2.6.1 Conventional approach of Data Processing 13

 2.6.2 Master curve technique 13

2.6.3 Interpretation using two-layer master curve 14

2.6.4 Types curves in resistivity sounding 15

2.6.5 Inversion Technique 17

2.6.6 Inverse Slope Method 17

2.6.7 Break Point Method 20

2.7 Typical Range of Resistivity Values 21

2.8 Limitations 21

2.9 Advantages 22

2.10 Past Studies 22

3. EXPERIMENTAL PROGRAM 24

3.1 Objective 24

3.2 Resistivity Meter 24

3.2.1 Measuring Unit 25

3.2.2 Power Supply Unit 26

3.2.3 Principle of Measurement 27

3.2.4 Current Unit 27

3.2.5 Measuring Unit 27

3.2.6 Field Layout and Data Acquisition 27

3.3 Field Testing Procedure 29

4. DATA ANALYSIS AND INTERPRETATION 31

5. CONCLUSION 45

6. REFERENCES / BIBLIOGRAPHY 46

Appendix 47

List of Figures

Figure

Caption

Page

Fig1.1

Relation between resistance and resistivity

1

Fig2.1

General range of electrical resistivities of common rocks and water

5

Fig2.2

Schematics of Resistivity Test

7

Fig2.3

Wenner Arrangement

9

Fig2.4

Schlumberger Arrangement

9

Fig2.5

Fraction of current flowing below depth Z1 for an electrode spacing L

11

Fig2.6

Comparison of penetration depth with electrode spacing.

12

Fig2.7

Current flow pattern

12

Fig2.8

Two Layer Master Curves

15

Fig2.9

Three Layer Curves in Resistivity Sounding

16

Fig2.10

Determination of geo-electric layers by Inverse Slope Method

18

Fig2.11

Plotting 1/R Vs. a (a= electrode separation) (after cherry IGIS)

19

Fig2.12

Plotting (AB/2)/ρa Vs. AB/2 (AB/2= electrode separation)

20

Fig3.1

Measuring unit

25

Fig3.2

Power supply unit

26

Fig3.3

Assembled parts of the instrument in the field

28

Fig3.4

Electric Resistivity Test in progress at Charfatak

29

List of Tables

Table

Caption

Page

Table2.1

Surficial geophysical methods

5

Table2.2

Typical Value of Soil Resistivity

21

Table3.1

Functional switches on measuring unit

25

Table3.2

Functional switches on power supply unit

26

Table3.3

Description of sites were test was conducted

30

Table4.1

Bore log with electrical resistivity for Ramgarhtal area of contaminated soil, Paidleganj, Gorakhpur

32

Table4.2

Bore log with electrical resistivity for MSW open dumping site at Belpar, Gorakhpur

33

Table4.3

Bore log with electrical resistivity for MSW open dumping site at Charfatak, Gorakhpur

34

Table4.4

Bore log with electrical resistivity for MSW open dumping site at Deoria Bridge

35

Table4.5

Bore log with electrical resistivity for MSW open dumping site at Nandanagar, Gorakhpur

36

Table4.6

Bore log with electrical resistivity for Ramgarhtal area, proposed retaining wall site, Gorakhpur

37

Table4.7

Bore log with electrical resistivity for MSW open dumping site at Nausarh mandi, Gorakhpur

38

Table4.8

Bore log with electrical resistivity for Rangarhtal area, Paidleganj, Gorakhpur

39

Table4.9

Bore log with electrical resistivity for Non contaminated area at MMM Engg. College campus, Gorakhpur

40

Table4.10

Bore log with electrical resistivity for MSW open dumping site at Rapti Bandh, Azad chowk, Gorakhpur

41

Table4.11

The resistivity value for various soil layers

42

Table4.12

The resistivity value for different contaminated soil

44

CHAPTER 1

INTRODUCTION

Geophysical techniques are important in deciphering the subsurface and structure. The electrical resistivity method, one of the significant geophysical methods, is based on the measurement of electrical property called electrical resistance, of the subsurface material. The electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made. Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as,

ρ = RA/L ................................................. 1.1

Where ρ=resistivity, L=length, A= cross-sectional area

Resistance is not the fundamental property of material through which current passes. It depends on the geometry of the material. Resistivity is defined as the resistance in the wire, times the cross-sectional area of the wire, and divided by the length of the wire. It is generally indicated by the Greek symbol ρ (Rho) in units of Ω· m. Resistivity describes how easily the material can transmit electrical current. A high value of resistivity implies that the material is easily that the material implies that the material is very resistant to flow electric current. Low values of resistivity imply that the material transmits electrical current very easily.

Fig 1.1 Relation between resistance and resistivity

It is an intrinsic property of a material. The resistivity of a conductor depends on its composition and its temperature. As a characteristic property of each material, resistivity is useful in comparing various materials on the basis of their ability to conduct electric current. As temperature increases, the resistivity of a metallic conductor usually increases. That’s why resistivity is used to interpret the electrical properties of the subsurface.

This method is very useful for,

Characterize subsurface hydrogeology,

Determine depth to bedrock/overburden

Determine depth to groundwater

Quantitative assessment of groundwater reserve of unconfined aquifer

Map stratigraphy

Map clay aquitards

Map saltwater intrusion

Map vertical extent of certain types of soil and groundwater contamination.

Estimate landfill thickness.

To understand the location of abandoned coal mine.

The primary objective of this study was to study the feasibility of using electric resistivity for deducting the contaminated soil. In order to achieve this objective, the field testing was carried out using electric resistivity test in conjunction with auger boring. Various contaminated soils were selected for this study. Electric resistivity tests were conducted using wenner configuration. The resistivity data was recorded with depth. Auger boring was also carried out to get the soils samples for classification of soil with depth. Resistivity data were compiled with the type of soil. It is found that the resistivity of contaminated soil ranges between 0-5.5 Ω-m where as the resistivity value of virgin soil is above 25 Ω-m. It is also found that resistivity decreases with saturation.

CHAPTER 2 LITERATURE REVIEW

2.1 Subsurface contamination

2.1.1 Introduction

Whether to investigate contaminated site or potential contaminated migration at an existing site, a proper characterization of the subsurface conditions and their interaction with surface features is critical to the process. For any contaminated site, it is absolutely essential to know what lies on and beneath the ground at a particular site.

A simplified cross-section shown in Fig 2.1 illustrates the contamination movement below a contaminated source and its propagation as contaminant plume. Inadequate incorrect subsurface characterization of soil contamination can lead to ineffective remediation of contaminated sites. Subsurface investigations must generate a clear definition of the site’s geological, hydrogeologic & geotechnical characteristics. The investigation must reveal the depth & extent of influence of contamination on subsurface.

2.1.2 Methods of site characterization

The methods of detection of subsurface contamination may be classified as –

(1) Indirect (non-intrusive)

(2) Direct (intrusive)

In employing indirect method, we measure some other parameter that can then be related to the parameter of interest. These methods could also be classified as non-intrusive because they do not generally require significant intrusion into the subsurface. In contrast, there are direct methods, such as boring, sampling, and in-situ testing, which generally more directly measure the parameter of interest. These methods could be classified as intrusive because they require substantial intrusion into the subsurface.

The advantage of indirect non-intrusive methods is that they can be used for planning for intrusive methods, interpolating between points of hand data, and optimization of drilling and sampling cost. The direct methods are usually somewhat limited because of their intrusive nature and their associated time and cost.

2.1.3 Geophysical / Non-intrusive methods

Non-intrusive / indirect investigation methods are often called geophysical methods. Geophysical techniques provide information about the subsurface conditions through the measurement of certain properties in the media (e.g. resistivity etc.) deduction of parameters of interest (e.g. depth of strata etc.) geophysical techniques generally rely on contrast to be successful. Interpretation of measure contrast is best left to those specifically trained for such fragments.

For most geophysical techniques, verification of interpretations is typically required by comparison with ground truth information. That is confirmatory drilling, sampling, and / or testing are completed at selected locations to verify the interpretations.

The various most commonly employed geophysical methods utilized. In general dry soils and rocks have very high resistance to electric flow whereas inorganically contaminated saturated soils have a relatively low resistance. The typical range of resistivity value for various soils / rock are tabulated in fig 2.2

 Fig 2.1 General range of electrical resistivities of common rocks and water (After Karanth, 1987)

Insitu for contamination detection are summarized in table 2.1

Table 2.1 surficial geophysical methods

Electric resistivity test have wider application in geo-environmental investigation hence, adopted for this study.

2.2 Electrical Resistivity Method:

Resistance is the property that impedes the flow of current through of a material. Resistivity is a function of material type porosity, water content & concentration of dissolved constituents in pure water and the electrical current flow geometry. When electricity passes through earth, it encounters resistance to its flow from the soil materials. The resistance offered to the current flow is dependent on the mineralogy, particle arrangement, water content and salinity of the underlying layers. (woods, 1994)

The true resistivity of an isotropic, homogeneous medium is constant. When the resistivity is not constant throughout the medium, as in invariably the case in practice; the effective resistivity actually measured in term the apparent resistivity or the mean resistivity.

2.2.1 Measurement of Electrical Resistivity of Soil

To illustrate the resistivity technique, consider a semi-infinite solid with uniform resistivity ρ in which a current I is introduce through the outer two electrodes at position C1 & C2 (Fig 2.1). The potential gradient E associated with this current is measure across two other middle electrodes as position P1 and P2 on the same surface. The arrangement (Fig 1) represents a very crude electrical resistivity unit. The one and only function of a resistivity instrument is to measure the ratio E/I (or the resistance R, according to the Ohm’s law).

Arrangement of Current Electrodes (C1&C2) and Potential Electrodes (P1&P2) The Potential E1&E2 at electrodes P1&P2respectively are given by

E1 = I ρ/2 (1/r1-1/r2) ....................................................... (2.2)

E2 =I ρ/2π (1/r3-1/r4) ....................................................... (2.3)

Where, r1, r2, r3 & r4 are distance between electrodes as marking in Fig 2.1

Fig 2.2 Schematics of Resistivity Test

The potential difference E measured by the voltmeter between electrodes

P1 & P2 is simply E1-E2.Subtracting Eq. 3 from Eq. 2 and solving for ρ gives:

ρ = 2πE/I [1/ {(1/ r1-1/r2) – (1/ r3 – 1/ r4)}] ............................. (2.4)

This is the fundamental equation of resistivity method. It gives the resistivity in terms of quantities (E, I and electrode spacing), which can be measured. The value of ρ is independent of positions of the electrodes and is not affected when the current and potential electrodes are interchanged.

:

2.2.1.1 Electrode

Four metal spices driven into ground along a straight line sever as electrodes. The electrode should reach moist earth it at all possible, otherwise water may be poured around the electrodes which using the direct current, the potential electrodes may be non-polarisable type, this to avoid electrochemical effects. A non-polarizing electrode consists of a porous pot containing a metal electrode immerses in an electrolyte of one of its own salts. For example a copper electrode is immersing in copper sulphate. The electrodes are placed in a ground in shallow pit about 150-200 mm deep. For ensuring proper contact with the ground a little water is poured in the pits few minutes before measurement is made.

2.2.1.2 Electrode spacing

The following guidelines should be kept in mind in adopting the electrode spacing.

1. Adopting the largest spacing at least three times (preferably 5 or 10 times) the      maximum depth of interest.

2. Keep the smallest electrode separation less than one-half the minimum depth at      which a change in material is expected, expects that spacing less than about 0.75 or      1 m is usually not needed.

3. Take readings at small intervals (spacing) for small electrode spacing and at large      intervals for large spacing.

2.2.2 Electrode Arrangement

2.2.2.1 Wenner Arrangement

The four electrodes are equally spaced along line with a constant spacing "a" between adjacent electrodes. The arrangement is shown in Fig.2.2 C1 and C2 are current electrodes, and P1 and P2 are the potential electrodes. For this case, Eq. 4 becomes:

ρ = 2πa E/I

= 2Ï€aR ................................................ (2.5)

Fig 2.3 Wenner Arrangement

2.2.2.2 Schlumberger Arrangement

The electrodes have unequal spacing (Fig. 2.3). The spacing ‘b’ of the potential electrodes are moved (i.e. L is changed) when changes in electrode spacing is required during field measurements. The ratio L/b is kept within the range of 3 to 30. For this arrangement, Eq. 4 becomes:

ρ = π b [(L/b) 2 - ¼] R ....................................... (2.6)

2.2.2.3 Comparison between Different Arrangements

The schlumberger arrangement permits easier discrimination between lateral and depth variations, and is also faster to use in the field. Interpretation of the reading is more complicated..

2.3 Applied Voltage for Resistivity Test

The tests voltage may be either D.C. or A.C. the A.C. test voltage gives an advantage of producing no polarization effect on the electrodes. The A.C. may, however, give erroneous results due to (i) redistribution of current flow in the earth (skin effect), (ii) electromagnetic coupling between the electrical cables, (iii) phase shift between current and voltage, the D.C. test voltage producing polarization effect on the electrodes

2.4. Types of Resistivity Surveys

There are two basic methods of conducting resistivity surveys:

i. Sounding

ii. Profiling

2.4.1 Sounding

Resistivity sounding is meant to provide information on the variation in subsurface conditions with depth. It is also called VES (vertical electrical sounding). In this method, a number of ρ values are measured at the same place by increasing the distance between the current electrodes each time after taking the reading. This kind of successive increase in distance makes the current penetrate more and more deeply. Hence the changes in ρ values measured indicate in the vertical (i.e. depth-wise) variations in the subsurface at the investigated point.

The depth of investigation in the Wenner arrangement is approximately equal to the electrode spacing "a" or 1/3 of current electrode spacing, and in Schlumberger arrangement it is approximate equal to L/2 to 2L/3, the distance of current electrodes from the centre.

2.4.2 Profiling

It is also called mapping or traversing, in which the electrode spacing remains constant during the survey. But centre of electrode spread is varying. This is done to deduct lateral change in resistivity. Keeping the electrode spacing constant, readings are taken either along a single line or along several parallel lines. Since the effective depth of investigation is related to the electrode spacing, this depth remains essentially constant for all the readings. Two variations of the method are used. In one method, known as the "single electrode spacing traverse," only one reading at a particular spacing ‘a’ is taken at each station. It is to determine the changes in the resistivity characteristics of materials from one location to other. In second method, known as "double electrode spacing traverse", two reading two electrode spacing a1 and a2 are taken at each station. This is done to distinguish between layers of different resistivity.

2.5 Depth of Current Penetration Vs Current Electrode Spacing:

When two current electrodes are moved in close proximity to one another, current flows along arc-shaped paths connecting the two electrodes. The following figure explains the current penetration as a function of electrode spacing. From the graph it can be understand that when L=2Z1 , half the current flow in the top upper layer and half penetrates below a depth.

1

Fig 2.5 Fraction of current flowing below depth Z1 for an electrode spacing L.

What this implies is that by increasing the electrode spacing, more of the injected current will flow to greater depths, as indicated in the Fig 2.5. Because the total resistance in the electrical path increases as electrode spacing is increased, to get current to flow over these longer paths requires a larger generator of electrical current. Thus, the maximum distance that current electrodes can be separated by is in part dictated by the size of the generator used to produce the current.

Fig 2.6: Comparison of penetration depth with electrode spacing.

Fig 2.7 current flow pattern

Assuming for a moment that we have a large enough generators to produce a measurable current in the ground at large current electrode spacing, this increase in the depth of current penetration as current electrode spacing increases suggests a way in which we could hope to decipher the resistivity structure of an area. If the current and potential electrodes are spread apart and the apparent resistivity remeasured then these measurements will incorporate information on deeper Earth structure.

2.6 Interpretation Theory:

2.6.1Conventional approach of Data Processing:

Conventionally the plot between apparent resistivity and electrode separation (a) in case of Wenner configuration and between apparent resistivity and half current electrode separation (AB/2), in case of Schlumberger configuration on double logarithmic scale is used for analysis of thicknesses and resistivity’s of the subsurface layers. The data density is between 6-8 per log cycle of electrode separation. Even if the data density is higher than that it would not give any additional advantage because of logarithmic plotting. There are various techniques for interpretation. For qualitative interpretation H, Q, A, K type curve technique is used and for quantitative interpretation Master curves matching technique, Inversion technique and Inverse slope technique can be used.

2.6.2 Master curve technique:

One of a set of theoretical curves, calculated for known models, against which a field curve can be matched. If the two fit very closely the model is considered to apply reasonably well to the field situation and the curve is known as the master curve. Master curves were used extensively in electrical resistivity depth sounding, but are being replaced by micro-computer curve matching which is much more accurate and more sensitive to real-life situations. To fit the field curve in the given master curve, we have to shift the x and y-axis of field curve to get a perfect fitting. After getting perfect fit, we determine the layer parameter. Theoretical curves, known as "Master Curve" or tables have been published for two layers, for three layers and for two, three and four layers. Set of curves are also available in the work of Bhattarcharya and Patra (1968) who have attempt to give a comprehensive idea and elaborate techniques of interpretation of both Schlumberger and Wenner method.

The theoretical curves are used for quantitative interpretation. The interpretation is carried out as follows:

1. Plot the field data on a double logarithmic graph on same the size double       logarithmic graph as used for the theoretical curve, with apparent resistivity ρ as       ordinate and electrode spacing a as abscissa.

2. Superpose the field curve on the various sets of theoretical curves until a good      match is found. Keep the axis parallel.

3. Read out the apparent resistivity value on the field curve this overlies the       ‘resistivity index’ line of theoretical curve.

2.6.3 Interpretation using two-layer master curve:

By using following procedure we will get the thickness and resistivity of the subsurface.

I. First plot ρa vs. on log-log transparent sheet of the same modulus of the master     curve.

II. Now superimpose field curve on the two layer master curve and shift field curve in       x-axis and y-direction to get perfect fit.

The value of origin on field curve gives the value of (ρ1, h1). Once we know the ρ1 and h1 the ratio is known we can compute ρ2. Thus we have determineρ1, ρ2 and h1. We interpret 3-layer cases in similar way.

Fig 2.8 Two Layer Master Curves

2.6.4 Types curves in resistivity sounding:

Depending on the variation of resistivity we give different name for a specific sounding.

Two layer case:

i. Ascending type (ρ2 > ρ1)

ii. Descending type (ρ1 > ρ2)

Three layer case:

i. K-type (ρ1 < ρ2 > ρ3)

ii. A-type (ρ1 < ρ2 < ρ3)

iii. H-type (ρ1 > ρ2 < ρ3)

iv. Q-type (ρ1 > ρ2 > ρ3)

Fig 2.9 Three Layer Curves in Resistivity Sounding

Four layer case:

i. KH-type (ρ1 < ρ2 > ρ3 < ρ4)

ii. KQ-type (ρ1 < ρ2 > ρ3 > ρ4)

iii. AA-type (ρ1 < ρ2 < ρ3 < ρ4)

iv. AK-type (ρ1 < ρ2 < ρ3 > ρ4)

v. HA-type (ρ1 > ρ2 < ρ3 < ρ4)

vi. HK-type (ρ1 > ρ2 < ρ3 > ρ4)

vii. QH-type (ρ1 > ρ2 > ρ3 < ρ4)

viii. QQ-type (ρ1 > ρ2 > ρ3 > ρ4)

Similarly for 5 layer case 16-different types of sounding curves will be possible.

6 layer ---32 curves

7 layer ---64 curves

HH and KK type curves can’t be drawn.

2.6.5 Inversion Technique:

A good inversion method must simultaneously minimize the effects of data error and model parameter errors. The necessary requirements for inversion of any geophysical data are a fast forward algorithm for calculating theoretical data for initial model parameters, and a technique for calculating derivatives of the data with respect to the model parameters. Unfortunately the second requirement is not readily available for two-dimensional (2D) or three-dimensional (3D) inverse resistivity problems.

disadvantages:

The resolving power of this method is poor and is particularly true for deeper         boundaries.

Due to principle of equivalence (i) a conductive layer sandwiched between two         layers of higher resistivities will have the same influence on the curve as long as         the ratio of its thickness to resistivity (h/ρ) remains the same and similarly (ii) a         resistive layer sandwiched between two conducting layers will have the same         influence on the curve as long as the product of its resistivity and thickness

2.6.6 Inverse Slope Method:

The stratigraphy has been interpreted based on the evaluation of the electrical resistivity tests results as well as visual observations. The principles for the analysis are as outlined herein. Based on the analysis of layered formations and electrical studies, Sanker Narayan & Ramanujachary (1967) have proposed a graphical procedure for computing the true resistivity of various layers. The analysis is called the "Inverse Slope Method" the procedure is as follows:

1. Plot electrical spacing ‘a’ verses ‘a/ρ’.

2. Draw the fitting straight line segments through the points, the intersections and read     off for depths.

3. The reciprocals of the corresponding slopes of the segments give the absolute      resistivity of the layers directly.

A sketch showing the procedure is presented below

Fig 2.10 Determination of geo-electric layers by Inverse Slope Method

Taking lead from this concept, Dr. KRR Chary of IGIS has invented an innovative approach to interpret the vertical electrical sounding data with Wenner configuration. According to this approach, the inverse of resistance measured (1/R) is plotted against the Wenner electrode separation ‘a’ on a linear graph. The plotted data points and align themselves on discrete line segments and are joined by straight lines. Each line segment represents a layer and the intersections of the line segments correspond to the depths to the particular layers.

Fig 2.11 Plotting 1/R Vs. a (a= electrode separation) (after cherry IGIS)

Slope of line segment = Δy/Δx = (1/R) / a

Inverse Slope = a / (1/R) = aR

Resistivity of the layer = 2Ï€ (Inverse Slope) = 2Ï€ (a R) = 2Ï€ a R

A small improvement of this is to plot (a / ρa) on the Y-axis instead of (1 / R). Then the Inverse Slope directly gives the resistivity of the layer (No need to multiply the inverse slope with 2π). This method required to correlate the observed value of resistivity form given standard data identify the type of soil / rock and lethology. Hence, the interpretation is essentially a data matching technique.

Since the resistivity ranges from the different soil type overlap, careful judgment is required to access likely soil stratum encountered In multilayered deposits, it is not sufficient to identify the layers based on one individual test; comparison with data from surrounding locations as bore hole is very important. It should be confirmed with physical profile as obtained from borehole or from near.

Fig 2.12 Plotting (AB/2)/ρa Vs. AB/2 (AB/2= electrode separation)

Originally, the Inverse Slope method was proposed for interpretation of Wenner sounding data. However, this method can also be used for Schlumberger data with a minor modification. For Schlumberger sounding the linear plot has to be prepared between (AB/2) on X-axis and {(AB/2)/ρa} on Y-axis. You should not use (1/R) for plotting since R depends on both AB/2 and MN/2. While the inverse slope of the line segments directly gives the true resistivity of the layers, the intersections of the line segments have to be multiplied with (2/3) to get the depths to the interfaces. The procedure of interpretation for both these soundings is described in the following tabular form.

2.6.7 Break Point Method

It is the simplest method of interpreting the data. Apparent resistivity is determined for electrode spacing using the Wenner formula (Eq.5). The apparent resistivity values (ohm m) are plotted against depth in meters. ‘Break’ occurring in the shape of the curves are assumed to indicate changes in geological formation.

2.7 Typical Range of Resistivity Values

The common rock forming minerals shows very high resistivity e.g. quartz has resistivity around 1011 ohm-m. Impervious rocks and porous but dry rocks have high resistivities. Resistivity decreases with increasing water saturation and salinity (Karanth, 1987). Whereas contaminated saturated soils have relatively low resistance (Kajartanson et al, 1998).

Typical soil resistivity information for uncontaminated soils & rocks are given in Table 2.2

Table 2.2 Typical Value of Soil Resistivity

S.No.

Material

Resistivity Range (ohm-m)

1

Clays

1-150

2

Sand

100-1500

3

Sand stone

102-103

4

Lime stone (dense)

104-107

5

Lime stone (porous)

102-103

6

Basalt (massive)

102-103

7

Granite

103-105

2.8 Limitations

The resistivity test is an indirect method of investigation and has its inherent limitations.

Some of these are:

(1) Advance knowledge of likely stratigraphy and groundwater conditions is essential      for proper interpretations.

(2) Where the strata dip steeply or where the stratigraphy is variable, the interpretation       from the test could be erratic.

(3) If the ground is steeply sloping or is undulating, serious errors may be introduced.

(4) The test cannot be done in a water-logged area or in flowing/standing water.

(5) Soil parameter required for geotechnical analysis, such as, shear characteristics      (c - ϕ), density, specific gravity, etc. cannot be obtained from the test. It will have      to be assessing from borehole data at nearby locations.

(6) In contaminated areas and in areas where localised inclusion, dykes or other       features are present, the interpretation has to take into account these factors and       requires a more detailed study.

2.9 Advantages

Used in the conjunction with borehole data, the resistivity test can provide a basis for geotechnical design. It can be used to identify the various layers and confirm the continuity of the various layer can also be assessed. The depth of water table and his salinity of the groundwater can also be obtained.

It cuts down the required for geotechnical investigation programme substantially. In the current scenario of fast track projects in the highway and infrastructure sector, it can be used to reduce the number of boreholes required to be drilled.

2.10 Past Studies

Electric resistivity of compacted clay was carried out by various researches such as Mc carter 1984; Benson et al (1994). It is reported by them that electric resistivity of compacted clay is sensitive to compaction condition and soil composition.

Jackson (1975) found that electric condition in clean sands and gravels occurs primarily in liquid contained in the pores. Urish (1981) reported that in the case of clayey coils and clay bearing rocks, electrical conductivity occurs in pores and on the surface of electric charged clay particles.

Mc Neill (1990) reported that the electric resistivity of a unsaturated soil ρ can be related to that of saturated soil ρsat by the following relationship

=S-B ................................................................ 2.8 Where s = degree of saturation and B is an empirical parameter.

Temperature also affects the electric resistivity of soil. Increasing the temperature increases the mobilization of ions and a result, decrease the electric resistivity of soil. Keller & Frischknecht (1966) reported that the electric resistivity of soil ρT at a temperature (T0c) can be related to a standard electric resistivity measured at 180 c (ρ18 ) by

ρT = ........................................................ (2.9)

where α = an empirical parameter that is approximately 0.0250C-1

Mazac et al (1990) concluded that the relationship between hydraulic conductivity and electric resistivity is inverse for a soil of a particular type e.g. saturated dense clean sands have lower porosity, lower hydraulic conductivity, and greater electrical resistivity than loose clean sands.

Abu-Huassanein et al (1996) have correlated electric resistivity with index property. They reported experimentally that soil. With higher liquid limit or plasticity index, a greater percentage of fines or clay, or a smaller coarse fraction have lower electric resistivity. They also found that electric resistivity of soils is sensitive to temperature and suggested a slightly higher value of α (refer to eqn 2.9) than the one suggested by Keller & Frischknecht (1966).

Sundaram & Gupta (2001) have used the electric resistivity test for geotechnical investigation for bridges in India. It is reported by them that resistivity tests in conjunction with borehole data can provide a basis for geotechnical design. However, for interpretation the services of an experienced person with thorough understanding of geo-physics are essential.

Jawaid et al (2006) have developed correlation between SPT and electric resistivity test for quick geotechnical site characterization.

CHAPTER 3

EXPERIMENTAL PROGRAM

3.1 Objective

The primary objectives of this study were:

(1) To study the feasibility of using electric resistivity for deducting the contaminated       soil.

(2) To develop the resistivity range for soil contaminated with the municipal solid       waste.

(3) To understand the effect of saturation on the resistivity of contaminated ground.

3.2 Resistivity Meter

The resistivity meter used for this study to acquire the electrical resistivity data was model SSR-MP-AT manufactured by Integrated Geo Instruments & Services Pvt. Ltd. (IGIS), Hyderabad. This equipment is a Resistivity Imaging System and can be employed for resistivity profiling as well as resistivity scanning of the subsurface.

In the presence of random (non-coherent) earth noises, the signal to noise ratio can be enhanced by √N where N is the number of stacked readingsThe SSR-MP-AT is programmable through user-friendly menu for its operation and entry of survey parameters like Survey No., Electrode Separations etc. The SSR-MP-AT Resistivity meter directly gives the Resistance (ratio of ΔV and current) with a resolution up to 10-5 ohms at 1 A input current.

The Multi-Electrode Resistivity Meter system comprises of:

The main measuring unit (Resistivity Meter)

Power supply unit to inject current into the ground

4 Stainless Steel electrodes

Connecting cables

3.2.1 Measuring Unit :

A photograph of the instrument is shown below figure various controls and other functional switches are enumerated below:

Table3.1 Functional switches on measuring unit

Fig 3.1 Measuring unit

3.2.2 Power supply unit:

A photograph of the power supply unit is shown below figure various controls and other functional switches are enumerated below:

Table 3.2 Functional switches on power supply unit

Fig 3.2 Power supply unit

3.2.3 Principle of Measurement:

The SSR-MP-AT contains mainly two unit’s Current unit and Microprocessor-based Measuring unit. The current unit sends bipolar signals into the ground at a frequency of about 0.5Hz. The receiver has a 4½ digit dual-slope analog to digital converter (ADC) unit, which can measure the ground potentials and current with resolution up to 100V and 100A respectively. The microprocessor controls the current unit, determines the attenuation level for potential measurements, computes the resistance values, averages the measured values, keeps the data in memory, displays and transfers the data to PC.

3.2.4 Current Unit:

This unit sends bipolar current into the ground. This unit has three voltage settings 50V, 150V and 350V, which are controlled by the microprocessor of the measuring unit. This unit is powered by 2 x 12 V rechargeable batteries.

3.2.5 Measuring Unit:

This is the main unit, which measures the current and potential values, calculates the resistance and apparent resistivity, stores in the memory and gives the output through the display. This unit is to be programmed. This unit is powered by the same batteries meant for current unit.

3.2.6 Field Layout and Data Acquisition:

Resistivity surveys are made to satisfy the need of two distinctly different kinds of interpretation problem:

The variation of resistivity with depth, reflecting more or less horizontal         stratification of earth material.

Lateral variation in resistivity that may indicate soil lenses, isolated ore bodies,         faults, or cavities.

For the first kind of problem, measurements of apparent resistivity are made at a single location (or around a single centre point) with systematically varying electrode spacings. This procedure is something called vertical electrical sounding (VES), or vertical profiling. Surveys of lateral variations may be made at spot or grid locations or along definite lines of traverse, a procedure sometimes called horizontal profiling. Either the Schlumberger or, less effectively, the Wenner array is used for sounding, since all commonly available interpretation methods and interpretation aids for sounding are based on these two arrays

Fig 3.3 Assembled parts of the instrument in the field

After finalizing the target point for Vertical Electrical Sounding, the other parameters to be considered are:

Depth to the investigated,

Depth resolution, and

Direction of required traverse.

After fixing of centre point of the array or sounding point, drives the current electrode (A, B) and potential electrodes (M, N) into the ground at fixed distance. Then connect these electrodes (A, B) and (M, N) through the wire of the spools to the C1, C2 and P1, P2 inserting point in the measuring unit respectively. The important point to note is that the electrodes are to be driven deep enough to get good galvanic contact with the ground; if necessary, wetting the ground at electrodes is recommended. Now complete the circuitry by joining the reference 1 and reference 2 of the G-Unit (Power Supply) to reference 1 and reference 2 terminals in the measuring unit and take the measurement for this AB/2 and MN/2 combination

3.3 Field Testing Procedure

Various municipal waste dumping sites as well as contaminated sites in and around Gorakhpur city was selected for this study. Electric resistivity tests were conducted at each sites in accordance with IS: 3043-1987. Wenner configuration was used in this study due to ease of testing. Experimental setup is shown in Fig 3.4

Fig 3.4 Electric Resistivity Test in progress at Charfatak

Auger Boring was also carried out at the same site for collecting the soil samples at various depths tests were conducted to make a bore log for comparison. Electric resistivity data was also reported in the bore log. The details of site selected for contamination detection are given in Table 3.3

Table 3.3 Description of sites were test was conducted

S.No.

Description of test site

1

Ramgarh Tal area of contaminated soil, Padleyganj, Gorakhpur

2

MSW open dumping site at Belpar, Gorakhpur

3

MSW open dumping site at Charfatak, Gorakhpur

4

MSW open dumping site at Deoria Bridge

5

MSW open dumping site at Nandanagar, Gorakhpur

6

Ramgarh Tal area, proposed retaining wall site, Gorakhpur

7

MSW open dumping site at Nausarh Mandi, Gorakhpur

8

Ramgarh Tal area, Padleyganj, Goraghpur

9

Non contaminated area at MMM engineering college, Gorakhpur

10

MSW open dumping site, Rapti Bandh, Azad chowk, Gorakhpur

The electrode spacing and resistivity (ρ) were recorded at each site and the same is enclosed here with as Appendix. The inverse slope method was adopted and electrode spacing ‘a’ and ratio of electrode spacing to apparent resistivity (a/ρ) was also calculated and reported in Appendix.

CHAPTER 4

DATA ANALYSIS AND INTERPRETATION

The resistivity data was plotted as a graph of ‘a’ versus ‘a/ρ’ and appended here as Fig 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, and 4.10. The data was analyzed by inverse slope method and results are plotted with bore logs. The bore logs of various sites along with resistivity (ρ) are given in Fig 4.1 to 4.10.

Table 4.1 Bore log with electrical resistivity for Ramgarhtal area of contaminated soil, Paidleganj, Gorakhpur

Resistivity

Ω-m

Log based on resistivity

Depth below ground level (m)

Log based on borehole data

Soil

Type

9.09

1

OL

200

2

SM

2.77

3

OL

2.85

4

5.5

5

3.125

6

1.886

7

1.78

8

1.85

9

2.7

10

4.54

11

100

12

SM

13

Table 4.2 Bore log with electrical resistivity for MSW open dumping site at Belpar, Gorakhpur

Resistivity

Ω-m

Log based on resistivity

Depth below ground level (m)

Log based on borehole data

Soil

Type

150

1

SM

2

3

50

4

SP

50

5

25

6

100

7

SM

250

8

5.6

9

OL

2.6

10

11

1.16

12

4

13

200

14

SM

15

Table 4.3 Bore log with electrical resistivity for MSW open dumping site at Charfatak, Gorakhpur

Resistivity

Ω-m

Log based on resistivity

Depth below ground level (m)

Log based on borehole data

Soil Type

25

1

SP

2

3

5

4

OL

3.33

5

10

6

1.25

7

OH

0.086

8

0.092

9

0.45

10

0.357

11

0.22

12

25

13

SP

Table 4.4 Bore log with electrical resistivity for MSW open dumping site at Deoria Bridge

Resistivity

Ω-m

Log based on resistivity

Depth below ground level (m)

Log based on borehole data

Soil

Type

50

1

SP

300

2

3

4

2.7

5

OI

5

6

2.5

7

1.11

8

0.28

9

1.05

10

1

11

50

12

SP

13

Table 4.5 Bore log with electrical resistivity for MSW open dumping site at Nandanagar, Gorakhpur

Resistivity

Ω-m

Log based on resistivity

Depth below ground level (m)

Log based on borehole data

Soil Type

200

1

SM

2

3

4

50

5

SP

4

6

OI

4

7

0.273

8

0.571

9

10

80

11

SP

12

Table 4.6 Bore log with electrical resistivity for Ramgarhtal area, proposed retaining wall site, Gorakhpur

Table 4.7 Bore log with electrical resistivity for MSW open dumping site at Nausarh mandi, Gorakhpur

Table 4.8 Bore log with electrical resistivity for Rangarhtal area, Paidleganj, Gorakhpur

Table 4.9 Bore log with electrical resistivity for Non contaminated area at MMM Engg. College campus, Gorakhpur

Table 4.10 Bore log with electrical resistivity for MSW open dumping site at Rapti Bandh, Azad chowk, Gorakhpur

Resistivity

Ω-m

Log based on resistivity

Depth below ground level (m)

Log based on borehole data

Soil Type

25

1

SP

2

3

25

4

5.5

5

OI

2.5

6

1

7

8

9

0.47

10

1.05

11

40

12

SP

13

The resistivity value for various soil layers reported from Table 4.1 to 4.10 are tabulated below (Table 4.11)

Table 4.11 The resistivity value for various soil layers

S.No.

Description of test site

Depth of GWT

Depth

(m)

Soil type

IS classification

Resistivity

(Ω-m)

1

Ramgarhtal area of contaminated soil, Paidleganj, Gorakhpur

2.0 m

0-1

Org. silt

OL

9.09

1-2

Silty sand

SM

200

2-11

Org. Silt

OL

1.78-5.50

11-13

Silty sand

SM

100

2

MSW open dumping site at Belpar, Gorakhpur

3.0 m

0-3

Silty sand

SM

150

3-6

Poorly graded Sand

SP

25-50

6-8

Silty sand

SM

100-250

8-13

Org. Silt

OL

1.16-5.6

13-15

Silty sand

SM

200

3

MSW open dumping site at Charfatak, Gorakhpur

6.0 m

0-3

Poorly graded Sand

SP

25

3-6

Org. Silt

OL

3.33-10

6-12

Org. Silt

OH

0.086-1.25

12-13

Poorly graded Sand

SP

25

4

MSW open dumping site at Deoria Bridge

4.0 m

0-4

Poorly graded Sand

SP

50-300

4-11

Org. Silt

OI

0.28-5

11-13

Poorly graded Sand

SP

50

5

MSW open dumping site at Nandanagar, Gorakhpur

5.0 m

0-4

Silty sand

SM

200

4-5

Poorly graded Sand

SP

50

5-10

Org. Silt

OI

0.273-4

10-12

Poorly graded Sand

SP

80

6

Ramgarhtal area, proposed retaining wall site, Gorakhpur

4.0 m

0-1

Poorly graded Sand

SP

25

1-4

Silty sand

SM

150

4-11

Org. Silt

OI

1.25-4.54

11-13

Silty sand

SM

200

7

MSW open dumping site at Nausarh mandi, Gorakhpur

4.2 m

0-3

Org. Clay

OH

0-1.25

3-4

Org. Silt

OL

10

4-13

Org. Clay

OH

0.194-1.17

13-15

Poorly graded Sand

SP

25

8

Rangarhtal area, Paidleganj, Gorakhpur

0.5 m

0-4

Org. Silt

OL

0-5

4-5

Poorly graded Sand

SP

25

5-8

Org. Silt

OL

1.25-2.85

8-9

Poorly graded Sand

SP

16.6

9-10

Org. Silt

OH

0.85

10-12

Poorly graded Sand

SP

50

9

Non contaminated area at MMM Engg. College campus, Gorakhpur

8.0 m

0-6

Poorly graded Sand

SP

250 - 1000

6-8

Silty sand

SM

3000 -5000

10

MSW open dumping site at Rapti Bandh, Azad chowk, Gorakhpur

4.5 m

0-4

Poorly graded Sand

SP

25

4-11

Org. Silt

OI

0.4-5.5

11-13

Poorly graded Sand

SP

40

Based on Table 4.11, it is evident that the resistivity values of organic soil / contaminated soils are within the range of 0 – 16 Ω-m whereas resistivity values of virgin soil starts from 25 Ω-m. Thus, resistivity values easily predict the organic soil layer / contaminated soil layers. Also, it is found that the resistivity values of various contaminated organic soil may be taken from Table 4.12 based on the values given in Table 4.11.

Table 4.11 The resistivity value for different contaminated soil

S.No.

Soil Name

Symbol

Resistivity Range(Ω-m)

1

Above water table

Low compressibility organic Silt / contaminated silt

OL

3.3 – 10

2

Medium compressibility organic Silt /  contaminated silt

OI

0.28 – 5.0

3

High compressibility organic Silt / contaminated silt

OH

0 – 1.25

4

Below water table

Low compressibility organic Silt / contaminated silt

OL

1.25 – 5.5

5

Medium compressibility organic Silt /

contaminated silt

OI

0.27 – 4.0

6

High compressibility organic Silt / contaminated silt

OH

0.086 –1.25

From the Table 4.11 & 4.12, it is for evident that the resistivity value decreases with the saturation. As can be seen from Table 4.11 & 4.12 that some of the ranges of resistivity value overlaps for different soils so, it is mandatory to interpret the resistivity data by comparing an adjacent borehole data. It is to mention that these ranges could vary even for similar nature of soils depending upon moisture content, salinity, level of ground water, degree of compaction, mineralogy, particle size distribution and other factors. Thus the above ranges are site specific.

CHAPTER 5

CONCLUSION

It is found that the resistivity test can provide useful data for geotechnical design if used in convention with the borehole data. It cuts down the time required for a geotechnical investigation / contamination / organic soil. Based on this study, the following conclusion may be drawn.

(1) Soil Resistivity test provides a continuous profile of the resistivity of various strata

available below the ground surface.

(2) The resistivity of contaminants / organic soil varies between 0 to 5.5 Ω-m.        However, the resistivity of virgin soil is more than 25 Ω-m.

(3) The resistivity of low compressibility organic silt / contaminated soil is more than        medium or high compressibility organic silt / contaminated silt.

(4) Resistivity value decrease with increase in degree of saturation.

(5) Soil parameters required for geotechnical analysis, such as shear characteristics,       density etc cannot be obtained from the resistivity test.

The resistivity test should not be considered as an alternative to borehole drilling. It should be used in conjunction with sufficient borehole data for realistic interpretations that match well with actual ground condition. Through knowledge of local conditions is essential so as to correlate the results with strata conditions. Prior to conducting the test, information on geology, geomorphology and anticipated stratigraphy of the project area should be collected.

CHAPTER 6

REFERENCES / BIBLIOGRAPHY

Abu-Hassanein, Z. (1994). "Using electrical-resistivity measurements as a quality-     control tool for compacted clay liners," MSc thesis, Univ. Of Wisconsin-    Madison,Wis.

Benson, C. and Daniel, D. (1994) "Minimum thickness of compacted soil liners: I-    stochastic models." J. Geotech. Engrg., ASCE, 120(1) 129-152.

Jackson, P. (1975). "An electrical-resistivity method for evaluating the in-situ      porosity of clean marine sands." Marine Geotechnical, 1(2), 91-115.

Jawaid, S. M. A., Sharma, P. K. and Kumar, A. (2006). "Application of electric      resistivity test in site characterization", Proc. National Confrence on Corrective      Engineering Practices in Troublesome Soils (CONCEPTS), JNTU College of      Engineering, pp. 11-13.

Keller, G., and Frischknecht, F. (1966). "Electrical methods in geophysical      prospecting", Pergamon Press, New Yark,N.Y.

Mazac, O., Milena, C., Kelley, W., and Landa, I. (1990). "Determination of hydraulic

conductivities by surface geoelectric methods." Geotech. And Envir. Geophys.,     Vol. 2, S.Ward, ed., 125-131.

McNeill, J. (1990). "Use of electromagnetic methods for groundwater studies."    Geotech. and Envir. Geophys., Vol. I, S. Ward, ed., 191-218.

Sanker Narayan, P.V. and Ramanujachary, K. R. (1967), "An Inverse Slope Method     of Determining Absolute Resistivity" Short Note, Geophysics, XXXII (6), pp 1036-    1040.

Sundaram, R. and Gupta, S. (2001), "Use of electrical resistivity tests for geotechnical

investigations for bridges"

Woods, R.D. (1994), "Geophysical Characterization of Sites", Volume prepared by

International Society of Soil Mechanics and Foundation Engineering, ISSMFE      Technical