Abstract:
A seismic measurement system and a method of obtaining seismic measurements are described. The seismic measurement system includes a cable and a plurality of sensors disposed at a first interval along the cable. The plurality of sensors receives reflections resulting from a seismic source and each of the plurality of sensors receives the reflection corresponding with a particular subsurface location. The system also includes a controller to turn on a first set of the plurality of sensors and turn off a second set of the plurality of sensors based on an area of interest.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 62/025,573 filed Jul. 17, 2014, entitled “CONTROLLED SPACED STREAMER ACQUISITION,” which is incorporated herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to seismic streamers towed by a vessel in a marine environment. In particular, embodiments detail dynamic adjustment of spacing of seismic samples. 
     BACKGROUND OF THE INVENTION 
     Seismic streamers are towed by a vessel in a marine environment such as an ocean. Each seismic streamer includes a series of the geophones or hydrophones (receivers) arranged along its length. Each of the receivers receives the seismic signals and converts them into electrical or other signals. The receivers arranged along the seismic streamers record seismic signals resulting from a reflection of a seismic source signal transmitted into the underwater environment below the seismic streamers. The seismic source signal may originate from the vessel, for example. The seismic streamers extend behind the vessel and can be several to tens of kilometers in length. Thus, the seismic streamers are unlikely to be linear but, instead, have shapes affected by factors like wind speed, direction, and marine current, for example. The seismic streamers are typically fashioned with the receivers arranged at regular intervals along each streamer. 
     SUMMARY OF THE INVENTION 
     According to an embodiment, a seismic measurement system includes a cable; a plurality of sensors disposed at a first interval along the cable, the plurality of sensors configured to receive reflections resulting from a seismic source and each of the plurality of sensors configured to receive the reflection corresponding with a particular subsurface location; and a controller configured to turn on a first set of the plurality of sensors and turn off a second set of the plurality of sensors based on an area of interest. 
     According to another embodiment, a method of obtaining seismic measurements includes disposing a cable in a marine environment, the cable configured to be towed by a vessel; disposing a plurality of sensors at a first interval along the cable, the plurality of sensors configured to receive reflections resulting from a seismic source and each of the plurality of sensors configured to receive the reflection corresponding with a particular subsurface location; and controlling the plurality of sensors to turn on a first set of the plurality of sensors and turn off a second set of the plurality of sensors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying figures by way of example and not by way of limitation, in which: 
         FIG. 1  is a simplified overhead view of a vessel towing a plurality of seismic streamers according to an embodiment of the invention; 
         FIG. 2  is a cross-sectional side view of a seismic sampling system according to an embodiment of the invention; 
         FIG. 3  illustrates a seismic streamer configured to be controlled according to embodiments of the invention; and 
         FIG. 4  is a process flow diagram of a method of dynamically adjusting seismic sample spacing within a seismic streamer according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not as a limitation of the invention. It will be apparent to those skilled in the art that various modifications and variation can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations that come within the scope of the appended claims and their equivalents. 
     As noted above, seismic streamers with receivers arranged along the length of the streamer (cable) can record seismic reflections produced based on a seismic source. The received reflections (reflection seismology) provide information about the subsurface environment in a similar way as sonar or echolocation, for example. The information may be used to generate a three-dimensional (3D) mapping of the subsurface environment that includes details such as, for example, density of the rock. Regular sampling using all the receivers may be adequate near to the source, but with increasing distance from the source, the bandwidth of the source wavelet decreases and the detailed regular sampling may no longer be necessary to adequately recover the signal. Fewer samples would also reduce the bandwidth required to recover the data from the streamers. That is, when the receivers arranged along the seismic streamer are too closely spaced, the density of data sampling that result can be prohibitive for timely analysis and processing. Furthermore, the additional data may not improve the quality of the final image and may even detract in resolution in same cases. On the other hand, when a particularly interesting subsurface feature is encountered, a more densely sampled area (receivers spaced closer together) may be desirable. For example, more sampled may be needed for structures that are dome like. Embodiments of the systems and methods described herein relate to dynamic adjustment of sample spacing to address both the need to reduce superfluous data and the need to increase sampling density in some cases. 
       FIG. 1  is a simplified overhead view of a vessel  140  towing a plurality of seismic streamers  110  according to an embodiment of the invention. The illustration in  FIG. 1  is for explanatory purposes regarding the components and is not to scale. The streamers  110  (cables) include a plurality of receivers  120  (e.g., hydrophones) that receive seismic reflections resulting from one or more seismic sources  160 . Although four streamers  110  are shown in  FIG. 1 , one or many streamers  110  may be towed behind the vessel  140 . A diverter, for example, may be used to separate the streamers  110  and spread them out behind the vessel  140 . The receivers  120  may be arranged at a distance d of 12.5 meters (m) from each other, for example, and this distance may be the spacing between each adjacent pair of receivers  120 . One or more magnetic compasses  130  may also be arranged along the seismic streamer  110 . A buoy  170  may be arranged at the opposite end of each streamer  110  from the vessel  140  and may carry additional equipment such as a gyroscope and an accelerometer, for example. The vessel  140  carries one or more controllers  150  to control and analyze data from the one or more sources  160  and sensors (e.g., receivers  120 , magnetic compasses  130 ). The controller  150  includes an input interface  152 , one or more processors  154 , one or more memory devices  156 , and an output interface  158 . 
       FIG. 2  is a cross-sectional side view of a seismic sampling system according to an embodiment of the invention. The vessel  140  in the water and a seismic streamer  110  towed below the water surface are shown. One or more geologic layers  210  may be above a sound reflection surface  220  beneath the surface of the water. Incident signals  230  originating at one or more sources  160  are reflected, and the reflected signals  240  can be received by the receivers  120  along the seismic streamer  110 . As  FIG. 2  illustrates, features of the sound reflection surface  220  may be lost if the receivers  120  are too far apart. That is, if one or more of the receivers  120  shown in  FIG. 2  were not operating, fewer reflections (from fewer areas of the sound reflection surface  220 ) would be received at the receivers  120  and, consequently, less detail would be recovered from those reflections. On the other hand, if more receivers  120  were interspersed between the receivers  120  shown in  FIG. 2 , each set of reflections would result in that much more data to be received, recorded, and analyzed. 
       FIG. 3  illustrates a seismic streamer  110  configured to be controlled according to embodiments of the invention. The controller  150  shown on the vessel in  FIG. 1  is discussed as the exemplary controller of the seismic streamer  110  for explanatory purposes, but each seismic streamer  110  may be controlled by a separate controller which may be located along the seismic streamer  110 , on the buoy  170 , or elsewhere in alternate embodiments. The exemplary seismic streamer  110  shown in  FIG. 3  includes a magnetic compass  130  and receivers  120  that may be arranged 1 meter (m) apart, for example. This physical spacing represents the minimum spacing (maximum spacing density) that is possible for the seismic streamer  110 . The controller  150  controls some of the receivers  120  to be on and some of the receivers  120  to be off.  FIG. 3  illustrates an area (A) of the seismic streamer  110  with all of the receivers  120  turned on by the controller  150 . This area A of the seismic streamer  110  may correspond with reflected signals  240  from a subsurface feature or topography of interest  310 , for example. The area A (the corresponding geology or topography of interest) may be identified based on forward modeling or acoustic modeling, for example, but is not limited to any particular basis for identification. The modeling output may be regarded as a trigger for dynamically controlling the sampling density. By dynamically controlling data collection from some of the receivers  120  to be on while other receivers are off, the controller  150  facilitates higher sampling density of geology and the topography of interest  310  (i.e., the topography that reflects signals received by the receivers  120  in area A) while preventing an increase in data volume from areas that are of less interest. 
     As  FIG. 3  indicates, the control of the receivers  120  may be truly dynamic rather than a selection of a preset pattern. That is, uniform spacing is not required between one or more receivers  120  that are on and one or more receivers  120  that are off. Thus, the distance d 1  (between two successive receivers  120  that are on) and the distance d 2  (between two successive receivers  120  that are on) need not be the same, for example. Further, over the length of the seismic streamer  110 , if two different areas of receivers  120  encounter topologies of interest (e.g.,  310 ), then the controller  150  may create two (or more) different areas of high density sampling (two different areas such as area A over the length of the seismic streamer  110 ), for example. In alternate embodiments, the area A with dense seismic sampling may move (different set of receivers  120  make up the area A shown in  FIG. 3 ) as the seismic streamer  110  moves past the topography of interest  310 . The controller  150  may return the receivers  120  to a default setting after a particular condition. The default setting may be every other receiver  120  being turned on or every third receiver  120  being turned on, for example. The condition (second trigger) for returning the receivers  120  to the default setting may be a period of time or an input indicating that the topological features of interest have been passed by the moving seismic streamer  110 . Receivers  120  of each of the other seismic streamers  110  may be controlled differently than the seismic streamer  110  shown in  FIG. 3  based on their relative position to the topography of interest  310  or their proximity to another feature of interest. 
     As noted above, the dynamic selection of which receivers  120  should stay on (record reflected seismic samples) and which receivers  120  should stay off at a given time provides two distinct advantages. Firstly, the dynamic selection facilitates higher resolution sampling of regions of interest. These regions may be of interest based on their geology or topography, for example. Secondly, the dynamic selection mitigates the problem of receiving too high a volume of data by turning off the receivers  120  that provide information that may ultimately be filtered out, for example. This also overcomes the bandwidth limitations of the main backbone cable for the data transmissions. 
       FIG. 4  is a process flow diagram of a method of dynamically adjusting seismic sample spacing within a seismic streamer  110  according to an embodiment of the invention. At block  410 , arranging the receivers  120  at a first spacing includes arranging the receivers  120  at the most dense sample spacing that may be needed. This is because the physical arrangement of the receivers  120  represents a hard limit on how closely spaced the seismic sampling can be performed. Based on input (e.g., forward modeling, acoustic modeling), a first trigger may be provided ( 440 ) to the controller  150  that results in turning on a first set of the receivers  120  and turning off a second set of the receivers  120  at block  420 , as shown in  FIG. 3 , for example. As noted above, the turning on and turning off receivers  120  (block  420 ) may be a process that moves along the seismic streamer  110  as the seismic streamer  110  moves over topography of interest  310 . 
     According to one embodiment, the first trigger may be based on a selection of the reconstruction algorithm that will be used to reconstruct the geology of the area based on the seismic signals. That is, based on the particular purpose of the seismic data gathering project and the area in which the streamers  110  are deployed, a particular reconstruction algorithm may need to be determined first. The determination of the reconstruction algorithm may be based on modeling, trials, or a combination. Once the reconstruction algorithm is chosen, the sampling scheme (which receivers  120  to turn on and off, the sampling rate, and other factors affecting the data collected) may be determined based on trials, for example. This determination of the appropriate sampling scheme for the reconstruction algorithm may be the first trigger (trigger  1 ) provided to the controller  150 . According to another embodiment, the implementation of compressive seismic imaging (CSI), which uses randomized subsampling, may act as the first trigger specifying the subset of receivers  120  to keep on. 
     At block  430 , reverting to a default setting for the receivers  120  may be done based on a second trigger being provided  450 . The second trigger may be a duration of time or may be based on an input (e.g., forward modeling, acoustic modeling) indicating that the topography of interest  310  is no longer in a relative position to the seismic streamer  110  to reflect source signals for reception by the receivers  120 , for example. 
     The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention.