Using attachable miniature temperature probes to measure sub-seafloor temperature gradients and estimate the Base of Gas Hydrate Stability Zone
Date Issued
2007
Date
2007
Author(s)
Lin, Su-Jer
DOI
zh-TW
Abstract
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.
Subjects
仿擬反射層
小型溫度探針
天然氣水合物
背景溫度梯度
圓柱體溫度衰減函數
Bottom simulating reflector
Miniature temperature probe
Gas Hydrate
Temperature gradient
Cylindrical temperature decay function
Type
thesis
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