双迭代偏移方法研究

DOUBLE ITERATIVE MIGRATION METHOD

1. 华东石油学院北京研究生部;2. 地矿部北京计算中心

碳酸盐岩地区地质构造复杂,有的地区地层陡峭,倾角超过45度,横向速度变化剧烈。为了研究复杂构造的成象问题,我们提出并进行了双迭代偏移方法试验。双迭代偏移包括迭代时间偏移和迭代深度偏移。迭代时间偏移是基于这样的思想:第一,时间偏移方法所用的速度参量大多是由迭加速度乘上某个加权系数,在横向速度变化大,断层发育,构造复杂的地带,这种参量的精度是不够的。我们利用时间偏移结果来修改速度参量。第二,对于超过45度倾角的地层,做二次(或多次)偏移。将前一次偏移的输出作为下一次偏移的输入,以提高偏移质量,在实施中将两者密切结合,做二次偏移的同时修改速度参量,以达到速度渐变逼近速度剧变。在迭代时间偏移之后,进行迭代深度偏移,以获得比较正确的地层界面形态和空间位置。我们经过合成记录和实际野外资料试验,可以直接从迭代时间偏移的最后输出剖面上提取初始速度界面模型(先验模型)。文中给出了迭代的准则和双迭代偏移方法处理复杂构造剖面的例子。这个例子说明方法是可行的,效果也是比较明显的。应该指出,本文提出的双迭代偏移方法为中小型计算机上处理复杂地区地震资料提供了一种有现实意义的途径。

On carbonate area, the geological structure is complicated. In some places, stratum were rather steep, their dip angles exceed 15° and the lateral velocities change abruptly.Generally, it is very difficult to obtain a desired result by using time migration for the first time. To generate imaging of complex structure, we studied ted tested the double iterative migration, i.e. the time iterative migration and the depth iterative migpation.Time iteratrie migration is based on the following ideas. Firstly, the velocity parameter used in the ordinary time migration is the stacking velocity multiplication by a certain weighting coefficient, it the lateral velocity is variant abruptly, the parameter usually can not meet our needs. In such case, We may modify the velocity parameter from the result of time migration. Secondly, when the dip of layes exceeds 45°, the next time migration is applied, that is the output of the first time migration is used as the input of the second one. They are based on the principle that the steep dip migration operator is equivalent to the phase migration, and the transnussion function of the steep dip wave equ ation migration can be expressed by the pure phase filter, and the effect of migration on amplitude is allowed. The second time migration (residual migration) can implore the result of migration.The combination of time migration and depth migration is useful iu practice. After the first time migration we modify the velocity parameter and then do the next time migration for complex structure.After time iterative migration, we perform depth iterative migration.We present implementation of depth migration. First, we produce a velocity interface model by using cubic .spline function. Second, we obtain a ray tracing picture by ray tracing along imaging ray. Last, we can do depth migration.If the result of depth migration deviat from velocity interface, we need modify the velocity interface model, by ray tracing and doing depth migration till the migration result agree with the velocity interface model essentially. Thus the method yields a correct space position of layer and also provides a reference velocity interface for interpretation. Here, an example of double iterative migration method applied to complex structure is described, It is obvious that the method is feasible and the effectiveness is very good.It is notable that the double iterative migration method is of practical importance for processing of seismic data in complex structure areas with medium or small-sized computer.