Abstract:
In one embodiment the invention comprises a method for performing a repeated marine seismic survey which includes towing at least one seismic source behind a seismic survey vessel substantially along the path of a previously conducted seismic survey, determining the position of the at least one seismic source during the repeated marine seismic survey and comparing the determined position of the at least one seismic source with a position of a seismic source during a previously conducted seismic survey. The position of the at least one seismic source is adjusted in response to said comparison.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     Not applicable  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     In performing marine seismic surveys, typically, a seismic survey vessel tows a plurality of streamer cables and a plurality of source arrays behind the vessel. The seismic source arrays, typically comprising a plurality of individual air guns, generate an acoustic signal upon command from a command and recording system, normally onboard the seismic survey vessel. The acoustic signals travel downwardly into the Earth&#39;s subsurface, and are reflected from the interfaces between subsurface strata having different acoustic impedances. The reflected signals are then detected by receivers, typically hydrophones, deployed in the plurality of streamer cables, and the detected signals are recorded by the command and recording system. Images of the subsurface are then generated from the detected and recorded seismic data, and these images are evaluated to predict regions that are favorable for the accumulation of petroleum.  
         [0004]     The towed streamer cables are towed along a chosen path to perform the survey in a chosen area. The movement of the vessel and the towed streamer cables is controlled to secure coverage of the desired areas. Seismic surveys which utilize a plurality of streamers laterally deployed behind the survey vessel are typically referred to as 3D (three dimensional) surveys. Recently, there has been an increased interest in 4D (four dimensional) surveys, in which seismic data are gather in an initial 3D survey, and the survey is then repeated at later times to determine changes in the subsurface that may have occurred with time. Especially, if petroleum is being produced from a reservoir, successively performed seismic surveys can provide an indication of fluid (either petroleum or brine) displacement in the reservoir as the reservoir is being produced. In the successively performed surveys it is important that the position of the streamer cables and the source arrays duplicate the positions of the streamer cables and source arrays from the previously performed surveys.  
         [0005]     It is known to the prior art to control the locations of the streamers during the subsequent surveys in order to duplicate the streamer locations from the initial survey. However, the accuracy of the comparison of data recorded during subsequent surveys with the data from previous surveys may be reduced if the seismic source locations differ between earlier and later surveys.  
       SUMMARY OF THE INVENTION  
       [0006]     In one embodiment the invention comprises a method for performing a repeated marine seismic survey which includes towing at least one seismic source behind a seismic survey vessel substantially along the path of a previously conducted seismic survey, determining the position of the at least one seismic source during the repeated marine seismic survey and comparing the determined position of the at least one seismic source with a position of a seismic source during a previously conducted seismic survey. The position of the at least one seismic source is adjusted in response to said comparison.  
         [0007]     In another embodiment the invention comprises apparatus for performing a repeated marine seismic survey which includes at least one seismic source being towed behind a seismic survey vessel during said repeated seismic survey, and means for determining the position of the at least one seismic source during the repeated marine seismic survey. Means are included for comparing the determined position of the at least one seismic source with a position of a seismic source during a previously conducted seismic survey; and for adjusting the position of the at least one seismic source in response to said comparison.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  shows a system for conducting a seismic survey in accordance with the present invention.  
         [0009]      FIG. 2  shows in more detail a portion of the system of  FIG. 1 .  
         [0010]      FIGS. 2A and 2B  show portions of subarrays of seismic energy sources.  
         [0011]      FIG. 3  shows an example of a deflector control assembly.  
         [0012]      FIG. 4  shows a schematic diagram of master controller.  
         [0013]      FIG. 5  shows one example of control panels that may be utilized in the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     The present invention enables repeatable, four-dimensional (4D) seismic data acquisition. The invention utilizes stored tracking data from an initial seismic data acquisition survey. Seismic data acquisition runs performed subsequently endeavor to generate acoustic signals and to detect the resulting signals with seismic sources (or source arrays) and streamer cables positioned at the same locations on the Earth&#39;s surface where the signals were generated and detected during the initial seismic survey.  
         [0015]     During the initial survey, the position of each source array, relative Earth coordinates, where each seismic signal is generated, is tracked and stored. This stored data may be referred to herein as archive data.  
         [0016]     When the initial seismic survey is conducted, which serves as the base survey for a 4D seismic survey operation, a Global Positioning System (GPS) receiver system is typically employed for continuously monitoring the precise geographical position of the seismic survey vessel, the seismic source arrays and the seismic streamers. In addition to the GPS receivers, typically mounted on the vessel, on the seismic source arrays, and at each end of the streamers, position detection systems of a type known to the art, such as acoustic detection systems and compasses, may also be employed. Algorithms known to those of ordinary skill in the art may utilize both the GPS position data and the acoustic data and/or compass data for calculating the precise geographic position of the seismic sources and the streamer cables.  
         [0017]     In accordance with the present invention, there is provided, during subsequently performed surveys, a system for controlling the position of the sources so that the seismic signals are generated during subsequently performed surveys in substantially the same locations relative Earth coordinates as during the initial survey.  
         [0018]      FIG. 1  shows a system for conducting a seismic survey in accordance with the present invention. A seismic survey vessel  10  is shown towing a plurality of seismic streamers  12  and two seismic source arrays A and B.  FIG. 2  shows in more detail a portion of the system of  FIG. 1 . In  FIG. 2  the streamer cables have been omitted for clarity.  FIG. 2  shows two seismic source arrays A and B, with each source array comprising three subarrays A 1 , A 2 , A 3  and B 1 , B 2 , B 3 , respectively. It is understood, however, that the invention is not limited to the number of source arrays, or the number of subarrays utilized for practicing the invention. Each subarray is shown towed by the seismic vessel by a cable  14 . A spreader line  16  is tied to the front end (the end closest to the vessel  10 ) of each of the subarrays and assists in maintaining lateral spacing therebetween. The separation between the centerlines of arrays A and B may typically be about 35 meters, and the spacing between the subarrays of an array may typically be about 12.5 meters.  
         [0019]     In each subarray, seismic sources  15 , typically air guns, are suspended beneath a float  13 . Portions of subarrays A 1  and B 1  are shown in more detail in  FIGS. 2A and 2B , respectively. In a particular implementation of the invention, a deflector  18  is connected to the keel (bottom) of a buoy (float)  13  supporting each subarray. However, in other embodiments, the invention may be performed with a deflector connected to less than all of the subarrays, for example, the invention may be implemented with a deflector connected only to the outermost of the subarrays within an array, such as, for example, subarrays A 1  and B 1 . To enable the deflectors to control the position of the source arrays, the deflectors are controllable so as to provide a variable angle between the deflector  18  and the longitudinal body of the float  13 . This angle, illustrated as angle  22  in  FIGS. 2A, 2B  and  3 , may be referred to herein as an “angle of attack”.  
         [0020]     A deflector  18  is affixed to the keel (bottom) of a buoy  13  by a deflector control assembly  24  that enables the angle  22  between deflector  18  and the buoy  13  to be varied. One example of a deflector control assembly is shown more clearly in  FIG. 3 , which shows the assembly of  FIG. 2B  in more detail. As shown in  FIG. 3 a  mounting bracket  26  is fixedly connected to the keel of buoy  13 . A deflector  18  comprising a plurality of vanes  30  is coupled to the mounting bracket  26  by means of deflector arm  20  and actuator arm  34 . Actuator arm  34  is rotatably connected to mounting bracket  26  to enable the angle  22  between the deflector  18  and the longitudinal axis of the buoy  13  to be varied. This angle variation is controlled by actuator arm  34 . In one embodiment, actuator arm  34  comprises a hydraulic mechanism whose extension is controlled by a signal from master control system  36 . (See  FIG. 4 .) Actuator arm  34  is rotatably coupled at one end to mounting bracket  26 , and the other end is rotatably coupled to connector rod  38  which is affixed in sliding engagement with deflector arm  20 . Thus, by controlling the length of actuator arm  34 , the angle of attack  22  is controlled in response to a control signal from master control system  36 .  
         [0021]     In a preferred embodiment, actuator arm  34  comprises a piston-cylinder assembly whose length is controlled in response to a signal from master control system  36 . Actuator arm  34  may be pressure compensated, so that the response of the actuator arm to the signal from the master control system  36  is substantially independent of depth below the water surface over the depth range at which it is anticipated the actuator arm  34  will need to operate.  
         [0022]     In a preferred embodiment of the invention, the deflector control system comprises a master control system  36 , normally located on the seismic survey vessel and an actuator  37  (not shown in detail) built into the deflector control assembly  24 . With reference to  FIG. 4 , master control system  36  comprises PLC (programmable logic controller)  40 , which receives input data from Integrated Navigation System  42  and from one or more control panels (which may be touch screens)  44 , and information from the actuators  37  providing the attack angle  22 , and generates control information for each of the actuators  37 . Input data from the Integrated Navigation System  42  include position deviation/error information derived from source position information and archive data. The functions of programmable logic controllers are well known to those of ordinary skill in the art and will not be described in detail herein.  
         [0023]     The invention may be performed in either a manual or an automatic mode. In a particular implementation of the invention, a capability may be included for switching between manual and automatic modes.  
         [0024]     Archive data providing locations relative Earth coordinates where seismic signals were generated during an initial seismic data acquisition survey may be stored in the Integrated Navigation System  42 . Global Positioning System (GPS) receivers mounted on the seismic source arrays may be utilized to determine the position of the seismic source arrays during the repeated (current) survey. GPS data may also be combined with data from other position detection systems, such as an acoustic sensor system, to improve the accuracy of source position determination. Source positioning data for a seismic source array is transmitted to the Integrated Navigation System onboard survey vessel  10 , typically through electrical or optical conductors in cables  14  or through an electromagnetic transmission link. The Integrated Navigation System then calculates the seismic source position and determines the difference between the desired position of a seismic source array (the position during the initial survey) and the actual position of a seismic source array.  
         [0025]     When operating in automatic mode, the Integrated Navigation System transmits a control signal for correcting the position of the seismic source arrays to the PLC  40 . Control signals for controlling each of the deflectors  18 , and hence the position of the seismic source arrays, are generated by the PLC  40  (in response to a signal from Integrated Navigation System  42 ) and transmitted to the actuators  37 . The actuators  37  may comprise an electrical motor and a gear system that controls the extension of actuator arm  34 , thereby controlling the angle of attack  22  of the deflector  18 . A position sensor externally mounted on the actuator monitors the angle of attack  22 , which information is transmitted to the PLC  40 . In a particular implementation of the invention the master control system  36  controls the deflectors  18  by transmitting to the actuator  37  a signal for the actuator to begin the movement of the deflector  18 , and whether the angle of attack  22  should be increased or decreased. The actuator  37  then varies the length of actuator arm  34  in order to vary the angle of attack of deflector  18 . Navigation data used to determine the actual location of the seismic source arrays (or subarrays) relative Earth coordinates is transmitted continuously back to the Integrated Navigation System  42 , as described above. The INS  42  calculates the difference between the desired position of the source arrays and the actual position of the source arrays. A signal representing this difference is then transmitted to the PLC  40  within master control system  36 . The master control system then calculates the individual deflector angles to minimize the position difference and maintain source separation. These data and control signals transmitted to and from master control system  36  may be transmitted via electrical or optical conductors included in cables  14 , or they may be transmitted by an electromagnetic signal utilizing techniques known to those of ordinary skill in the art.  
         [0026]     By properly regulating the angle of attack  22  the source arrays may be displaced either to the starboard or port side of the vessel, thereby controlling the position of the arrays.  
         [0027]      FIG. 5  shows one example of control panels that may be utilized in the invention, including automatic control panel  48  and manual control panel  46 . In various implementations of the invention a plurality of duplicate control panels, as indicated in  FIG. 4 , may be positioned at different locations on the survey vessel to facilitate operations; however, the invention will be described with reference to a single manual and automatic control panel.  
         [0028]     In a particular implementation of the invention, the crossline deviation (CD) and the source separation (SS) may be displayed on the automatic control panel  48 . These parameters are continuously calculated by the Integrated Navigation System  42  and automatic control panel  48  is updated at frequent intervals, such as, for example, every 50 milliseconds. The crossline deviation (CD) is the lateral difference between the actual path of the seismic source arrays and the desired path, and the source separation (SS) may be the actual distance between the center points of the two source arrays or the difference between the desired separation between the two source arrays and the actual separation. These displays enable an operator to monitor the operations of the system to verify the system operation quality.  
         [0029]     At the discretion of an operator the system may be switched from automatic to manual mode. In a particular implementation of the invention, a control panel  44 , which may be a touch screen, such as shown in  FIG. 5 , is utilized to reconfigure the operation of PLC  40  from automatic to manual mode by touching an appropriate control display, such as the location labeled M. Once the system is switched to manual mode, each of the deflectors  18  are operated manually, with the operator selecting the attack angle  22  for each deflector. On manual control panel  46  there is shown control buttons for varying the attack angle  22  for each of three subarrays comprising the port side source array and for controlling each of three subarrays comprising the starboard source array, respectively. Control buttons  50 A and  50 B increment or decrement, respectively, the attack angle  22  for the outermost subarray A 1 , of source array A. Control buttons  51 A and  51 B increment or decrement, respectively, the attack angle  22  for the center subarray A 2  of source array A, and control buttons  52 A and  52 B increment or decrement, respectively, the attack angle of the innermost subarray A 3  of source array A. Further control buttons  53 A and  53 B increment or decrement, respectively, the attack angle  22  for the outermost subarray B 1  of source array B. Control buttons  54 A and  54 B increment of decrement, respectively, the attack angle  22  for the center subarray B 2  of source array B, and control buttons  55 A and  55 B increment or decrement, respectively, the attack angle  22  of the innermost subarray B 3  of source array B. Above the control buttons for each of the source subarrays is an indicator of current attack angle for the particular subarray. Pressing a particular control button may increment or decrement the attack angle by a selected amount, for example one percent. Automatic control panel  48  will continue to display an indication of crossline deviation (CD) and source separation (SS) while the deflector control system is operating in manual mode, to aid an operator in controlling the source array positions.  
         [0030]     In a preferred embodiment, the present invention provides for control of the crossline separation. Arrays A and B may both be shifted to one side or the other of the vessel while keeping the separation of the two arrays constant. Arrays A and B may also be shifted independently of each other to vary the crossline separation. Further, in a particular implementation of the invention the attack angle of all of the actuators may be reduced to essentially zero degrees, to facilitate retrieval of the source arrays onto the survey vessel.  
         [0031]     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.