地球統計学的手法による亀裂性花崗岩体の3次元透水係数モデリングと広域地下水流動解析への応用

3D hydraulic conductivity modeling of fractured granitic body using geostatistical techniques and its application to regional groundwater flow analysis

久保 大樹*; 小池 克明*; Liu, C.*; 栗原 新*; 松岡 稔幸

Kubo, Taiki*; Koike, Katsuaki*; Liu, C.*; Kurihara, Arata*; Matsuoka, Toshiyuki

広範囲の水理地質構造を明らかにするためには、調査によって得られた限られた情報を3次元的に拡張し、複数の情報を有機的に統合するための空間モデリング法が必要である。本研究では、地下水流動の支配要因である亀裂と透水係数の空間分布について、地球統計学を応用した推定手法を考案し、花崗岩体深部の水理構造の把握を試みた。作成した透水性分布モデルを用いた地下水流動シミュレーションの結果が、既往の調査結果と整合したことから、モデリング手法の妥当性が確認された。また同シミュレーションにより、対象地域において主要な地下水の流出域や、断層深部を通過する地下水の流れの存在が示唆されるなど、新たな知見を得ることができた。

Numerical simulations have been the most effective method for estimating flow pattern, flux, and flow velocity of the groundwater to precisely characterize large-scale groundwater systems. Spatial modeling of the 3D distribution of hydraulic conductivity over a study area is indispensable to obtain accurate simulation results. However, such spatial modeling is difficult in most cases due to the limitations of hydraulic conductivity data in terms of their volume and location. To overcome these problems and establish an advanced technique, we adopt geostatistics and combine a fracture distribution model with measured conductivity data, selecting the Tono area situated in Gifu Prefecture, central Japan for the field study. The size of the main target area covers 12 km (E-W) by 8 km (N-S), with a depth range of 1.5 km, and it is chiefly underlain by Cretaceous granite. Because the distribution of 395 hydraulic test data acquired along the 25 deep boreholes was biased, the data values were compared to the dimensions of simulated fractures using GEOFRAC. As a result, a positive correlation was identified. Using a regression equation for the correlation, hydraulic conductivity values were assigned to every simulated fracture. Then, a sequential Gaussian simulation (SGS) was applied to construct a 3D spatial model of hydraulic conductivity using those assumed values and actual test data. The plausibility of the resulting model was confirmed through the continuity of high and low permeable zones. The next step is a groundwater flow simulation using MODFLOW and the model. The simulation results were regarded to be appropriate because distribution of hydraulic head, locations of major discharge points, and anisotropy of hydraulic behavior of the Tsukiyoshi fault correspond to the results of observations.