Hot spring water flow path estimated by electromagnetic survey -An example in Yuda Hot Spring, Yamaguchi Prefecture-

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  • 電磁探査法による温泉水の流動経路の推定―山口県湯田温泉における適用―
  • デンジ タンサホウ ニ ヨル オンセンスイ ノ リュウドウ ケイロ ノ スイテイ : ヤマグチケン ユダ オンセン ニ オケル テキヨウ

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Abstract

<p>　Yuda Hot Spring in Yamaguchi City, Yamaguchi Prefecture, is an alkaline simple hot spring that flows up into the plains. This hot spring yields an abundant amount of water (2,000 tons/day), but its flow path has not yet been clarified. In this study, we analyzed the water chemistry of Yuda Hot Spring and performed resistivity measurements of geological samples around the spring. In particular, the resistivity structure was examined by electromagnetic survey, core observations were made by spring source boring, and a numerical simulation was carried out to elucidate the flow path of the hot spring water. The electromagnetic survey revealed a difference in resistivity between the bedrock and the overlying sedimentary layer, and the upper surface of the bedrock in the region was found to be depressed, sandwiched between multiple faults crossing the Yuda Hot Spring area. However, a resistivity structure could not be obtained that indicated the flow path of the hot spring. The measured electrical conductivity (EC) of the hot spring water was about 100 mS/m, an order of magnitude higher than that of groundwater in surrounding rock. The resistivity of a rock sample with groundwater present as pore water was half that of a rock sample with hot spring water present as the pore water. The obtained core sample was primarily composed of hard sandy schist and pelitic schist, and the frequency of fractures was 1.25 fractures/m. On the other hand, cracks were present in a part where a quartz porphyry with a width of about 0.6 m intruded at high angles, so it is possible that the cracks in intrusive rock such as quartz porphyry serve as the flow path of the hot spring water. Based on these results, we constructed a geological model of Yuda Hot Spring and conducted a numerical simulation. The results showed that resistivity would be difficult to detect when the intrusive rock where cracks are predominant, which we regarded as the flow path, has a width of 100 m or less. These results suggest that the flow path of Yuda Hot Spring is less than 100 m wide.</p>

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