NEW ADVANCES IN NEAR-SURFACE ELECTRICAL RESISTIVITY TOMOGRAPHY: UNDERSTANDING THE HYDROGEOLOGICAL PROPERTIES AND BEHAVIOR OF A VERY SLOW-MOVING LANDSLIDE IN THE SEMI-ARID THOMPSON RIVER VALLEY, BRITISH COLUMBIA, CANADA

Globally, transportation infrastructure, public safety, the environment, and natural resources are increasingly at risk from landslides triggered by extreme weather events, earthquakes, volcanoes; changes in sea level, storms, floods, drought, wildfires; and anthropogenic activities. A key role for both the Geological Survey of Canada (GSC) and British Geological Survey (BGS) is to provide fundamental earth science knowledge to ensure resilience to multi-geohazard risks. For example, since the late 19^th Century, landslides along a 10-km reach of Thompson River south of Ashcroft, British Columbia, Canada, have repeatedly damaged vital national railway infrastructure, in addition to threatening salmon runs, potable water supplies, cultural heritage features and public safety. Government agencies, universities, and the railway industry are presently focusing research efforts on the very slow-moving Ripley Landslide to better manage geohazard risk in this critical transportation corridor. Here, we characterize the landslide’s composition and structure through hydrogeological and electrical resistivity imaging using a Proactive Infrastructure Monitoring and Evaluation (PRIME) system, which addresses 74 electrodes positioned across the slide body and head scarp, and is calibrated by laboratory testing of resistivity of earth materials under controlled moisture and temperature conditions. Daily monitoring of electrical resistivity since November 2017 reveals complex hydrogeological pathways in the slide body, with differences in electrical resistivity of earth materials reflecting a combination of hydrogeological characteristics, responses to variations in ground temperature and precipitation, and the subsurface distribution of solute-bearing soil water. These new hydrogeological and geophysical datasets: 1) enhance our understanding of the composition, internal structures, and groundwater flow paths through the landslide - critical information to develop moisture thresholds for slope failure; and, 2) provide important context for interpreting other multi-year slope monitoring efforts underway in the valley.