Abstract:
A marine towed array has a plurality of tow bodies of positive buoyancy each containing a marine acoustic source, such as a vibrator source. Each tow body uses a reel containing an electro-mechanical tow cable connecting the sources, whereby reeling and unreeling of the cable, controlled from a towing ship, changes the spacing of the tow bodies. One of the tow bodies at an end of the array is attachable to the towing ship.

Description:
This application claims the priority benefit of United States Provisional Application No. 60/140,392, filed Jun. 22, 1999. 
    
    
     DESCRIPTION 
     The present invention relates a towed acoustic source array of marine sources such as vibrator sources (Vibroseis type) of MULTIPULSE™ sources, for example of the type described in U.S. Pat. Nos. 4,131,178 issued Dec. 26, 1978, U.S. Pat. No. 4,207,962 issued Jun. 17, 1980, and U.S. Pat. No. 4,362,214, issued Dec. 30, 1986. The invention is especially suitable for use as a towed geophysical array for underwater geophysical exploration applications. 
     The array may use geophysical sources such as air guns as well as hydraulically driven marine vibrator units and MULTIUPLSE™ sources. 
     It is the principal feature of the invention to provide a towed array system where the depth and lateral position of each source may be set under tow and maintained and also where the separation of the sources may be varied so as to facilitate stowing thereof on a towing vessel and deployment to prescribed spacing between the sources. 
     Geophysical and other source arrays for marine applications have heretofore been proposed. See, for example, Bouyoucos, etal U.S. Pat. No. 5,841,733, issued Nov. 24, 1998. A towed source array system, particularly a geophysical source array for marine applications, which can readily be stowed aboard a vessel and then deployed at prescribed depths and with lateral positioning and maintained during operation at such depths and positioning has heretofore not been provided. 
     Briefly described, a marine towed array in accordance with the invention uses a plurality of tow bodies each containing an acoustic source. Each tow body including the source and any associated power supply and control systems are positively buoyant and contain control planes for driving the bodies under tow to a prescribed depth and lateral position and holding them there. Preferably the bodies are interconnected to the ship and to each other in series by electro-mechanical umbilical, tow cables. The bodies may contain reels from which the cables may be wound and unwound to change the spacing of the sources. The first tow body, at one end of the array, is attachable to the towing ship and the bodies, their positioning, their sources and reels are controllable from the ship. 
    
    
     The foregoing and other objects, features and advantages of the invention will become more apparent from a reading of the following description in connection with the accompanying drawings in which. 
     FIGS. 1A &amp; 1B are a broken away side view, and an end view, respectively, of a unitary marine vibrating unit with a power supply and controls in a positively buoyant tow body with active control surfaces for lateral and depth positioning while under tow; 
     FIG. 2 is a schematic view showing three vibrating unit bodies under tow with individual depth sensors and control surfaces to maintain prescribed depth and lateral position; 
     FIG. 3 is a schematic plan view showing a seismic survey vessel towing a plurality of marine vibrator unit sub-arrays each having a plurality of units such as shown in FIGS. 1A and 1B; 
     FIG. 4 is a broken away side view of a marine vibrator tow body with an integral cable reel to aid deployment and recovery, especially in heavy seas; 
     FIG. 5 is a view similar to FIG. 4 showing three marine vibrator units deployed at eight meter separation in a transmit array configuration; 
     FIG. 6 is a schematic view illustrating the array shown in FIG. 5 spaced out for boarding on a towing ship; 
     FIG. 7 is a schematic view illustrating the boarding of the array shown in FIG. 5 on approach to the towing ship; 
     FIG. 8 is a view similar to FIG. 7 showing the first tow body in process of being boarded on the towing ship; 
     FIG. 9 is a view similar to FIG. 8 showing the boarding of the second tow body on the ship; 
     FIG. 10 is a view similar to FIG. 9 showing the boarding of the three tow bodies completed onto the stowing deck of the ship; 
     And FIG. 11 is a plan view schematically showing the storage deck of a GEM class seismic towing ship, and particularly the stowing or sheltered deck thereof, with stowage tracks providing for captured movement of the tow bodies for deployment, retrieval and maintenance. 
    
    
     Referring to FIG. 1 there is shown a single marine tow body unit  10  and its associated hydraulic power, energy storage and control unit  12 . This control unit receives control signals from the towing ship via an umbilical tow cable. The tow body contains a marine vibrator unit  13  and the hydraulic power storage and control unit  12 . The tow body also contains floatation material sufficient to make this body positively buoyant. Control surfaces on the body are in the form of a vertical stabilizer  16  having a control surface  18  which is pivotable about a vertical axis, and pivotal diving planes  20  extending laterally from the body  14 . Forward and aft brackets  22  are provided for attachment of the umbilical towing cable. 
     The control surfaces  18  and planes  20  are provided to drive the unit under tow to a prescribed depth and lateral position and hold it there. The control surfaces may be retractable so that they are not damaged during deployment and recovery of the array. 
     The horizontal planes  20  may be used in conjunction with depth sensors  24 , which may be of the ultrasonic type, to detect the depth of the unit below the surface  26  of the water under which the array is disposed. See FIG.  2 . The signals from the depth sensors and control signals from the towing ship operate the controls unit  12  to maintain a depth prescribed by a master control unit aboard the towing ship. 
     The sensors  24  and the water (sea) surface  26  under which the array is towed is shown in FIG. 2 which illustrates a deployed array of three tow bodies  28 ,  30  and  32 , and interconnecting umbilical tow cables  34  and  36  and a tow cable  38  connected to the first body  28  and to the towing ship. 
     The weight of the tow body, including the vibrator unit  10  and the power and control unit  12  may be about 7,600 pounds in air and 350 to 500 pounds (buoyant) submerged. The integral buoyant feature eliminates the need for a separate surface float and can significantly simplify the handling of the array. 
     FIG. 3 illustrates a seismic vessel  38  towing six sub-arrays each containing three sub-arrays of tow bodies indicated at  40 . Streamers  42  around the arrays may also be used in a manner similar to that used with air gun arrays. 
     In all cases the units may be driven deep to remain deployed and to ride out unanticipated heavy weather. 
     By operating the control planes  18  and  20 , lateral source to source string (series) spacing, such as 8 meters, may be obtained even with the units driven deep, thereby avoiding entanglement of the units and their cables. 
     FIG. 4 illustrates a unit  44  similar to that shown in FIG. 1A but with addition of a cable reel to the tow body  14  to facilitate changing of the spacing between bodies with all bodies deployed, and also providing for deployment and recovery of one body at a time, well separated from neighboring bodies. 
     FIG. 5 shows three bodies  48 ,  50  and  52  on a string or in series in a transmits configuration. 
     FIG. 6 illustrates the bodies  48 ,  50  and  52  spaced out for boarding. 
     FIG. 7 illustrates the towing vessel  54 , which may be of the GEM class, having a stowing deck also known as a shelter deck  56  at the aft end thereof. Note that the bodies  48  and  50  are spaced away from each other during boarding. 
     Referring to FIG. 8 the aft end of the vessel has a ramp or chute  58  extending rearwardly from the aft end onto the deck  56 . The cable  38  is attached to a wench  60  on which it is wound during boarding and retrieval of the array. 
     FIG. 9 shows the boarding of the second body  50 . Note that the cable between the second and third bodies is spaced out to avoid entanglements while the cables between the bodies as they move onto the deck. The spacing is shortened to conserve space on the deck during stowage. 
     FIG. 10 illustrates all three units  48 ,  50  and  52  stowed in close proximity on the deck  56 . 
     Referring to FIG. 11 there is shown a multiple sub-array system such as illustrated in FIG. 3 stowed on the deck  56  of the vessel  54 . A track system consisting of a deployment and retrieval track  64  and transverse stowage tracks  66  allows for the lateral and longitudinal captured movement of the bodies of the sub-array to facilitate deployment, retrieval, storage and maintenance. 
     For deployment of the array system, whether of one or multiple sub-arrays, the recovery procedures described in connection with FIGS. 5 to  10  are reversed. 
     From the foregoing description it will be apparent that there has been provided an improved source array system for marine applications which is especially suitable for use in geophysical exploration. Variations and modifications of the herein described system, within the scope of the invention, will undoubtedly suggest themselves to those skilled in the art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.