江陵凹陷北部高速玄武岩下二维广角地震初探

江汉油田江陵凹陷北部高速玄武岩发育，利用常规地震资料很难得到高速玄武岩下的地震反射。分析研究表明，广角地震有利于玄武岩下的地震成像。通过综合正演和实际地震数据完成了广角地震资料的处理，获得了江陵凹陷北部高速玄武岩下的反射地震信息。在开展广角地震波场的识别和利用研究过程中，从正演模型出发，探讨了在不同地质条件下高速层对地震波场的影响，并针对靶区资料讨论了如何识别广角地震信息。在上述基础上，通过对实际地震资料做精细的分析和处理，最终完成了高速玄武岩下地震资料的成像。

A Preliminary Study on 2-D Wide Angle Seismic Prefiling in Area of High Velocity Basalt Layer in the Northern Jiangling Depression, Hubei Province

The problem of high velocity impedance has become one of the obstacles in the oil and gas (exploration) in south China. A special survey should be adopted for getting the seismic imaging beneath the high velocity basalt layer.In area of the high velocity impedeace, the reflection signals are weak, so that we obtain the seismic image hardly. It is very important to increase length of offset in the seismic data acquisition to get the deep seismic reflection. There are three methods for the wide angle seismic profiling to solve the high velocity impedance problem. The first is to delineate the structure of the top and bottom of the impedance layer and to obtain the velocity of the basement from the refraction seismic data. The second is to get rid of the noise near offset to improve the quality of the imaging using the wide reflection. The third is to get image for the low velocity layers below the high velocity impedance with converted wave in the high velocity layer. All of them can be combined by using various data to improve the quality of the imaging. Jiangling depression is one of the key prospecting areas in south China. The extensive distribution of basalt in this area deteriorates the seismic data seriously. The reflections are not clear, the S/N ratio is low, the event continuity is bad, and the multiples are strong.In order to get the clear structure,we increased the length of the offset. Unfortunately, more difficulty in seismic data processing appeared. Besides the complex seismic waves and signals, the most important problem is the obvious anisotropic characteristics alongside the length increasing. Much research work has been done about the time-distance curve of the seismic waves in the various media. Conventional NMO is not available to achieve seismic imaging of the long offset. The deep seismic imaging and the analysis of seismic amplitude characteristics can be achieved only based on the phase correction. A fourth order correction can be made by the three terms of Taylor series given by Taner and Koehler. Using more than two terms of the Taylor series can improve velocity analysis and CMP stacking. However, a simple truncation generally causes a significant travel time error at very large offset-to-depth ratio. To have a travel time approximation with good accuracy at large offsets, a high order long offset NMO correction (LNMO) was used.