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海底下浅部の地質構造を明らかにすることは, 伏在断層の抽出, 水質の空間分布の解釈, 地下水の湧出経路の特定において重要である. 海底下の活断層の位置や分布は音波探査により調査されている. しかし, 音波探査では地下水についての情報は得られない. そこで, 地下水を含む海底下約50 mの範囲の地質構造をイメージングするために, 九州中部の有明・八代海を対象として, 曳航型海底電気探査を適用した. 両海は日本を代表する閉鎖性海域であり, 地下水資源が豊富な熊本・八代平野に面している. 測定に使用したケーブルの長さは250 mで, これに20本の電極を設置した. 有明・八代海にそれぞれ4本, 8本の計26 kmの測線を設定し, 信頼性の高い比抵抗データを得るために3極法を採用した. 海水の厚さを考慮した電位補正法と1次元インバージョン解析によって, 宇土半島周辺で正断層を示唆する特徴, および地下水湧出経路と考えられる幅200 mの高比抵抗帯の存在が明らかになった. 白川・緑川の河口では, 後氷期の厚い海成粘土層( 有明粘土層)の中に周囲に比べて高い比抵抗帯が現れた. これは海底地下水が陸源性野の淡水に起因し, この大規模な分布が高比抵抗帯に関連すると解釈した. 八代海測線のインバージョン解析結果からは, 日奈久断層の延長線上で比抵抗が大きく変化することがわかった. これは日奈久断層の繰り返し, かつ新しい動きによって, 地下水湧出経路が形成されているためと考えられる.Clarifying geological structure in shallow depth under the seam bottom is important to detect latent fault, interpret spatial distribution of water quality, and identify paths of groundwater discharge at the sea bottom. Although sonic prospecting has been used widely for investigating the position and distribution of submarine active faults, it cannot detect the features related to groundwater. The Ariake and Yatsushiro Seas in central Kyushu, southwest Japan are representative closed seas in Japan, facing the Kumamoto and Yatsushiro Plains being rich in groundwater resources. For imaging the geological structure containing groundwater down to 50 m depth, a towing electric sounding method was applied to these seas with a cable of 250 m length and 20 electrodes made in the cable. Four and eight measuring lines in total 26 km length were set in the Ariake and Yatsushiro areas, respectively, and a three electrode method was used to obtain accurate data. By a potential correction that considers the thickness of sea water and a 1D inversion analysis of the corrected data, a weak feature of a normal fault and a clear high resistivity zone with 200 m wide that may be related to the groundwater discharge were clarified near the Uto peninsula. For the zones near the mouths of the Midori and Sira rivers, high resistivity zones were detected in the thick post-glacial marine clay (the Ariake clay) and its cause was interpreted as the existence of the large volume of submarine groundwater. From the inversion results of the Yatsushiro Sea lines, remarkable change of the resistivity was highlighted at the extension line of the Hinagu fault. Consequently, recent and repeated movements of the Hinagu fault that formed permeable paths of the groundwater discharge were clarified

ōyō chishitsu/ouyou chishitsu

Imaging of Geologic Structures and Groundwater-discharge Paths in the Shallow Depths under the Sea Bottom of Ariake and Yatsushiro Seas by Electric Sounding Survey