Integrated geophysical survey in a sinkhole-prone area: Microgravity, electrical resistivity tomographies, and seismic noise measurements to delimit its extension

Detection, forecasting, early warning, and effective monitoring are key aspects for the delimitation of sinkhole-prone areas and for susceptibility assessment and risk mitigation. To attain these goals, direct and indirect techniques can be employed, and the integration of different indirect/non-invasive geophysical methods including 2D- and 3D-electrical resistivity tomography, microgravity, and single-station seismic noise measures was carried out at “Il Piano” (Elba Island - Italy), where at least nine sinkholes occurred between 2008 and 2014. The most likely origin for these sinkholes had been considered related to net erosion of sediment from the alluvium, caused by downward water circulation between the aquifer hosted in the upper layer (Quaternary alluvial deposits) and that in the lower (Triassic brecciated dolomitic limestone and Cretaceous slate). The integrated geophysical survey, therefore, was carried out a) to differentiate shallower from deeper geological layers, b) to detect possible cavities that could evolve into sinkholes, c) to suggest possible triggers, and d) to delimit the sinkhole-prone area. The results of the integrated geophysical surveys suggest that the study area is mainly characterised by paleochannels, and that the sinkhole-prone area boundaries correspond to these paleochannels.