Microseismic monitoring has become necessary as daily operational work for various underground energy development projects, e.g., unconventional oil and gas or enhanced geothermal system projects. The primary purpose of microseismic monitoring is to record the effect of fluid injections and to infer the shape of the created reservoir. Thus, hypocenter location is the first order information. Various seismological analysis technics provide chances to extract more information such as fault orientations from microseismicity. However, the quality of microseismic waveforms is limited, i.e., they have a small amplitude. In this study, we propose the novel concept to introduce information of natural fractures inferred from borehole logging into microseismic analysis as prior information. In seismology, we solve reverse problems by analyzing seismograms. We cannot access in-situ geophysical data in seismogenic zones since basically natural earthquakes occur mostly at much greater depth than we can reach. In the field of induced seismicity, we can access geophysical parameters related to induced seismicity directly through borehole measurements. This offers possibilities to integrate geophysical information in seismological analyses geomechanical and geological theory.
We focus on natural fracture information acquired by borehole log analysis and apply it to focal mechanism analysis of induced seismicity. In this work we use data from the Basel EGS project to test our concept. We estimate the focal mechanism of induced seismicity from many first motions of P-wave arrivals, but it is challenging to constrain the solution due to usually small numbers of monitoring stations in real induced seismicity cases. We use the distribution of natural fracture orientations as prior information and build a statistical model of possible fault orientations of induced seismicity. Then, the range of possible focal mechanisms from the seismological analysis is constrained by superposing the statistical model. Introducing such prior information leads to a significant reduction of the range of focal mechanisms.