Item

Abstract

Two fluid injection experiments were conducted at the German Continental Deep Drilling Site (KTB) in 1994 and 2000. Microseismicity occurred at different depth intervals. Hypocenter locations were precisely localized. Here, microseismicity is analyzed in terms of its spatio-temporal evolution characteristics. An approach is applied which assumes microseismicity to be triggered by a diffusive process of pore pressure relaxation. The method yields estimates of hydraulic parameters of rocks on large spatial scales. At the KTB site the method enables us to study hydraulic diffusivity at two different depth intervals. We observe significant variations in the evolution characteristics of the seismic activity at different depths. Estimates of hydraulic diffusivity for shallower parts of the crust seem to be much smaller than for deeper regions. To understand reasons for this, we have analyzed spatial relations of hypocenter locations to the distribution of intensities of seismic reflections. Low values of hydraulic diffusivity correlate with low reflection intensities and high diffusivities with large intensities, respectively. The analysis confirms the hypothesis that the process of pore pressure relaxation along pre-existing and critically stressed natural fractures is an important triggering factor for induced microseismicity.