It is possible to obtain the information of the preferred orientation of microcracks in granite by the measurement of the P-wave velocity, because the value of P-wave velocity is directly related to the density of microcracks. Therefore, we can obtain the information of three-dimensional distribution of microcracks in granite by measuring P-wave velocity in various directions. Assuming that the three-dimensional distribution of microcracks in granite was caused by the stress release during coring, we can assess the stress state at the location of coring in the underground considering the relationship between three-dimensional distribution of P-wave velocity and microcracks. Differential Strain Curve Analysis (DSCA) is one of the methods to evaluate the stress state in the underground using rock samples based on the initiation of microcracks due to the stress release. Therefore, if the three-dimensional distribution of P-wave velocity is related to the orientation of microcrack distribution determined by DSCA, the stress state in the underground can be evaluated from the measurement of P-wave velocity.
Based on the above consideration, we try to investigate the relationship between the three-dimensional distribution of P-wave velocity and that of the crack parameter determined from DSCA by conducting both P-wave velocity measurements in various directions using a polyhedral specimen and DSCA using granite samples obtained from the same location. In addition, we consider the possibility to evaluate the stress state (directions and ratio of principal stress) in the underground from only P-wave velocity measurement.
From wave velocity measurement using a polyhedral specimen, we could obtain three-dimensional distribution of P-wave velocity. From DSCA, the values of crack parameters were obtained. A negative correlation between the P-wave velocity and the crack parameter obtained from DSCA was observed for granite. Additionally, by using the value of the P-wave velocity corresponding to the crack-free granite, we could evaluate the directions and ratio of principal stress in the underground. Consequently, it is possible to evaluate the directions and ratio of principal stress in the underground from P-wave velocity measurement in granite.
