Resistivity and IP Tomography to determine Overburden-Bedrock Interface: A case study of Ilam Embankment dam

Determination of the overburden-bedrock interface with fine-grained sediments in a high-fold sedimentary environment is a challenging geophysical issue. Electrical Resistivity Tomography (ERT) is considered one of the most effective geophysical approaches for mapping subsurface layers based on the conductivity distribution of materials. The surveys are often performed in two dimensions to investigate lateral and depth variations of resistivity and chargeability values of subsurface layers. The resistivity method, influenced by the volumetric properties of empty spaces, is defined by the ability to transfer charge in subsurface medium, but the induced polarization method depends upon the geometric properties of the pore spaces (grain surface size). Despite the advantages of geo-electrical methods in imaging subsurface structures, due to the high dependency of resistivity and induced polarization parameters on the physical and hydrogeological conditions of the layers, it is not possible to fully match the geological and geo-electrical sections.



One of the applications of geophysical studies is to determine the contact zone between overburden and bedrock in engineering structures such as embankment dams. In cases where the conductivity contrast between the overburden and the bedrock is low, the exact determination of this boundary with the help of geo-electrical methods confronts high uncertainty. In this study, the efficiency of electrical resistivity tomography and induced polarization is investigated by measuring several parallel profiles with the aim of imaging the boundary between overburden and bedrock and determining the possibility of a water escape zone at the left bank of the Ilam embankment dam. According to the results obtained from the inversion of the field measurements, rechargeable sections would be ascribed to the shale region as well as marl limestone containing pyrite particles.



The main objectives of this study include determining the general condition of the overburden concerning the bedrock, geometric imaging of the bedrock, and identification of parts of the bedrock eroded over time. The significant challenge of this geophysical study is the low conductivity contrast between clay and silt overburden and limestone bedrock interbedded with shale and marl. Due to the size of the study area, the studies were performed based on tomographic measurements of electrical resistivity and induced polarization. The field surveys were conducted using four almost parallel profiles (according to the topographic conditions of the area) and with relatively different lengths and through a Pole-Dipole array in forward and reverse measurements.



Geological data, as well as borehole information, are used to validate the geo-electrical sections to better interpret the models obtained from the collected data (i.e., geo-electrical measurements). Finally, due to the high topography of the area and to better show the trend of subsurface structures using two-dimensional models obtained from electrical resistivity tomography and induced polarization as well as drilled boreholes, a three-dimensional view of sections and boreholes has been prepared. Based on the models obtained from the geo-electrical data, it can be concluded that geophysical studies (electrical tomography) have been able to successfully determine the eroded region of the bedrock surface as well as the bedrock-overburden contact which correlates well with boreholes drilled in the area.