利用大地電磁法探勘宜蘭平原南部之地熱構造研究;The geothermal structure inferred from magnetotelluric data beneath southern part of Ilan Plain, north-eastern Taiwan

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Title:	利用大地電磁法探勘宜蘭平原南部之地熱構造研究;The geothermal structure inferred from magnetotelluric data beneath southern part of Ilan Plain, north-eastern Taiwan
Authors:	楊志賢;Yang, Zhi-Xian
Contributors:	地球科學學系
Keywords:	大地電磁法;電阻率;地熱構造;熱液蝕變;magnetotellurics;electrical resistivity;geothermal structure;thermal alteration
Date:	2018-07-23
Issue Date:	2018-08-31 13:10:19 (UTC+8)
Publisher:	國立中央大學
Abstract:	宜蘭平原區為台灣最具地熱發電潛力的區域之一，從過去大地測量、速度構造、空中及大地磁測、震測等地球物理方法皆揭露了宜蘭平原地下可能的地熱潛藏分布。本研究鎖定宜蘭紅柴林地區，選定東西向12公里、南北向6公里的範圍作為研究區域，利用大地電磁法(Magnetotellurics, MT)進行地下電阻率探勘研究，藉由國外諸多地熱與岩石電阻率構造之相關研究經驗，對比宜蘭平原區地下電阻率構造的分布及特徵，研判可能的地熱儲熱區及導水通道。研究期間重新整理過去在宜蘭平原區內量測的MT資料，淘汰資料品質較差、受人為雜訊干擾較嚴重的測點，並於今年度4月、7月及9月在研究區域內增加新MT測點，同時使用遠端參考技術提升資料品質，最後結合新、舊測站中品質較佳的資料，將各測站資料平滑化以去除區域性雜訊後，進行全三維模型逆推。由觀測資料對比逆推模型預測資料的擬和程度分析，顯示逆推結果對於觀測資料的擬合程度良好，此逆推模型可信度高。　　在蘭陽溪南方可發現一深一淺垂直分布的東西向延伸低電阻帶，此低電阻區電阻率大約為10 Ω-m，並於此低電阻率下方可發現一同樣為東西向延伸、電阻率略高的低電阻率構造，電阻率大約為30至100 Ω-m，此二低電阻構造並且被極高電阻率構造包圍(>1000 Ω-m)，低電阻率構造之東西向延伸趨勢與火成岩侵入造成之高磁力異常延伸趨勢相似，暗示低電阻率構造與入侵火成岩體之關係密切，另由淺部低電阻率構造向北延伸之趨勢，對比於鄰近地熱觀測井井溫剖面，說明了低電阻率構造確實由高溫構造所導致，高溫構造向北延伸至耕莘井下方，並略為延伸至二號井，造成升溫速率較背景地溫梯度快速。將宜蘭地下電性構造對比其他國家於地熱區電阻率構造研究，此低電阻構造可能與黏土礦物因接觸高溫熱液產生熱液蝕變作用有關，生成低電阻率礦物進而導致小區域的異常低電阻帶，研判此低電阻帶為南宜蘭平原地熱潛藏可能性最高的區域。 ;Ilan plain is identified as a western extension of the Okinawa Trough in the northern Taiwan subduction system and has been considered as one of the most productive geothermal area in Taiwan. An east-west extension magma-like body caused by the westward of the Okinawa Trough has been revealed from seismic and geomagnetic studies. Some geothermal wells have been drilled around this area, but the temperature and water volume are lower than that expected, indicating the limitation of the distribution of the geothermal providing area. To find out the location of the geothermal providing area, we investigated the electrical resistivity structure beneath Ilan Plain using the magnetotelluric method. At data collecting steps, we carried out the survey of more than 100 sites, and employeed more than one sites for remote reference technique to promote data quality. After carefully visual inspection of the data of each sites, 84 sites were remnant with perfect data quality within study area. At data processing steps, we used moving average method to keep the smoothness of the data based on basic conceptual of MT method. MT data were analyzed and modeled using 3D inversion scheme. The fitness revealed by the pseudosection of the observed and model predicted data is good at relative shallow part, indicating this resistivity model is reasonable and can be accepted. The result shows that the geothermal providing area is a few hundreds of meters southward from those observation wells. The northward extension of the shallow conductor can be explained by the trace of hot water, and is matched to the temperature logging data. Comparing with the geothermal resistivity conceptual model, the shallow conductor(~10 Ω-m) and relative deeper conductor(~30-100 Ω-m) can be related to secondary mineralization altered by geothermal fluids which is typically investigated from geothermal area. The shallow conductor is related to the smectite-zeolite zone(70-220°C), and the relative deeper conductor is related to chlorite-illite zone(180-240°C). We suggest that this geothermal providing area may connected to deeper intrusive magma body.
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