On Density Derived From Borehole Gravity

The application of borehole gravity meters (BHGM) information evaluation is to estimate bulk density from which porosity and fluid saturation are determined. The fundamental equation for the determination of rock density from borehole gravity is where p is the calculated density, F the free-air gradient, Az the vertical separation between two adjacent measurement stations, Ag the difference in gravity readings, and G the universal gravitational constant. Equation (1) has been given for the infinite uniform horizontal slab model in the seminal papers of borehole gravimetry (Hammer, 1950; Smith, 1950). The real world is different from this ideal model. The purpose of this note is to explain the meaning of each term of equation (1); i.e., the difference approximation of the vertical gravity gradient Ag/Az, the free-air gradient F, and the BHGM-derived density p.