______ gravity anomaly refers to small regional variations in the Earth's gravity field resulting from density variations in underlying rocks.

• A. Bouguer
• B. Free-air
• C. Helmert
• D. Stokes

Answer: (A)

Seeing small regional variations in gravity that come from density differences in the rocks beneath requires removing the influence of the mass of the surface and near-surface rocks. The Bouguer gravity anomaly does exactly that: it subtracts the gravitational pull of the topography and the material between the observation point and a reference level using an assumed density. What remains highlights subsurface density contrasts in the underlying rocks, such as changes in rock type, crustal blocks, or buried structures, making it the best descriptor for these regional density variations.

In contrast, the free-air anomaly only corrects for height above the reference level and still carries the gravity effect of the topography, so it doesn’t isolate deeper density variations as cleanly. Helmert and Stokes refer to other correction formulations or calculations but do not define the signal from underlying rock density contrasts in the same direct way Bouguer does.