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Geophysics - Vibration & Noise - Rock Mechanics

Providing Quality Geophysical Consulting Services since 1984

The Gravity Method

The gravity method detects and measures lateral variations in the earth’s gravitational field that are associated with near-surface changes in density. Variations in the density of near surface materials produce perturbations in the earth’s gravitational field. These perturbations are very small compared to the total gravitational field of the earth. It is not uncommon for gravitational anomalies to be less than one ten-millionth of the earth’s total gravitational field. Extremely sensitive instruments are required to resolve such small differences in gravitational force. Several instruments are commercially available which are suitable for high resolution gravity surveys.
Gravity surveys may be conducted on land, in the air or in water. The lateral spacing between reading locations depends upon the desired penetration depth and resolution of the survey. The measured gravity at any given location will generally be influenced by the density of the material beneath the location, elevation, topography and latitude. In order to relate gravity data to subsurface density, the other factors must be accounted for. This is accomplished by reducing the gravity data to a common datum equipotential surface. In addition, regional trends in gravity data are often removed to provide residual profiles which accentuate anomalies produced by near surface features.

The distribution of subsurface materials are determined from gravity data using non linear least squares inversion of the data to provide a model which best fits the observed data. Gravity data and computed models are typically presented as linear profiles or as contour maps.