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How can gravity surveys help locate underground cavities and old mine workings?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

Gravity surveys are an effective tool for identifying underground cavities and old mine workings because they detect local variations in the Earth's gravitational field caused by differences in subsurface density.1

When a gravimeter—a highly sensitive instrument capable of detecting minute differences in gravitational acceleration—is used to map an area, it identifies anomalies where the density of the subsurface material deviates from the surrounding rock. Because underground cavities and abandoned mine workings represent voids or areas of significantly lower density compared to solid rock, they produce measurable negative gravity anomalies.1

By conducting precise surveying on a grid, geophysicists can map these anomalies to pinpoint the location of subterranean voids. While these surveys are powerful, the data must be carefully corrected for external factors such as elevation and latitude to ensure that the detected anomalies accurately reflect subsurface geologic features rather than surface topography.12