Gravity Surveys

All materials in the earth influence gravity but because of the inverse-square law of
behaviour, rocks that lie close to the point of observation will have a much greater effect
than those farther away. The bulk of the gravitational pull of the earth (g) has little to do
with the rocks of the earth’s crust but rather is caused by the enormous mass of the
mantle and core. Only about 0.3% of g is due to materials contained within the crust and
of this small amount roughly 15% (0.05g) is accounted for by the uppermost 5 kilometres
of rock. Changes in the densities of rocks within this region will produce variations in g
which generally do not exceed 0.01% of its’ value anywhere. Fluctuations in the value of
g which may be associated with bodies that have a commercial mineral value are unlikely
to exceed even a small fraction of this minute amount, perhaps 10-5 g altogether. Thus
geological structures contribute very little to the earth’s gravity but the importance of that
small contribution lies in the fact that it has a point-to-point variation that can be mapped.
The gravitational field of the earth has a world-wide average of ~980 gals with a total
range of variation from equator to pole of about 5 gals, or 0.5%. Mineral ore bodies and
geological structures of interest seldom produce fluctuations in g exceeding a few
milligals and for practical purposes of exploration, a reading sensitivity of 0.01 milligals
is required. This represents about 1 part in 108 of the gravitational field of the earth. No
instrumentation is available that can measure g absolutely to this accuracy. Modern day
gravimeters respond to variations in g by measuring minute changes in the weight of a
small object as it is moved from place to place and can achieve reading sensitivities of
0.001 mgals.
Surface gravity measurements are affected by several factors, including such things as the
tidal forces generated by the moon, local topography and the ellipticity of the earth.
These factors can generate changes in the measured gravity that are several orders of
magnitude greater than those generated by the density variations in the underlying rocks.
Compensation for these factors requires precise geographical survey precision. For a
typical survey, the distance from the equator must be measured to within ~3 metres and
the absolute elevation to within 2-3 cm. For small, localized surveys, topographic
features within several hundred metres of the measurement location are considered. For
more regional surveys, major topographic features (mountains, lakes, oceans) within a
radius of 150 kilometres must be included in the data reduction procedures.
In the past, topographic surveys of this accuracy often accounted for the bulk of survey
costs. Recent advances in global positioning (GPS) technology have reduced these costs
considerably.
Gravity exploration typically involves taking measurements of the earth’s gravimetric
field across a surface grid. These data are processed to compensate for the various effects
described above to produce a map showing the relative strength of the earth’s gravity
across the area of interest. The presence of an anomalous mass beneath the surface will
be superimposed on the background field. By estimating this regional field and
subtracting it from the observed data, one obtains the field due to this anomalous mass.
Characteristics of this field can be used to estimate the properties of the anomalous body.