SOURCE ARRAY CONFIGURATION FOR REPEATED MARINE SEISMIC SURVEYING OF THE SAME AREA

Abstract

A method for conducting a seismic survey for collecting seismic data off shore. It comprises a first seismic survey providing a first set of data for an area using individual seismic sub-source arrays (2). It further comprises a second seismic survey of the same area for providing a second set of data. The individual seismic sub-source arrays (2) are similar to the sub-source arrays used during the former survey and are arranged in more than two shot-unit sources (3). Each shot-unit source (3) comprises a pair of neighboring individual seismic sub-source arrays (2) arranged to be fired substantially at the same time.

Claims

1-15. (canceled)

16. A method for conducting a seismic survey for collecting seismic data off shore comprising: towing a seismic source array comprising individual seismic sub-source arrays, towing the seismic source array behind a marine vessel in a towing direction generating desired acoustic signals by activating the seismic source array by releasing acoustic shots means, and said acoustics signals are reflected when hitting a seabed, said reflected signals are picked up by streamer cables each carrying a plurality of receivers, and said reflected acoustic signals are transferred into seismic pictures, the method comprises a first seismic survey of an area comprising firing the individual seismic sub-source arrays arranged in two sources: a first source and a second source placed with an average-distance L between them measured in a direction perpendicular to the towing direction, said second source is fired after the first source when a certain distanced T in the towing direction has been completed, and whereby a first set of data is provided by repeating the shooting, the method further comprises a second seismic survey of the same area for providing a second set of data, where the individual seismic sub-source arrays are similar to the sub-source arrays used during the former survey, and a pair of neighboring individual seismic sub-source arrays are arranged to be fired substantially at the same time providing a shot-unit source wherein the individual seismic sub-source arrays are arranged in more than two shot-unit sources, that each shot-unit source is released after a forgoing release of a shot-unit source has taken place, and each shot-unit source comprises a pair of neighboring seismic sub-source arrays, both sub-source arrays being different from the pair of seismic sub-source arrays being fired as the following shot-unit source by which a first series of fired shot-unit sources are provided.

17. The method according to claim 16, wherein an outermost placed first shot-unit source is fired, and an opposite outermost placed second shot-unit source arranged at the distance L from the other outermost placed first shot-unit source is fired when the distance T is completed, and that at least one shot-unit source is placed in-between said outermost placed shot-units and fired before the opposite outermost placed second shot-unit source is fired.

18. The method according to claim 16, wherein the first series of fired/released shot-units sources are repeated following same pattern of releasing the shot-units sources as the first series.

19. The method according to claim 16, wherein the number of fired/released shot-unit sources in a series and fired between the two outermost placed shot-unit sources during travelling the distance T is the same, as the number of shot-unit sources fired between the last fired outermost placed shot-unit source in a series and a firing start of a subsequent series.

20. The method according to claim 16, wherein the individual seismic sub-source arrays arranged in more than two shot-unit sources comprise at least six seismic sub-source arrays arranged substantially parallel to each other and to the towing direction and with the same distance between them, said the seismic sub-source arrays are numbered consecutive starting with number one at one of the outermost placed sides, and the seismic sub-source arrays are paired for firing in the following sequence: firing number 1 and 2, then firing number 3 and 4, then firing number 1 and 2, then firing number 5 and 6, then firing number 2 and 3 and finally firing number 4 and 5, by said sequence a first series of shot-unit sources 3 are provided said sequence is to repeated.

21. The method according to claim 16, wherein the first series are repeated, and that the distance in the towing direction between each released shot-unit source is ⅓ of the distance T, and the number of individual sub-source arrays is six.

22. The method according to claim 16, wherein the first seismic survey has been performed by a dual seismic source array comprising six sub-source arrays arranged substantially parallel in relation to each other in the towing direction, said the seismic sub-source arrays are numbered consecutive starting with number 1 at one of the outermost placed sides, obtaining seismic data for a base line survey and providing a 3D picture of the seabed, and that a corresponding seismic source array is used after a time period where the individual seismic sub-source arrays are fired in the sequence, where two neighboring seismic sub-source arrays are fired simultaneously thereby providing the shot-unit sources, firing number 1 and 2, then firing number 3 and 4, then firing number 1 and 2, then firing number 5 and, then firing number 2 and 3 and finally firing number 4 and 5, by said sequence the first series of fired shot-units sources are provided and which are to be repeated, whereby a duplicate of the fired dual seismic source is obtained for the first shot-unit sources and the fourth shot-unit source.

23. The method according to claim 16, wherein the distance between the individual sub-source arrays is L/4, and that the new set of data provides a grid having a size of L/8*T/3.

24. The method according to claim 16, wherein the first series is repeated, whereby the shot-unit sources are released in the same order as the first series, whereby a duplicate of a position of the dual seismic source is provided for shots provided by shot-unit sources having the number 1, 4, 7 - - - that is number 1+n*3 where n is a consecutive integer.

25. The method according to claim 16, wherein the number of sub-source arrays used for collecting the second set of data is the same as the number of sub-source arrays used for collecting the first set of data.

26. The method according to claim 16, wherein the first source comprises three sub-source arrays, and the second source comprises three sub-source arrays.

27. The method according to claim 16, wherein the seismic data obtained are processed in a process-unit such as a data processor, whereby the data are analyzed in connection with a former baseline survey.

28. The method according to claim 16, wherein each seismic sub-source array comprises a cluster of guns spaced along a longitudinal means such as a wire or a cable, said each gun on a seismic sub-source array is activated independent of the other guns on the same seismic sub-source array, and that a fired gun is reloaded in the time interval between firing of said gun has taken place and firing of other guns, whereby said gun is ready to be fired again.

29. The method according to claim 16, wherein a data-grid for providing the second set of data for analyzing the seabed is obtained.

30. The method according to claim 16, wherein the data-grid is in the size of 6.25m*6.25 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] FIG. 1 is a view of a known dual source system arranged to be fired alternately between first source and second source.

[0065] FIG. 2 is a view of the same system but arranged to be fired as a five shot-unit sources.

[0066] The invention will be explained with reference to the figures.

[0067] FIG. 1 shows a seismic source array 1 arranged as a dual source comprising a first source 12 and a second source 13. Each source 12, 13 comprises three parallel arranged sub-source arrays 2—gun-strings—comprising a cluster of guns spaced along a cable. The sources 12,13 are arranged symmetrical around a towing direction shown with arrow Y. First source 12 comprises gun-string number 4 denoted D, gun-string number 5 denoted E and gun-string number 6 denoted F; second source comprises gun-string number 1 denoted A, gun-string number 2 denoted B and gun-string number 3 denoted C.

[0068] The towing direction of the seismic source array is the Y direction and the line perpendicular to the towing direction is the X-direction. The crossing between X line and Y line is starting point (0, 0). The mean-distance L between the first source 12 and the second source 13 in the X-direction is determined as the distance between the central axis of the first source 12 and the central axis of the second source. In the example the distance from zero to the first source 12 is 25 m and marked with the number 4 being the center axis of the first source and the distance from zero to the second source 13 is −25 m and marked with the number 5 being the center axis of second source. Further, the shooting interval T in the towing direction is chosen to be 25 meter.

[0069] The first survey of a seabed area is provided with the dual source shown in FIG. 1.

[0070] The result is as follows as shooting takes place with the interval of 25 meter in the Y direction and in X direction alternating between first source and second source:

TABLE-US-00001 TABLE 1 Dual Source at 25 m SPI (interval in Y-direction between shooting) Shot X Y 1 −25 0 2 25 25 3 −25 50 4 25 75 5 −25 100 6 25 125 7 −25 150 8 25 175 9 −25 200 10 25 225 11 −25 250 12 25 275 13 −25 300 14 25 325 15 −25 350 16 25 375

[0071] A similar or the same seismic source array 1 is used for providing a new set of data for the same area. This may be done month or years after.

[0072] However, the sub-source arrays 2 are arranged and fired in another way.

[0073] The gun strings 2 are paired and fired two and two in such a way that paired neighboring gun strings 2 are fired substantially at the same time thereby making a more intensive shooting and each paired gun-string 2 provides a shot-unit source 3.

[0074] The distance between neighboring gun-strings 2 is the same in the X-direction and is L/4 which in this case is 12.5 meter. The new set of data provided by using the seismic source array 1 shown in FIG. 2 will then cause that the grid-size in X direction is 12.5/2 meter, that is 6.25 meter.

[0075] The shot-unit sources 3 are provided as follows:

[0076] number A and B are providing the first shot-unit source 6; number C and D are providing the second shot-unit source 7; number E and F are providing the third shot-unit source 9; number B and C are providing the fourth shot-unit source 10 and D and E is providing the fifth shot-unit source 11.

[0077] This sequence provides a series that are repeated. The first shot-unit source 3 is used twice in a series.

[0078] The paired gun strings A,B and E,F are at the outermost sides. In the X direction they are placed such that the middle line between two paired gun strings 2 is placed with the same X-coordinate as is the case with the central axis of first source and of second source during the sampling of the first dataset (In this case +25 m and −25 meter.)

[0079] Source 1, 2, 3, 4, 5 in table 2 refer to the sequence of firing the paired sub-source arrays 2, and in FIG. 2 shown with the same numbers placed in a band connecting neighboring sub-source arrays/gun-strings 2.

[0080] The sequence takes place as follows:

TABLE-US-00002 TABLE 2 Penta Source at 8.3 m SPI Shot-unit Source # Shot X Y Gun Strings 1 1 −25 0.0 1 + 2 2 2 0 8.3 3 + 4 1 3 −25 16.7 1 + 2 3 4 25 25.0 5 + 6 4 5 −12.5 33.3 2 + 3 5 6 12.5 41.7 4 + 5 1 7 −25 50.0 1 + 2 2 8 0 58.3 3 + 4 1 9 −25 66.6 1 + 2 3 10 25 75.0 5 + 6 4 11 −12.5 83.3 2 + 3 5 12 12.5 91.6 4 + 5 1 13 −25 100.0 1 + 2 2 14 0 108.3 3 + 4 1 15 −25 116.6 1 + 2 3 16 25 125.0 5 + 6 4 17 −12.5 133.3 2 + 3 5 18 12.5 141.6 4 + 5 1 19 −25 150 1 + 2 2 20 0 158.3 3 + 4 1 21 −25 166.6 1 + 2 3 22 25 175 5 + 6 4 23 −12.5 183.3 2 + 3 5 24 12.5 191.6 4 + 5 1 25 −25 200 1 + 2 2 26 0 208.3 3 + 4 1 27 −25 216.6 1 + 2 3 28 25 225 5 + 6

[0081] The line is a marking end of a series and the beginning of a new.

[0082] The shooting interval in the Y-direction is now T/3 that is 25/3 meter. The table shows a shooting sequence that allows 8.33 m inline shot interval—that is in Y-direction—for the five designated shot-unit sources 3 where every third shot repeats a shot position of the dual source baseline shots shown in table 1.

[0083] Thereby the additional sub-source arrays increase the overall density of 3D sampling while maintaining as a subset the shot positions and 3D grid used for the original baseline survey.

[0084] The example shown in table 1 and table 2 is shown using six gun-strings/sub-source arrays 2. However, the number of gun strings 2 could be more or less if convenient. The number of gun strings used for providing the second set of data just has to be the same as is the case when providing the first set of data.