Patent Publication Number: US-11662491-B2

Title: Repeating a previous marine seismic survey with a subsequent survey that employs a different number of sources

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit to the filing date of U.S. Provisional Application 63/082,419, filed Sep. 23, 2020, the contents of which are hereby incorporated by reference as if entirely set forth herein. 
    
    
     BACKGROUND 
     Marine seismic surveys are often used to improve the quality of decisions associated with locating or producing natural resources, such as hydrocarbons, in a geographic area of interest. 
     It is sometimes the case that multiple marine seismic surveys are performed over the same geographic area at different times. For example, a first survey may be performed over a given area either before or immediately after a production field is established to extract hydrocarbons from that area. Later, after production has begun, one or more subsequent surveys may be performed over the same area in order to detect changes in subsurface structures or characteristics over time. Information about changes in the subsurface can be used to improve efficiency in hydrocarbon extraction from the field. 
     The process of performing multiple surveys over the same area at different times for this purpose is often referred to as “4D” surveying. Each of the surveys performed during the process is a 3D survey (giving information in three spatial dimensions) and comprises a temporal snapshot of the subsurface. The term “4D” refers to the fourth dimension of time, as the comparison of results from the various 3D surveys gives information about changes in the subsurface as time advances. The first survey in a 4D surveying process is often referred to as the “baseline” survey. Subsequent surveys are often referred to as “monitor” surveys. Thus, a baseline survey is always a previous survey relative to any monitor survey, and a given monitor survey may be a previous survey relative to a subsequently performed monitor survey. The terms “baseline” and “previous” are used interchangeably herein. 
     One of the goals in 4D surveying is to ensure that a comparison of results from the baseline and monitor surveys, or between different monitor surveys, will yield meaningful information. Conventionally, therefore, industry participants have endeavored to perform monitor surveys using the same source and streamer layouts as were used during the baseline survey. This is so that changes observed in monitor survey results are more likely to reflect changes in the subsurface than they are to reflect changes in the surveying methods employed during the surveys themselves. In this sense, a 4D monitor survey attempts to repeat a previous baseline survey. The degree to which this is achieved is often referred to as “repeatability” and can be measured as the sum of the differences in source positions, dS, and the differences in receiver positions, dR, between a monitor survey and a previous survey. The lower the sum, dS+dR, the better the repeatability achieved by the monitor survey. 
     Other goals in marine surveying, however, include efficiency and data quality. As surveying methods and equipment advance in sophistication over time, these goals can come into conflict with the goal of repeatability in the context of 4D surveying. This is because adopting new surveying techniques or equipment requires deviating from the techniques and equipment that were employed during the baseline survey. Doing so has been thought to undermine repeatability. 
     Embodiments described herein enable efficiency and data quality improvements to be captured during 4D survey processes while preserving repeatability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an overhead view illustrating a representative towed streamer marine seismic survey system. 
         FIG.  2    is a side elevation view of the marine seismic survey system of  FIG.  1   . 
         FIG.  3    is a schematic view illustrating a motion sensor collocated with a pressure sensor inside a streamer. 
         FIG.  4    is a schematic view illustrating a motion sensor collocated with a group of pressure sensors inside a streamer. 
         FIG.  5    is a rear view schematically illustrating several aspects of a dual source marine seismic survey. 
         FIG.  6    is a top view of a monitor survey according to embodiments in which a monitor survey vessel tows four sources asymmetrically. 
         FIG.  7    is a top view of a monitor survey according to embodiments in which a monitor survey vessel tows five sources symmetrically. 
         FIGS.  8  and  9    are rear views schematically illustrating several aspects of the monitor survey configuration of  FIG.  7   . 
         FIG.  10    is a rear view schematically illustrating aspects of a transition from a dual source baseline survey to a triple source monitor survey in accordance with embodiments. 
         FIG.  11    is a top view illustrating a transition from a dual source baseline survey to a triple source monitor survey in accordance with embodiments. 
         FIGS.  12 - 14    are top views schematically illustrating a class of monitor survey embodiments in which pairs of sources in the monitor survey are used either to repeat shot points from a previous survey or to provide additional shot points, or both. 
         FIG.  15    is a top view schematically illustrating a baseline survey in which source activation dither times are employed. 
         FIG.  16    is a top view schematically illustrating a monitor survey according to embodiments in which source activation dither times from the baseline survey of  FIG.  16    are used to repeat the shot points of the baseline survey. 
         FIG.  17    is a top view schematically illustrating a monitor survey according to embodiments in which shot point locations from the baseline survey of  FIG.  16    are used to repeat the shot points of the baseline survey. 
         FIG.  18    is a block diagram illustrating an example computer system suitable for use in implementing methods according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure describes multiple embodiments by way of example and illustration. It is intended that characteristics and features of all described embodiments may be combined in any manner consistent with the teachings, suggestions and objectives contained herein. Thus, phrases such as “in an embodiment,” “in one embodiment,” and the like, when used to describe embodiments in a particular context, are not intended to limit the described characteristics or features only to the embodiments appearing in that context. 
     The phrases “based on” or “based at least in part on” refer to one or more inputs that can be used directly or indirectly in making some determination or in performing some computation. Use of those phrases herein is not intended to foreclose using additional or other inputs in making the described determination or in performing the described computation. Rather, determinations or computations so described may be based either solely on the referenced inputs or on those inputs as well as others. The phrase “configured to” as used herein means that the referenced item, when operated, can perform the described function. In this sense an item can be “configured to” perform a function even when the item is not operating and is therefore not currently performing the function. Use of the phrase “configured to” herein does not necessarily mean that the described item has been modified in some way relative to a previous state. “Coupled” as used herein refers to a connection between items. Such a connection can be direct or can be indirect through connections with other intermediate items. Terms used herein such as “including,” “comprising,” and their variants, mean “including but not limited to.” Articles of speech such as “a,” “an,” and “the” as used herein are intended to serve as singular as well as plural references except where the context clearly indicates otherwise. 
       FIGS.  1  and  2    present top and side elevation views, respectively, of an example towed-streamer marine seismic survey system  100 . Survey system  100  is representative of a variety of similar geophysical survey systems in which a vessel  102  tows an array of elongate sensor streamers  104  in a body of water  106  such as an ocean, a sea, a bay, or a large lake. Vessel  102  is shown towing twelve streamers  104  in the illustrated example. In other embodiments, any number of streamers may be towed, from as few as one streamer to as many as twenty or more. Embodiments to be described below have useful application in relation to towed-streamer surveys such as that depicted in  FIGS.  1  and  2   . They may also have useful application in other environments in which different types of seismic sensor are used. For example, they may be employed in environments in which the seismic sensors are housed in ocean bottom cables or in ocean bottom nodes. 
     During a typical marine seismic survey, one or more seismic sources  108  are activated to produce acoustic energy  200  that propagates in body of water  106 . Energy  200  penetrates various layers of sediment and rock  202 ,  204  underlying body of water  106 . As it does so, it encounters interfaces  206 ,  208 ,  210  between materials having different physical characteristics, including different acoustic impedances. At each such interface, a portion of energy  200  is reflected upward while another portion of the energy is refracted downward and continues toward the next lower interface, as shown. Reflected energy  212 ,  214 ,  216  is detected by sensors  110  disposed at intervals along the lengths of streamers  104 . In  FIGS.  1  and  2   , sensors  110  are indicated as black squares inside each of streamers  104 . Sensors  110  produce signals corresponding to the reflected energy. These signals are collected and recorded by control equipment  112  located onboard vessel  102 . The recorded signals may be processed and analyzed onboard vessel  102  and/or at one or more onshore data centers to produce images of structures within subsurface  218 . These images can be useful, for example, in identifying possible locations of hydrocarbon reservoirs within subsurface  218 . 
     In the illustrated example, vessel  102  is shown towing a total of two sources  108 . In other systems such as those to be described below, different numbers of sources may be used, and the sources may be towed by other vessels, which vessels may or may not tow streamer arrays. Typically, a source  108  includes one or more source subarrays  114 , and each subarray  114  includes one or more acoustic emitters such as air guns or marine vibrators. A distinction between a “source” as used herein and a source subarray is that the crossline distance between two or more “sources” towed during a survey is greater than the crossline distance between subarray elements within any one of the two or more sources. Another distinction is that separate “sources” as used herein are capable of independent activation, whereas the subarray elements within a single source are typically not capable of independent activation, but rather may only be activated in tandem, responsive to a single source activation signal. 
     Each subarray  114  may be suspended at a desired depth from a subarray float  116 . Compressed air as well as electrical power and control signals may be communicated to each subarray via source umbilical cables  118 . Data may be collected, also via source umbilical cables  118 , from various sensors located on subarrays  114  and floats  116 , such as acoustic transceivers and global positioning system (“GPS”) units. Acoustic transceivers and GPS units so disposed help to accurately determine the positions of each subarray  114  during a survey. In some cases, subarrays  114  may be equipped with steering devices to better control their positions during the survey. 
     Streamers  104  are often very long, on the order of 5 to 10 kilometers, so usually are constructed by coupling numerous shorter streamer sections together. Each streamer  104  may be attached to a dilt float  120  at its proximal end (the end nearest vessel  102 ) and to a tail buoy  122  at its distal end (the end farthest from vessel  102 ). Dilt floats  120  and tail buoys  122  may be equipped with GPS units as well, to help determine the positions of each streamer  104  relative to an absolute frame of reference such as the earth. Each streamer  104  may in turn be equipped with acoustic transceivers and/or compass units to help determine their positions relative to one another. In many survey systems  100 , streamers  104  include steering devices  124  attached at intervals, such as every 300 meters. Steering devices  124  typically provide one or more control surfaces to enable moving the streamer to a desired depth, or to a desired lateral position, or both. Paravanes  126  are shown coupled to vessel  102  via tow ropes  128 . As the vessel tows the equipment, paravanes  126  provide opposing lateral forces that straighten a spreader rope  130 , to which each of streamers  104  is attached at its proximal end. Spreader rope  130  helps to establish a desired crossline spacing between the proximal ends of the streamers. Power, control, and data communication pathways are housed within lead-in cables  132 , which couple the sensors and control devices in each of streamers  104  to the control equipment  112  onboard vessel  102 . 
     Collectively, the array of streamers  104  forms a sensor surface at which acoustic energy is received for recording by control equipment  112 . In many instances, it is desirable for the streamers to be maintained in a straight and parallel configuration to provide a sensor surface that is generally flat, horizontal, and uniform. In other instances, an inclined and/or fan shaped receiving surface may be desired and may be implemented using control devices on the streamers such as those just described. Other array geometries may be implemented as well. In various embodiments, streamers  104  need not all have the same length and need not all be towed at the same depth or with the same depth profile. Moreover, prevailing conditions in body of water  106  may cause the depths and lateral positions of streamers  104  to vary at times. 
     Sensors  110  within each streamer  104  may include one or more different sensor types such as pressure sensors (e.g. hydrophones), velocity sensors (e.g. geophones), and acceleration sensors such as micro-electromechanical system (“MEMS”) devices.  FIGS.  3  and  4    illustrate two example arrangements according to embodiments for disposing sensors  110  in a streamer or cable  104 . In both illustrations, pressure sensors are indicated with white squares, while vector sensors such as velocity or acceleration sensors are indicated with shaded squares. In the arrangement of  FIG.  3   , a pressure sensor  300  is collocated with a vector sensor  302  inside a streamer  104 . In the arrangement of  FIG.  4   , a set of pressure sensors  300  forms a single pressure sensor group  400  inside a streamer  104 . A vector sensor  302  is disposed substantially at the center of pressure sensor group  400 . Typically, the signals generated by sensors forming a sensor group are combined or aggregated in some way, such as by summation and/or averaging. Such combination or aggregation may be accomplished in any suitable manner, such as in an analog domain using appropriate electrical coupling, or in a digital domain using digital data processing. In general, a sensor group may include any number of sensors and may comprise either pressure sensors or vector sensors. Normally, however, only measurements of the same type in a group (e.g. pressure, velocity, or acceleration) would be subject to combination or aggregation. Thus, in the arrangement of  FIG.  4   , the measurements of pressure sensors  300  may be combined or aggregated into a single signal, while the measurements of vector sensor  302  would be preserved as a separate signal. In other embodiments, streamers may be employed that contain only pressure sensors and no vector sensors. 
     Repeating a Previous Survey with Asymmetric Quad Sources 
     The survey layout depicted in  FIGS.  1  and  2    may be taken to represent a class of baseline surveys known as “dual source” surveys. In a dual source baseline survey, vessel  102  tows a total of two sources  108  to acquire the survey. Depending on the survey design, the two sources  108  may be activated at the same time or at different times. In general, such a baseline survey may include multiple sail passes. In each sail pass, vessel  102  follows a sail line  134  that defines the sail pass. 
     An arbitrary Cartesian xyz coordinate system is shown in  FIG.  1    for reference. In the illustration, the x direction is shown generally parallel to sail line  134  and is referred to as the “inline” direction. The y direction is orthogonal to the x direction and parallel to the surface of body of water  106 . The y direction is generally referred to as the “crossline” direction, as it crosses sail line  134 . The z direction points downward from the xy plane toward the subsurface  218  and is generally referred to as the “depth” direction. As can be seen in  FIG.  1   , the crossline positions of the centers of the two sources  108  define a crossline source spread  136 . In the general case for surveys that employ more than two sources, the crossline source spread would correspond to the distance between the centers of the outermost sources in the crossline direction (the “crossline outermost” sources). In the specific case of a dual source survey, the crossline source spread is simply the crossline distance between the centers of the two sources. 
       FIG.  5    is a rear view of a dual source configuration. In this drawing, vessel  102  is shown projected 90 degrees out of the xy plane for clarity of illustration. In reality, vessel  102  would be oriented in the xy plane with its stern facing the viewer. Moreover, sources  108  are shown at the surface of body of water  106  for clarity of illustration. In reality, each of sources  108  would normally be towed at a non-zero depth in accordance with various survey requirements, as depicted in  FIGS.  1  and  2   .  FIGS.  8 ,  9  and  10    make similar simplifications. Also note that, in each of  FIGS.  6 - 10  and  12 - 14   , a monitor survey configuration is shown superimposed over at least one previous survey configuration. This is done only for clarity of discussion and should not be construed to mean either that the monitor survey and previous survey are performed simultaneously, or that they be must or should be performed by the same survey operator. 
       FIG.  5    illustrates an arbitrary reflection surface  500 , which may be located within subsurface  218 . Shaded triangles  502  on reflection surface  500  illustrate crossline midpoints between the starboard source  108  and sensor streamers  104 . Similarly, white triangles  504  on reflection surface  500  illustrate crossline midpoints between port source  108  and sensor streamers  104 . Energy  506  generated by the starboard source  108  is shown being reflected at  502 . Energy  508  generated by the port source  108  is shown being reflected at  504 . As can be seen in the drawing, the crossline distance between the shaded and the white reflection points defines a crossline bin size d. This crossline bin size corresponds to a crossline resolution that is inherent in the survey design—i.e., in the number of and spacing between the sources, and the number of and spacing between the streamers. 
       FIG.  6    illustrates a top view of a monitor survey  600  according to embodiments in which monitor survey vessel  602  tows four sources  608  (the “monitor sources”) asymmetrically. In these embodiments, sources  608  are towed such that the crossline spacing between them is uniform, but such that their crossline center  609  exhibits a non-zero crossline offset relative either to sail line  134  or to the crossline center  607  of the sources  108  that were used in the previous survey. (In the embodiment shown, the crossline center of sources  108  is aligned with sail line  134 . In other embodiments, center  607  may have a non-zero crossline offset relative to sail line  134 .) Thus, sources  608  are towed asymmetrically relative either to sail line  134  or to the crossline center  607  of the baseline dual sources  108 . The term “asymmetrically” as used herein refers to either configuration. Any suitable towing technique may be employed to tow the four sources in the asymmetrical arrangement shown, such as with the use of conventional source steering devices. The same number of streamers  604  may be towed in the monitor survey as were towed in the baseline survey, and at the same crossline positions. Two of the monitor sources  608  occupy the same crossline positions as did the dual sources  108  of the baseline survey. 
     Source activation positions (“shot points”) from the baseline survey are shown in the drawing as stars inside of circles. The locations of baseline shot points  610  may be obtained from a “post plot” of the baseline survey. A post plot is a data set indicating, among other things, where each source was fired and when during a survey. As can be seen in the illustration, monitor survey  600  repeats shot points  610  that were produced during the baseline survey, and also produces shot points  612 ,  614  that are additional relative to baseline shot points  610 . As used herein, the term “repeated shot points” means shot points that were produced during a previous survey and that are being repeated in a monitor survey. The term “additional shot points” means shot points produced during a monitor survey that were not also produced during a previous survey that is being repeated by the monitor survey. 
     The additional shot points produced by any one source (or by any one pair of sources) during the monitor survey may occur at a non-constant interval, as depicted in the embodiment of  FIG.  6   , or they may occur at a constant interval. The choice of the interval or intervals to be used for producing additional shot points may take various factors into consideration such as, for example, air compressor recharge times and capabilities of the source controller hardware to be used during the monitor survey. In some embodiments, a programmable source controller may be used in order to produce non-constant shot point intervals for some or all of the additional shot points. 
     In embodiments corresponding to  FIG.  6   , some of the additional shot points ( 614 ) are located inside the crossline source spread  136  of the baseline survey, while others of the additional shot points ( 612 ) are located outside the crossline source spread  136  of the baseline survey. Among the benefits gained by additional shot points  612 ,  614  is that the crossline bin size of the monitor survey will be half that of the baseline survey, or d/2. Thus, the monitor survey achieves twice the crossline resolution as does the baseline survey, providing an enhancement in data quality relative to the baseline survey while also faithfully repeating the baseline survey. 
     In some embodiments, source steering can be employed such that the four monitor sources  608  are towed asymmetrically to the starboard side during one part of a monitor survey, and asymmetrically to the port side during another part of the monitor survey. That is, when towing asymmetrically to the starboard side, the crossline center  609  of the monitor sources would have a crossline offset to the starboard side of either the sail line  134  of the previous survey or the crossline center  607  of the previous survey source spread. When towing asymmetrically to the port side, the crossline center  609  of the monitor sources would have a crossline offset to the port side of either the sail line  134  of the previous survey or the crossline center  607  of the previous survey source spread. In some embodiments, changing between the port and starboard asymmetric configurations during the monitor survey may be performed based on a post plot of the previous survey, such that the configuration chosen at any given time is the one that will require the lesser amount of source steering during a corresponding portion of the monitor survey. In still further embodiments, changing between the two configurations may be performed within a single sail line of the monitor survey. 
     Repeating a Previous Survey with Symmetric Penta Sources 
       FIG.  7    is a top view illustrating a class of embodiments in which a monitor survey  700  employs five monitor sources  708 . In the illustration, monitor survey  700  is shown repeating a previous dual source survey, as in the example of  FIG.  6   . Monitor survey vessel  702  is shown superimposed over previous survey vessel  102 . Locations of the sources  108  of the previous survey are shown with circles. In monitor survey  700 , monitor vessel  702  tows monitor sources  708  symmetrically. That is, they are towed such that sources  708  have a uniform crossline separation between them and such that their crossline center  709  coincides with either a sail line  134  of the previous survey or with a crossline center  707  of the source spread used during the previous survey. The term “symmetrically” as used herein refers to either configuration. The same number of streamers  704  may be towed in the monitor survey as were towed in the baseline survey, and at the same crossline positions. 
     Monitor survey  700  produces repeated shot points  710 , indicated as stars within circles, and also produces additional shot points  712 ,  714 . Some of the additional shot points ( 712 ) are located outside the crossline source spread  136  of the previous survey, while others of the additional shot points ( 714 ) are located inside the crossline source spread  136  of the previous survey. As can be seen, the configuration of  FIG.  7    achieves the same doubling of crossline resolution as was achieved by the quad-source configuration of  FIG.  6   . That is, the crossline bin size of the monitor survey, d/2, is half the crossline bin size, d, of the baseline survey. 
       FIGS.  8  and  9    are rear views illustrating how a symmetric penta-source configuration such as that shown in  FIG.  7    may be used in some embodiments to mimic an asymmetric quad-source configuration such as that shown in  FIG.  6   . The technique shown in  FIGS.  8  and  9    provides several benefits. First, it provides the same doubling of crossline resolution (halving the crossline bin size) as do the configurations of  FIGS.  6  and  7   . Second, the configuration of  FIGS.  8  and  9    does not require as much source steering to maintain as does the configuration of  FIG.  6   . Third, the configuration of  FIGS.  8  and  9    produces additional shot points that are both inside and outside the crossline source spread of the previous survey. 
     In both of  FIGS.  8  and  9   , subsurface reflection points  716 - 722  of the monitor survey are shown with triangles having similar shading as corresponding ones of the monitor sources  708 . Similarly, subsurface reflection points  724 - 726  of the previous survey are shown with triangles having similar shading as corresponding ones of the previous survey sources  108 . As was the case in  FIG.  7   , the same number of streamers  704  may be towed in the monitor survey as were towed in the baseline survey, and at the same crossline positions. 
     During one part of the monitor survey ( FIG.  9   ), the port-most monitor sources, indicated at  709 , are activated so as to mimic four sources towed asymmetrically to the port side. During another part of the monitor survey ( FIG.  8   ), the starboard-most monitor sources, indicated at  711 , are activated so as to mimic four sources towed asymmetrically to the starboard side. In some embodiments, as in the quad-source configuration of  FIG.  6   , changing between the port-most and the starboard-most configurations may occur within a single sail line of the previous survey. In additional embodiments, changing between the two configurations may be performed based on a post plot of the previous survey. 
     Transitioning from Dual Source Baseline Surveys to Triple Source Monitor Surveys 
       FIGS.  10  and  11    are rear and top views, respectively, illustrating a class of embodiments in which a transition may be achieved from a dual source baseline survey  100  to a triple source monitor survey  1000 . 
     Referring first to  FIG.  11   , vessel  102  performs a baseline survey  100  by towing two sources  108  and an array of streamers as shown. The crossline outermost streamers define a streamer spread  140  for the baseline survey. The coverage area achieved by this configuration is indicated generally at  113 . Vessel  102  is shown at three different times during the baseline survey, towing the streamer spread first along sail line  134 , then along sail line  137 , and then along sail line  139 . As can be seen, the alignment of the coverage areas  113  yields full coverage over the area being surveyed. 
     Later in time, vessel  702  performs a monitor survey  700  over the same area by towing five sources  708  and a first monitor survey streamer spread. The crossline outermost streamers in the first monitor survey streamer spread define a streamer spread  740  that is the same as streamer spread  140 . As was the case with the baseline survey, vessel  702  follows each of sail lines  134 ,  137  and  139 , and the alignment of coverage areas  713  yields full coverage over the area. 
     Later still, vessel  1002  performs a second monitor survey  1000  over the area by towing three sources  1008  and a second monitor survey streamer spread. The crossline outermost streamers in the second monitor survey streamer spread define a streamer spread  1040  that is twice the width of streamer spreads  140  and  740 . In other embodiments, streamer spread  1040  may be more than twice the width of streamer spreads  140  and  740 . For example, the streamer array towed during the second monitor survey may contain more streamers than were used in either of the previous surveys. The configuration of survey  1000  creates a coverage area  1013  that is larger than coverage areas  113  and  713 . Thus, vessel  1002  need not follow each of sail lines  134 ,  137  and  139  to yield full coverage over the survey area. Instead, vessel  1002  need only follow every other one of the sail lines that were followed during the previous surveys. For example, in the drawing, vessel  1002  is shown following only sail lines  134  and  139 . As can be seen in the drawing, however, the alignment of coverage areas  1013  during the second monitor survey nevertheless yields full coverage of the area. Because only every other one of the previous survey sail lines must be followed during the second monitor survey, efficiency is gained relative to both of the previous surveys. 
     At the same time, both data quality and repeatability are preserved. This can be seen more easily with reference to  FIG.  10   . In  FIG.  10   , the three sources  1008  of monitor survey  1000  are shown with shaded stars. These sources are shown superimposed over the five sources  708  of monitor survey  700 , which are illustrated with white stars. As  FIG.  10    illustrates, monitor vessel  1002  tows sources  1008  with uniform crossline separations between them. The three monitor sources  1008  coincide with crossline positions of three of the sources  708  that were used in the previous survey. Specifically, their positions coincide with the crossline outermost sources from the previous survey and with the center source from the previous survey. As  FIG.  10    also illustrates, streamers  1004  (shown with shaded circles), may be towed during monitor survey  1000  with greater crossline separation between them relative to the crossline separation of streamers  704  of the previous survey (shown with white circles). In the illustrated embodiment, the total number of monitor streamers  1004  is the same as the total number of streamers  708  and  108  used in the previous surveys. Thus, streamers  1004  may be towed with twice the crossline separation of the previous surveys. In other embodiments, different numbers of streamers and different crossline separations may be used. As was mentioned above, streamer spread  1040  may be designed to be more than twice that of streamer spreads  740  and  140 . For example, the number of streamers in streamer spread  1040  may be greater than the number of streamers in either of streamer spreads  740  or  140 . 
       FIG.  10    illustrates how data quality and repeatability are preserved by the second monitor survey. Subsurface reflection points  1010  of monitor survey  1000  coincide with subsurface reflection points  1012  of monitor survey  700 . Thus, comparison of results between survey  1000  and survey  700  is meaningful. Moreover, the crossline bin size, d, corresponding to monitor survey  1000  is the same as the crossline bin size, d, of baseline survey  100 . Thus, crossline resolution is maintained relative to baseline survey  100 . In this manner, a transition can be achieved from a dual source baseline survey to three-source (“triple source”) monitor surveys, achieving improvements in survey efficiency while preserving both data quality and repeatability. 
     Source Pairing for Repeated Shot Points and Additional Shot Points 
       FIGS.  12 - 14    illustrate a class of embodiments in which pairs of sources in a monitor survey are used either to repeat shot points from a previous survey or to provide additional shot points, or both. 
     In monitor survey  1200  ( FIG.  12   ), monitor vessel  1202  tows monitor streamers  1204  and five monitor sources  1208  having distinct crossline positions and having uniform crossline separation between them. Monitor sources  1208  are towed asymmetrically. That is, there is a crossline offset between the crossline midpoint of sources  1208  and either sail line  134  or a crossline midpoint  1207  between sources  108  from the previous survey. In all of the embodiments of  FIGS.  12 - 14   , paired monitor sources may be activated together or separately. Monitor source pairs  1216  and  1220  are used to repeat shot points  1210  from the previous survey. Monitor source pair  1218  is used to provide additional shot points  1214  located inside the crossline source spread  136  of the previous survey, while monitor source pair  1222  is used to provide additional shot points  1212  located outside the crossline source spread  136  of the previous survey. Moreover, each of monitor sources  1208  may be actuated individually or in other pairings to yield yet further additional shot points (not shown) that were not produced in the previous survey. 
     In monitor survey  1300  ( FIG.  13   ), monitor vessel  1302  tows monitor streamers  1304  and six monitor sources  1308  having distinct crossline positions and having uniform crossline separation between them. Monitor sources  1308  are towed symmetrically. That is, the crossline midpoint of sources  1308  coincides with either sail line  134  from the previous survey or with a crossline midpoint  1307  between sources  108  from the previous survey. Monitor source pairs  1318  and  1322  are used to repeat shot points  1310  from the previous survey. Monitor source pairs  1316  and  1324  are used to produce additional shot points  1312  located outside the crossline source spread  136  of the previous survey, while source pair  1320  is used to produce additional shot points  1314  located inside the crossline source spread  136  of the previous survey. Moreover, each of monitor sources  1308  may be actuated individually or in other pairings to yield yet further additional shot points (not shown) that were not produced in the previous survey. 
     In monitor survey  1400  ( FIG.  14   ), monitor vessel  1402  tows monitor streamers  1404  and ten monitor sources  1408  having distinct crossline positions and having uniform crossline separation between them. Monitor sources  1408  are towed symmetrically. That is, the crossline midpoint of sources  1408  coincides with either sail line  134  from the previous survey or with a crossline midpoint  1407  between sources  108  from the previous survey. As in the configurations of  FIGS.  12  and  13   , some of monitor source pairs  1416 - 1428  are used to repeat the shot points of the previous survey, while others are used to produce additional shot points  1412 ,  1414  that are located outside and inside the crossline source spread  136  of the previous survey, respectively, and as shown. In other embodiments, different numbers of source pairs may be used. 
     In  FIG.  14   , note that previous survey shot points  1410 ,  1411  and  1413  were all produced by one previous survey source  108 ′ that was located on the starboard side of vessel  102  during the previous survey. During monitor survey  1400 , however, three different monitor source pairs,  1430 ,  1428  and  1426 , are used to repeat shot points  1410 ,  1411  and  1413 , respectively. Each of pairs  1426 ,  1428  and  1430  represents a subset of monitor sources  1408 , but none of the subsets is identical to the other two subsets. Rather, three distinct monitor source subsets are used at different times to repeat shot points that were produced by a single source during the previous survey. Among the benefits achieved by this technique is that source steering may be minimized during monitor survey  1400 , even in circumstances where the post plot of the previous survey exhibits crossline variations in shot point locations attributable to one or more of the previous survey sources. A similar technique may be employed in any of the previously described configurations in which three or more sources are used during the monitor survey. 
     In still other embodiments, source pairs not being used to reproduce shot points from the previous survey may be used to produce additional shot points, and those or other source pairs may be used to mimic the asymmetric quad source arrangement of  FIG.  6    according to the techniques described in relation to  FIGS.  8  and  9   . 
     Use of Additional Monitor Sources to Enhance Previous Survey Dithering Patterns 
       FIGS.  15 - 17    illustrate a class of embodiments in which additional sources provided in a monitor survey may be used to repeat dithered shot points from a previous survey or to provide additional dithered shot points during the monitor survey, or both. The additional shot points may be used to enhance or complement the dithering pattern that was used in the previous survey. 
     “Dithering” may be understood with reference to  FIG.  15   , which illustrates a three-source baseline survey  1500 . Vessel  1502  tows sources at different crossline positions, as shown. Constant shot point intervals are indicated in the drawing by black-filled circles  1504 . One illustrative example of a constant shot point interval would be activating one of the three sources every 12.5 meters. Other examples of constant shot point intervals are also possible. A sequence in which each of the sources is activated once is referred to as an “activation sequence.” One such activation sequence is indicated in the drawing at  1506 . In each of  FIGS.  15 - 17   , source activations are illustrated by shaded stars. 
     In a dithered survey such as the survey of  FIG.  15   , rather than activating the sources at regular distances corresponding to a constant shot point interval, vessel  1502  instead activates the sources with varying positive or negative delays  1550 - 1560  relative to a constant shot point interval. Such delays are known as “dither times” or “dither values” and, collectively, constitute a “dithering pattern.” In the embodiment shown, the delays vary in time a range between 0 and 1 seconds. Other delay time ranges are also possible. Dither times typically vary from one activation sequence to another and may do so in a systematic, random or pseudo random manner. Such variations may be artificially introduced by means of manipulating source activation times, or they may be produced in a so-called “natural” manner by relying on variations in the environment in which the survey is being performed, such as varying ocean currents. The variations may also be produced by other influences such as random errors occurring in GPS units that are used during the survey. Regardless of which dithering pattern or dithering method is employed, a post plot of the survey typically captures the actual firing times and locations for each shot point produced. 
       FIGS.  16  and  17    illustrate two different techniques according to embodiments for repeating a dithered survey such as survey  1500 . In monitor surveys  1600  and  1700 , any of the source and streamer configurations previously described herein may be employed and, thus, additional sources may be used during the monitor surveys relative to the number of sources that were used in the previous survey. For clarity of illustration, however, not all of the monitor survey sources appear in  FIGS.  16  and  17   , and only the shot points being repeated by the monitor survey are shown. 
     In the method of  FIG.  16   , monitor vessel  1602  activates the monitor survey sources by generating source activation signals at times that are based on the dither times, or the dithering pattern, that was employed during the baseline survey. That is, monitor vessel  1602  repeats the baseline survey shot points by introducing the same delays  1550 - 1560  in source firing times, relative to regular shot point intervals  1604 , as were introduced during the baseline survey. 
     In the method of  FIG.  17   , monitor vessel  1702  activates the monitor sources according to shot point locations  1562 - 1572  that were recorded during the baseline survey. That is, vessel  1702  repeats the baseline survey shot points by following a post plot of shot point positions corresponding to the previous survey and by activating the monitor sources accordingly. 
     In either of the above two methods, the monitor vessel may also produce additional shot points by using any of the techniques previously described herein. According to embodiments, these additional shot points may be produced at times and/or locations that are calculated to exhibit incoherence compared to relevant ones of the repeated shot points. In this context, relevant ones of the repeated shot points would normally include those whose reflected energy will be recorded simultaneously with reflected energy from a given additional shot point under consideration. For example, relevant ones of the repeated shot points may include shot points that fall within a given activation sequence or shot points that are adjacent to a given activation sequence, or both. The incoherence so introduced may comprise incoherence with respect to time, or with respect to one or more spatial dimensions, or may comprise a combination of these. In this manner, both the repeated shot points and the additional shot points produced during the monitor survey may be efficiently deblended from one another according to known techniques that exploit such incoherency. (“Deblending” refers to known processes for separating, from recorded seismic data, energy that is attributable to an activation of one source during a survey from energy that is attributable to an activation of a different source used during the same survey.) 
     One method of analyzing and/or planning source activations for the additional shot points is to consider them over a period of time or a unit of distance that is of interest. The period of time or the unit of distance over which the source activations are considered may vary. For example, the period of time or the unit of distance may correspond to one activation sequence of the sources to be used during a survey. As another example, the period of time or the unit of distance may correspond to all source activations that will occur during one full sail line of a survey. Other examples are also possible. In either case, a nominal shot time interval or shot distance interval may be visualized as the center of a horizontal axis that represents time or distance, as appropriate. Each source activation being planned may then be placed on the horizontal axis at an offset that represents a difference between the given source activation time or distance and the nominal source activation time or distance. In embodiments, the timing and/or the spacing of the additional shot points to be produced during a 4D monitor survey may be designed such that the offsets from nominal for all of the source activations that will occur during a period of interest (including both the additional shot points and those being repeated from a prior survey) are substantially evenly distributed. Stated differently, the timing and/or the spacing of the additional shot points to be produced during the 4D monitor survey may be planned so as to avoid clustering of source activation offsets when the planned additional shot points and relevant ones of the previous survey shot points are considered together, and wherein the offsets represent deviations from a nominal source activation interval. In this manner, the dithering pattern used during a monitor survey may enhance or complement the dithering pattern of a survey that is being repeated. 
     Computer System 
       FIG.  18    is a block diagram illustrating an example computer system  1800  that may be used to perform any of the methods described above. A computer system such as computer system  1800  may also be used to produce a computer-readable survey plan comprising instructions that, if followed by navigation and control equipment onboard or otherwise associated with a survey vessel, cause the equipment to perform any of the methods described above. 
     Computer system  1800  includes one or more central processor unit (“CPU”) cores  1802  coupled to a system memory  1804  by a high-speed memory controller  1806  and an associated high-speed memory bus  1807 . System memory  1804  typically comprises a large array of random-access memory locations, often housed in multiple dynamic random-access memory (“DRAM”) devices, which in turn are housed in one or more dual inline memory module (“DIMM”) packages. Each CPU core  1802  is associated with one or more levels of high-speed cache memory  1808 , as shown. Each core  1802  can execute computer-readable instructions  1810  stored in system memory  1804 , and can thereby perform operations on data  1812 , also stored in system memory  1804 . 
     Memory controller  1806  is coupled, via input/output bus  1813 , to one or more input/output controllers such as input/output controller  1814 . Input /output controller  1814  is in turn coupled to one or more tangible, non-volatile, computer readable media such as computer-readable medium  1816  and computer-readable medium  1818 . Non-limiting examples of such computer-readable media include so-called solid-state disks (“SSDs”), spinning-media magnetic disks, optical disks, flash drives, magnetic tape, and the like. Media  1816 ,  1818  may be permanently attached to computer system  1800  or may be removable and portable. In the example shown, medium  1816  has instructions  1817  (software) stored therein, while medium  1818  has data  1819  stored therein. Operating system software executing on computer system  1800  may be employed to enable a variety of functions, including transfer of instructions  1810 ,  1817  and data  1812 ,  1819  back and forth between media  1816 ,  1818  and system memory  1804 . 
     Computer system  1800  may represent a single, stand-alone computer workstation that is coupled to input/output devices such as a keyboard, pointing device and display. It may also represent one node in a larger, multi-node or multi-computer system such as a cluster, in which case access to its computing capabilities may be provided by software that interacts with and/or controls the cluster. Nodes in such a cluster may be collocated in a single data center or may be distributed across multiple locations or data centers in distinct geographic regions. Further still, computer system  1800  may represent an access point from which such a cluster or multi-computer system may be accessed and/or controlled. Any of these or their components or variants may be referred to herein as “computing apparatus” or a “computing device.” 
     Instructions  1817  may correspond to algorithms for performing any of the methods described herein or for producing a computer-readable survey plan for implementing one or more of such methods. In such embodiments, instructions  1817 , when executed by one or more computing devices such as one or more of CPU cores  1802 , cause the computing device to perform methods described herein, or to perform operations described herein on data  1819 , producing results that may be stored in one or more tangible, non-volatile, computer-readable media such as medium  1818 . In some embodiments, data  1819  may correspond to marine seismic sensor measurements or other signals recorded during a marine geophysical survey performed according to methods described herein or may correspond to a survey plan for implementing any of the methods described herein. 
     In such embodiments, medium  1818  constitutes a geophysical data product that is manufactured by using the computing device to perform methods described herein and by storing the results in the medium. Geophysical data product  1818  may be stored locally or may be transported to other locations where further processing and analysis of its contents may be performed. If desired, a computer system such as computer system  1800  may be employed to transmit the geophysical data product electronically to other locations via a network interface  1820  and a network  1822  (e.g. the Internet). Upon receipt of the transmission, another geophysical data product may be manufactured at the receiving location by storing contents of the transmission, or processed versions thereof, in another tangible, non-volatile, computer readable medium. Similarly, geophysical data product  1818  may be manufactured by using a local computer system  1800  to access one or more remotely-located computing devices in order to execute instructions  1817  remotely, and then to store results from the computations on a medium  1818  that is attached either to the local computer or to one of the remote computers. The word “medium” as used herein should be construed to include one or more of such media. 
     In any of the above-described embodiments, such a computing device may be used to generate first and second distinct data sets to represent results of a 4D monitor survey. The first data set may correspond just to the set of repeated shot points that were performed during the 4D monitor survey, such that each of the shot points in the first data set corresponds to a shot point from the previous survey. The second data set may include both the set of repeated shot points and the set of additional shot points that were produced during the 4D monitor survey. 
     Conclusion 
     Multiple specific embodiments have been described above and in the appended claims. Such embodiments have been provided by way of example and illustration. Persons having skill in the art and having reference to this disclosure will perceive various utilitarian combinations, modifications and generalizations of the features and characteristics of the embodiments so described. For example, steps in methods described herein may generally be performed in any order, and some steps may be omitted, while other steps may be added, except where the context clearly indicates otherwise. Similarly, components in structures described herein may be arranged in different positions or locations, and some components may be omitted, while other components may be added, except where the context clearly indicates otherwise. The scope of the disclosure is intended to include all such combinations, modifications, and generalizations as well as their equivalents.