Gravity surveys can provide useful information where other methods do not work. For example, gravity may be used to map bedrock topography under a landfill, where seismic refraction is limited. The gravity method measures the gravitational attraction exerted by the earth at a measurement station on the surface. The strength of the measured field is due to a mass difference between the subsurface at the point of measurement and the larger force of gravity due to the total mass of the earth.
By precise measurement of gravity and by careful correction for variations in the larger component due to the total mass of the earth, a gravity survey can sometimes detect natural or manmade voids, variations in the depth of bedrock, and geologic structures of engineering interest.