Using relative gravity measurements between surface and underground stations to assess the hydrology of the soil layers in between

The absolute-gravity (AG) station in Riga is situated about 10 m underground, at the bottom of a silo-like vertical structure made of concrete and probably founded on the limestone basement. During the first absolute gravity measurement in 1995 by the FGI two excenter stations were established on the surface and tied to the absolute station by relative measurements. During absolute measurements by the BKG and by the NGA in 1996 and by the FGI in 2007 the relative ties were re-measured; BKG even established an additional station halfway down the silo and another on top of it. The purpose of the relative measurements was to isolate the gravity effect of the variable water storage in the sediment layers between the bottom and the surface stations: at the bottom the attraction is upwards, on the surface downwards, and in the gravity difference it appears double, with most other sources of variation like loading by regional water storage eliminated. The relative ties could therefore facilitate a hypothesis-free correction for the local hydrology to the AG series.

The relative measurements during the AG campaigns were too few for validating the results. Therefore after the fifth AG measurement (by the FGI in 2013), a monthly series of relative campaigns was started by the LGIA, using two Scintrex CG-5 gravimeters. To date these measurements cover more than two annual cycles. The variation in the gravity difference top-to-bottom is 16 microgal (peak-to-peak), quite well explained by the variation in groundwater level (0.5 m peak-to-peak).  Obviously the groundwater level here serves as a proxy indicator for the variation in soil moisture as well. We discuss the role of rainfall and snow cover, and use gravity to determine the specific yield of the aquifer. In principle, one can now use the groundwater reading to contribute to a real-time estimate of the current gravity value of the stations.