Gravity Survey Method_31072012

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Gravity Survey Method



Medieval artistic representation of

a spherical Earth - with

compartments representing Earth,

air, and water

The Earth as seen from the

Apollo 17 mission.

Better understanding of Human Being over the centuries

http://en.wikipedia.org/wiki/Image:The_Earth_seen_from_Apollo_17.jpg

http://upload.wikimedia.org/wikipedia/commons/e/e2/John_Gower_world_Vox_Clamantis_detail.jpg



Three-dimensional visualization of geoid undulations, using units

of Gravity

•



How Is the Gravitational Acceleration, G, Related to Geology?



The Relevant Geologic

Parameter is NotDensity, But Density

Contrast

This simple thought

experiment forms the physical basis on which

gravity surveying rests.



Gravity Measurement

Over a Buried Sphere



Principal Sources:

1.Basement Structures( Horst and Graben)

2. Basement Fault

3. Fault in Sediments

4. Sedimentary Structures

5. Salt Diapir

6. Variation in Overburden Thickness

Origin of Gravity Anomaly



How do we measure Gravity?

Falling Body Measurement :

One drops an object and directly computes theacceleration, the body undergoes, by carefullymeasuring the distance and the time as thebody falls.

Pendulum Measurement :Gravitational acceleration is estimated bymeasuring the period of oscillation of apendulum.

Mass on a Spring Measurement :By suspending a mass on a spring and observinga how much the spring deforms under the forceof gravity, an estimate of the gravitational

acceleration can be determined



Portable Pendulums were

used in Oil Exploration

till 1930’s.

Pendulum Measurement



Mass on a Spring Measurement

Use started in early 1930’s and replaced the preexisting

gravity surveys.

These are extremely sensitiveMechanical Balances with

mass supported by a spring.

Small changes in gravity move

the weight against the

restoring force of the spring




kx = mg, where, k is spring constant, xis length of spring, m is mass of spring , g is acceleration due to gravity

kΔx = mΔg (Any change in gravity

i.e. Δ g should produce a proportionalchange Δ x in the stretch of spring)

The natural period of oscillation is

T = 2 Π m / k •Mass = 10 gm, T < 10 second whatwill be length of spring.




kx = mg, where, k is spring constant, x is length of

spring, m is mass of spring , g is acceleration due togravity

kΔx = mΔg (Any change in gravity i.e. Δ g should

produce a proportional change Δ x in the stretch of

spring)

The natural period of oscillation is T = 2 Π m / k

Survey requirement for portability of instrument

Mass = 10 gm, T < 10 second what will be length of

spring.



Data Acquisition

• Instrument

–Gravimeter based on mass spring

balance• Field Method

–Areal coverage,

–Distance between control points will

decide resolution of data



Gravity Measurements in exploration

For relative measurements the difference

between the gravity values is determined(which is easy to measure).

The instruments, which are used, formeasurement are known as Gravimeters.

The difference in gravity value in a particulararea is due to the difference in distribution of masses.

This can be attributed to variation in density of

various lithological units.The relative gravity values in turn are indicative

of distribution of densities in a given area.



Gravimeters

1. All gravimeters are extremely

sensitive balance in which mass issupported by a spring.

2. These spring balances carry aconstant mass

3. Variation in the weight of mass is

caused by variation in gravity; causethe length of spring to vary and togive a measure of change in gravity.



Static and Unstable static Equilibrium

• Objects that are not moving in any way either in

translation or rotation in a frame of reference , theyare in static equilibrium

• If a force can displace a body and end the

equilibrium, the body is in unstable staticequilibrium.

• (An unstable equilibrium is a situation in which all

forces on an object resolve to zero, but if any oneof the forces is changed slightly, the object will fallover, dash off in some direction, etc., and come torest in a different place )



Stable gravimeter

This type of gravimeter has linear dependence on gravity

over a large range. They require considerableamplification of the minute changes in length of the

spring. This amplification may be mechanical, optical,

electrical, or a combination of these. They are in state of

stable equilibrium or static equilibrium. (Ref Fig.)

Disadvantages:

1. Very High Period

2. Extremely sensitive to other physical factors such as

pressure, temp, and small magnetic and seismic

variations.

3. Bulky and Heavy due to thermostats



Stable Gravimeter Working Principle

g =4* Sq. of s / Sq. of T



Unstable gravimeter: Warden Gravimeter

A small very light Quartz element of the mass Mweighs only 5 mg.

Thus it is not necessary to clamp the movement between stations.

The system is enclosed in vacuum flask reducessensitivity to P & T.

Automatic temperature compensatingarrangement to reduce the variation of T.

Warden gravimeter is small (24’ in height and10’ in diameter) and weighs about 6 lbs. The

power requirement of two cells is used toilluminate the scale.



g k/M)(b/a) (cos2 cos s

Basic Principle of Warden Gravimeter



Diagrammatic View of Interior of Worden Gravimeter



g =(k/M)*(b/a)*(z/s)*(y/s)* s

Lacoste-Romberg Gravimeter



• For a given change in g, one can make s as

large as possible by decreasing one or morefactors on the right hand side; moreover thecloser the spring is to zero length, smaller thez and larger s.

• In operation, this is used as a null instrument,a second spring being used which can beadjusted to restore the beam to the horizontal

position. The sensitivity is about 0.01 mgal.



Zero Length Spring

P i i l f G it S



Principle of Gravity Survey



Field Method of Gravity Survey



Record of Gravity Survey



Processing of Gravity Data

•

Factors that affect the ‘g’ – Temporal Variations –

• These are changes in the observed acceleration that are

time dependent. In other words, these factors cause

variations in acceleration that would be observed even if we didn't move our gravimeter.

– Spatial Variations –

• These are changes in the observed acceleration that are

space dependent. That is, these change the gravitationalacceleration from place to place, just like the geologic

affects, but they are not related to geology.

F t th t ff t th ‘ ’



Factors that affect the ‘g’

Instrument Drift - Changes in the observed

acceleration caused by changes in the responseof the gravimeter over time.

Tidal Affects - Changes in the observedacceleration caused by the gravitationalattraction of the sun and moon.



http://upload.wikimedia.org/wikipedia/commons/0/00/Geoida.svg



1. Ocean

2. Ellipsoid

3. Local plumb

4. Continent

5. Geoid : Being an equipotential surface, the geoid is by definition

a surface to which the force of gravity is everywhere perpendicular

http://upload.wikimedia.org/wikipedia/commons/0/00/Geoida.svg



Instrument Drift

• A gradual and unintentional change in the reference value with

respect to which measurements are made.• Even if the instrument is handled with great care (as it always

should be - new gravimeters cost ~$30,000), the properties of

the materials used to construct the spring can change with

time.

• These variations in spring properties with time can be due to

stretching of the spring over time or to changes in spring

properties related to temperature changes. To help minimize

the later, gravimeters are either temperature controlled or

constructed out of materials that are relatively insensitive to

temperature changes. Even still, gravimeters can drift as much

as 0.1 mgal per day.



Notice the general increase in the gravitational acceleration with

time. This is highlighted by the green line.This line represents a least-squares, best-fit straight line to the

data.

This trend is caused by instrument drift. In this particular

example, the instrument drifted approximately 0.12 mgal in 48

hours.



Accounting for Elevation Variations: The Free-Air Correction



Variations in Gravity Due to Excess Mass



Correcting for Excess Mass: The Bouguer Slab Correction

• Corrections based on simple slab approximation are referred

to as the Bouguer Slab Correction.

• It can be shown that the vertical gravitational acceleration

associated with a flat slab can be written simply as :

.04193 h. Where the correction is given in mgal, is the

density of the slab in gm/cc, and h is the elevation difference

in meters between the observation point and elevation

datum.

• h is positive for observation points above the datum level and

negative for observation points below the datum level.



C ti i G it D t N b T h



Correction in Gravity Due to Nearby Topography

• Terrain Corrected Bouguer Gravity ( gt) - The Terrain

correction accounts for variations in the

• observed gravitational acceleration caused by variations in

topography near each observation

• point. The terrain correction is positive regardless of

whether the local topography consists of a

• mountain or a valley. The form of the Terrain corrected,

Bouguer gravity anomaly, gt , is given by;

• gt = gobs - gn + 0.3086h - 0.04193r + TC (mgal)

• where TC is the value of the computed Terrain correction.



This completes the data

acquisition and processing

of gravity data

This data is to be interpreted

Interpretation Techniques:Qualitative Methods




a) Represents a contour map, whichdoes not bring out any anomalousfeature, i.e. a homogeneous rock

mass without any anomalousdensity distribution.

b) Profile falls from one end to another,or contour shows parallel lines of gravity, it may correspond to sloping basement. The down slope of basement will be towards decreasinggravity values.

c) If contours are parallel, and become more closely spacingalong an axis, following possibilities exist,

1. If contour values are greater than other, it is a sharpcontact or fault, If the contour values are lower on eitherside,

2. it may be an anticline or ridge.

I t t ti T h i Q lit ti M th d




If profile shows a high or low, or

the contour contains a clearhigh, or low closure, theanomalous body occurs at depth.

Feature Density of Anomalous body

Density of Surrounding

High High Low

Low Low High

Profile Position of Body

ContourPattern

Symmetric

Vertical Symmetric

Asymmetric

Dipping Asymmetric



I t t ti T h i Q lit ti M th d (C td



Interpretation Techniques: Qualitative Methods (Contd

Sometimes anomaly may besuperimposed over the other, e.g. d

and f, this is due two different bodies

present at different depths.

The large wavelength anomaly

reflects deeper bodies and small

wavelength anomaly represents

shallow bodies.

A shallow body may give larger

wavelength, but sharper fall along theedges of a body will expose its

shallowness.



Thin Dipping Rod

Rod having inclination α and

cross section A. The gravityeffect

at p due to an element dl is given

by δgr = g σ A dl / r2

The total effect of rod when therod is vertical, is

g = 2.03X 10-3 σ A [1/ (z2+ x2)

1/2 - 1/ {(z+L) 2 + x2}1/2]

The total effect of rod when the

rod is inclined, isg = 2.03X 10-3 σ A / x sin α



T=1000 ft, h1 = 2500 ft, h2 = 4500 ft, σ=1gm /cc, α = ?

What type of fault it is? For Inclined Fault with inclination α g = 4.07 X 10-3 σ t [π + tan-1{(x/h

1

) + cot α} - tan-1{(x/h2

)+ cot α}]

Faulted Horizontal Slab



T=1000 ft, h1 = 2500 ft, h2 = 4500 ft, σ=1gm /cc, α = ?

What type of fault it is? For Inclined Fault with inclination α g = 4.07 X 10

-3

σ t [π + tan-1

{(x/h1) + cot

α} - tan-1

{(x/h2)+ cot

α}]

Faulted Horizontal Slab



The circular gravity anomaly Two Possible interpretation

Interpretations of Gravity Profiles

Gravity Survey Method_31072012

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