Which geophysical method measures tiny gravity variations to detect subsurface rock density?

• A. Gravity survey
• B. Seismic reflection
• C. Magnetic survey
• D. Electrical resistivity

Answer: (A)

The main idea is that gravity surveys detect density differences underground by measuring tiny changes in the gravitational field. A dense rock body adds more mass beneath the surface, causing a small increase in gravity at the surface. By taking precise gravity measurements along lines and applying corrections (to account for elevation, terrain, and regional gravity), geophysicists map gravity anomalies that highlight where density contrasts occur. This lets them infer where dense ore bodies or dense rocks lie and estimate their shape and size.

This method is best for detecting subsurface rock density because gravity responds directly to how mass is distributed below, unlike the other methods. Seismic reflection relies on how rocks reflect sound waves and reveals boundaries and layering rather than density alone. Magnetic surveys map how rocks respond to magnetic fields, which depends on magnetic minerals rather than bulk density. Electrical resistivity looks at how easily electricity flows through rocks, tying to fluids and porosity rather than density.

Tiny gravity variations are the clue, making gravity surveys the tool that directly targets subsurface density contrasts to reveal buried structures.