The Relevant Geologic Parameter is Not Density, But Density Contrast

Contrary to what you might first think, the shape of the curve describing the variation in gravitational acceleration is not dependent on the absolute densities of the rocks. It is only dependent on the density difference (usually referred to as density contrast) between the ore body and the surrounding soil. That is, the spatial variation in the gravitational acceleration generated from our previous example would be exactly the same if we were to assume different densities for the ore body and the surrounding soil, as long as the density contrast, d₂ - d₁, between the ore body and the surrounding soil were constant. One example of a model that satisfies this condition is to let the density of the soil be zero and the density of the ore body be d₂ - d₁.

The only difference in the gravitational accelerations produced by the two structures shown above (one given by the original model and one given by setting the density of the soil to zero and the ore body to d₂ - d₁) is an offset in the curve derived from the two models. The offset is such that at great distances from the ore body, the gravitational acceleration approaches zero in the model which uses a soil density of zero rather than the non-zero constant value the acceleration approaches in the original model. For identifying the location of the ore body, the fact that the gravitational accelerations approach zero away from the ore body instead of some non-zero number is unimportant. What is important is the size of the difference in the gravitational acceleration near the ore body and away from the ore body and the shape of the spatial variation in the gravitational acceleration. Thus, the latter model that employs only the density contrast of the ore body to the surrounding soil contains all of the relevant information needed to identify the location and shape of the ore body.

*It is common to use expressions like Gravity Field as a synonym for gravitational acceleration.

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