Detecting gypsum caves with microgravity and ERT under soil water content variations (Sorbas, SE Spain)

Abstract

The detection of caves developed in gypsum and marls through geophysical methods is tested in the gypsum karst of Sorbas (SE Spain). We applied microgravity and electrical resistivity tomography (ERT) in the Covadura cave system, which features a large variety of shapes developed in a multilayer structure. The response of caves in different conditions of host rocks and cave dimensions is analysed by means of synthetic models. Gravity studies require very accurate conditions in data acquisition and processing to highlight the associated minima, given the low density contrast between caves and gypsum or marl host rock. Different microgravity station spacing was tested to find the best configuration for the detection of such caves. The suitability of ERT under different conditions of soil humidity is discussed in conjunction with the low resistivity contrast between gypsum bedrock and cave conduits. After measuring 2D ERT parallel profiles coincident in space, yet in wet versus dry soil conditions, the results were compared and it was determined that the ERT profiles measured under wet soil conditions are better than in dry ones for cave detection. A 3D model was drawn up for the two different humidity conditions to obtain the fitted morphology of the cave. The best results for cave detection are obtained when combining gravity and ERT in wet conditions, as they provide for a higher contrast in physical properties and an optimal setting for instrumental measurements.