Methods and systems for conducting reconnaissance marine seismic surveys

Methods, systems, and apparatuses are disclosed for conducting reconnaissance marine seismic surveys. In one example method of acquiring a marine seismic survey, a plurality of streamers are towed behind an acquisition vessel, the plurality of streamers defining a swath. An independent source is towed by an independent source vessel above one or more of the plurality of towed streamers.

BACKGROUND

The present invention relates to seismic acquisition in general and, in particular, to methods and systems for conducting reconnaissance marine seismic surveys.

Petrochemical products such as oil and gas are ubiquitous in society and can be found in everything from gasoline to children's toys. Because of this, the demand for oil and gas remains high. In order to meet this high demand, it is important to locate oil and gas reserves in the Earth. Scientists and engineers conduct “surveys” utilizing, among other things, seismic and other wave exploration techniques to find oil and gas reservoirs within the Earth. These seismic exploration techniques often include controlling the emission of seismic energy into the Earth with a seismic source of energy (e.g., dynamite, air guns, vibrators, etc.), and monitoring the Earth's response to the seismic source with one or more receivers in order to create an image of the subsurface of the Earth.

Conventional marine seismic surveys generally involve towing one or more streamer cables with a plurality of receivers behind an acquisition vessel. Each receiver includes, for example, a pressure sensor and/or a particle motion sensor in proximity to one another. The pressure sensor may be, for example, a hydrophone that records scalar pressure measurements of a seismic wavefield. The particle motion sensor may be, for example, a three-component geophone that records vectorial velocity measurements of the seismic wavefield. By observing the reflected seismic wavefield detected by the receiver(s) during the survey, the geophysical data pertaining to reflected signals may be acquired and these signals may be used to form an image indicating the composition of the Earth near the survey location.

It can be very expensive to acquire and process the data collected in towed-streamer marine seismic surveys. The acquisition can be expensive because of the large amount of equipment and coordination required to plan and carry out a towed-streamer survey. Conventional 3D towed-streamer acquisition geometries typically include 5-15 streamers towed, together with one or more sources, behind an acquisition vessel, with the streamers spaced at 50-100 meters and towed at a depth of about 10 meters. The vessels usually tow the streamers in a “racetrack” geometry, with swaths defined by the streamers overlapping by 50% or more in successive acquisition lines. In many cases, the reason for such large overlap is because of the relatively sparse coverage in the near offset range (i.e., for the receivers closest to the vessel and source) that needs to be infilled in subsequent acquisition lines.

DETAILED DESCRIPTION

Features generally relating to one or more improved systems, methods, and/or apparatuses for conducting marine reconnaissance seismic surveys are described. InFIGS. 1A to 11, various acquisition geometries are described for acquiring relatively sparse seismic data in a first, so-called reconnaissance seismic survey. A reconnaissance seismic survey may be an initial exploration survey acquired at a relatively lower cost and/or in a relatively faster manner (as compared with traditional narrow or wide azimuth seismic surveys). As described inFIGS. 1A to 11, the data from one or more of these reconnaissance seismic surveys may be used to target further, more detailed seismic surveys, and the data from the initial, reconnaissance survey may optionally be combined with the data from the second, more detailed survey to process the seismic data in forming, for example, a subsurface image. However, in other embodiments, data from a reconnaissance survey may be used alone and independent of data from any other surveys.

Various embodiments described herein may omit, substitute, or add various procedures or components as appropriate. For instance, it should be appreciated that the methods may be performed in an order different than that described, and that various steps may be added, omitted or combined. Also, aspects and elements described with respect to certain embodiments may be combined in various other embodiments. It should also be appreciated that the following systems, methods, devices, and software may individually or collectively be components of a larger system, wherein other procedures may take precedence over or otherwise modify their application.

Referring first toFIGS. 1A and 1B, one example of a marine seismic acquisition system100suitable for reconnaissance seismic surveys is shown, including an acquisition vessel105towing a plurality of streamers110. Each of the streamers110includes a plurality of receivers115, and each of the receivers115includes one or more sensors, such as a pressure sensor or a particle motion sensor. As shown inFIG. 1A, the streamers110may be towed at a depth D that is deeper than in conventional towed streamer surveys. For example, the streamers110may be towed at a depth of 20-30 meters. In some embodiments, the streamers110may be variable depth or slanted streamers, with the receivers115on a single streamer being towed at a plurality of different depths.

An independent source vessel125tows an independent source130above one or more of the towed streamers110. The independent source vessel125tows the independent source130above the one or more towed streamers110in that the source130is positioned over one or more portions of one or more of the streamers110in the vertical dimension of the water column. In this manner, at different points during the survey, the source130may be positioned directly over various receivers115of the streamers110, thus allowing for zero-offset and near-offset measurements to be made corresponding to the source130. It will be appreciated that the streamers110are generally towed at a depth D that operationally allows for the independent source vessel125and the source130to be towed above the streamers110without tangling or causing other overlapping problems. Hence, as mentioned above, the streamers110may be towed at a depth D of, for example, 20-30 meters.

In some embodiments, and as shown inFIGS. 1A and 1B, the independent source130replaces an acquisition-vessel source (or sources) conventionally towed by the acquisition vessel105. By eliminating the source(s) conventionally towed by the acquisition vessel105, the streamers110may be able to be towed with greater crossline offset spacing than in conventional towed streamer seismic surveys (i.e., a wider overall swath width of the streamers110may be achieved). This is illustrated inFIG. 1B, in which the streamers110-a-1,110-a-2,110-a-3,110-a-4,110-a-5,110-a-6,110-a-7,110-a-8have a relative sparse crossline offset CL—which may be, for example, approximately 150 meters between each adjacent pair of streamers110-a-1,110-a-2,110-a-3,110-a-4,110-a-5,110-a-6,110-a-7,110-a-8. However, it will also be appreciated that in some embodiments of the present disclosure, the acquisition vessel105may still tow one or more acquisition-vessel sources. Also, it will be understood that 150 meters is merely one example of crossline offset CL spacing that may be used, and that the crossline offset CL spacing between the streamers may be more or less than this, including traditional, narrow crossline offset CL spacing. Also, while eight streamers110-a-1,110-a-2,110-a-3,110-a-4,110-a-5,110-a-6,110-a-7,110-a-8are shown inFIG. 1B, this is merely illustrative, and more or fewer streamers may be towed by the acquisition vessel105.

Still referring toFIG. 1B, the plurality of streamers110-a-1,110-a-2,110-a-3,110-a-4,110-a-5,110-a-6,110-a-7,110-a-8may define a swath120, which is the overall width of the streamers110-a-1,110-a-2,110-a-3,110-a-4,110-a-5,110-a-6,110-a-7,110-a-8. More specifically, the swath120may be defined in some embodiments as the width between the leftmost and rightmost receiver115on the streamers110-a-1,110-a-2,110-a-3,110-a-4,110-a-5,110-a-6,110-a-7,110-a-8when towed over a given subsurface region. It is generally understood that the swath120may be wider or narrower than the coverage of Common MidPoint (CMP) bins in the subsurface region—in other words, the width of well-covered subsurface portions maybe wider or narrowed than the width of the streamers.

WhileFIGS. 1A and 1Bshow a single independent source vessel125towing a single independent source130, in other embodiments, including several described below, multiple independent source vessels may tow multiple independent sources (i.e., one or more additional independent source vessels may tow one or more respective additional independent sources above the one or more towed streamers). Also, in some embodiments, each independent source vessel (including in those embodiments with only a single independent source vessel) may tow two or more independent seismic sources. In those embodiments where a plurality of independent sources are towed by either one or a plurality of independent source vessels, the independent sources may be towed at the same or at different depths. So, as just one example, if a first independent sources is towed at a depth of 5 meters, a second independent source may be towed at a depth of 12 meters. Towing multiple independent sources130over the streamers110may provide several different advantages. For example, the multiple sources130may provide a greater diversity of CMP coverage data because of the greater number of source-receiver pairs available. Multiple sources130may also allow one of the sources to be taken temporarily out of service (e.g., if the source130is malfunctioning or needs service) while the other sources130continue to operate. In some embodiments, the sail paths of the remaining sources130may need to be adjusted to account for the source130that was taken out of service. It will be appreciated that by allowing additional sources130to continue to operate even after one source130is taken out of service may improve the overall availability of the system100to continue to acquire seismic data—because, for example, if only a single source was used and that single source is taken offline, the acquisition may need to be stopped until the single source returns to service. Nonetheless, in some instances, only a single source130may be used, as illustrated inFIGS. 1A and 1B.

In some embodiments, the independent source vessel125(and hence the independent source130as well) may undulate as it is towed above the streamers110. To help understand the region over which the independent source vessel125and independent source130may undulate,FIGS. 2A-2Cillustrate various source navigation regions205-a,205-b,205-c. The source navigation regions205-a,205-b,205-cillustrated inFIGS. 2A-2Cgenerally define the width, and in some cases the length, of the area over which the independent source vessel125and/or the independent source130undulate. Of course, in other embodiments, the independent source vessel125may not undulate, but may follow a relatively straight sail line, similar to the sail line of the acquisition vessel105, or generally may move within a defined source navigation region in any manner, including in a pseudorandom pattern, a pattern of moving back and forth, crooked lines, and so forth.

InFIG. 2A, the source navigation region205-ais approximately aligned with the swath defined by the streamers110-a-1,110-a-2,110-a-3,110-a-4,110-a-5,110-a-6,110-a-7,110-a-8—more specifically, the left and right (or outer) sides of the source navigation region205-aare generally aligned with the respective left and right (or outer) sides of the swath defined by the leftmost streamer110-a-8and the rightmost streamer110-a-1. InFIG. 2B, the left and right (or outer) sides of the source navigation region205-bextend beyond the respective left and right (or outer) sides of the swath defined by the streamers110-a-1,110-a-2,110-a-3,110-a-4,110-a-5,110-a-6,110-a-7,110-a-8. InFIG. 2C, the left and right (or outer) sides of the source navigation region205-care approximately aligned with a sub-swath defined by a subset of the plurality of streamers—specifically inFIG. 2C, the left and right sides of the source navigation region205-care approximately aligned with the streamers110-a-5and110-a-6, respectively. In describing the source navigation regions205-a,205-b,205-c, left and right are used here to refer to the crossline width of the streamers with a view towards the acquisition vessel105. Also, whileFIGS. 2A-2Cillustrate three examples of source navigation regions205-a,205-b,205-c, these are provided as examples only, and it will be understood that other source navigation regions may be defined—including for example source navigation regions spanning different subswaths and so forth.

As mentioned above, in some embodiments, the independent source vessel125and the independent source130undulate within the source navigation region. Understanding the different source navigation regions205-a,205-b,205-cpossible fromFIGS. 2A-2C, and now turning toFIGS. 3A-3G, the undulation pattern of the independent source vessel(s)125(e.g., the pre-plotted course planned for the independent source vessels) and the independent source(s)130will now be described. The source navigation region205inFIGS. 3A-3Gmay be any of the source navigation regions205-a,205-b,205-cillustrated inFIGS. 2A-2Cor a different source navigation region altogether.

FIG. 3Aillustrates a single independent source vessel125-a(towing one or more independent sources—not shown) undulating between a left side305of the source navigation region205and the right side310of the source navigation region205in a substantially sinusoidal pattern. The independent source vessel125-aadvances in the same general direction as the acquisition vessel (not shown inFIG. 3A) as it undulates within the source navigation region205.FIGS. 3B-3Esimilarly show one or more source vessels undulating within the source navigation region in a substantially sinusoidal pattern, although the number of independent source vessels, the spacing of the vessels, and the specific sinusoidal pattern followed vary in each figure.

TakingFIG. 3Bnow, three independent source vessels125-b-1,125-b-2,125-b-3are shown undulating within the source navigation region205, each in a substantially sinusoidal pattern. The first independent source vessel125-b-1, however, undulates closer to the left side305than the second and third independent source vessels125-b-2,125-b-3, the third independent source vessel125-b-3undulates closer to the right side310than the first and second independent source vessels125-b-1,125-b-2, and the second independent source vessel125-b-2undulates between the first and third independent source vessels125-b-1,125-b-3such that the paths of the three independent source vessels125-b-1,125-b-2,125-b-3substantially do not overlap. Stated differently, the three independent source vessels125-b-1,125-b-2,125-b-3each undulate in a sinusoidal pattern over respective first, second, and third substantially non-overlapping sub-source navigation regions. The non-overlapping of these sub-source navigation regions may reduce the risk of collision when multiple independent source vessels125-b-1,125-b-2,125-b-3are employed, as inFIG. 3B.

InFIG. 3C, three independent source vessels125-c-1,125-c-2,125-c-3all undulate between the left and right sides305,310of the source navigation region205, but are spaced such that the sinusoidal sail lines they traverse are substantially equally spaced, thereby creating a helix-like pattern of interwoven sinusoids.FIG. 3Dis similar toFIG. 3C, except only two source vessels125-d-1,125-d-2undulate between the left and right sides305,310in spaced apart sail lines. InFIG. 3E, a single independent source vessel125-eis shown, but the “frequency” of the sinusoidal undulation is not kept constant, and instead varies to give a greater diversity of source-receiver pairs in the data recorded.

Referring now toFIGS. 3F and 3G, undulation patterns other than sinusoids are illustrated. InFIG. 3F, two independent source vessels125-f-1,125-f-2undulate in a saw tooth pattern, and inFIG. 3G, a single independent source vessel125-gundulates in a square wave pattern. it will thus be appreciated that the source vessels need not undulate in a sinusoidal pattern, but generally can undulate in any given manner. Further, as illustrated inFIG. 3E, the undulation pattern need not remain constant, but can vary as the independent source vessel advances in the same general direction as the acquisition vessel. In general, whileFIGS. 3A-3Gillustrate some examples of undulation patterns for one or more independent source vessels, it will be understood that there any many variations that are within the scope of the present disclosure. As one example of one such variation, consider that the pattern shown inFIG. 3Eis generally a sinusoid with varying “frequency,” but note that the “amplitude” of the undulation could also be varied (i.e., by having the independent source vessel not traverse all the way to the edges of the source navigation region on some turns, but all the way or beyond the edges on other turns). Also, note that the paths shown inFIGS. 3A-3Gmay be idealized paths for the independent source vessels to follow and that the actual paths followed may vary from those shown due to environmental factors (e.g., choppy water), navigational constraints (e.g., inaccuracies in navigation, a relatively large turn radius of the source vessels), and so forth.

Referring still toFIGS. 3A-3G, and also back toFIGS. 1A and 1B, the position of the independent source vessel(s)125relative to the acquisition vessel105will now be described. In some embodiments, the independent source vessel(s)125may be towed as close to the acquisition vessel105as operationally feasible in order to maximize the offset range of data acquired from the receivers115on the posterior end of the streamers. For example, the first independent source vessel may be towed no more than 500 meters following the independent source vessel, or may be towed a distance following the independent source vessel that is approximately one-half the width of the swath defined by the streamers (e.g., 800 meters). If more than one independent source vessel is used, each of the independent source vessels may be towed at different distances following the acquisition vessel in order to, for example, reduce the risk of collision and provide greater diversity of source-receiver pairs.

In other embodiments, however, the independent source vessel(s)125may be towed near the posterior end of the streamers110(e.g., with the last independent source vessel being towed no more than 500 meters from the posterior end of the streamers110), which may again maximize the offset range of data acquired from the receivers115, except in this case the far offsets would be the receivers115closest to the acquisition vessel.

Turning now toFIG. 4, some of the benefits of acquiring a seismic survey as described inFIGS. 1A-3Gwill now be described. As shown in dotted lines inFIG. 4, a previous acquisition line in the same region was acquired, with the swath defined by the streamers from the previous pass shown as120-d-1. In a subsequent acquisition line, the vessel105is again shown towing the streamers defining a second swath120-d-2, with a spatial overlap region405shown between the first and second swaths120-d-1,120-d-2. In one embodiment, the width (in the crossline direction) of the spatial overlap region405may be no greater than 5, 10, 15, 20, 25, or 30% of the overall width (in the crossline direction) of one or both of the swaths120-d-1,120-d-2, or in other embodiments, there may be no overlap region and instead there may be a gap between the swaths120-d-1,120-d-2. The overlap between successive acquisition lines, if any, may be less than in conventional surveys because the diversity of data acquired from the undulating source vessels may provide enough data to regularize a seismic image using interpolation during the seismic data processing stage. In other words, by acquiring data as described above with reference toFIGS. 1A-3Gsubstantial overlap (e.g., 50%) can be avoided during successive acquisition lines because of the diversity of source-receiver (including near offset coverage) data acquired by towing one or more independent source vessels over the one or more acquisition streamers.

FIG. 5is a flowchart illustrating a method500of acquiring seismic data using the system100shown inFIGS. 1A-4, in accordance with some aspects of the present disclosure. At block505, a plurality of streamers are towed behind an acquisition vessel, with the plurality of streamers defining a swath. At block510, one or more independent sources are towed by one or more respective independent source vessels above one or more of the plurality of streamers. As described above, the one or more independent source vessels may undulate within a defined source navigation region.

In another contemplated embodiment, a survey region encompassing a spread of ocean bottom cables or ocean bottom nodes (i.e., in place of the towed streamers described above) may be defined, and one or more independent sources may be towed by one or more independent source vessels in an undulating manner (e.g., as described above with reference toFIGS. 3A-3G) within a source navigation region, the source navigation region corresponding to one of a plurality of subregions within the survey region. In one example the subregions of the survey region may define a grid, with the grid including at least two subregions in its width dimension and at least two subregions in its height dimension. The grid and subregions may or may not be rectangular, and the subregions may or may not be equally sized. In some embodiments, each of the plurality of subregions of the survey region may be smaller than the overall survey region (e.g., less than 50%, 40%, 30%, 20%, 10%, 5% etc. the size of the overall survey region). The independent source vessel may undulate or move in any manner described above within the subregions, and the subregions may be less than a predefined width and a predefined length, each of which is smaller than the respective overall width and overall length of the survey region. The survey region may be aligned with the placement of the ocean bottom cables or ocean bottom nodes, or may extend outside the placement of such.

In some embodiments, the ocean bottom cables or the ocean bottom nodes may define a grid over which one or more independent source vessels may undulate. In these embodiments, the one or more independent source vessel may undulate over the grid defined by the ocean bottom nodes or the ocean bottom cable in that the independent source vessel traverses over the gridlines defined by the individual cables or nodes, rather than traversing along the gridlines defined by the cables or nodes. The traversing over the gridlines defined by the cables or nodes may be similar to any type of source vessel movement described above, including the undulations shown inFIGS. 3A-3G—for example, the independent source vessel may undulate back and forth between two gridlines (or generally any source navigation region based on the gridlines of the cables or nodes).

Referring now toFIG. 6, another example of an acquisition system600suitable for reconnaissance seismic surveys is shown, and inFIG. 7, a flowchart illustrating a method700of acquiring seismic data using the system600shown inFIG. 6is shown, in accordance with some aspects of the present disclosure. At block705, and as illustrated inFIG. 6, a plurality of streamers are towed behind a primary acquisition vessel, the plurality of streamers defining a primary swath. At block710, and as illustrated inFIG. 6, an independent source is towed by an independent source vessel near an outer edge of a posterior end of the plurality of streamers.

Referring still toFIGS. 6 and 7, in some embodiments, the independent source may be a first independent source, the independent source vessel may be a first independent source vessel, and the outer edge may be a left-most streamer of the plurality of streamers, and the method700may further include towing a second independent source by a second independent source vessel near the posterior end of a right-most streamer of the plurality of streamers. The method700may also further include towing one or more streamers behind each of the first and second independent source vessels, and a length of the one or more streamers towed behind each of the first and second independent source vessels may be less than half a length of the plurality of streamers towed behind the primary acquisition vessel. For example, each of the one or more streamers towed behind each of the first and second independent source vessels may be no more than 2 kilometers long.

In some embodiments, at least one of the one or more streamer cables towed behind one of the first or second independent source vessels may be towed within the primary swath defined by the plurality of streamers towed by the primary acquisition vessel, and/or at least one of the one or more streamer cables towed behind one of the first or second independent source vessels may be towed outside of the primary swath defined by the plurality of streamers towed by the primary acquisition vessel. The plurality of streamer cables towed behind the primary acquisition vessel and the one or more streamer cables towed behind the first and second independent source vessels may in some embodiments all record data associated with the first independent source, the second independent source, and a third source associated with the primary acquisition vessel, and the data recorded by the one or more streamer cables towed behind the first and second independent source vessels may be used to provide near-offset coverage lacking in data recorded by the plurality of streamer cables towed behind the primary acquisition vessel.

In some embodiments, the plurality of streamers may be towed behind the primary acquisition vessel along a first sail line, the primary swath may be a first primary swath associated with the first sail line, and the method700may further include towing the plurality of streamers behind the primary acquisition vessel along a second sail line adjacent to the first sail line, thereby defining a second primary swath. The first and second primary swaths may spatially overlap by no more than 20%, or may substantially no overlap. The primary acquisition vessel may not be towed between the first and second sail lines.

In some embodiments, the posterior end of the plurality of streamers may be a free end of the streamers furthest away from the primary acquisition vessel. Also, the independent source may be towed 100 meters or more following the posterior end of the plurality of streamers, and/or may be towed adjacent to a left-most or right-most streamer of the plurality of streamers but ahead of the posterior end of the plurality of streamers.

In some embodiments, the independent source vessel may undulate between a leftmost edge of a source navigation region and a rightmost edge of the source navigation region. A source may be towed behind the primary acquisition vessel. In some embodiments, the plurality of streamers may be fanned out to form a wedge, or may be towed in straight lines parallel to one another.

In some embodiments, the independent source vessel may be towed within a predetermined distance from the outer edge of the posterior end of the plurality of streamers, with the predetermined distance being, for example, no more than 15% of an overall length of the plurality of streamers.

Referring now toFIG. 8, another example of an acquisition system800suitable for reconnaissance seismic surveys is shown, and inFIG. 9, a flowchart illustrating a method900of acquiring seismic data using the system800shown inFIG. 8is shown, in accordance with some aspects of the present disclosure. At block905, and as illustrated inFIG. 8, a first plurality of streamers are towed behind a first primary acquisition vessel. At block910, and as illustrated inFIG. 8, a second plurality of streamers are towed behind a second primary acquisition vessel, the second plurality of streamers being towed laterally offset from and behind the first plurality of streamers. At block915, and as illustrated inFIG. 8, an independent source may be towed by an independent source vessel in between the first plurality of streamers and the second plurality of streamers.

Referring still toFIGS. 8 and 9, in some embodiments, a bow of the second primary vessel may never advance ahead of a posterior end of the first plurality of streamers. Also, in some embodiments, the independent source may be towed behind a posterior end of the first plurality of streamers and/or ahead of a bow of the second primary vessel.

In some embodiments, the first plurality of streamers may be towed along a first sail line, the second plurality of streamers may be towed along a second sail line parallel to the first sail line, and the independent source vessel may undulate between the first and second sail lines as it advances. The first and second pluralities of streamers may be fanned out as they are towed behind the respective first and second primary acquisition vessels. The first and second pluralities of streamers may define a first combined swath as they are towed behind the first and second primary acquisition vessels along respective first and second sail lines, and the method900may further include towing the first and second pluralities of streamers behind the first and second primary acquisition vessels along respective third and fourth sail lines adjacent to the first and second sail lines, thereby defining a second combined swath, wherein the first and second combined swaths spatially overlap by no more than 12.5%, 15%, or 25%.

In some embodiments, the method900may further include towing a first acquisition-vessel source behind the first primary acquisition vessel and a second acquisition-vessel source behind the second primary acquisition vessel, and the first plurality of streamers may be towed along a first sail line, the second plurality of streamers may be towed along a second sail line parallel to the first sail line, and the independent source may be towed along a third sail line, with the third sail line being planned so as to provide near offset coverage missing from source-receiver pairs associated with the first and second acquisition-vessel sources. In some embodiments, this third sail line may be closer to first sail line than to the second sail line.

Referring now toFIG. 10, another example of an acquisition system1000suitable for reconnaissance seismic surveys is shown, and inFIG. 11, a flowchart illustrating a method1100of acquiring seismic data using the system1000shown inFIG. 10is shown, in accordance with some aspects of the present disclosure. At block1105, and as illustrated inFIG. 10, a first plurality of streamers are towed behind a first primary acquisition vessel. At block1110, and as illustrated inFIG. 10, a second plurality of streamers are towed behind a second primary acquisition vessel, with the second plurality of streamers being substantially parallel to the first plurality of streamers, the first and second pluralities of streamers defining a combined swath. At block1110, and as illustrated inFIG. 11, an independent source may be towed by an independent source vessel laterally offset from the combined swath.

Referring still toFIGS. 10 and 11, in some embodiments, the first and second primary acquisition vessels may advance abreast along respective first and second sail lines, and independent source vessel may also advance abreast of the first and second primary acquisition vessels along a third sail line. The first, second, and third sail lines may be substantially parallel to one another, and the second sail line may be substantially equidistant from the first and third sail lines. In other embodiments, the independent source vessel may advance along a third sail line ahead of the first and second primary acquisition vessels.

In some embodiments, the independent source vessel may undulate between the first and second sail lines, or generally in between any given two lines. The combined swath may be a first combined swath, and in some embodiments, the method1100may further include towing the first and second pluralities of streamers along respective third and fourth sail lines adjacent to the first and second sail lines, thereby defining a second combined swath, wherein the first and second combined swaths spatially overlap by no more than 10%, or do not substantially overlap at all.

In some embodiments, the first and second plurality of streamers may be towed as close together as operationally feasible—e.g., such that no more than 1500 meters separates a leftmost streamer of the first plurality of streamers and a rightmost streamer of the second plurality of streamers. Also, in some embodiments, the first plurality of streamers may be towed at a first depth and the second plurality of streamers may be towed at a second depth greater than the first depth so that the streamers are less likely to tangle (e.g., in case one primary acquisition vessel needs to cease acquisition while the other continues).

Referring now to the systems100,600,800,1000and the associated methods described above with reference toFIGS. 1A-11, it will be appreciated that the systems100,600,800,1000may be fit for the purpose of relatively sparse seismic data acquisition, with broad swaths, acceptable data quality, and regionally interpretable data, all achieved within a shorter timeframe and/or in a less costly manner than conventional towed streamer seismic surveys through the use of efficient acquisition geometries.

FIG. 12is a flowchart illustrating a method in accordance with still additional aspects of the present disclosure. At block1205seismic data may be acquired in a first, reconnaissance-type seismic survey (using, for example, one of the systems100,600,800,1000described above). At block1210, additional seismic data may be acquired in a second, more detailed seismic survey, and at block1215, the data from the first and second surveys may be combined, and the combined sets of data may be processed together (e.g., to form an image of the subsurface). In general, the first and second seismic surveys may differ in the crossline offset spacing of the streamers, in the speed of the acquisition vessel (and/or the independent source vessel, if any), and so forth.

Specific details are given in the description to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, well-known structures and techniques have been shown without unnecessary detail in order to avoid obscuring the embodiments.