[G04-P] Poster

[G04-P-04] Time-lapse relative gravity measurements between surface and underground stations for studying the local hydrology

^○Jaakko Makinen¹, Ivars Liepins², Viesturs Sprogis², Janis Sakne², Kalvis Salmins³, Janis Kaminskis⁴, Reinhard Falk⁵, David Stizza⁶ (1.Finnish Geospatial Research Institute (FGI), National Land Survey, Masala, Finland, 2.Department of Geodesy and Cartography, Latvian Geospatial Information Agency (LGIA), Riga, Latvia, 3.Institute of Astronomy, University of Latvia, Riga, Latvia, 4.Institute of Geodesy and Geoinformation, Riga Technical University, Riga, Latvia, 5.Division of Geodesy, Federal Agency for Cartography and Geodesy (BKG), Frankfurt am Main, Germany, 6.National Geospatial-Intelligence Agency (NGA), St. Louis, USA (at the time of the measurements))

The present work originated in the study of Fennoscandian postglacial rebound using repeated absolute-gravity (AG) measurements. The AG station in Riga is situated about 10 m underground, at the bottom of a silo-like concrete structure on the limestone basement. During the first AG measurement in 1995 by the FGI two excenter stations were established on the surface and tied to the AG station by relative measurements. It was thought that repeating the relative ties at every AG measurement would give an opportunity to isolate the gravity effect of the variable water storage in the sediment layers for correcting AG. At the AG station at bottom the attraction is upwards, at the surface stations downwards, and in the gravity difference it appears approximately double. 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 also established an additional station halfway down the silo and another on its roof 5 m above the ground surface.
The relative measurements during the AG campaigns were too few for validating the results. Therefore after the fifth AG measurement performed by the FGI in 2013, a monthly series of relative campaigns was started by the LGIA, using two Scintrex CG-5 gravimeters. The variation in the gravity difference top-to-bottom is 16 microgal (peak-to-peak). The terrain is flat and the assumption of laterally homogeneous 2-D local hydrology (only perforated by the silo and the piers of the surface stations) is a reasonable first approximation. Then we can invert the relative gravity series for both variation in subsurface water mass and for the mean depth of the layers with variable water content. At the second stage we combine the inversion results with the observations of groundwater level in an on-site access tube. Comparison shows that in addition to the groundwater, soil moisture content contributes appreciably to variation in water mass and in gravity.