The Gravity Method paper associates geothermal activity with gravity anomalies: “ The most commonly used processed data are known as Bouguer gravity anomalies, measured in mGal. The interpretation of Bouguer gravity anomalies ranges from just manually inspecting the grid or profiles for variations in the gravitational field to more complex methods that involves separating the gravity anomaly due to an object of interest from some sort of regional gravity field. From this, bodies and structures can be inferred which may be of geothermal interest.
Volcanic centres, where geothermal activity is found, are indicators of cooling magma or hot rock beneath these areas as shown by the recent volcanic flows, ashes, volcanic domes and abundant hydrothermal activities in the form fumaroles and hot springs. Gravity studies in volcanic areas have effectively demonstrated that this method provides good evidence of shallow subsurface density variations, associated with the structural and magmatic history of a volcano. There is a correlation between gravity highs with centres of recent volcanism, intensive faulting and geothermal activity. For example, in the Kenya rift, Olkaria, Domes and Suswa geothermal centres are located on the crest of a gravity high. ”
An investigation showing the frequencies under which gravity is searched for: https://www.researchgate.net/publication/252817283_Airborne_gravimetry_An_investigation_of_filtering