天然气水合物相关的Slipstream海底滑坡体速度结构模型反演

针对目前用于建立二维速度结构模型的RAYINVR软件对四分量海底地震仪(OBS)记录的转换横波无法自动反演, 建模效率低的问题, 采用MATLAB遗传反演算法, 对RAYINVR软件进行改进, 实现对横波速度结构模型各层和层中区块泊松比的自动同步反演, 可以为天然气水合物勘探调查提供泊松比和杨氏模量等重要弹性参数。利用采自Slipstream海底滑坡的OBS数据, 通过同步反演方法, 获得该地区较为精细的纵波和横波速度结构模型, 并可与附近U1326钻孔的测井数据进行对比, 验证了横波速度结构同步反演建模方法的有效性。模型揭示了两个泊松比差异大的结构面: 代表水合物稳定底界的似海底反射界面(BSR)(海底之下230±10 m)以及浅部异常高速体(有可能是高饱和度水合物富集的砂体)的底界(海底之下75~100 m), 后者与滑脱面大致重合, 指示水合物与Slipstream海底滑坡的形成有关。

Model Inversion of Velocity Structure for Slipstream Submarine Slide Related to Gas Hydrate

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.