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• Density of the underlying rocks.

The ability for rock density to be detected using gravity variations is the basis for the use of Gravity
Surveys in mineral exploration.
A gravity survey is designed to pick up density variations below the surface.

Gravity Anomaly Map of New Jersey

If the earth was a perfect sphere and was composed of the
same material throughout, gravity would be the same
anywhere on the surface. However because the planet is
slightly squashed, gravity at the poles is slightly higher
than at the equator due to the slightly shorter distance to
the center of the earth at the poles. The ability for rock
density to be detected using gravity variations is the basis
for the use of Gravity Surveys in mineral exploration.
Rock below the earth’s surface is not homogeneous. It is
composed of material of different densities, both
horizontally and vertically. The density of rock varies with
the amount of mass contained within them. Denser rocks
contain more mass and therefore exert a greater force of
gravitational attraction. Rock density varies with
different mineral compositions and the amount of pore
space between the individual mineral grains.. Metals tend
to be denser than surrounding rocks, meaning that a
gravity anomaly may pick up a mineral concentration that
is otherwise invisible to other geological exploration
methods.

Gravity Anomalies in Detail
Gravity anomalies come in all sorts of shapes and sizes. Positive gravity anomalies are produced by
regions of higher than average density. Lower than average density regions produce negative
anomalies. The areal extent of an anomaly defines the lateral extent of the body producing the anomaly.
A sharp, short wavelength anomaly indicates a shallow, dense object. A broad long wavelength anomaly
points to the presence of a deeply buried massive body.
Variations such as these provide an insight into different rock types and geological structures that are
otherwise hidden. Three examples of the gravitational response above different rock types and
structures are given below.

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