POLARIZATION OF CRACK WAVES ALONG AN ARTIFICIAL SUBSURFACE FRACTURE

We analyze three-dimensional (3D) particle motions of crack waves whic h propagate along a fracture to determine fracture orientation. Crack waves were measured at an artificial subsurface fracture at a depth of approximately 370 min the Higashihachimantai Hot Dry Rock model field in Japan. Dispersion was observed in crack waves. Velocities of crack waves below 100 Hz are 100 m/s - 150 m/s along this artificial fractu re. Therefore, crack waves arrived after compressional wave and shear wave. A coherence matrix is used to analyze the 3D particle motions in the frequency domain. Noise, which is outside of the band of the dete ctor, is excluded in the coherence matrix analysis. We examine the pol arization of 3D particle motions of crack waves using Principal Compon ent Analysis. The longest axis of an ellipsoid which approximates the 3D particle motions shows a direction consistent with estimates obtain ed by core-sample measurements and tectonic stress measurements. The d ifferences in fracture orientation obtained from the crack-wave analys is and the core sample analysis are less than 16 degrees in azimuth an d less than 20 degrees in dip.