The gravity method can be utilized during hydrogeologic and engineering investigations to:
Map regional geologic structure
Map basement fault zones
Locate underground caverns
Locate abandoned mine shafts
Map basement topography and sediment thickness
The gravity method involves measurement of the gravitational attraction exerted by the earth at a measurement station on the surface. The strength of the gravitational field is directly proportional to the mass, and therefore, the density of subsurface materials. Anomalies in the earth's gravitational field result from lateral variations in the density of subsurface materials. The intensity of the force of gravity due to a buried mass difference is superimposed on the larger force of gravity due to the total mass of the earth. Thus two components of gravity forces are measured at the earth's surface, a general and relatively uniform component, and a second component of much smaller size which varies due to lateral density changes.
By very precise measurement of gravity and by careful correction for variations in the larger component due to the total mass of the earth, a gravity survey can sometimes detect natural or man-made voids, variations in the depth of bedrock, and geologic structures of engineering interest.
