Resistivity modelling forclay layer characterisation, possibilities and limitations

Abstract

Quick clay plays an important role in Norway due to the occurrence of several landslides that caused severe damage. Hence, it is of high importance to understand the properties of quick clay and locate it in the subsurface.\rIn this report, 2D resistivity modelling was done in order to study the possibilities and limitations of mapping quick clay and other material. Models of different horizontal layers, dipping interfaces and lenses were analysed. The following configurations were used for the modelling: Dipole-dipole, Gradient, Pole-dipole and Wenner. The forward modelling was done with RES2DMOD whereas the inversion was performed with RES2DINV. The protocol files that were used for the forward modelling simulate the measuring procedure in the field with an ABEM Lund system which is based on a layout of 4 cables with total 81 electrode positions. Smooth and robust inversion was carried out for all the models in order to compare the different responses. Additionally, vertical\/horizontal filter values of 1 and 0.5 were tested and the effect on the inversion models analysed. Results from some models are presented with two colour scales to enhance different conditions.\rIn general, the smooth inversion is better for detecting quick clay in a low resistivity environment. However, if bedrock is present with higher and sharper contrasts, robust inversion can improve the mapping of bedrock. Mostly a vertical\/horizontal filter of 0.5 showed slightly better results, since a lot of models tend to have horizontal formations and therefore a filter value of 0.5 aligns the structures more along the horizontal axis.