Watlet, A. ; Poulain, A. ; Francis, O. ; Hallet, V. ; Kaufmann, O. ; Rochez, G. ; Van Camp, M.

Talk presented at International Geologica Belgica 2016 Congress on 2016-01-26

Abstract: Monitoring gravity continuously is an integrating technique that has already proved its efficiency to provide valuable hydrogeological information in different contexts. Most of the time, the aim of such a method consists in the monitoring of aquifer recharge. Applying it to follow groundwater variations within unsaturated zones has also been successfully tested, even in karst environments. The particularity of such areas concerns temporary perched aquifers that can occur in the subsurface due to changes of weather conditions, reduced evapotranspiration and the vertical gradients of porosity and permeability. Thus, vadose zones in karst systems may show higher and more noticeable variations in their gravity signal. We present a microgravimetric monitoring installed at the Rochefort Cave Laboratory (RCL), located in the Variscan fold-and-thrust belt (Belgium), a region that has many karstic networks within Givetian limestone units. Our investigations cover two years of hydrogeophysical monitoring. It involves a superconducting gravimeter continuously measuring at the surface of the RCL. Early in 2015, a second relative gravimeter was installed in the underlying cave system located 35 meters below the surface. These relative measurements are calibrated using data of an absolute gravimeter measuring approximately every month at RCL. While measuring only from the surface already give useful information on the groundwater variations within the vadose zone, it also record changes in the saturated zone. Typically, flash floods events in the cave are highly noticed by the gravimeter. When adding a gravimeter permanently measuring in the cave, it allows removing most of the saturated zone gravity signal by subtracting surface gravimetric data to those recorded at depth. Gravity changes within the vadose zone are therefore highlighted, bringing information on the seasonal recharge of potential groundwater reservoir. We also investigated the spatial gravity variations within the studied area using a spring gravimeter. It involves 12 additional stations (7 at the surface, 5 in the cave) monitored on a monthly basis. Such measurements are useful but tricky in such a small area (1 ha). The influence zone of the gravimeter comprises a cone below (and above) the gravimeter with a diameter that varies as 20 times its depth. This allows investigating only variations in the 2-3 first meters of below each of these temporary stations; such depths are comprised in the epikarstic layer, where most changes are expected though. Direct measurments are also used to calibrate both gravity signals coming from the vadose and the saturated zone. Level monitoring and tracer tests performed in the saturated zone are used as well as dripwater monitoring in the cave, informing on the vadose zone discharge and response to rainfall events.

Keyword(s): Gravimetry ; Hydrogeology ; Hydrogeophysics ; Karst