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Variations in the gravitational field may be mapped by the pendulum, gravimeter, and torsion balance. The pendulum and gravimeter measure relative gravity, whereas with the torsion balance, the variations of gravity forces per unit horizontal distance, also known as “ gradients ” of gravity are determined. Since gravitational effects of geologic bodies are proportional to the contrast in density between them and their surroundings, gravity methods are particularly suitable for location of structures in stratified formations.
Pendulum methods. 
Pendulum may be used to determine not only time but gravity as well. Gravity pendulums are kept as contrast as possible in length so that variations in period indicate changes in gravity only. The most common method for securing the necessary accuracy in pendulum observations is the “ coincidence ” or beat method whereby the gravity is compared with a chronometer or another pendulum of nearly equal period.
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