The RAYINVR software, which is widely used in academia for 2-D velocity structure model from fourcomponent OBS (ocean bottom seismometer) data, is unable to invert for the converted S-wave automatically, resulting in the low-efficiency of modeling process. Using MATLAB’s genetic algorithm, the RAYINVR software is improved and able to automatically and synchronously invert for Poisson’s ratios of each layer with all subblocks for the S-wave velocity structure model, and thus can provide Young’s modulus, Poisson’s ratio and other important mechanical information for gas hydrate survey. This method is applied to process the OBS data collected at the Slipstream submarine slide, and a fine P- and S-wave velocity structure model is obtained, which is comparable to the logging data of nearby borehole U1326. Therefore, the validity of the auto-synchronous inversion method is verified for the S-wave velocity structure modeling. The optimal velocity model reveals two structural interfaces with large Poisson’s ratio contrast. One is BSR (bottom simulating reflector) at 230±10 mbsf (meter beneath sea floor), which represents the bottom boundary of the gas hydrate stability zone, and the other is the basal boundary of a shallow abnormal high-speed body (possibly a sand body enriched with high saturation gas hydrate) at 75?100 mbsf. The latter agrees roughly with the glide plane of Slipstream submarine slide, indicating that the hydrate is related to the formation of submarine landslide.

It is the scientific basis for seismic exploration and resource evaluation of natural gas hydrate to study the acoustic properties of gas hydrate-bearing unconsolidated sediments in the laboratory. For this purpose, an instrument was designed to experimentally study the change in acoustic velocity of water saturated unconsolidated sediments during gas hydrate formation. The improved ultrasonic transducer was employed to measure the acoustic velocity. The saturation of methane hydrate was continuously monitored as referring to the gas pressure change in the reactor, which was then related to the corresponding acoustic measurement. With the artificial sample simulating the sediments of core HY-3 from SH-7 borehole in Shenhu area of South China Sea, several round experiments of methane hydrate formation and dissociation were conducted. The results showed that the measured compressional and shear wave velocities were located between the curves predicted as to the rock physics models with and without hydrate cementing sediment grains in pore space. Although methane hydrate preferred to suspend in pore space or precipitate on sediment grains in dissociation and formation processes, there was always a very small amount of hydrate cementing the grains, and it significantly increased the acoustic velocities of sediments.