Calibration data from Mount Hamilton, California, for USGS Volcano Science Center gravimeters

Temporal changes in gravity, also called microgravity in volcano geodesy and exploration geophysics applications, provide a means of measuring mass change beneath the surface—for example, due to the accumulation or withdrawal of water, magma, or other fluids. Relative spring-based gravimeters, however, require calibration to establish consistency between instruments, and to assess potential changes in the elastic properties of the spring over time. Calibration is best done on a line of survey points with gravity values established by absolute measurements. The U.S. Geological Survey Volcano Science Center performs gravimeter calibrations on a survey line located in the San Francisco Bay Area of California between the city of Menlo Park and the summit of Mount Hamilton, site of the Lick Observatory. The line has a gravity range of over 300 milligals and spans an elevation of about 1,200 meters. Repeated measurements using different gravimeters provides a basis for intercomparison, and for assessing instrument stability over time.