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BACKGROUND OF THE INVENTION
The invention relates to towed marine cable control practice and more
particularly but without limitation to the positioning of a towed seismic
cable in a desired 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 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 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 presence 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 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 reposition the cable along the track.
It is therefore also desirable to be able to controllably move selected
sections of 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 track on the far side of the obstacle
while avoiding a lengthy detour.
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 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
controlled device for laterally or laterally and vertically positioning a
streamer or other towed marine cable. Each device is mounted around the
cable and, depending upon the embodiment, is provided with pairs of
vertical or vertical and horizontal fins rotatable for directional
control. Waters discloses a different embodiment of a depth and lateral
positioning apparatus comprising vertically oriented hydrofoil supported
from the sea surface by a float and in turn supporting a length of the
towed cable by resilient connecting means. In Weese, an undefined signal
is generated and transmitted in an undescribed fashion along wires within
the cable being towed to appropriate sensing and actuating equipment
within each lateral positioning device which accomplishes the rotation of
the fins. Waters further discloses a control method and acoustic apparatus
for 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 apparatus described by Waters is unsuited for
selectively positioning individual lateral positioning devices.
Several means for determining and monitoring the location of a towed cable
are known. Weese and Waters 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 equipment 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 alternately suggests monitoring the
location of the cable by horizontal ranging sonar, presumably on the
towing vessel.
An entirely different method of locating a streamer or other marine cable
with respect to a towing vessel is described in U.S. Pat. No. 3,953,827 to
Le Moal et al. Le Moal et al 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 supplied 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 determined 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 tangent 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 interpolation. Compared with the transducer
locating methods described by Weese and Waters, the Le Moal et al method
allows the more accurate determination of inflections 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 correction purposes. The Le Moal
et al method does not suffer from loss of sight of the cable which may be
encountered 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 convenient for seismic data correcting.
U.S. Pat. No. 4,068,208 to Rice, Jr. et al discloses yet another marine
streamer position determining system which, however, is unsuited for cable
monitoring use with lateral positioning 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
tangent headings to determine the relative horizontal position of a cable
with respect to a towing vessel and to produce a visual display of the
towing vessel and cable for monitoring purposes.
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
estimation of the location of seismic sensors also spaced along the cable.
It is yet another object of the invention to estimate the location of
selected points along the cable by monitoring the headings of tangents to
the cable at known points along its length.
Accordingly, a plurality of remotely controlled, lateral 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 vessel and first
lateral positioning device and between adjacent lateral positioning
devices for providing signals 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 signals representing
the horizontal position coordinates of the vessel and of the lateral
positioning devices relative 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 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 provided on-board the vessel for determining
corrections to allow for the local magnetic compass variations.
It is a further object of the invention to provide operative means for
controlling selected lateral positioning devices whereby the location of
segments of the cable between such selected devices may be individually
controlled.
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 are provided for
generating a coded signal representing the identification of the lateral
positioning device selected for control and information for the activation
and operation of the motor means within the selected lateral positioning
device for accomplishing a desired control surface adjustment. The
aforesaid control signal is generated 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 coordinate signals. Transmission
means are provided for converting 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 determine if the signal was directed to that
device. In the selected lateral positioning device, circuitry further
decodes the coded control signal. Motor actuation means are controlled by
the decoder means in accordance with the information provided in the coded
control word and operate motor means for the adjustment of the lateral
positioning control surfaces in the designated direction and to the
desired degree. Feedback is provided by updated vessel and lateral
positioning device coordinate signals. Additional control signals may be
generated in the previously described manner to further position the
device or to commence the relocation 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 automatically returning its vertical control
surfaces to a neutral orientation upon operator command.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a seismic exploration system utilizing 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 coordinates with respect to the vessel;
FIG. 4 represents a truth table for locating the bearing 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 vessel 10 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 employs a plurality of hydrophones or other suitable
seismic 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
typically towed by the vessel 10 to generate seismic waves, the
reflections of which are received by the hydrophones. 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 of lateral 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 determined in a manner
subsequently described. Each lateral positioning device 14 is provided
with a plurality of vertical control surfaces which can be adjusted by
remote 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 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 control unit 12B is addressed
with its appropriate code number by an interrogator 23, a Model 290 Data
Acquisition 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 compass headings from the control units 12B
to information 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 Model 101 Magnetic Sensor of
Digicourse, Inc. The 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 transfer the compass
headings to an external header unit in a field recorder 27, such as a DFS
V Digital Field Recorder 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 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 information required
to compute the absolute positions of the vessel and cable and the position
of the cable relative 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 purpose 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
selected heading to determine the coordinates of the lateral 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 coordinates 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 positioning devices is
provided on a suitable visual display device such as a cathode ray tube
31. The computer 30 also provides the bearing and range of each lateral
positioning 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 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 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 computer 30 is programmed to respond to
the operator entered control command to generate an appropriately coded
digital command word which is outputted from the computer 30 is a suitable
transmission means 34 for subsequent transmission along wires 35 in the
cable 11 to the lateral positioning devices 14, one of which is depicted
functionally. Alternatively, the computer can be programmed to respond
directly to the coordinate information to automatically generate suitable
lateral positioning device control signals.
A suitable receiver means 36 in each lateral positioning 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
automatically by the computer for final positioning of each device 14.
Having generally described the invention in conjunction 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
location 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 specified). When the sensor polling operations of the
interrogator 23 are complete, it outputs a signal to the transmitter 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 gyrocompass 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 30 has previously been supplied with
information regarding 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 coordinates of each lateral positioning device
14 in a cartesian system centered on the vessel with the +X direction
being defined as heading off the stern of the vessel at 180.degree. to the
aforesaid selected course made good heading and the +Y direction heading
off the starboard of the vessel at 90.degree. to the course made good
heading. 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 outputted to the recorder/printer 32 for display.
Determinations of the location of the lateral positioning 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 circular
arcs. When tangents to two points along each arc and the distance between
the points are known, the 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 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) heading.
Referring more particularly to FIG. 3 there is illustrated an example
configuration for a seismic cable 11 being towed by a vessel P.sub.0 and
mounting three lateral positioning devices P.sub.1, P.sub.2, P.sub.3 and
six magnetic compasses C.sub.ij, i=1-3, j=1-2. A compass pair (j=1,2) is
provided along the cable before each lateral positioning device (P.sub.i,
i=1-3). It should be realized that the configuration of the cable depicted
in FIG. 3 is exaggerated for purposes of illustration although such a
configuration 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 circular arc for that cable section. If desired,
more than two cable heading sensors can be provided between the vessel and
the leading lateral positioning device or between adjoining devices, and
circular arcs estimated for each cable segment between such adjoining
sensors. Moreover, although it is possible to locate the compasses at any
point along the length of arc between the 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 C.sub.ij,
j=1,2, be located an equal distance from the boat or nearest lateral
positioning device. Thus, in FIG. 3 the distance along the cable of the
first magnetic compass C.sub.11 from the vessel 10 is equal to the
distance between the second compass C.sub.12 and the first lateral
positioning device P.sub.1. In this way, each chord between the points
P.sub.i-1 and P.sub.i (such as chord D.sub.1 between the vessel P.sub.0
and lateral positioning device P.sub.1) will be parallel to the chord
d.sub.i between the compasses C.sub.i1 and C.sub.i2 in that segment of
cable (i.e., chord d.sub.1 between C.sub.11 and C.sub.12). For example,
each magnetic compass C.sub.ij may be located a distance from the proximal
lateral positioning device P.sub.i or P.sub.i-1 equal to one quarter of
the arc length S.sub.i between each positioning device P.sub.i and
P.sub.i-1. The other pertinent terms and relationships of FIG. 3 are
defined as follows:
c.sub.o =course made good heading of the vessel P.sub.0 with respect to
magnetic north,
c.sub.ij =cable headings with respect to magnetic north for each compass
C.sub.ij,
d.sub.i =chord subtending the arc of curvature of the cable between
adjacent compass pairs C.sub.i1 and C.sub.i2 (e.g., i.e. d.sub.1 between
C.sub.11 and C.sub.12),
D.sub.i =chord subtending the arc of curvature of the cable between the
vessel and/or the adjacent lateral positioning devices (P.sub.i-1 and
P.sub.i) located at the ends of each arc i, i=1,3 (i.e. D.sub.1 -D.sub.3),
s.sub.i =cable length between adjacent compass pairs C.sub.i1 and C.sub.i2
in each arc i, i=1-3, (i.e. s.sub.1 -s.sub.3),
S.sub.i =cable length between points P.sub.i-1 and P.sub.i (vessel Po and
devices P.sub.1, P.sub.2, and P.sub.3), (i.e. S.sub.1 -S.sub.3),
a.sub.i =angle between the parallel chords d.sub.i and D.sub.i and the
tangent line for the cable heading c.sub.i1, (i.e. a.sub.1 -a.sub.3)
b.sub.i =angle between a line pointing in the direction of the vessel's
course made good heading C.sub.0 and the chords d.sub.i and D.sub.i (i.e.
b.sub.1 -b.sub.3).
Each chord D.sub.i defined in the above manner becomes a directed line
segment with vector components X.sub.i and Y.sub.i. Computation of each
coordinate pair X.sub.i, Y.sub.i and the distance R.sub.i and bearing
.theta..sub.i from the vessel (P.sub.0) to each lateral positioning device
P.sub.i is as follows (assuming equal spacing of compass pairs C.sub.i1
and C.sub.i2 from adjoining points P.sub.i-1 and P.sub.i, respectively):
a.sub.i =1/2(c.sub.i1 -c.sub.i2)
b.sub.i =a.sub.i +c.sub.0 -c.sub.i1
d.sub.i =(180.multidot.s.sub.i /.pi..multidot.a.sub.i).multidot.sin a.sub.i
D.sub.i =(180.multidot.s.sub.i /.pi..multidot.a.sub.i).multidot.sin
(a.sub.i .multidot.S.sub.i /s.sub.i)
X.sub.i =D.sub.i .multidot.cos b.sub.i ; Y.sub.i =D.sub.i .multidot.sin
b.sub.i
R.sub.i =.sqroot. [(.SIGMA..sub.i X.sub.i).sup.2 +(.SIGMA..sub.i
Y.sub.i).sup.2 ]
.theta..sub.i =180.degree.-tan.sup.-1 (.SIGMA..sub.i Y.sub.i /.SIGMA..sub.i
X.sub.i)
Examination of the signs of the .SIGMA..sub.i X.sub.i and .SIGMA..sub.i
Y.sub.i give the bearing .theta..sub.i with respect to the selected
(course made good) heading c.sub.o 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 provided 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, drilling towers,
etc., may also be displayed with their X-Y coordinates. It is envisioned
that the range and heading of the obstacle is determined by suitable means
such as the vessel's radar or sonar. The cable control operator enters the
obstacle's range and heading into the computer 30 by means of the input
keyboard of the recorder/printer 32. Suitable programming in the computer
responds to the information entry, and generates a signal representing the
X-Y coordinates of the obstacle which is entered in the matrix of the
display. A corresponding cell, such as that indicated by the * is
illuminated. Updated range and heading information can be periodically
entered by the operator through the recorder/printer 32. Updated X-Y
coordinates of the obstacle are generated by the computer which enters the
coordinates into the matrix of the cathode-ray tube device 21. It is
envisioned that suitable radar and/or sonar equipment may be provided for
automatic periodic entry of the obstacle range and bearing information
into the computer 30 for display update.
The cable control operator selectively controls one or more lateral
positioning devices 14 to bring the cable into line along the selected
track or to otherwise steer the cable in a desired manner by entering a
lateral positioning device command into the computer 30 by a suitable
computer interface means such as the keyboard of the printer/recorder 32.
The operator command identifies the particular lateral positioning device
to be controlled, and the direction and magnitude of the desired movement
of its plurality of vertical, adjustable control surfaces. For example, a
command "P1+010" might be used to indicate a rotation of ten degrees
("010") in a plus ("+") direction (clockwise or counterclockwise, however
defined) in the first lateral positioning device ("P1") along the cable.
In response to the operator command the computer 30 generates a coded
digital word identifying the selected positioning device and the necessary
actuation of motor means in the selected lateral positioning device to
accomplish the commanded adjustment. The information contained in the
coded digital word regarding motor actuation will vary depending upon the
method and apparatus selected for use in the lateral positioning devices
to control their operation. For example, in the preferred embodiment of a
lateral positioning device described herein, a DC stepper motor is
provided in each lateral positioning device for the rotation of a pair of
vertical fins 44 (see FIGS. 2, 6 and 7). The computer 30 is provided with
tables converting degrees of fin rotation to stepper motor pulses. The
coded digital word would therefore contain the direction of current
supplied to the motor (so as to control its direction of rotation) and the
numbers of pulses required to accomplish the operator selected rotation.
It is further preferred that computer 30 be capable of generating, in
response to an operator command, a unique digital word commanding a
selected lateral positioning device to return its adjustable control
surfaces to a neutral position. Again, in the embodiment of the system
being described, each lateral positioning device is provided with
rotatable, vertically oriented control surfaces or fins 44 (see FIGS. 6
and 7). The neutral position of the fins 44 is an essentially zero angle
of attack with respect to the central longitudinal axis of the lateral
positioning device running essentially parallel to the central
longitudinal axis of the cable 12. It is envisioned, for example, that the
computer generated digital control word be provided with a bit that
indicates either return of the fins to the neutral position or some other
specified rotation, the specifics of which are contained in other bits of
word. The coded digital word is passed through an output channel of the
computer 30 to the transmission means 34. The transmission means 34
comprises a buffer 34A, a signal generator 34B and an amplifier 34C. The
buffer 34A contains registers or other suitable means for storing in
sequence each coded digital command word outputted by the computer 30. A
signal from the interrogator 23 indicates when interrogation of the
various compasses is completed so that transmission of the coded digital
words to the lateral positioning devices can commence. Signals between the
vessel and magnetic sensors 12 and between the vessel and lateral
positioning devices 14 must be transmitted through the seismic cable 12 at
separate times and during breaks between the seismic data recording cycles
to prevent crosstalk and other signal interference. Upon receiving the
appropriate signal from the interrogator 23 indicating the end of the
cable sensor polling operation, circuitry in the buffer 34A passes the
digital control words from the registers of the buffer 34A in sequence to
the signal generator 34B which converts the digital control words into an
encoded signal which is passed through amplifier means 34C and onto wires
35 passing through the length of the cable 11.
The control components of each lateral positioning device 14 are depicted
representatively, in block diagram forms, in FIG. 2 and consist of
receiver means 36, decoder means 38, actuator means 40, motor means 42
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