| 講演要旨(和文) | | 宮古島において最も活用されている上水道水源である白川田湧水において，琉球石灰岩中の大きな地下空洞の有無を調べるための精密重力探査を行った．三つの探査測線（それぞれ，340，160，144 m）で，ラコストD型重力計（D68）を用いて重力を測定した．全観測点176点のうち54点については，QC（品質管理）を目的に日時を変えて重複測定を行い，平均的な測定値のばらつきとして0.004 mgal(標準偏差)を達成した．正確なローカルブーゲー異常値を得るために，地形補正については，1 m DEMを使用して周囲 350 m 以内の範囲で計算した．残差重力値のプロファイルからは，大きな地下空洞に対応する負の重力異常シグナルは得られなかった．一方，幅約100 mにわたる正の重力異常（約0.03 mgal）が見出されたが，これは，周辺のボーリングコアに見られる再結晶化部分に対応すると思われる． |
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| | 講演要旨(英文) | | In limestone area, finding out about underground caves to avoid collapse is important to maintain water resources being used. We applied microgravity survey to find underground caves in Shirakawata water resources area, miyakojima city. On three survey lines (340, 160 and 144 m length) including 176 gravity points, gravity was measured by using Lacoste and Romberg D type gravimeter (D68). We conducted duplicate measurements at 54 points for quality control and consequently got the averaged measurement variation of 0.004 mgal (SD). Three profiles of local Bouguer anomaly were calculated with accuracy using the terrain correction calculated from 1m DEM within 350 m range. Usually, a negative gravity anomaly is expected for underground cave, however positive signals with approximately 100 m width are found around the water resource. This positive signals infer an absence of underground cave. On the other side, this signals are consistent with the presence of recrystallized limestone zone which was find in borehole cores around the water resource. |
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