Peter Starzec, Michael Fehler, Roy Baria, and Hiroaki Niitsuma
Spatial correlation of seismic slip at the HDR-Soultz geothermal site; qualitative approach
Bulletin of the Seismological Society of America (December 2000), 90(6):1528-1534

Abstract:
One major goal of monitoring seismicity accompanying hydraulic fracturing of a reservoir is to obtain information about the pattern, size, and orientation of fractures. The traditional method is to analyze the pattern of locations of the induced microearthquakes. To investigate whether additional information about the fracture pattern may be obtained from induced microearthquake datasets, we applied variogram analysis to investigate the spatial distribution of shear displacement at the source of microearthquakes. Variography is a method for analyzing spatial interdependence of a variable. The method was applied to a set of induced microearthquakes observed in the vicinity of the GPK1 injection well at Soultz-sous-Forets Hot Dry Rock geothermal site (Alsace, France). Variograms obtained for 20 vertical slices (zones) along the wellbore were compared with the orientation of the fractures derived from a Formation Micro Imager, which is a resistivity log for measuring position and orientation of fractures in a borehole. We found that variograms exhibiting spatial dependency correlated well with zones where fractures belong to one dominant orientation set and variograms with no or low spatial dependency corresponded to zones with two or more fracture orientations/sets. We propose that this correlation results from the relation between the spatial variation of shear displacement, the physical properties of fractures, and their orientations.