克希霍夫叠前时间偏移走时算法应用效果评价
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摘要
地震资料成像效果与走时精度有关,在克希霍夫叠前时间偏移中,通常有两种计算旅行时的算法,即直射线法和弯曲射线法,不同的旅行时算法成像精度有一定差别。直射线法根据均方根速度计算旅行时;弯曲射线法利用层速度模型计算旅行时,虽然考虑了速度的横向变化,但仍然假设炮点或成像点两侧的走时是对称的。非对称走时Kirchhoff叠前时间偏移算法中含有水平源像距的多次项以及关于速度的横向导数(非对称项),能够有效提高走时计算精度,从而改善地震波的聚焦效果。对3种旅行时算法从理论公式、对速度的适应性方面进行应用效果分析,并在SB地区实测,认为弯曲射线与非对称走时的速度建模较为相似,两者速度的适应性较好。横向速度相对变化较大时,直射线较非对称走时所需均方根速度偏高。在速度横向变化较小的平层,效果相当;在速度横向变化较大时,非对称走时算法计算精度较弯曲射线稍高,较直射线有较大优势;直射线走时算法对断点和断面的分辨率略低。
Seismic data imaging effects associate with travel time accuracy,and each travel time algorithm has some difference in imaging precision.There are two kinds of the algorithm for calculating travel time in Kirchhoff pre-stack time migration: straight ray method and bending ray method.Travel time is calculated ac-cording to RMS velocity in the straight ray method.Layer-velocity model is applied to the calculation of trav-el time for the bending ray method.During the calculation process,the lateral variation of velocity is considered,but it is assumed that shot or image point is symmetrical on both sides of the walk.The equation for calculating asymmetrical travel time in Kirchhoff pre-stack time migration contains polynomial items of hori-zontal source image distance and horizontal derivatives of the velocity,which can improve the calculation accuracy of travel time and focusing of seismic waves.The application effect of three algorithms of the travel time in the SB area is analyzed from the formula and the velocity adaptability.Bending ray is similar with non-symmetrical travel time on velocity modeling.They both have better adaptability to the velocity.The RMS velocity for the straight ray travel time is higher than that for the asymmetric travel time when horizontal velocity variation is large,and they are fair when the velocity variation is small.The calculation accuracy of the asymmetric algorithm is better than the other algorithms when the horizontal velocity variation is large,and resolution of the breakpoint and section is slightly lower for the straight ray travel time algorithm.
Seismic data imaging effects associate with travel time accuracy,and each travel time algorithm has some difference in imaging precision.There are two kinds of the algorithm for calculating travel time in Kirchhoff pre-stack time migration: straight ray method and bending ray method.Travel time is calculated ac-cording to RMS velocity in the straight ray method.Layer-velocity model is applied to the calculation of trav-el time for the bending ray method.During the calculation process,the lateral variation of velocity is considered,but it is assumed that shot or image point is symmetrical on both sides of the walk.The equation for calculating asymmetrical travel time in Kirchhoff pre-stack time migration contains polynomial items of hori-zontal source image distance and horizontal derivatives of the velocity,which can improve the calculation accuracy of travel time and focusing of seismic waves.The application effect of three algorithms of the travel time in the SB area is analyzed from the formula and the velocity adaptability.Bending ray is similar with non-symmetrical travel time on velocity modeling.They both have better adaptability to the velocity.The RMS velocity for the straight ray travel time is higher than that for the asymmetric travel time when horizontal velocity variation is large,and they are fair when the velocity variation is small.The calculation accuracy of the asymmetric algorithm is better than the other algorithms when the horizontal velocity variation is large,and resolution of the breakpoint and section is slightly lower for the straight ray travel time algorithm.
引文
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[3]周纳,刘继勇.非对称走时叠前时间偏移方法及应用[J].油气地球物理,2010,8(2):48-52.
[4]马在田.地震偏移成像[M].北京:石油工业出版社,1989.
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