Geophysical Method: Gravitational Methods

GRAVITATIONAL METHODS

Principle 

Gravity or the force by which the earth attracts other objects towards itself is a well-known principle in Physics. It can be theoretically calculated for any part of the earth I from the relationship:

where g is the normal gravity value at sea level and 0 is the latitude of the place under consideration.

Ø     In these calculations it is assumed that the earth is ideally homogenous in nature. Actually, however, the earth is far from homogenous.

Ø  Thus, we have another means to locate the materials which are ' less' or

' more' dense than the ideal material of the earth.

Ø     Procedure is simple: for any given area, we would know its latitude and thus calculate the value of normal gravity (the theoretical value).

Ø    Then observations are made with sensitive instruments to determine the actual value of gravity at that place.

Ø     This is called observed value. Under ideal conditions the two values should be identical but when there is considerable difference, a gravity anomaly is believed to exist and that would be a pointer towards existence of some unusual rock mass below the place of gravity anomaly.

Ø    The unit for gravity is gal, which is acceleration of 1 cm/sec/ sec and that of gravity anomaly a milligal (which is a thousandth part of a gal).

Methods.

Ø     A number of methods and instruments are available to determine the value of gravity.

Ø     The earlier used pendulum method and the torsion balance method are almost obsolete now.

Ø    The gravimetric method is most commonly used at present. In this method, the value of gravity is measured directly by instruments known as gravimeters.

Ø     Two important versions of these instruments are: the stable gravimeter and

the unstable gravimeter.

In the stable gravimeter,

v       the spring in the gravimeter remains unchanged in its position if the gravity pull is same.

v       when there is a change in the value of gravity at a place, there is a change in the length of the spring: it increases or decreases compared to original length.

With the help of an external element, the spring can be made to acquire the original position and thereby indicate the amount of change.

In the unstable type

v the spring once disturbed due to change in gravity at the place of measurement is not brought back to the balanced state there and then.

v       instead its deflections are recorded directly on a suitable magnified scale, which give a measure of gravity anomaly.

vi     Isogams or lines passing through points of same gravity anomaly are drawn as a result of gravimetric observations which could be then interpreted to reveal important conclusions.