Safety assessment of landslides by electrical tomography: A case study from Ardabil, Northwestern Iran



Introduction: Occurrence of landslides depends on several factors such as the composition and structure of earth materials, rainfall level, temperature, groundwater regime and cover crops. Landslides are a main natural hazard in Iran, because of its special geological conditions and its mountainous active tectonic regime. Linear civil structures, such as roads, highways and railways, face serious problems in Iran, because of their long length which exposes them to several various geological features. For analyzing the stability of these structures, several geophysical methods, such as electrical tomography methods are frequently used. The main purposes of geophysical surveys are reconstruction of landslide geometry, detection of sliding surface (between sliding mass and bed rock), and exploration of the groundwater flow regime which is a stimulant factor before landslide occurrence.
Regional Geology: Based on the geological map of Sarab (1:100000), Rhyolitic and Dacitic lavas form the bed rock of the region with basaltic highlands. A major portion of road on the path consisted of alluvial river terraces and their great ability to absorb water which caused muddy materials to overflow and spontaneously move towards the valley. The main reason for this hazard is the base loss in down range. The landslide occurred several years after the utilization of the road, indicating the involvement of another factor which is the increase in intense rainfall during the previous months. This causes the region's high vulnerability and lack of base, during landslides.
Electrical Tomography: Electrical tomography or electrical resistivity tomography (ERI) is a geophysical technique for imaging sub-surfaces structures (sliding surface in this case) from electrical measurements made at the surface or boreholes. So the first stage in our electrical tomography was sending an electric current into the ground and then measuring the response of the earth in voltage. In the next step for building the inversed resistivity model, the algorithms of well know software Res2dinv (for making two-dimensional inversed model) and Res3dinv (for making three-dimensional inversed model) was used.
A simple way to utilize blocky inverse modeling is the application of the ordinary least square equations. In this method, the model that best fits the data is achieved through a method of optimization.
Discussions and Conclusion: The Schlumberger array was used for data acquisition. From 99 soundings which were carried out in this area, 90 soundings are selected for inversed modeling in 6 profiles. Due to the high noise levels in the surface, more damping (attenuation) coefficient was considered for surface data. Subsurface materials due to water absorption have more moisture with low resistivity, in contrast with the mass above the sliding surface, with relatively higher resistivity. 2D Electrical Tomography generally shows the location of Landslide Valley that can be seen most particularly in place of resistivity profiles P4 and P5. 3D Electrical Tomography has also been used for acquiring a 3D view from the sliding surface. In these models, the sliding surface is at depths up to 60m. Application geotechnical data may improve the inverse modeling images for upcoming exploration programs.