Seismic Data Processing and the Characterization of a Gas Hydrate Bearing Zone Offshore of Southwestern Taiwan


Various seismic attributes of gas hydrate bearing sediments were analyzed in the accretionary prism offshore of southwestern Taiwan utilizing seismic imaging, velocity analysis, AVO analysis, and AVO inversion of large offset seismic data. A bottom-simulating reflector (BSR) is clearly observed on the seismic section with a reversed polarity compared to that of the seabed reflection. Instantaneous amplitude sectioning clearly shows lateral variations of the BSR. The zero-phase waveform of the BSR is distinct and the weak reflectors above the BSR can be observed on the instantaneous phase section. AVO analysis shows the absolute value of the negative BSR amplitude increasing with offset. A low P-wave interval velocity layer was found below the strong BSR by detailed velocity analysis. Both the P (normal incident P-wave reflection coefficient) and G (AVO gradient) values are highly negative for the strong BSR on the P and G sections, and they lie in the third quadrant of the P and G cross-plot section. The P+G (reflectivity of Poisson¡¦s ratio) value is also negative on the P+G section and the P-G (normal incident S-wave reflection coefficient) value is approximately zero on the P-G section along the same strong BSR. All the seismic characters described above suggest that a gas hydrate layer exists together with a free gas layer below it along strong BSRs in the area offshore southwestern Taiwan.

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