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

Abstract

在台灣西南海域的大陸斜坡及大陸隆起處，於震測剖面上發現了許多海底仿擬反射層。為了能更準確的求得天然氣水合物穩定生成的底部，即海床的地溫梯度與天然氣水合物穩定曲線的交點。我們將5根小型溫度探針繫綁於岩心採樣器之外管壁上，在岩心採樣時同時量取海床底下之溫度。 由於附著式小型溫度探針需以支架繫掛於大而重之長岩心器上，因此磨擦熱產生的起始溫度也不再是單一的脈衝形狀而是一前一後的複雜波形，在資料處理方法中我們發現使用緊貼於岩心管的矮支架所得的地溫資料，雖然所受到各種的磨擦熱源影響較大，但優點是這些磨擦熱會在短時間內抵達探針，因此我們可選擇遠離溫度波峰較遠(後)的資料來處理，即可將此複合式的升高溫度形狀視為是只有單一的複合波，如此便可利用溫度衰減函數匹配推求背景溫度及梯度；反之高支架的地溫資料，顯示溫度在衰減過程中會有二次增溫的現象，導致我們無法利用圓柱體溫度衰減函數去匹配推求背景溫度，僅能利用較前段之尚未升溫之資料推估背景溫度是其缺點。另外我們也發現直接從溫度梯度的資料用該函數來推求最終之背景溫度梯度的方法又優於先用該函數推求背景溫度再求背景梯度的方法。 由於長岩心採樣系統尚未能正常運作，只能將小型溫度探針繫掛於直徑為11公分長6公尺之岩心器上，並沿着有明顯海底仿擬反射層處收集了12站的資料(但僅有10站的資料較好)，如以靜水壓模式推求天然氣水合物穩定帶的底部，在台灣西南海域被動式大陸邊緣的6站深度測站為134至438 mbsf，而在主動式大陸邊緣的4站卻深達359至473 mbsf，初步研判後者之所以偏高是受高沉積速率的影響而降低了溫度梯度值。但整體而言，與以往所收集的資料相較，趨勢尚稱一致，即天然氣水合物穩定帶的底部之所以未隨海水深度而增加，是因溫度梯度有隨水深而增加的關係，說明了天然氣穩定生成的深度受地溫梯度的影響較大。

Bottom simulating reflectors (BSRs) have been found on the seismic profiles collected from continental slopes and rises, and most of which are associated with the accretionary prism off southwest Taiwan. In order to obtain a more accurate prediction of intersection between the temperature gradient measured at sub-seafloor and the gas hydrate phase boundary curve (i.e. BGHS；Base of Gas Hydrate Stability), we attached 5 miniature temperature probes to the outside walls of a sediment corer to measure the temperature and thermal gradient of sub-seafloor when taking a core sample. Since the probes are attached to a big and heavy sediment corer, the frictional temperature pulse is no longer a spike-like wavelet, but an elongated and complex shape. In data processing, we found that the temperature recordings obtained from probes attached to the corer with short fins will lead higher frictional temperature than that produced by using tall fins. The advantage is the frictional temperature may reach to the probe in a short period of time. We can choose the temperature data away from the initial temperature surge for processing. That is, we can consider the raised temperature a single impulse and predict the ambient temperature and temperature gradient of the sediments by fitting with the cylindrical temperature decay function. If the tall fins were used, the secondary raised temperature may appear on the temperature record after a period of time following the primary raised temperature peak. So, we couldn’t derive the ambient temperature by the previous method. Only the anterior data following immediately the earlier temperature decay (before the secondary temperature rising) are usable for data reduction. In addition, we found that to obtain ambient temperature gradients by fitting the cylindrical temperature decay function to the temperature gradient data directly is better than that obtained by applying the function to fit the temperature data first and then to compute the temperature gradients. Fail to use the new long-coring equipment in time, we can only attach the miniature temperature probes to a core of 11 cm in diameter and 6 m in length. Though we collected 12 stations of temperature data along obvious BSR, only 10 stations of the data are usable. Assuming the hydrostatic model is applicable to the area; from the gas hydrate stability curve we may estimate the base of gas hydrate stability. The depths of BGHS at sites on passive margin range from 134 to 438 mbsf while at sites on active margin range from 359 to 473 mbsf. The depth of BGHS for later sites are much deeper because of the effects of high sedimentary rate which reduced the temperature gradients beneath seafloor. The BGHS does not obviously increase with the water depth, but it depends on the temperature gradients greatly. From the compiled contour map, we have found that the BGHSs are deeper than the previous results; however, the general trend is consistent with each other.