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Speleological Geophysics - Methodology for the Application of Geophysics in Iron Cave Studies

The brazilian speleological heritage is protected by federal environmental laws, and in the case of mining, the licensing procedures require scientific studies that demonstrate that caves will not have impacts on their ecosystems. Blasting near mining caves is the activity that offers the greatest risk of damage to their physical structures. To mitigate the problem, geotechnical and geostructural studies are performed through conventional surveys and mapping, but in an invasive and not always possible way. In this context, the application of geophysical methods has been proving to have a great potential, much more extensive and complementary to the direct methods, allowing to know the interior of the massif and its structural framework, better subsidizing, therefore, stability studies of the caves. In order to organize and sequence the flow of the involved processes, this work proposes a methodology for the systematic use of near surface geophysics in speleological studies, in this case, the electrical resistivity, and presents an example of its application in a cave near the N4EN iron mine in Carajás, Brazil. Introduction The Brazilian federal laws, from 1988, include natural underground caves as Union assets. Since 2008, with the high demand for mineral commodities, this legislation has become more restrictive, requiring for the licensing of any enterprise a series of long and complex scientific studies that ensure the integrity of the cave ecosystem during operations (MMA, 2004 e Brasil, 2008). The mineral industry was one of the most impaired when in cave regions, since blasting explosives is undoubtedly the greatest risk to their physical integrity. Therefore, to know the geostructural conditions and the geomechanical quality of the massif, as well as the degree of structural stability of the caves, are actions pursued by companies today, in order to comply with the legislation, trying to accelerate the licensing of areas. In general, studies of the physical environment of natural caves, combining the different disciplines such as, structural geology, geophysics, geotechnics and rock mechanics, are well developed in the literature for carbonate caves, but very scarce for caves in ferriferous terrains (iron caves), whose peculiarity are the low depths (up to 30m) where they occur, usually at the edge of plateaus. However, there has been a substantial increase in these studies for iron caves in recent years, mainly in Brazil, due to the legislation. In the geophysical literature there are important researches and projects with electrical, radar and seismic geophysical methods, practically all carried out in carbonate caves, even though they provide precious foundations for studies of iron caves. Some researches deserve attention, such as Pellerin (2002), that applied electrical and electromagnetic methods for geotechnical investigation of caves, and Manney et al. (2005), who explored the response of electroresistivity in a cave under different layered structural directions. More recent geophysical projects for iron caves are described in Barbosa et al. (2016a, b), Barbosa et al. (2017), Prosdocimi et al. (2018) and Gama et al. (2018), and show that near surface geophysics can detail relevant underground geostructural aspects, imperceptible to conventional mapping methods, especially in the portions between the caves ceilings and the surface of the terrain, identifying more precisely their real zones of fragility. In order to organize and sequence the flow of the processes involved in geophysical surveys of caves, the present work draws a methodology, called Speleological Geophysics, that was applied to an iron cave near the N4EN iron mine in the region of Carajás, southeast of the state of Pará, northern Brazil (Figure 1). Figure 1 – Study area location map.