| dc.description.abstract | Gas hydrate and free gas in Yung-An Ridge, Offshore SW Taiwan are existed in unconsolidated marine sediment and give a large effect on the elastic properties. The elastic properties such as porosity, density, P-wave velocity (Vp), S-wave velocity (Vs), Vp/Vs, Poisson’s Ratio, Lamdha-Rho (λρ), Mu-Rho (μρ), Lamdha/Mu (λ/μ) are varied a lot. So, we purpose to distinguish the physical properties of gas hydrate and other mineral bearing in unconsolidated marine sediment, and try to characterize the gas hydrate and free gas reservoir in Yung-An Ridge. Detecting gas hydrate and free gas’ indicator and knowing the structure, morphological or depositional processes features, stratigraphy, even for rock or pore fluid properties can be done through seismic attribute method. The attribute study become the initial effort to allocate the potential seismic responses associate with gas hydrate stability zone and provide the suggestion on area where high potential GH concentration and continuous supply of gas below the hydrate zone can exist. Next step is to characterizing the gas hydrate and free gas through post-stack and pre-stack inversion.
The approach to perform post-stack inversion with the limitation of budget, there is no well log information, we do post-stack modeling. The post-stack modeling tries to define a 1D velocity model and treat the stacked data at some selected CMP locations as a individual pseudo-log with efforts by manually define the P- wave velocity model. With 1D model defined from these selected pseudo-log, post-stack inversion were performed first through Model-Based Inversion (MBI) method. The MBI produces P-Impedance (Zp), and with the linear equation in the HRS Software, Vp can be determined, and then density can be estimated. Vp and density from Model-Based Inversion can be used to initial guess for pre-stack inversion. Simultaneous Inversion (SI) is the famous method to perform the pre-stack inversion. SI try to calculate reflectivity based on Fatti’s equation with angle range 0o to 40o. The reflectivity that obtained in this inversion is representative by reflectivity of the P-impedance, reflectivity of the S-Impedance, and reflectivity of the density. Hence, the elastic properties such as P-wave, S-wave, density, Zp, Zs, and Vp/Vs can be obtained in a single inversion. Other parameters of elastic properties such Poisson’s Ratio, λρ, μρ, and λ/μ can be derived through equations. The parameters which are very sensitive with the gas hydrate and free gas in Yung-An Ridge are λρ, and μρ. They are separated from background trend that can be shown through cross plot. The lithology can be separated through crossplot too. To enhance the result of this interpretation, we use unsupervised Machine Learning algorithm such as K-means cluster to distinguish gas hydrate-and free gas from the lithology. By using MSCL Data that show information that at least, there are 7 lithologies in the Yung-An Ridge. With additional information from Elbow method to choose the optimal K-value then K-means cluster can be performed. Silhoutte Score with value -1 to 1 can be used judge to K-means cluster’s result. Gas hydrate can be identified with high P-, S-, Zp, Zs, λ? and slightly higher Vs/Vp, slightly lower λ/μ and Poisson’s ratio (ν) while free gas can be identified with low P-, AI, EI, none-varying S- impedance, high Vs/Vp, low λ/μ, λ? and ν.
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