The GeoModel at Kiel University: A Full Scale Model for Hydro- and Biogeophysical Studies in the Vadose Zone

Our GeoModel, a full scale tank analog (3x5x2 m3), is a new approach to develop novel integrated hydro-biogeophysical 4D techniques of high spatiotemporal resolution (in the range of cm and seconds). Fully controlled experiments are conducted at vadose soils simulating various structural, textural, lithological and hydrological properties in their natural scale. The GeoModel is a bridge between scaled laboratory models (typical size of 1 m3) and field surveys (several km3) and takes advantage of both. The GeoModel is equipped with a multitude of hydrogeophysical instruments including dc resistivity, GPR, TDR, tensiometer and tracers. A special device simulates varying irrigation scenarios of precipitation, ponding and drip of different rates, intensities and contaminations. A bottom filter pebble layer is divided into different segments for monitoring lateral distribution of flow. A self-developed vacuum suction device is installed to compensate for the hydraulic capillary barrier at the interface between the pebble filter layer and the overlying fine sand. The hardware and software allow complex 3D data acquisition, processing and inversion using fine electrode grids from all sides in numerous arrays, radar antenna configurations from surface and boreholes, and a distribution of hydrological probes for water content and tension. Performance tests showed the very high quality and resolution of the data and the capability of the GeoModel for new experiments to determine petro-hydrogeophysical parameters. We present examples of 3D mapping of fine structures and monitoring (4D) of water and dye tracer flow in the GeoModel. The inversion of reference data sets resolves cells with a length smaller than 1 cm. In many infiltration experiments the monitored radargrams and resistivity models image clearly the proceeding of a heterogeneous preferential flow front with time. Radargrams show the continuously increasing travel times from a reference reflector caused by increasing water content. Time dependent measurements enable us to monitor water flux. We established empirical calibration relationships similar to that of Archie and Topp for the substrate of the GeoModel and transformed the resistivity and velocity models into water content distributions.