**1. Introduction**

Geophysics is a field of study that deals with the study of the physical properties of the earth's interior generally by direct measurement on or above the earth's surface in order to find the quantitative value of the earth attraction force on a given buried material below the subsurface on the basis of Newton's law of gravitation. Geology and geophysics appeared difficult for one to define the broader line between the two. But in a broader sense, it can be put that, geology deals with study of the physical properties of the earth by direct observations and analysis of handpicked samples from field (i.e., on the ground). While geophysics involves the study of those buried physical properties of the earth using an appropriate measuring instruments on or above the earth's surface. It also involves the interpretation of such measured data to make inferences about the basement structures. As a matter of fact, the two branches of geoscience are interwoven. For instance, well logs are done for geological interpretation whereas borehole geophysics has to do with such measurements.

In a nutshell, geophysics provides the measuring instruments for the measurement of the composition and structures of the earth's interior. All that comes from underneath the earth's surface to limited certain depths to which boreholes or mine shaft penetrated come from geophysical measurements. The knowledge of the existence and properties of the earth's crust, mantle and the core came from observations of the seismic waves by earthquakes, gravity and magnetic measurements and thermal properties of the earth.

There are practically two related aspects of all geophysical deliberations, "pure" and "applied". First of which deals with the understanding of the dynamics of the

earth whereas the second one deals with the economic applications which is of prime importance to mankind. However, for pragmatic design and execution of geophysical survey/exploitation, a perceptive understanding of the structure, evolution of the crust and the uppermost mantle and various processes operating on the earth are essential.

Gravity surveys play vital role in recognition of geological structures such as sedimentary basins, faults, caves and other archeological structures [1–5]. There are varieties of techniques and methods for the interpretation of gravity anomalies over or due to sedimentary basins [6–12]. The structure of the sedimentary basins is often derived from the gravity anomalies with constant density contrast throughout the section of the basins [13]. However, the density contrast of sedimentary rocks is not practically constant [9]. The gravity method for hydrocarbon exploration is firstly used in 1924 in the Gulf coast of the United States and Mexico [14]. Till date, the structures in which hydrocarbons are entrapped exhibit such large density variation when compared to the densities of the surrounding rock formations [15]. Gravity method is very useful in deciding an appropriate location for drilling. For suitable geology of an area, gravity data can provide whether the sedimentary thickness beneath the subsurface is sufficiently thick enough to justify further geophysical investigation. This can be done very easily, since the densities of different sedimentary rock formations are usually lower than those of the basement rocks. Whenever this large contrast exists, it is easier to map out and determine the depth distributions of the sedimentary basins [16].

Gravity data/method is also useful in determining the positions/locations and sizes of key source features/structures in which hydrogeological aquifers, enormous base metals, iron ores, salt domes are entrapped or hosted [17, 18]. Gravity anomaly at long wavelength usually suggests undulations possibly in the topographic interface and the lateral variations in its physical properties (densities). While, short wavelength anomaly may suggest density variations related to the nature of the basin fill. These could encompass the compaction, facies changes and basic to intermediate intrusive [17]. Gravity data can be used to study the internal tectonic and stratigraphic framework, basement and crustal structure. Thus, understanding the structural basement framework, thickness and the physical properties of crust and mantle down to lithosphere is very important more especially in hydrocarbon prospect. Moreover, gravity method is still widely used as an exploration tool to map subsurface geology and estimate ore reserves for some massive ore bodies.

In mineral exploration, gravity method plays more applicability especially in search for ores like Chromite bodies [18]. The density contrast between the chromite bodies and the material that surround them can be so large that they can easily be located through direct gravity measurement. In the same sense, buried channels beneath the earth's subsurface containing gold or uranium can be located since the channel fill is usually less dense than the rock in which it is hosted. Regional gravity studies are also very important in delineating major geological structures like faults/ lineaments in which minerals are probably accumulated [19].
