Joint Inversion of Seismic Refraction and Electrical Resistivity Tomography to Investigate Sinkholes

A joint inversion of seismic and resistivity data was carried out for site investigation to delineate subsurface expression of dissolution induced sinkholes and assess the risk for future development of the study area. The results of the 2D seismic refraction tomography are compared with results of the 2D electrical resistivity tomography and show good compatibility at a depth of about 20 meters. The results of cluster analysis of the joint inversion indicate that the subsurface section of the study area consists of four layers; dry sand, saturated sand, till/clay and saturated chalk layers. The joint inversion results and the lithology from boreholes confirm that sandy sediments are dominant on top of the chalk close to the observed sinkholes and to the discontinuities in reflection seismic sections instead of till/clay which is dominant towards North. Most of the discontinuities and observed sinkholes are located close to the boundary between the dry sand and the shallow till/clay layer. This geologic boundary and the permeability of the sand layer allow for direct groundwater contact with the bedrock (chalk) surface and may lead to the dissolution and formation of sinkholes at the surface.