Introduction Geophysical -VI

7/28/2019 Introduction Geophysical -VI

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G.Vijayakumar, SACET, Chirala 1

GEOPHYSICAL INVESTIGATIONS

Introduction

Geophysics is study of earth by making use of established principles of physics. These studies can deals the Earths Crust including oceans and atmosphere. It is measuring certain physical properties and interpreting them mainly in terms of

subsurface geology.

Branches of Geophysics

Based on the aims of investigations, geophysics has two branches namely

1. General geophysics: The study of earth as a whole and its major surface features)

Ex: Earth Structure, their intervening major discontinuities

2. Exploration geophysics: Its aim is to make specific investigations of economic importance.

Ex: oil and gas, ore deposits, groundwater for solving of certain problems of Engg.Geology

Necessity of Geophysical Investigations

To ensure safety, success and economy in construction of major civil engineeringstructures.

It is necessary to be thoroughly aware of the geology of the concerned site.There are two approaches

1. Direct observations

2. Indirect observations

Direct observations: it can be made by digging, trenching, or drillingthe ground. Such

processes are expensive.

Indirect observations:

These provide quick, expensive, easy and fairly reliable means to get subsurfacedetails.

Generally these investigations are followed by making drillings or trenches at someselected places and checking the inferred data with the observed data.

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Importance of Geophysical Investigations

1. These investigations are carried out quickly. This means large areas can beinvestigated in a reasonably short period and hence time is saved

The geophysical instruments used in the field are simple. And can be operated easily.This means the field work is not laborious.

Since the work carried out quickly and only physical observations are made withoutthe use of consumables (like chemicals), it is economical too.

Different inference to suit different purpose can be drawn from the same field data. These are the multipurpose.

Ex: Electrical Resistivity data can be interpreted for knowing rock types, GW

conditions, ore deposits, depth to bed rock..etc.,

6. Reconnaissance surveys are enough. In the scale format of 1: 100,000.Engineering Geophysics

There are number of important civil engineering problems in which geophysical methods find

extensive applications. Most of the involve

1. Foundation problems2. Surface rock formations, their attitudes3. Associated geological structures(Faults and Joints) in places where dams, reservoirs,

or any other constructions are planned.

4. In such areas geophysical investigations area) in the determination of the thickness of overburden (the depth of bed rock)

b) The bed rock profile along a given direction such as axis of a dam.c) Locating fault zones, shear zones etc.,d)

Determination of GW potential, and its favorable conditions in CE. Like tunnels,reservoirs.


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Classification of Geophysical Methods

There are Six types

1) Gravity Methods2) Magnetic Methods3) Electrical Methods4)

Seismic Methods5) Radiometric Methods

6) Geothermal MethodsGravity Method

The gravity method involves measurement of the gravitational attraction exerted bythe earth at a measurement station on the surface.

The strength of the gravitationalfield is directly proportional to the

mass and, therefore, the density ofsubsurface materials.

The normal value and the Observevalue of deflection is Called

anomaly.

Method: The instruments which

are commonly used to measure

gravitational deflections are

1. Pendulum2. Torsion balance and3. Gravity meter

Of these gravity meter is most

useful. For covering larger areas

rapidly airborne gravity survey is done.

In the area of search, traverse are laid at suitable intervals. Then the values of

gravitational deflections are measured at predetermined points. The readings thus the

obtained are plotted on a graph with distances on x- axis and deflection on the y-axis.

If a dense rock or a massive ore body is present in the area, the graph will show an

anomaly in the form of a peak as shown in Figure: 1.

The gravity data can also interpreted by contouring the anomaly. In this case

the gravity anomaly for each station is plotted on a base map and then lines of equal

gravity anomaly are drawn in the same way as contour lines. Units of gravity meter is

mGal (MilliGal)

Fig: 1


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Magnetic Methods

These are used to measure the magnetic intensity of the ground at various stations. For

covering large areas rapidly, airbrone magnetic surveys are conducted

i) The magnetic surveys have been used widely for the exploration of oil andmagnetic ore bodies such as deposits magnetite, pyrrhotite and ilmenite.

ii)

At places faults may bring together rocks of different magnetic properties. Hencethey may be delineated from magnetic data.

iii)The magnetic and pyrrhotite are more abundant in basic igneous rocks than in acidrocks. Hence the former can be detected by the magnetic surveys.

iv)Certain mineral deposits which contain magnetic minerals are in subordinateamount, such as magnetite with asbestos and pyrrhotite, with base metals, can be

detected by the magnetic surveys.

Method:

The magnetometers are used to measure the magnetic intensity of the ground at various

stations. For covering large areas rapidly, airbrone magnetic surveys are conducted.

In the area search, traverses are laid at suitable intervals. Then the values of magnetic

intensities are measured at closely spaced stations, For each station, the observed value is

compared with the normal value. The difference between them is called magnetic

anomaly.

The values of anomaly are plotted on a base map. Then the lines of equal magnetic

anomaly are drawn in the same way as contour lines. From such a map, the area of the

magnetic body can be readily delineated. The anomaly data may also be interpreted by

constructing magnetic profiles in the same way as done for gravity data. Figure.1

Electrical Methods

These are used mainly for the exploration of metallic mineral deposits.

The electrical survey methods are four types

Self potential method Equipotential method. Electromagnetic method Resistivity method

Self potential method

These currents are called telluriccurrentsIn this process the electrical

energy produced by the ore body itself is

directly measured no outside energizing force is required.


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Equipotential method

The equipotential method is best suited to shallow

Deposits in the regions not to wet. It can be used to locate

ore bodies in the glacial Drift and for determining

structure beneath the soil. It is also used for geologicalvertical formations

Electromagnetic method

This is most favored method of ore bodies, it is More

precise and yields greater information regarding shape, size

and position of the hidden ore body

Resistivity method

The amount of resistance met by an electric current which is passed through a portionof the earth, is measured.

The measure of resistivity is presumed to be measure of the fluid content and porosityof rocks.

This is distinction between Saturated rocks and unsaturated rocks, and porosity.Uses:

The resistivity surveys are very effective in the investigation of horizontal or gentle dipping

rocks. these are used in detecting the following.

1) The thickness of overburden or depth to bed rock is determined very accurately.2)

The resistivity surveys have been used in the exploration of the placer deposits and

bedded deposits

3) The resistivity methods have been used widely for the exploration of groundwater. Inregions of gentle dips the presence of aquifers can be determined.

4) Fault zones may be determined as they contain electrolyte in solution.5) These surveys can be used for discovering the subsurface structure and lithology. The

buried aniticlines can be traced by detecting depths to strata of greater or lesser resistivity.

Hence they are also used the exploration of petroleum.


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These are different types

1. Profiling2. Sounding3. Self potential method

Profile is done to detect lateral changes in resistivity. This throws light on the change in the

subsurface lithology or structure from palce to place. Sounding(Which is popularly known as

vertical electrical sounding, VES) is done to determine vertical changes in resistivity.

There resistivity types are

1. Schlimberger array

2. Wenner array

3. DipoleDipole array

1. The Wenner configuration: This method was

developed by Wenner in 1915. In this

configuration, the outer electrodes, C1 and C2

are used to send current into the ground and the

inner electrodes, P1 and P2 are used to measure the potential. The important feature of this

setup is that the distance between any two successive electrodes is equal. The apparent

resistivity measured in the Wenner method is given by Pa = 2a(V/I) where

a = Electrode separation

v = Potential difference measured

I = Current sent into the ground

Pa = apparent resistance (ohm-mts )

Profiling Method: This is also known as Lateral Electrical investigation. In this process, the

electrode array i.e., setup as a whole is moved from place to place with same intervals

(constant electrode spacing) along a given line and the Pa value at each of the station is

determined. The changes in pa indicate lateral variations in the subsurface to a certain

depth.

It is obvious that the profiling technique will be useful in detecting only the dyke bodies or

vertical beds. The presentation of profiling data is done on a ordinary graph sheet on X-axis

(station) and true resistivity values on Y-axis (ohm-mts). The interpretation of profiling data

can demarcate the high and low resistivity values of the sub-surface.

2. The Schlumberger Configuration: This method was developed by Schlumberger in

1916. In this method, potential electrodes are kept at smaller side compared to the current

electrodes. In general, the electrode separation MN < 1/5 AB relation is maintained in this

investigation. Here A and B are the current electrodes and M and N are the potential

electrodes. The apparent resistivity measured in the Schlumberger configuration is given by

Pa= K (V/I) where K is a constant and the value varies as per AB distance


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Sounding: This method is popularly known as Vertical Electrical Sounding (VES). In this

method, a number ofpa values are measured at the same place by increasing the

distances between the current electrodes each time

after taking the reading. The successive increasing

in distance makes the current penetrate more and

more deeply. Generally, the depth of penetration of

the current (ie depth) is about 1/3rd of the distance

between the current electrodes. It is necessary to

cancel the self-potential before taking V and I readings

for every electrode separation. From I/V values of

each electrode separation, pa is obtained and it is multiplied by the configuration constant

(K) to obtain true resistivity values.

The values are plotted on loglog sheet by plotting the electrode separation

(station/distance) on x-axis and true resistivity values (ohm-mts) on y-axis. The obtained

curve is to be matched with master curves.Seismic Methods

The variations in the seismic wave velocity are measured in different rock layers

Method

In seismic surveys, truck mounted drilling rigs and recording systems are used. Small

charges of explosive are detonated in shallow breoholes drilled in the surface rocks. the

seismic waves thus generated are transmitted through the rocks and are picked up by a

series of geophones carefully spaced along lines of traverse

Fig:1: Showing shot points and geophones along a line of traverse

The geophones record the vibrations on a rapidly moving photographic paper.

These are two methods

Refraction method: The seismic waves undergo reflection and refraction at the rock

boundaries. In the refraction method only the refracted waves are recorded and used for

determining the structure of rocks. The refraction survey is commonly used for the following.

For determining the structure of rocks lying at relatively shallow depth. It is widelyused for Civil Engineering explorations

For location of shallow salt domes and the oil pools associated with them.


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Reflection method: this is generally used for deep explorations (600 m ore more). It is

widely used for oil explorations. This method accurately delineates the subsurface structural

traps and salt domes.

Radiometric Method:

The nuclei of elements are unstable spontaneously into the nuclei of other elements

This change is accompanied by emission of radiations known as radioactivity .

These are three types

- Rays

Rays

Rays

- Rays: these are the beams of positively charged particles.These particles are the nuclei of

helium atoms(2 protons + 2 neutrons). They travel with velocities of thousands of km/s

Rays

These are the beams ofnegatively charged particles.

They travel faster than - rays

- Rays

These are the rays of very shortwave length like x-rays.

The normal radioactivity indifferent types of rocks. In

Igneous rocks it decreases with

decreasing acidity .i.e.,

In granites it is more and in basic or ultrabasic rocks. it is the least, In sedimentaryrocks, clays have high radioactivity. Shales, sandstones and limestones have

successively decreasing activity.


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Geothermal Methods

A geothermal method deals with measurements of the physical properties of the earth. The

emphasis is mainly related to temperature and fluid content of the rocks.

The aim can be to delineate a geothermal resource, to locate aquifers, or structures that may

control aquifers etc

Temperature distribution on the surface of the earth is due to three different sources.They are:

(i) Heat received from the sun ( varies with the time of the day and with the season )up to a few meters depth only.

(ii)Heat conveyed from the hot interior of the earth due to conduction and convectionprocesses

(iii)Heat due to decay of radioactive minerals in the crust of the earth.

By eliminating (i) and (ii), the solar heat component and the heat contribution of radioactive

mineral decay, the only one is to interpret the values of temp ofthe earths surface. For the

measurement of the temperature on the surface of the earth, in shallow holes or in deep bore

holes, THERMISTOR THERMOMETERS and PLATINUM RESISTANCEHERMOMETERS can achieve an accuracy of 0.01oC are used. The geothermal

methods find application to locate structural bodies, oil and gas structures, ore

deposits, ground water studies etc

Thermal methods include direct measurements of temperature and/or heat, and thus correlate

better with the properties of the geothermal system than other methods. To measure

temperatures close to the surface, in the uppermost part of the earth crust is fairly simple.

Knowledge about status at deeper levels is based on the existence of wells, usually shallow

gradient wells (e.g. 30-100 m deep), from which the thermal gradient can be calculated and

possibly the depth to the exploitable geothermal resource. Drilling is though usually fairly

expensive, and puts practical limits to the use of the method. Furthermore, shallow wells arenot always adequate to get reliable values on the thermal gradient.

The heat exchange mechanism in the earth is important for interpretation of thermal methods.

A distinction is made between:

Conduction, which is based on atomic vibrations, and is important for transfer of heat in the

earth's crust;

Convection, which transfers heat by motion of mass, e.g. natural circulation of hot water;

and

Radiation, which does not influence geothermal systems.

The parameter k, the thermal conductivity (W/mC), is a material constant, which ranges

between 1 and 5 W/mC, with the low values usually associated with sedimentary formations

and the higher for crystalline rocks. The thermal gradient, )T/ )z, gives information on the

increase of temperature with depth, and its distribution can be important information for

understanding and delineation of the geothermal resource, both on a regional scale and local

scale. If the conductive hear transfer, Q, is 80- 100 mW/m2 or higher, it may indicate

geothermal conditions in the subsurface

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Introduction Geophysical -VI