Systems and methods for randomizing firing times of simultaneous sources in marine surveys

1. A method for activating multiple sources towed by one or more survey vessels, the method comprising:

generating a set of seed time delays;

assigning each seed time delay to one of the multiple sources;

for each shot interval of a marine-survey-time line,

randomly selecting a time delay that is less than a duration of the shot interval;

pseudo-randomly shifting the seed time delays based on the time delay to generate shifted time delays within the shot interval, each shifted time delay associated with one of the multiple sources; and

firing the multiple sources within the shot interval based on the assigned shifted time delays.

2. The method of claim 1 , wherein generating the set of seed time delays includes randomly generating seed time delays that are within the duration of the shot intervals.

3. The method of claim 1 further comprises

dividing a duration of the shot intervals of the marine-survey-time line into a set of duration groups;

pseudo-randomly arranging the duration groups into a schedule of duration groups; and

for each shot interval of the marine-survey-time line,

pseudo randomly selecting a duration group from the set of duration groups;

randomly selecting the time delay from the selected duration group; and

pseudo-randomly shifting the seed time delays to generate the shifted time delays by

assigning the time delay to the source of the multiple sources with a shortest seed time delay,

calculating a difference between the time delay and the shortest seed time delay, and

adding the difference to the seed time delays of remaining sources to generate shifted time delays of the remaining sources.

4. The method of claim 3 , wherein dividing the duration of the shot intervals into the set of duration groups includes determining the number of duration groups based on a ratio of the number of duration groups to the number of traces in an operator length of a data window, the ratio equal to one or greater than one.

5. The method of claim 3 , pseudo randomly selecting the duration group includes:

randomly arranging the duration groups in the set into a sequence; and

for each shot interval, selecting the duration group according to the sequence.

6. The method of claim 3 , wherein calculating the shifted time delays for the multiple sources includes:

selecting a time delay in the randomly selected duration group;

calculating a difference between the time delay and the selected time delay; and

adding the difference to the time delay of each of the multiple sources to generate a shifted time delay for each source.

7. The method of claim 6 further comprises when a shifted time delay associated with a source is greater than the duration of the shot intervals, subtracting the duration of the shot interval from the shifted time delay to generate a wrap-around shifted time delay associated with the source, the wrap-around shifted time delay within the shot interval.

8. The method of claim 1 further comprises

resetting the multiple sources for a subsequent shot interval; and

allowing a waiting period to elapse between shot intervals.

9. The method of claim 1 wherein firing the multiple sources within the shot interval based on the shifted time delays includes firing the sources in order from the shortest shifted time delay to the longest shifted time delay with respect to the beginning of the shot interval.

10. The method of claim 1 wherein each source further comprises air guns, water guns, marine vibrators, or an electromagnetic-field source.

11. A computer system for pseudo-randomizing the order in which multiple sources are fired in simultaneous source acquisition, the computer system comprising:

one or more processors;

one or more data-storage devices; and

a routine stored in one or more of the one or more data-storage devices that when executed by the one or more processors controls the computer system to carry out:

generating a set of seed time delays;

assigning each seed time delay to one of the multiple sources towed by one or more marine survey vessels;

for each shot interval of a marine-survey-time line,

randomly selecting a time delay that is less than a duration of the shot interval;

pseudo-randomly shifting the seed time delays based on the time delay to generate shifted time delays within the shot interval, each shifted time delay associated with one of the multiple sources; and

firing the multiple sources within the shot interval based on the assigned shifted time delays.

12. The system of claim 11 , wherein generating the set of seed time delays includes randomly generating seed time delays that are within the duration of the shot intervals.

13. The system of claim 11 further comprises

dividing a duration of the shot intervals of the marine-survey-time line into a set of duration groups;

pseudo-randomly arranging the duration groups into a schedule of duration groups; and

for each shot interval of the marine-survey-time line,

pseudo randomly selecting a duration group from the set of duration groups;

randomly selecting the time delay from the selected duration group; and

pseudo-randomly shifting the seed time delays to generate the shifted time delays by

assigning the time delay to the source of the multiple sources with a shortest seed time delay,

calculating a difference between the time delay and the shortest seed time delay, and

adding the difference to the seed time delays of remaining sources to generate shifted time delays of the remaining sources.

14. The system of claim 13 , wherein dividing the duration of the shot intervals into duration groups includes determining the number of duration groups based on a ratio of the number of duration groups to the number of traces in an operator length of a data window, the ratio equal to one or greater than one.

15. The system of claim 13 , pseudo randomly selecting the duration group includes:

randomly arranging the duration groups in the set into a sequence; and

for each shot interval, selecting the duration group according to the sequence.

16. The system of claim 13 , wherein calculating the shifted time delays for the multiple sources includes:

selecting a time delay in the randomly selected duration group;

calculating a difference between the time delay and the selected time delay; and

adding the difference to the time delay of each of the multiple sources to generate a shifted time delay for each source.

17. The system of claim 16 further comprises when a shifted time delay associated with a source is greater than the duration of the shot intervals, subtracting the duration of the shot interval from the shifted time delay to generate a wrap-around shifted time delay associated with the source, the wrap-around shifted time delay within the shot interval.

18. The system of claim 11 further comprises

resetting the multiple sources for a subsequent shot interval; and

allowing a waiting period to elapse between shot intervals.

19. The system of claim 11 wherein firing the multiple sources within the shot interval based on the shifted time delays includes firing the sources in order from the shortest shifted time delay to the longest shifted time delay with respect to the beginning of the shot interval.

20. The system of claim 11 further comprising storing the shifted time delays in the one or more data-storage devices.

21. The system of claim 11 wherein each source further comprises air guns, water guns, marine vibrators, or an electromagnetic-field source.

22. A computer-readable medium having machine-readable instructions encoded thereon for enabling one or more processors of a computer system to perform the operations of

generating a set of seed time delays;

assigning each seed time delay to one of two or more sources towed by one or more marine survey vessels;

randomly selecting a time delay that is less than a duration of the shot interval;

generating pseudo-randomly shifted time delays based on the time delay for each shot interval of a marine-survey-time line, each shifted time delay assigned to one of the two or more sources; and

firing the two or more sources within each shot interval based on the assigned shifted time delays.

23. The medium of claim 22 further comprises

dividing a duration of the shot intervals of the marine-survey-time line into a set of duration groups;

pseudo-randomly arranging the duration groups into a schedule of duration groups; and

for each shot interval of the marine-survey-time line,

pseudo randomly selecting a duration group from the set of duration groups;

randomly selecting the time delay from the selected duration group; and

pseudo-randomly shifting the seed time delays to generate the shifted time delays by

assigning the time delay to the source of the multiple sources with a shortest seed time delay,

calculating a difference between the time delay and the shortest seed time delay, and

adding the difference to the seed time delays of remaining sources to generate shifted time delays of the remaining sources.

24. The medium of claim 23 , wherein dividing the duration of the shot intervals into duration groups includes determining the number of duration groups based on a ratio of the number of duration groups to the number of traces in an operator length of a data window, the ratio equal to one or greater than one.

25. The medium of claim 23 , pseudo randomly selecting the duration group includes:

randomly arranging the duration groups in the set into a sequence; and

for each shot interval, selecting the duration group according to the sequence.

26. The medium of claim 23 , wherein calculating the shifted time delays for the sources includes:

selecting a time delay in the randomly selected duration group;

calculating a difference between the time delay and the selected time delay; and

adding the difference to the time delay of each source to generate a shifted time delay for each source.

27. The medium of claim 26 further comprises when the shifted time delay associated with a source is greater than the duration of the shot intervals, subtracting the duration of the shot interval from the shifted time delay to generate a wrap-around shifted time delay associated with the source, the wrap-around shifted time delay within the shot interval.

28. The medium of claim 23 further comprises

resetting the two or more sources for a subsequent shot interval; and

allowing a waiting period to elapse between shot intervals.

29. The medium of claim 23 wherein firing the sources within the shot interval based on the shifted time delays includes firing the sources in order from the shortest shifted time delay to the longest shifted time delay with respect to the beginning of the shot interval.

30. The medium of claim 22 wherein each source further comprises air guns, water guns, marine vibrators, or an electromagnetic-field source.

31. A marine seismic data acquisition system comprising:

multiple sources; and

a computer system for operating the seismic source comprising:

one or more processors;

one or more data-storage devices; and

a routine stored in one or more of the one or more data-storage devices that when executed by the one or more processors controls the computer system to carry out:

generating a set of seed time delays;

assigning each seed time delay to one of the multiple sources towed by one or more marine survey vessels;

for each shot interval of a marine-survey-time line,

randomly selecting a time delay that is less than a duration of the shot interval;

pseudo-randomly shifting the seed time delays based on the time delay to generate shifted time delays within the shot interval, each shifted time delay associated with one of the multiple sources; and

firing the multiple sources within the shot interval based on the assigned shifted time delays.

32. The system of claim 31 , wherein generating the set of seed time delays includes randomly generating seed time delays that are within the duration of the shot intervals.

33. The system of claim 31 further comprises

dividing a duration of the shot intervals of the marine-survey-time line into a set of duration groups;

pseudo-randomly arranging the duration groups into a schedule of duration groups; and

for each shot interval of the marine-survey-time line,

pseudo randomly selecting a duration group from the set of duration groups;

randomly selecting the time delay from the selected duration group; and

pseudo-randomly shifting the seed time delays to generate the shifted time delays by

assigning the time delay to the source of the multiple sources with a shortest seed time delay,

calculating a difference between the time delay and the shortest seed time delay, and

adding the difference to the seed time delays of remaining sources to generate shifted time delays of the remaining sources.

34. The system of claim 31 , wherein dividing the duration of the shot intervals into the set of duration groups includes determining the number of duration groups based on a ratio of the number of duration groups to the number of traces in an operator length of a data window, the ratio equal to one or greater than one.

35. The system of claim 31 , pseudo randomly selecting the duration group includes:

randomly arranging the duration groups in the set into a sequence; and

for each shot interval, selecting the duration group according to the sequence.

36. The system of claim 31 , wherein calculating the shifted time delays for the sources includes:

selecting a time delay in the randomly selected duration group;

calculating a difference between the time delay and the selected time delay; and

adding the difference to the time delay of each of the multiple sources to generate a shifted time delay for each source.

37. The system of claim 36 further comprises when a shifted time delay associated with a source is greater than the duration of the shot intervals, subtracting the duration of the shot interval from the shifted time delay to generate a wrap-around shifted time delay associated with the source, the wrap-around shifted time delay within the shot interval.

38. The system of claim 36 further comprises

resetting the sources for a subsequent shot interval; and

allowing a waiting period to elapse between shot intervals.

39. The system of claim 31 wherein firing the sources within the shot interval based on the shifted time delays includes firing the sources in order from the shortest shifted time delay to the longest shifted time delay with respect to the beginning of the shot interval.

40. The system of claim 31 further comprising storing the shifted time delays in the one or more data-storage devices.

41. The system of claim 31 wherein each source further comprises air guns, water guns, marine vibrators, or an electromagnetic-field source.

42. A method of generating source activation times, the method comprising:

generating a random sequence of seed time delays;

assigning one of the seed time delays to each source of multiple sources; and

for each shot interval of a marine-survey-time line,

randomly selecting a time delay that is less than a duration of the shot interval;

randomly generating a time shift based on the time delay, and

applying the time shift to each of the seed time delays.

43. The method of claim 42 , wherein generating the random sequence of seed time delays includes generating the seed time delays within a duration of the shot intervals.

44. The method of claim 42 wherein randomly generating the time shift, and applying the time shift to each of the seed time delays further comprises:

dividing the duration of the shot intervals into time duration groups;

randomly ordering the duration groups into a round-robin schedule;

for a first shot interval of the shot intervals of the marine-survey-time line,

assigning the random sequence of seed time delays to the sources as source firing time, and

identifying the duration group in the round-robin schedule that contains the shortest time delay of the sequence of time delays; and

for each shot interval after the first shot interval,

randomly selecting the time delay in a next duration group of the round-robin schedule,

calculating the time shift as a difference between the shortest time delay in the sequence of time delays and the randomly selected time delay,

adding the time shift to the seed time delays assigned to each of the sources, and

assigning the shifted time delays to the sources as source firing times.

45. The method of claim 44 , further comprising when a shifted time delay associated with a source is greater than the duration of the shot interval, subtracting the duration of the shot interval from the shifted time delay to generate a wrap-around shifted time delay associated with the source within the shot interval.

46. The method of claim 44 , wherein dividing the duration of the shot intervals into the duration groups includes determining the number of duration groups based on a ratio of the number of duration groups to the number of traces in an operator length of a data window, the ratio equal to one or greater than one.

47. The method of claim 42 , further comprising

resetting the multiple sources for a subsequent shot interval; and

allowing a waiting period to elapse between shot intervals.

48. The method of claim 42 wherein further comprising firing the sources in order from the shortest shifted time delay to the longest shifted time delay with respect to the beginning of the shot interval.

49. The method of claim 42 , further comprising firing the sources in each shot interval with the assigned time delays shifted by the time shift generated for the shot interval.

50. The method of claim 42 , further comprising storing shifted time delays for each shot interval in a physical data storage device.