Environmental effects in time-series of gravity measurements at the Astrometric-Geodetic Observatorium Westerbork (The Netherlands)

Abstract

The effect of air pressure variations, soil moisture variations and groundwater level variations on time-series of gravity measurements acquired with the tidal gravimeter ET-15 at the Astrometric-Geodetic Observatorium Westerbork (WAGO), The Netherlands, has been investigated. The gravity measurements have been corrected for gravimeter drift, Earth body tide, and ocean loading effects. The residual signal has been smoothed using a variational smoothing algorithm. An agro-hydrological model provided the change in soil moisture in the vicinity of the gravity bunker over the measurement period. A precise finite element model of the gravity bunker and the surrounding layers was developed to compute the impact of changes in soil moisture content and ground water level variations on gravity. The combined effect of soil moisture variations and groundwater level variations varies between-7 μGal and-3 μGal. It is dominated by soil moisture variations in the layers above the gravimeter between-7 μGal and-5.5 μGal. Soil moisture variations below the gravimeter and groundwater level variations contribute between 0 μGal and 2.5 μGal. The analysis of two years of gravity and local air pressure variations show high correlation factors above 80% for periods of one day and shorter, whereas periods between one day and half a week are correlated between 50% and 80%. Over periods shorter than half a week, an admittance factor of-0.37 μGal/mbar has been obtained. The correlation for periods longer than half a week is very low, i.e. these periods should not be used to determine an admittance factor from local air pressure data. Rainfall events have a significant influence on gravity measurements at the WAGO site. A proper modeling requires measurements of precipitation, evaporation and run-off. This is the subject of future studies. © Springer-Verlag Berlin Heidelberg 2007.