Geophysical prospections using 2D-ERT coupled with EMI survey for the spatial variability assessment of landslide related pedological discontinuities in the Turbolo basin (Calabria Region, Italy).

In the framework of the ongoing project “SOIL SHADES – SOIL features and pedogenic processes as predisposing factors of SHAllow landsliDES”, funded by Next Generation EU, National Recovery and Resilience Plan (PNRR) of Italy, M4.C2.1.1., National Research Programme (PNR)–Research Projects of Significant National Interest (PRIN), an integrated multi-scale and multi-analytical approach was applied in the Turbolo Stream catchment, in northwestern Calabria region (southern Italy). Due to its peculiar geological-geomorphological and pedological characteristics, this basin has been selected as pilot study area representative of several Mediterranean environments. It is about 30 km² wide, elevation ranges between 75 and 1015 m asl and displays a dendritic pattern in mountainous sub-basins along with a trellis-like pattern in hilly reaches. Paleozoic metamorphic rocks (gneiss, phyllites, schists, metabasites interbedded with metapelites and metalimestone) outcrop in the western sector, while Miocene to Pleistocene deposits (clay, sand and conglomerate) in the eastern part, and Holocene sediments in the valley floor. The western sector is dominated by high relief and steep slopes dissected by deep V-shaped valleys, whereas the eastern hilly reaches are characterised by gentler slopes, fluvial terraces and broad valleys. The study area is recurrently affected by rainfall-triggered landslides damaging agricultural land, infrastructure and settlements. Geophysical prospections using 2D-electrical resistivity tomography (ERT) have been combined with electro-magnetic induction (EMI) surveys for the identification of possible shallow sliding surfaces, due to the effectiveness of both techniques in the detection of geological and pedological discontinuities in terms of particle size distribution, mineralogy, porosity, water content, solute concentration, etc. To support the geophysical data, several field observations were conducted along the landslide area. The most representative soil profile was selected at about 130 m asl on the southern slope of a Pleistocene fluvial terrace, in the eastern hilly reaches of the basin. A very deep soil profile (approximately 3 m of depth) was described on the scarp of a rotational slide that developed for some tens of meters downslope. Soils appear moderately to deeply weathered and have a matrix color ranging from reddish to yellowish brown with red and grey mottles indicating the persistence of stagnic conditions during the rainy season. Evidence of clay illuviation processes (i.e., clay coatings) is found in both the topsoil and the bottom soil, very likely due to alternating phases of slope stability and surface soil erosion. The soil texture varies from sandy loam to clay loam with relevant changes in the amount of subrounded to subangular coarse fragments. The soil reaction is from slightly acid to neutral, consistently with the absence of carbonates and the illuviation process evidence. Results of the geophysical surveys displayed some changes in the measured parameters in the surface layers, which are consistent with the depth of the landslide scarp and of the soil profile, as well as of the potential depth of the failure surface.