七、超深水海底飞行节点地震仪研发与地球物理探测
Ocean Bottom Flying Node Seismometer (OBFN) Development and Geophysical Exploration
针对当前超深水(>1500m)海底节点地震勘探存在成本高、周期长、数据处理解释有瓶颈等问题，我室“地球物理勘探技术”团队在国家重点研发计划“深海关键技术与装备”重点专项的资助下，开发了带有导航定位与自主航行功能的海底飞行节点地震仪(Ocean Bottom Flying Node, OBFN)。该技术采用海底长基线实时定位技术对多台OBFN进行导航定位，提高了节点布放与回收效率；研制了带有时标功能的长基线定位基站，降低了数据采集授时成本；实现采集模块与飞行节点运载器之间软链接，可保证采集模块与海底良好耦合，提高地震信号记录质量，同时也方便后期数据下载与维护处理。
海底飞行节点地震仪海试成功
2022年在南海北部超深水盆地荔湾凹陷开展了OBFN海试，最大下潜水深达到2861.4 m，在国际上首次实现了超深水海底飞行节点地震仪数据采集，获得了高质量海底四分量地震记录。此次海试发展了海底四分量地震数据弹性波成像与定量解释新技术，实现了OBFN仪器研发、布放与回收、数据采集、处理与解释全链条创新，OBFN所有元器件全部实现国产化，整体处于国际并跑-领跑水平。
图7.1 南海北部超深水盆地荔湾凹陷海试区
图7.2 海底飞行节点地震仪布放、回收与海试团队
针对OBFN地震数据特点，发展了基于声-弹耦合方程的四分量多参数全波形反演方法，以实现中浅层速度建模；利用上-下行波P/S解耦的反射走时与波形反演方法，以实现中深层的速度建模；提出了共检波点域弹性波高斯束矢量成像方法，以实现纵波与转换波的高效成像；基于声-弹耦合方程的地震逆时偏移成像方法和最小二乘偏移成像方法，以实现高精度阻抗成像。这些方法联合构成了海底多分量地震数据弹性波成像技术。理论模型测试表明，该技术建模准确，成像深度误差<0.5%。
将该技术应用于我国南海深水荔湾凹陷OBFN实测地震数据，获得了U1501站位所在区域的地下纵、横波速度结构和成像剖面，地质构造合理。将该技术应用于我国东海浅水OBN四分量实测地震数据，成功获得了东海秋月探区的地下高波数纵、横波速度结构和成像剖面，揭示了中浅层的构造特征，与测井吻合较好。
深海地球物理方法进展
深海地层岩石物理响应机理和建模技术是基于弹性波地震数据进行岩性和孔隙度预测的物理基础，研发了最小二乘框架下弹性波PP和PS联合叠前AVO反演技术，相比于单纯的PP反演结果，PP-PS联合反演对薄层和沉积地层的连续性有更好刻画。对南海海试OBFN弹性波数据开展PP-PS联合AVO反演测试，获得了表征深海沉积地层的纵波速度、横波速度和密度分布。利用随机森林机器学习算法建立弹性波速度与岩性和物性参数的映射关系，结合PP-PS联合反演结果对深水沉积地层的岩性和孔隙度开展预测，岩性和孔隙度地震预测结果能够较好的刻画深水沉积地层的非均质性分布。
图7.3 针对南海深海沉积地层开展基于PP反演和PP-PS联合反演的(上)岩性和（下）孔隙度预测结果对比
主要参考文献：
He, W. and Geng, J. Elimination of free-surface-related multiples by combining Marchenko scheme and seismic interferometry, Geophysics, 2022, 87(3): Q1-Q14.
Liu, Y., Liu, W., Wu, Z., Yang, J. Reverse time migration with an exact two-way illumination compensation, Geophysics, 2022, 87(2): S53-S62.
In order to address the high cost, long cycle, and bottleneck in data processing and interpretation of seismic exploration for deep-sea (>1500m) underwater nodes, the Geophysical Exploration Technique Group led by Prof. Jianhua Geng has developed an Ocean Bottom Flying Node Seismometer (OBFN) with autonomous navigation capabilities, under the support of the National Key Research and Development Program for "Key Technologies and Equipment for Deep-sea". This technology uses underwater long-baseline real-time positioning technology to navigate and locate multiple OBFNs, improving the efficiency of node deployment and recovery.
In 2022, an OBFN sea trial was conducted in the Lixiwan depression in the northern part of the South China Sea's ultra-deepwater basin. The maximum diving depth reached 2861.4 meters, and for the first time internationally, data was collected from a super-deep-sea seabed flying node seismograph, obtaining high-quality seafloor four-component seismic records. This sea trial developed new technology for elastic wave imaging and quantitative interpretation of seafloor four-component seismic data, achieving full-chain innovation in the development, deployment, recovery, data collection, processing, and interpretation of OBFN instruments. All components of the OBFN instrument have been domestically produced, and overall, the technology is at the forefront of the international community. Under the framework of least-squares, an elastic wave PP-PS joint pre-stack AVO inversion technology was developed. Compared with the results of single PP inversion, the PP-PS joint inversion can better characterize the continuity of thin layers and sedimentary formations.