Fraturas e halos de subsidência em torno de dolinas, Semiárido do Brasil

Abstract

Carbonate rocks are recognized for their great heterogeneity and for the presence of structures associated with dissolution. This is very important in fractured regions, since fractures can induce dissolution processes. Among the consequences generated are secondary porosity and increased permeability, which is essential in regions with oil reservoirs. In addition, the increase in dissolution is responsible for the formation of collapse structures, which can occur both at the surface and in the subsurface. The collapses are responsible for several problems in urban areas built on this type of terrain, such as the collapse of buildings and roads, which can cause serious social and environmental problems. The work focused on the study of dolines, which are the most expressive collapse structures in carbonate rocks, and their relation with preexisting fractures. The occurrence of dolines in outcrops can help to answer questions intrinsic to the problems mentioned, such as if there is any interference in the structural and petrophysical properties of the affected rocks, or even to improve the prediction about the effects that the collapses generate in the topography. The studied area has four sets of preexisting fractures, one N-S / EW and another NE-SW / NW-SE, which concentrate the main dissolution in the region. The presence of these fractures allowed the formation of the collapse doline. The data showed that in areas where collapses occur, there is the formation of what are being called subsidence halos. This zone is suffering subsidence due to the main collapse, and the original relief is affected, plunging towards the dolines. Topographic relief variations greater than 10 meters in relation to the non-affected areas were measured. It was also observed that the affected area around the dolines has on average twice the radius of the dolines. This process generates a change in the pattern of fractures of the region, with the formation of a new set, called collapse fracture. These fractures are circular in shape and occur around the dolines. An increase in the aperture and density of these fractures when approaching the dolines was also observed through scanlines. This represents an indicator of permo-porous quality improvement in these areas. In addition, it shows that there is an increase in structural instability, rising the risk of accidents in areas built on soluble rocks, since the affected area may be much larger than previously predicted.