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Relative variation in observed gravity readings over different geological structures.
Gravity Anomaly #1 – Diatreme

The first example is a diatreme where granite has been broken up and redeposited within a volcanic vent.
A reduction in density has occurred due to an increase in porosity. Although a sandstone layer covers
the diatreme it is detectable by the gravity survey method because a mass deficit exists. A negative
gravity anomaly is the result.
Gravity Anomaly #2 – Dyke
The next example shown is a dolerite dyke intruding granite. Both rocks have virtually the same
porosity so this is not the cause for the different densities. The mineral composition of the dyke means
it a much denser than the granite. Again, the overlying sandstone unit is no barrier to the gravity survey
method. Due to the extra mass associated with the dyke, a positive gravity anomaly is recorded in the
survey results
Gravity Anomaly #3 – Hidden Fault
Faults can also be detected by the gravity surveys. Granite has been brought next to sandstone because
of fault action. Later, a sandstone layer has covered the fault however the structure is still detectable by
the gravity survey method because of the mass differences immediately below the sandstone layer.
Granite has a higher density than sandstone and therefore contains more mass than the sandstone in a
given volume. A positive response occurs over the granite while a negative response occurs over the
sandstone.

How Is A Gravity Survey Done?
Gravity Meter. Image CC
Gravity surveys can be can be either aerial, ship-based on land-based. On
land, surveys use four wheel drive vehicles and quad bikes. The gravity meter
or gravimeter is a very delicate instrument as. It is highly sensitive and great
care is required, as the unit is easily damaged by vibrations and shocks in
transit. Minor vibrations may also affect the readings as they are being taken
at each locality.
Once the gravity measurements have been collected data are processed by
the geophysicist. The data is then plotted as a computer-generated map. The
geophysical anomalies mapped are then studied by the geologist who
overlays geochemical and other geophysical data. The geologist looks for
coincident anomalies which can then be followed up with field work and
drilling.
Iron oxide-copper-gold (IOCG) deposits can be identified quite successfully
using the gravity survey method. These deposits give a strong positive gravity
anomaly due to the abundance of hematite, an iron oxide mineral, which is
much denser than the surrounding non-mineralised rock. Due to strong alteration of magnetite to

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