Dispersion analysis of crack-waves in an artificial subsurface fracture using two crack models

The authors investigated crack-wave dispersions in an artificial subsurface fracture both experimentally and numerically using a wavelet analysis and two crack models. Crack-waves are seismic modes that propagate along a fracture. The dispersion characteristics of crack-waves depend on the geometry and physical properties of a fracture. The authors measured crack-waves at an artificial subsurface fracture in Higashi-Hachimantai Hot Dry Rock model field, Japan. This subsurface fracture is at a depth of about 370 m. During a measurement, they injected water into the fracture and changed the interface conditions of the fracture. A wavelet analysis provided the dispersion of the arrival times of crack-waves. The crack-waves showed positive velocity dispersion; i.e., low frequency components arrived later. As wellhead pressure increased due to water injection, the dispersion characteristics changed. A low-velocity-layer (LVL) model and a crack-stiffness model were examined to explain crack-wave dispersion. In the LVL model, rock layers with a low velocity surround a fluid layer. There is no contact between the LVLs. On the other hand, the crack-stiffness model considers crack stiffness due to contact between asperities on fracture surfaces. The arrival-time curves calculated by the crack-stiffness model showed a good fit to the measured values. As wellhead pressure increased, crack stiffness decreased and thickness of a fluid layer increased. In contrast, the LVL model did not adequately duplicate the measured data.