The gravity technique is based on measuring localised variations in the Earth’s gravitational field which are caused by materials of different densities. Typical targets for high resolution ‘microgravity’ surveys include (i) low density targets such as caves, fault zones, mine-workings and basements, and high density targets such as dykes, shallow rock, buried walls etc.
The presence of an anomalously high (or low) density body in the subsurface causes a localised high (or low) anomaly in the measured gravitational field.