利用海底仿擬反射訊號深度估算台灣西南海域地溫梯度分布研究;Geothermal Gradient Estimation from the Depth of Bottom Simulating Reflectors Offshore Southwestern Taiwan

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題名:	利用海底仿擬反射訊號深度估算台灣西南海域地溫梯度分布研究;Geothermal Gradient Estimation from the Depth of Bottom Simulating Reflectors Offshore Southwestern Taiwan
作者:	謝宜廷;Hsieh, I-Ting
貢獻者:	地球科學學系
關鍵詞:	台灣西南海域;海底仿擬反射訊號;地溫梯度分布
日期:	2020-03-16
上傳時間:	2020-06-05 17:12:31 (UTC+8)
出版者:	國立中央大學
摘要:	天然氣水合物是一種極具潛力的未來能源，並且已經被廣泛的研究。由科技部及經濟部中央地質調查所資助的數個海洋科學調查航次中，多頻道反射震測航次所提供的資料顯示海底仿擬反射訊號（Bottom Simulating Reflector, BSR）廣泛分布於台灣西南海域。BSR普遍被視為一個重要的分界指標，其上部為富含天然氣水合物的沉積層，下部則不含水合物，而是夾雜著甲烷氣。天然氣水合物的行為深受地溫梯度變化影響，因此詳細調查天然氣水合物賦存地區的熱流變化極具意義。在過去數十年中，前人已經利用利氏高解析度熱探針對台灣西南海域進行高密度的熱流值量測，而本研究利用BSR深度與溫鹽深儀（CTD）提供之底水溫度等資料，估算地溫梯度，並與熱探針現地量測之地溫梯度進行比較。此舉的目的是希望藉由了解兩者間的轉換關係，再透過更為連續與密集之BSR資料獲得更細緻的地溫梯度分布。估算的過程中，對於沉積層的成分組成，以及其平均密度對壓力估算的影響，前人有諸多爭論，主要分為應使用靜岩壓力或靜水壓力兩個派別。根據本研究結果，在足夠深的水域中，不論在靜岩壓力或靜水壓力條件下，利用BSR深度所估算之地溫梯度並無太大差異。而結果也顯示，利用BSR深度估算與熱探針量測所得之地溫梯度大致都相近，但在地質構造有快速變化區域則差異較大，例如變形前緣東側一帶相差約10℃／km，而西側幾乎沒有差值。本研究認為這樣的分布可能是由於BSR所代表的地溫梯度應為短期地質事件發生前沉積層穩定狀態下的結果，而熱探針量測值則代表量測當下近地表環境之地溫梯度所導致。因此，利用BSR深度估算地溫梯度之方法，在較穩定區域的確可以簡便而有效地提供在穩定狀態下更為連續之地溫梯度資料。但一區域之地質環境之改變，則會增加兩者的差值，可以作為初步判斷該環境地質條件改變的根據。;Gas hydrate is a major potential resource of energy and has been widely studied. An abundant distribution of Bottom Simulating Reflectors (BSRs), recognized as the boundary between the upper gas-hydrate bearing sediments and the lower non-bearing sediments, was determined from the seismic profiles collected during numerous oceanographic cruises supported by the Central Geological Survey (CGS) and Ministry of Science and Technology (MOST) in Taiwan. The gas hydrate behavior depends deeply on the variation of geothermal gradient. Therefore, a good understanding about the heat flow variation in the gas-hydrate enriched area is of a great importance. Over the past decades, heat flow measurements have been conducted extensively offshore Taiwan area by using Lister-type high-resolution heat probe. Taking this opportunity, we aim to estimate the geothermal gradient based on the depth of BSRs and the sea bottom temperature collected from Conductivity-Temperature-Depth (CTD) data and compare them with the in-situ measurements. The objective is to evaluate the possibility of obtaining a more detailed and continuous image of the heat flow values based on the relatively wider spread BSR depth data. To do that, we collect seafloor temperature from CTD data, and calculate the temperature at the depth of the BSRs using the pressure estimated from the empirical law. Then the geothermal gradient can be obtained from all these parameters. While calculating the pressure at the depth of BSR, the density of the sedimentary layer containing gas hydrate need to be considered. Intuitively, lithostatic condition is reasonable; however, some other researchers prefer to assume it under the hydrostatic condition. However, our estimations demonstrate that the geothermal gradients under two different conditions show no differences. Hence, this observation may suggest that the BSRs-based geothermal gradients represent the steady state geothermal gradients before the short term geological event, and the geothermal gradients measured by heat probe represent the in-situ condition at the measuring time. By comparing our result with the in-situ measurements, the values estimated in our study are slightly higher than that obtained from in-situ measurements with a magnitude of 10℃/km in the eastern side of Deformation Front, and there is almost no difference between them in the western side. The differences also show the abnormal high differences in the rear segment nearby Yung-An Ridge. Hence, the BSRs-based estimation gives us the more continuous geothermal gradients under the steady state. Having the contraction with the geothermal gradients by heat probe can make effort to evaluate the regional geological environments.
顯示於類別:	[地球物理研究所] 博碩士論文

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