Non Seismic Geophysical Applications-Gravity Method

Abstract

Abstract

Objectives- The object of the gravity method is to determine information about the earth's subsurface. One can just carry out a qualitative examination of the grid of gravity values, contour maps or the gravity profiles to determine the lateral location of any gravity variations or one can perform a more detailed analysis in order to quantify the nature (depth, geometry, density) of the subsurface feature causing the gravity variations. Anomalies in the earth's gravitational field result from lateral variations in the density of subsurface materials and the distance to these bodies from the measuring equipment. The method can infer location of faults, permeable areas for hydrothermal movement. It is however, more commonly used in determining the location and geometry of heat sources.

Procedure-A gravity meter or gravimeter measures the variations in the earth's gravitational field. The variations in gravity are due to lateral changes in the density of the subsurface rocks in the vicinity of the measuring point. Because the density variations are very small and uniform, the gravimeters have to be very sensitive so as to measure one part in 100 million of the earth's gravity field (980 gals or 980,000 mgals) in units of mgals or microgals.

Result- Gravity readings on earth are not the same everywhere. The planet's rotation and polar flattening are well known and easy to correct for, as are the small and predictable diurnal variations due to tidal forces of the Moon and the Sun. After these and other corrections, we are interested in map-scale gravity variations attributable to lateral changes in the density of local rocks. More valuable for oil than mineral exploration, in marine and continental-margin regions the more elaborate isostatic and enhanced isostatic gravity reductions often produce superb results. On a regional scale, useful results are sometimes obtained by applying a version of the Bouguer reduction offshore.

Conclusion-The gravity method responds directly to a mass excess or deficit. The method requires a relatively small amount of instrumentation and is an unobtrusive method able to be conducted in environmentally sensitive areas. It is a straightforward geophysical technique that can be applied to a variety of engineering and environmental problems as well as exploration for geothermal energy including the location of heat sources and faults.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016