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.