Application of multi-dimensional electrical resistivity tomography datasets to investigate a very slow-moving landslide near Ashcroft, British Columbia, Canada

Landslides in the Thompson River valley, British Columbia, Canada, have historically impacted vital transportation infrastructure, the environment and natural resources, cultural heritage features, communities, public safety, and the economy. To better understand and manage geohazard risks in Canada’s primary national railway corridor, government agencies, universities, and railway industry partners are focusing research efforts on Ripley Landslide, 7 km south of Ashcroft. Electrical resistivity tomography (ERT) datasets collected in November 2013 (on land) and November 2014 (over water) were successfully combined and inverted into a pseudo-3D model that produced significantly deeper resistivity values than previously available in 2D profiles. The lithology, degree of saturation, porosity, presence of dissolved electrolytes, and temperature all influence electrical resistivity of earth materials in the landslide. Continuous (real-time) ERT monitoring began in November 2017 to characterize the long-term hydrological behavior of geological units in the landslide. Seventy-two electrodes were positioned in two arrays across the slide body and connected to a proactive infrastructure monitoring and evaluation (PRIME) system with internet access. PRIME resistivity results corroborate data from other geophysical techniques and hints at an unusual distribution pattern for surface moisture and groundwater in fractured bedrock and overlying clay-rich sediments containing vertical tension cracks and discrete sub-horizontal planar features interpreted as slide surfaces within pre-sheared zones. A greater understanding of the composition and internal structure of slope failures in the valley is gained at the site from terrain analysis and modeling of multi-dimensional geophysical datasets. This insight helps with the interpretation of multi-year monitoring datasets and will guide future efforts to record landslide activity in the valley, reducing stakeholder risks.