Limitations in detectability of air-filled gypsum karst cavity by electrical resistivity tomography: a case study from the Baltic Devonian sedimentary basin

Abstract

Karst terrain is widely distributed globally, posing one of the most significant issues for civil engineering and public safety. Electrical resistivity tomography (ERT) is regarded as the most suitable method for exploring subsurface karst features. Nevertheless, ambiguities in the ERT inversion process can arise due to specific geological conditions. In this study, we used measurements obtained in the area next to a recently developed gypsum karst sinkhole in Latvia and 3D geophysical modelling to specifically analyse the limitations in identifying near-surface air-filled karst voids by ERT. Our results emphasise that due to the 3D effect, even the recent sinkhole may be undetectable in ERT data, despite the placement of ERT profiles directly above the overhanging part of the 7-m-deep sinkhole. The 2D synthetic modelling results suggest that a karst sinkhole of similar size to the one surveyed in the field should be easily recognised. In contrast, the results obtained with 3D synthetic modelling reveal almost no indication of a sinkhole in the modelled profiles. We conclude that 2D synthetic modelling cannot always be used to assess the possibilities of identifying subsurface cavities with the ERT method. Reliable assessment can only be achieved using 3D synthetic modelling techniques. Our results demonstrate that problems with detecting air-filled karst sinkholes can arise not only in areas where surrounding rocks have a high electrical resistivity but also where surrounding rocks have a low resistivity.