| 講演要旨(和文) | | 沖縄県多良間島では琉球石灰岩を帯水層として,塩水上に賦存する淡水レンズが水源となっている.このような島における今後の水利用計画や地球温暖化による海面上昇の影響を検討するためには,淡水レンズの分布を把握する必要がある.そこで,琉球石灰岩中の地下水面および地下水の塩分濃度境界が比抵抗変化としてとらえられることを期待し,TDEMによる比抵抗構造調査を行った.その結果,比抵抗構造の第1層基底はほぼ地下水面に一致し,第2層層厚は島の中心部で大きく,縁辺部で小さくなることが明らかになった.第2層の形状は淡水レンズのそれに調和するが,観測井で把握されている淡水レンズ層厚より有意に大きい.淡水レンズの形状を探査するためには,垂直電気探査との比較,探査システムの改良を行う必要がある. |
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| | 講演要旨(英文) | | The fresh water lens that is on seawater is a water source on Tarama island in Okinawa Prefecture. To examine the plan for the water supply and the influence of global warming on such an island, we should understand the distribution of the fresh water lens. Then, we investigated the resistivity structure of Tarama island by TDEM expecting the boundary of the density of salinity of the groundwater and groundwater surface in the Ryukyu limestone to be understood as a resistivity change. As a result, the base of the first layer of the resistivity structure was almost corresponding to the groundwater surface. The second layer is thick in the center part and is thin in the edge part on the island. The thickness of the second layer was more intentionally larger than that of the fresh water lens confirmed in the observation wells, though the shape of the second layer harmonized with that of the fresh water lens. It is necessary to improve the survey system of TDEM and to compare with VES to clarify the shape and distribution of the fresh water lens accurately. |
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