Gravity and magnetic methods

Abstract

Gravitational method is the study of the distribution of mass in the subsurface with the observation point at the earth's surface. The gravity technique provides information regarding the density distribution in the subsurface and can identify anomalous geological features (of varying density) in order to detect structural or lithological contrasts in the subsurface. The success of the gravity method depends on the different earth materials having different bulk densities (mass) that produce variations in the measured gravitational field. These variations can then be interpreted by a variety of analytical and computers methods to determine the depth, geometry and density that causes the gravity field variations. The gravity method produces an ambiguous, non-unique solution for the subsurface structures. Therefore precise gravity interpretation require a number of data reductions methods so as to eliminate all other effects and only be left with those that are caused by geological variation in the sub-surface. On the other hand magnetic method is a geophysical exploration method used in the study of the distribution of magnetic minerals in the upper sub-surface of the earth's crust. Magnetic method may also be used to estimate the thickness of the crust or to constrain temperatures in the crust using the Curie isotherm (the temperatures at which minerals lose their strong magnetic properties), whichever is shallower. It can also be used to record variations in the magnetic field due to lateral variability in the magnetization of the crust. These lateral variations may produce anomalous regions which are indicative of structural or lithological contrasts in the subsurface.