Hydrology and gravity at the Membach station, Belgium

Van Camp, M. [] Vanclooster, Marnik [UCL] Dassargues, A. [] Crommen, O. [] Petermans, T. [] Verbeeck, K. [] Meurens, B. [] et al.[show all]

This paper investigates hydrological processes and their influence on gravity at the underground Membach station (eastern Belgium), where absolute (AG) and superconducting (SG) gravity measurements have been performed since 1996. Seasonal and short term effects are observed. The 3 μGal seasonal effect reflects long-wavelength component of annual continental water storage variations. The short-term effects consist of fast gravity decrease lasting minutes to a few days. These variations are anticorrelated with rainfall events in 80 % of the cases. The largest decrease was 4 muGal after a significant rainfall (150 mm in 3 days). Apart from rainfall, the Newtonian effect of vertical air mass distribution plays also an essential role (Meurers et al., this session). The gravity station was excavated in low-porosity argillaceous sandstone. Geophysical prospecting showed that the thickness of the weathered zone covering this bedrock can be highly variable between zero and 10 meters. In 2004, soil moisture and temperature probes were installed in the shallow upper 60 cm partially saturated soil, 45 m above the station. Based on a digital elevation model and geological studies, we investigated the soil moisture data and their relationships with high temporal resolution rainfall and gravity time series. This work can be essential to correct local effects that can mask regional effects such as changes in continental water storage. Local effects, indeed, could prevent the combination of satellite data (e.g. GRACE) with ground-based gravity measurements. On the other hand, studying the local seasonal variations also contributes to investigate the influence of the water storage variations in small river basins on the time dependent gravity field.