GRAVITY SURVEYING METHOD

Among all surveying methods, gravity method is also one of the important methods to investigate the subsurface geology. Gravity surveying method investigates on the basis of variation on Earth’s gravitational field arising due to the variance of density between the materials. The rocks on the surface or in the subsurface of Earth’s crust possess different densities according to their nature due to which they posses different gravity values. Hence the gravity surveying method deals with the variation of gravitational field along with density of materials and it is often called as gravity anomaly.

Gravity anomaly is defined as the difference between gravity values on a surface which may be resulted from unusual concentration of masses in a particular region. As different materials posses different gravity values within a certain area and the small change in the gravity value often regarded as gravity anomaly.

 

Principle:

Gravity surveying method is based on principle of two laws derived by Newton, namely “Universal Law of Gravitation” and his “second law of motion”.

F = G M1.M2/R^2……………1),

F = mg ………………2)

And,

g = GM/R^2…………….3)

As, magnitude of acceleration due to gravity is directly proportional to the mass (M) and inversely proportional to square of distance (R).

 

 

Process:

As Gravity survey measures the minute difference in the gravity value of earth’s materials. Based on this variation the density as well as the composition of a material can be determined. It generally admits that, “higher the gravity value the denser the rock beneath”. By comparing the gravity values of the rocks the variation can be determined at different section along a traverse. So the gravity values obtained are estimated by correcting the gravity data for elevation, topography, latitude, tidal effect, etc. As the distribution of materials are not considered to be uniform on the Earth so by knowing gravity value we can measure the density and nature of the materials in the subsurface.

 

Equipment:

Gravity meter / Gravimeter is the instrument to measure the gravity value of subsurface geology.

Gravimeter is like a spring balance system consisting a constant mass on it. If the weight of a mass varies in a gravimeter due to the variation of the gravity value at any particular point causing the change in the length of spring balance of gravimeter and give the measure of change in gravity of that point with respect to another. There are two types of gravimeters:

1)               Absolute: It measures the absolute value of gravity at any particular point in absolute units.

 

2)               Relative: It measures the ratio of gravity value at two different points.

 

 

Applications:

1)               It is used to reveal the subsurface form of igneous intrusion like granite intrusion.

2)               Gravity surveying is extensively used to locate sedimentary basin.

3)               It is used in hydro geological investigation like to determine the position of aquifers.

4)               It is used to determine the density and nature of the rock beneath the earth surface.

5)               It is used in petroleum industry to locate the subsurface hydrocarbons.

6)               It is mostly used in mineral exploration as it helps in the determination of tonnage of the ore.

 

                Limitations:

1)               Gravity surveying method is very slow to execute.

2)               Different correction should be made while conducting gravity surveying methods.

3)               It is considered as quite expensive surveying methods.