All extrapolation operators face the problem of choosing the referencevelocity due to continuation in depth.
此类波场外推算子沿深度层进行波场外推,都存在参考速度选择问题。
2
The velocity of the center of mass has not changed as seen from your frame of reference, because the velocity of center of mass is always the same, remember, because momentum is conserved.
Now, a word about placeholder substitution: Velocity will print any unrecognized reference as plain text, as illustrated by the next two bold lines in Listing 6.