Correcting time shifts

A computer-implemented method of preparing Common Depth Point (CDP) gathers by:

• performing, by a computer system, Normal MoveOut (NMO) correction on a first Common Depth Point (CDP) gather, the NMO correction based on a reference velocity, the NMO correction creates first NMO data comprising a plurality of traces;
• calculating, by the computer system, a zero-offset time shift based on the reference velocity;
• adding, by the computer system, the zero-offset time shift to each trace of the first NMO data;
• and then performing, by the computer system, reverse NMO correction on the first NMO data;
• and wherein the improvement comprises:
• selecting, by the computer system, a first estimated velocity that makes travel time represented in the traces of the first NMO data substantially the same;
• performing, by the computer system, NMO correction on the first CDP gather based on a final estimated velocity to create second NMO data, the final estimated velocity selected based on the first estimated velocity;
• wherein calculating the zero-offset time shift further comprises calculating, by the computer system, the zero offset time shift that represents a difference in travel time as between the reference velocity and the first estimated velocity at a zero-offset trace;
• wherein adding the zero offset time shift further comprises adding, by the computer system, the zero-offset time shift to each trace of the second NMO data;
• and
• wherein performing reverse NMO correction further comprises performing, by the computer system, reverse NMO correction on the second NMO data to create first corrected CDP gather;
• thereby adjusting the first CDP gather to the reference velocity compensated for variances in speed of sound in the water during collection of the first CDP gather with respect to the reference velocity.