SEISMIC ACQUISITION USING MULTIPLE SOURCES AND SEPARATE SHOOTING VESSELS
This method is directed to marine seismic data acquisition systems and, in one aspect, such systems with a streamer, streamers, and/or one or more seismic sources towed by one or more powered vehicles. Such vehicles may be manned or unmanned and tethered, or not, to a host vessel. This invention is related to the field of Common Mid-Point (CMP) marine seismic data acquisition.
In the field of marine seismic data acquisition, seismic signal sources, e.g. air guns, are towed behind a vessel, which may also tow a set of seismic sensors. The sensors are normally contained in streamers that are comprised of seismic signal receivers, e.g. hydrophones, which are sensitive to reflections and refractions from sound impulses emitted by seismic sources.
It has long been known that for high quality data, multiple samplings of the same subsurface areas are required. The addition of multiple signals from the same sub-surface reflection point results in an increase in the information signal with a cancellation of the noise. This process is commonly known as the Common Mid-Point method.
Figure 1 A shows schematically a marine seismic survey system. A ship 14 tows a seismic source 5 of any known type, for instance air guns. Also towed by ship 15 in this example are one or more sets of seismic sensors 13 usually referred to as streamers. The seismic sensors in the streamers may be single
sensors or groups of sensors. Often streamers and sources are towed by the same vessel, but this varies with survey acquisition logistics. Streamer 13 contains as an example for the illustration 15 sensors or groups of sensors, sensor 11 is closest to source 5 while sensor 19 is furthest away. Modern streamers contain hundreds of sensors or sensor groups.
Figure IB shows how seismic energy propagates from the seismic source 5 to the streamer 13 sensors. Each time a seismic impulse is generated by a source, there is a corresponding midpoint (or Common Mid-Point, CMP) for every seismic sensor in a streamer. Seismic energy generated by source 5 travels to midpoint or CMP 1 and is reflected back to seismic sensor 11, the nearest sensor to source 5 of streamer 13. Seismic energy also reflects off of CMP 2 and travels to seismic sensor 19, the furthest sensor from source 5 of streamer 13. Of course, there are midpoints or CMPs corresponding to each seismic source impulse with each of the streamer 13 sensors. For example, the streamer shown in Figure IB contains 15 seismic sensors, and so each seismic impulse generated by source 5 would produce 15 CMPs from CMP 1 to CMP 2 inclusive.
Figure 1C shows how the addition of another seismic source 7 towed, for example by ship 18, will produce CMPs from a source following the acquisition system in the direction of survey as demonstrated in Figures 1A and IB. Here, the seismic energy travels from seismic source 7 to CMP 4 and back to the seismic sensor 19 nearest the source 7. Seismic energy travels from source 7 to CMP 3 and then the seismic sensor 11 furthest from source 7. Again, there are CMPs for each seismic sensor and seismic source pair, each time a source generates a seismic impulse.
As is known in the art, the number of times the same subsurface midpoint areas are sampled is commonly referred to as the "fold" of the survey. Increase in fold results in an increase in signal-to-noise ratio, which will increase temporal subsurface resolution, and therefore, seismic surveys with a high fold are desirable and operational changes that result in a decrease in fold are not desirable.
Another way to improve subsurface resolution is to increase the areal resolution of a seismic survey by moving sampling bins closer together by decreasing the size of the subsurface sampling areas known as midpoint or CMP bins. Subsurface sampling areas (e.g. CMP spacing) need to be made as small as operational and economic constraints will allow.
Marine seismic operations are often large, complex and expensive. There are recognized needs for improving the efficiency of acquisition equipment deployment and operations. A system and method that will either decrease the size of the CMP spacing (thereby increasing areal resolution) or increase areal coverage per unit of survey time without decreasing fold, all with minimal increases in acquisition equipment deployment and operational logistics is highly desirable. The present invention addresses these recognized needs.
Accordingly, there is a need for a method and system to maintain the fold while increasing the areal coverage in a marine survey without increasing the number of streamers. There is a need for a method and system of reducing the CMP bin spacing without increasing the number of streamers or towing streamers closer together. The prior art discloses a variety of marine seismic systems with one or more streamers and/or one or more seismic sources, some of
which include a main or host vessel and other manned or unmanned vessels, vehicles, or apparatuses such as floats, paravanes, or buoyant members which may be connected to the host vessel by lines, cables or tethers. However, the prior art does not disclose systems or methods to improve the efficiency of the use of acquisition equipment as embodied in the present invention.
The present invention, in certain embodiments, discloses methods of acquiring seismic data and increasing the areal density of CMP coverage or increasing the areal coverage of CMP lines or both by towing a plurality of streamers with one or more vessels wherein the streamers comprise a plurality of seismic sensors for receiving and recording data. The methods include towing at least one seismic source on a first source vessel wherein one or more sources contribute to data acquisition for a first set of CMP survey lines. The methods include towing at least one seismic source on at least one other source vessel wherein at least one source of said at least one other source vessel contributes to CMP survey lines separate from said first set of CMP survey lines.
The novel features which are believed to be characteristic of the invention, both as to organization and methods of operation, together with the objects and advantages thereof, will be better understood from the following detailed description and the drawings wherein the invention is illustrated by way of example for the purpose of illustration and description only and are not intended as a definition of the limits of the invention: FIG. 1A - 1C are illustrations of traditional marine seismic data acquisition. FIG. 2A and 2B illustrate marine seismic data acquisition. FIG. 3 A and 3B illustrate an embodiment of the invention.
FIG. 4 illustrates an alternate embodiment of the invention. FIG. 5 illustrates another embodiment of the invention with lateral source separations substantially equidistant.
FIG. 6 illustrates an embodiment of the invention where areal survey coverage may be increased.
FIG. 7 illustrates an alternative embodiment of the invention where areal survey coverage may be increased.
For the purpose of clarity and explanation, the method of this invention will be described by way of example, but not by way of limitation, with respect to a marine seismic data acquisition system containing four seismic sources with two or four streamers. It is to be clearly understood that the method may be applied to any marine seismic data recording geometry or acquisition system and is not limited in terms of the number of streamers, the number of sources or the particular spacing measurements as used in the example embodiments. The distances between sources and streamers in the 'inline' directions are determined by operational and geophysical considerations. In these examples individual vessels may tow individual streamers and sources, but this is for illustration only. In practice the same vessel may tow many sources and streamers.
Figure 2A shows in plan view how CMPs fall between a source and receiver. Figure 2A shows a seismic survey proceeding substantially parallel to the 'Survey Direction' arrow. A ship or tow vessel 104 with source 50 precedes streamer 130 (towed by vessel 105) in the survey direction. The midpoint between source 50 and sensor 119 of streamer 130 is CMP 20. CMP 20 falls on line of seismic survey 6453. The midpoint between source 50 and sensor 111 of
streamer 130 is CMP 10, also on line of survey 6453. From CMP 20 to CMP 10 on line 6453 include the fifteen midpoints between source 50 and the fifteen example sensor groups of streamer 130. The lateral offset (perpendicular to the direction of survey) between streamer 130 and source 50 is 25 meters in this example, with the result the line of survey 6453 parallel to the direction of survey is 12.5 meters offset in the crosslme direction from streamer 130 and source 50.
Figure 2B shows how bins from a source 70 towed by vessel 108 following streamer 130 in the direction of the survey fall along CMP line of survey 7300. The midpoint between source 70 and sensor 119 is CMP 40. The midpoint between source 70 and sensor 111 is CMP 30. Source 70 is offset 50 meters from streamer 130. CMP line 7300 is 25 meters from both source 70 and streamer 130. Source 70 with streamer 130 receiver 119 will create CMP 40 on line 7300. Source 70 with streamer 130 receiver 111 will create CMP 30 on line 7300.
Figure 3A shows an example 4 streamer with 4-source configuration without midpoint bins represented. Figure 3A shows an example acquisition configuration with sources preceding and following the streamers in the survey direction. This configuration results in substantially uniform distribution of CMP spacings in the crossline and inline directions. The streamers are 100 meters apart. Tow vessel 407 is shown towing streamer 410. Streamer 420 is towed by vessel 405, streamer 400 is towed by 409 and streamer 430 is towed by vessel 403. Sources 470 and 480, separated by 25 meters in this example, are towed by vessel 402. Source 450 is towed by tow vessel 406. Source 460 is
towed by vessel 404. Sources 450 and 460 are separated 75 meters in the crossline direction. In this configuration the lateral separation between sources preceding the streamers is different from the separation of the sources following the streamers in the survey direction. Figure 3B shows all the CMP lines of survey for the four source four- streamer configuration of Figure 3A. Two seismic sources will be preceding the streamers in the survey direction and two behind the streamers. The lines of CMP created by all the sources in Figure 3B are equidistant from adjacent CMP lines of survey. This configuration will lead to 12.5 meter crossline spacing for adjacent CMP lines when 100 meters separate adjacent streamers, as is the case in this example. In this configuration each source contributes to separate sets of CMP lines of survey. Source 450 creates 4 midpoint CMP lines: CMP line 504 with streamer 400, CMP line 514 with streamer 410, CMP line 524 with streamer 420 and CMP line 534 with streamer 430. Source 460 creates 4 midpoint CMP lines: CMP line 604 with streamer 400, CMP line 614 with streamer 410, CMP line 624 with streamer 420 and CMP line 634 with streamer 430. Source 470 creates 4 midpoint CMP lines: CMP line 704 with streamer 400, CMP line 714 with streamer 410, CMP line 724 with streamer 420 and CMP line 734 with streamer 430. Source 480 creates 4 midpoint CMP lines: CMP line 804 with streamer 400, CMP line 814 with streamer 410, CMP line 824 with streamer 420 and CMP line 834 with streamer 430.
A very similar configuration is displayed in Figure 4 where a similar pattern of CMP lines is produced from different source spacing. Source 450 towed by vessel 406 has been replaced by source 490 towed by vessel 408. Here
the lateral source spacing for sources 460 and 490 following the streamers in the direction of survey is substantially equal to the lateral source spacing between 470 and 480 preceding the streamers, in this example, 25 meters. In this configuration each source contributes to separate sets of CMP lines of survey. The difference in replacing source 450 in Figure 3B with 490 in Figure 4 is that CMP lines 504, 514 and 524 are replaced by CMP lines 914, 924 and 934. CMP line 534 has been dropped and CMP line 904 has been added.
Another configuration as shown in Figure 5 is to spread the distribution of sources laterally such that adjacent CMP lines are produced by sources alternately preceding the streamers in the direction of survey and then following the streamers. The streamers are the same as in Figure 3B. Here sources 570 and 580 towed by vessel 501 preceding the streamers are 50 meters apart. Likewise the sources following the streamers, 550 and 560 towed by vessel 503, are 50 meters apart. In this configuration the lateral separation between sources preceding the streamers is substantially equal to the separation of the sources following the streamers in the survey direction. In this configuration each source contributes to separate sets of CMP lines of survey. Source 550 creates 4 midpoint CMP lines: CMP line 505 with streamer 400, CMP line 515 with streamer 410, CMP line 525 with streamer 420 and CMP line 535 with streamer 430. Source 560 creates 4 midpoint CMP lines: CMP line 605 with streamer 400, CMP line 615 with streamer 410, CMP line 625 with streamer 420 and CMP line 635 with streamer 430. Source 570 creates 4 midpoint CMP lines: CMP line 705 with streamer 400, CMP line 715 with streamer 410, CMP line 725 with streamer 420 and CMP line 735 with streamer 430. Source 580 creates
4 midpoint CMP lines: CMP line 805 with streamer 400, CMP line 815 with streamer 410, CMP line 825 with streamer 420 and CMP line 835 with streamer 430.
The acquisition configurations described may be used to either increase areal density in a 3D survey leading to an increase in imaging resolution or to increase the areal coverage leading to an increase in seismic survey acquisition efficiency. These benefits are obtained with present equipment resources and the methods of the present invention. Figures 3 through Figure 5 have shown how the method may be used to efficiently increase imaging resolution by decreasing the CMP bin width spacing. In the cases shown in Figure 3 and
Figure 5, the CMP bin spacing is halved in the crossline direction. The increase in CMP bin density resulting in increased seismic resolution may come with a modest 10% increase in acquisition cost.
An application of the method outlined in either Figure 3B or Figure 5 would be to use 6 to 10 streamers as desired, each for example 4800 meters in length, separated by 100 meters with the sources configured as outlined above to produce CMP lines separated by 12.5 meters in the cross line direction. The sources may then be sequentially fired every 12.5 meters as the survey proceeds. This would produce 48 fold coverage on lines 12.5 meters apart, rather than the traditional coverage of 48 fold coverage on lines 25 meters apart.
Typically the firing sequence of multiple sources will alternate between sources preceding the streamers and then sources following the streamers. For example first a source from the sources preceding the streamers will be fired, such as one of 470 and 480 of Figure 3B, subsequently a source following the
streamers, such as one of 450 and 460 of Figure 3B. The sequence would then alternate back to firing a source preceding the streamers that had not yet fired, and so on. A typical sequence firing order, sources firing in the case of Figure 3B every 12.5 meters, would be 480 followed by 460 followed by 470 followed by 450 and then repeating. Alternatively the sequence could begin with a source following the streamer so a sequence could be 470 followed by 460 followed by 480 followed by 450 and then repeat.
Doubling the seismic footprint using the present invention may increase areal seismic coverage acquisition efficiency as may be seen Figure 6, a doubling of the footprint relative to the parameters of Figure 3B. The streamer separation is 200 meters. Typically the firing sequence will be as described in the previous examples, for example sequentially firing the sources every 12.5 meters. As an example, the lateral separation of 150 meters for sources 650 and 660 following the streamers in the direction of survey is combined with a lateral separation of 50 meters for sources 670 and 680 preceding the streamers. The resulting crossline separation of CMP lines of survey is 25 meters. The example tow vessel 607 tows streamer 610. Tow vessel 609 tows streamer 620. Vessel 603 tows sources 670 and 680. Source 660 is towed by vessel 606. Source 650 is towed by vessel 608. In the usual case one seismic vessel may be capable of towing all the streamers and several of the sources used.
The CMP lines for the example configuration of Figure 6 are 25 meters apart from adjacent CMP lines. Source 650 produces CMP line 651 with streamer 610 and produces CMP line 652 with streamer 620. Source 660 produces CMP line 661 with streamer 610 and produces CMP line 662 with
streamer 620. Source 670 produces CMP line 671 with streamer 610 and produces CMP line 672 with streamer 620. Source 680 produces CMP line 682 with streamer 620.
Figure 7 is a doubling of the seismic footprint relative to Figure 5. The example tow vessels are only for illustration. Tow vessel 707 tows source 730 and streamer 710. Vessel 701 tows source 760. Vessel 709 tows streamer 720. Vessel 706 tows source 750 and vessel 708 tows source 740. The lateral source separation of the sources 730 and 760 preceding the streamers in the survey direction is 100 meters. The lateral source separation of the following sources, 740 and 750, is also 100 meters. The resulting crossline separation of CMP lines of survey is 25 meters. Source 730 produces CMP line 731 with streamer 710 and produces CMP line 732 with streamer 720. Source 740 produces CMP line 741 with streamer 710 and produces CMP line 742 with streamer 720. Source 750 produces CMP line 751 with streamer 710 and produces CMP line 752 with streamer 720. Source 760 produces CMP line 761 with streamer 710 and produces CMP line 762 with streamer 720.
The acquisition method based on the relative lateral positions of sources and streamers as outlined in Figure 6 and Figure 7 while using 4800 meter long streamers will result in CMP lines with 48 fold. The resulting crossline separation of adjacent lines will be 25 meters. Using this invention may lead to savings in the field effort of about 40% compared to traditional methods.
Utilization of sources positioned preceding the streamers in the survey direction and following the streamers has advantageous effects on illumination of the subsurface. Increasing the lateral crossline distance between sources as
outlined above also leads to better subsurface illumination. Positioning the sources preceding and following the streamers so that the sources are substantially equal distances from the nearest sensor of the streamer a source is nearest to may contribute to uniformity of distribution for offsets and dip illumination characteristics of the acquired dataset.
While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope and spirit of the appended claims be embraced by the foregoing disclosure.