System and method for towing acoustic source arrays

[0001] In a variety of marine environments, seismic surveys are conducted to gain a better understanding of geological formations beneath a body of water. Relatively large marine regions can be surveyed by a surface vessel or vessels towing seismic streamer cables through the water. Another vessel, or the same vessel, can be employed in providing seismic sources, such as compressed air guns utilized to generate acoustic pulses in the water. The seismic sources can be arranged in source arrays. In some applications, for example, air guns are suspended from a float and towed behind a seismic vessel. During towing, however, drag forces on the source array deform the shape of the array which can be detrimental to obtaining accurate seismic data.

[0002] Currently, a common technique for creating seismic source arrays is to position all the guns at one layer and to suspend the guns from a float. A tow rope extending from the towing vessel is coupled directly to the gun array. In other applications, the tow rope is connected directly to the float or float layer. In either application, the source array deforms substantially during towing.

[0003] Attempts also have been made to construct source arrays with the guns arranged in layers separated vertically. However, the towing of multilayered gun arrays tends to be far more complex than towing a single gun layer. Depending on how towing ropes and distance ropes are arranged, the shape of the source array can be severely distorted during towing relative to its nominal position. The towing shape is substantially altered due to the drag forces exerted by the water against the source array.

[0004] In general, the present invention provides a methodology and system for the production of acoustic pulses used in marine seismic surveys. Generally, a source array system is created by a plurality of acoustic sources suspended from a float in a plurality of layers positioned at different vertical levels. The source array system comprises a bridle coupled to the plurality of layers to enable towing of the source array system by a suitable tow line. The bridle is constructed and connected in a manner such that the arrangement of acoustic sources substantially retains a desired shape during towing.

[0011] In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible.

[0012] The present invention generally relates to a technique for improving the production of acoustic pulses used in marine seismic surveys. The technique enables retention of a desired shape of a towed configuration, such as retention of the nominal shape of a towed source array with separated vertical layers of acoustic sources. A bridle is connected to the layers of acoustic sources in a manner that facilitates retention of the desired shape of the source array when towed.

[0013] In many applications, the towed configuration comprises a gun array with two vertically separated layers. The present technique enables the system to be towed at varying towing velocities while retaining the desired shape of the source array. The tow line is connected to the gun array by the bridle which connects to the individual layers of acoustic sources at the front of one or more gun arrays.

[0014] Referring generally to FIG. 1, an example of a seismic survey system 20 is illustrated according to one embodiment of the present invention. As illustrated, system 20 comprises an acoustic source array system 22 that is towed through a marine seismic survey area 24 to conduct a marine seismic survey. The acoustic source array system 22 may be towed by a surface vessel 26 connected to acoustic source array system 22 via a tow line 28. In this example, the acoustic source array system 22 comprises one or more acoustic source arrays 30 connected to tow line 28 by one or more corresponding bridles 32.

[0015] One example of acoustic source array system 22 is illustrated in FIG. 2. In the embodiment illustrated, the acoustic source array 30 comprises a plurality of acoustic sources 34 suspended below a float 36 designed to float along the surface of the water during towing by surface vessel 26. By way of example, the acoustic sources 34 may comprise acoustic guns, such as air guns operated to provide acoustic pulses. The acoustic sources 34 are arranged in a plurality of layers that are separated vertically. For example, the acoustic sources 34 may be arranged in a first layer 38 located above a second layer 40.

[0016] The first layer 38 of acoustic sources 34 may be suspended from float 36 by generally upright lines 42 that may be formed from suitable ropes, cables, or other types of suspension lines. The generally upright lines 42 can be individual lines or groups of lines connected between float 36 and the acoustic sources 34 of first layer 38. The second layer 40 of acoustic sources 34 also may be suspended from float 36 by generally upright lines 42. For example, the second layer 40 may be suspended by lines 42 that are connected between the acoustic sources 34 of first layer 38 and the acoustic sources 34 of second layer 40. Additionally, the acoustic sources 34 of each layer 38, 40 can be connected by generally horizontal/distance lines 44. Similar to upright lines 42, the generally horizontal lines 44 may comprise flexible lines formed of suitable ropes, cables, or other types of appropriate connector lines. The generally horizontal lines 44 can be positioned to horizontally align acoustic sources 34 of the upper layer 38 and to horizontally align acoustic sources 34 of the lower layer 40.

[0017] In the embodiment illustrated, bridle 32 is connected to first layer 38 and to second layer 40 by a first bridle line 46 and a second bridle line 48, respectively. Bridle lines 46, 48 can be formed as flexible lines of suitable rope, cable, or other lines able to withstand the towing forces. The bridle lines 46, 48 have connection ends 50 by which the bridle 32 is connected to a forward (or front) end of acoustic source array 30. For example, connection ends 50 can be coupled to the leading acoustic source 34 of each acoustic source layer 38, 40. The opposite ends of bridle lines 46, 48 are connected together at a tow connection end 52 which, in turn, is coupled to tow line 28.

[0018] The arrangement of bridle 32 can be described with reference to a coordinate system in which a z-axis is the vertical axis and an x-axis is a horizontal axis that extends back along the length of acoustic source array 30. As illustrated, the layers 38 and 40 lie at different vertical levels along the z-axis. For example, first layer 38 is located at a first vertical level 54 disposed below float 36, and second layer 40 is located at a second vertical level 56 below the first vertical level. The distance between float 36 and first layer 38 is a non-zero value that can be defined as D1, and the distance between first layer 38 and second layer 40 is a non-zero value that can be defined as D2. The connection ends 50 of bridle 32 are joined to the acoustic source array 30 at the vertical levels 54 and 56.

[0019] The tow connection end 52 of bridle 32 is disposed ahead of acoustic source array 30 by a non-zero distance, defined as L2, along the x-axis. Additionally, the tow connection end 52 is located at a vertical level 58 below the first vertical level 54 and above the second vertical level 56. In other words, the lengths of first bridle line 46 and second bridle line 48 are selected such that upon towing of acoustic source array 30, the tow connection end 52 automatically moves to the desired vertical level 58 between the vertical levels of first layer 38 and second layer 40. Consequently, the end of tow line 28 connected to bridle 32 also moves to the desired vertical level 58. The vertical distance between first vertical level 54 and vertical level 58 of tow connection end 52 can be defined as E1. Similarly, the vertical distance between second vertical level 56 and vertical level 58 of tow connection end 52 can be defined as E2. The vertical distances to tow connection end 52 establish the equation: E1+E2=D2. Additionally, the position of second layer 40 along the z-axis is less than the z-axis position of tow connection end 52 which is less than the z-axis position of first layer 38.

[0020] The arrangement of bridle 32 ensures that acoustic source array 30 retains its desired, e.g. nominal, shape while being towed by surface vessel 26. The coupling of connection ends 50 to first layer 38 and second layer 40, respectively, while joining the opposite ends of the bridle lines generally at a common point (located at the vertical and horizontal positions described above) prevents disruption of acoustic source array shape during towing and operation of the acoustic source array. As a result, more accurate data can be obtained during marine seismic surveys.

[0021] Referring generally to FIG. 3, another embodiment of acoustic source array system 22 is illustrated. In this embodiment, an optional third bridle line 60 is connected between tow connection end 52 and float 36 of the surface float layer. In some applications, towing of the acoustic source array 30 is facilitated by applying a portion of the towing force to float 36 from a tow point located between the vertical level 54 of first layer 38 and the vertical level 56 of second layer 40.

[0022] The type of components utilized and the overall structure of acoustic source array system 22 may be varied from one seismic survey application to another. For example, the number of acoustic sources 34 in each layer 38, 40 can vary, and the structure, size, and type of acoustic sources can be selected according to the parameters of a given application. For example, the acoustic sources 34 may comprise air guns that are arranged individually or in clusters. As illustrated in the embodiment of FIG. 4, for example, the acoustic sources 34 are in the form of guns and each acoustic source 34 comprises a gun cluster 62 having a plurality of cooperating guns 64.

[0023] With further reference to FIG. 4, additional variations are illustrated. For example, the bridle lines can be connected to tow line 28 at sequential points along the tow line. In the illustrated example, the bridle line 60 that extends to float 36 is connected to tow line 28 at a position 65 ahead of the location at which bridle lines 46 and 48 are joined via tow line 28. In another variation, the lower layer 40 of acoustic sources 34 is suspended directly from float 36 by independent lines 42, as illustrated, rather than being suspended from float 36 by attaching lines 42 to the acoustic sources of upper layer 38.

[0024] Depending on the specific seismic survey application, the acoustic source array system 22 also may comprise one or more acoustic source arrays 30. In the example illustrated in FIG. 5, a plurality of acoustic source arrays 30 are employed and towed behind the surface vessel 26. By way of example, the acoustic source arrays 30 may be arranged generally in parallel and separated by a support structure 66 designed to maintain the acoustic source arrays 30 at a desired horizontal spacing with respect to each other. The support structure 66 may have a variety of forms, including a rigid bar or other structure connected to the forward ends of the acoustic source arrays 30.

[0025] The examples discussed above are just a few of the possible acoustic source array system 22 configurations that benefit from the incorporation of bridles for controlling the tow point location as discussed above. In addition to variation in the number and arrangement of acoustic sources, the system may utilize a variety of floats and float layers. Many types of connection lines also can be used to connect acoustic sources with each other and with the float. The lower layer of acoustic sources may be suspended from other acoustic sources and/or the float layer. Additionally, the vertical distance between acoustic source layers as well as the horizontal distance between acoustic sources may be different from one seismic survey application to another.

[0026] Although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Accordingly, such modifications are intended to be included within the scope of this invention as defined in the claims.