Integrated time-lapse geophysical surveys for hydrogeological characterisation and monitoring of a clay-rich landslide in North Yorkshire, UK
Authors: Whiteley Jim , Uhlemann Sebastian , Watlet Arnaud , Boyd Jimmy , Chambers Jonathan , Kendall Michael 
Published: March 2020

&lt;p&gt;Landslides triggered by hydrological factors pose a risk to human safety and socioeconomic activities across the world. Detailed knowledge of the spatial extents of hydrogeological units in the landslide system, combined with an understanding of how moisture dynamics within these units vary over time, is crucial for identifying failure mechanisms and predicting future slope destabilisation. For landslide systems in which point-source monitoring information is sparse or depth-limited, spatially high-resolution time-lapse geophysical surveys can be used to both characterise the subsurface and infer changes in the saturation state in areas for which no point-source observations are available. Hence, geophysical characterisation and monitoring approaches can be used to improve local landslide early-warning systems, the majority of which predominantly rely on surface observations, or sparse subsurface data, to inform failure predictions.&lt;/p&gt;&lt;p&gt;Here, we present the results of an integrated geophysical characterisation and monitoring campaign undertaken at the Hollin Hill Landslide Observatory in North Yorkshire, UK. The observatory is situated in Lias Group mudrocks, comprising the failing clay-rich Whitby Mudstone Formation overlying the more stable Staithes Sandstone Formation. The landslide displays accelerated displacement during periods of high antecedent ground moisture and increased rainfall, driven by increased pore water pressures at the contact between the mudstone and sandstone. Over a period of 22 months, eleven co-located electrical resistivity tomography and seismic refraction tomography surveys were undertaken at the site. This campaign has the aim of characterising and monitoring the subsurface at resolutions and depths greater than exclusively using on-site surface or near-surface sensors (piezometers, moisture content and water potential sensors, etc.) or intrusive observations (boreholes, trial-pits, etc.).&lt;/p&gt;&lt;p&gt;Using a combined analysis of geoelectrical and seismic data, the subsurface of the landslide is discretised into hydrogeological units, which have distinct geoelectrical and seismic relationships corresponding to spatial variations in lithology and saturation. Variations in resistivity over time within these units are sensitive to changes in moisture content, and established site-specific petrophysical relationships between resistivity and moisture content are used to monitor the saturation state of the subsurface. Similarly, seismic derivatives, in particular P- to S-wave ratio and Poisson&amp;#8217;s ratio, are sensitive to changes in elastic properties induced by increases in moisture, providing information on the volumetric changes of subsurface units in relation to changes in saturation. The integrated monitoring provided by these combined geoelectrical and seismic methods reveals relative spatiotemporal variations in material properties including saturation, shear strength and shrink-swell state, all of which are important when considering slope destabilisation. This study highlights the need for incorporating high-spatial resolution monitoring approaches for managing and mitigating future landslide failures, and underscores geophysical monitoring methods as a powerful tool to be included when providing early-warning of slope destabilisation.&lt;/p&gt;