In gravity prospecting, geophysicists measure minute variations in the force of gravity from rocks up to a few miles beneath the earth's surface. Different types of rocks have different densities, and the denser rocks have the greater gravitational attraction.  

If the higher-density rock formations are arched upward in a structural high, such as an anticline, the Earth's gravitational field will be greater over the axis of the structure than along its flanks. A salt dome, on the other hand, which is generally less dense than the rocks into which it is intruded, can be detected from the low value of gravity recorded above it compared with that measured on either side.  

The anomalous variations in gravity that reveal the geologic structures sought in oil exploration may represent only one-millionth or even one-ten millionth of the Earth's total field!