Electrical resistivity contrasts in the geotechnical assessment of iron caves, N4EN mine, Carajás, Brazil

Abstract

Although environmental laws and regulations vary among countries regarding natural caves near mine operations, all caves require studies for safety, preservation, and/or operational licensing purposes. Therefore, understanding the cave’s stability features and their risks of collapse are important considerations for mining companies. A near-surface electric resistivity survey applied to natural caves in the surroundings of an iron mine in Carajás, state of Pará, Brazil, identified three electrical patterns as indicative of points/zones of cave ceiling lithostructural weaknesses and instabilities: (1) low-resistivity contrast, in which the signatures abruptly pass to a low resistivity ([Formula: see text]) within a high-resistivity domain ([Formula: see text]), (2) high-intermediate resistivity contrast, a transition zone at the edge of the cave between a high resistivity at the center of the cave and a resistivity of approximately 900 ohm-m in the background, and (3) angular-resistivity contrast, a zone in which the section indicates vertical geometric patterns. We found five examples of these types of anomalies. When mining operations occurred closer to a known cave, a total of 15 rock falls were mapped. We found that 11 of these (73%) had geoelectrical anomalies similar to the patterns that we identified. These results contribute to improving the predictability of cave damage and its monitoring issues in which caves should be preserved during mining operations in their vicinity.