How gravity and geophysical tools can monitor time and spatial variations of an epikarstic reservoir

Present knowledge of karst systems has evidenced the importance of the unsaturated zone on the water dynamics. Temporary perched aquifers can appear in the epikarst layer due to changes of climate conditions, the lack of evapotranspiration in the winter and differences of porosity relative to deeper layers. We present results covering one year of geophysical monitoring at the Rochefort Cave Laboratory, which is located in the Variscan fold-and-thrust belt (South Belgium), in a region that shows many karstic networks within Devonian limestones units.
A microgravimetric monitoring is a reliable technique to inform on the amount of water stored in the epikarst. Relative gravimetric monitoring calibrated with data of an absolute gravimeter has been set up in November 2013. It records continuously gravity changes in the surface laboratory of the Rochefort Cave. Ten additional stations (seven at the surface, three in the cave) were monitored on a monthly basis since December 2013 by a handle gravimeter. These latter datasets are giving crucial information on the spatial variations of the epikarstic groundwater reservoir through time.
To support these gravimetric measurements, the field site has been equipped with an Electrical Resistivity Tomography (ERT) monitoring system recording data on a daily basis since February 2014. This system provides valuable data to infer variation of the saturation rate within the epikarst. A seismic ambien noise study over this ERT profile is an additional tool to confirm spatial epikarstic extensions.