Estimation of underground fracture density of granitic rocks body using P-wave velocity tomography and crack tensor theory

A crystalline rock such as granitic rocks is one of a candidate for deep underground excavations for disposal of high-level radioactive waste. Such a rock is fractured medium, and such fractures cause alternation of the mechanical properties such as strength and deformability, and the hydrological properties such as permeability and diffusion coefficient since the fractures function as a water-pathway. Therefore, the fractures are a very important parameter, when we determine hydro-mechanical properties of the crystalline rock at great depth. When the underground water flow is estimated by numerical simulation, distribution of the underground permeability is an important initial value to make hydro-geological model. However, it is difficult to obtain the underground permeability. Underground permeability is able to represent permeability tensor Pij (Oda, 1987), and Pij is function of the geometry of cracks such as density, aperture and connectivity, therefore, we are able to estimate underground permeability using the geometry of cracks. A non-dimensional second rank tensor Fij, called the crack tensor, has successfully been introduced to deal with crack geometry. We seek the possibility of using the longitudinal wave velocities to overcome the difficulty associated with the determination of crack tensors. A new second-rank tensor Vij is introduced by takemura and oda (2005), such that the longitudinal wave velocity is represented in terms of the tensor, and the crack tensor Fij is then given as a function of Vij. In this study, we try to estimate two-dimensional underground permeability using the distribution of underground longitudinal wave velocity, which is tomography image of the longitudinal wave, Vij and permeability tensor based on crack tensor theory. In addition, the aperture of fractures under confining pressure needs to estimate permeability tensor, and we propose experimental formula to estimate the aperture of fracture in underground.

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