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The instruments, field operations, data reduction, and mapping described above produce a gravity map which, ideally and theoretically, is free of planetary effects of the earth as a whole and from the visible topography. If the earth's crust were composed of uniform horizontal sheets with no horizontal variations in density, the gravity map would be featureless. All of the features which it shows have their origin in horizontal discontinuities at any depth from the grass roots down. The objective of gravity interpretation is to deduce the geological character of the subsurface from variations in the gravitational field.

The interpretation is inherently ambiguous. In the first place, there is no single mathematical solution to the determination of the source of a gravity field no matter how precisely that field may be mapped. Figure 36 shows a simple positive gravity anomaly which has been calculated as the gravity effect from a sphere (or point mass) shown as body no. 1. The same anomaly could be accounted for completely by a thin lenticular body. no. 3 in the figure, at a very shallow depth or by an intermediate narrower, and thicker body such as no. 2. There is a “cone of solutions” as indicated by the dotted lines. The spherical mass, no. 1, is the deepest possible solution as any deeper mass would produce a broader anomaly but solutions at all shallower depths to the surface are possible. Also combination solutions are possible for which part of the total mass is in different bodies. The one thing that the several possible solutions have in common is that their total mass (i.e., the product of the volume times the density contrast of the anomalous material) must be the same, the oretically, for each solution.

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