The force of gravity at any point is the total attraction of the entire earth at that
point. If the earth was perfectly spherical and all layers within the earth were
concentric and homogeneous, the force of gravity would be exactly the same at all
points on the earth’s surface (Newton’s law of gravity). One of the several departures
from this ideal situation in practice is the fact that different rock types in the
earth have different densities. For this reason the acceleration due to gravity is very
slightly less over alluvium than a rock. There are other causes of departure from this
ideal situation, for example effects of tides, latitude, terrain and height etc.
But they are accurately predictable and may be taken into account, to leave just the
gravity anomalies that are due to variations in rock densities.
All materials within the earth produce effects on gravity value but because of inverse
square law rocks/masses/structures lying close to the surface will have greater effect
than those at greater depth. The gravity method relies on detecting very small
differences in the gravity between one point on the earth surface and another.
These differences are due to the inhomogeneous distribution of masses/bodies having
different densities. The gravity effect of a mass depends on a number of factors
• The closer the body (mass) is to the point of measurement the bigger the effect.
• The greater the density contrast between the body and it’s surrounding.
• The greater the volume of the body, the greater the effect.
The gravity data processing is carried out in two stages. First is the reduction of data and second is the preparation of maps. The reduction of gravity data is carried out to remove unwanted affects e.g drift of the instrument, elevation correction etc. The difference between the corrected gravity values and the standard value is known as the gravity anomaly. The gravity value of the base station is considered to be the standard value of the surveyed area and theoretically it must be constant at all the points on the reference plane. But practically it does not happen with the corrected values due to variation in density distribution in the subsurface. The corrected gravity value at each station is posted and contoured to produce a gravity anomaly map that is later interpreted. The reduction of gravity data will be carried out on K-tron Gravity Modeling System (GMS).
Qualitative interpretation is the description of the anomalies seen on the map in terms of geological features e.g. gravity anomaly lows over an area where low-density contrast is expected and highs over high-density magnetic ores. It is possible to quantitatively predict the gravity anomaly that will occur over a given geometrical body that has a given density contrast with its surrounding rock. Therefore, we often calculate “theoretical” anomalies in this way and compare them with the anomaly observed in the field in order to obtain numerical information such as depth, shape, size etc. The anomalies will be quantitatively interpreted in terms of their depth and geological structure.
The magnetic field variations observed on the surface of the earth are attributable to the magnetic susceptibility differences in the rocks or changes in the subsurface geological structures. The susceptibility depends upon the content of ferromagnetic minerals in rocks. Therefore, igneous and metamorphic basement rocks, which are more mafic, have a higher susceptibility than sedimentary rocks and thus are the source of magnetic anomalies. Amplitude and gradient of the anomalies depend upon the depth of the source (basement). The magnetic field data will be processed and magnetic map produced. The depth of the basement of different anomalies will be calculated and a map of the same prepared using Magm, version 5.50, 2011 Geometrics. This map later shall be integrated for final interpretation.