[19]
`United States Patent
`[45] Oct. 28, 1980
`Neeley
`
`[11]
`
`4,231,111
`
`
`
`[54] MARINE CABLE LOCATION SYSTEM
`Walter P. Neeley, Irving, Tex.
`[75]
`Inventor:
`
`[73] Assignee:
`
`Mobil Oil Corporation, New York,
`NY.
`
`4,036,632
`
`4/1978
`
`Lions ............................. 343/112 PT
`
`-
`Primary Examiner—Stephen C. Buczinski
`Attorney, Agent. or Firm—C. A. Huggett; William J.
`Scherback
`
`[21] Appl. No.: 885,916
`[22] Filed:
`Mar. 13, 1978
`
`Int. CLJ ............................................... 601V 1/38
`[51]
`[52] U.S. Cl. ... 3667/196; 131:7/21533,
`8
`F' ld f S
`h
`343/5 E1214:/37/Tl071R 7/PCO
`[5 ]
`1? 34g/13335ll4/244253 367”’9 166 130:
`'
`‘
`’
`’
`EMS/g EM’
`
`[56]
`
`$3322.?
`3,981,008
`4,068,208
`
`References Cited
`US. PATENT DOCUMENTS
`340 7 R
`4
`B
`lgjigié
`Nil-332:1].""""""""" 3402 R
`
`343/5 EM
`9/1976 Mann .....
`
`.
`..................... 340/7 R
`1/ I978
`Rice, Jr. e
`
`ABSTRACT
`[57]
`A marine cable location system includes a plurality of
`magnetic compasses located at known spaced intervals
`along a cable being towed by a marine vessel. These
`compass readings are recorded along With an onboard
`magnetic compass reading, an onboard gyrocompass
`reading, and satellite navigational information. From
`7 these recordings, the X—Y coordinates of cable com-
`passes with reSpect to vessel heading are determined.
`These X-Y coordinates are recorded along with the
`vessel’s position and heading .0" magnetic. tape and a
`cathode—ray tube so as to prowde a Visual display of the
`cable position with respect to the vessel.
`
`6 Claims, 5 Drawing Figures
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`KEY BOARD
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`ION 1010
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`1
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`

`

`US. Patent
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`Oct. 28, 1980
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`

`4 US. Patent
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`Oct. 28, 1980
`
`Sheet 2 of 2
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`4,231,111
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`-c4
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`FIG. 4
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`9% = OBSTACLE LOCATION
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`3
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`

`1
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`4,231,111
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`MARINE CABLE LOCATION SYSTEM
`
`BACKGROUND OF THE INVENTION
`
`This invention relates to seismic exploration and
`more particularly to marine exploration. In marine ex-
`ploration, seismic energy is generated in the water and
`reflections of such energy from subsurface interfaces
`are detected by a linear string of detectors or hydro—-
`phones. The seismic energy sources and the detectors
`are towed through the water by means of cables extend-
`ing from a marine vessel. Signals received by the detec-
`tors are transferred to the vessel through the cable wir-
`ing. In many instances, groups of detectors are com-
`bined to form arrays within the cable, and the signals
`received by each such array are combined and trans-
`ferredrrto the vessel.
`,
`One method for determining the instantaneous posi-
`tion of each detector or array of detectors along the
`cable as the cable is towed through the water is dis-
`closed in U.S. Pat. No. 3,953,827 to Le Moal et al. A
`plurality of detectors or hydrophones are distributed
`along a towed cable. The position of each detector is
`determined by the interpolation of values of the angle of
`the tangents to the cable with a fixed and known direc-
`tion, such as magnetic north, at a plurality of measuring
`points. At each measuring point along the cable there is
`located preferably a magnetic compass. There is also
`provided means for coding and transmitting the mea-
`sured values by means of electronic pulses to a central
`station. Such means includes a multiplex device. The
`position of each measuring point is determined by as-
`similating that part of the towed cable located between
`compasses to an arc of a circle, the length of which is
`known from the construction of the cable, while the
`angular value of the arc is determined from the differ-
`ences between the angles measured by the compasses
`between the tangent to the cable and the fixed and
`known direction. The positions of the detectors along
`the cable are then determined by interpolation between
`the positions of the compasses along the cable.
`SUMMARY OF THE INVENTION
`
`In accordance with the present invention there is
`provided a system for determining the position in X-Y
`coordinate a plurality of points along a cable towed by
`a marine vessel and for visually displaying such points
`for use by the vessel’s operator in steering the vessel
`past other vessels or obstacles.
`In the development of marine exploration, the seismic
`detector cables have become quite long, extending for
`one mile, two miles, or even farther behind the marine
`vessel. Such lengths can cause problems in accurately
`determining the position and configuration of the cable
`as it is towed through the water since it is unlikely that
`cables of such lengths will extend in a straight line be—
`hind the towing vessel or even be configured in the
`shape of a single arc of curvature. Rather, the cable may
`have one or more inflection points in its curvature and
`may extend laterally to one or even both sides of the
`towing vessel simultaneously as illustrated in FIG. 1.
`One of the primary concerns in towing such a long
`and curved cable is in the steering of the towing vessel
`past other marine vessels or obstacles such as drilling
`towers, etc., in such a way that the projected path of the
`cable does not intersect such other vessels or obstacles.
`This is true not only when the vessel passes such obsta-
`cle in a straight line but also when the vessel is in a
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`directional turn. Under certain conditions the vessel
`could even turn sharply enough to cross the cable itself
`as it extends one or more miles behind the vessel.
`It is therefore a specific aspect of the present inven-
`tion to provide a system for visually displaying the
`position of a towed cable for use by the towing vessel‘s
`operator. A plurality of sensors are located at select
`points along the towed cable to provide signals repre-
`sentative of the heading of tangents to the cable at such
`select points. A sensor located onboard the vessel pro-
`vides a signal representative of the heading of the vessel
`itself. A navigational system onboard the vessel pro-
`vides signals identifying the X-Y coordinate of the ves-
`sel. The heading signals from the cable sensors and the
`vessel sensor along with the vessel’s X—Y coordinate
`signal from the navigational system are used to deter-
`mine the X-Y coordinates for the cable sensors. A visual
`display having a matrix of display squares records the
`X-Y coordinates of the vessel and the cable sensors.
`In one aspect of the invention the vessel sensor and
`the able sensors are magnetic compasses producing
`signals representing headings with respect to a direction
`of magnetic north. In this aspect, there is also included
`a gyrocompass onboard the vessel for producing a sig~
`nal representative of the true north heading of the ves-
`sel. The magnetic variations of the vessel compass and
`the cable compasses from true north of the vessel’s
`heading are determined in identifying the X-Y coordi-
`nates of the select points along the cable. These X-Y
`coordinates are displayed in a plus X direction off the
`stern of the vessel and a plus Y direction off the star-
`board of the vessel.
`In a further aspect, the X-Y coordinates of obstacles
`in the path of the cable as it is being towed are identi-
`fied. These X-Y coordinates are entered into the matrix
`of squares of the visual display along with the X-Y
`coordinates of the vessel and of the select points along
`the towed cable as illustrated in FIG. 3.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates a seismic exploration system em-
`ploying a marine vessel and towed seismic cable.
`FIG. 2 illustrates seismic recording equipment em-
`ployed with the marine exploration system of FIG. 1.
`FIG. 3 illustrates a visual display of cable-positioning
`data determined by the recording equipment of FIG. 2.
`FIG. 4 illustrates the geometric configuration utilized
`in determining cable compass X-Y coordinates.
`FIG. 5 represents a truth table for locating the hear—
`ing of the farthest compass from the vessel.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`
`In seismic marine exploration, the marine vessel 10 of
`FIG. 1 tows a seismic detector cable 11 along a line of
`exploration. Such a cable 11 conventionally employs a
`plurality of detectors, or hydrophones, (not shown)
`spaced along its length for receiving seismic reflections
`from the subsurface layer below the ocean floor. The
`cable also employs a plurality of magnetic heading sen-
`sors 12 equally spaced along its length, five such sensors
`being illustrated in FIG. 1. Each sensor provides a sig—
`nal representing the magnetic heading or direction of
`the tangent to that particular point of the cable. By
`knowing the heading of the tangents to the cable at such
`plurality of points along the cable and the distances
`
`4
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`

`4,231,111
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`’4
`ters of interrogator 13 to the external header unit of the
`field recorder 17.
`the magnetic differences
`.
`In the field recorder 17,
`between the magnetic compass readings and the gyro-
`compass reading are determined as indications of the
`magnetic variations of the compass readings from true
`north of the vessel’s heading. The compass readings are
`then read out of the field recorder 17 and into the cable
`location computer 20 approximately one second before
`each firing of the seismic acoustic source and the re-
`cording of the resulting seismic reflection data. Approx-
`imately 12 seconds are utilized between each such
`source firing.
`Following transfer of the corrected compass head-
`ings and the navigation information from the field re-
`corder 17, the computer 20 determines the X and Y
`coordinates of each compass, with plus X direction
`being headed off the stern of the ship and plus Y being
`headed off the starboard of the ship. Also, the bearing
`and range of each compass with respect to the ship are
`determined. Such determinations are based upon the
`theory that when the tangent of a plurality of points
`along the cable (i.e., as indicated by the compass read-
`ings) is known and the distances between such points
`along the cable are known, then the lengths and direc-
`tions of the chords between such points can be deter-
`mined, assuming the cable between such points is an arc
`of a circle. These chords can then be stacked as a prev
`diction of the cable’s position with respect to the ves-
`sel’s heading as the cable is towed through the water.
`Such stacking will also yield a single vector indicating
`the distance of the farthest cable compass from the
`vessel and the bearing of such compass with respect to
`the vessel‘s bearing. Referring more particularly to
`FIG. 4, there is illustrated an example configuration for
`a towed cable with respect to vessel heading wherein:
`c0=vesse1 heading with respect to magnetic north,
`c,-=cable headings with respect to magnetic north at
`the select points of the cable compasses (i.e., ci—cs),
`i=chord subtending the arc of curvature of the
`cable between adjacent compasses (i.e., do—d4),
`an: angle between the chord d; and the tangent line
`for the cable heading ci,
`bi=angle between a line pointing in the direction of
`the vessel's heading and the chord di, and
`si=cable arc length between adjacent compasSes.
`Each chord d,- defined in the above manner becomes a
`directed line segment with vector components Xi and
`Yi. Computation of the distance R and bearing 0 from
`the vessel to the last cable compass is as follows:
`Ui=l(fr=m i)
`
`b,'=a,-+co—c,-
`
`d;=(180.r,‘/1rai) sin a,-
`
`Xi=di cos bi: Y;=d,- sin b;
`
`R :V<zx,~)2+(2 Y»?
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`0:180” flan ~ lo: maxi)
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`3
`between each of such points, the location of the cable
`along its entire length can be estimated.
`In the preferred embodiment, each sensor 12 includes
`a magnetic compass 12a and a binary control unit 12b. A
`Model 319 Magnetic Sensor supplied by Digicourse,
`Inc., is utilized for each magnetic compass 120. and a
`Model 350 Binary Control Unit of Digicourse, Inc., is
`utilized for each binary control unit 12b. The readings
`of the compasses are multiplexed by the associated bi-
`nary control units onto a single pair of wires running the
`length of the cable to the onboard cable location com-
`puting system as illustrated in FIG. 2. Each binary con-
`trol unit is addressed with its appropriate code number
`by the interrogator 13. A Model 290 Data Acquisition
`Unit of Digicourse, Inc., is utilized for such interroga-
`tor. A start pulse from the cycle timer l4 initiates the
`multiplexing of the magnetic compass headings to infor-
`mation registers in the interrogator 13. Also applied to
`an information register in the interrogator 13 is the
`heading from an onboard magnetic compass 15, such as
`the Model 101 Magnetic Sensor of Digicourse, Inc. The
`compass heading in any one of the six information regis-
`ters can be visually displayed on the sensor display 16,
`such as a Model 102 Sensor Display of Digicourse, Inc.
`The information registers of the interrogator l3 trans-
`fer the compass headings to an external header unit in
`the field recorder 17. Such field recorder is preferably
`the DFS IV digital field recorder of Texas Instruments
`Incorporated. Also applied to such external header unit
`is the true north reading from an onboard gyrocompass
`18 and a satellite-positioning reading of latitude and
`longitude from the onboard navigational system 19. The
`field recorder 17 therefore contains all the information
`required by the cable location computer 20 to compute
`cable position relative to the vessel's gyroheading. Pref-
`erably, the computer 20 is the Model 980B of Texas
`Instruments Incorporated. This computer converts the
`compass data into X and Y coordinates for recording on
`magnetic tape unit 24, plus X direction being headed off
`the stem from the ship and plus Y being starboard from
`the ship. A simple plot of the ship coordinates and the
`cable compasses appears on the cathode—ray tube dis
`play 21. The computer also provides the bearing and
`range of each compass with respect to the ship. A zero
`degree (0") reference is used for the gyroheading of the
`'ship. Therefore, compass bearings will generally be
`around 180°. Such bearing and range information are
`recorded on the recorder 22, preferably a Silent 730
`KSR Keyboard Recorder/Printer of Texas Instruments
`Incorporated.
`Having generally described the invention in conjunc-
`tion with the block schematic of FIG. 2, a more detailed
`description of the operation of the various units of FIG.
`2 will now be described in conjunction with the loca-
`tion of the cable 11 during a towing operation.
`During seismic exploration operations, each seismic
`recording cycle is initiated at time zero by a ground
`signal of at least 40 milliseconds but not greater than 100
`milliseconds from the cycle timer 14. If, however, seis-
`mic exploration operations are not being carried out,
`the ground signal can be supplied to the interrogator 13
`from the cable location computer 20. This ground signal
`is utilized by the interrogator 13 to successively address
`each binary control unit 12b for 100 milliseconds. It will
`therefore require 500 milliseconds to read the five cable
`compasses 12a. The onboard magnetic compass is ready
`every 25 milliseconds. It requires 60 milliseconds to
`output the compass readings from the information regis—
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`Examination of the signs 2X; and EYi gives the bearing
`9 with respect to vessel heading as shown in the truth
`table of FIG. 5.
`A typical plot of such cable location with respect to
`the ship for the fivevcompass cable of the preferred
`embodiment is illustrated in FIG. 3. Such plot is based
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`5
`on a matrix of display squares wherein the entire square
`in which a determined X'Y coordinate falls is bright—
`ened on the face of the cathode-ray tube display 21.
`This five-point plot of X-Y coordinates is updated prior
`to each seismic firing cycle or approximately every 12
`seconds. Should the vessel change its direction and
`position as indicated by the arrow 25 in FIG. 3, each
`compass position is changed to a new X-Y coordinate
`on the display matrix as indicated by the dashed lines.
`In addition to the visual display of the location of the
`marine vessel and its towed cable, various obstacles that
`lie in the path of the vessel and its cable, such as other
`vessels, drilling towers, etc., may also be displayed with
`their X-Y coordinates. Should the vessel’s radar 23
`identify a possible obstacle, the vessel’s operator may
`enter the obstacle’s X-Y coordinate into the display
`matrix by means of the input keyboard of the recorder/—
`printer. The display square in which the obstacle’s X—Y
`coordinate falls is brightened on the face of the cathode-
`ray tube display 21. For example, the square marked
`with an asterisk, *, in FIG. 3 indicates the position of an
`obstacle lying in a collision path with the cable.
`Various modifications may be made to the foregoing-
`described embodiment of the present invention without
`departing from the scope and spirit of the present inven-
`tion as set forth in the appended claims. For example, in
`lieu of the use of magnetic compasses for the cable
`sensors 12a, there may be employed encoding gyros,
`strain gauges, or even flow sensors connected in bridge
`circuits to detect such things as pressure or temperature
`unbalance across the cable. Also, navigational antennas
`may be located on cable support buoys for sending
`cable heading signals to the vessel. Further, various
`types of recording and display equipment may be em-
`ployed on the vessel for indicating cable location.
`I claim:
`1. A system for visually displaying the position of a
`cable towed by a marine vessel, comprising:
`(a) means for providing navigational
`information
`signals indentifying the X-Y coordinate of said
`marine vessel,
`(b) a sensor located on said marine vessel for provid-
`ing a signal representative of the heading of said
`
`6
`marine vessel as it tows said cable through the
`water,
`(0) a plurality of sensors located at select points along
`said cable for providing signals representative of
`the heading of tangents to the cable at said select
`points,
`(d) means responsive to the signals from said cable
`sensors, from said marine vessel sensor, and from
`said navigational information means for determin—
`ing the X-Y coordinate of each of said plurality of
`cable sensors,
`(6) a visual display having a matrix of display squares,
`(0 means for entering the X-Y position of said marine
`vessel along with the X-Y position of said plurality
`of selected cable points into the matrix of said vi-
`sual display, whereby squares of the display matrix
`identified with said entered X-Y positions are visu-
`ally distinguishable from the remaining squares to
`display a locus of points defining the configuration
`of the towed cable,
`(g) means for identifying the X-Y coordinates of ob-
`stacles in the path of said cable as it is being towed
`by said marine vessel, and (h) means for entering
`the X-Y coordinates of said obstacles into the ma-
`trix of said visual display to visually display a mark
`representing the position of the obstacle relative to
`the cable.
`2. The system of claim 1 wherein said cable sensors
`and said marine vessel sensor are magnetic compasses.
`3. The system of claim 2 further including a gyrocom-
`pass on the marine vessel for producing a signal repre-
`sentative of the true north heading of said marine vessel.
`4. The system of claim 3 further including means for
`determining the magnetic variations of the cable com—
`passes and the marine vessel compass from the true
`north of said marine vessel’s heading.
`5. The system of claim 1 wherein the X-Y coordinates
`of said cable sensors are identified as a plus X direction
`off the stern of the marine vessel and a plus Y direction
`off the starboard of the marine vessel.
`6. The system of claim 1 wherein said visual display is
`a cathode—ray tube. 58
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