Three-dimensional geophysical anatomy of an active landslide in Lias Group mudrocks, Cleveland Basin, UK

Abstract

A geoelectrical investigation of a slow moving earth slide–earth flow in Lower Jurassic Lias Group rocks of the Cleveland Basin, UK, is described. These mudrock slopes are particularly prone to failure and are a major source of lowland landslides in the UK, but few attempts have been made to spatially or volumetrically characterise the subsurface form of these slides. The primary aim of this study was to consider the efficacy of fully three-dimensional geoelectrical imaging for landslide investigation with reference to a geological setting typical of Lias Group escarpments. The approach described here included a reconnaissance survey phase using two-dimensional electrical resistivity tomography (ERT), resistivity mapping, self-potential (SP) profiling and mapping, followed by a detailed investigation of an area of the landslide using three-dimensional (3D) ERT and self-potential tomography (SPT). Interpretation of the geophysical data sets was supported by surface observations (aerial LiDAR and differential GPS geomorphological surveys) and intrusive investigations (boreholes and auger holes). The initial phase of the study revealed the existence of a strong SP signature at the site consistent with a streaming potential source and established the relationships between the main geological units, the geomorphologic expression of the landslide, and the resistivity of the materials in and around the study area. The 3D SPT model generated during the second phase of the study indicated drainage patterns across the landslide and preferential flow from the low permeability mud rocks into the underlying more permeable sandstone formation. Because of favourable resistivity contrasts between the clay-rich Whitby Mudstone Formation landslide material and the underlying Staithes Sandstone Formation, the volumetric 3D ERT image allowed a number of surface and subsurface landslide features to be identified and spatially located. These included the lateral extent of slipped material and zones of depletion and accumulation; the surface of separation and the thickness of individual earth flow lobes; and the dipping in situ geological boundary between the Whitby Mudstone and Staithes Sandstone bedrock formations.