Theory
Different subsurface materials have different bulk densities. Gravity surveys seek to detect areas of contrasting or anomalous density by collecting surface measurements of the Earth’s gravitational Field. A gravity meter is a highly sensitive instrument that measures the acceleration due to gravity. When positioned above a dense material it records the acceleration (g) as a relative high (a positive gravity anomaly). When positioned above a low density feature (e.g. an air filled cavity) a relative gravity low (or negative gravity anomaly) is recorded. Many gravity meters have a sensitivity of 0.01 milligal. This allows the detection of a change of one part in 100 million of the earth’s gravitational field.
Applications

• Detection of subsurface cavities (micro-gravity), e.g. mine workings, caves, solution features, sinkholes,  tunnels
• Locating buried tanks and reservoirs
• Regional geological studies
• Hydrocarbon exploration and reservoir monitoring
• Exploration for, and mass determination of, mineral deposits
• Monitoring of CO2 containment underground
• Location of buried rock valleys
• Determination of glacier thickness
• Tidal oscillations
• Archaeogeophysics (e.g. location of tombs, crypts)
• Shape of the earth (geodesy) 
• Hydrological changes in the geoid.