Experimental study on imaging hydraulic conductivity of rock masses based on elastic wave velocity dispersion

弾性波の分散を利用した岩盤の透水性評価手法の開発を目指し，室内試験および原位置試験による基礎的な検討を実施した。流体で満たされた岩盤中を弾性波が伝播する際には，周波数に応じた固体と流体との相互作用により速度と減衰率が変化する分散現象が生じその周波数特性と透水性との相関性を示す理論が提案されている。本稿では，弾性波の分散と透水性との関係について数値実験を実施し，次いで室内試験を実施した。その結果，透水性の異なる花崗岩試料に対して，異なる分散特性が認められ，理論式と整合することを確認した。さらに，周波数に依存した弾性波速度の分散が原位置で測定可能かどうかを検討した。その結果1.5mから70mの孔間で0.8kHzから80kHzの周波数帯において分散現象の可能性が高い試験結果が得られた。孔間距離1.5mで，発振周波数が20kHzおよび40kHzの弾性波トモグラフィ測定を実施し，地球統計学的手法を利用した試解析により透水係数断面を得た。得られた透水係数断面中央付近のボーリングを用いて透水試験を実施し，推定透水係数と比較したところ，両者の良好な一致が認められた。
The objective of this study is to establish a technique to obtain hydraulic conductivity distribution in granite rock masses. This technique is based on the characteristic that elastic wave velocity disperses in fully saturated porous media on frequency. Velocity dispersion is governed by fluid/solid interaction which corresponds to the hydraulic conductivity such as described by Biot and BISQ theory. In order to verify the validity of these theories in granite rock, laboratory tests were performed. The results of experiments show that the elastic wave velocity dispersion for granite rock can be well described by BISQ theory. To test the applicability of this approach in real rock masses, we performed in-situ experiments at the Grimsel test site in Switzerland. Dispersion phenomena were detected in the frequency range from 0.8 kHz to 80 kHz, in the borehole distance range from 1.5m to 70m. Cross-hole tomography measurements, with frequency at 20 kHz and 40 kHz in a distance between boreholes of 1.5 meter, were carried out. To obtain the hydraulic conductivity image, geostatistical technique was tried. A limited number of hydraulic test data were used to find the correlation between the velocity difference and hydraulic conductivity. With this correlation, a hydraulic conductivity image was constructed. The location of the high hydraulic conductivity zone from the result agrees with that estimated using geological information. Additional hydraulic tests were conducted in a borehole located in the middle of the tomography plane. These hydraulic tests show a good agreement between the measured hydraulic conductivity and the hydraulic conductivities predicted from the cross-hole tomography. These result of experimental studies show that the possibility as a practical method to determine the hydraulic conductivity image in granite rock.