Ex. PGS 1033
`
`Ex. PGS 1033
`
`
`
`
`
`

`
`United States Patent [191
`Zacbariadis
`
`[54] SYSTEM FOR LATERALLY POSITIONING A
`TOWED MARINE CABLE AND METHOD OF
`USING SAME
`
`[75]
`
`Inventor: Robert G. Zacbariadis, Dallas, Tex.
`
`[73] Assignee: Mobil Oil Corporation, New York,
`N.Y.
`
`[21] Appl. No.: 221,733.
`
`[22] Filed:
`
`Dec. 31, 1980
`
`Int. CI.l ............... : ............................... G01V 1/ 38
`[51]
`[52] U.S. CI . ........................................ 367/ 19; 367/17;
`114/246
`[58] Field of Search ..................... 367/16, 17, 19, 106;
`114(242, 246
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,581,273 5/1971 Hedberg ................................ 367/19
`3,605,674 9/1971 Weese ............................. 114/235 B
`3,953,827 4/1976 LeMoal et al . ..................... 340/7 R
`4,033,278 7/1977 Waters ................................ 114/245
`4,066,993 1/1978 Savit ...................................... 367/77
`4,068,208 1/1978 Rice. Jr. et al. ............. ....... 34017 R
`4,086,504 4/1978 Ezell et al ............................. 367177
`4,087,780 5/1978 !tria et al. ............................. 367/17
`4,152,691 5/1979 Ward ..................................... 367/77
`4,231,111 10/1980 Neeley .................................. 367/19
`4,323,989 4/1982 Huckabee et al. .................... 367/17
`
`FOREIGN PATENT DOCUMENTS
`10660 5/1980 European Pat. Off . .............. 367177
`18053 10/1980 European Pat. Off . .............. 367/17
`
`[11]
`
`[45]
`
`4,404,664
`Sep. 13, 1983
`
`OTHER PUBLICATIONS
`U.S. Patent Application 885,916, Inventor Walter P.
`Neeley, filed Mar. 13, 1978.
`Syntron, Inc., Remote-Controlled "Cable Leveler,"
`brochure.
`Primary Examiner-Nelson Moskowitz
`Attorney, Agent, or Firm- Alexander J. McKillop;
`James F. Powers, Jr.; George W. Hager, Jr.
`[57]
`ABSTRACf
`A marine cable positioning system which includes a
`plurality of magnetic compasses and a plurality of lat(cid:173)
`eral positioning devices spaced at known intervals
`along the cable being towed by a marine vessel. Read(cid:173)
`ings from the co mpasses together with readings from a
`magnetic compass and gyrocompass onboard the tow(cid:173)
`ing vessel are gathered and used with a selected refer(cid:173)
`ence heading to generate X -Y coordinates of the lateral
`positioning devices with respect to the towing vessel
`and selected reference heading. These coordinates are
`reco rded and provided to a cathode ray tube for visual
`display of the relative position of each lateral position(cid:173)
`ing device with respect to the vessel and selected head(cid:173)
`ing. Coded digital commands are generated and trans(cid:173)
`mitted to each lateral positioning device for adjustment
`of its control surfaces whereby the lateral thrust pro(cid:173)
`d uced the device as it is towed through the water is
`varied and the horizontal position of the portion of the
`cable to either side of the device controlled. Feedback
`of changes in the cable's location are provided by up(cid:173)
`dated compass readings and visual displays. Each lat(cid:173)
`eral positioning device responds to a command for a
`specific adjustment of its control surfaces or for the
`return of the control surfaces to a neutral orientation
`where lateral thrust is not produced.
`
`1 Claim, 8 Drawing Figures
`
`15
`
`10
`
`~----~--------
`
`13
`
`14
`
`14
`
`Ex. PGS 1033
`
`

`
`U.S. Patent
`15
`
`18
`
`Sep. 13, 1983
`
`Sheet 1 of 5
`
`4,404,664
`
`,10
`13
`___ __ _/_ - - - - - - - -
`
`12
`F'iq.l
`
`14
`
`SELECTED
`(COURSE
`MADE
`GOOD)
`HEADING
`
`X=-
`Y=+
`
`X=-
`Y=-
`
`X=+
`Y=+
`
`X=+
`y =-
`
`Fiq.4
`
`" <
`
`F'iq. 5
`
`Ex. PGS 1033
`
`

`
`POWER SUPPLY 46
`
`ACTUATOR MEANS 40
`
`DECODING CIRCUITRY 38B
`- lillfs1tif38A --
`
`44
`
`38
`
`35
`
`II
`
`AMPLIFIER 36B
`36-~-----l~------
`INDUCTION COIL 36A
`
`COMPASS
`
`12A
`
`Pig. 2
`
`14
`
`35
`
`TRANSMISSION
`MEANS 34
`
`SIGNAL
`AMPLIFIER 1
`I GENERATOR 348
`34C
`
`I
`
`SENSOR
`
`26
`
`24
`
`25
`
`28
`
`GYRO(cid:173)
`COMPASS
`
`31
`
`CRT DISPLAY
`
`~ .
`Cl'.l .
`
`-\0
`
`00 w
`
`N
`0 .....
`VI
`
`Ex. PGS 1033
`
`

`
`U.S. Patent
`
`Sep. 13, 1983
`
`Sheet 3 of 5
`
`4,404,664
`
`MAGNETIC
`NORTH
`
`\
`t
`
`SELECTED
`(COURSE Co
`MADE ..--...L-~~~=---+---­
`GOODl
`HEADING
`
`Piq. 3
`
`Ex. PGS 1033
`
`

`
`U.S. Patent
`
`Sep. 13, 1983
`
`Sheet 4 of 5
`
`4,404,664
`
`92b
`
`90c
`
`90b
`
`92c
`
`lF'iq. B
`
`64
`
`59
`
`II
`
`54-
`
`I
`I
`86
`I
`I
`66
`
`I +--7
`
`Piq.6
`
`Ex. PGS 1033
`
`

`
`_.,...-------
`//56
`L_ _ _ _
`
`35
`
`72
`
`44
`
`66
`
`56
`
`58
`
`60
`
`c .
`en .
`
`DIRECTION
`OF TOW
`
`tO
`..._. ~
`
`VI
`0 ....,
`VI
`
`Ex. PGS 1033
`
`

`
`SYSTEM FOR LATERALLY POSITIONING A
`TOWED MARINE CABLE AND METHOD OF
`USING SAME
`
`BACKGROUND OF THE INVENTION
`The invention relates to towed marine cable control
`practice and more particuhirly but without limitation to
`the positioning of a towed seismic cable in a desired 10
`horizontal orientation by remote control.
`Marine seismic exploration is often conducted by
`means of a marine cable containing a multiplicity of
`seismic sensors and known in the art as a streamer
`which is towed beneath the surface of the water by a 15
`vessel over an area to be seismic surveyed. Generally
`such surveys are conducted by towing the streamer
`along a predetermined line or track over the seafloor
`creating seismic disturbances in the water along that
`track by suitable means, and recording signals produced 20
`by the seismic sensors as a result. Cross-currents tend to
`drive the cable laterally off the track, a condition which
`degrades the quality of the seismic data collected. Thus
`it is desirable to be able to tow a seismic cable in a
`straight line along a selected heading despite the pres- 25
`ence of cross-currents and the like which tend to drive
`segments or the entire cable laterally from the selected
`track.
`Obstacles are sometimes encountered which force the
`towing vessel to break away from the track so as to 30
`avoid damaging the towed cable. As these cables are
`often two miles or more in length, a time consuming
`detour must be made to avoid the obstacle and to repo(cid:173)
`sition the cable along the track. It is therefore also desir(cid:173)
`able to be able to controllably move selected sections of 35
`a marine cable laterally to the direction in which the
`cable being towed so as to, for example, steer the cable
`around the obstacles and to reposition the cable along
`~~~~~h~nd:~~::.r side of the obstacle while avoiding a 40
`Lateral positioning of a towed cable comprises two
`basic aspects: determining the existing position of the
`cable and moving it to a desired position. Examining the
`latter aspect first, designs for remotely controllable 45
`lateral positioning devices for use with seismic and
`other towed marine cables are disclosed in U.S. Pat.
`Nos. 3,605,674 to Weese and 4,330,278 to Waters.
`Weese discloses several variations of a remotely con(cid:173)
`trolled device for laterally or laterally and vertically 50
`positioning a streamer or other towed marine cable.
`Each device is mounted around the cable and, depend(cid:173)
`ing upon the embodiment, is provided with pairs of
`vertical or vertical and horizontal fins rotatable for
`directional control. Waters discloses a different embodi- 55
`ment of a depth and lateral positioning apparatus com(cid:173)
`prising vertically oriented hydrofoil supported from the
`sea surface by a float and in tum supporting a length of
`the towed cable by resilient connecting means. In
`Weese, an undefined signal is generated and transmitted 60
`in an undescribed fashion along wires within the cable
`being towed to appropriate sensing and actuating equip(cid:173)
`ment within each lateral positioning device which ac(cid:173)
`complishes the rotation of the fins. Waters further dis(cid:173)
`closes a control method and acoustic apparatus for 65
`automatically positioning a multiplicity of the lateral
`positioning devices along a straight line at a selected
`heading from the towing boat. The control method and
`
`1
`
`4,404,664
`
`2
`apparatus described by Waters is unsuited for selec(cid:173)
`tively positioning individual lateral positioning devices.
`Several means for determining and monitoring the
`location of a towed cable are known. Weese and Waters
`5 both suggest locating and monitoring the position of the
`towed cable by means of sonar transponders positioned
`at selected points along the cable. The transponders can
`be used in a variety of ways with complementary equip(cid:173)
`ment in the towing vessel to provide data from which
`the range and heading of each transponder with respect
`to the towing vessel can be calculated. Weese alter(cid:173)
`nately suggests monitoring the location of the cable by
`horizontal ranging sonar, presumably on the towing
`vessel.
`An entirely different method oflocating a streamer or
`other marine cable with respect to a towing vessel is
`described in U.S. Pat. No. 3,953,827 toLe Moal eta!. Le
`Moal et a! discloses determining the locus of a towed
`cable by identifying the angles of tangents to the cable
`with respect to a fixed and known direction, such as
`magnetic north, at a plurality of known measuring
`points along the cable. The angular information is sup-
`plied by a suitable sensor located at each measuring
`point, preferably a magnetic compass. Means are also
`provided for coding and transmitting the measured
`angular values by means of electronic pulses to a central
`station. The position of each measuring point is deter-
`mined by approximating that part of the towed cable
`located between the sensors to a circular arc, the length
`of which is known from the spacing of the sensors,
`while the angular value of the arc is determined from
`the differences between the angles formed by the tan(cid:173)
`gent to the cable at the measuring points and the fixed
`and known direction. The positions of other known
`points along the cable are then determined by interpola(cid:173)
`tion. Compared with the transducer locating methods
`described by Weese and Waters, the Le Moal et al
`method allows the more accurate determination of in(cid:173)
`flections in the cable between the measuring points.
`This information is of particular importance in seismic
`surveying where the precise location of the streamer
`sensors providing the seismic data is desired for correc(cid:173)
`tion purposes. The Le Moal et al method does not suffer
`from loss of sight of the cable which may be encoun(cid:173)
`tered by horizontal ranging sonar when the distal end of
`the towed cable is shielded under certain orientations by
`deflections of the cable lying in the line of sight of the
`sonar. Furthermore, the horizontal ranging sonar cable
`monitoring method does not identify the location of the
`lateral positioning devices along the cable nor does it
`provide cable positioning information in a form conve(cid:173)
`nient for seismic data correcting. U.S. Pat. No.
`4,068,208 to Rice, Jr. et a! discloses yet another marine
`streamer position determining system which, however,
`is unsuited for cable monitoring use with lateral posi-
`tioning devices.
`U.S. patent application Ser. No. 885,916 filed Mar.
`13, 1978, by Walter P. Neeley, which has been assigned
`to the assignee of this invention, discloses a marine
`seismic cable location system which utilizes cable tan(cid:173)
`gent headings to determine the relative horizontal posi(cid:173)
`tion of a cable with respect to a towing vessel and to
`produce a visual display of the towing vessel and cable
`for monitoring purposes.
`
`
`
`Ex. PGS 1033
`
`

`
`4,404,664
`
`4
`vice coordinate signals. Additional control signals may
`be generated in the previously described manner to
`further position the device or to commence the reloca-
`tion of another lateral positioning device.
`In a further aspect of the invention, coordinates of
`obstacles in the path of the towing vessel and/or cable
`are identified with respect to the vessel's position and
`entered into the computer by suitable means such as an
`operator keyboard for entry into the matrix of the visual
`display for presentation with the aforesaid coordinates
`of the vessel and lateral positioning devices.
`In yet another aspect of the invention each lateral
`positioning device is provided with means for automati(cid:173)
`cally returning its vertical control surfaces to a neutral
`15 orientation upon operator command.
`
`5
`
`3
`SUMMARY OF THE INVENTION
`It is an object of th invention to provide a system for
`controlling the lateral position of a cable being towed
`through the water.
`It is another object of the invention to provide a
`system for controlling a plurality of lateral positioning
`devices secured to a seismic cable at selected points
`utilizing cable monitoring means which permit the esti(cid:173)
`mation of the location of seismic sensors also spaced 10
`along the cable. ·
`It is yet another object of the invention to estimate
`the location of selected points along the cable by moni(cid:173)
`toring the headings of tangents to the cable at known
`points along its length.
`Accordingly, a plurality of remotely controlled, lat(cid:173)
`eral positioning devices are mounted at selected points
`along the length of a cable being towed. Pluralities of
`positioning sensors, such as magnetic compasses, are
`located at other selected points between the towing 20
`vessel and first lateral positioning device and between
`adjacent lateral positioning devices for providing sig(cid:173)
`nals representing the heading of tangents to the cable at
`the location of the position sensors. The position sensor
`generated signals are used by a computer to produce 25
`signals representing the horizontal position coordinates
`of the vessel and of the lateral positioning devices rela(cid:173)
`tive to the vessel and to a selected heading, such as the
`course made good heading of the vessel. The coordinate
`signals are provided to a display matrix of a suitable 30
`device such as a cathode ray tube for display of the
`relative positions of the vessel and lateral positioning
`devices with respect to the selected heading. Where
`magnetic compasses are used along the cable, other
`means such as a magnetic compass and gyrocompass are 35
`provided on-board the vessel for determining correc(cid:173)
`tions to allow for the local magnetic compass variations.
`It is a further object of the invention to provide oper(cid:173)
`ative means for controlling selected lateral positioning
`devices whereby the location of segments of the cable 40
`between such selected devices may be individually con(cid:173)
`trolled.
`It is a further object to control the lateral positioning
`devices of such a system through the use of a computer.
`Accordingly, means such as the aforesaid computer 45
`are provided for generating a coded signal representing
`the identification of the lateral positioning device se(cid:173)
`lected for control and information for the activation and
`operation of the motor means within the selected lateral
`positioning device for accomplishing a desired control 50
`surface adjustment. The aforesaid control signal is gen(cid:173)
`erated in response to a signal provided by operator
`means, such as a keyboard, or in direct response to the
`aforesaid vessel and lateral positioning device coordi(cid:173)
`nate signals. Transmission means are provided for con- 55
`verting the coded control signal into a form suitable for
`transmitting through conductors in the seismic cable.
`Suitable circuitry in each lateral positioning device
`senses and examines the coded control signal to deter(cid:173)
`mine if the signal was directed to that device. In the 60
`selected lateral positioning device, circuitry further
`decodes the coded control signal. Motor actuation
`means are controlled by the decoder means in accor(cid:173)
`dance with the information provided in the coded con(cid:173)
`trol word and operate motor means for the adjustment 65
`of the lateral positioning control surfaces in the desig(cid:173)
`nated direction and to the desired degree. Feedback is
`provided by updated vessel and lateral positioning de-
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 illustrates a seismic exploration system utiliz(cid:173)
`ing the invention;
`FIG. 2 illustrates in block diagram form the cable
`monitoring and positioning equipment of the invention
`employed with the exploration system of FIG. 1;
`FIG. 3 illustrates the geometric configuration utilized
`in determining cable lateral positioning device coordi(cid:173)
`nates with respect to the vessel;
`FIG. 4 represents a truth table for locating the bear(cid:173)
`ing of a lateral positioning device;
`FIG. 5 illustrates a visual display of the coordinates
`of the vessel and selected points along the towed cable
`as determined by the equipment of FIG. 2;
`FIG. 6 is a front view of a lateral positioning device
`mounted to a cable;
`FIG. 7 is a section view of the lateral positioning
`device of FIG. 6 as viewed along the section line 7-7.
`FIG. 8 is a section view of the device of FIG. 7 as
`viewed along the section line 8-8.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`In seismic marine exploration, a surface vessel 10 of
`FIG. 10 tows a seismic detector cable or streamer 11
`along a selected line of exploration or track 13. The
`heading 15 of the vessel10 is varied depending upon the
`absence or presence of cross-currents so that the vessel
`10 follows a net or "course made good" heading 18
`along the track 13. The cable 11 conventionally em(cid:173)
`ploys a plurality of hydrophones or other suitable seis(cid:173)
`mic wave detectors (not shown) spaced along its length
`for receiving seismic wave reflections from geophysical
`strata beneath the seafloor. One or more seismic wave
`sources (not depicted), such as airguns, are also typi(cid:173)
`cally towed by the vessel 10 to generate seismic waves,
`the reflections of which are received by the hydro(cid:173)
`phones. Conductors running through the streamer 11
`carry seismic sensor generated signals back to the vessel
`for recording, retransmission and/or display.
`Positioned at known distances along the cable 11 are
`a plurality of cable tangent heading sensors 12, six of
`which are illustrated in FIG. 1, and a plurality oflateral
`positioning devices 14, three of which are illustrated in
`FIG. 1. Each sensor 12 provides a signal representing
`the magnetic heading of the tangent to the cable in the
`horizontal plane at that particular point. By knowing
`the headings of tangents to the cable at a plurality of
`points along its length and the distances between each
`of such points, the location of the cable can be deter(cid:173)
`mined in a manner subsequently described. Each lateral
`positioning device 14 is provided with a plurality of
`
`
`
`Ex. PGS 1033
`
`

`
`5
`
`5
`vertical control surfaces which can be adjusted by re(cid:173)
`mote control as will also be later described so as to vary
`the magnitude of the lateral component of force to
`which each device is subjected at any given time as it is
`towed through the water.
`FIG. 2 depicts in schematic, block diagram form, a
`preferred embodiment of the invention. Each sensor 12,
`one of which is depicted, includes a magnetic compass
`12A such as a Model 319 Magnetic Sensor supplied by
`Digicourse, Inc., and a binary control unit 12B, such as 10
`a Model 350 Binary Control Unit also by Digicourse,
`Inc. The readings of the compasses 12A are multiplexed
`by the associated binary control units 12B on to a single
`pair of wires 12C running the length of the cable to the
`on-board cable location computing system. Each binary 15
`control unit 12B is addressed with its appropriate code
`number by an interrogator 23, a Model 290 Data Acqui(cid:173)
`sition Unit of Digicourse, Inc. or similar functioning
`device. A start pulse from either a computer 30 or a
`cycle timer 24 initiates the multiplexing of the magnetic 20
`compass headings from the control units 12B to infor(cid:173)
`mation registers in the interrogator 23. Also applied to
`an information register in the interrogator 23 is the
`heading from an on-board magnetic compass 25, such as
`the ModellOl Magnetic Sensor ofDigicourse, Inc. The 25
`compass heading in any one of the information registers
`can be visually displayed on a suitable sensor display 26
`such as a Model 102 Sensor Display of Digicourse, Inc.
`or a similarly functioning device.
`The information registers of the interrogator 23 trans- 30
`fer the compass headings to an external header unit in a
`field recorder 27, such as a DFS V Digital Field Re(cid:173)
`corder of Texas Instruments, Inc. or other comparable
`seismic recording system. Also applied to such external
`header unit is the heading of the vessel with respect to 35
`true north from an on-board gyrocompass 28 and the
`absolute coordinates of the vessel in a geodetically fixed
`coordinate system from an on-board navigation system
`29. The field recorder 27 therefore contains all the in(cid:173)
`formation required to compute the absolute positions of 40
`the vessel and cable and the position of the cable rela(cid:173)
`tive to the vessel. The compass headings and absolute
`coordinates of the vessel are outputted from the field
`recorder 20 to a digital computer 30 which may be
`dedicated to cable location/control or a general pur- 45
`pose device used in connection with other exploration
`activities. A recorder/printer 32 having a keyboard,
`such as a Texas Instruments, Inc. Silent 730 KSR, is
`provided for operator entry of a selected heading to the
`computer 30. The computer 30 uses the aforesaid se- 50
`lected heading to determine the coordinates of the lat(cid:173)
`eral positioning devices in a two dimension, cartesian (X
`and Y) coordinate system centered on the vessel and
`oriented with respect to the selected heading. The X
`and Y coordinates together with the absolute coordi- 55
`nates of the vessel and the various compass headings are
`outputted to a suitable device such as a magnetic tape
`unit 24 for recording and later use. A simple plot of the
`X and Y coordinates of the ship and the lateral position(cid:173)
`ing devices is provided on a suitable visual display de- 60
`vice such as a cathode ray tube 31. The computer 30
`also provides the bearing and range of each lateral posi(cid:173)
`tioning device 14 with respect to the slip and selected
`heading, on the keyboard recorder/printer 32. When
`the course made good heading of the vessel is selected 65
`by the operator, as is suggested when towing a seismic
`cable along a track, and entered into the computer 30
`through the keyboard 32, the compass bearings to the
`
`4,404,664
`
`6
`positioning devices will be generally reciprocal to the
`course made good heading. The displacement of each
`lateral positioning device 14 from the selected track is
`visible on the display unit 31.
`Control commands for a selected lateral positioning
`device 14 are entered by a cable positioning operator
`through the recorder/printer 32 for inputting to the
`computer 30. The cgmputer 30 is progra11;1med to re(cid:173)
`spond to the operator entered control command to
`generate an appropriately coded digital command word
`which is outputted from the computer 30 is a suitable
`transmissi~n means 34 for subsequent transmission
`along wires 35 in the cable 11 to the lateral positioning
`devices 14, one of which is depicted functionally. Alter(cid:173)
`natively, the computer can be programmed to respond
`directly to the coordinate information to automatically
`generate suitable lateral positioning device control sig(cid:173)
`nals.
`A suitable receiver means 36 in each lateral position(cid:173)
`ing device 14 detects and reconstitites the digital word
`and passes it to the decoder means 38 which examines
`the coded control word to determine if it is directed to
`that device 14. If found to apply to that device 14, the
`decoder means 38 provides in response to the coded
`word an appropriate signal to actuator means 40 within
`the device 14 which in turn provides power to a motor
`42 to adjust a plurality of vertical control surfaces 44 in
`the desired direction and to the desired extent. Changes
`in the location of the selected lateral positioning device
`14 with respect to the vessel will be disclosed through
`subsequent readings of the magnetic compasses 12A and
`generation of new coordinates. In response to the new
`position, corrective commands may be made by the
`operator through the recorder/printer 32 or automati(cid:173)
`cally by the computer for final positioning of each de(cid:173)
`vice 14.
`Having generally described the invention in conjunc(cid:173)
`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(cid:173)
`tion and lateral positioning of the cable 11 during a
`towing operation.
`During seismic operations each seismic recording
`cycle is initiated at time zero by a ground signal from
`the cycle timer 24. If seismic operations are not being
`carried out, the computer 30 can supply the ground
`signal to the interrogator 23. This signal is utilized by
`the interrogator 23 to successively address each binary
`control unit 12B for an appropriate length of time (100
`milliseconds for the Digicourse unit in this example).
`The on-board magnetic compass 25 is also read by the
`interrogator 23 (requiring 60 milliseconds to accomplish
`with the compass and interrogator previously speci(cid:173)
`fied). When the sensor polling operations of the interro(cid:173)
`gator 23 are complete, it outputs a signal to the transmit(cid:173)
`ter means 32. The magnetic compass readings are then
`outputted from information registers (not depicted) of
`the interrogator 23 to the external header unit of the
`field recorder 27. Also passed to the field recorder 27
`are the true north heading of the ship from the gyro(cid:173)
`compass 28 and the absolute coordinates of the vessel
`from the on-board navigation system 29. The compass
`readings and on-board navigation system coordinates
`are then read out of the field recorder 27 and into the
`computer 30. The course made good heading to be
`followed by the vessel during towing is inputted into
`the computer 30 by the cable control operator through
`the keyboard in the recorder/printer 32. The computer
`
`
`
`Ex. PGS 1033
`
`

`
`4,404,664
`
`8
`c0 =course made good heading of the vessel Po with
`respect to magnetic north,
`Cij=cable headings with respect to magnetic north
`for each compass Cij,
`d;=chord subtending the arc of curvature of the
`cable between adjacent compass pairs cil and ci2
`(e.g., i.e. dt between Ct 1 and Ctz),
`D;=chord subtending the arc of curvature of the
`cable between the vessel and/or the adjacent lat(cid:173)
`eral positioning devices (P;.t and P;) located at the
`ends of each arc i, i= 1,3 (i.e. Dt-03),
`s;=cable length between adjacent compass pairs Ci!
`and Cain each arc i, i= 1-3, (i.e. St-S3),
`S;=cable length between points P;.J and P;(vessel Po
`and devices Pt, Pz, and P3), (i.e. St-S3),
`a;= angle between the parallel chords d; and D; and
`the tangent line for the cable heading c;t, (i.e. a1-a3)
`b;=angle between a line pointing in the direction of
`the vessel's course made good heading Co and the
`chords d; and D; (i.e. bt-b3)-
`Each chord D; defined in the above manner becomes a
`directed line segment with vector components X; and
`Y;. Computation of each coordinate pair X;, Y; and the
`distance R; and bearing e; from the vessel (Po) to each
`lateral positioning device P; is as follows (assuming
`equal spacing of compass pairs Ci! and Ca from adjoin(cid:173)
`ing points P;.J and P;, respectively):
`
`7
`30 has previously been supplied with information re(cid:173)
`garding the spacing of each of the magnetic compasses
`12A and lateral positioning devices 14 along the cable
`11. The computer 30 then determines the X and Y coor(cid:173)
`dinates of each lateral positioning device 14 in a carte- 5
`sian system centered on the vessel with the +X direc(cid:173)
`tion being defined as heading off the stern of the vessel
`at 180° to the aforesaid selected course made good
`heading and the+ Y direction heading off the starboard
`of the vessel at 90° to the course made good heading. 10
`The bearing and range of each lateral positioning device
`14 with respect to the ship and the course made good
`heading are determined by the computer 30 and output(cid:173)
`ted to the recorder/printer 32 for display.
`Determinations of the location of the lateral position- 15
`ing devices are based upon the theory that between the
`lateral positioning devices 14, the cable 11 will assume a
`curve which can be approximated by one or more circu(cid:173)
`lar arcs. When tangents to two points along each arc
`and the distance between the points are known, the 20
`location of any point along the arc and chords between
`any points of the arc can be determined. Thus, chords
`between the towing vessel and each lateral positioning
`device can be determined. These chords can then be
`stacked to depict the cable's position with respect to the 25
`course made good heading as the cable is towed
`through the water. Such stacking will also yield a single
`vector indicating the distance of the terminus of any
`chord from the vessel and the bearing of such terminus
`with respect to the selected (course made good) head- 30
`in g.
`Referring more particularly to FIG. 3 there is illus(cid:173)
`trated an example configuration for a seismic cable 11
`being towed by a vessel Po and mounting three lateral
`positioning devices Pt, Pz, P3 and six magnetic com- 35
`passes Cij, i=l-3, j=l-2. A compass pair 0=1,2) is
`provided along the cable before each lateral positioning
`device (P;, i= 1-3). It should be realized that the config(cid:173)
`uration of the cable depicted in FIG. 3 is exaggerated
`for purposes of illustration although such a configura- 40
`tion might be achieved during a turning maneuver of
`the towing vessel. At least two cable tangent headings
`along a length of cable are required to estimate a circu(cid:173)
`lar arc for that cable section. If desired, more than two
`cable heading sensors can be provided between the 45
`vessel and the leading lateral positioning device or be(cid:173)
`tween adjoining devices, and circular arcs estimated for
`each cable segment between such adjoining sensors.
`Moreover, although it is possible to locate the com(cid:173)
`passes at any point along the length of arc between the 50
`boat and the first lateral positioning device or between
`adjoining lateral positioning devices, it is suggested for
`simplification that each compass of a compass pair Cij,
`j = 1,2, be located an equal distance from the boat or
`nearest lateral positioning device. Thus, in FIG. 3 the 55
`distance along the cable of the first magnetic compass
`CII from the vessel 10 is equal to the distance between
`the second compass C12 and the first lateral positioning
`device Pt. In this way, each chord between the points
`P;.J and P;(such as chord Dt between the vessel Po and 60
`lateral positioning device Pt) will be parallel to the
`chord d; between the compasses C;t and Cain that seg(cid:173)
`ment of cable (i.e., chord dt between CII and C12). For
`example, each magnetic compass Cij may be located a
`distance from the proximal lateral positioning device P; 65
`or P;.J equal to one quarter of the arc length S;between
`each positioning device P;and P;.J. The other pertinent
`terms and relationships of FIG. 3 are defined as follows:
`
`b;=ai+co-cn
`
`d;={l80-s;/7r·a;)·sin a;
`
`D;= (180·s;/7r·a;)·sin (a;.S;/s;)
`
`X;=D;·cos bb· Y;=D;·sin b;
`
`Examination of the signs of the L;X; and L;Y; give the
`bearing 0; with respect to the selected (course made
`good) heading c0 as shown in a truth table depicted in
`FIG. 4.
`A typical plot of locations with respect to the course
`made good heading of the vessel for the three lateral
`positioning devices of FIG. 1 is illustrated in FIG. 5.
`Such plot is based on a matrix of display cells wherein
`the entire cell in which a determined X-Y coordinate
`falls is brightened on the face of the cathode ray tube
`display 21. This four point plot of X-Y coordinates is
`updated prior to each firing cycle (approximately every
`12 seconds) or, if seismic data is not being gathered, as
`often as is initiated by the computer 30. If desired,
`greater positional definition of the cable can be pro(cid:173)
`vided by computing and displaying the X-Y coordinates
`of other points along the cable such as the locations of
`the cable mounted magnetic compasses 12A.
`In addition to the location of the marine vessel and
`the lateral positioning devices, the locations of various
`obstacles that lie of the path of the vessel or its cable,
`such as other vessels,