Chapter 1

Introduction

1.1 Introduction

Geophysics, a subject of geology connected with physics and

mathematics, is utilized in the study of earth in exploring the subsurface Geology.

Geophysical investigations involve simple methods of study made on the surface

detail. Geophysical methods of investigations provide quick, inexpensive, easy

and fairly reliable means to get subsurface details.

Gilbert's discovery that the earth behaves as a great and rather irregular

magnet and Newton's theory of gravitation may be said to be the beginning of

Geophysics. Mining and research for metals began at the earliest but the

scientific record began with the publication in 1556 of the famous treatise De re

metallica by Georgius Agricola. The initial step in the application of Geophysics to

the research for minerals probably was taken in 1843 when Von Wrede pointed

out that the magnetic theodolite used by Lamont to measure variations in the

earth's magnetic field might also be employed to discover magnetic ore bodies.

This idea was published in Professor Robert Thalen's book in 1879 entitled, "On

the Examination of Iron Ore Deposits by Magnetic Methods". The Thalen-Tiberg

magnetometer manufactured in Sweden and later the Thomson-Thalen

instrument furnished the means of locating the strike, dip and depth below the

surface of magnetic dikes (Telford, 1988).

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The aim of geophysical survey is to obtain geological information before

the subsurface excavation. In order to make sure that the inferences drawn from

geophysical investigations are correct, it is necessary to make drillings or

trenches at some selected places and check the inferred data with the

observation data (Meidav 1960, Daily et al 1992, LaBrecque et al 1996a, Lennox

and Carlson 1967). So the geophysical investigations reduce the amount of test

drilling by giving a better selection of test borehole locations. Geophysics devoted

to the exploration of minerals including groundwater is called by different names,

viz. Applied Geophysics, Exploration Geophysics, Geophysical prospecting etc.

1.2 Geophysical Survey

Geophysical exploration, commonly called applied geophysics or

geophysical prospecting, is conducted to locate economically significant

accumulations of oil, natural gas and other minerals including groundwater.

Geophysical investigations are also employed with engineering objectives in

mind, such as predicting the behaviour of earth materials in relation to the

foundations for roads, railways, buildings, tunnels, dams and nuclear power

plants. Surveys are generally identified by the property being measured namely,

electrical, gravity, magnetic, seismic, thermal, or radioactive properties. Deep

electrical sounding provides valuable information about the internal structure of

the earth's crust and mantle. Electrical surveys are based on natural sources of

potential and current. This technique has also become important in the scientific

investigation of environmental problems.

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1.2.1 Sources of Study

For the present study the information obtained from outcrop, surface

geophysics, and drilling data have been utilized. Natural or man made outcrops

are the essential source of geological information. Two or three dimensional

pictures of subsurface are obtained by surface geophysical techniques. They are

important tools for the exploration of subsurface, since it gives the information of

shape and arrangements of beds, their nature, petrophysical properties, fluid

content etc. This hypothesis drawn by the interpretation of the information is

verified by drilling.

The continued expansion in the demand for metals of all kinds and the

enormous increase in the use of oil and natural gas during the past fifty years

have led to the development of many geophysical techniques of ever increasing

sensitivity for the detection and mapping of unseen deposits and structures. The

development of new electronic devices for field equipment and the widespread

application of the digital computer in the interpretation of geophysical data

advance the geophysical techniques. Several of the devices now used by

geophysicists were developed from methods used for locating guns, submarines

and aircraft during the two world wars.

The variation in electrical conductivity and in natural currents in the earth,

the rate of decay of artificial potential differences introduced into the ground, local

changes in gravity, magnetism and radioactivity provide information to the

geophysicist about the nature of the structures below the surface to locate the

mineral deposits.

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1.2.2 Flow Chart of Stages of Geophysical Survey

The geophysical surveys using specific techniques consist of three steps.

They are Surveying, Data Processing and Data interpretation. A successful

survey will yield more information on the geological target, its existence, location,

shape, size etc. Analyzing the sequence of geophysical survey steps is shown in

(Fig. 1.1) flowchart.

Research

Operations

Physical properties Attributes of the ground

Instrumentation and

Geophysical surveyingfield techniques and data processing

Interpretation theory

Interpretation of dataand analog modeling

Desired information

Figure 1.1 Flow chart showing the steps involved in the geophysical survey(Palacky, 1983)

New instruments were tested over known targets and if successful, they

were put to routine use. Then interpretation theory for a particular technique was

developed later.

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1.3 Geophysical Methods

Geophysical methods for detection of subsurface anomaly causing bodies

may be classified into surface methods and subsurface methods. Surface

methods include many kinds of geophysical methods used in surface geophysical

prospecting. The surface geophysical methods are used generally for planning

efficient and economical test drilling programmes and to have general preliminary

ideas of the lithological conditions. Though this can never replace drilling, yet this

can provide adequate information so that the number of drillings may be held to a

minimum and the depths of the exploratory boreholes can be estimated.

Experience has shown that most subsurface structures can be identified

and located if detectable differences in physical properties exist. The basic

properties that are exhibited by the common rocks and formations which are

generally used for geophysical prospecting are density, magnetic susceptibility,

elasticity and electrical conductivity.

Based upon four basic properties mentioned above the following methods

are generally used for geophysical exploration. They are:

1. Gravity and Magnetic methods

2. Seismic methods

3. Electrical and Electromagnetic methods

4. Radioactive methods and

5. Subsurface methods which include Well logging methods.

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1.3.1 Gravity Methods

In gravity methods, the nature of distribution of gravity on the surface is

analyzed. The gravity is influenced positively if the causative body is heavier,

larger and occurs at a shallow depth. The physical property, density of the

material, is the controlling factor of this method. The gravimeter is the suitable

equipment for measurements in this method.

1.3.2 Magnetic Methods

The magnetic methods are based on the fact that the magnetic bodies

present in the earth subsurface contribute to the magnetic field of the earth. The

main controlling physical property in magnetic methods is magnetic susceptibility.

The magnetometer is the instrument used for the magnetic survey.

1.3.3 Seismic Methods

Seismic methods of study are based on the principle that subsurface rock

formations bear different elastic properties. Because of this, the velocities of the

propagation of seismic wave undergo changes with changes occurring in

lithology. Measurement of the seismic wave velocities of rock formations,

therefore, provides a scope to distinguish different subsurface lithological units.

Elastic property differences in rock are the controlling property. The instrument

used in Seismic prospecting is the most complicated and expensive. Geophone,

amplifier, galvanometer and seismic timer all together make the instrument for

seismic measurements in one station.

1.3.4 Electrical and Electromagnetic Methods

Electrical and electromagnetic methods, based on the measurement of the

electrical resistivity or conductivity of the subsurface minerals and formations are

very useful for the exploration of minerals and groundwater. Various methods,

such as Self potential method, Resistivity method, Inductive method, Telluric

method, Magneto-Tell uric method, Induced polarization method, Charged body

method, Radio wave method, and Transient method are used in the electrical and

electromagnetic studies.

Surface self potential studies measure the naturally occurring potential at

the ground due to shallow subsurface conductors having differences in pH

concentration at the top and the bottom. Equipotential line method records the

distortion in regular equipotential lines produced by artificial current sent into the

ground and thereby locates the ore deposits producing the anomaly.

In resistivity method a current is introduced into the ground by two or

more current electrodes and the potential difference is measured between two

points suitably placed with respect to the current electrodes. The potential

difference for unit current sent through the ground is a measure of the electrical

resistance of the ground between the probes. The measured resistance is a

function of the geometrical configuration of the electrodes and the electrical

parameters of the ground.

Inductive method of prospecting makes use of the alternating current as

an artificial source which induces eddy currents on the conductors giving rise to a

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secondary electromagnetic response which helps in detecting the anomaly.

These methods are very useful in shallow mineral or groundwater exploration.

Telluric method of prospecting uses the electric field due to naturally

flowing earth currents caused by activities in the ionosphere to investigate depths

beyond the range usually penetrated by direct current resistivity method. Telluric

method is used for mapping basement structures below the thick sedimentary

column.

Magneto Telluric method of prospecting makes use of the electromagnetic

fields due to telluric earth currents and measures time variations for the deep

subsurface structures of the order of a few kilometers. Induced polarization

method based on over voltage effect makes measurements of the decay voltage

(in time domain or frequency domain) produced due to polarization of the

electrode interfaces at the subsurface after the current flow has stopped.

Charged body method is used to find the extent of an ore body which has been

located at least in one borehole. Radio wave method makes use of the field due

to electromagnetic waves transmitted from radio broadcasting stations as the

source. The depth of investigation is limited and only the shallow conductors and

water saturated zones may be detected.

1.3.5 Radioactive Method

Radioactive methods of prospecting are based on the measurement of the

spontaneous disintegrated alpha, beta and gamma rays by various radioactive

materials and detect sources of such disintegration by means of GM counter or

Scintillation counter.

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1.3.6 Subsurface Well Logging Methods

The continuous recording of a geophysical parameter along a bore hole

produces a geophysical well log. The value of the measurement is plotted

continuously against depth in the well. Well logging was first invented by

Schlumberger and Doll in 1927 (Rider, 1986). Various well logging techniques

such as Self potential (SP) log, Resistivity log, Radiation log, Sonic log,

Temperature log, Caliper and micro caliper logs are used for detailing ore

deposits and for mineral, oil and water exploration.

SP log gives a record of the naturally occurring potential with depth and is

utilized for distinguishing "Porous and Permeable" beds against shales.

Resistivity log gives the value of the true resistivity of the formation. Gamma ray

log measures the natural radioactivity of the formations and is used to identify

shales against sand. Neutron log records the response due to neutron capture

gamma rays which depends on the hydrogen content of the formation.

Sonic log records the time required for a sound wave to travel through unit

length of the formation from which the sonic velocity is noted and porosity

calculated. Temperature log records the variation in temperature with depth.

Caliper and micro-caliper logs measure the effective diameter of the bore hole.

1.4 Significance of Electrical Methods

Among these methods the surface electrical methods are the most popular

one because they are successful in dealing with a variety of problems like

groundwater studies, subsurface lithology, ore deposits and many others. So, the

geoelectrical ones are of first rate importance for many reasons. Electrical

measuring instruments with their accuracy, range and wide applicability are

superior to other measuring devices. Furthermore measurements of the electrical

soil properties can be carried out simply and speedily. The soil remains

undamaged in its original state.

Conrad Schlumberger started his pioneering work on electrical prospecting

in 1912, and approximately at the same time Wenner developed the same idea in

the USA (Schlumberger, 1920; Kunetz, 1966). Geoelectrical method is the high

resolution in near surface exploration problems (Akos Gyulai, 1999). The

determination and presentation of electrical properties of geological formations,

particularly of their resistivity represent a very valuable criterion for the

delineation of geostructural and hydrogeological relations of the subsurface.

Many changes in geological structure and hydrogeological characteristics of the

subsurface can be clearly recognized as changes of electrical parameters.

Among geoelectrical and hydrological parameters there exist some well known

functional relationships which can be measured and calculated (Thiele 1952,

Fritsch 1956).

The modern developments and refined techniques of interpretation using

high speed computers have increased the depth of investigation to the order of

8-10 kms with higher precision. Geoelectnc exploration consists of exceedingly

diverse principles and techniques and utilizes both stationary and variable

currents produced either artificially or by natural process.

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1.5 Development of Resistivity Method

Measurements of the electrical conductivity or resistivity have been

applied for soil salinity surveys for many years (Rhoades and lngvalson 1971;

Austin and Rhoades, 1979; Rhoades et al, 1990). The most common method is

the electrical profiling using four electrode probes in the Wenner configuration.

The probes are applied on the soil surface as well as in borehole logging

(Rhoades and Schilfgaarde, 1976; Halvorson and Rhoades, 1976; Rhoades,

1979).

Vertical electrical sounding was applied for the stratification of soils and

sediments and estimation of their hydraulic conductivity (Mazac et al., 1990) and

texture (Banton et al., 1997). Although the method of vertical electrical sounding

(VES) is very popular in conventional geophysical studies, such as gas, oil, and

coal exploration (Verma and Bandyopadnyay, 1983), it is rarely used in shallow

subsurface and soil studies. Barker (1990) applied VIES to a landfill outlining at a

40 m depth. Smith and Randazzo (1975) demonstrated that for Wenner arrays, a

one to one association between electrode spacing and effective depth of current

penetration can be assumed in shallow, saturated, Florida limestones.

The Geological Society Engineering Group Working Party (1988)

recommends that resistivity testing is an excellent approach with well developed

analysis techniques for identifying depth to rock, stratigraphy and lithology as well

as for groundwater exploration. Surface geophysics can be used in conjunction

with geologic, hydrologic, and borehole geophysical investigations to optimize

well siting (Jansen and Jurcek, 1997), or as a stand alone method of fracture

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detection (Lewis and Haeni, 1987; Lieblich and others, 1991; Haeni and others,

1993). DC resistivity methods have been successfully used by a number of

investigators to detect bedrock fractures (Habberjam, 1975; Risk, 1975;

McDowell, 1979; Palacky and others, 1981; Soonawala and Dence, 1981; Taylor,

1982; MaIlik and others, 1983; Leonard Mayer, 1984a, 1984b; Ogden and Eddy,

1984; Taylor, 1984; Taylor and Fleming, 1988; Lieblich and others, 1991; Ritzi

and Andolsek, 1992; Lane and Ha€ni, 1995; Powers and others, 1999). DC

resistivity surveys using a square array have been conducted to detect productive

fracture zones in crystalline bedrock for groundwater supply (Darboux Afouda

and Louis, 1989; Sehli, 1990). DC resistivity measurements have been used in

long-term groundwater monitoring programmes (Aaltonen and Olofsson, 2002).

Electrical resistivity mapping was conducted to delineate boundaries and

architecture of the Eumsung Basin Cretaceous (Ji Soo Kim et al, 2001).

Resistivity measurements were carried out utilizing the Wenner array, the single

pole array, the half Wenner array and the half Schlumberger array to evaluate the

resolution of resistivity techniques in detecting and locating anomalies of vein like

bodies in the Al Quweira area, southwest Jordan as part of an economic deposit

and ore exploration project (Batayneh, 2001). The development of direct current

resistivity imaging technique has been discussed by Torleif Dahlin (2001).

Using geoelectrical depth soundings, potential groundwater pollution was

assessed on a former shipyard of the Newport Naval Base, Rhode Island, USA

(Froehlich and Urish, 2002). A series of two dimensional resistivity profiles

collected in the Blue Ridge Province of southwest Virginia and the results from

numerical modelling of synthetic data reveal substantial differences in depth of

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investigation, resolution, and sensitivity using Wenner, Wenner—Schiumberger,

dipole—dipole, and pole—pole data collection techniques (Seaton and Burbey,

2002). Resistivity sounding curves are used to derive ground anisotropy

parameters using square arrays (Senos Matias, 2002). The saline water intruded

zone in Korean paddy fields near the seashore was diagnosed accurately by joint

exploration with VES and chemical anaysis (Sang Ho Lee et al, 2002). The salt

water intrusion mapping by geoelectrical imaging surveys was made by Abdul

Nassir et al (2000). The 2D electric imaging technique was employed to map the

thick soft Pusan clay deposits in four reclamation sites (Giao and others, 2003).

1.6 Development of Self Potential Method

Self potential methods are used for quantitative interpretation in many

fields of applied Geophysics. Petrowsky (1928) and Yungul (1950) interpreted

self potential data on polarized spheres as the source body. Edge and Laby

(1931) worked out the expression for potential. De Witte (1948) developed a

method which involves field self potential values. Roy and Chowdhury (1959) and

Meiser (1962) made detailed quantitative interpretation methods on self potential.

Rao (1953), Semenov (1974) Schiavone and Quarto (1984) worked on

hydrogeological studies using SP method. Zachos (1963), Sivenas and Beales

(1982) worked on self potential anomalies associated with ore bodies. Paul

(1965) interpreted self potential anomaly caused by infinite inclined sheets. Sato

and Mooney (1960), Heinrichs (1966), Logn and Bolviken (1974), Carry (1985)

reported self potential case histories in the literature concerning mining.

Zohdy et al (1973), Anderson and Johnson (1976), Corwin and Hoover (1979),

Corwin et al (1981), Fitterman and Corwin (1982) worked on self potential

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measurements in Geothermal exploration. Sill (1983) proposed that the SP

anomalies that occur over massive and disseminated mineral deposits are those

of principal concern in mineral exploration.

Bogoslovsky and Ogilvy (1970a, b, 1973), Bogoslovsky et al (1979),

Gex (1980), Filterrnan (1983a), Hadley (1983), Black and Corwin (1984),

Butler (1984) worked in dam and embankment seepage control on the basis SP

method. Bhattacharya and Roy (1981), Rao and Babu (1984), Murty and Han

chandran (1985) derived numerical estimates of the source parameters from the

analysis of the field self potential anomaly diagram.

Kilty (1984) carried out the origin and interpretation of self potential

anomalies. Pal and Dasguta (1984) and Pal and Mukerjee (1986) studied the

electrical potential in an inhomogeneous an isotropic half space. Corwin and

Morrison (1977), Fitterman (1978), Morrison et al (1979), Varotsos and

Alexopoulos (1984a, b), Patella (1991), Di Bello et al (1994) studied forecasting

earthquakes using the SP method. Corwin (1990) worked on the self potential

method for environmental and engineering applications. Eskola and Hongisto

(1987) and Furness (1992) made physical model for the self potential to study

mineral deposit.

Bogoslovsky and Ogilvy (1977), Bogoslovsky, Ogilvy and Strakhova

(1977), Patella, et al (1995) worked in engineering geophysics for land slide with

the help of SP measurements. Wynm and Sherwood (1984), Cammarano et al

(1995) used SP method in the study of archeological surveying. Zablocki (1976),

Di Maio and Patella (1994), Di Maio et at (1996) studied volcanic eruptions with

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the help of self potential methods. Patella (1997) presented a new approach to

SP data interpretation for the recognition of a buried causative SP source system.

A review of some applications of the self potential method (SP) in the field of the

subsurface prospecting was made by (Aubert, 1997).

A self potential survey of the summit zone of Karthala volcano (Grande

Comore) was made by Jean Francois Lenat et al (1998). A self potential survey

was carried out in the Kestanbol area, Turkey in order to investigate the fault

zones that might be associated with geothermal activity (Ilyas Caglar and

Mustafa, 1999). The influence of transverse ground anisotropy on the formation

of self potential (SP) anomalies produced by polarized bodies is studied by

Skianis and Hernandez (1999).

Visual interpretation of superposed self potential anomalies in mineral

exploration was made by Ilyas Caglar (2000). Aubert et at (2000) applied self

potential method to investigate the internal structure of the Merapi summit in the

area between the crater Pasabubar and the dome. Self potential anomalies with

unexplained high values in Cerro de Pasco and Hualgayoc areas (Peru) have

been explained by Vagshal and Belyaev (2001) on the basis of electrokinetic

effect obtained when a topographic high occurs above a rock layer through which

water filtrates and which itself lies over a highly resistive layer. Methods of self

potential, electrical profiling, vertical electrical sounding and non-contacting

electromagnetic profiling were applied to urban soils in Astrakhan, Russia and

Kiev, Ukraine (Larisa Pozdnyakova et al 2001). The surveys based on a

combination of self potential (SP), direct current (DC) resistivity, induced

polarisation (IF), and transient electromagnetic (TEM) methods have been

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carried out at the Ranger mine site in the Northern Territory, Australia to

investigate their use in detecting any seepage from structures used to store ore

processing tailings (Buselli and Kanglin Lu, 2001).

Electrical resistivity method was used to delineate zones favourable for

seepage, whereas self-potential (SP) method was used to delineate the seepage

paths at two of the four Saddle dams of the Som Kamla Amba project, Rajasthan

State, India (Panthulu et al, 2001). Self potential field survey over the oil

reservoirs in Albania was presented by Alfred Frasheri (2002). The experimental

study on variations of self potential and unsaturated water flow with time in sandy

loam and clay loam soils was investigated by Claude Doussan et al (2002). Two

conductive tracers, one deep and one on the surface, were injected and

monitored for direct evaluation of the groundwater flow vector using resistivity

and self potential methods (Sandberg et al 2002).

Nimmer (2002) made an experimental study on direct current and self

potential monitoring of an evolving plume in partially saturated fractured rock in

Columbia River basalt. The study of fluid circulation of the Stromboli Island using

a dense coverage of self potential (SP) and soil CO2 data was made by Anthony

Finizola et al (2002). A self potential survey has been conducted around Waita

volcano, Kyushu, Japan by Yasukawa et al (2003). Resistivity and self potential

changes associated with volcanic activity in Japan was monitored by Zlotnicki

et al (2003).

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The log shapes are used to find the fades and environments by several

authors. Krueger (1968), Galloway (1968), Fisher (1969), Eons (1969), Pirson

(1970), Goetz et al (1977), Coleman and prior (1982), Galloway and Hobday

(1983) defined the relationship between log shape and grain size in sandstone

bodies and classified sand bodies on the basis of the shape of the Self potential

log. Shields and Gahan (1974), Wolff and Pelissier-combescure (1982)

emphasized the significance of log shape character. Davies & Ethridge (1975),

Tixier and Alger (1967), Serra and Abott (1980), Selley (1976), Sarg and Skjold

(1982), Poupon et al (1970) used the log shapes to identify the depositional

environment.

Of the various surface and subsurface geophysical methods, the

combination of surface resistivity and surface self potential methods are used in

the present study.

1.7 Aim and Objectives

The range of resistivities of rocks is not unique and a considerable

overlapping of several rock types is noticed (Table 1.1). As a result resistivity is

an extremely variable parameter not only from formation to formation but even

within a particular formation (or type of rock). There is no general agreement of

lithology with resistivity. The resistivity of porous sedimentary formations is highly

variable, depending on the degree of saturation and the nature of the pore

electrolytes. Since the resistivity range overlaps for the different rock and soil

types, it becomes necessary to assess the soil stratum encountered with

available borehole data.

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Table 1.1 Range of resistivity of some common rocks and minerals (Loke andBarker, 1996b)

Material Resistivity (ohm-m)

Igneous and Metamorphic

Granite 5 x 103— 106

Basalt 103— 106

Slate 6 x 102 _4 x 10

Marble 102_2.5 x 108

Quartzite 102_2 x 108

Sedimentary Rocks

Sandstone 8-4x10

Shale 20-2 x

Limestone 50-4 x 102

Soils and Waters

Clay 1-100

Alluvium 10-800

Groundwater (fresh) 10-100

Seawater 0.2-1

The range of resistivity of soils depends upon moisture content, salinity,

level of groundwater, degree of compactness, mineralogy and other factors

(Karanth, 1987; Woods R.D, 1994). Generally, rock skeletons without clay matrix

and fluid content are bad conductors. Reduction of porosity by cementing

process in pore spaces of the rocks increases the resistivity values. The clay in

the moisture state encourages ion exchange process, and as a result the

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conductivity increases manifold. The highly conductive brine in saturation has

tended to mask the real character of the bed rock (Kailasam, 1963A).

The resistivity curves obtained in depth soundings readily indicate whether

two or three layers are involved. It is restricted to identify only a few numbers of

layers. Mathematical studies and laboratory experiments also have established

that the apparent resistivity curves for ideal layers do not show breaks or other

irregularities of the type depending upon the empirical methods of interpretation

(Rao, 1993). The resistivity method sometimes fails to locate the conducting

minerals in the disseminated form (Apparo et al, 1997).

The resistivity values of the rock formation are mainly controlled by the

fluid type and the compactness of the sedimentary formation and it is difficult to

give explanation for the following:

1. To recognize more number of different sedimentary layers in shallow

depth.

2. To estimate the depth of the sedimentary layer boundaries

3. To identify the disseminated form of conducting minerals

4. To distinguish the salt water sand from clay bed (in wet state)

5. To differentiate the hard rock from the clay bed (in dry condition)

6. To make a distinction of the limestone from sandstone

7. To explain masking effect of true resistivity of rock formation due to the

saline water incursion

The self potential (SP) i.e. the naturally occurring current, depends on the

chemical and the physical nature of the rock formations and also it is indicative of

permeable or porous zones. Therefore, the SP method in conjunction with

resistivity method has been used to find out the explanation for the problems

mentioned above.

The following are the main objectives of the present study.

1. To establish the depth of the SP source and the Current penetration

depth in VIES study

2. To identify the mineral formation and also to reconstruct lithological

sequences through the surface level SP and the resistivity study

3. Recognition of the depositional sedimentary environments through

the log shape analysis.

It is also planned to delineate the depthwise bed boundaries, thickness,

and porous and permeable properties of the rock formations and the nature of

fluid type.

1.8 Order of Presentation

The work presented here deals with various aspects pertaining to the

geophysics delineated using geophysical techniques, surface level SP and

resistivity methods, mineral deposits and lithology of subsurface formation and

the depositional environment. The thesis has been divided into seven chapters

and the investigations covered are as follows

Chapter 1 deals with the general aspects of the study comprising the

geophysical survey, geophysical methods, development of resistivity and self

potential methods and the objectives of the studies. This chapter briefly describes

the principles of conventional geophysical methods, such as self potential,

electrical profiling, vertical electrical sounding, electromagnetic induction, and non

contact electromagnetic profiling.

Chapter 2 furnishes an account of each of the electrical methods (SP and

resistivity), outlining the general principles on which they are based and the

instruments and equipments used.

Chapter 3 describes the detection of depth of different rock formations on

the basis of inflection points observed in the SP and resistivity curves. For this

study, laboratory tank model experiment and field experiments were carried out.

Field experiments in two levels, one at shallow level near Tuticorin and another at

deeper level at Neyveli open cast mine area were made.

Chapter 4 explains the identifications of minerals like gypsum and caicrete

beds in the disseminated forms in the black caftan soil of Tuticorin region and the

clay minerals like ball clay and fire clay; sand, sandstone, lignite and marcasite in

the Neyveli mine area are accomplished on the basis of the nature of deflections

observed on the SP curve and the highs and lows of apparent resistivity curve of

the surface level composite logs.

In chapter 5, the curve pattern analysis gives detailed depthwise

lithological sequences of sand, sandstone, limestone, clay and intercalated sand

and clay of the study areas: Govangadu, Spic area, Karapad, and Tuticorin. The

combination of curve elements (peak, trough, etc) of both SP and apparent

resistivity curves at a particular depth leads to the identification of the litho unit of

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the subsurface. After identifying all the litho units of a particular location,

depthwise lithological column was prepared. This was verified with the level of

depth limits of the core and the rock cuttings collected from the bore hole of the

respective experimental sites.

Chapter 6 explains the recognition of depositional sedimentary

environment of Manapad and Neyveli through the SP log shape (bell, funnel, and

cylinder) analysis. The interpretations of depositional environment and

depositional sequence were made after establishing lithology and electrofacies of

the sedimentary rocks of the Manapad and Neyveli formations.

Chapter 7 concludes the thesis by giving the summary and the significant

findings related to various analyses. Documentation of the work has been carried

out with adequate number of figures and tables followed by detailed references.