MODELLING AND INTERPRETATION OF GRAVITY DATA ON LAND AND IN BOREHOLE

Abstract

The gravity survey is one of the most important geophysical prospecting in geophysical exploration and is well known for oil exploration, mineral exploration and to find out structure of the subsurface. This survey is cheaper than seismic prospecting and is important in engineering and archaeological studies and well known for reconnaissance survey. The gravity prospecting is a natural source method such as radioactive, magnetic, and some electrical technique. In this thesis, the analytical expressions will be derived for the gravity and different components of gravity gradient along borehole and then comparison would be done for gravity, gradient and curvature factors & torsion balance quantities by taking examples of sphere, horizontal cylinder and fault (examples of 2D, 21/2 and 3D body sources). A model have been constructed for a random body sources (31) body sources) by fitting rectangular vertical prism and expressions would be found out for the gravity anomalies on the land and gravity gradient along the borehole (as a subsurface). Then finally, the gravity real data (gravity data) would be interpreted theoretically and interpreted taking real field data. The gravity real data is collected during the field survey. The survey is done in 1998 in the Khetri (also known as Khetri Copper Belt or Khetri Copper Mineralisation Belt) and its surrounding areas, Jhunjhuna, Rajasthan, India under guidance of Prof. R.G.S. Sastry. The area comes under Aravalli Hill Ranges of Jhunjhuna, Rajasthan, India trending about 76 Km from NE (North East) to SW (South West). In interpretation part, different types of expressions would be derived for depth rule for determining depth under different cases (such as sphere, cylinder and etc) excess and deficit buried anomalous masses, edges of the buried body sources and the approximate thickness. Finally the density of the lithology along borehole would be derived. It is called as direct interpretation. In the indirect interpretation, Talwani's algorithm has been identified which is used in case study.