The densities of rocks (Tab. 1.1), naturally, depends on the mineral composition of particular rock. However, not only mineral composition, but also other factors influences densities. The next main factor, mainly when dealing with sedimentary rocks, is the porosity and kind of media filling the pores. Increasing porosity decreases the density, since air (or any other media filling the pores (water, gas, oil, etc.) has lower density than any of minerals. The other factors are weathering of rocks, fractures and joints, etc.
Combining all these factors clearly explains the high variance of measured values reported in a literature. Densities needed for a data and interpretation interpretation (e.g. for the Bouguer anomalies) could be either measured in laboratory (keeping in mind the laboratory values could differ from the reality mainly due to change of the media filling the pores in the nature and in laboratory) or could be estimated from the gravity measurements.
One of widely used methods for density estimation is the Nettleton’s method (Nettleton 1939). This method is based on the fact that the Bouguer anomaly depends on the density of rocks as well as on the topography. If the topography along the profile is changing but the density is constant then, according to the equation (2.7), the Bouguer anomaly should be constant as well. If it is not constant then the density estimate is wrong and the topography changes are not compensated well.
Therefore, if we compute a set of Bouguer anomaly curves with different densities and compare them with the topography the Bouguer curve which correlates the least with a topography is the curve with a correct density estimate.
