This paper examined the variation of fresh-salt water interface in a coastal aquifer and the applicability of electrical prospecting to detect the fresh-salt water interface with multi-layer geology. Both electrical prospecting by Wenner method and measurements of vertical electrical-conductivity profile in a test boring were carried out at the same point of coastal farming area. As a result, it is confirmed that fresh-salt water interface keeps changing for the influence of tides. Vertical electrical resistivity sounding curves demonstrated that the aquifer consisted of a basic three-layer model, and the boundaries of each layer were estimated. It was estimated that the upper layer corresponded to the surface clay-layer geology. The lower boundary of the middle layer corresponded to the top of the fresh-salt water transition zone. This result verifies that electrical prospecting can detect the fresh-salt water interface in multi-layer geology.

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