Geoelectrical,strain and tilt investigations of hydrological processes at the broadband Geodynamical Observatory Moxa,Germany

The Geodynamic Observatory Moxa, located in Thuringia/Germany, is dedicated to studies of temporal deformations of the earth's crust and of variations of the gravity field. One of the essential issues with respect to these investigations is the reduction of the hydrological impact on the data of the gravimeters, strainmeters and tiltmeters. In order to optimise the reductions, we investigated the changes in the hydrological conditions in the woody mountain slope above the observatory with time-lapse electrical resistivity tomography (ERT), and analysed the strain and tilt measurements for prominent signatures of pore pressure induced subsurface deformations.Here we present the results for two profiles – parallel and perpendicular to the slope – measured with ERT during 33 campaigns between June 2007 and April 2010. Resistivity changes and variations of apparent soil moisture, inferred from ERT sections, were found to primarily occur in the first two metres of the subsurface. These variations can be related to subsurface flow in the upper two metres induced by precipitation events and snowmelts. Trees close to the profiles only show a minimum impact on the resistivity and soil moisture changes.Furthermore, systematic hydrologically induced deformations can be observed in hodographs of strain and tilt measurements for large precipitation events (> 80 mm) and snowmelts. In the strain data a short-term (< 3 days) dilatational signal is found with an amplitude of 20 nstrain to 60 nstrain and a long-term (> 7 days) compressional signal between 40 nstrain and 180 nstrain. The preferential N–S direction of long-term deformational signals (> 1 week) is also observed in the tilt data. The direction of tilt changes (25 nrad–120 nrad) is nearly parallel to the drainage direction of the nearby Silberleite creek indicating variations of pore pressure gradients during hydrological events.The results of these hydrological studies at the Geodynamic Observatory Moxa can be used for removing the time dependent hydrological signal in strain and tilt data and, thus, better correction algorithms for hydrological impacts can be developed to enhance the value of the data for geodynamic studies.