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The importance of later arrivals for migration in complex media is demonstrated by prestack images of the Marmousi data set.


Whenever one cell of a message overflows the buffer of an ATM switch, the algorithm causes the switch to discard other cells of the message (including later arrivals).


strong "horizontal reflections" at later arrivals.


Later arrivals tended to be limnophilic species that probably came from other bodies of water.


Development in an estuarine fouling community: The influence of early colonists on later arrivals

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 Analyzing the records obtained from the deep seismic soundings in North China Plain and SaudiArabia, it has been noticed that the first arrival Pn can be correlated continuously without interruption up to a distance of 500 km from the shot point. No phase shift occurs at this interval. At a distance of 350500 km from the shot point, there are one or two strong later arrivals, with the same velocity as Pn. In comparison with the results obtained from the field experiments in shallow sedimentary layers,... Analyzing the records obtained from the deep seismic soundings in North China Plain and SaudiArabia, it has been noticed that the first arrival Pn can be correlated continuously without interruption up to a distance of 500 km from the shot point. No phase shift occurs at this interval. At a distance of 350500 km from the shot point, there are one or two strong later arrivals, with the same velocity as Pn. In comparison with the results obtained from the field experiments in shallow sedimentary layers, it is concluded that in the region of lower lithosphere, one or two high velocity thin layers with thickness less than 1 km are embedded in a normal uppermost mantle medium of lower velocity. The existence of embedded high velocity thin layers in lower lithosphere may be of significance in the evolution of the lithosphere.  我们分析了中国华北平原以及沙特阿拉伯深地震测深的记录,发现P_n可以连续追踪到500公里以上,中间并无间断现象,而且在接收距离350—500公里之间,P_n后面有一到两个很强的续至震相,它们的速度与P_n相似,而能量比P_n更强。经与沉积层中薄层的野外实验结果以及室内超声波模型实验的结果相比较,我们得到这样的结论:岩石圈下部存在一至数个高速薄层,其厚度不超过一公里。地幔顶部物质的正常速度可能比薄层略低。岩石圈中薄层的存在对于岩石圈的演化过程具有重要的意义。  To the most of velocity fields, the traveltimes of the first break that seismic waves propagate along rays can be computed on a 2D or 3D numerical grid by finitedifference extrapolation. Under ensuring accuracy, to improve calculating efficiency and adaptability, the calculation method of firstarrival traveltime of finitedifference is derived based on any rectangular grid and a local plane wavefront approximation. In addition, head waves and scattering waves are properly treated and shadow and caustic... To the most of velocity fields, the traveltimes of the first break that seismic waves propagate along rays can be computed on a 2D or 3D numerical grid by finitedifference extrapolation. Under ensuring accuracy, to improve calculating efficiency and adaptability, the calculation method of firstarrival traveltime of finitedifference is derived based on any rectangular grid and a local plane wavefront approximation. In addition, head waves and scattering waves are properly treated and shadow and caustic zones cannot be encountered, which appear in traditional raytracing. The testes of two simple models and the complex Marmousi model show that the method has higher accuracy and adaptability to complex structure with strong vertical and lateral velocity variation, and Kirchhoff prestack depth migration based on this method can basically achieve the position imaging effects of wave equation prestack depth migration in major structures and targets. Because of not taking account of the later arrivals energy, the effect of its amplitude preservation is worse than that by wave equation method, but its computing efficiency is higher than that by total Green′s function method and wave equation method.  对于大多数速度场 ,地震波沿射线传播的初至波走时 ,可以用有限差分外推的方法在二维或三维数值网格上计算出来 .在保证精度的条件下 ,为提高计算效率和适应性 ,本文推导了基于任意矩形网格和局部平面波前近似的有限差分初至波走时计算方法 .另外 ,该方法对首波和散射波做了合适的处理 ,而且不会碰到传统射线法存在的阴影区和焦散区等问题 .简单模型和复杂的Marmousi模型试算的结果表明 ,该方法精度较高并适用于强纵、横向变速的复杂介质 .基于该方法的Kirchhoff叠前深度偏移 ,在主要构造和目的层位置的成像效果上基本达到了波动方程法叠前深度偏移的位置成像效果 .由于未考虑续至波等有效能量 ,在成像的保幅性上不如波动方程法叠前深度偏移的效果 ,但其计算效率则明显高于全格林函数法和波动方程法 .  
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