Abstract:
A system and method for deployment of a plurality of seismic recorder assemblies from a survey vessel on the ocean bottom is disclosed. The seismic recorder assemblies are self contained, autonomous nodal devices which are capable of receiving and recording reflected seismic energy and storing the data locally while operating for an extended period of time. The assemblies each have two or more attachment points for the connection of separate connecting cable segments.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to marine seismic surveying. More specifically, the invention relates to a system and method for deploying seismic recorders. 
     2. Description of the Related Art 
     Seismic exploration is widely used to survey subterranean geological formations to determine the location of hydrocarbon formations in the earth. Reflection seismology is used to estimate the properties of the subsurface from reflected seismic waves. In reflection seismology, generated acoustic waves are propagated down through subterranean strata and reflect from acoustic impedance differences at the interfaces between various subterranean strata. The reflected seismic energy is recorded and processed to create an image of the subsurface structures. Because many commercially viable hydrocarbon formations are located beneath bodies of water, marine seismic methods have been developed. 
     In marine seismic survey systems it is typical to use geophone, hydrophones, or other seismic recorders (also referred to as sensors) to detect reflected seismic energy that is emitted from one or more seismic sources. These recorders are generally deployed in an array that may constitute one or more parallel lines. There are numerous ways seismic recorders can be deployed in a marine environment. In some instances, a streamer carrying seismic recorder sensors is towed near the surface behind a survey vessel. The streamer typically contains wiring to interconnect the sensors. Examples of these types of systems are found in, for example, U.S. Pat. Nos. 4,450,543 and 5,561,640. Because the survey cable is, in most instances, of unitary design and contains the seismic recorders wired together within the cable, one cannot change the spacing between the seismic recorders within the cable as may be desirable given a specific geological objective of the survey at hand. Also, because the reflected acoustic energy propagates through the water before being received by the seismic recorders in the streamer, noise significantly distorts the reflected energy. Also, because water has no shear strength, the aforementioned method is only capable of recording the vertical or pressure component of the full seismic wavefield. 
     In other instances, interconnected seismic recorders are placed directly on the bottom in a method typically known as Ocean Bottom Cable, or simply “OBC.” The seismic recorders in an OBC system are interconnected by reinforced cables that provide power and transmit data from the seismic recorders to a distal storage device. The rigid cables often allow good coupling with the bottom only along the major axis of the cable, significantly reducing the ability of the system, if equipped with three-dimensional geophones, to record the shear components of the full seismic wavefield. Additionally, one cannot readily change the spacing between the sensors within the cable as may be desirable given a specific geologic objective for a particular survey. Because this system relies on cables for power and telemetry, any damage to the cables or connectors, which is common in the marine environment, prevents the recording of data and contributes significant downtime and increased survey cost while the system is retrieved, repaired and redeployed. Also, the data and power cables contribute significant weight to the system which, combined with the reliability concerns, effectively prevents OBC systems from being deployed in deep water. 
     In other instances, autonomous nodal recorders are attached to a main cable by individual tethers, as disclosed in U.S. Pat. No. 6,024,344 to Buckley et al. The tethers interconnect a single attachment point on the recorder to a single attachment point on the cable. There are significant drawbacks with this type of arrangement. It is often necessary to remove the recorders from the main cable when the units are retrieved for charging, downloading and moving and then reattach the recorders immediately prior to re-deployment, which increase the handling effort and cost and also complicate the task of ensuring that the individual seismic recorders are deployed in the desired sequence. There is also a risk that the tethers could wrap around and get tangled in the main cable during deployment and potentially compromise data quality. If the main cable consists of a single length of cable and is damaged, the entire cable may have to be replaced at a significant financial cost and delay of operations. If the seismic recorders attach to the main cable at fixed attachment points, it is difficult to adjust the spacing between recorders, as may be necessary given the objective of a particular survey. Also, because the seismic recorders attach to the main cable only at a single point, the recorders will land on the ocean bottom at a completely random orientation relative to the other recorders and the survey geometry in general. 
     SUMMARY OF THE INVENTION 
     In one embodiment, the invention is directed to a system and method for deploying a plurality of seismic recorder assemblies from a survey vessel on the ocean bottom. The seismic recorder assemblies are self contained, autonomous nodal devices which are capable of receiving and recording reflected seismic energy and storing the data locally while operating for an extended period of time. The assemblies each have two or more attachment points for the connection of separate connecting cable segments. 
     In one embodiment, a seismic recorder array is configured to be deployed on the bottom of a body of water. The array includes a plurality of autonomous seismic recorders interconnected by separate connecting segments (also referred to as cable segments). The connecting segments or cable segments are not attached to one another. Adjacent pairs of recorders are connected by a respective connecting segment. The maximum distance between the adjacent recorders is established by the length of the respective connecting segment. When the seismic recorder array is fully stretched or is under tension, the distance between the adjacent recorders will be greatest, which is equal to the length of the connecting segment that connects the adjacent recorders. 
     In one embodiment, a method of deploying a seismic recorder array on the bottom of a body of water includes affixing a first connecting segment to a first attachment point on a first recorder and affixing the first connecting segment to a first attachment point on a second recorder. The method further includes affixing a second connecting segment to a second attachment point on the second recorder, and disposing the recorders into the body of water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For detailed understanding of the invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings in which reference characters designate like or similar elements throughout the several figures of the drawings. 
         FIG. 1  is a side view of an exemplary seismic recorder assembly for use with the present invention. 
         FIG. 2  is a top view of the seismic recorder assembly shown in  FIG. 1 . 
         FIG. 3  is an end view of the seismic recorder assembly shown in  FIGS. 1 and 2 . 
         FIG. 4  is an isometric view of an exemplary connecting segment used to interconnect two seismic recorder assemblies. 
         FIG. 5  is a side view of an exemplary recorder array, in accordance with the present invention, during deployment. 
         FIG. 6  is a side view of the recorder array shown in  FIG. 5 , now at a further point during deployment. 
         FIG. 7  is a side view of the recorder array shown in  FIGS. 5 and 6 , now fully deployed. 
         FIG. 8  illustrates a technique for storing a recorder array constructed in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIGS. 1-3  depict an exemplary seismic recorder assembly, or recorder assembly,  10  in accordance with one embodiment. The recorder assembly  10  generally includes an outer housing  12 . The housing  12  contains one or more seismic energy sensors  14  capable of sensing reflected seismic energy. The seismic energy sensors  14  are preferably geophones, hydrophones or other sensor devices known in the art. The sensors  14  are electrically connected to a circuit  16 . The circuit  16  may include one or more programmable processors of a type known in the art for controlling operation of the sensors  14  and a data storage device for recording the received seismic waves. The circuit  16  may also include a clock configured to provide timing signals to the sensors and provide time stamp to the recorded seismic waves. Exemplary seismic sensors, storage devices and a clock circuit for use in this application are described in further detail in U.S. Patent Publication No. 2008/0049550, entitled “Autonomous Seismic Data Acquisition Unit,” which is hereby incorporated by reference in its entirety. 
     The housing  12  also encloses a power source  18 , which is electrically connected to the sensors  14  and the circuit  16  to provide power for those components. The power source  18  is preferably a rechargeable battery which is sufficient to power the components within the housing  12  for the required duration of time. In one implementation, the seismic recorder assembly  10  is a self-contained, autonomous, nodal recorder assembly which is capable of detecting and recording seismic energy without the need for an external power or data cable to be connected to it during use. A data and power cable plug  20  is incorporated into the housing  12  and is interconnected with the power source  18  as well as the data storage device. A charging and downloading cable (not shown) can be coupled to the data and power plug  20  when the recorder assembly  10  is aboard the survey vessel and not in use, in order to recharge the power source  18  and/or retrieve recorded seismic data from the data storage medium. The individual components  14 ,  16 ,  18 , and  20  contained within the housing  12  are preferably sealed and water-tight as well as pressure-resistant to withstand the water pressures associated with deep marine environments. These components can be sealed together, in groups or individually. Preferably, the housing  12  can serve to create the sealed and pressure-resistant environment for the internal components. 
     The housing  12  is depicted as having a generally rectangular body with generally opposite end portions  22  and  24 . The housing  12  may be cylindrical, spherical, tubular, conical, or have any other suitable shapes. In addition, the housing  12  may be symmetrical or non-symmetrical. In one implementation, the end portions  22 ,  24  are located at generally opposite ends of the housing  12 . First and second attachment points  26 ,  28  are formed on the housing  12  proximate to the respective end portions  22 ,  24 . The attachment points  26 ,  28  are shaped and sized to be suitable for the reversible attachment of connecting segments, as will be described. In the depicted embodiment, the attachment points  26 ,  28  are apertures through which a snap link may be disposed. 
       FIG. 4  depicts an exemplary connecting segment  30  which is used to interconnect two recorders  10 . In the depicted embodiment, the connecting segment  30  includes a non-metallic rope segment  32  with two end loops  34 ,  36 . It should be understood that, while the segment  32  material is shown in the form of a non-metallic rope, it may also take the form of a metallic cable, coated cable, chain, or similar element. In addition, the connecting segment  30  may be of any desired length. Typically, the connecting segments  30  are of the same length, but their lengths may differ. The connecting segment  30  also includes two swivels  33  and two snap links  38 ,  40  of a type known in the art to facilitate attachment of the cable segment  32  to the seismic recorder assemblies  10 . 
       FIG. 5  illustrates a recorder array  42  in accordance with one embodiment during deployment into ocean  46  from a survey vessel  44 . The term “ocean,” as used herein, is intended to refer generally to all navigable bodies of water, including freshwater lakes and rivers as well as seas. An anchor  48  is affixed to a first connecting segment  30   a . The anchor  48  could take the form of an anchor, chain or other suitable weighted object. A first recorder assembly  10   a  is affixed to the first connecting segment  30   a  and a second connecting segment  30   b . A second recorder assembly  10   b  (not shown) is interconnected to the first recorder assembly  10   a  via the second connecting segment  30   b . The second connecting segment  30   b  is separate from and not attached to the first connecting segment  30   a . It is noted that the use of the anchor  48  is optional. To attach the first recorder assembly  10   a  to the anchor  48 , the snap link  38  of the first connecting segment  30   a  is connected to attachment point  28  on the first recorder assembly  10   a , and the other snap link  40  is affixed to the anchor  48 . In an alternative embodiment of the invention, the first recorder assembly  10   a  is deployed into the ocean  46  without an anchor  48  attached to the first attachment point  28  via a first connecting segment  30   a.    
     The greatest or maximum distance between adjacent recorder assemblies is established by the length of the connecting segment that connects the adjacent recorder assemblies. Thus, the greatest or maximum distance between the recorder assemblies  10   a  and  10   b  is established by the length of the connecting segment  30   b . It will be apparent that when the recorder array is fully stretched or under tension, the distance between the adjacent recorder assemblies  10   a  and  10   b  will be greatest, which is established by the length of the connecting segment  30   b.    
       FIG. 6  depicts the exemplary recorder array  42  now in a further point during deployment. Three recorders  10   a ,  10   b  and  10   c  have now been assembled with connecting segments  30   b  and  30   c  interconnecting them. It should be understood that the recorder array  42  is preferably assembled by securing the snap link  40  of connecting segment  30   b  to one attachment point  26  of the first recorder assembly  10   a  and the other snap link  38  of the connecting segment  30   b  to one attachment point  28  on the second recorder assembly  10   b . Then, the third recorder assembly  10   c  is interconnected to the second recorder assembly  10   b  by attaching one snap link  40  of the connecting segment  30   c  to attachment point  26  on the second recorder assembly  10   b  and the other snap link  38  of the connecting segment  30   c  to the attachment point  28  of the third recorder assembly  10   c . The snap link  40  of the next connecting segment  30   d  is then attached to the connecting point  26  of the third recorder assembly  10   c.    
     As the components are assembled, they can be placed into the water  46  from the vessel  44  generally in the order in which they are assembled or the components can be pre-assembled in advance of deployment. The anchor  48  and connecting segment  30   a  are placed into the water  46  and allowed to sink toward the sea floor or bottom  50 . Thereafter as the survey vessel moves along the planned path of the recorder array, the first recorder assembly  10   a  and connecting segment  30   b  are placed into the water  46  and allowed to sink toward the sea floor  50 . This is repeated with the second recorder assembly  10   b  and second connecting segment  30   c  and so on. During deployment, the vessel  44  is preferably moving in a line above the area where it is desired to place the recorder array  42 . The speed of the survey vessel  44  and the points at which the recorder assemblies  10  and connecting segments  30  are placed in the water are set to facilitate the units landing on the bottom  50  at a desired position and interval. 
       FIG. 7  illustrates the recorder array  42  now in a fully deployed configuration. Recorder assemblies  10   a ,  10   b ,  10   c ,  10   d , and  10   e  are deployed along the ocean bottom  50 . When the desired number of recorder assemblies  10  is deployed into the water  46 , the array  42  is preferably terminated with a floatable marker  54 . The marker  54  may be a floating buoy or a known device that rests on the ocean bottom  50  and then is inflated upon receipt of a remote trigger signal so that it rises to the surface  56  of the water  46 . A marker connecting segment  30   f  is used to interconnect the marker  54  with the last recorder assembly  10   e . One or more markers might also be attached to other connecting segments  30  or recorders  10 . 
     Although only five recorder assemblies  10  are depicted, those of skill in the art will understand that the array  42  may include many more recorder assemblies  10  which are interconnected to each other in the same manner as these. It is noted that the recorder array  42  is preferably disposed in a substantially linear configuration along the ocean bottom  50 . 
     In order to retrieve the recorder array  42  into the survey vessel  44 , the marker  54  is retrieved into the vessel  44  along with the connecting segment  30   f . Thereafter, the recorders  10  and connecting segments  30  are retrieved into the vessel  44  in the reverse order from which they were deployed. Finally, the anchor  48  is retrieved into the vessel  44 . Preferably, a winch  52  or similar device is used to help draw the components into the vessel  44 . 
     Once the recorder array  42  has been retrieved, the receiver array  42  is generally left intact and stored on the survey vessel  44  as a single unit awaiting redeployment in another location. In an alternate embodiment of the invention, the recorder array  42  may be disassembled into its major components, the recorder assemblies  10  and the connecting segments  30 , as it is retrieved onto the survey vessel  44 . If this is done, the various connecting segments  30  may be secured to one another in an end-to-end fashion to form a single continuous strand. This strand may then be coiled up or wound onto a reel.  FIG. 8  illustrates an exemplary storage rack  58  upon which the recorder array  42  might be stored. As illustrated, recorder assemblies  10   a ,  10   b  and  10   c  are stored upon the rack  58 . 
     Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.