| dc.description.abstract | Seismic exploration was acquired along Fangliao Basin. The anomalous target zones had been identified along MGL0908-TST line as high potential hydrates concentration. The conventional semblance velocity estimation showed that the anomalous velocity distribution occurred with the presence of low velocity existing below Bottom Simulating Reflectors (BSR) and high velocity above BSR. The presence of methane hydrate is broadly inferred by BSR on the seismic profile. It means that lithology changes play a big role to generate this condition.
The AVO/AVA effect is investigated based on amplitude preservation data processing workflow strategy to measure the effected thin layer geometry along CMP 3541 to 3700 and relative reflection coefficient (RC) curves at different CMP locations. The post-stack AVO modeling suggests that the BSR has a thickness of approximately 14 m. The pre-stack AVO/AVA modeling and analysis determined reflection coefficient trends that provide information about fluid changes. The pre-stack AVO/AVA modeling establishes the 1-D velocities (Vp, Vs) and density (ρ) for some typical CMP gathers along with the anomalous zone. The model is crucial to initiate the inversion procedure. The further AVA modeling through relative and absolute AVA curves at several specified interfaces is necessary to refine the predefined 1-D velocities and density model. The relative AVA responses are measured with RC 0.27 to 0.1 for seafloor, -0.01 to -0.12 for the top of BSR and 0.02 to 0.18 for bottom BSR as a function of incidence angles. The absolute AVA curves calculation measures by computing all possible combination of Vp, Vs, and rock physics parameters based on Zoeppritz approximation, thus representing the relative AVA responses. According to the fitting and searching cluster of data points, we could estimate the acoustic (Ip) and elastic (Ip and Is) impedance and obtain the better-constrained velocities (Vp, Vs) and density distribution of the layer above and below the seafloor, BSR-Top, and BSR-bottom which were used for references.
The rock physics parameters are used as an initial guess values for post-stack acoustic impedance (PoSAInv) and pre-stack elastic impedance (PreSEInv) inversions. The inversion revealed the detail estimations of Vp, Vs, ρ, and Vp/Vs by iteratively refining the acoustic impedance (AI), elastic impedance (EI) and elastic modulus models. The refined spatial distribution of physical properties superimposed on the seismic data are further used for interpretation. In addition, the inverted solutions can be treated as pseudo-log to reveal the thickness of gas hydrate bearing sediment zone (GHBSZ), BSR, and free gas zone (FGZ). Furthermore, the inverted model can be further verified by pre-stack AVA analysis. The proposed approach quantified the thickness and rock properties of three major layers and its potential gas hydrate-bearing sand system within the study area. Those three major layers are the GHBSZ zone, BSR, and FGZ. Based on Biot-Gassmann’s approximation, the predicted porosity and saturation of gas hydrate-bearing sediments are approximately 40% and 26%, respectively. From here thereafter, we can estimate the potential gas-hydrate reserved through proposed approach.
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