The impact of soil moisture changes on gravity residuals obtained with a superconducting gravimeter

Abstract

Summary Time dependent observations of the gravity field are carried out to gain a better understanding about the dynamics of the Earth. Terrestrial measurements are made with gravimeters which monitor temporal changes in the vertical component of the Earth’s gravity field. At present, superconducting gravimeters (SG) are the most precise instruments for long-term stationary continuous observations. Such an instrument, as part of a worldwide gravimeter network, is located at the Geodynamic Observatory Moxa, Germany. The measurements of the SG are affected among other environmental influences by hydrological mass balance changes. The temporal and spatial variability of water distribution in the surrounding of the SG makes a quantification and reduction of such effects a challenging task. For further investigations of these influences an interdisciplinary research project which comprises scientists from hydrology, hydrogeology, geophysics and soil sciences has been launched in the surrounding of the observatory. For this project, the small (≈2 km 2 ) catchment which surrounds the observatory has been instrumented with soil moisture and groundwater probes at various locations as well as additional precipitation gauges and a climate measurement station for monitoring of climatological and hydrological parameters in high spatial and temporal resolution. Secondly, a distributed and spatially high resolution hydrological model, based on the modelling system J2000, has been set up for continuous simulation of the hydrological processes in the catchment using daily and hourly time steps. The main purpose of the project is to explore the types and magnitudes of hydrological influence on gravity monitored by the very sensitive superconducting gravimeter. Preliminary assessment showed that the gravimeter records contain noticeable influence due to variations of groundwater, soil water content and snow coverage. These influences interfere with the geodynamic signals, but the observed gravity changes carry interesting information from a hydrological point of view, in that a method for direct, integrative and non-invasive measurement of soil water contents and groundwater variations would be available if the hydrological influence on the response of the gravimeter records is extracted and utilised. This paper concentrates on the measurement and spatially distributed simulation of soil water contents and their effect on the signal of the SG. This is done by qualitative comparison of the volumetric soil water content with the observed gravity variations. Additionally, examples of how the modelled water volumes can be translated into gravity variations by means of a 3D gravimetric model is demonstrated and briefly discussed towards the end of the paper.