Gravity measurements can be obtained either from airborne (remote) or ground surveys. The most sensitive surveys are currently achieved from the ground.
Gravity surveys in geology measure the variations in the gravitational pull of the Earth. These variations are due to local changes in rock density and therefore depend on the type of rocks beneath the surface. Sedimentary rocks are, for example, less dense than granite which is in turn less dense than basalt.
- High Density
- Extrusive Igneous Rocks, Eg. Basalt
- Metamorphic Rocks
- Intrusive Igneous Rocks, e.g. Granite
- Sedimentary Rocks
- Low Density
Gravity is a result of the attractive forces between objects. The larger the mass of an object, the greater the attractive force. The mass of the Earth, for example, is much greater than the mass of a human and so the Earth's gravity attracts humans so much that it is an effort to jump away from the Earth. A person does have their own gravitational pull on the Earth but this is ineffectual due to their relatively small mass.
The variations in the density of the crust and cover are presented on a gravity anomaly map. A gravity anomaly map looks at the difference between the value of gravity measured at a particular place and the predicted value for that place.
Gravity anomalies form a pattern which may be mapped as an image or by contours. This gives a map which looks very similar to the contoured magnetics map illustrated above. The wavelength and amplitude of the gravity anomalies gives geoscientists an idea of the size and depth of the geological structures causing these anomalies.
Deposits of very dense and heavy minerals will also affect gravity at a given point and produce an anomaly above normal background levels.
