Application of combined time‐lapse seismic refraction and electrical resistivity tomography to the analysis of infiltration and dissolution processes in the epikarst of the Causse du Larzac (France)
Résumé
In geophysical and hydrological studies, it can be challenging to achieve a complete understanding of infiltration processes in the upper zone of a karstic aquifer since this type of medium is highly heterogeneous and may contain a perched aquifer. In an effort to further investigate such aquifers, time‐lapse electrical resistivity tomography and time‐lapse seismic refraction tomography were carried out at three different epochs and at two sites on a dolostone plateau of Southern France. The first site has a sinkhole, whereas the second covers a less efficiently drained area, which is characterized by temporary lakes during periods of heavy rain.
These studies show that shallow time‐lapse electrical resistivity tomography and time‐lapse seismic refraction tomography analyses are correlated with hydrological data because resistivities and propagation velocities decrease with increasing rainfall. Nevertheless, the Biot–Gassmann relationship does not provide an adequate explanation for the strong variations in velocity observed in the upper parts of the models. This could be explained by the dissolution process called ghost‐rock weathering, which directly attacks the dolostone rock frame of the studied area. The assessment of such processes can lead to an improved understanding of velocity variations and to the localization of dissolution processes that may affect karstic landscapes or even lead to their collapse.