Analysis of sensitive clays from the Gjerdrum landslide in Norway: Dielectric spectroscopy with modeled TEM IP-responses, and x-ray diffraction analysis

Dielectric spectra of pure and silty clays of varying sensitivity from the Romerike clay deposit were obtained to model induced polarization (IP) signatures in TEM (transient electromagnetics) data obtained with stationary current loops on the ground. The Maxwell–Wagner (MW) relaxation frequencies that characterizes the dielectric spectra were in the range 6–11 kHz, and the results indicate higher relaxation frequencies for more sensitive clay. X-ray diffraction analysis showed nearly identical mineral composition of all the clay samples, with chlorite and illite as the main components. The results indicate that both the relaxation frequency and clay sensitivity increase with silt contents. Although pore water conductivity (due to salt and other ions) did not correlate well to sensitivity and relaxation frequency in the current samples, it is still likely that the ionic composition influences these parameters. Relaxation frequency variation in clay layers a few meters below the surface can be detected with TEM provided that the dielectric strength of the clay is sufficiently large (above about 1500), and that the resistivity of the overlying soil layer is sufficiently large (above about 100 $\Omega m$). Hence, TEM could be used to determine subsurface clay sensitivity under the right conditions, once the link between the relaxation frequency and clay sensitivity is fully understood.