Several types of geophysical data are used in hydrocarbon exploration. For example, measurements of the earth’s gravity field allow sediment thicknesses to be estimated, boundaries of salt domes and salt ridges to be mapped, and areas containing volcanic intrusive rocks to be identified. Other geophysical measurements create gross pictures of subsurface distributions of rock types, mineral concentrations, rock porosities, and pore fluids by detecting variations in the earth’s magnetic and electrical fields.