Patent Publication Number: US-11029433-B2

Title: Calibration of streamer navigation equipment

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application 62/435,177, filed Dec. 16, 2016, which is incorporated by reference. 
    
    
     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 sources below the water surface and over a subterranean formation to be surveyed for mineral deposits. 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 air guns, marine vibrators, 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 water surface. The receivers thereby measure a wavefield that was initiated by the actuation of the source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perspective view of an apparatus for calibration of streamer navigation equipment. 
         FIG. 2  illustrates a front view of an apparatus for calibration of streamer navigation equipment. 
         FIG. 3  illustrates a side view of an apparatus for calibration of streamer navigation equipment. 
         FIG. 4  illustrates a diagram of a system for calibration of streamer navigation equipment. 
         FIG. 5  illustrates a diagram of a machine for calibration of streamer navigation equipment. 
         FIG. 6  illustrates a method flow diagram for calibration of streamer navigation equipment. 
         FIG. 7  illustrates a front elevation or xz-plane view of marine surveying in which acoustic signals are emitted by a source for recording by receivers. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure is related generally to the field of marine surveying. Marine surveying can include, for example, seismic surveying or electromagnetic surveying, among others. For example, this disclosure may be related to marine surveying, in which one or more sources are used to generate wave-fields, and receivers (towed and/or ocean bottom) receive energy generated by the sources and affected by the interaction with a subsurface formation. The receivers thereby collect survey data, which can be useful in the discovery and/or extraction of hydrocarbons from subsurface formations. 
     A towed object, such as a source, a receiver, or a streamer, may be towed behind a marine survey vessel to collect the survey data. A streamer can be a marine cable assembly that can include receivers and electrical or optical connections to transmit information collected by the receivers to the marine survey vessel. The streamer can include receivers such as seismic receivers (e.g., hydrophones, geophones, etc.) or electromagnetic receivers. The streamer can include a streamer telemetry unit (STU) with a compass. The compass along with the STU may be referred to herein as CSTU. The compass can be used to determine a heading or additional spatial and navigational information for the streamer. A multi-dimensional calibration can be performed to render high quality heading values. In some previous approaches, in-sea calibration can be performed after streamer deployment in a body of water. However, it can be beneficial to calibrate the compass prior to deployment and have the compass generate these high quality heading values upon first deployment (and be operational without performing calibration after deployment). However, embodiments of the present disclosure do not preclude further calibration after deployment. 
     A two-dimensional roll of the CSTU may not be sufficient to generate a heading dataset to perform a calibration. As described further below, an apparatus that allows for rotation about at least two axes can be used to calibrate the compass within the CSTU. For example, rotation about an axis that adjusts a roll of the CSTU can be performed. Rotation about an axis that adjusts a pitch of the CSTU can be performed. In this way, the CSTU can be placed in different orientations that facilitate calibration readings that correspond to the different orientations. The calibration telemetry data recorded can be compared to an expected telemetry field shape. The differences between the recorded data and the expected data can be used to determine calibration values and/or heading values to correct for deviations in the compass of the STU being tested. A calibration value can include calibration data acquired from the CSTU for analysis in calibrating the CSTU during in-field use. A heading value can indicate a heading of the CSTU based on a position and/or location of the CSTU. 
     It is to be understood the present disclosure is not limited to particular devices or methods, which may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting. 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 (having the potential to, being able to), not in a mandatory sense (must). The term “include,” and derivations thereof, mean “including, but not limited to.” The term “coupled” means directly or indirectly connected. 
     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,  110  may reference element “ 10 ” in  FIG. 1 , and a similar element may be referenced as  210  in  FIG. 2 . Multiple analogous elements within one figure may be referenced with a reference numeral followed by a hyphen and another numeral or a letter. For example,  121 - 1  may reference element  21 - 1  in  FIGS. 1 and 121-2  may reference element  21 - 2 , which can be analogous to element  21 - 1 . Such analogous elements may be generally referenced without the hyphen and extra numeral or letter. For example, elements  121 - 1  and  121 - 2  may be generally referenced as  121 . 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  illustrates a perspective view of an apparatus  100  for calibration of streamer navigation equipment. The apparatus  100  can include a base assembly  110  and a pivot assembly  112 . The base assembly  110  can be a rectangle-shaped base for the apparatus  100 . The pivot assembly  112  can be generally rectangle-shaped and used to hold a compass for the streamer navigation equipment. The apparatus  100  can include a carriage  122  that is used to adjust a roll when calibrating the streamer navigation equipment. The apparatus  100  can include a fixture  125  for holding a compass streamer telemetry unit (CSTU)  126 . The apparatus  100  can include a compass streamer telemetry unit (CSTU)  126  used to collect and transmit telemetry data from the CSTU  126  to an associated marine vessel and/or receive telemetry data from the marine vessel. The CSTU  126  can include a streamer telemetry unit (STU) and a compass combined into one unit. The CSTU  126  is illustrated as above the pivot assembly  112  for ease of description and illustration. A first end  127 - 1  of the CSTU  126  can be inserted into a first coupler  124 - 1  and a second end  127 - 2  can be inserted into a second coupler  124 - 2 . The first end  127 - 1  can be a male end and can be inserted into the first coupler  124 - 1  which has a female end. The first end  127 - 1  can include electrical connections and other equipment for transferring and/or receiving navigational data. The first coupler  124 - 1  can include electrical connections to receive and/or transfer navigational data to and from the CSTU  126 . The second coupler  124 - 2  can be a female end and the second end  127 - 2  can be inserted into the second coupler  124 - 2 . A pitch wheel  120  can be used to adjust a pitch and a first roll wheel  121 - 1  and a second roll wheel  121 - 2  can be used to adjust a roll associated with the apparatus  100 . 
     The pivot assembly  112  can be rotatably coupled to the base assembly  110 . Rotatably coupled can refer to a first component being coupled to a second component where the first component has limited movement in some directions but can be rotated about the point of coupling. For example, the pivot assembly  112  can rotate about a lateral axis  114  while remaining coupled to the base assembly  110 . The pivot assembly  122  is limited in movement along the lateral axis  114  in relation to being coupled to the base assembly  110 . While rotating about the lateral axis  114 , a first end  116 - 1  of the pivot assembly  112  can be raised (indicated by the up arrow of double-sided arrow  118 - 1 ) while a second end  116 - 2  is lowered (indicated by the down arrow of the double-sided arrow  118 - 2 ). Alternatively, the first end  116 - 1  of the pivot assembly  112  can be lowered (indicated by the down arrow of double-sided arrow  118 - 1 ) while the second end  116 - 2  is raised (indicated by the up arrow of double-sided arrow  118 - 2 ). The pivot assembly  112  can be rotated by rotating, indicated at arrow  115 , a pitch wheel  120  where the pitch wheel  120  is fixedly coupled to the pivot assembly  112  and the base assembly  110  remains stationary during the rotation. As the pitch wheel  120  rotates, the pivot assembly  112  rotates an equal amount, in degrees, about the lateral axis  114 . The lateral axis  114  can be perpendicular to the pitch wheel  120 , as illustrated, and the lateral axis  114  can run through a center of the pitch wheel  120 . 
     A pitch of the fixture  125  can be adjusted by rotating the pitch wheel  120  and causing the carriage  122  and thereby the fixture  125  to rotate about the lateral axis  114 . By rotating the pitch wheel  120  clockwise in relation to arrow  115 , the first end  116 - 1  of the pivot assembly  112  is raised, as indicated by up arrow of  118 - 1 , and the pitch of the carriage  122 , and thereby the pitch of the fixture  125  and the CSTU  126 , is increased. Alternatively, as the pitch wheel  120  is turned counter-clockwise in relation to arrow  115 , the first end  116 - 1  of the pivot assembly  112  is lowered, as indicated by down arrow of  118 - 1 , and the pitch of the carriage  112 , and thereby the pitch of the fixture  125  and the CSTU  126 , is decreased. In at least one embodiment, the pivot assembly  112  is prevented from making a full rotation about the lateral axis  114  and may be limited by the setup of the apparatus  100 . For example, the pivot assembly  112  may be able to rotate up to a particular amount, such as 60 degrees, 90 degrees, etc., from its illustrated position (where the fixture  125  is illustrated as having a 0 degree pitch). While the CSTU  126  is not illustrated as on the fixture  125 , it should be understood that as a pitch and/or roll of the fixture  125  is adjusted, the pitch and/or roll of the CSTU  126  is also adjusted in kind. For example, as the fixture  125  is rotated a particular amount (to adjust the roll), the CSTU  126  is rotated the particular amount. Likewise, as a pitch of the fixture  125  is adjusted a particular angle, the pitch of the CSTU  126  is adjusted the particular angle. 
     A carriage  122  can be rotatably coupled to the pivot assembly  112 . The carriage  122  can rotate about a longitudinal axis  128 . As a first roll wheel  121 - 1  on the second end  116 - 2  is rotated, indicated by arrow  130 - 1 , the carriage  122  and the first roll wheel  121 - 1  can rotate, as indicated by arrow  140 , an equal amount, in degrees, about the longitudinal axis  128  because the first roll wheel  121 - 1  is fixedly coupled to the carriage  122 . The longitudinal axis  128  can be perpendicular to the first roll wheel  121 - 1 , as illustrated. The longitudinal axis can run through a center of the first roll wheel  121 - 1 . The carriage  122  is illustrated as rotating counter-clockwise by arrow  140  with respect to the first roll wheel  121 - 1 , but the carriage can also rotate clockwise with respect to the first roll wheel  121 - 1 . A second roll wheel  121 - 2  of the first end  116 - 1  rotates, as indicated by arrow  130 - 2 , along with the carriage  122  and the first roll wheel  121 - 1  because both the first roll wheel  121 - 1  and the second roll wheel  121 - 2  are fixedly coupled to the carriage  122 . Embodiments are not limited to the carriage  122  being coupled to two roll wheels. In at least one embodiment, the carriage  122  can be coupled to only one roll wheel on either end of the carriage  122 . 
     A fixture  125  can be fixedly coupled to the carriage  122 . The carriage  122  can be shaped to receive fixture  125 . The fixture  125  can be positioned into the carriage  122  as the carriage  122  is shaped for insertion of the fixture  125 . The carriage  122  can be configured to secure the fixture  125  in place. The fixture  125  can be secured to the carriage  122  by fastening mechanisms (such as a bolt, a screw, a pin, etc.). 
     A roll of the fixture  125  can be adjusted by rotating the first roll wheel  121 - 1  and causing the carriage  122  and thereby the fixture  125  to rotate about the longitudinal axis  128 . By rotating the first roll wheel  121 - 1  clockwise, in relation to arrow  130 - 1 , a roll of the carriage  122  is adjusted as the carriage  122  is likewise rotated clockwise about the longitudinal axis  128 . Alternatively, by rotating the first roll wheel  121 - 1  counter-clockwise, in relation to arrow  130 - 1 , the roll of the carriage  122  is adjusted as the carriage  122  is likewise rotated counter-clockwise about the longitudinal axis  128 . In at least one embodiment, the carriage is fully rotatable 360 degrees and can be rotated more than one full 360 degree rotation. 
     The fixture  125  is coupled to a first cabling  132 - 1  that extends from the fixture  125  in the direction of the first end  116 - 1  and a second cabling  132 - 2  that extends from the fixture  125  in the direction of the second end  116 - 2 . Cabling refers to cables that transfer data. For example, the first cabling  132 - 1  and the second cabling  132 - 2  can transfer navigation and/or calibration data from the CSTU  126  to additional streamer navigation equipment. In some embodiments, calibration data can be transferred through the first cabling  132 - 1  and the second cabling  132 - 2  to an external processor to analyze the calibration data. The first cabling  132 - 1  can extend out from the first side  116 - 1  and be placed up and across a U-shaped portion of a first cable support post  134 - 1 . The second cabling  132 - 2  can extend out from the second side  116 - 2  and be placed up and across a U-shaped portion of a second cable support post  134 - 2 . The carriage may be prevented from unlimited rotation as the first cabling  132 - 1  and the second cabling  132 - 2  may wrap around the carriage  122 . In at least one embodiment, the first cabling  132 - 1  and the second cabling  132 - 2  may not go through the first roll wheel  121 - 1  or the second roll wheel  121 - 2 , but can extend from the ends of the CSTU  126  and directly up toward the first cable support post  134 - 1  and the second cable support post  134 - 2  respectively. The first cabling  132 - 1  can rest on the first cable support post  134 - 1  and the second cabling  132 - 2  can rest on the second cable support post  134 - 2 . As the carriage  122  is rotated, the first cabling  132 - 1  and the second cabling  132 - 2  are wrapped around the carriage  122  as many times around as rotations of the carriage  122  are performed. Further, the first cabling  132 - 1  and the second cabling  132 - 2  can extend across the pivot assembly  112  from the ends of the fixture  125 . 
     The CSTU  126  can be secured into the fixture  125  by a first side locking mechanism  142 - 1  and a second side locking mechanism  142 - 2 . A respective end closest to the center of the apparatus  100  of each of the first side locking mechanism  142 - 1  and second side locking mechanism  142 - 2  can pivot up and away from the center of the apparatus  100  to be in an unlocked position (not illustrated). While in the unlocked position, the CSTU  126  can be removed from the fixture  125 . And, in the reverse, the first side locking mechanism  142 - 1  and second side locking mechanism  142 - 2  can pivot downward and toward the center of the apparatus  100  to be in a locked position (as illustrated) to hold the CSTU  126  into the fixture  125 . A top locking mechanism  144  can pivot down over the CSTU  126  to lock the CSTU  126  in place. 
     The pitch wheel  120  can include a first fastening mechanism  136 - 1  and a second fastening mechanism  136 - 2  that can each be inserted into a respective aperture, which can be analogous to the aperture  138 - 1 . Once the pitch wheel  120  is rotated to a position (and therefore a particular pitch of the carriage  122  and the fixture  125 ) for recording calibration data, the first fastening mechanism  136 - 1  and the second fastening mechanism  136 - 2  can each be inserted into the apertures and locked into place so that the pitch wheel  120  is prevented from further rotation. A first handle  137 - 1  and a second handle  137 - 2  can be used to more easily rotate the pitch wheel  120 . The first roll wheel  121 - 1  can include a similar fastening mechanism  136 - 3  and a plurality of apertures, such as aperture  138 - 2 . The second roll wheel  121 - 2  can include a similar fastening mechanism  136 - 4  and a plurality of apertures. Once the first roll wheel  121 - 1  and the second roll wheel  121 - 2  are rotated to a position (and therefore a particular roll of the carriage  122  and the fixture  125 ) for recording calibration data, the fastening mechanism  136 - 3  can be inserted into one of the number of apertures and locked into place so that the first roll wheel  121 - 1  and the second roll wheel  121 - 2  are prevented from further rotation. The first roll wheel  121 - 1  can include a first handle  137 - 3  and a second handle  137 - 4 . The second roll wheel  121 - 2  can include a first handle  137 - 5  and a second handle  137 - 6 . The handles can be used to more easily rotate the roll wheels. Either the first roll wheel  121 - 1  or the second roll wheel  121 - 2  can be used to adjust the roll because they are both fixedly coupled to the carriage  122 , on opposite sides of the pivot assembly  112 . 
       FIG. 2  illustrates a front view of an apparatus  200  for streamer navigation equipment calibration. The apparatus  200  is analogous to the apparatus  100  illustrated in  FIG. 1  and includes a pivot assembly  212  and a base assembly  210 . The pivot assembly  212  is rotatably coupled to the base assembly  210 . The pivot assembly is fixedly coupled to the pitch wheel  220 . As described in  FIG. 1 , as the pitch wheel  220  is rotated clockwise about a longitudinal axis  214  (as illustrated by the left arrow of  215 ), a first end  216 - 1  of the pivot assembly  212  is raised, indicated by the up arrow of  218 - 1  and a second end  216 - 2  is lowered, indicated by the down arrow  218 - 2 . As the pitch wheel  220  is rotated counter-clockwise about the longitudinal axis  214  (as illustrated by the right arrow of  215 ) the first end  216 - 1  is lowered, indicated by the down arrow of  218 - 1 , and the second end  216 - 2  is raised, indicated by the up arrow of  218 - 2 . The pitch wheel  220  is fixedly coupled to the pivot assembly  212  through an aperture in the base assembly  210  such that as the pitch wheel  220  rotates, the pivot assembly  212  rotates an equal amount, in degrees, while the base assembly remains stationary. In this way, a pitch of the pivot assembly  212  can be adjusted. The pitch wheel  220  can include a first handle  237 - 1  and a second handle  237 - 2  that can aid in rotating the pitch wheel  220 . A first fastening mechanism  236 - 1  and a second fastening mechanism  236 - 2  can be inserted into one of a plurality of apertures along the pitch wheel  220  (such as aperture  238 - 1 ) and prevent the pitch wheel  220  from continuing to rotate and lock it in place. 
     A carriage  222  can be rotatably coupled to the pivot assembly  212 . A first roll wheel  221 - 1  and a second roll wheel  221 - 2  can be fixedly coupled to the carriage  222  such that as the roll wheels are rotated, as indicated by arrows  230 - 1  and  230 - 2 , about a longitudinal axis  228 , the carriage is rotated, as indicated by arrow  240 , an equal amount, in degrees, about the longitudinal axis  228 . The first roll wheel  221 - 1  can include a fastening mechanism  236 - 3  and a first handle  237 - 3 . The second roll wheel  221 - 2  can include a second fastening mechanism  236 - 4  and a second handle  237 - 5 . A fixture  225  can be fixedly coupled to the carriage  222 . A compass streamer telemetry unit (CSTU)  226  can be inserted onto the fixture  225  and held in place by a first side locking mechanism  242 - 1 , a second side locking mechanism  242 - 2 , and a top locking mechanism  244 . As the locking mechanism  242 - 1  is raised upward, a first coupler  224 - 1  can move toward the left, as illustrated. As the locking mechanism  242 - 2  is raised upward, a second coupler  224 - 1  can move toward the right, as illustrated. In this way, the first coupler  224 - 1  and the second coupler  224 - 2  can move outward from the center of apparatus  200  and make room for the CSTU  226  to be inserted between them. Once inserted, the first locking mechanism  242 - 1  and the second locking mechanism  242 - 2  can be lowered and the first coupler  224 - 1  and the second coupler  224 - 2  can move inward toward the center of the apparatus  200  and lock the CSTU  226  in place. The fixture  225  can include first cabling  232 - 1  that exits the apparatus  200  to the left and second cabling  232 - 2  that exits the apparatus  200  to the right, as illustrated. The first cabling  232 - 1  can be placed up and over a U-shaped portion of the first cable support posts  234 - 1 . The second cabling  232 - 2  can be placed up and over a U-shaped portion of the second cable support posts  234 - 2 . 
       FIG. 3  illustrates a side view of an apparatus  300  for calibration of streamer navigation equipment. The apparatus  300  is analogous to the apparatus  100  illustrated in  FIG. 1 . A pitch wheel  320  can be used to rotate the pivot assembly  312  (illustrated in  FIG. 3  as the second end  316 - 2  of the pivot assembly  312 ) to adjust pitch. The pitch wheel  320  can be rotatably coupled to a base assembly  310  such that as the pitch wheel  320  rotates, the base assembly  310  remains stationary while the pitch wheel  320  and the pivot assembly  312  rotate. The pitch wheel  320  can include a handle  337 - 2  to turn the pitch wheel  320  and a fastening mechanism  336 - 2  to lock the pitch wheel  320  in place. Rotating the pitch wheel  320  can rotate the pivot assembly  312  about a lateral axis  314 . 
     A first roll wheel  321 - 1  can be fixedly coupled to a carriage  322  and rotatably coupled to the pivot assembly  312 . As the first roll wheel  321 - 1  is rotated, the carriage  322  rotates an equal amount, in degrees, about the longitudinal axis  328 . Rotating the first roll wheel  321 - 1  and the carriage  322  can adjust a roll of the carriage  322  (and therefore a roll of a compass of a STU coupled to the fixture  325 ). The first roll wheel  321 - 1  can include a first handle  337 - 3  and a second handle  337 - 4  to assist in rotating the first roll wheel  321 - 1 . The first roll wheel  321 - 1  can include a plurality of apertures (one such aperture being 338-2) and a fastening mechanism  336 - 3  to lock the first roll wheel  321 - 1  and prevent it from further rotating. A cable support post  334 - 2  can be fixedly coupled to the base assembly  310  to hold cables that may extend from the fixture  325 , such as fixture  125  in  FIGS. 1 and/or 225  in  FIG. 2 . A top locking mechanism  344  can be used to lock a CSTU in place onto the fixture  325 . 
       FIG. 4  illustrates a diagram of a system  404  for calibration of streamer navigation equipment. The system  404  can include a database  450 , a subsystem  452 , and a number of engines, such as a first receive engine  454 , a first compare engine  456 , a second receive engine  458 , and a second compare engine  460 . The subsystem  452  and engines can be in communication with the database  450  via a communication link. The system  452  can include additional or fewer engines than illustrated to perform the various functions described herein. The system can represent program instructions and/or hardware of a machine such as the machine  505  referenced in  FIG. 5 , etc. As used herein, an “engine” can include program instructions and/or hardware, but at least includes hardware. Hardware is a physical component of a machine that enables it to perform a function. Examples of hardware can include a processing resource, a memory resource, a logic gate, etc. While this example illustrates the system  404  with a database  450 , examples are not so limited. In some examples, calibration data from a flow of calibrations can be stored in individual files that are not stored in a database format. 
     The number of engines can include a combination of hardware and program instructions that is configured to perform a number of functions described herein. 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 first receive engine  454  can include a combination of hardware and program instructions that is configured to receive a first set of telemetry calibration data points. The first set of telemetry calibration data points is associated with a first set of positions of a compass of a STU. The first set of positions can be associated with a fixed first pitch and a plurality of roll positions of the compass. For example, the first set of positions can be associated with rotating a carriage (e.g., rotating the first roll wheel  121 - 1  and thereby carriage  122 ) to adjust a roll of a compass while maintaining a fixed first pitch of the compass (e.g., not rotating pitch wheel  120  and thereby not rotating the pivot assembly  112 ). Fixed can refer to something that is held constant, For example, a fixed pitch can refer to a pitch that is held constant. The first set of telemetry data points can be acquired while adjusting the roll as the compass goes through the first set of positions. The first compare engine  456  can include a combination of hardware and program instructions that is configured to compare the first set of telemetry calibration data points to an expected telemetry field shape based on the first number of positions. 
     The second receive engine  458  can include a combination of hardware and program instructions that is configured to receive a second set of telemetry calibration data points associated with a second set of positions of the compass. The second set of positions can be associated with a fixed second pitch and a plurality of roll positions of the compass. For example, the second set of positions can be associated with rotating a carriage (e.g., rotating the first roll wheel  121 - 1  and thereby carriage  122 ) to adjust a roll of a compass while maintaining a fixed second pitch of the compass (e.g., not rotating pitch wheel  120  and thereby not rotating the pivot assembly  112 ). The second set of telemetry data points can be acquired while adjusting the roll as the compass goes through the second set of positions. The first pitch is a different pitch than the second pitch. 
     The second compare engine  460  can include a combination of hardware and program instructions that is configured to compare the second set of telemetry calibration data points to an expected telemetry field shape based on the second position. The second set of telemetry calibration data points can be compared to the expected telemetry field shape based on a calibration method. For example, the second set of telemetry calibration data points can be compared to the expected telemetry field shape using algebraic fitting, or orthogonal fitting, among other methods. 
     In addition, though not illustrated, a third receive engine can include a combination of hardware and program instructions that is configured to receive a third set of telemetry calibration data points associated with a third set of positions of the compass. The third set of positions can be associated with a third fixed pitch and a plurality of roll positions of the compass. For example, the third set of positions can be associated with rotating a carriage (e.g., rotating the first roll wheel  121 - 1  and thereby carriage  122 ) to adjust a roll of a compass while maintaining a fixed third pitch of the compass (e.g., not rotating pitch wheel  120  and thereby not rotating the pivot assembly  112 ). The third set of telemetry data points can be acquired while adjusting the roll as the compass goes through the third set of positions. In at least one example, the third pitch is a different pitch than the first pitch and the second pitch. 
     In addition, though not illustrated, a third compare engine can include a combination of hardware and program instructions that is configured to compare the third set of telemetry calibration data points to an expected telemetry field shape based on the third position. The third set of telemetry calibration data points can be compared to the expected telemetry field shape based on a calibration method. For example, the third set of telemetry calibration data points can be compared to the expected telemetry field shape using algebraic fitting, or orthogonal fitting, among other methods. 
     In at least one embodiment, a first set of telemetry calibration data points can be associated with a first set of positions of a compass of a STU. The first set of positions can be associated with a fixed pitch and a plurality of roll positions of the compass. A second set of telemetry calibration data points can be associated with a second set of positions of the compass. The second set of positions can be associated with a fixed roll and a plurality of pitch positions of the compass. For example, the second set of positions can be associated with rotating a pivot assembly (e.g., rotating pivot assembly  112  and thereby adjusting the pitch of carriage  122  in  FIG. 1 ) to adjust a pitch of a compass while maintaining a fixed roll of the compass (e.g., not rotating the carriage  122  and thereby not adjusting the roll). The second set of telemetry data points can be acquired while adjusting the pitch as the compass goes through the second set of positions. In the at least one example, the first set of calibration data points and the second set of calibration data points can be compared to an expected telemetry field shape based on the calibration method. 
     In at least one embodiment, a fixed pitch of the compass can be maintained while adjusting a roll of the compass during a first set of data acquisitions and a fixed roll of the compass can be maintained while adjusting the pitch of the compass during a second set of data acquisitions. In at least one embodiment, a first pitch of the compass can be maintained while adjusting a roll of the compass during a first period of data acquisition and a second pitch of the compass can be maintained while adjusting the roll of the compass during data acquisition during a second period of data acquisition. In addition, a first roll of the compass can be maintained while adjusting a pitch of the compass during a third period of data acquisition and a second roll of the compass can be maintained while adjusting a pitch of the compass during a fourth period of data acquisition. 
     While adjustment of the roll and pitch is illustrated as using wheels (such as pitch wheel  120  and roll wheels  121 - 1  and  121 - 2  in  FIG. 1 ) to perform manually, embodiments are not so limited. For example, the roll and pitch of the compass can be adjusted automatically using a servomotor and/or additional electrical and mechanical equipment to hold the compass at particular pitch angles and roll positions while automatically acquiring the calibration data at these corresponding angles and positions. 
     The following is a description of how telemetry calibration data points are used to determine a heading of a streamer and corresponding calibration parameters. A geomagnetic field and gravity of earth can be described by two vectors: 
     
       
         
           
             
               
                 
                   
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                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The magnetic inclination angle can be given by δ, and the magnetic field strength and the gravity field strength can be given by B and g respectively. The axes of the compass of the STU can be described by an NED (North (x), East (y), Down (z)) coordinate system and rotations about these axes can be described by rotation matrices. The compass can include an accelerometer sensor and a magnetometer sensor. The accelerometer sensor can be used to detect changes in acceleration of the compass and the magnetometer can be used to detect a magnetic field and can detect fluctuations in the Earth&#39;s magnetic field. The rotation matrices define how to rotate an object about the axes of the coordinate system. The rotation matrices R x (θ x ), R y (θ y ) and R z (θ z ) represent rotation about the x, y and z axes respectively. 
                         R   x     ⁡     (     θ   x     )       =     [         1       0       0           0         cos   ⁡     (     θ   x     )             sin   ⁡     (     θ   x     )               0         -     sin   ⁡     (     θ   x     )               cos   ⁡     (     θ   x     )             ]       ,     
     ⁢         R   y     ⁡     (     θ   y     )       =     [           cos   (     θ   y     ⁢           )         0         -     sin   (     θ   y     ⁢           )               0       1       0             sin   ⁡     (     θ     y   ⁢               )           0         cos   ⁡     (     θ   y     )             ]       ,     
     ⁢         R   z     ⁡     (     θ   x     )       =     [           cos   ⁡     (     θ   z     )             sin   ⁡     (     θ   z     )           0             -     sin   ⁡     (     θ   z     )               cos   ⁡     (     θ   z     )           0           0       0       1         ]               (   3   )               
The output from an accelerometer sensor of the compass can be described as G p , and can be a function of the CSTUs orientation.
 
                     G   p     =       [           acc   ⁢           ⁢   X               acc   ⁢           ⁢   Y               acc   ⁢           ⁢   Z           ]     =         R   x     ⁡     (     θ   x     )       ·       R   y     ⁡     (     θ   y     )       ·       R   z     ⁡     (     θ   z     )       ·     [         0           0           g         ]                 (   4   )               
Similarly, the output from a magnetometer sensor of the compass can be described as:
 
                     B   p     =       [         magX             mag   ⁢           ⁢   Y               mag   ⁢           ⁢   Z           ]     =         R   x     ⁡     (     θ   x     )       ·       R   y     ⁡     (     θ   y     )       ·       R   z     ⁡     (     θ   z     )       ·     B   ⁡     [           cos   ⁡     (   δ   )               0             sin   ⁡     (   δ   )             ]                   (   5   )               
To be able to calculate the heading of the CSTU, the accelerometer is used to calculate the pitch and roll angles. These angles can be expressed as:
 
                     tan   ⁡     (     θ   x     )       =     accY   accZ             (   6   )                 tan   ⁡     (     θ   y     )       =       -   accX         accY   ·     sin   ⁡     (     θ   x     )         +       accZ   ·   cos     ⁢           ⁢     (     θ   x     )                   (   7   )               
The roll angle is associated with (6) and the pitch angle is associated with (7). In order to perform calibration, the roll and pitch angles (by rotating the carriage to adjust roll and rotating the pivot assembly to adjust pitch) can be used to “up-rotate” the magnetometer to get values for the magnetic horizontal components (x and y). An example of how the telemetry calibration data points are compared to an expected telemetry field shape is described below.
 
     B p  can be recalculated and rewritten to express the heading (θ z ). 
                     tan   ⁡     (     θ   z     )       =         mag   ⁢           ⁢     Z   ·     sin   ⁡     (     θ   x     )           -     mag   ⁢           ⁢     Y   ·   cos     ⁢           ⁢     θ   x                   mag   ⁢           ⁢     X   ·   cos     ⁢           ⁢     (     θ   y     )       +     mag   ⁢           ⁢     Y   ·     sin   ⁡     (     θ   y     )       ·   sin     ⁢           ⁢     (     θ   x     )       +               mag   ⁢           ⁢     Z   ·     sin   ⁡     (     θ   y     )       ·   cos     ⁢           ⁢     (     θ   x     )                       (   8   )               
tan(θ z ) can express how to calculate the heading if the CSTU was oriented along the axes of the CSTU and if the sensors are providing accurate readings. The simplified operations (4) and (5) above can be updated to include calibration matrices and correct for module orientation.
 
                     G   p     =         W   g     ·       R   z     ⁡     (   α   )       ·       R   y     ⁡     (   β   )       ·       R   x     ⁡     (     θ   x     )       ·       R   y     ⁡     (     θ   y     )       ·       R   z     ⁡     (     θ   z     )       ·     [         0           0           g         ]       +     V   g               (   9   )                 B   p     =         W   B     ·       R   z     ⁡     (   α   )       ·       R   x     ⁡     (     θ   x     )       ·       R   y     ⁡     (     θ   y     )       ·       R   z     ⁡     (     θ   z     )       ·     B   ⁡     [           cos   ⁡     (   δ   )               0             sin   ⁡     (   δ   )             ]         +     V   B               (   10   )               
The offset vectors V g  and V B  can be 3×1 vectors describing the offset magnitude on the x, y and z sensor axes (referred to as W g  below). The offset vector V B , incorporates the offsets generated by the sensor bias and the hard iron effect.
 
                     W   g     =     [           a   x         0       0           0         a   y         0           0       0         a   z           ]             (   11   )               
The accelerometer sensor&#39;s sensitivity is described by a diagonal matrix and the misalignment is a rotation matrix defining the orientation of the sensor related to the orientation of the CSTU. The CSTU can be intentionally mounted with a −45° angle about the z-axis (a). The CSTU can be mounted as such to get all three axes activated during a CSTU roll. The misalignment angle β could be close to 0°. The calibration matrix W B , shown below, can be more complicated. For example, W B  can be described by three 3×3 matrices. However, the form of the three matrices can vary.
 
 W   B   =S   f   ·S   m   ·S   si   (12)
 
The matrices S f , S m  and S si  describes the scale factor, the nonorthogonality and the soft iron effect respectively and, in its most general form, can consist of 9 independent variables:
 
                     W   B     =     [           m   xx           m   xy           m   xz               m   yx           m   yy           m   yz               m   zx           m   zy           m   zz           ]             (   13   )               
The output from the calibrated sensors can be written:
 
                     G     p   cal       =       [           accX   cal               accY   cal               accZ   cal           ]     =         R   y     ⁡     (     -   β     )       ⁢       R   z     ⁡     (     -   α     )       ⁢       W   g     -   1       ⁡     (       G   p     -     V   g       )                   (   14   )                 B     p   cal       =       [           magX   cal               magY   cal               magZ   cal           ]     =         R   z     ⁡     (     -   α     )       ⁢       W   B     -   1       ⁡     (       B   p     -     V   B       )                   (   15   )               
The reason for emitting the rotation matrix R y (−β) in the magnetometer calibration is due to the general form of W B . β will be close to zero and any deviation will be captured in W B .
 
     Distortions of the earth&#39;s magnetic field can be a result of external magnetic influences generally referred to as either a hard iron effect or a soft iron effect. If no distorting effects are present, rotating a magnetometer through a minimum of 360 degrees and plotting the resulting data as a y-axis vs. x-axis can result in a circle centered around coordinates (0,0). However, anomalies due to hard or soft iron effects can produce perturbations of the circle as a simple offset from (0,0) in the case of a hard-iron effect or deform the circle to produce an ellipse in the case of a soft-iron effect. Hard-iron effect distortion can be produced by material that exhibits a constant, additive field to the earth&#39;s magnetic field, thereby generating a constant additive value to the output of each of the magnetometer&#39;s axes. If the orientation and position of the magnet relative to a sensor is constant, the field and associated offsets will be constant. Compensating for these distortions can include determining the x and y offsets and applying these constants to the data. Soft-iron distortions can be the result of material that influences, or distorts, a magnetic field, but is not additive. Thus, soft-iron distortion cannot be compensated for with a simple constant and uses more complicated procedures. A soft-iron distortion is usually exhibited as a perturbation of the ideal circle into an ellipse. 
     Optimal calibration parameters and rotation angles can be determined by estimating calibration variables, such as α, β, W b , W g , V B  and V g . Two fitting models can be used to determine the calibration parameters and rotation angles. The first is referred to as algebraic fitting and the second is referred to as orthogonal fitting. Algebraic fitting refers to use of an ellipsoid to solve for parameters of A, B, C, D, E, F, G, H, and I directly. An operation can have a quadratic form of: 
                   Q   ⁢     =   def     ⁢       Ax   2     +     By   2     +     Cz   2     +     2   ⁢           ⁢   Dxy     +     2   ⁢           ⁢   Exz     +     2   ⁢           ⁢   Fyz     +     2   ⁢           ⁢   Gx     +     2   ⁢           ⁢   Hy     +     2   ⁢           ⁢   Iz     -   0             (   16   )               
The operation can have nine independent variables and can be written in matrix form as:
 
 x   T   A x   q   x= 0  (17)
 
Here, x is given by
 
                   x   =     [         x           y           z           1         ]             (   18   )               
and A q  is given by
 
                     A   q     =     [         A       D       E       G           D       B       F       H           E       F       C       I           G       H       I         -   1           ]             (   19   )               
The center of the ellipsoid can be found where the gradient of Q vanishes:
 
                     ∇   Q     =       [         ∂   Q       ∂   x       ,       ∂   Q       ∂   y       ,       ∂   Q       ∂   z         ]     =   0             (   20   )               
and can be found by solving:
 
                       [         A       D       E           D       B       F           E       F       C         ]     ⁡     [           x   0               y   0               z   0           ]       =     [           -   G               -   H               -   I           ]             (   21   )               
The ellipsoid can be translated to the origin by multiplying x with the translation matrix (T).
 
                   T   =     [         1       0       0         x   0             0       1       0         y   0             0       0       1         z   0             0       0       0       1         ]             (   22   )                     (   Tx   )     T     ⁢       A   q     ⁡     (   Tx   )         =   0           (   23   )                     x   T     ⁡     (       T   T     ⁢     A   q     ⁢   T     )       ⁢   x     =   0           (   24   )               
The rotation matrix R can now be expressed in terms of A q  and T.
 
                   R   =         T   T     ⁢     A   q     ⁢   T     =     [           r   11           r   12           r   13           r   14               r   21           r   22           r   23           r   24               r   31           r   32           r   33           r   34               r   41           r   42           r   43           r   44           ]               (   25   )                   x   T     ⁢   Rx     =   0           (   26   )               
The eigenvectors and the semi-axes of the ellipsoid can be found by solving the eigenvalue problem of matrix S defined as:
 
                   S   =     -       1     r   44       ⁡     [           r   11           r   12           r   13               r   21           r   22           r   23               r   31           r   32           r   33           ]                 (   27   )               
If the offset vector, v, is defined as
 
                   v   =     [           x   0               y   0               z   0           ]             (   28   )               
the ellipsoid can be represented using the new matrices.
 
( x−v ) T   S ( x−v )=1  (29)
 
To obtain the linear transformation from an ellipsoid to a sphere, the square root of S can be used. The square root of S is equal to w −1  and can be applied as shown in (14) and (15) for the magnetometer and accelerometer. The construction of S where A q  is symmetric and T is a translation matrix can impose a symmetric constraint on S. A real symmetric matrix can be diagonalized by:
 
 S=QΛQ   T   (30)
 
In (30), Q can be a real orthogonal matrix holding the eigenvectors of S and Λ as a diagonal matrix with the real eigenvalues of S. If the principal axes of the ellipsoid are along the sensor axes, the eigenvectors (e i ) will simply be:
 
                 e   1     =     [         1           0           0         ]       ,       e   2     =     [         0           1           0         ]       ,       e   3     =     [         0           0           1         ]             
The sensor sensitivities can further be calculated from the eigenvalues, λ 1 , λ 2  and λ 3 
 
                       s   x     =     1       λ   1           ,     sy   =     1       λ   2           ,     sz   =     1       λ   3                   (   31   )               
In the example of a skewed ellipsoid, the eigenvectors can have cross terms and a 3×3 sensitivity correction matrix can be computed as:
 
                     S   sqrt     =       w     -   1       =           [           e   1           e   2           e   3           ]     ⁡     [           1   /   sx         0       0           0         1   /   sy         0           0       0         1   /   sz           ]       ⁡     [           e   1           e   2           e   3           ]       ⁢   T               (   32   )               
The sensitivity correction matrix S sqrt  is the inverse of the W g  and W B  matrices for the accelerometer and the magnetometer respectively. S sqrt  will take the form as:
 
                     S   sqrt     =     [           s   xx           s   xy           s   xz               s   xy           s   yy           s   yz               s   xz           s   yz           s   zz           ]             (   33   )               
and includes 6 independent variables. (11) shows that W g  is described by 3 variables. For W g , the cross terms s xy , s xz  and s yz  are zero. In the construction of S sqrt , a symmetric constraint and rotation of the axes cannot be determined by this calibration type. Further, this particular solution may be more sensitive to noise compared to orthogonal fitting.
 
     Orthogonal fitting can be less affected by noise. Orthogonal fitting can use an iterative solver to find its minimum. The optimization operation for orthogonal fitting includes: 
                     min       w   ij     -   1       ,     x   0     ,     y   0     ,     z   0         ⁢       ∑   i     ⁢       (                w     -   1       ⁡     (       [           x   ⁡     [   i   ]                 y   ⁡     [   i   ]                 z   ⁡     [   i   ]             ]     -     [           x   0               y   0               z   0           ]       )            2     -   1     )     2               (   34   )               
There can be different ways of writing w −1 . In the most general form w −1  is written as:
 
                     w     -   1       =     [           s   xx           s   xy           s   xz               s   yx           s   yy           s   yz               s   zx           s   zy           s   zz           ]             (   35   )               
The optimization problem will consist of 12 independent variables. The criteria of magnetometer measurements to lay on the surface of a sphere after calibration are not enough to uniquely define the 12 parameters. For (34) to uniquely define a set of calibration parameters, w −1  needs to be restricted to be symmetric. However, by including additional cost functions the full set of 12 parameters can be estimated.
 
       FIG. 5  illustrates a diagram of a machine  505  for calibration of streamer navigation equipment. The machine  505  can utilize software, hardware, firmware, and/or logic to perform a number of functions. The machine  505  can be a combination of hardware and program instructions configured to perform a number of functions. The hardware, for example, can include processing resources  562  and memory resources  566 , such as a machine-readable medium or other non-transitory memory resources  566 . The memory resources  566  can be internal and/or external to the machine  505 . For example, the machine  505  can include internal memory resources 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, for example, an action such as receiving a first set of telemetry calibration data points. The set of machine-readable instructions can be executable by one or more of the processing resources  562 . The memory resources  566  can be coupled to the machine  505  in a wired and/or wireless manner. For example, the memory resources  505  can be an internal memory, a portable memory, a portable disk, or a memory associated with another resource, for example, enabling machine-readable instructions to be transferred or executed across a network such as the Internet. As used herein, a “module” can include program instructions and/or hardware, but at least includes program instructions. 
     Memory resources  566  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 a solid state drive, etc., as well as other types of non-transitory machine-readable media. 
     The processing resources  562  can be coupled to the memory resources  566  via a communication path  564 . The communication path  564  can be local or remote to the machine  505 . Examples of a local communication path  564  can include an electronic bus internal to a machine, where the memory resources  566  are in communication with the processing resources  562  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  564  can be such that the memory resources  566  are remote from the processing resources  562 , such as in a network connection between the memory resources  566  and the processing resources  562 . That is, the communication path  564  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. 
     As shown in  FIG. 5 , the machine-readable instructions stored in the memory resources  566  can be segmented into a plurality of modules  554 ,  556 ,  558 , and  560  that when executed by the processing resources  562  can perform a number of functions. As used herein a module includes a set of instructions included to perform a particular task or action. The number of modules  554 ,  556 ,  558 , and  560  can be sub-modules of other modules. For example, the first receive module  554  can be a sub-module of the first compare module  556 , and the second receive module  558  and the second compare module  560  can be contained within a single module. Furthermore, the number of modules  554 ,  556 ,  558 , and  560  can comprise individual modules separate and distinct from one another. Examples are not limited to the specific modules  554 ,  556 ,  558 , and  560  illustrated in  FIG. 5 . 
     Each of the number of modules  554 ,  556 ,  558 , and  560  can include program instructions or a combination of hardware and program instructions that, when executed by a processing resource  562 , can function as a corresponding engine as described with respect to  FIG. 4 . For example, the first receive module  554  can include program instructions or a combination of hardware and program instructions that, when executed by a processing resource  562 , can function as the first receive engine  454 . The first compare module  556  can include program instructions or a combination of hardware and program instructions that, when executed by a processing resource  562 , can function as the first compare engine  456 . The second receive module  558  can include program instructions or a combination of hardware and program instructions that, when executed by a processing resource  562 , can function as the second receive engine  458 . The second compare module  560  can include program instructions or a combination of hardware and program instructions that, when executed by a processing resource  562 , can function as the second compare engine  460 . 
     The program instructions can be executed by the processing resource  562  to generate a calibration value and a heading value for the CSTU based on calibration described in association with  FIG. 4 . The program instructions can be executed to determine a set of calibration values and heading values associated with the compass. A calibration value and a heading value can be used to determine an accuracy of a first set and a second set of telemetry calibration data points that are acquired while performing calibration acquisitions (e.g., adjusting rolls or pitches, as described in association with  FIGS. 1-3 ). The calibration value and the heading value can be used to adjust the calibration values to bring them to a full accuracy value so that future acquired navigational data is in line with what would be expected. A full accuracy value refers to an accuracy value that indicates that the heading value and calibration values acquired are completely accurate with respect to an actual heading value and calibration value of the navigation equipment. The program instructions can be executed to determine an accuracy value of the first set of telemetry calibration data points and the second set of telemetry calibration data points in comparison to the expected telemetry field shape. The accuracy value indicates how accurate the first and second sets of telemetry calibration data points are when compared to the expected telemetry field shape. The program instructions can be executed to receive a third set of telemetry calibration data points associated with a third position of a compass. The third position can be associated with a third pitch and a third roll of the compass. The program instructions can be executed to compare the third set of telemetry calibration data points to an expected telemetry field shape based on the third position. 
     In accordance with at least one embodiment of the present disclosure, a geophysical data product may be produced. Geophysical data may be obtained and stored on a non-transitory, tangible computer-readable medium. The geophysical data product may be produced by processing the geophysical data offshore or onshore either within the United States or in another country. 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. 
       FIG. 6  illustrates a method flow diagram for calibration of streamer navigation equipment in accordance with at least one embodiment of the present disclosure. At block  670 , the method can include receiving a first set of telemetry data points associated with a first set of positions of a compass housed on a carriage. The first set of telemetry data points can be received while maintaining a fixed first pitch of the carriage and adjusting the roll of the carriage. 
     At block  672 , the method can include receiving a second set of telemetry data points associated with a second set of positions of the compass housed on the carriage. The second set of telemetry data points can be received while maintaining a fixed second pitch of the carriage and adjusting the roll of the carriage. 
     The method can include receiving a third set of telemetry data points associated with a third set of positions of the compass housed on the carriage. The third set of telemetry data points can be received while maintain a fixed third pitch of the carriage and adjusting the roll of the carriage. 
     The method can include comparing the first set and the second set of telemetry data points to an expected telemetry field shape. The first set of telemetry data points can be compared to the expected telemetry field shape to determine whether there are navigational deviations in a compass from what would be expected based on the particular pitch and roll of the compass and other navigational parameters. In response to the first set of telemetry data points and the expected telemetry field shape being compared as expected, additional heading value corrections may not be necessary. In addition, in response to the second set of telemetry data points and the expected telemetry field shape being compared as expected, additional heading value corrections may not be necessary in association with the second set of telemetry data points. Further, the third set of telemetry data points can be compared to the expected telemetry field shape to determine whether there are navigational deviations in the compass from what would be expected based on the particular pitch and roll of the compass and other navigational parameters. Additional heading value corrections may not be necessary based on the comparisons. 
     In response to the first, the second, and the third set of telemetry data points not being what is expected based on comparison with the expected telemetry field shape, the method can include determining a set of calibration values based on differences between the first and the second set of telemetry data points and the expected telemetry field shape, between the first, the second, and the third set of telemetry data points and the expected telemetry field shape, etc. For example, the set of calibration values can be used to adjust the first and the second set of telemetry data points in order to bring the first and the second set of telemetry data points in line with the expected telemetry field shape so that the compass operates as expected. In this way, the heading values of the compass can be adjusted so that the compass operates as expected in the field while gathering navigational data. These calibrations, calculations, comparisons, etc. can be performed while in a pre-field setting (e.g., a factory, a location prior to launching streamers on a marine vessel, etc.). 
       FIG. 7  illustrates an elevation or xz-plane  799  view of marine surveying in which acoustic signals are emitted by a source  796  for recording by receivers  792 . 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.  FIG. 7  illustrates a domain volume  707  of the earth&#39;s surface comprising a subsurface volume  776  of sediment and rock below the surface  774  of the earth that, in turn, underlies a fluid volume  778  of water having a water surface  779  such as in an ocean, an inlet or bay, or a large freshwater lake. The domain volume  707  shown in  FIG. 7  represents an example experimental domain for a class of marine surveys.  FIG. 7  illustrates a first sediment layer  780 , an uplifted rock layer  782 , second, underlying rock layer  784 , and hydrocarbon-saturated layer  786 . One or more elements of the subsurface volume  776 , such as the first sediment layer  780  and the first uplifted rock layer  782 , can be an overburden for the hydrocarbon-saturated layer  786 . In some instances, the overburden may include salt. 
       FIG. 7  shows an example of a marine survey vessel  788  equipped to carry out marine surveys. In particular, the marine survey vessel  788  can tow one or more streamers  790  (shown as one streamer for ease of illustration) generally located below the water surface  779 . The streamers  790  can be long cables containing power and data-transmission lines (electrical, optical fiber, etc.) to which receivers may be coupled. The streamers  790  can each include a compass streamer telemetry unit (CSTU) with a compass used for navigational purposes. The compasses can be calibrated by adjusting a pitch and/or a roll of the compass during telemetry data collection to verify and/or adjust heading values acquired by the compass. In one type of marine survey, each receiver, such as the receiver  792  represented by the shaded disk in FIG.  7 , 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 hydrophone that detects variations in pressure. In one type of marine survey, each marine survey receiver, such as marine survey receiver  792 , comprises an electromagnetic receiver that detects electromagnetic energy within the water. The streamers  790  and the marine survey vessel  788  can include sensing electronics and data-processing facilities that allow marine survey receiver readings to be correlated with absolute positions on the sea surface and absolute three-dimensional positions with respect to a three-dimensional coordinate system. In  FIG. 7 , the marine survey receivers along the streamers are shown to lie below the sea surface  779 , with the marine survey receiver positions correlated with overlying surface positions, such as a surface position  794  correlated with the position of marine survey receiver  792 . The marine survey vessel  788  can also tow one or more marine survey sources  796  that produce signals as the marine survey vessel  788  and streamers  790  move across the sea surface  779 . Marine survey sources  796  and/or streamers  790  may also be towed by other vessels, or may be otherwise disposed in fluid volume  778 . For example, marine survey receivers may be located on ocean bottom cables or nodes fixed at or near the surface  774 , and marine survey sources  796  may also be disposed in a nearly-fixed or fixed configuration. For the sake of efficiency, illustrations and descriptions herein show marine survey receivers located on streamers, but it should be understood that references to marine survey receivers located on a “streamer” or “cable” should be read to refer equally to marine survey receivers located on a towed streamer, an ocean bottom receiver cable, and/or an array of nodes. 
       FIG. 7  shows source energy illustrated as an expanding, spherical signal, illustrated as semicircles of increasing radius centered at the marine survey source  796 , representing a down-going wavefield  798 , following a signal emitted by the marine survey source  796 . The down-going wavefield  798  is, in effect, shown in a vertical plane cross section in  FIG. 7 . The outward and downward expanding down-going wavefield  798  may eventually reach the surface  774 , at which point the outward and downward expanding down-going wavefield  798  may partially scatter, may partially reflect back toward the streamers  790 , and may partially refract downward into the subsurface volume  776 , becoming elastic signals within the subsurface volume  776 . 
     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.