层状各向异性介质转换波克希霍夫叠前时间偏移

Kirchhoff prestack time migration of PS-wave data for the layered anisotropic medium

在克希霍夫叠前时间偏移处理中,地震波走时的计算方法是决定大偏移距地震资料成像品质的重要因素.在常规的三维转换波各向异性叠前时间偏移公式中,走时的计算是基于等效单层各向异性介质的非双曲线方法.用这种方法处理的成像道集,在偏移/深度比超过一定阈值后,成像道集中的反射同相轴将出现过偏现象,这种偏移不平的同相轴将影响偏移叠加的最佳响应,使得偏移成像波组呈低频化特征,最终降低三维转换波偏移成像质量.我们采用层状介质的走时计算方法代替常规算法,并且利用了常规方法的转换波各向异性偏移速度模型.基于层状介质的算法能够提高大偏移距转换波走时计算精度,克服中浅地层大偏移距远道成像道集中反射同相轴逐渐上翘的问题.两个地区的三维转换波资料处理结果证实,基于层状各向异性介质的转换波克希霍夫叠前时间偏移方法,明显改善了反射成像剖面的连续性和分辨率,提高成像剖面构造的可解释性.

The method of calculating travel times of seismic wave in Kirchhoff pre-stack time migration （PSTM） plays an important role of determining the quality of imaging for large offsets. The travel times are computed by the non-hyperbolic equation based on single layer anisotropic medium in the conventional 3D PS-wave anisotropic PSTM equations so that the events are over-migrated in common imaging point gathers （CIP） when the offset/depth ratio is greater than a threshold value. The uneven events have negative effects on migrated stack response, decrease frequency and lead to contaminating the quality of imaging. We proposed a new method to calculate travel times based on the layered anisotropic medium in PS-wave PSTM technology utilizing the migrated velocity model which was generated in the conventional 3D PS-wave anisotropic PSTM. The new method can improve the precision of travel times for the large offsets and overcome the over-migration events for a large offset/depth ratio in CIP gathers. The realistic 3D PS-wave data from two areas of West China demonstrated that the continuity and resolution of imaging section had obviously been improved by using the new PSTM comparing to conventional PS-wave PSTM and enhanced the interpretation of structures on imaging section.