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4.2.2有限差分偏移为了阐述有限差分偏移的物理基础,我们来回顾上节图4-21港湾里的例子。我们下取沿着海滩记录包含绕射双曲线的剖面,而是将其收敛得到图4-27的偏移剖面,考虑下面另外外的方法。用从沿海滩记录的波场开始(图4-28a),假设堤坝离海滩1250m处,把接收电缆移到水中离海滩250m,从平面波到达堤坝的瞬间开始记录,记录的剖面显示在图4-28b上。再将电缆移到离海滩500m处,记录的剖面为图4-28c。图4-28d是离海滩750m处的记录,最后离海滩1000m记录的剖面显示在图4-28e上。可注意到,每一次记录均获得一条双曲线,其顶点越来越靠近时间零。在每幅图顶部的实红为记录电缆的实际长度。如果我们在堤坝上面离海滩1250m记录的话,那么,其
4.2.2 Finite Difference Offsets To illustrate the physical basis of finite difference offsets, let’s review the example in the previous section, Figure 4-21. Instead of recording the profile containing the diffraction hyperbolas along the beach, we converge to obtain the offset profile of Figure 4-27, taking into account the following additional methods. Starting from the wave field recorded along the beach (Figure 4-28a), assume that the dam is 1250m away from the beach, move the receiving cable into the water 250m from the beach, and start recording from the moment when the plane wave arrives at the dike. The recorded profile is shown in Figure 4- 28b. Move the cable to 500m away from the beach and record the section as Figure 4-28c. Figure 4-28d is the record at 750m from the beach and the last record at 1000m from the beach is shown in Figure 4-28e. Notice that each record gets a hyperbola, with its vertices getting closer and closer to time zero. The real red at the top of each figure is the actual length of the cable recorded. If we record 1250m from the beach above the dam, then, it