Using Finite Elemente Method to Correct Topography Effect of Sub-bottom Thermal gradient and Estimate the Base of Gas Hydrate Stability Zone
Date Issued
2009
Date
2009
Author(s)
Lou, Wen-Lin
Abstract
By the effect of rough topography, heat flow will tend to be preferentially convergent towards valleys or low areas, and divergent from ridges or peaks on the sea floor. Hence, the measured result of the seafloor gradient cannot correctly estimate the real thermal gradient of the sub-bottom. In order to calculate the variation in temperature more efficiently, we use ANSYS software using the finite element method to correct such effects and compute the Base of Gas Hydrate Stability Zone; BGHS.uring the preliminary test, we found that the temperature of the sub-bottom deviated for more than one hundred meters due to topography effects in the offshore of southwestern Taiwan. This means that it is necessary to correct for the topography effects. To achieve this, we try to remove the topographic effects on the temperature gradients from cruise No. 786 of the R/V Ocean Researcher I.hen applying ANSYS to calculate the temperature gradients, we adopted the data collected from one site as the boundary condition, and corrected the errors that resulted from the topography effect in this area. The calculated seafloor thermal gradient of the other site, situated on a similar sediment deposited depression, is close to the measured value; but, on the third site, the result is significantly different from the measured value. We believe the difference is due to the third site being located on the ridge, thus causing heat to be refracted away from depressions deposited with thick sediments. This example emphasizes that not only the topographic effects, but also the sedimentation is important to the temperature gradient calculation, and therefore the estimate of BGHS. For the application, we use ANSYS to correct the topographic effects on sites KP-4, KP-5-1, and KP-5-2 which are drilling sites for gas-hydrate investigation proposed by the Central Geological Survey. After the correction we obtained, the BGHSt (corrected BGHS) for the three sites are 311, 298, and 350 mbsf, respectively. The results are useful information for borehole drilling reference. he model proposed in this study is limited by the lack of sedimentary data and excludes the basement relief effect. Actually, the sub-bottom thermal gradient variation is affected by the topography, the thickness of the sediments, the basement relief, and the structure of the rock. It will be better if we can get more information about the structures, the thickness of the sediments, and thermal conductivities for each layer.
Subjects
Thermal gradient
Topography correction
Finite Element
Base of Gas Hydrate Stability Zone
Type
thesis
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