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SJ Geophysics Ltd. - Contracting Services: Gravity Surveys Applications:
Examples:
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. Similar Service: Magnetic
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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.
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.