Analysis of the microseismicity induced by a fluid injection in a granitic rock mass

Abstract

The microseismicity induced by a fluid injection at a depth of 443 m in a granitic rock mass has been monitored with a 16 station network. The location of the five events observed on the near totality of the network suggests that flow away from the well occurred along a preexisting fracture nearly parallel to the maximum horizontal principal stress direction and not along the fractures observed at the wellbore. Focal mechanisms of these events are coherent with shear motions along fissures which intersect the main flow path. A difficulty has been encountered in the determination of source parameters from spectral analysis of signals because of attenuation effects. Indeed, for uncorrected spectra the corner frequency was found to decrease as the source to station distance increased. This demonstrates the effect of attenuation on the signals and therefore the necessity of correcting spectra for attenuation effects before estimating source parameters.