SOURCE ARRAY FOR MARINE SEISMIC SURVEYING

Abstract

The invention provides a system (100) for marine seismic surveying, comprising a towing vessel (110) with a controller, a source array (120) and a receiver array (130) with several streamers (131). The source array (120) comprises n>4 identical subarrays (121) configured as at least (n1) seismic sources S.sub.1, . . . , S.sub.n-1, wherein adjacent subarrays (121) are part of at least two sources S.sub.i, S.sub.j at different times.

Claims

1-10. (canceled)

11. A system for marine seismic surveying, comprising: a towing vessel with a controller, a source array and a receiver array with several streamers, wherein the source array comprises n4 identical subarrays configured as at least (n1) seismic sources S.sub.1, . . . S.sub.n-1, wherein adjacent subarrays are part of at least two sources S.sub.i, S.sub.j at different times.

12. The system according to claim 11, wherein two sources S.sub.i, S.sub.j fired within a minimum time interval are separated by a minimum distance.

13. The system according to claim 11, wherein each source S.sub.i comprises at least two adjacent subarrays.

14. The system according to claim 11, wherein the controller is configured to release at least one acoustic pulse from each seismic source S.sub.i during each period of twice the recharging time T for a subarray.

15. The system according to claim 11, wherein a source S.sub.i is fired with a random offset t in consecutive periods T.

16. The system according to claim 11, wherein the source array is displaced laterally from a centreline through the receiver array.

17. The system according to claim 11, wherein the source array is located behind the receiver array.

18. The system according to claim 11, wherein the receiver array has a fanned configuration.

19. The system according to claim 11, wherein the receiver array has a curved configuration.

20. The system according to claim 11, wherein the receiver array has a feathered configuration due to underwater currents at a towing depth.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention will be described by way of example and reference to the accompanying drawings, in which:

[0030] FIG. 1 illustrates a system according to the invention;

[0031] FIG. 2 illustrates a general scheme for source configuration;

[0032] FIG. 3 illustrates a special case of the scheme in FIG. 2;

[0033] FIG. 4 illustrates an embodiment with a fanned streamer configuration;

[0034] FIG. 5 illustrates other obvious configurations of a discrete data acquisition device; and

[0035] FIG. 6 illustrate a fanned and feathered configuration common in the art.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0036] The drawings are schematic and intended to illustrate the invention. Thus, they are not to scale, and numerous details known to one skilled in the art are omitted for clarity.

[0037] FIG. 1 illustrates a system 100 for marine seismic surveying, the system comprises a seismic survey vessel 110 towing a source array 120 and a receiver array 130. Here, an x-axis along the centreline of the survey vessel 110 indicates the direction of towing and a y-axis indicates a crossline direction.

[0038] The source array 120 comprises n subarrays 121 numbered 1 to n arranged in the crossline direction. Subarrays 1 and n are too far apart to form a source, so n subarrays form at most n1 sources S.sub.1-S.sub.n-1. Each source S.sub.i is located on the line between subarray i and the adjacent subarray i+1. The main benefit is that each of n1 sources emits twice the energy of a single subarray at the cost of one extra subarray. The acoustic pulses emitted during the survey should be as equal as possible, so the sources S.sub.1-S.sub.n-1 should have identical specifications. In this case, the resolution is half the source separation. That is, the space between sources may, for example, be 12.5 m for a crossline resolution of 6.25 m. The source arrays can also be mirrored in order to control directivity in shallow water zones.

[0039] Specifically, each source S.sub.i should be small in time and space compared to seismic wavelengths of interest. If not, the approximation of a pulse with Dirac's delta localised in time and space becomes uncertain. Passing a significant uncertainty through a nonlinear process may render the resulting model of the underground even more uncertain or invalid. Moreover, the shots should contain approximately the same amount of energy distributed in narrow pulses of similar width and height. Thus, we use two subarrays per source in the following examples. However, a source may comprise 3 or more subarrays depending on the seismic wavelength of interest and the size of the subarrays. Likewise, the subarrays may be arranged in a polygon rather than in a row. In practice, this would mean towing the subarrays at different depths to obtain separations comparable to that of two sub arrays side by side. We believe the added complexity outweighs the benefit of an n.sup.th source in a source array already containing (n1) sources in most practical embodiments.

[0040] The receiver array 130 in FIG. 1 has eight streamers 131. However, it is fully feasible to tow 12 or more streamers as noted in the introduction. Lead-in cables, paravanes, birds and other means to tow, spread and steer the streamers 131 are omitted from FIG. 1, but will be part of a real embodiment. Each streamer 131 comprises several seismic receivers 132, e.g. hydrophones of known design, and a tail buoy 133, also of a known type. Today, streamers 131 are typically 1-20 km from their head end to the tail buoy 133. An uneven separation of streamers from inner (closest to centreline) to outer (furthest from centreline) may be utilised to design a pattern of CMP locations per area covered by the in-sea equipment and taken advantage of in order to increase acquisition efficiency.

[0041] In that case, the distance between the streamers closest to the centreline x is smaller than the distance between the outermost streamers. Thus, the midpoints between the receivers 132 and one of the sources 121 vary. This increases the density of reflection points as described by Greene mentioned in the introduction.

[0042] FIG. 2 illustrates that the sources S.sub.i must be separated in time and space to be discernible from each other. In this example, we assume that adjacent sources S.sub.i and S.sub.i+1 must be separated by a minimum time interval t.sub.min and that two sources S.sub.i and can be fired within this time interval if they are not adjacent, i.e. with t<t.sub.min if j(i1).

[0043] In FIG. 2, each subarray is represented by a circle indicating a shot and an open arrow indicating a time T required for recharging. Source S.sub.1 comprises subarrays 1 and 2 and is fired at t.sub.0=0. Source S.sub.3 is fired at t.sub.min for a reason explained below. S.sub.5 is neither adjacent to S.sub.1 nor to S.sub.3 and may thus be fired at an arbitrary time t<t.sub.min.

[0044] At T+t.sub.min, recharging subarrays 3 and 4 is complete, and subarray 3 is combined with subarray 2 into source S.sub.2. Subarray 4 is also recharged, and might be combined with subarray 5 into S.sub.4. However, S.sub.4 and S.sub.5 are adjacent, and must thus be separated at least by t.sub.min. Keeping in mind that t is arbitrary and can be close to zero, S.sub.4 cannot be fired before T+2t.sub.min. S.sub.3 was not fired until t.sub.0+t.sub.min for the same reason.

[0045] If we require T+2t.sub.min<2T, it follows that t.sub.min<T/2. With this requirement, t.sub.0 may be shifted to t and the process above repeated with S.sub.5 replacing S.sub.1 and S.sub.4 replacing S.sub.3.

[0046] The arbitrary interval t may be fixed, e.g. 0 or t.sub.min/r, where r is a real scalar. Alternatively, t may be a random variable. Pseudorandom noise with a triangular probability density functions (pdf) added to the input is generally known to minimise autocorrelation between signals and input in discrete systems, so a pseudorandom t with a triangular pdf may be preferred.

[0047] Moreover, the scheme in FIG. 2 applies to any number n4 subarrays. For example, removing subarray 6 would remove S.sub.5, but leave S.sub.1-S.sub.4 intact. There would still be room for a fixed or random t in the interval [t.sub.min, T>.

[0048] Further removing subarray 5 would leave S.sub.1-S.sub.3 intact and permit firing of S.sub.2 within 2T from t.sub.0 when S.sub.1 was fired. The survey may need a minimum time separation t.sub.min<2T/3, e.g. because the real filtering is done after a Fourier transform to an fk-domain. In this case, subarray 2 defines a minimum time 2T for completing the firing sequence S.sub.1, S.sub.3, S.sub.2 in FIG. 2. Adding subarray >6 would permit extra arbitrary variables t.sub.p.

[0049] FIG. 3 illustrates a round-robin shot sequence with fixed intervals. The period of the round-robin scheme has historically been dictated by the desired seismic record length in milliseconds due to the inability to record and subsequently separate overlapping records.

[0050] Advances in acquisition and processing technology now permit this invention to become practical. The period dictates the seismic fold. As in FIG. 2, six subarrays form 5 sources S.sub.1-S.sub.5, each comprising two adjacent subarrays i and i+1. For convenience, only the indices of the sources are shown in FIG. 3. We assume a recharge time of 6 seconds. Noting that sources 1 and 2 include subarray 2, which needs 6 seconds for recharging, source 2 is not fired immediately after source 1. Rather, the sources are fired in the order 1, 3, 5, 2, 4 at fixed intervals of 3 seconds. The column Distance illustrate the distance traveled with a typical towing speed.

[0051] The scheme in FIG. 3 is a special case of the general scheme in FIG. 2. For example, setting t=0 and t.sub.min=T/2 in FIG. 2 would yield an alternative shot sequence 1, 5, 3, 2, 4. In both FIGS. 2 and 3, the recharge time for source S.sub.4 extends beyond 2T.

[0052] FIG. 4 shows an embodiment with two source arrays 120a and 120b displaced from the towing line. One or both source arrays 120a, 120b may be towed by the vessel 110 or by separate vessels. Either way, the subarrays are combined into sources as described above in order to improve the illumination of the underground from different angles.

[0053] FIG. 4 also show a fanned out streamer configuration, i.e. a configuration in which each streamer 131 forms an angle 0 with the centreline. The main benefit of a fan is that a larger area is covered in each leg of the survey. The main challenge is towing the fan in adjacent legs to provide a sufficient overlap between the outermost streamers, yet not so much that the benefit of the fan disappears. This will be further discussed with reference to FIG. 6.

[0054] FIG. 5 further illustrates configurations lacking an inventive step as such. Specifically, the vessel 110 may have any location, speed and heading determined by the survey at hand. Similarly, it is irrelevant whether source array 120a is towed by vessel 110 or another vessel as long as the locations of each source and each receiver 132 in time and geodetic coordinates are sufficiently accurate. The location of the source array 120c behind the receiver array 130 may affect a moveout correction, but does not alter any principle for discrete sampling of a wavefield or marine seismic data acquisition. Finally, it is generally known that a freely suspended cable assumes a catenary or hyperbolic shape to minimise tension, stress and strain. Likewise, it is generally known that the hyperbolic shape changes to a parabolic shape when an inline pull is applied to the cable. Thus, minimising the noise from birds generally means to use birds as little and possible, and allow the streamers to assume the parabolic shape in FIG. 5. Using birds as little as possible is not inventive. Neither is the resulting parabolic shape of the fanned streamers 131 in FIG. 5.

[0055] In FIG. 6, the survey vessel tows the receiver array in FIG. 3 to cover an area 201. Due to currents, the centreline of the receiver array is displaced from the towing direction by a feather angle . Such feathering may be significant. For example, =1 causes a crossline deviation of 175 m for a receiver 10 km from the leading end.

[0056] The dotted towing vessel illustrates an adjacent return path covering the area 202. The areas 201 and 202 overlap in the overlap area 203, which should be wide enough to ensure proper coverage by the sparsely spaced aft receivers, but not so wide that the number of measurements becomes unnecessarily highas would the time and cost for the survey. Such feathering is well known to anyone of ordinary skill in the art, and may affect the position and orientation of a source array. As indicated above, the configuration of the data acquisition device is irrelevant as long as it provides a proper discrete sampling of the responses or wavefield caused by a series of Dirac's deltas localised in time and space.

[0057] Thus, the invention defined in the appended claims regards an inventive source configuration and shot sequence, not configurations of a discrete data acquisition device that are known or obvious as such. The skilled person will recognise the above and other obvious embodiments within the scope of the present invention.