The greater part of the Earth's gravity is due to the planet's enormous mass, concentrated in the Earth's core.
If the Earth did not rotate and was perfectly spherical and homogeneous, the Earth's surface would have the same force of gravity. As we know, this is not the case. A flattening effect at the poles suggests the surface is 21 km closer to the centre of gravity than at the equator. This proximity, combined with centrifugal force, suggests that the force of gravity is greater at the poles than along the equator. Moreover, in addition to the unequal distance to the centre of the Earth's mass and the Earth's rotation, the force of gravity is also dependent on the structure, location and density of the bedrock.
Measurement of the gravity field variations
If we measure the gravity field above a light rock type, the gravity value will be less than the norm for that line of latitude because there is a lighter mass directly beneath the observation point. On the contrary, above an ore deposit or a rock type with high density, we will register a higher gravity value than normal.
Precise measurement of the gravity field or, in reality, the gravitational acceleration (g), is both time-consuming and complicated. When instruments were developed that could rapidly measure relative values or differences in gravity, gravimetry then received widespread application within ore prospecting and for estimating the geometry of small geological structures.
Gravity acquisition in Norway
NGU has two gravity meters, one Scintrex and one LaCoste & Romberg, both of which are spring based. In a location where the gravitational acceleration (g) is great, the mass in the spring is dragged farther down than at a place with lower g. The actual extension of the spring is, thus, a measure of the value of g at the observation point.
The unit Gal (cm/sec²) is still used for measuring gravity, but in gravimetry we use milligal (mGal). At our latitudes, g is normally c. 9.81 m/s² = 981 Gal = 981,000 mGal. On the LaCoste & Romberg gravimeter we can register variations in the Earth's gravitational field to within 0.01 mGal.
Processing and correction of gravity measurements
When the gravity measurements are ready for processing, we must first introduce a few corrections, such that the anomalies obtained reflect only the properties of the rocks in the subsurface. In addition to the corrections that are made in calculating the Bouguer anomalies (measurements adjusted to a datum plane such that one can then compare measurements acquired from different measuring sites), the measurements are also corrected for day-to-day operations. This is due to gravitational effects from the Sun and Moon as well as to mechanical variations in the instrument. Gravity at one and the same measuring station is recorded at regular intervals during the period of investigation such that it can be corrected for this time-dependent variation.
Corrections included in the calculations of the Bougueranomaly are as follows:
- Latitude correction. Here we correct for a calculated normal field. The correction takes into account the Earth's rotation, as well as the fact that the distance to the centre of the Earth's mass varies with latitude.
- Free-air correction. A correction is made for the reduction in the gravity field from sea level to the altitude of the measuring site.
- Bouguer correction. Here we subtract the effect of the rock mass between the measuring site and sea level.
- Terrain correction. Account is taken of the topography in the vicinity of the measuring station. Here it is necessary to have a good map (preferably digital) of the area around the observation site.
The registered measurements are relative. We measure the differences in the gravity field at the different measuring stations. The relative measurements are adjusted to absolute values by measuring at a site with a known absolute value. After all these corrections have been made, and after subtracting the theoretical value of gravity at that particular site, one is left with the corrected Bouguer anomaly.
A gravity anomaly can result from an infinite number of combinations of density contrasts and dimensions in the exposed and concealed rock mass at the measuring site. However, as we generally know the actual density values of the rocks in the vicinity of the site, as well as information on the geology and structure, then this limits the number of possible interpretations.
We interpret a gravity anomaly by devising probable models and calculating what kinds of anomaly these models are likely to generate. We then compare these calculated anomalies with the observed anomalies and vary the dimensions of the models until we obtain the same anomalies as those observed at the measuring sites.