Delineation of Clay Layers in a Landslide Area in Norway Using Frequency Domain Helicopter-borne EM Survey

Abstract

There was a quick-clay landslide in Byneset on 1st January 2012. The landslide area is surrounded mostly by agricultural lands. The geology in the area consists of old ocean floor with outcropped bedrocks at several places. Prehistorically, the sea-level was ~160 m higher than the present sea-level. The sediment in the area mainly consists of marine clay. Subsequent leaching by fresh groundwater alters the chemical composition of the pore water and “quick clay” may develop. Quick clay completely liquefies when remolded under stress and results in large landslides. Resistivity survey is a powerful tool to investigate such clay layers but it is time consuming. Airborne EM survey can be a faster way to investigate large areas. Seismic refraction survey is useful to delineate depth of bedrock. Therefore frequency domain helicopter EM (FHEM), resistivity and refraction seismic surveys were performed in the area last year. There is a good agreement of the results from all the geophysical surveys. Resistivity and FHEM data show various clay layer boundaries well in case of thin marine clay deposits. However, FHEM data is poor to resolve geological boundaries in case of thicker marine clay deposits due to low skin depth in a conductive environment.