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地質調査結果と経験的解釈に基づく花崗岩体中の亀裂密度分布の推定; 瑞浪超深地層研究所のボーリングデータを用いた検討事例

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

栗原 新; 天野 健治  ; 小池 克明*; Liu, C.*

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

花崗岩体中に発達する断層・節理等の不連続面(以下、亀裂)は、岩盤中の物性や透水性の不均質分布を生じさせる主な要因として、地下水流動や物質移動のほか、応力状態等さまざまな地質現象に影響を与えると考えられている。したがって、限られたデータから亀裂の三次元分布を高い精度でモデル化することは、資源探査・地下構造物建設・放射性廃棄物の地層処分等の工学的評価において、特に重要な技術的課題の一つである。本研究では、瑞浪超深地層研究所周辺のボーリング孔において確認した土岐花崗岩中の亀裂密度に関して、規模の大きな地質構造の分布との空間的相関構造に着目し、既存の地質調査結果と文献情報に基づく地質構造分布の経験的解釈から、データの得られていない領域の亀裂密度の分布傾向を推定できる可能性のあることが明らかとなった。

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.

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