Patent Publication Number: US-2021190985-A1

Title: Marine Survey Data Acquisition at a Tow Line

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application 62/949,666, filed Dec. 18, 2019 and to U.S. Provisional Application 63/026,899, filed May 19, 2020, which are incorporated by reference as if entirely set forth herein. 
    
    
     BACKGROUND 
     In the past few decades, the petroleum industry has invested heavily in the development of marine survey techniques that yield knowledge of subterranean formations beneath a body of water in order to find and extract valuable mineral resources, such as oil. High-resolution images of a subterranean formation are helpful for quantitative interpretation and improved reservoir monitoring. For a typical marine survey, a marine survey vessel tows one or more marine survey sources (hereinafter referred to as “sources”) below the sea surface and over a subterranean formation to be surveyed. Marine survey receivers (hereinafter referred to as “receivers”) may be located on or near the seafloor, on one or more streamers towed by the marine survey vessel, or on one or more streamers towed by another vessel. The marine survey vessel typically contains marine survey equipment, such as navigation control, source control, receiver control, and recording equipment. The source control may cause the one or more sources, which can be impulsive sources such as air guns, non-impulsive sources such as marine vibrator sources, electromagnetic sources, etc., to produce signals at selected times. Each signal is essentially a wave called a wavefield that travels down through the water and into the subterranean formation. At each interface between different types of rock, a portion of the wavefield may be refracted, and another portion may be reflected, which may include some scattering, back toward the body of water to propagate toward the sea surface. The receivers thereby measure a wavefield that was initiated by the actuation of the source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevation or xz-plane view illustrating marine surveying in which signals are emitted by a source for recording by receivers. 
         FIG. 2  is a plan or yx-plane view of a marine survey setup illustrating examples of embodiments of receivers associated with tow lines. 
         FIG. 3A  is a plan or yx-plane view of a marine survey setup illustrating an example of a critical distance. 
         FIG. 3B  is a plot of time versus offset illustrating a critical distance. 
         FIG. 3C  is an elevation or xz-plane view of a state representing marine surveying illustrating a critical distance as well as primaries and multiples represented as rays. 
         FIG. 4  illustrates an exemplary embodiment of a system for marine survey data acquisition at a tow line. 
         FIG. 5  illustrates an exemplary embodiment of a method for marine survey data acquisition at a tow line. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure is related to a marine survey data acquisition at a tow line. A marine survey can be a seismic survey, electromagnetic survey, or other types or combinations of types of marine surveys. Marine survey data is data obtained from a marine survey and can include geophysical data, which is data describing the earth. The marine survey can measure physical properties of the subsurface, along with anomalies in those properties, which can be used to detect or infer the presence and position of economically useful geological deposits such as hydrocarbons. As mentioned above, for a typical marine survey, receivers may be located on or near the seafloor or on one or more streamers towed by a marine survey vessel. Some marine surveys may include the use of nearfield receivers, such as hydrophones, in the vicinity of the source in order to receive a source wavefield directly from the source without it being reflected or refracted. In contrast, according to at least one embodiment of the present disclosure, a receiver is associated with a tow line, and the receiver is configured to receive marine survey data at a location of the tow line. That the receiver is associated with the tow line means that the receiver is coupled to or integrated with the tow line. A tow line is a piece of marine survey equipment that is used to tow an object behind a marine survey vessel in the water. The tow line is coupled to the marine survey vessel and to the towed object. Examples of tow lines, described in more detail below with respect to  FIG. 2 , include lead-ins and umbilicals, among others. Streamers, which include receivers, are not tow lines. A streamer is a towed object, which can be towed by a tow line such as a lead-in. 
     At least one embodiment of the present disclosure can advantageously provide for the acquisition of marine survey data at negative source-receiver offsets. An offset is the distance between the source and a receiver. Typically, the direction downstream of the source (behind the source in the direction of travel) is characterized as being the positive direction. Therefore, as described in more detail below, negative source-receiver offsets refer to the positioning of receivers upstream of the source (ahead of the source in the direction of travel) such that the offset between the source and receiver is characterized as being negative. According to at least one embodiment of the present disclosure, this acquisition at negative source-receiver offsets can be achieved without the use of additional tow lines beyond those that would otherwise be used to tow objects, such as streamers or sources, as part of the marine survey. Accordingly, acquisition at negative source-receiver offsets according to at least one embodiment of the present disclosure does not increase the complexity of the towing operation of the marine survey, nor does it significantly increase the cost of performing the marine survey. 
     The present disclosure describes various embodiments relating to primary reflections (“primaries”) and multiple reflections (“multiples”). Primaries are generally up-going reflections of a down-going source wavefield. Multiples can be divided into down-going multiples and up-going multiples. A down-going multiple is a reflection of a wavefield (whether a primary or a multiple) off of a sea surface. An up-going multiple is a reflection of a down-going multiple. 
     As used herein, the singular forms “a”, “an”, and “the” include singular and plural referents unless the content clearly dictates otherwise. Furthermore, the word “may” is used throughout this application in a permissive sense (i.e., having the potential to, being able to), not in a mandatory sense (i.e., must). The term “include,” and derivations thereof, mean “including, but not limited to.” The term “coupled” means directly or indirectly connected and, unless stated otherwise, can include a wireless connection. 
     The figures herein follow a numbering convention in which the first digit or digits correspond to the drawing figure number and the remaining digits identify an element or component in the drawing. Similar elements or components between different figures may be identified by the use of similar digits. For example, 126 may reference element “26” in  FIG. 1 , and a similar element may be referenced as  226  in  FIG. 2 . Analogous elements within a Figure may be referenced with a hyphen and extra numeral or letter. See, for example, elements  240 - 1 , and  240 - 2  in  FIG. 2 . Such elements may be referenced generally without the hyphen and extra numeral. As will be appreciated, elements shown in the various embodiments herein can be added, exchanged, and/or eliminated so as to provide a number of additional embodiments of the present disclosure. In addition, as will be appreciated, the proportion and the relative scale of the elements provided in the figures are intended to illustrate certain embodiments of the present invention and should not be taken in a limiting sense. 
       FIG. 1  is an elevation or xz-plane  130  view illustrating marine surveying in which signals are emitted by a source  126  for recording by receivers  122 . The recording can be used for processing and analysis in order to help characterize the structures and distributions of features and materials underlying the surface of the earth. For example, the recording can be used to estimate a physical property of a subsurface location, such as the presence of a reservoir that may contain hydrocarbons.  FIG. 1  shows a domain volume  102  of the earth&#39;s surface comprising a subsurface volume  104  of sediment and rock below the surface  106  of the earth that, in turn, underlies a fluid volume  108  of water having a sea surface  109  such as in an ocean, an inlet or bay, or a large freshwater lake. The domain volume  102  shown in  FIG. 1  represents an example experimental domain for a class of marine surveys.  FIG. 1  illustrates a first sediment layer  110 , an uplifted rock layer  112 , an underlying rock layer  114 , and a hydrocarbon-saturated layer  116 . One or more elements of the subsurface volume  104 , such as the first sediment layer  110  and the uplifted rock layer  112 , can be an overburden for the hydrocarbon-saturated layer  116 . In some instances, the overburden may include salt. 
       FIG. 1  shows an example of a marine survey vessel  118  equipped to carry out marine surveys in accordance with the present disclosure. In particular, the marine survey vessel  118  can tow one or more streamers  120  (shown as one streamer for ease of illustration) generally located below the sea surface  109 . The streamers  120  can be long cables containing power and data-transmission lines (e.g., electrical, optical fiber, etc.) to which receivers may be coupled. In one type of marine survey, each receiver, such as the receiver  122  represented by the shaded disk in  FIG. 1 , comprises a pair of sensors including a geophone that detects particle displacement within the water by detecting particle motion variation, such as velocities or accelerations, and/or a receiver that detects variations in pressure. However, embodiments are not so limited. Surveys can include receivers  122  that are only pressure sensors, pressure and motion sensors, or electromagnetic sensors, among others. In one type of marine survey, each receiver, such as the receiver  122 , comprises an electromagnetic receiver that detects electromagnetic energy within the water. The streamers  120  and the marine survey vessel  118  can include sensing electronics and data-processing facilities that allow receiver readings to be correlated with absolute locations on the sea surface and absolute three-dimensional locations with respect to a three-dimensional coordinate system. In  FIG. 1 , the receivers along the streamers  120  are shown to lie below the sea surface  109 , with the receiver locations correlated with overlying surface locations, such as a surface location  124  correlated with the location of receiver  122 . 
     The marine survey vessel  118  can tow a source  126  that produces signals as the marine survey vessel  118  and streamers  120  move across the sea surface  109 . The source  126  and/or streamers  120  may also be towed by other vessels or may be otherwise disposed in fluid volume  108 . For example, the receivers may be located on ocean bottom cables or nodes fixed at or near the surface  106 . For the sake of efficiency, illustrations and descriptions herein show receivers located on streamers, but it should be understood that references to receivers located on a “streamer” or “cable” should be read to refer equally to receivers located on a towed streamer, an ocean bottom receiver cable, and/or an array of nodes. 
       FIG. 1  shows acoustic energy illustrated as an expanding, spherical signal, illustrated as semicircles of increasing radius centered at the source  126 , representing a down-going wavefield  128 , following a signal emitted by the source  126 . The down-going wavefield  128  is, in effect, shown in a vertical plane cross section in  FIG. 1 . The outward and downward expanding down-going wavefield  128  may eventually reach the surface  106 , at which point the outward and downward expanding down-going wavefield  128  may partially scatter, may partially reflect back toward the streamers  120 , the receivers  122  associated therewith, and the receivers  123  associated with various tow lines. The wavefield  128  may partially refract downward into the subsurface volume  104 , becoming elastic signals within the subsurface volume  104 . 
     According to at least one embodiment of the present disclosure, an additional receiver  123  is associated with a tow line. For example, a first receiver  123 - 1  is illustrated as being associated with the tow line coupled to the source  126  and a second additional receiver  123 - 2  is illustrated as being associated with the tow line coupled to the streamer  120 . Embodiments are not limited to a particular quantity or location of the additional receivers associated with the tow lines. The receivers  123  are configured to receive marine survey data at a location of the tow line with which they are associated. The additional receivers  123  can includes sensors such as those described above with respect to the receiver  122 . The additional receivers  123  can include sensors such as passive accelerometer-based sensors on piezoceramic piezoelectric structures or microelectromechanical systems (MEMS) sensors, among others. Alternatively, the additional receivers can be optical fibers that are internal to or external to the tow lines with which they are associated, as described in more detail with respect to  FIG. 4 . For example, the additional receiver  123 - 1  can be an optical fiber that runs along an interior space of the tow line coupled to the source  126  and configured to receive marine survey data at a location of that tow line. 
     The marine survey vessel  118  can include a controller  119 . Although not specifically illustrated in  FIG. 1 , the marine survey vessel  118  can include an interrogator coupled to or integrated with the controller  119  for embodiments in which the receivers  123  comprise optical fibers. The controller  119  can generally control the source  126  and receivers  122 ,  123 . The controller  119  can include an actuation system and a recording system, which may be separate systems that communicate data between each other, or they may be sub-systems of an integrated system. The control system can be configured to selectively actuate the source  126 , while the recording system can be configured to record the signals received by receivers  122 ,  123  in response to the energy imparted into the water and thereby into subterranean formations below the solid surface  104 . The recording system can be configured to determine and record the geodetic positions of the source  126  and the receivers  122 ,  123  at any time. Source actuation and signal recording by the receivers  122 ,  123  may be repeated a plurality of times while the marine survey vessel  118  moves through the water. Each actuation record may include, for each receiver  122 ,  123 , signals corresponding to the energy produced by the source  126 . 
       FIG. 2  is a plan or yx-plane  232  view illustrating a marine survey setup including examples of embodiments of receivers  223  associated with tow lines.  FIG. 2  shows an example of a marine survey vessel  218  equipped to carry out marine surveys and to tow one or more towed objects through water. The towed objects can be streamers  220 , which can include one or more receivers  222 . Other examples of towed objects include sources  226 . Although only one source  226  appears in  FIG. 2  for ease of illustration, embodiments are not so limited. The towed objects can be coupled to marine survey equipment, particularly towing equipment, that maintains the towed objects at selected positions, including depth and lateral positions, with respect to each other and with respect to the marine survey vessel  218 . 
     The towing equipment includes various tow lines. Tow lines are lines coupled between the marine survey vessel  218  and a towed object. Although not required for any particular embodiment, but shown for illustration, each of the tow lines illustrated in  FIG. 2  includes and associated receiver  223 . Although the receivers  223  are illustrated as circles at particular locations along the tow lines, embodiments are not so limited. The receivers  223  can be in different positions than those illustrated along the tow lines, or can be distributed along the tow lines, such as in the case of an optical fiber receiver  223 . At least a portion of the optical fiber receiver  223  can be positioned with respect to the tow line to be towed in water at a negative offset from the source  226 . At least a portion of the optical fiber receiver  223  can be positioned with respect to the tow line to be towed in water within a critical distance from the source  226 . The critical distance is described in more detail with respect to  FIG. 3 . The tow lines and associated receivers  223  illustrated in  FIG. 2  are just one example. Other towing configurations and arrangements of receivers associated with tow lines can be used in different embodiments. 
     The tow lines illustrated in  FIG. 2  include umbilicals  234 - 1 ,  234 - 2  coupled between the marine survey vessel  218  and the source  226 . The umbilicals  234  can include power cables, air cables, or other cabling used to communicate with or provide energy to the source  226 . The umbilicals  234  also provide the tension necessary to tow the source  226  through the water. The first umbilical  234 - 1  is illustrated as having an associated receiver  223 - 1  and the second umbilical  234 - 2  is illustrated as having an associated receiver  223 - 2 . The receivers  223 - 1 ,  223 - 2  associated with the umbilicals  234 - 1 ,  234 - 1  can be located at a negative source-receiver offset because the receivers  223 - 1 ,  223 - 2  are upstream from the source  226  (ahead of the source  226  in the direction of travel, in the in-line direction  229 ). 
     The in-line direction  229  is generally colinear with the x-axis and in line with the direction of travel of the marine survey vessel  218 . For embodiments in which the streamers are towed in a straight-line configuration (as opposed to a fanned configuration, for example) the in-line direction is parallel to the streamers  220 . The crossline direction  231  is perpendicular to the in-line direction  229  and crosses the length of the streamers  220 . The streamers  220  are generally spaced apart in the crossline direction  231 . Although not specifically illustrated, in at least one embodiment, the streamers  220  can be towed in a curved path. 
     The tow lines illustrated in  FIG. 2  include lead-ins  236  coupled between the marine survey vessel  218  and the streamers  220 . The lead-ins  236  thereby allow the marine survey vessel  218  to tow the streamers  220 . The lead-ins  236  can include electrical or optical connections between the appropriate components in the recording system on the marine survey vessel  218  and the receivers  222  on the streamers  220 . The first lead-in  236 - 1  is illustrated as having an associated receiver  223 - 3 , the second lead-in  236 - 2  is illustrated as having an associated receiver  223 - 4 , the third lead-in  236 - 3  is illustrated as having an associated receiver  223 - 5 , and the fourth lead-in  236 - 4  is illustrated as having an associated receiver  223 - 6 . 
     In at least one embodiment, the receivers  223 - 3 ,  223 - 4 ,  223 - 5 ,  223 - 6  can make use of the electrical or optical connection provided by the lead-ins  236  to communicate data received thereby with the recording system on the marine survey vessel  218 . It may be cost and material efficient to use the same connection, for example, where the receivers  223 - 3 ,  223 - 4 ,  223 - 5 ,  223 - 6  are internal to the lead-ins  236 . In at least one embodiment, the receivers  223 - 3 ,  223 - 4 ,  223 - 5 ,  223 - 6  can communicate data received thereby with the recording system on the marine survey vessel  218  with electrical or optical connections separate from the electrical or optical connection provided by the lead-ins  236 . It may be more cost effective to provide a separate connection for the receivers  223 - 3 ,  223 - 4 ,  223 - 5 ,  223 - 6 , for example, where the lead-ins  236  are pre-provided as a closed system or where the receivers  223 - 3 ,  223 - 4 ,  223 - 5 ,  223 - 6  are external to the lead-ins  236 . In at least one embodiment, one or more of the receivers  223  can communicate wirelessly with the recording system on the marine survey vessel  218  or with a recording system located elsewhere. In at least one embodiment, one or more of the receivers can store received data locally for later download without transmitting the data to a recording system located on the marine survey vessel  218 . 
     The receivers  223 - 3 ,  223 - 4 ,  223 - 5 ,  223 - 6  can be positioned with respect to the lead-ins  236  so as to provide data at a desired offset from the source  226 . For example, if data (e.g., additional to that provided by the receivers  223 - 1 ,  223 - 2 , when so employed) is desired with a negative source-receiver offset, the receivers  223 - 3 ,  223 - 4 ,  223 - 5 ,  223 - 6  can be positioned along the lead-ins  236  so as to be closer to the marine survey vessel in the in-line direction  229  than the source  226  during towing. If data is desired with a near-zero source-receiver offset (in the in-line direction  231 ), then the receivers  223 - 3 ,  223 - 4 ,  223 - 5 ,  223 - 6  can be positioned along the lead-ins  236  so as to be at an equal distance from the marine survey vessel  218  in the in-line direction  229  as the source  226  during towing. If data is desired with a near, but positive source-receiver offset, then the receivers  223 - 3 ,  223 - 4 ,  223 - 5 ,  223 - 6  can be positioned along the lead-ins  236  so as to be farther from the marine survey vessel  218  in the in-line direction  229  than the source  226  during towing. 
     The tow lines illustrated in  FIG. 2  include other lead-ins referred to in the art as super-wide ropes  240  coupled between the marine survey vessel  218  and paravanes  238 . The term “super-wide” should not be read to imply anything about the width of the spread of streamers  220  towed by the marine survey vessel  218 . Rather, the term “super-wide” refers to the fact that the super-wide ropes  240  are towed outside the width of the spread of streamers  220  in the crossline direction  231 . The first super-wide rope  240 - 1  is illustrated as having an associated receiver  223 - 7  and the second super-wide rope  240 - 2  is illustrated as having an associated receiver  223 - 8 . The receivers  223 - 7 ,  223 - 8  can be positioned with respect to the super-wide ropes  240  so as to provide data at a desired offset from the source  226 . For example, if data (e.g., additional to that provided by the receivers  223 - 1 ,  223 - 2 , when so employed) is desired with a negative source-receiver offset, the receivers  223 - 7 ,  223 - 8  can be positioned along the super-wide ropes  240  so as to be closer to the marine survey vessel in the in-line direction  229  than the source  226  during towing. If data is desired with a near-zero source-receiver offset (in the in-line direction  231 ), then the receivers  223 - 7 ,  223 - 8  can be positioned along the super-wide ropes  240  so as to be at an equal distance from the marine survey vessel  218  in the in-line direction  229  as the source  226  during towing. If data is desired with a near, but positive source-receiver offset, then the receivers  223 - 7 ,  223 - 8  can be positioned along the super-wide ropes  240  so as to be farther from the marine survey vessel  218  in the in-line direction  229  than the source  226  during towing. Although not specifically illustrated, the umbilicals  234  can be coupled to the marine survey vessel  218  by way of winches or other towing equipment to enable adjusting the deployed length of each umbilical  234 , lead-in  236 , or super-wide rope  240 . 
     Although not specifically illustrated, the super-wide ropes  240  can be coupled to the paravanes  238  via bridles, as is known in the art. The paravanes  238  are each configured to provide a lateral force component to the various towed object coupled thereto when the paravanes  238  are towed in the water. The paravanes  238  can maintain a spread of the streamers  220  in the crossline direction  231 . The combined lateral forces provided by the paravanes  238  separate the paravanes  238  from each other constrained by spreader line  244  coupled between the paravanes  238 . In at least one embodiment, the paravanes  238  can be coupled directly to the spreader line  244 . In at least one embodiment, as illustrated, the first paravane  238 - 1  can be coupled to the spreader line  244  by a first spur line  242 - 1  and the second paravane  238 - 2  can be coupled to the spreader line  244  by a second spur line  242 - 2 . 
     In at least one embodiment, the marine survey vessel  218  includes a controller  219  coupled to an interrogator  221  via one or more communication links as described in more detail with respect to  FIG. 4 . The interrogator  221  can be coupled to the tow lines to control and receive data from the receivers  223  associated therewith. However, at least one embodiment does not include the interrogator  221 . 
       FIG. 3A  is a plan or yx-plane  332  view of a marine survey setup illustrating an example of a critical distance  346 .  FIG. 3B  is a plot of time  356  versus offset  358  illustrating a critical distance  346  for the marine survey setup illustrated in  FIGS. 3A and 3B .  FIG. 3C  is an elevation or xz-plane  330  view of the marine survey setup of  FIG. 3A .  FIGS. 3A, 3B, and 3C  are correlated positionally with one another, but are not drawn to scale. For example, the position of the source  326  is the same in  FIGS. 3A and 3C  along their respective x-axes. Furthermore, the critical distance  346  is the same along the x-axes of  FIGS. 3A, 3B, and 3C . 
     The marine survey setup in  FIGS. 3A-3C  can be analogous to that illustrated in  FIG. 1  or  FIG. 2 , or a different setup.  FIG. 3A  includes an illustration of a source  326  and a tow line, specifically an umbilical  334 . The umbilical  334  is coupled to the source  326 . Although not specifically illustrated, the marine survey setup can include a marine survey vessel coupled to the umbilical  334  and can include additional sources  326  and umbilicals  334 . The umbilical  334  has a receiver  323  associated therewith, as described herein. Although not specifically illustrated, the marine survey setup can include tow lines, such as lead-ins, for the streamers  320 , where the lead-ins can also be associated with respective receivers.  FIG. 3A  includes an illustration depicting the convention for what is referred to as positive source-receiver offsets  348  and negative source-receiver offsets  350 . Any receiver in the marine survey setup positioned opposite the marine survey vessel side of the sources  326  is considered to have a positive source-receiver offset  348  in the in-line direction  329 . Any receiver positioned on the marine survey vessel side of the sources  326  is considered to have a negative source-receiver offset  350  in the in-line direction  329 . 
     In the art, the critical distance  346  is defined as the source-receiver offset at which the reflection time equals the refraction time for the energy from the source to the receiver as illustrated in  FIG. 3B . Stated differently, the critical distance  346  is the shortest offset where a refraction  354  is first received (or would be received if a receiver was so positioned). The times  356  and offsets  358  at which the refraction  354  are received are indicated by the straight line, which begins at an offset  358  equal to the critical distance  346 . The times  356  and offsets  358  at which the reflection (first up-going primary  352 - 1 ) are received are indicated by the curved line. The refraction  354  is not illustrated in  FIG. 3C . 
     The state illustrated in  FIG. 3C  includes a sea surface  309 , a solid surface  304 , and a subsurface reflector  360  underlying the solid surface  304 . The sea surface  309 , solid surface  304 , and subsurface reflector  360  are illustrated as straight lines for convenience and simplicity, but are not indicative of real Earth layouts. The state includes a source  326  and various receivers  322 ,  323 . For ease of illustration, the receivers are located at a same depth as the source  326 , however embodiments are not so limited. Typically, receivers  322  associated with streamers  320  are towed at a greater depth than the source  326 . The receiver  323  associated with the umbilical  334  may be at approximately the same depth as and/or shallower than the source  326 . 
     The source  326  can emit a down-going source wavefield represented as a first down-going ray  328 - 1  and a second down-going ray  328 - 2 . The state includes a plurality of receivers  322 - 1 ,  322 - 2 ,  322 - 3 ,  322 - 4  that are associated with the streamers  320  in  FIG. 3A  at convenient locations for purposes of illustration. The first down-going ray  328 - 1  can reflect off of the solid surface  304  as a first up-going primary  352 - 1 , which can be received by a first receiver  322 - 1 . The offset between the source  326  and the first receiver  322 - 1  is equal to the critical distance  346 . The first up-going primary  352 - 1  can continue and reflect off of the sea surface  309  as a down-going multiple  362 . The down-going multiple  362  can reflect off of the solid surface  304  as an up-going multiple  364 , which can be received by a first receiver  322 - 3 . 
     The second down-going ray  328 - 2  can reflect off of the solid surface  304  as a second up-going primary  352 - 2 , which can be received by the receiver  323 , which is associated with the umbilical  334 . The receiver  323  is positioned within the critical distance  346  from the source and has a negative offset  350  from the source  326 . This allows additional marine survey data to be received within the critical distance  346  and allows marine survey data to be received at negative offsets from the source  326 . 
     A portion of the energy from the first down-going ray  328 - 1  can pass through the solid surface  304  and continue to the subsurface reflector  360 , where it is reflected as a third up-going primary  352 - 3 . The up-going primary  352 - 3  can be received by a fourth receiver  322 - 4 . 
     There is a unique critical distance for every reflecting interface in the earth (such as the solid surface  304  and the subsurface reflector  360 ). The critical distance for shallower reflectors (such as the solid surface  304 ) is shorter than for deeper reflectors (such as the subsurface reflector  360 ). Regardless of the depth below the solid surface  304  of any reflector, the critical distance becomes a bigger issue when operating in shallower water. Marine survey data processing methods designed to enhance the desired signal, remove undesired noise or multiples, or attenuate undesired noise or multiples make use of the source wavefield that that penetrated the geological depths. The ability to interpret that wavefield is affected by cascading critical distances relevant to each interface between the solid surface  304  and the depth being considered. 
     In some marine survey locations, shallow water and/or highly reflective near-surface geology limits the critical distance  346  to short source-receiver offsets, which can make multiple removal difficult. In this context, “short source-receiver offsets” means that the source-receiver offset is shorter than a source receiver offset defined by the source  326  and a closest receiver on a closest streamer  320 . Therefore, the more traces that are available within the critical distance  346 , the better. Towed streamer  320  operations typically tow the sources  326  in front of the streamers  320 , so data is received only for a small portion (or none) of the circle comprising the critical distance  346 . Moreover, the typical setup only provides for positive source-receiver offsets. In many cases, the source-receiver offset for the closest receiver in an outer streamer  320  is not within the critical distance  346 . In any event, typically, the common midpoint fold available for imaging near-surface reflectivity from outer streamers is low. The critical distance  346  in  FIGS. 3A-3C  is just an example for a given survey location and setup, but illustrates that a critical distance  346  can exist with little or no overlap of the streamers  320  and the receivers located thereon. This emphasizes a beneficial use case for at least one embodiment of the present disclosure that includes a receiver associated with a tow line, such as the umbilicals  323 , to be towed in water within the critical distance  346  based on the relative position of the source  326  and the underlying geology. Shallow geology that is not typically imaged with the seismic data recorded in a typical manner can be imaged according to at least one embodiment of the present disclosure. Furthermore, imaging (or improved imaging) of that shallow geology according to at least one embodiment of the present disclosure can help remove noise or multiples that affect imaging of deeper geologies. 
     Surface-related multiple elimination (SRME) is just one example of multiple removal, which may also be referred to as a “demultiple method.” Embodiments of the present disclosure are not limited to any particular demultiple method. SRME, by way of example, uses a shallow reflectivity model to predict surface multiples, and in some cases, all orders of surface multiples. The model of multiples is adaptively subtracted from recorded data during processing. If the reflectivity model from the recorded data imperfectly captures the reflectivity characteristics of the seafloor and near-surface geology, the multiples are imperfectly modeled and subtracted. 
       FIG. 4  illustrates an exemplary embodiment of a system  468  for marine survey data acquisition at a tow line. The system  468  includes a controller  419  coupled to an interrogator  421  via one or more communication links. The controller  419  includes processing resources  470 - 1  and memory resources  472 - 1 . The interrogator  421  can include processing resources  470 - 2  and memory resources  472 - 2 . The interrogator  421  can be an optoelectronic device coupled to fiber optic cables  480  at ports  478 . Although not specifically illustrated, the ports  478  can be coupled to an optical source  474  and an optical receiver  476 , or each port can have its own optical source and/or optical receiver. The optical source  474  can be a laser, a light emitting diode, or another type of optical source. The fiber optic cables  480  can be coupled to or integrated with tow lines such as super-wide ropes, lead-ins, or umbilicals. A fiber optic cable  480  includes at least a core optical fiber surrounded by cladding. In at least one embodiment, the fiber optic cables  480  also include a buffer around the cladding and a jacket around the buffer. The system  468  can function as a distributed acoustic sensing (DAS) system. 
     Rayleigh scattering based DAS systems use fiber optic cables  480  to provide distributed strain sensing. Rayleigh scattering is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scattering particle, the amount of scattering is inversely proportional to the fourth power of the wavelength. 
     In DAS, the optical fiber is the sensing element. Measurements can be made, and in part processed, using the interrogator  421 . Such a system allows acoustic frequency strain signals to be detected over distances and in harsh environments. A coherent laser pulse can be sent along an optical fiber and scattering sites within the fiber cause the fiber to act as a distributed interferometer with a gauge length approximately equal to the pulse length. The intensity of the reflected light can be measured as a function of time after transmission of the laser pulse. This is known as coherent Rayleigh optical time domain reflectometry. When the pulse has had time to travel the full length of the fiber and back, the next laser pulse can be sent along the fiber. Changes in the reflected intensity of successive pulses from the same region of fiber are caused by changes in the optical path length of that section of fiber. This type of system is sensitive to both strain and temperature variations of the fiber and measurements can be made almost simultaneously at all sections of the fiber. 
     Single mode fiber enables one type of light mode to be propagated at a time. Multimode fiber means the fiber can propagate multiple modes. The difference in core diameter between single and multimode fiber (multimode generally has a larger core diameter), along with wavelength and light source bandwidth allows for this propagation difference. An example of a single mode fiber core diameter is nine micrometers, whereas an example core diameter for multimode fiber is fifty micrometers. Conventionally, DAS systems tend to have limited signal-to-noise ratios because Rayleigh scattering is weak. Conventional DAS deployments may use conventional fiber optic cable, which is designed primarily to transmit telecommunication signals, and therefore to minimize scattering. However, according to at least one embodiment of the present disclosure, DAS systems, such as arrays, can use fiber optic cables designed specifically for the purpose of DAS, and therefore scatter a larger proportion of the light pulse. This produces a significant improvement in the signal quality. The improvement in the measurement sensitivity has been achieved by advancing the state of the DAS optoelectronics interrogator  421  architecture, together with the introduction of next generation engineered optical fiber. The optical fiber is engineered with bright scatter centers along its length to capture and reflect more light back to the interrogator  421 . This is achieved without introducing significant loss to the forward propagating laser pulses. The DAS noise performance with engineered optical fiber is much lower (on the order of 20 decibels less) compared to that when using standard optical fiber. The DAS performance is comparable to that of geophones or hydrophones around 10 hertz but can far exceed the response of geophones or hydrophones in the 1 hertz range. The sensitivity of engineered optical fiber DAS systems can therefor approach or exceed that of typical seismic sensors such as geophones and hydrophones. 
     In at least one embodiment, the optical fiber has a linear shape. The optical fiber in a DAS system is most sensitive to strain along its longitudinal axis. Therefore, in at least one embodiment, the optical fiber can run along a longitudinal axis of the fiber optic cable  480  in a helical shape in order to provide sensitivity to signals received at angles not parallel to the fiber optic cable  480 . 
     Examples of processing resources  470  include a processor, combinational logic, a field programmable gate array, an application specific integrated circuit, etc. The controller  419  and the interrogator  421  can include a combination of hardware and machine-readable instructions, which may also be referred to as program instructions. The program instructions can be stored in the memory resources  472  or embodied in the processing resources  470 , which are configured to perform a number of functions described herein. For example, the program instructions can be analogous to those described with respect to  FIG. 5 . The program instructions, such as software, firmware, etc., can be stored in a memory resource such as a machine-readable medium, etc., as well as hard-wired program such as logic. Hard-wired program instructions can be considered as both program instructions and hardware. 
     The controller  419  and the interrogator  421  can utilize software, hardware, firmware, and/or logic to perform a number of functions. The controller  419  and the interrogator  421  can be a combination of hardware and program instructions configured to perform a number of functions and/or actions. The hardware, for example, can include processing resources  470  and memory resources  472 , such as a machine-readable medium or other non-transitory memory resources. Although illustrated as being internal to the respective controller  419  and interrogator  421 , the memory resources  472  can be internal and/or external thereto. In at least one embodiment, the controller  419  or the interrogator  421  can include internal memory resources  472  and have access to external memory resources. The program instructions, such as machine-readable instructions, can include instructions stored on the machine-readable medium to implement a particular function. The set of machine-readable instructions can be executable by one or more of the processing resources  470 . The memory resources  472  can be coupled to the respective controller  919  or interrogator  421  in a wired and/or wireless manner. For example, the memory resources  472  can be an internal memory, a portable memory, a portable disk, and/or a memory associated with another resource, for example, enabling machine-readable instructions to be transferred and/or executed across a network such as the Internet. 
     The memory resources  472  can be non-transitory and can include volatile and/or non-volatile memory. Volatile memory can include memory that depends upon power to store data, such as various types of dynamic random-access memory among others. Non-volatile memory can include memory that does not depend upon power to store data. Examples of non-volatile memory can include solid state media such as flash memory, electrically erasable programmable read-only memory, phase change random access memory, magnetic memory, optical memory, and/or a solid-state drive, etc., as well as other types of non-transitory machine-readable media. 
     The processing resources  470  can be coupled to the memory resources  472  via a communication path. The communication path can be local or remote to the respective controller  419  or interrogator  421 . Examples of a local communication path can include an electronic bus internal to a machine, where the memory resources are in communication with the processing resources via the electronic bus. Examples of such electronic buses can include Industry Standard Architecture, Peripheral Component Interconnect, Advanced Technology Attachment, Small Computer System Interface, Universal Serial Bus, among other types of electronic buses and variants thereof. The communication path can be such that the memory resources are remote from the processing resources, such as in a network connection between the memory resources and the processing resources. That is, the communication path can be a network connection. Examples of such a network connection can include a local area network, wide area network, personal area network, and the Internet, among others. 
       FIG. 5  illustrates an exemplary embodiment of a method for marine survey data acquisition at a tow line. At  590 , the method can include receiving marine survey data with an optical fiber at a location of a tow line coupled between a marine survey vessel and a towed object. At  592 , the method can include modeling a seismic multiple based at least in part on the marine survey data. At  594 , the method can include removing the modeled seismic multiple from the marine survey data. In at least one embodiment, the method includes receiving the marine survey data at a negative source-receiver offset, as described herein. In at least one embodiment, the method includes receiving marine survey data in a near-offset range, thereby improving removal of the modeled seismic multiple from the marine survey data. A near-offset range is dependent upon the offset and the depth of the survey. An example of a near-offset range is for an offset-to-depth ratio of less than 0.5, however embodiments are not so limited. 
     In at least one embodiment, the towed object can be a source. The method can further include towing the source with the marine survey vessel and towing a streamer with the marine survey vessel. Receiving the marine survey data can include receiving signals between the marine survey vessel and the streamer, where signals are not typically received according to some previous approaches. Receiving the marine survey data can include receiving signals between the marine survey vessel and the source, where signals are not typically received according to some previous approaches. 
     In at least one embodiment, the towed object can be a streamer. The method can further include towing the streamer with the marine survey vessel. Receiving the marine survey data can include receiving signals between the marine survey vessel and the streamer, where signals are not typically received according to some previous approaches. The method can further include towing a source with the marine survey vessel. Receiving the marine survey data can include receiving signals between the source and the streamer, where signals are not typically received according to some previous approaches. Receiving the marine survey data can include receiving signals between the marine survey vessel and the source, where signals are not typically received according to some previous approaches. 
     In accordance with at least one embodiment of the present disclosure, a geophysical data product may be produced or manufactured. Geophysical data may be obtained from a marine survey and stored on a non-transitory, tangible machine-readable medium. The geophysical data can be obtained with a receiver at a location of a tow line coupled between a marine survey vessel and a towed object. The geophysical data product may be produced by processing the geophysical data offshore or onshore either within the United States or in another country. Processing the geophysical data can include modeling a seismic multiple based at least in part on the geophysical data and removing the modeled seismic multiple from the geophysical data. The geophysical data product can be recorded on a non-transitory machine-readable medium, thereby creating the geophysical data product. If the geophysical data product is produced offshore or in another country, it may be imported onshore to a facility in the United States. In some instances, once onshore in the United States, geophysical analysis may be performed on the geophysical data product. In some instances, geophysical analysis may be performed on the geophysical data product offshore. 
     Although specific embodiments have been described above, these embodiments are not intended to limit the scope of the present disclosure, even where only a single embodiment is described with respect to a particular feature. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise. The above description is intended to cover such alternatives, modifications, and equivalents as would be apparent to a person skilled in the art having the benefit of this disclosure. 
     The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Various advantages of the present disclosure have been described herein, but embodiments may provide some, all, or none of such advantages, or may provide other advantages. 
     In the foregoing Detailed Description, some features are grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the disclosed embodiments of the present disclosure have to use more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.