A mobile atomic gravimeter for gravity surveys

Mobile gravimetry is important in geodesy and geophysics, and measurements have been traditionally made with relative gravimeters. Compared to existing instruments based on springs, superconducting coils, or falling corner cubes, atomic gravimeters use matter-wave interferometry to determine the absolute free-fall acceleration of an atomic cloud. The laser wavelength defines the photon momentum with high precision, and no mechanical movement is involved with the measurement. Atomic gravimeters could be among the most accurate mobile gravimeters, but they are currently constrained by being complex and fragile. Here, we demonstrate a more robust and mobile atomic gravimeter, measuring tidal gravity variations in the laboratory as well as surveying gravity in the field. The mobile atomic gravimeter is based on a magneto-optical trap inside a pyramid mirror with a through-hole. Atom interferometry is performed using Doppler-sensitive two-photon Raman transitions as the cold cesium cloud freely falls under the pyramid mirror. This geometry offers advantages of fast cycling rate, good signal-to-noise, and simple vibration isolation. The tidal gravity measurements achieve a resolution of 37 μGal (1 μGal=10 nm/s²) in an averaging time of 1 s and a resolution of better than 2 μGal in an averaging time of 0.5 h. The data reveal ocean tidal loading effects from the nearby San Francisco Bay and record seismic waves from several distant earthquakes. We survey gravity at stations in the Berkeley Hills, California, with a resolution of ~40 μGal and estimate the density of the subsurface rocks from the vertical gravity gradient. These gradients agree with results from spring-based, relative gravimeter surveys. With robust mobility and sensitivity, our atomic gravimeter is now capable of absolute gravity field surveys.