Shear-wave splitting detected by using downhole triaxial seismic detector during dilation of artificial subsurface fracture

A small change in traveltime and shear-wave splitting in elastic waves transmitted through a pressurized single fracture has been detected through downhole triaxial cross-hole seismic measurements. A field experiment was carried out at Higashi-Hachimantai Field of Tohoku University, Japan, where an artificial single fracture had been created at 369 m by hydraulic fracturing in intact welded tuff. P and S waves transmitted through the pressurized single fracture were measured by using a downhole three-component seismic detector installed in a borehole near the fracture. In our previous studies, a small change in traveltime of about 0.1 ms was detected by using the cross-spectral moving window technique. In this recent study, the arrival times of split shear waves and the polarization directions were detected by using the wavelet transform of a three-component signal. The traveltime difference between the first and second shearwaveswas 0.05-0.12 ms,when the wellhead pressure was increased up to 3.4 MPa and the incident angle of waves into the main fracture was changed from 12° to 20°. A fracture model of a single fracture and a microcrack zone in the vicinity of the main fracture was used to explain the shear-wave splitting phenomenon. The width of the reopening microcrack zone and the crack density were quantitatively evaluated by using the fracture model and shear-wave splitting analysis. The best-fit theoretical curve to the observed data suggested that the width of the reopening zone was 0.8 m and the crack density was 0.39.