Adjustment of a Crustal Geoelectric Model from Commissioning Data of HVDC Ground Electrodes: A Case Study from the Northeastern Paraná Basin, Brazil

Small-scale near-surface galvanic scatters are known to be a bane when using natural field EM data to get quantitative information about the subsurface resistivity structure. Specifically, on magnetotelluric measurements, the effect of local surficial inhomogeneities is almost inevitable and produces a static shift in the measured apparent resistivities over the whole range of sampled frequencies. We present here the methodology used to account for static shift effects and make a correct interpretation of the resistivity distribution in a region beneath the northeastern part of the volcanic-covered Parana basin, SE Brazil. Within this region are located the ground electrodes of the Rio Madeira HVDC transmission system (Bipoles I and II). The available data includes geophysical soundings made for the electrodes site selection before their installation and data acquired in the subsequent commissioning measurements after their construction (electrode resistance, soil surface potentials and pipe-to-soil potentials). Iterative calculations for the simultaneous adjustment of the measured electrode resistance and the local geoelectric model were developed, based on the commissioning data of the electrode. The accuracy of the adjusted geoelectric model was confirmed by the excellent fit obtained for the measured and calculated pipe-to-soil potentials at Gasbol, the Bolivia-Brazil gas pipeline located 26 km away from the electrode. (C) 2020 Published by Elsevier B.V.