gravity survey[′grav·əd·ē ′sər‚vā]
a method of measuring the force of gravity in drill holes by means of gravimeters in order to determine the mean density values of rocks at different depths in their natural bed. Changes in density are associated with the lithology of the rocks and, in a number of cases, may indicate directly the location of a mineral deposit (oil, gas. coal, rock salt, ore bodies). The combined use of gravity and electrical surveys permits greater reliability in differentiating geologic profiles, determing rock porosity, and so on.
Measurement of the gravitational attraction of a rock stratum, the density of which is determined by gravity surveying, also ensures greater accuracy in the interpretation of gravity anomalies that show up as a result of surface gravimeter surveys, especially in studies of the deep structure of the earth’s crust.
Gravity surveying is conducted with gravimeters adapted for measuring increases in the pull of gravity in drill holes. The gravimeter is operated by remote control, and readings are taken from a control panel on the earth’s surface. The results of the density measurements are presented in tables and graphs. With several drill holes located on the same profile, charts are compiled showing lines of equal density values (isodenses) in a vertical plane passing through these drill holes. When drill holes are spaced out over a surface area, isodense charts on horizontal planes for different stratigraphic horizons may be compiled in addition to the vertical-plane isodense charts.
REFERENCESpravochnik geofizika, vol. 5. Moscow, 1968.
P. I. LUKAVCHENKO