Presenter: Yu-Hua Liou
Date: 2017/06/08
Abstract
Seismic exploration was acquired along Fanglio 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 BSR and high velocity above BSR. The presence of methane hydrate is broadly inferred by Bottom Simulating Reflectors (BSRs) on 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 pre-stack data processing workflow strategy to measure relative reflection coefficient (RC) curves at different CMP locations. The study determined reflection coefficient trends that provide information about fluid changes. AVO responses are measured with RC 0 to 0.5 for seafloor, - 0.17 to -0.25 for top of BSR and 0.06 to 0.3 for bottom BSR. According to the AVO/AVA and rock physics modeling, we computed 1-D velocity model for some typical CMP number along the anomalous zone. We estimated velocity (Vp, Vs) and density distribution of the layer above and below BSR which were used for references of theoretical AVO/AVA curve.
A synthetic AVO/AVA curve generated from theoretical reflection coefficient, was obtained by computing all possible combination of Vp,Vs, and rock-physics parameters based on Zoeppritz approximation. The estimated relative curves obtained from observed data were used to fit the theoretically derived absolute AVO/AVA curves. We can estimate the acoustic (Ip) and elastic (Ip and Is) impedance by comparing the picked relative RC curve with theoretically generated curve. By fitting cluster of data-points within some particular CMP range, we can obtain better constrained Vp, Vs, and density distributions. The theoretical parameters were referred as Initial model to quantify pre-stack and model-based- post-stack AVO inversion. It reserved detail estimation related to Vp, ρ, and Vp/Vs by iteratively refining the model, based on data distribution. The refined parameters were superimposed to the data and reserved estimation for interpretation. The estimation indicates that for hydrate layer above BSR, the P-wave velocity (Vp) range from 2.0 to 2.1 km/s, the S-wave velocity (Vs) is fairly low which range from 0.95 to 0.975, and the density is 1.93 to 2.2 g/cm3. BSR has thickness about 12 ms time-depth (2248-2260 ms) with Vp 1.9 to 1.97 km/s, Vs 0.98 to 1.0 km/s and density 1.95 to 2.03 g/cm3. Free gas occurred below BSR, the Vp is relatively equal with BSR : 1.85 to 1.95 km/s, Vs is lower than BSR: 0.8 to 0.95 km/s, and density is about 2.05 to 2.15 g/cm3.
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