海底高频地震仪观测系统优化设计及其在南海天然气水合物勘探中的应用

An optimal design of a high-frequency ocean bottom seismometer(HF-OBS)and its application to the natural gas hydrate exploration in the South China Sea

海底高频地震仪（HF-OBS）可获得含天然气水合物（以下简称水合物）层的纵横波速度、波阻抗等多种有效地震反射信息,对准确估算水合物的资源量具有重要意义。根据我国南海北部海底水合物HF-OBS勘探实例,基于射线追踪法、双聚焦法、波动方程的正演模拟法3种方法,对HF-OBS观测系统进行了优化设计,并利用纵横波联合走时反演获得了该区含水合物层的纵横波速度关系。结论认为：①采用射线追踪法模拟目的层射线分布和叠加次数,同时结合波动方程的正演模拟结果,可以有效地进行观测系统优化设计;②双聚焦理论的参数模拟能够进一步验证观测系统设计的合理性,针对特定的水合物目标体,为了获取矿体内部精细的速度结构,HF-OBS的分布形态应采用长方形或正方形的矩阵分布,分布间距以300~500m为宜;③优化后的观测系统有利于获得高质量的HF-OBS地震数据,而纵横波联合反演获得的速度信息减小了水合物识别的不确定性。

A high-frequency ocean bottom seismometer（HF OBS）can not only receive multi-component displacement records of the bottom of the sea, but acquire various seismic information of P-wave and converted S-wave, which is of great significance to accu rately evaluate the hydrate reserves of localities. Therefore, in a case study of natural gas hydrate exploration in the South China Sea, we made an optimal design of the HF-OBS geometry system through three forward modeling simulation methods of ray tracing, focal beam analysis, and wave equation algorithms. And we adopted a hybrid inversion method of wave-equation travel time inversion （WT） and waveform inversion to obtain the relationship between P- and S-wave velocity of hydrate layers in this study area. The following findings were concluded. （1） The ray tracing method simulates the Gamma distribution and stacking folds （superimposition times） of pay zone and integrates the results of wave equation algorithms to effectively improve the optimal design of HF-OBS geometry system. （2） Simulation parameters of the focal beam analysis can further validate the reasonability of the observing system design and as for the specific hydrate target, the matrix distribution of a rectangle or a square with the space between of 300 - 500 m should be employed for the HF-OBS to acquire the very fine inner velocity structure of the hydrate target. （3） The optimized observation system is conducive to obtain HFOBS seismic data, while velocity obtained by P- and S-wave joint inversion decreases the uncertainty of gas hydrate recognition.