The problem is resolved, at least in principle, by means of Snell waves. 15 Time of multiple depends on sum of all times. Incompleteness of the data causes us to have problems with most inversion methods. Data can be incomplete in time, space, or in its spectrum. Any recursive method must be analyzed to ensure that an error made at shallow depths will not compound uncontrollably during descent.
All data is spectrally incomplete.
Also, with all data there is uncertainty about the shot waveform. To solve this problem, Taner built a special auxiliary recording system. The methods to be discussed next are before-stack methods, but they do not require downward continuation much below the sea floor.