Publication

Abstract : Developing efficient in-situ monitoring of areas prone to landslides is a key problem when looking at hazard mitigation. Point-scale measurements, e.g. rainfall or soil moisture content sensors, provide rapid information on the slope status. Combining such conventional approaches with monitoring based on geophysical imaging can bring additional information on spatial and temporal hydraulic processes inside unstable slopes, thereby providing an opportunity to identify precursor processes to slope failure. An Electrical Resistivity Tomography (ERT) monitoring system (PRIME, developed by BGS) was installed in October 2017 near the town of Munnar, in the Western Ghats mountain range of South India. The site is more than 2000 m above sea level with an annual rainfall of between 2500 and 3500 mm. It experienced a complex rotational slide-debris flow landslide in 2005 and is considered vulnerable to further major landslide events. Since 2009, it has been instrumented by Amrita Vishwa Vidyapeetham with a wireless sensor network, including pore pressure, soil moisture, strain, tilt sensors, and a weather station. The ERT monitoring has been providing 4D resistivity imaging of the slope since March 2019. It is composed of four parallel lines of 32 electrodes. In the first stage of deployment, two of these lines have been collecting ERT data on a daily basis since November 2017, while suffering from several gaps due to issues related to the challenging tropical conditions, which have been mitigated in the second stage of deployment. Preliminary results indicate strong resistivity contrasts in the subsurface of the slope, which are likely to be linked with inhomogeneous weathering features and geological variations. Imaging the resistivity in the near surface during extreme weather conditions (i.e. dry and monsoon seasons) helps the understanding of the complexity of hydraulic processes affecting the slope. Time-lapse resistivity images suggest that the irregular subsurface weathering results in spatially variable moisture content variations, which might be the driver of local failure processes. This calls for a greater integration of geophysical and point-based monitoring systems in unstable slopes in mountainous monsoonal regions.