Geophysical investigation

“Geophysical Investigations for Engineering Projects”, 05-07 February, 2014, CWPRS, Pune

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MAGNETIC METHOD

R.S.Ramteke, Joint Director

1.0 INTRODUCTION

Majority of geophysical techniques depends on the contrasts in the physical properties of earth

materials. These contrasts may be in the acoustic impedances between sedimentary layers used in seismic

reflection work, or variations in the electrical conductivity of the subsurface materials used in electrical

surveys. In the case of magnetic methods, the source of the signals derives from lateral changes in the

magnetization of the earth materials. The methods, therefore, are adapted to finding horizontal changes

produced by either vertical displacement of homogeneous layers or by inherent lateral in homogeneities in

the earth materials themselves. Horizontal layers in which these properties are constant provide no signal

and consequently are “invisible” to the techniques.

Magnetic surveying enjoys more limited engineering applications but is useful in mapping the

geometry of igneous bodies such as dykes, sills, and other intrusions and detecting type may also be

determined from magnetic data.

2.0 BASICS

2.1 Magnetic field intensity:

By the extension of coulomb’s law for electrostatic force between two point poles S1 and S2 is

The magnetic field intensity is usually measured in oersted while magnetic induction is measured in

Gauss. The geomagnetic field varies from about 0.28 Gauss near the magnetic equator to about 0.66 Gauss

at the Magnetic Pole. The practical unit is nanotesla (nT) where

… (1)

… (2)


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1 nT=10-5

Gauss … (3)

2.2 Magnetic Susceptibility

When a magnetizable body is subjected to an external magnetizing field it acquires a magnetization

that is lost when the applied field is removed. Such magnetization I is said to be induced by the applied field.

I = KT … (4)

K is called magnetic susceptibility and is a characteristic constant for magnetizable material. Table

1 shows the magnetic susceptibility of various rock types from sedimentary rock which have small

susceptibilities reflecting small concentration of magnetic materials to basic igneous rocks rich in magnetic

materials.

Table. 1 Magnetic Susceptibility of Rocks and Minerals.

Mineral/Rock Type Susceptibility

(KX 10-6

SI units)

Quartz ( Diamagnetic) -15

Gypsum ( Diamagnetic) -13

Rock Salt ( Diamagnetic) -10

Slate 0 - 1200

Sandstone 35 – 950

Gneiss 0 – 3000

Granite ( with magnetite ) 20 - 40000

Basalts 500 – 80000

Hematite (ore) 420 – 10000

Magnetite (ore) 7.0 x 104 - 14.0 x 10

6

3.0 THE EARTHS MAGNETIC FIELD

3.1 The Geomagnetic Element and Poles

The geomagnetic field F is a vector quantity which requires the specification of three elements for a

complete statement of its magnitude and direction at any point. A common combination comprises the

vertical component, Z, the horizontal component H and the declination D which is angle between the

direction of the horizontal component (i.e. the magnetic north) and the true or geographic north. An

alternative set of elements is the total field intensity F, its inclination, I with respect to the horizontal and

declination, D, occasionally the field components are directly referred to geographical co-ordination north

X, east Y and vertically down Z.


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The axis of the best-fitting central dipole intersect the earth’s surface at two points which are referred

to as “geomagnetic poles” These poles are located approximately at 78.8º N, 71º W (in north west Green

land) and 78.8º S, 109º E ( in Antarctica).

Fig.1 Main elements of the geomagnetic field. D and I are the declination and inclination,

respectively, of the total field vector F

The widely accepted standard to provide reliable spatial distribution of the geomagnetic field is called the

“International Geomagnetic Reference Field “(IGRF) and it is revised every five years. A map of the total

intensity of the magnetic field over the earth surface (IGRF 2000) is shown in Fig.2.

3.2 Secular and Diurnal Variation

Long terms changes in the geomagnetic field which are progressive over decade or centuries are

known as the secular variation. They are immediately apparent from the yearly averages of the values of

geomagnetic elements recorded by magnetic observations all over the world. The longest record of secular

variation are from London and Paris, which are sufficiently close together to show nearly the same pattern

Fig.3 shows the change in declination and inclination at London and Boston over the past four centuries.


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Fig.2 Total Intensity of Earth’s magnetic field for the epoch 2010

Fig.3 Records of long-term changes in declination and inclination

of Geomagnetic field at London and Boston.

Short period changes in the intensity of the field follow a daily cycle and shows local variation of

some tens of nTs during normal quite day. On the other hand the “disturbed day” variation are irregular and

extreme in magnitude, amounting to several hundreds of nTs within an hour or so. They are associated with

magnetic storms which are related in some way with the increased solar activity during sun spot cycle. Fig.5

shows the record of a base magnetometer reading registered during a magnetically stormy day at Alibag,

India ( Fig.4) Such storm usually last for several days.


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Fig.4 Magnetic observatories in India

Fig.5 Record of H –Magnetogram at Alibag Observatory

Diurnal variations have considerable practical significance in magnetic surveying for geological

mapping and mineral prospecting. Of course during disturbed days magnetic surveying operation have to be

discontinued. Since there is no satisfactory way of allowing for their unpredictable effect on magnetic field

data.

3.3 Origin of the Main Field

A spherical harmonic analysis of the magnetic field observed over the surface of the earth shown that

the main field and its secular variation originate within the earth and is probably associated with an intense

current loop circulating in the core Fig.6. It is presumably derived from local eddy current produced in the

liquid core by friction due to the earth’s rotation.


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Fig. 6 The Earth’s Magnetic field associated with current - loop

3.4 Magnetic Field Anomaly

As the direction of the magnetic field varies over the surface of the earth from vertically down wards

at the North pole to horizontal at the magnetic equator to vertically upwards at the South Pole, the anomalies

due to essentially the same body ( same depth, same thickness, same length, same strike, etc.) vary

considerably. This is illustrated in Fig.7 where the anomaly due to same body is shown for various magnetic

latitudes. Anomaly also changes with a change in strike direction from due north to a strike of due east.

Thus in interpreting magnetic data, care must be taken to allow for these variation is anomaly shape that are

not due to variations in the shape of the body or its depth.

Fig. 7 Variation in total magnetic intensity with change in magnetic latitude.

3.5 Factors Controlling the Anomalies

The for most of the physical factors that controls the contribution of geological objects to the

measured geophysical parameter is the physical property of the object. As is well known for the success of

any geophysical method there should be sufficient physical property contrast between the objects to be

located and the surrounding and overburden rocks. It is evident that as this contrast increases, the anomalies


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in geophysical method enhance and the objects can be more reliably detected. The size and depth of the

causative body have a strong influence on the anomalies (Fig.8) The chances of detectability of object

increase with their increasing dimension and with decreasing depths.

Fig. 8 Magnetic Anomaly with physical property contrast

4.0 INSTRUMENTATION

Among the most widely used instruments for magnetic surveying are the fluxgate and proton

precession magnetometers. Ground based surveys use these instruments exclusively. In high resolution, air-

borne surveys including gradiometer surveys optically pumped alkali vapor magnetometers are used.

Although these are more expensive than the fluxgate and proton precession magnetometers, they are capable

of greater sensitivity.

4.1 Fluxgate magnetometer

The operation of this magnetometer depends on the saturation magnetization of a ferromagnetic core

when placed in low magnetic fields similar to that of the earth. An alternating current (freq. = 1,000 Hz)

passed through the primary coil drives the core to saturation producing a time varying magnetization shown

in fig.9. A secondary coil wrapped around the core and primary coil records an alternating electrical current


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whose magnitude is a function of the rate of change of the magnetizing of the core. In the instrument, two

identical cores are used around which the primary coil is wound in opposite senses. The resulting secondary

currents in each coil are equal in amplitude but out of phase by 180 and consequently add destructively to

produce zero total current. When the two core system is placed in the Earth’s magnetic field with its axis

aligned along the ambient field direction, the Earth’s field adds to one core’s magnetization while at the

same time subtracting from the other. The result is that the secondary current from each core are no longer

exactly 180 out of phase and consequently do not add destructively as before. The displacement in time of

the two secondary currents is a measure of the strength of the Earth’s field.

The fluxgate is frequently used in surveys to measure the vertical component of the Earth’s field.

Modern instruments are capable of 0.1 n T accuracy but in field operation 1 nT is often more realistic.

Fig. 9 Flux gate magnetometer circuit

4.2 Proton precession magnetometer:

The proton precession magnetometer uses the properties of the spin magnetic moment of the proton

to determine the field strength. Fig. 10 shows the design of a typical precession magnetometer. The protons

in water (H20) whose magnetic moment is initially randomized are subjected to a polarizing current, which

aligns the moments along a direction approximately perpendicular to the Earth’s field. The current is passed

for several seconds to ensure complete alignment of the magnetic moments. The polarizing current is then

switched off and the protons are allowed to relax in presence of the Earth’s magnetic field. In relaxing the

protons, precess about the ambient field direction with a frequency, which is directly proportional to the

strength of the field. By measuring the precessional frequency of the protons, the field strength may be

determined. Proton precession magnetometers have typical accuracies of 0.1n T although some manufactures

indicate better accuracies.


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Fig. 10 Block diagram of Proton precession magnetometer.

4.2 Alkali vapor magnetometer

The alkali vapor magnetometer (Fig.11) is based upon the splitting of electron energy levels in alkali

elements (typically Cesium and Rubidium) by the Zeeman effect. The energy gap produced (E in Fig.12) lies

in the radio-frequency range and is therefore usually detected in an oblique manner, is measured. When level

A1 is depleted, the incident light beam will no longer be absorbed but will pass through the vapor and be

detected by the photocell. If now the correct radio-frequency signal is applied, transitions from A2 to A1 will

occur. This will result in the light being absorbed once again. Because the exact radio frequency required is

unknown a varying RF signal is applied which sweeps through a range of probable frequencies. At the time

that the photocell voltage drops indicating absorption by the vapor, the RF being applied has the necessary

frequency to produce A2 to A1 transitions. This frequency then is related to the energy gap, E, between

A1 and A2, which in turn is controlled by the strength of the field producing the Zeeman splitting. Thus by

measuring the RF accurately, the total field vector can be determined. Accuracies of such instruments are

typically 00.1 nT to 01 nT but better accuracies have been quoted by several manufactures.

Fig. 11 Block diagram of Alkali vapor magnetometer


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Fig. 12 Splitting of energy levels in Alkali elements.

5.0 FIELD PROCEDURE

Magnetic surveys are usually carried out on a more or less regular grid with station spacing

appropriate for the target of interest. Thus for example for the detection of shallow bodies such as clay field

cavities at less than 20 m depth a spacing of the order of 5 to 10 m is more appropriate. For small scale

shallow bodies the station spacing may be only a few meters. An approximate rule of thumb is that the

station spacing should be at lease ½ depth of interest. In this survey, care must be taken to avoid all

extraneous sources of magnetism. Such sources will generally have a large effect on the measurement.

Magnetic surveys are also affected by temporal changes in the Earth’s magnetic field. It is therefore

necessary for a continuously recording magnetometer be placed at the base station to monitor this

observation. These variations may be several tens of n Ts each day and consequently the measurement at

each station must be corrected for this variation.

At each station, usually approximately 5 values of the magnetic field are obtained using a

magnetometer. The sensing head of the instrument is frequently placed on a tall (3m) staff so as to provide a

significant height above the ground to filter out very shallow extraneous sources.

6.0 DATA PROCESSING

Processing of magnetic data is relatively straight forward and involves only two main corrections 1)

Main field correction (sometimes called the Normal correction), 2) Diurnal variation correction. Magnetic

storms are rarely corrected for, as their influence is too variable and erratic. Data gathered during such

storms are usually discarded.


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1) The main field correction removes long period (100’s of years) temporal variation of the field, which

originates in the interior of the Earth. These secular variations have amplitude of 10 to 100 nTs per year and

are not usually important over the duration of a signal survey. The main role of this part of the correction,

however, it to allow survey data gathered in different years to be integrated without serious datum problems

(Fig.13)

Fig. 13 Variation in total magnetic field during 1960-1985.

To make the main field correction one simply uses one of the existing module of the internal field of

earth e.g. International Geomagnetic Reference Field (I.G.R.F). These are published on a periodic basis so

as to better approximate the secular changes in the field. The information required to obtain the main field is

the location of the site (lat and long), elevation and time of year. As the field typically varies by 1 – 3 nT/km

the positional accuracy required is not great. The vertical variation in the earth’s field is very small and

consequently elevations do not need to be known accurately.

2. The diurnal variation correction is used to remove the influence of ionosphere current on the measured

magnetic field values. Since the degree of ionization and therefore the resulting currents, is largely

controlled by the local sun angle, the magnetic field variation have a substantial regular component (Fig.14).

The field is therefore monitored within the survey area using a continuously recording magnetometer and the

measured field values arrested on the basis of these records. If the survey is close to a permanent

geomagnetic observatory then the records from such an observatory may be used.


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Fig. 14 Daily variation of vertical magnetic field

7.0 DATA ANALYSIS

After regular data processing, we are left with a composite signal that contains contributions from

many sources. Consequently, the next task is to identify the signals of interest. This is usually achieved in

one of two ways: either the unwanted signals are removed from the complex signal or the desired signal is

amplified in some way with respect to the other unwanted signals. The approaches are termed anomaly

separation and anomaly enhancement respectively. Techniques of anomaly separation result in a “pure”

signal, which may be directly interpreted in a quantitative manner whereas anomaly enhancement usually

results in data that are qualitatively interpreted. The results from both techniques provide important insights

into the distribution and overall nature of subsurface sources.

7.1 Anomaly Separation

There are many techniques used to separate out anomalies of interest; usually, although not always,

much of this effort is devoted to filtering out the effects of signals with longer spatial wavelengths (e.g. the

regional field) than a in the wavelength range of interest. Generally, in geo-engineering studies the desired

signals have wavelengths of a few meters to, at most, a few hundreds of meter. Anomaly separation

techniques include spatial convolution (e.g. grid residual methods) and frequency filtering methods.

One of the methods used in anomaly separation is that of the ring residual in which the data on a

regular grid are averaged around the periphery of a circle and then this average value subtracted from the

observed value at the circle’s center. The choice of circle radius is critical to the effectiveness of the

procedure and it is common to repeat the method for a selection of different radii (usually 2 or 3).A large


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radius results in longer wavelengths being removed while a smaller radius may begin to attenuate the desired

signal. The techniques therefore require care in the selection of an appropriate radius. Other techniques are

used which involve more than one circle and have the added problem of deciding on which circle to place the

greater emphasis (weighting).

Transforming the data to the frequency domain and then applying a frequency dependent filter to the

various components today invariably achieve anomaly separation. The anomaly separated data are then

transformed back to the spatial domain for visual inspection. The procedure is similar for profile data where

an assumption of two-dimensionality is made. A selection of frequency dependent filters can be tried (in a

similar way to the manner in which frequency filters are used on seismic sections) and the results inspected.

The major advantages of this approach are its speed and the fact that the wavelengths removed from the

original data are known.

In addition to these automated methods, there is a popular “hands on” or graphical technique in

which the interpreter uses his/her skill and extensive experience to determine which wavelengths are to be

removed. To achieve the separation, the interpreter fits a smooth, low order curve to the observed (Firg.15)

and then subtracts this to produce a residual anomaly. Although the technique is subjective and required

considerable time and effort where large data sets are involved, it has the benefit of flexibility in that it

permits individual data sets to be used independently in the determination of suitable “regional” for different

parts of the map. However, the amount of effort together with its arbitrary nature has resulted in a decline in

its popularity in recent years.

Fig. 15 Graphical technique of anomaly separation

7.2 Anomaly Enhancement

In the case of anomaly enhancement the signal of interest is to be amplified relative to other

unwanted signals, i.e. the signal to “noise” (i.e. unwanted signals) ratio is to be improved. Because the

signals of interest usually lie at short spatial wavelengths while the majority of the coherent “noise” is at

longer wavelengths, the techniques usually seek to emphasize the localized nature of such anomalies.


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Several parameters can be used but one of the more popular and effective parameters is the curvature or

second derivative of the anomaly field. Localized anomalies have high curvature while broad scale regional

features have low curvature. A contour map of the second derivative will tend to highlight localized features

while suppressing the regional anomalies. The maximum (or minimum) curvature will therefore tend toile

over the center of the causative body with the contours of the high curvature approximating the outline of the

body. The method is extremely useful in delineating “interesting” areas, which may not be apparent on the

original map. This is especially true wherever the broad scale features contribute a steep regional gradient to

the data thus masking the influence of small –scale features.

Other techniques of anomaly enhancement include field continuation (i.e. upward and more often,

downward continuation of the anomaly field) and various kinds of strike filtering, which will “being out”

subtle trends in the data. Downward continuation is especially popular, as this method will greatly amplify

localized small amplitude anomalies.

8.0 INTERPRETATION

8.1 Qualitative Interpretation

The qualitative interpretation of a magnetic anomaly map begins with a visual inspection of the

shape and trend of the major anomalies. After delineation of the structural trends, a closer examination of

the characteristic features of each individual anomaly is made. These feature are a) the relative location and

amplitude of the positive and negative part of the anomaly, b) the elongation and areal extent of the contour,

c) the sharpness of the anomaly as sent by the spacing of contours. In many cases, meaningful geological

information can be deciphered directly by looking at the map, without any calculation.

8.2 Quantitative Interpretation

After completing the qualitative study, it is important to extract some quantitative information from

the magnetic data. From the relative spreads of the maximal and minima of the anomaly, the approximate

location and horizontal extent of the causative body may be determined. The geometrical parameters must

then be translated into structural terms in the light of known geology. Finally, from the amplitude of the

anomaly, the magnetization contrast may be determined.


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For making rapid quantitative estimate, it is necessary to examine in detail the shape pattern of

magnetic anomalies of the most commonly used models in magnetic interpretation. In magnetic mapping of

two models have been widely used to approximate the shape of probable anomaly sources. One is bottom

less vertical Sided prism model. The other model commonly used is the two dimensional rectangular strip

which serves to approximate many geological features of extensive strike length i.e. thick dyke and broad

magnetic zones cutting the host rock.

Fig. 16 Magnetic profile showing measurements for slope and half slope parameters.

Some empirical depth rules have also been desired which when applied with care may furnish useful

results. Of these only two which have been most widely used. These based on the “maximum slope” and

half slope “parameter of the anomaly profiles”. On magnetic profiles a line of maximum slope is drawn

through a point of inflation 0. Maximum slope coincides very closely with the magnetic profile for a certain

distance and then breaks away from the straight line at consistently measurable position S1 and S2. The

distance S between these points is the slope parameter. The half slope parameter is the distance between the

point at which a straight line with half the maximum slope is tangent to the anomaly curve below and above

the straight line slope ( Fig.16) from these measurement, as a general rule, the depth to the sources is

approximately equal to S and also approximately equals to P/2.

9.0 APPLICATIONS

Because the magnetization of soils is quite small, the magnetic method has fewer applications in

geo-engineering problems. The magnetization contrast between various basement rocks and between dyke


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material and sedimentary formations, however, is sufficiently large that basement faults and dykes are

generally detectable. Where the basement is deep, more subtle shallow sources may be discerned. These

include areas of fill material, clay pockets and certain kinds of shale’s and sandstone. Intra sedimentary

volcanic material such as sills and dykes are associated with large magnetization contrasts, produce large

anomalies with locally steep gradients, and as such are readily detectable. Where the magnetic data are

carefully interpreted, information regarding the depth, attitude and thickness of such bodies may be obtained

although it is perhaps more common to simply use the magnetic method to indicate the presence/absence of

such material within survey areas. Among the more prevalent engineering uses of the magnetic method is

the detection of disused mine working where the mineshafts have been inadequately sealed and therefore

pose a significant risk to future structures. In such applications, the magnetization contrast between the fill

plus lining of the shaft and the surrounding soil will be sufficient to produce anomalies of several tens to

sometimes hundreds of nTs. Thus where contrasts in magnetization are associated with changes in other

physical properties, the magnetic anomalies may be used to draw attention to such areas where more detailed

studies using other geophysical and geological techniques are required.

10.0 CASE STUDIES

10. 1 Magnetic survey for delineation of Dolerite Dyke Cutting Across Omkareshwar Dam Axis.

Magnetic survey was carried out in the vicinity of Omkareshwar Dam Khandwa District, Madhya

Pradesh to delineate the dolerite dyke and to establish its continuation across Narmada and Kaveri rivers

from the nose of Manedhata Island. The dam is located77 km away from Indore (Latitude 20º 14’ 25’’ N

longitude 70º 9’15’’E). The area around proposed dam site consist of hard compact quartzite and fissile

quartzite rocks. The quartzite at some places is interblended with siltstone and shale. The rocks towards the

right bank are cut across by a dolerite dyke and exhibit sheared contacts.

The field investigation was made with hand held proton precession magnetometer manufactured by Scintrex

Ltd., Canada which measures total field with an accuracy of 1 gamma. For recording or monitoring the daily

variation in the earth’s magnetic field, similar equipment with recorder was used at the base station (Fig.17).

Fig. 18 indicates isoanomaly map contoured at 100 gamma interval for Omkareshwar dam site. This

map depict irregularly shape magnetic contours, however, one prominent anomaly trend along X1 X 2

corresponding to dolerite dyke could be picked up, extending roughly ENE-WSW direction. The nature of

magnetic anomaly contours indicate that the dolerite dykes which extends from upstream, takes a turn in the

vicinity of dam axis between Ch.500 and 530m. Total magnetic anomaly for the dyke is of the order of 5000

gamma. This high anomaly can be due to thick and wide dyke. The results indicate that the length of the


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dyke is more than 200m extending from the nose of Mandhata Island towards West, cutting across the

junction of Narmada, Kaveri rivers and terminating at Panthia village.

Fig. 17 Location map of Omkareshwar dam axis.

Fig. 18 Total Magnetic field anomaly


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10.2 Magnetic survey to delineate lineaments/fractured zones for Groundwater Explorationat Usha

Ispat, Sindhudurg Dist.Maharashtra

The integrated steel plant being set up by M/s Usha Ispat Ltd at Satarda, Sawantwadi Taluka,

Sindhudurg Dist. Maharashtra requires about 1500m/hr of water for its operation. Since the demand of

water is substantial and cannot be met from surface sources alone, exploration of groundwater resources was

attended in the area.

Generally, the occurrence of Ground water in hard rock terrain is associated with the geological

structural features like lineaments, fractures/ fissures, fault zones etc. Using magnetic contrast between these

zones and basement massive can identify these features/ impervious rock is sufficiently high enough to

produce significant anomaly.

Magnetic survey was carried out in the entire area (Fig.19) to demarcate the weathered/ fractured

zone and lineaments that are favorable for groundwater. A total eight profiles were taken in a near NW-SW

direction. The length of these profiles varied from 450 m to 1200 m with a station interval of 25 m.

The measured magnetic values were first corrected for diurnal variation through frequent repetition

of base value. Regional – residual separation was carried out using graphical approach. The criteria adopted

for separating this anomaly was to fit a smooth low order curve with the observed one and then subtract this

to produce residual anomaly. The magnetic anomaly values (measured in nT) were plotted against the

distance traversed, as shown in Fig.20.

Fig. 19 Location of Magnetic Profiles


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In each profile, the low order magnetic anomalies were identified and correlated visually with those

on the adjacent profile. Low order magnetic anomalies were observed mostly across the lineament along

which a stream also runs and due to the effect of running water along this stream, the disintegration as well

as decomposition of the laterite rock might have taken place. This might have caused reduction in the

magnetic field and hence produced the low order magnetic anomalies. Thus, the fractured, weathered zones

and lineaments were delineated. These weak zones are shown in Fig.20. However, another minor

discontinuous lineament was also observed by the magnetic method, parallel to the above lineament in the

southern portion of the area, which is concealed by a thick soil cover.

Fig. 20 Map showing magnetic results

10.3 Magnetic Survey for detecting disused or abandoned mine

The study area in an industrial estate where light engineering factories and warehouses are located.

The soil is predominantly black fill, colliery waste overlying a silty clay with gravel sized rock fragments.

There are therefore, opportunities for magnetic anomalies produced by contrast in soil magnetization as well

as those due to presence of mine shaft.

The total field anomalies contoured at 50 nT are shown in Fig.21 and 22 although there are several

anomalies present in the study area; only one is a major anomaly. This anomaly it’s composed of 25 nT

positive to the south and – 150 nT negative to the north. Such a composite anomaly is typical of the

anomalies found over abandoned shafts. Boring in the vicinity of the anomaly revealed the presence of

disused mine shaft. Upon excavation the top of the shaft was found to contain large amount of coiled were

rope.


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Fig. 21 Location map of study area

Fig. 22 Magnetic anomaly map

10.4 Bedrock topography from Magnetic anomalies

In the hard rock terrains, the thickness of weathered layer is an important parameter that determines

the quantity of groundwater accumulated in the unconfined aquifer above the basement .The basement rock,

in the process of weathering, loses it s magnetic properties and becomes much less magnetic. Therefore, the

magnetic response is mostly due to the unweathered hard basement rock and the depths of magnetic sources

obtained from the analysis gives us the top of the basement. Information about the thickness of the

weathered layer would help in assessing the groundwater potential of the region.


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The ground magnetic map (Fig.23) of a part of Hydrabed city has been analyzed to derive the

basement top configuration. The basement depth obtained from magnetic anomalies is in general

agreement with those obtained from drilling (Fig.24)

Fig. 23 Magnetic anomaly map of part of Hyderabad city

Fig. 24 Depth contour map

10.5 Vertical Magnetic Survey for detecting ore body.

A high magnetic anomaly in old working near Garividi of the Kodur manganese belt Srikakulam

District, Andhra Pradesh was found to be associated with a manganese ore body possibly partly worked out

some decade ago, but now concealed by dumps. The dumps near the ore body also contributed to the

magnetic anomaly, making it broader in shape as shown in the Figure 25.


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In this area manganese ore deposits did not always give rise to magnetic anomalies. It was

established by laboratory studies that the area show high magnetic properties and give rise to sizeable

magnetic anomalies only when associated with certain manganese minerals like vredenbergite and magnetite.

Fig. 25 Magnetic anomaly map of ore body

11.0 REFERENCES

Bhimasankaram V.L.S., 1977, Exploration Geophysics, AEG Hyderabad.

Dobrin, M.B., 1976, Introduction to Geophysical Prospecting, McGraw Hill.

Ghosh N and Hall S.A., 1992, Cross over analysis of magnetic data over the Colombian Basin, Western

Caribbean Sea, Geological Society of India Memoir No. 24, 93-101.

Hooper W and McDowell P 1977, Magnetic survey for buried mine shaft, Ground Engineering.

Nettleton L.L., 1976 Gravity and Magnetics in oil Prospecting, McGraw Hill.

Rambabu H.V., Kameswara Rao N and Vijay Kumar V 1991, Bedrock topography from magnetic

anomalies.- An aid for groundwater exploration in hard rock terrains. Geophysics, Vol 56, 1051 – 1054.

Ramteke R.S. and Rangarajan G.K. 1982, Correlated long-term changes in Solar wind and Geomagnetic field

at low latitude, Proceeding of Space Science Symposium, Vol.II 323.


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Ramteke R.S. and Mutta K.S., 1988, Magnetic Survey for Delineation of Dolerite Dyke Cutting across

Omkareshwar Dam Axis, Journal of Association of Exploration Geophysics, Vol.IX 131-135.

Ramteke R.S., Venugopal K.,Ghosh N., Krishnaiah C, Panvalkar G.A., and Vaidya SD, 2001 Remote

Sensing and Surface Geophysical Technique in the exploration of Ground Water at Usha Ispat Ltd

Sindhudurg Dist., Maharashtra Journal of Geophysical Union, Vol 5, 41-50.

Rangarajan G.K., Ramteke, R.S. & Krishnamoorty, G.V. 1981 Severe Geomagnetic Disturbance of 19 Dec.

1980, Indian Journal of Radio & Space Physics, Vol 10, 76-77.

Rangarajan G.K. and Ramteke R.S. 1982, Solar wind Intraction region and low latitude Geomagnetic field,

Indian Journal of Radio & Space physics, Vol.13, 16-18.

Sharma, P.V., 1986, Geophysical Methods in Geology, Elsevier publication.

Geophysical investigation

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