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

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  • 電気探査法による有明・八代海の海底下浅部の地質構造のイメージングと地下水湧出経路推定への応用
  • デンキ タンサホウ ニ ヨル アリアケ ・ ヤツシロカイ ノ カイテイ カセンブ ノ チシツ コウゾウ ノ イメージング ト チカスイ ユウシュツ ケイロ スイテイ エ ノ オウヨウ

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Abstract

 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.

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