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Three-dimensional inverse modeling of two large-scale cross-hole hydraulic tests in fractured granite at the Mizunami Underground Research Laboratory site, Japan

瑞浪超深地層研究所における亀裂性岩盤を対象とした大規模孔間水理試験の3次元逆解析モデリング

Illman, W. A.*; Liu, X.*; Yeh, J.*; 安藤 賢一*; 竹内 真司; 三枝 博光

Illman, W. A.*; Liu, X.*; Yeh, J.*; Ando, Kenichi*; Takeuchi, Shinji; Saegusa, Hiromitsu

瑞浪超深地層研究所において2回の孔間水理試験を実施した。その結果を非定常水理トモグラフィ(THT: Transient Hydraulic Tomography)コードを用いて解析を行い、透水係数と比貯留係数の3次元分布を比較した。その結果、高い透水係数と低い比貯留係数が550mから1200mの範囲にわたって分布し、これらが2つの断層帯を形成していることが示された。また透水係数と比貯留係数は負の相関を有し、このため水圧伝播が早いことが明らかとなった。さらにTHTコードによる解析により、低透水性ゾーンの存在が表現された。これは、既に予測している北北西走向の遮水性断層と整合している。解析結果の妥当性の確認については、地質学的な情報,揚水に伴う応答時間,伝播速度などにより実施した。

Two cross-hole tests were conducted at separate locations in deep boreholes at the MIU construction site in central Japan. We analyze the two cross-hole hydraulic tests using the Transient Hydraulic Tomography (THT) code of Zhu and Yeh [2005] to compute the hydraulic conductivity (K) and specific storage (Ss) distributions as well as their uncertainties in three-dimensions. The equivalent K and Ss were obtained using the asymptotic analysis served as the initial parameter estimates for the 3D stochastic inverse modeling effort. Results show 2 distinct high K and low Ss zones that are continuous over distances ranging from 550 - 1,200 m, which delineate 2 separate fault zones. The fault zones that are imaged through THT correlate well with available geological data and drawdown records. The analysis of the tests with the THT code also identified a low K zone which corresponds with a known fault zone trending NNW and has been found to compartmentalize groundwater flow at the site.

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