Hydrological aspects of geophysical borehole measurements in crystalline rocks of the Black Forest

Abstract

One of six wells drilled into the crystalline rock of the Black Forest basement during the German Continental Deep Drilling Project (KTB) site survey has been used to detect hydraulically active fractures and to identify their character by a variety of logging tools such as caliper and televiewer logging, vertical seismic profiling (VSP) and electrical and thermal measurements in addition to standard logging recordings. In the 265 m Moosengrund well drilled into the Triberg granite body these methods were applied successfully to discriminate between fractures and breakouts and to identify open and closed fractures and water-consuming or water-expelling fractures, respectively. Fracture strike, dip and width were determined from televiewer data. VSP measurements were used to determine the seismic velocities in the unfractured rock and the fracture zones. Estimates of permeability (10–55 Darcy) and transmissivity (0.1–1.0cm 2s −1) of the fracture zones were obtained by the analysis of P waves and tube waves in the VSP records. The summed values of the transmissivity yield an upper estimate of the overall transmissivity determined by a pumping test.