Vertical Gravity Profile in a 10 m Atom Interferometer

Absolute gravimeters are used in geodesy, geophysics, and physics for a wide spectrum of applications. Stable gravimetric measurements over timescales from several days to decades are required to provide relevant insight into geophysical processes. Instrumental accuracy is established by the comparison with a reference apparatus. However, since no reference gravimeter of higher-order accuracy currently exists, absolute gravimeters participate in group comparisons led by the International Committee for Weights and Measures. The construction of stationary, large scale atom interferometers paves the way towards new absolute gravimetry references with a potential stability better than 1 nm/s$^2$ at 1 s integration time. At the Leibniz University Hannover, we are currently building such a very long baseline atom interferometer with a 10 m long interaction zone. The knowledge of local gravity and its gradient along and around the baseline is required to establish the instrument's accuracy budget and enable transfers of gravimetric measurements to nearby devices for comparison and calibration purposes. We therefore established a control network for relative gravimeters and repeatedly measured its connections during the construction of the atom interferometer. We additionally developed a 3D model of the host building and studied the impact of mass changes due to hydrology on the gravity field around the reference instrument. The adjusted model fits the results of the latest gravimetric measurement campaign with 95% confidence, opening the way for transfers of gravimetric measurements beyond the 10 nm/s$^2$ level.