Modelling microfracture geometry to assess the function of a karst system (Vizfo spring catchment area, Western Mecsek Mountains, Hungary)

One of the current objectives of karst research is to understand the spatial dynamics of karstic processes that increase or decrease pore volume. Although we can construct 3D numerical models, it is a complex, multi-step process. These modelling approaches combine a dissolution algorithm, a flow and/or transport model and an algorithm to reconstruct the spatial geometry of fracture networks. The paper focuses on the last task, and does not consider the first two problems. We applied the RepSim code, a DFN (discrete fracture network) type fracture geometry modelling software that uses fractal behavior of fracture patterns, for simulations. This method simulates fracture systems at a reservoir scale. The input parameters (length distribution, aperture, orientation and fractal dimension of the fracture midpoints) were determined using field measurements and evaluation of digitized images. The primary images were of outcrops from the surface and from two caves. The examined area is the karstic block of the Mecsek Mountains in SW Hungary, which is one of the most explored karstic regions in that country. There are seven small and one relatively large (Vizfo) catchment areas in the mountains. The large catchment was used as the study area, which lithologically consists of sandstones and limestones, both intensely fractured by subsequent tectonic events. The spatial distribution of cave entrances and dolinas is uneven across the study area. This phenomenon has not yet been investigated. Here, a relationship is inferred between the original (prekarstic) microfracture network geometry and the spatial distribution of the aforementioned karst forms. The results show that the region can be divided into two zones that fractured in distinctly different ways. Their fracture network communication features, porosity and permeability differ. Additionally, sub-regions could develop inside the catchment area where dissolution-cementation processes could have been differently effective, determining the spatial distribution of the dissolution governed karstic forms (e.g. caves, dolinas).