臺灣西南海域之天然氣水合物穩定帶底部與沉積物熱導係數異常之探討

The Base of Gas Hydrate Stability Zone and the Anomalous Sediment Thermal Conductivity Offshore of SW Taiwan

李信宏(Shin-Hung Lee)

指導教授 ： 徐春田

國立臺灣大學/理學院/海洋研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.00312

摘要

在臺灣西南海域大陸斜坡至大陸隆堆的區域，於反射震測剖面發現有廣泛分佈的海底仿擬反射面(Bottom Simulating Reflector；BSR)。經由多年的探測，我們一共收集了80個測站的地熱資料，根據其地溫梯度推估天然氣水合物穩定帶底部（Base of Gas Hydrate Stability Zone；BGHS）的深度，並與反射震測資料所推估的BSR深度做比較，結果除了部份插入深度過淺處的地溫資料所推估的BGHS與BSR深度有較大的差異之外，BSR深度值與BGHS值相當吻合。
我們發現在某些測站的熱導係數會出現異常現象，可能是沉積物中含有天然氣水合物的結果。現象之一是當我們用利氏探針（Lister-type heat probe）下插海床時，發現僅有些微或幾乎沒有因摩擦熱而使探針溫度上升；現象之二是當我們用細熱導探針（needle probe）量測剛從海底收回之沉積物岩心時，會發現一開始的熱導係數極高，但經過一段時間後會遽減至穩定的正常值，我們認為這很有可能是因為含有天然氣水合物的沉積物遇到摩擦熱或溫度上升及壓力減少時，天然氣水合物解離吸熱的效應。上述兩現象可協助吾人研判沉積物中是否含有天然氣水合物的依據，雖然我們也發現曾有一處沒有因摩擦而升溫的現象發生在極鬆的軟泥上。
為驗證上述說法，我們除在實驗室中以恆溫槽做實驗及數值模擬外，也先後兩次在同一地熱測站以利氏探針及細熱導探針量測熱導係數做比對，結果也都相當支持上述推論。

關鍵字

海底仿擬反射面 ； 天然氣水合物穩定帶底部 ； 含天然氣水合物之沉積物 ； 熱導係數異常 ； 臺灣西南海域

並列摘要

We have collected 80 sites of heat flow data for many years over Bottom Simulating Reflectors (BSRs) appeared on the seismic profiles collected from continental slopes and rises offshore of southwestern Taiwan. The Bases of Gas Hydrate Stability Zones（BGHSs）estimated by thermal gradients agree with BSRs except the data obtained from shallow penetration.
We found two abnormal phenomena that maybe due to the hydrate bearing sediment.One is we could not detect the temperature rise induced from the friction heat when the Lister-type heat probe was inserted into seafloor sediment over BSR and the other one is we found the thermal conductivity very high at the beginning, but gradually decreased to a stable vaule when measuring it with a needle probe in a laboratory on board the ship. These could mean the gas hydrate dissociation and endothermic process occurred due to penetration friction heat, temperature increase, and pressure decrease. These two phenomena, in general could be taken as an indication of gas hydrate bearing sediments. However, we also found one site on area covered by the extremely soft and loose mud spurted by the mud diaper without penetration friction heat raised temperature.
In order to prove our hypothesis, we did experiments in constant temperature baths, numerical modeling, and compared the thermal conductivities measured by the Lister-type heat probe and that by the needle probe at the same site. The results confirmed the hypothesis.

並列關鍵字

Bottom Simulating Reflector ； Base of Gas Hydrate Stability Zone ； Gas hydrate bearing sediment ； Thermal conductivity ； Offshore of southwestern Taiwan

參考文獻

林書晢 使用附著式小型溫度探針量測海底沉積物之溫度梯度並推估天然氣水合物穩定帶底部之研究，國立台灣大學海洋研究所碩士論文，2007。

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