Reconstructing the rotational landslide near Frixa (Greece, Peloponnese) with a combination of different geophysical methods and terrestrial laser scanning

After the 2007 wildfires in the western Peloponnese (Greece) we started an investigation in 2009 in this area at different sites, aiming at the reconstruction of different landslide types and to investigate the relation between fire-induced deforestation and landslides. The village Frixa, particularly its southern part, has massive problems with big rotation slides, shallow slides and erosional features. Tertiary Flysch units and Neogene deposits in the Pyrgos area are generally landslide prone. The slopes in the area have the typical morphological features of a “landslide landscape”. Many recent effects from landslides like slope failures, cliff break ups, road failures, destroyed retention walls, and cracks in houses(structural damage) can be observed in the burned areas even 2 years after the great fires. We assume that the intensity and the frequency of shallow landslides and rotation slides are increasing due to the wildfires, since the lack of vegetation results in a lowered retention potential.
   In this case study we present our preliminary results of the slide investigations in Frixa near the ancient city of Olympia. For the study we used different geophysical methods (the capacitive coupled DC geoelectrics system “OhmMapper” and Ground Penetrating Radar, GPR) and a remote sensing tool (ground based t-LiDAR). The terrestrial laser scanning (TLS) is an effective remote sensing technology for reconstruction and observation of natural phenomena or geohazards as it is well founded of high spatial and temporal resolution. TLS was used for the reconstruction of the landslide geomorphology. To ensuring the complete recording of the landslide morphology it is necessary to scan the object from different angles. The entire scan sequence in this case study includes six different scan positions with approximate 2.5 million points with around 4 cm point distance. The t-LiDAR data allowed achieving a 0.5 m digital terrain model after the data processing (alignment the different scan windows, data filtering and cleaning, data interpolation).
   We used 100 MHz and 270 MHz antennae and the SIR-3000 data collection system (GSSI) for the GPR investigations. Since penetration depth and spatial resolution depend on the antenna frequency, we used two different antennae that cover a depth of up to 7 m and a resolution of up to 7 cm, depending on the underground conditions. Penetration depth is in inverse ratio to conductivity, so clayey and humid materials lead to a high attenuation of the radar waves.
   The OhmMapper was used to determine the resistivity distribution in the soils up to a depth of approx. 5 m. Layer depths determined by GPR can be used to improve geoelectrics data inversion, while information from geoelectrics measurements help to interpret the GPR signals. The combination of geophysical surveying and remote sensing allows mapping the surface topography and the thickness of the landslide bodies, thus enabling us to create a three-dimensional model of slides.