THE EQUIVALENT SOURCE TECHNIQUE

The inherent ambiguity of potential field interpretation can be put to advantage. Bouguer anomaly measurements on an irregular grid and at a variety of elevations can be synthesized by an equivalent source of discrete point masses on a plane of arbitrary depth below the surface. By keeping the depth of the plane within certain limits relative to the station spacing, we can ensure that the synthesized field closely approximates the true gravity field in the region close to and above the terrain. Once the equivalent source is obtained, the projection of the Bouguer anomaly onto a regularly gridded horizontal plane is easily done. In addition, the equivalent source can then be economically used to carry out vertical continuation. The technique is illustrated by a hypothetical example and a case history of a local gravity survey in precipitous topography.