3D fracture networks in the crystalline upper crust ‐ A new seismic model of the Continental Deep Drilling Site (South Germany)

Abstract

ABSTRACTBased on three‐dimensional seismic reflection data, we present the first comprehensive three‐dimensional model of the fault and fracture inventory of the crystalline upper crust penetrated by the Continental Superdeep Drillhole (Kontinentale Tiefborung, Oberpfalz, Southeast Germany). The investigated volume spans ∼19×19 km2 down to 16‐km depth. It can be regarded as a typical example of metamorphic crust that has undergone numerous phases of ductile and brittle deformation since the start of the Variscan orogeny. We developed an automated workflow for identifying and quantifying the crustal fracture network in order to prepare a basis for an assessment of fluid pathways and geothermal potential of crystalline crust. The workflow comprises the following steps: determination of (i) a three‐dimensional model of major faults by structural tensor analysis and (ii) a three‐dimensional model of middle‐scale fractures by log‐Gabor filtering and image processing; (iii) validation of results by comparison with geophysical borehole data, with independent seismic and seismicity data; and (iv) definition of a three‐dimensional fracture density function serving for a statistical assessment of fracture connectivity and “relative permeability.” This assessment is based on probabilistic fractionation and percolation theories. The derived three‐dimensional distribution of “relative permeability” may be used as a kernel function for inverting hydraulic permeability from hydrothermal field experiments. By comparing borehole and three‐dimensional seismic data, we could confirm that the faults and fractures of the Kontinentale Tiefbohrung area follow a fractal law that is consistent with Turcotte's fractionation model. An important conclusion from this is that upscaling and downscaling between one‐dimensional borehole and three‐dimensional seismic data appear possible. The corresponding one‐, two‐, and three‐dimensional fractal dimension are about 0.8, 1.9, and 2.8, respectively.