Abstract

In 1993, during the development of the European hot-dry-rock geothermal field at Soultz-sous-Forets (France), 44,000 m3 of water were injected to stimulate the natural fracture system of this granitic rock mass. Some 16,000 microseismic events were recorded by a network consisting of a hydrophone and three four-component seismic sensors cemented in the granite at the bottom of observation wells. Multiplets (close earthquakes with similar waveforms) are retrieved in order to find local structures and correlate them with major fractures observed in the well by acoustic imaging and flow logs. Among the eleven identified and relocated multiplets, eight of them deline a planar structure, which is presumed to be a fracture plane. Two of the multiplets lie on a linear structure and one of them is the intersection of two previous planar multiplets. This leads to a description of the fracture network within a few tens of meters around the borehole. In addition, in order to compare the geometrical rupture parameters of the "multiplet" fractures with those of the "borehole" fractures, focal mechanisms of multiplets are modeled by using a statistical approach. This is done by combining the geometry of the fracture planes, the P wave polarity and the S wave polarization, as seen only at the four stations. The S waves polarization shows an hexagonal anisotropy with a North- South horizontal symmetry axis, which is consistent with the regional maximum horizontal stress direction.

P. 545

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