Gravitational Anomaly of a subsurface body
The gravitational anomaly (delta_g) changes linearly with the density contrast, and proportionally to the inverse square of depth. That's simply a version of the 'Universal Law of Gravitation'.
Here is an example for the anomaly created by an spherical anomalous spherical body with a density difference of delta_rho relative to the surrounding density. G is the known Gravitational Constant: 


From Turcotte and Schubert, 2002, Geodynamics