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How do gravimeters help locate underground geologic structures?

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Faux amphibolite — geology A rock formation called “faux amphibolite,” from the Nuvvuagittuq greenstone belt in northern Quebec, dated to 4.28 billion years ago.

Gravimeters are highly sensitive instruments used in geology to detect minute variations in the Earth’s gravitational field caused by differences in the density of subsurface rocks.12

Rather than measuring absolute gravity, these devices—typically consisting of a weight suspended from a spring—measure relative differences in gravitational acceleration. When a gravimeter passes over underground structures, the spring stretches or contracts in response to local density changes. These variations, known as gravity anomalies, provide critical data for identifying geologic features:123

Positive Anomalies: Often caused by uplifts that bring denser rocks closer to the surface or by faults that displace rocks of varying densities.1
Negative Anomalies: Frequently produced by salt domes, as salt is typically less dense than surrounding rock formations.1

By mapping these anomalies, geologists can locate oil and mineral deposits, as well as hidden caverns or old mine workings. Because these measurements are influenced by factors like elevation and latitude, data must be carefully corrected to ensure accuracy in prospecting and surveying efforts.12