Estimation of fracture density distribution in a granite body based on geological investigation and interpretation; A Case study of data from boreholes around the Mizunami Underground Research Laboratory, central Japan

Kurihara, Arata; Amano, Kenji  ; Koike, Katsuaki*; Liu, C.*

The spatial distribution of hydrogeologically-relevant geological structures such as fracture and fault is usually very heterogeneous, varying, in general, according to geological history, scale of observation and rock type. With the objective of improving analytical methods to better understand structural heterogeneities, this paper describes the results of investigations of the spatial relationship between regional geological structures (Tsukiyoshi fault and Tsukiyoshi paleo-channel) and fracture density with special emphasis on low-angle fractures in the Toki granite, central Japan. As a result of the analysis, positive correlations between average of fracture spacing data and the horizontal distances from two regional geological structures have been observed. Because the positive correlations can be approximated with a polynomial function of the response surface, we have obtained an improved understanding of the spatial distribution of low-angle fractures with respect to the location of regional geological structures. We have interpreted that the observed spatial distribution of low-angle fracture density conforms to the tectonic setting, on the assumption that the Tsukiyoshi fault and Tsukiyoshi paleo-channel are probably associated with the pull-apart basin by strike-slip faulting from the past investigations and geophysical surveys for this research area. In addition, the proposed spatial modeling techniques and geological interpretations are able to evaluate certainty of interpreted faults and fractures distributions, quantify general trend in structural heterogeneities and detect unknown faults.