摘要
To investigate the distribution and velocity attributes of gas hydrates in the northern continental slope of South China Sea, Guangzhou Marine Geological Survey conducted four-component （4C） ocean-bottom seismometer （OBS） surveys. A case study is presented to show the results of acquiring and processing OBS data for detecting gas hydrates. Key processing steps such as repositioning, reorientation, PZ summation, and mirror imaging are discussed. Repositioning and reorientation find the correct location and direction of nodes. PZ summation matches P- and Z-components and sums them to separate upgoing and downgoing waves. Upgoing waves are used in conventional imaging, whereas downgoing waves are used in mirror imaging. Mirror imaging uses the energy of the receiver ghost reflection to improve the illumination of shallow structures, where gas hydrates and the associated bottom-simulating reflections （BSRs） are located. We developed a new method of velocity analysis using mirror imaging. The proposed method is based on velocity scanning and iterative prestack time migration. The final imaging results are promising. When combined with the derived velocity field, we can characterize the BSR and shallow structures; hence, we conclude that using 4C OBS can reveal the distribution and velocity attributes of gas hydrates.
为了调查南海北部陆坡的天然气水合物的分布及其速度特征,广州海洋地质调查局在该海域进行了一系列的四分量(4-Component,简称4C)海底高频地震仪(Ocean Bottom Seismometer,简称OBS)测量。本文对该局在该海域采集的四分量OBS站点地震数据进行成像处理。处理的关键步骤包括重定位、重定向、水陆检合并、镜像偏移。站点的重定位与重定向是使得四分量数据正确归位。水陆检合并是指水检和陆检的匹配,相加和相减分离上行波和下行波。上行波用于常规成像,下行波可用于镜像偏移。由于节点稀疏,上行波的常规成像无法识别天然气水合物分布。镜像偏移是利用检波点的下行波进行成像,既改善浅地层的构造照明,又提高了天然气水合物矿体的成像精度,可建立精确的速度模型。通过下行波成像,镜像偏移剖面构造特征清晰,分辨率高,海底、天然气水合物地震识别标志-似海底反射(Bottom Simulating Reflection,简称BSR)与高分辨率剖面和速度特征相吻合。
基金
supported by the National Hi-tech Research and Development Program of China(863 Program)(Grant No.2013AA092501)
the China Geological Survey Projects(Grant Nos.GZH201100303 and GZH201100305)
