Long term linked geoelectrical / hydro-mechanical monitoring of an active slow-moving landslide

Abstract:

Understanding the hydrological and geological conditions present within an unstable slope is crucial for anticipating the likelihood of failure. The majority of landslides tend to be moisture-induced and as they occur in many regions of the world, they represent a global geohazard. Numerous recent geophysical studies have been conducted in order to characterize unstable hillslopes, often highlighting geoelectrical methods as a possible hydrological monitoring tool. However, landslides pose specific challenges for processing long-term monitored geophysical data. First, the sensor arrays can move during the monitored period, necessitating more sophisticated data processing. A second challenge is the interpretation of results in terms of hydro-mechanical parameters which are critical to understanding slope stability. Here we present two phased approach for processing long-term geoelectrical data from an active slow-moving landslide, Hollin Hill, situated in Lias rocks in the southern Howardian Hills, UK. The active monitoring period on the landslide is approximately 8.5 years, over which time various metre scale movements were detected, distorting the initial setup of the monitoring array. To avoid artefacts in the time-lapse electrical resistivity tomography (ERT) images we model landslide surface movement from discrete surface markers as a series of thin plate spines. The time-lapse landslide surface model is periodically calibrated with digital elevation models acquired by Unmanned Aerial Vehicle (UAV)-based photogrammetry and terrestrial laser ranging and detection scans (sourced retrospectively). This permits the recreation through time of the landslide topography and the positions of the electrodes (which were buried just beneath the surface). Both of these are necessary for accurate modelling of electrical current flow in time-lapse ERT. We then suggest that electrical resistivity can be used as a proxy for pore pressure in the hillslope. A petrophysical relationship is developed in the laboratory between electrical resistivity, soil tension and gravimetric moisture content and applied to infield modelled resistivities, yielding insights into the hillslope hydrology.