Gravity

Gravity surveys are passive, non-invasive measurements of variations in the Earth’s gravitational field. The measurements are sensitive to density contrasts, allowing for detection/delineation of subsurface geology. Gravity surveys are effective for determining overburden thickness and geometry, estimating depth to bedrock, mineral/energy resource exploration, geologic structure delineation, hydrogeologic studies, and near-surface void detection.

Image source: https://www.kgs.ku.edu/Geophysics/Reports/B226/index.html