Hydrogeophysical investigation of clay-rich landslides through combined electrical and electromagnetic methods

Abstract

Landslides are complex systems, which are commonly investigated using data from punctual sensors, e.g., installed in boreholes. Assuming lateral variations in the subsurface fabric by interpolating the data from sparse boreholes may provide biased insight into the processes and architecture of landslides. Geophysical methods can be used to overcome this issue, gaining information about the physical properties (e.g., electrical conductivity, seismic velocity) of the subsurface covering large areas with high resolution. In landslides, geophysical methods have been used to investigate the geometry of the geological units and the depth to the bedrock, of the position of sliding planes and to compute the volume of mobilized material. Moreover, recent studies have demonstrated the ability of geophysical methods to quantify variations in the hydrogeological properties in an imaging framework. While the use of borehole data helps to reduce ambiguities in the interpretation of the geophysical images, the combination of more of different geophysical methods allows to enhance the coverage and resolution of the investigation as well as to reduce modeling uncertainties. In this contribution, we present the combination of electrical and electromagnetic methods for the hydrogeological characterization of a clay-rich landslide located in Upper Austria (Austria). The investigation considers a two-step approach: (1) mapping at the large scale using electromagnetic methods at low induction number, and (2) selection of particular areas for the conduction of spectral induced polarization (SIP) transects. The first step aims resolving the main variations of clay content as well as to identify preferential flow paths for near-surface run-off; while in the second step SIP measurements are used to quantify hydraulic conductivity and water content. EMI mapping was conducted using vertical and horizontal configurations with two different instruments, each one consisting of three receivers, resulting in mapping information along 12 different geometries reaching a maximal nominal depth of investigation of 7 m. SIP measurements were collected at 12 different frequencies in the range between 0.25 and 225 Hz using 64 electrodes in each transect, with a spacing of 2.5 m to reach a depth of investigation of ca. 50 m. Maps of the electrical conductivity gained by EMI measurements reveal strong lateral variations in clay content across the entire site. The inversion of the SIP data permits to quantify vertical and lateral changes in the hydraulic conductivity and water content along the transects. Our results demonstrate that an adequate processing of the data and the use of cascade inversion of multi-frequency SIP data permit to resolve for consistent hydraulic properties using different petrophysical approaches. Inversion of the EMI data along the SIP profiles reveals consistent results in the variations of electrical conductivity, permitting to validate the SIP results in shallow areas. Additionally, we investigate the relationship between electrical and hydraulic conductivity along the SIP transects and use it for a quantitative interpretation of the EMI maps; thus, permitting a hydrogeological investigation of the entire study area. Our results reveal the potential of combining EMI and SIP for quantitative investigations of landslides.