Landslide surveys in Tertiary soft rock areas using HEM

Abstract

This paper describes case histories for landslide surveys in Tertiary soft rock areas in Japan using a Helicopter-borne Electromagnetic (HEM) system operating at five frequencies between 220 Hz and 137,500 Hz. The usual method for conducting landslide surveys consists of airphoto interpretation, field investigation, and test borings. The ground dc resistivity method is seldom used for landslide surveys, because of the intermixture of topographic effects, the lack of uniformity of data and the difficulty of obtaining closely spaced survey lines along the slopes. On the other hand, HEM data are densely sampled, are very uniform over a survey area and are not as likely to suffer from topographic effects since the HEM "footprint" is relatively small compared with terrain changes. These factors make HEM measurements ideal for detecting the small resistivity changes caused by the landslides. HEM results show that the landslide locations coincide well with anomalously low resistivity zones in HEM apparent resistivity profiles and maps. This correlation occurs because the sliding surfaces of Tertiary type landslides are always accompanied by montmorillonite clay, which lowers the resistivity of the rocks. Furthermore, landslide locations very often correspond to an upheaval or raised pattern of low-resistivity zones in imaged AEM-resistivity sections. Shallow low-resistivity zones such as sliding surfaces also lower the apparent resistivity values at lower frequencies. The HEM method is effective in detecting and delineating landslides associated with cohesive soil, collapsed soil, or weathered zones in areas of Tertiary soft rocks.