We investigate a transfer function obtained from crack waves. Crack waves propagate along a fracture and their energy is concentrated in the liquid layer between rock layers. We measured crack-wave propagation in an artificial subsurface fracture. The subsurface fracture was created by hydraulic fracturing at a depth of 370 m in a layer of welded tuff with no significant natural cracks. Two boreholes intersected the fracture. We controlled contact conditions by hydraulic pressurization. We used a downhole air gun as a wave source in one borehole and detected the crack waves with a hydrophone in the other borehole. We examine the transfer function calculated from the crack waves. This transfer function has two peaks at frequencies of 65 Hz and 160 Hz. When the wellhead pressure is increased from 0 MPa to 3.0 MPa, although these frequencies do not change, the amplitudes of the peaks increase. The size of the fluid-filled fracture is 1 m by a simple approximation based on acoustic resonance.
Crack-wave measurement is a promising seismic technique for characterization of fractures because crack waves propagate along the fracture and their energy is concentrated at the fracture. Therefore, the propagation characteristics of crack waves depend upon the geometry and mechanical properties of the fracture. Many researchers have studied the dynamic motion and propagation characteristics of crack waves in numerical fracture models (Chouet 1986; Ferrazzini and Aki 1987; Hayashi and Sato 1992). We recently measured crack waves in a field (Nagano et al. 1995). When we applied hydraulic pressure to a subsurface fracture, the velocity and amplitude of the crack waves varied as a function of the reservoir pressure. Furthermore, we have used crack-wave velocities to estimate crack stiffness at a subsurface fracture (Nagano and Niitsuma, in press). In this paper, we describe a transfer function of a fluid- filled fracture obtained from crack waves. We examine the peaks of the transfer function when the subsurface fracture was pressurized hydraulically.
Crack waves were detected in a field measurement at the Higashi-Hachimantai Hot Dry Rock model field in Iwate prefecture, Japan (Niitsuma 1989). Figure 1 illustrates the crack-wave measurement. An artificial sub- surface fracture was created in intact welded tuff at a depth of 369.0 m in well F-I by hydraulic fracturing. During the hydraulic fracturing, 40-mesh sand was injected as a propping agent. Core samples showed no significant joint or crack before fracturing. Well EE-4 was drilled into the artificial fracture after the hydraulic fracturing, and intersected the artificial fracture at a depth of 358.2 m. The distance between the points at which wells F-I and EE-4 intersected the artificial fracture was 6.7 m. The radius of the fracture was about 60 m (Niitsuma 1989). A transmissibility test showed that the fracture aperture was about 0.08 mm without pressurization and 0.2 mm at a wellhead pressure of 3.0 MPa (Hayashi and Abe 1989). The velocities of compressional and shear waves in intact rock in this field were 3000 m/s and 1750 m/s, respectively.
