Simultaneous gravity measurements using two superconducting gravimeters to observe temporal gravity changes below the nm s-2 level: ocean tide loading differences at different distances from the coast
Gravity monitoring might require observation of temporal changes of gravity below the nanometre per second squared (nm s-2) level, which can be achieved by precise isolation of the signal of interest from all other disturbing effects. One method of signal isolation is elimination of disturbing effects by taking the difference between gravity changes measured simultaneously using two gravimeters installed close together. Herein, we describe differences in temporal gravity changes below the nm s-2 level in the tidal frequency bands as observed through simultaneous measurements taken with three superconducting gravimeters (SGs) located 80, 93 and 94 m from the coastline in Tomakomai, Hokkaido, northern Japan. Those changes are consistent with differences in ocean tide loading effects on gravity at the SG locations computed using the software package GOTIC2, which uses a highly accurate land-sea boundary and ocean tide model near our site. The observed ocean tide loading differences were found to result from Newtonian attraction of the ocean tide mass within an angular distance of 0.003° from the SG locations. This result suggests that coastal observations of differential tidal gravity variations at different distances from the coast help to validate ocean tide loading computation models in the immediate vicinity of the SG stations. This method enables observation of non-periodic gravity changes occurring below the nm s-2 level over a few hours. Its salient benefit is that rapid and simple observation can be achieved without long-term continuous measurements, which is necessary for observing that level of gravity change with only one SG.