ABSTRACT: We measured guided wave trapped in a subsurface artificial fracture in Higashi-Hachimantai Hot Dry Rock model field, Japan. The subsurface fracture was produced at about 370 m depth in an intact welled tuff layer by hydraulic fracturing. We used a downhole air gun as a wave source. Because energy of the guided waves concentrates in a fluid layer, a hydrophone was used to detect the guided waves. We determine the arrival of guided wave by an analysis of tube wave, to which the guided wave converts at an intersection of a borehole and the subsurface fracture.
We found that travel time of the guided wave increased corresponding to pressurization of fracture. This phenomenon cannot be explained by the simple parallel wall fracture model. Observed velocity decrease is thought to be caused by effects of actual properties of the subsurface fracture; contact of asperities of the fracture surfaces and elastic variation of the rock due to pressurization.
