Gypsum Karst Investigation Using Electrical Resistivity Tomography: Case Study from Allazi, Latvia

This report is a case study on use of Electrical Resistivity Tomography (ERT) alongside other methods to have a better understanding of surface depressions and their formation in gypsum karst region of Latvia. Study territory is well known gypsum karst region. It is a relatively flat area with up to 5m thick Quaternary cover overlaying Upper Devonian Salaspils Formation gypsum sediments. Previous studies have identified several sinkhole formations in the area. Remote sensing methods like analysis of Digital Elevation Model (DEM) and different orthophoto maps were used to identify surface karst features. Further, geophysical measurements using GPR and ERT were performed over some of the identified surface depressions to understand the geological conditions and their impact on formation of these depressions. Combination of GPR and ERT methods have been used previously by authors in similar geological setting [1] and shown good results. By comparing different orthophoto maps an approximate formation year of one sinkhole was detected. ERT measurements were performed using multichannel instrument Syscal Pro Switch (IRIS instruments) with 72 electrodes, using Wenner electrode array with 2 m electrode spacing. Data were processed in Res2DInv program and the resulting 2D model interpreted based on existing borehole information about regions geology. Low electrical resistivities in the upper part were interpreted as locations where moraine sediments have been eroded and potential collapse or gradual land subsidence could occur. Results showed low resistivity zones under all 3 surface depressions that the ERT profile crossed. Also, one potential location where sinkhole could form in the future was detected based on low resistivity anomaly.