1. . 
The Earth's gravitational field, or the attractive force produced by the mass of the Earth. Variations in the gravitational field can be used to map changes in the density of formations in the Earth. Gravity surveys can be used to map the extent or depth of sedimentary basins or even individual hydrocarbon prospects.
See: anomaly, Bouguer anomaly, Bouguer correction, caprock effect, deep tow, density contrast, density profile, downward continuation, free-air correction, geophysics, gravimeter, gravimetry, gravity anomaly, isostatic correction, potential field, remote sensing, spherical divergence, upward continuation