FRACTURED-KARST-SYSTEM INVESTIGATION USING ERT: FROM SHALLOW DISSOLUTION-ENLARGED FRACTURES TO DEEP KARST INTERVALS PASSING THROUGH KARST CONDUITS

摘要

The structure of karst-systems is complex and karst aquifers may be highly heterogeneous.Karst-systems may comprise different karst features such as dissolution-enlarged fractures on the ground,sinkholes,epikarst,infiltration zones,karst conduits,cavities and karst intervals,Karst-system exploration is challenging for ground-based geophysical methods due to the highly variable and unpredictable target characteristics.Fractures and sinkholes are near-surface indicators of deep karst aquifers which may comprise different underground active karst features such as conduits,cavities and karst intervals.Electrical resistivity tomography(ERT)is as a cost-effective solution for investigating karst features such as dissolution-enlarged fractures on the ground,sinkholes,epikarst,karst conduits,cavities and deep karst intervals because of the ability of the technique to detect resistive features and discriminating subtle resistivity variations.ERT surveys were undertaken to investigate the fractured-karst system of the Hydrogeological Experimental Site(HES)of Poitiers,France.Here,Dogger Limestone occurs at a depth ranging between 30 and 120 m and is overlain by argillaceous limestone.ERT data were collected using a Wenner-Schlumberger(WS)array which delivers large vertical resolution and signal amplitude and a Pseudo-Pole-Dipole(PsPD)array which shows large lateral resolution and penetration depth and,even if is more susceptible to noise contamination,may present a good compromise between resolution and signal strength.A hybrid array has been obtained by the combination of WS and PsPD arrays.A three-dimensional resistivity imaging was obtained from full inversion of combined 2-D ERT data.This study compares the imaging obtained from different arrays in detecting shallow dissolution-enlarged fractures that do not show any surface expression,middle depth karst conduits and deep karst intervals.The study also provides some insights on the benefits arising from combining and jointly inverting data obtained with arrays differing in depth of investigation and vertical resolution.