3D geoelectric modeling to characterize ferruginous caves in Brazil, using numerical and physical simulation

Abstract

In Brazil, the discovery of ferruginous pseudokarst caves in laterite profiles on iron mining areas has been significant. As a result, speleological legislation has restricted mining activity to protect caves considered to be most relevant, however, with subjective criteria. This requires the collection of precise data to better guide them. The application of techniques for the identification and characterization of caves is still limited, due to the irregular topography, vegetation and the obstruction of conduits and saloons by material that fell from the ceiling and walls, and it was also mobilized by miners. A relatively cheap and quick alternative to mapping the caves, compared to classical methods such as boreholes, is Electrical Resistivity (ER). Literature data are apparently controversial, as caves filled with air sometimes have high resistivity (tens to hundreds of thousands of ohms. meter), and sometimes low (up to a few thousand ohms. meter). Thus, the objectives were to investigate the electrical resistivity duality that can be observed in these caves, and present a non-invasive cave mapping methodology and a 3D geoelectric model (inverted), to be used as a reference in future field research. To this end, direct and inverse 3D geoelectrical numerical modeling was carried out to predict the electrical resistivity of typical elements in the ferruginous pseudokarst system of the Quadrilátero Ferrífero (QF), southeast region, where hundreds of caves were cataloged. Field surveys, laboratory analysis of collected material and simulations in a reduced physical model validated the predictions. The results showed that moisture around ferruginous caves can attenuate the electrical signal or even mask the high resistivity of conduits and saloons filled with air. Therefore, it was the main factor considered in the geoelectric model.