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`Ex. PGS 1029
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`EX. PGS 1029
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`

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`United States Patent
`
`[191
`
`[ll] Patent Number:
`
`5,200,930
`
`Rouquette
`
`[45] Date of Patent:
`
`Apr. 6, 1993
`
`|l||l|lllllllllllllllllllllIlllllllllllllllllllllllllllllllllllllilllllllll
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`COMMUNICATION srsrm
`
`[75]
`
`Inventor: Robert E. Rouquem: Kenner, La
`
`[73] Assignee: The Laitram Corporation, Harahan,
`-
`La.
`
`[2!] Appl. No; 825,!!!7
`
`[22] Filed:
`
`Jan. at, 1992
`
`[51]
`Int. Cl.5 ............................................... 001V 1/22
`
`[52] U.S. Cl. ............................ 367/76
`
`[58] Field of Search ............................. 367/20, 76, 80;
`340/87018, 870.26, 854.9
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,958,216 5/1976 Chapman .............................. 367/80
`4,219,8l0
`8/1980 Joostcn
`340/853
`
`4.9l2.684
`3/1990 Fowler
`..... 367/76
`4,967.40) 10/1990 Woods .................................. 367/21
`
`Primary Examiner—Ian J. Lobo
`Attorney, Agent, or Firm—James T. Cronvich
`
`ABSTRACT
`[57]
`For use with marine seismic streamers, a two-wire,
`mum-channel communication system capable of han-
`dling the high throughput necessary for effective com-
`munication between a central controller aboard a tow
`vessel and the many sensors deployed along the
`
`streamer. The central controller includes an intelligent
`modem with the capability of transmitting and receiv-
`ing frequency-modulated message signals on one or
`more signal
`lines, such a conventional
`twisted-pair
`wires, over a number of individual inbound and out-
`bound frequency channels. In the preferred embodi-
`ment, seventeen channels are spread over a frequency
`band ranging from about 20 kHz to l00 kHz, thereby
`making available for communication a bandwidth much
`wider than available in conventional single-channel
`streamer communication. In this way, many positioning
`sensors, such as compasses, depth sensors, cable-level-
`ing birds, and acoustic-ranging transceivers, attached to
`the streamer and each having a transmitter and receiver
`tuned to one of the modem’s inbound and outbound
`
`channels, respectively, can be put in communication
`with the modem. To take advantage of its high through-
`put capability, the intelligent modem refers to a stored
`table of individual sensor parameters. such as sensor
`type, transmit channel, and receive channel, to schedule
`an efficient scan of the sensors. As a diagnostic tool, the
`communication system also monitors the quality and
`performance of the communication link by measuring
`and recording such parameters as the transmitted and
`received signal strengths, signal-to-noise ratios, and
`number of incorrectly received messages.
`
`25 Claims, 9 Drawing Sheets
`
`
`
`EX. PGS 1029
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`Ex. PGS 1029
`
`

`

`1
`
`
`
`5,200,930
`
`
`
`TWO-WIRE MULTI-CHANNEL STREAMER
`
`
`COMMUNICATION SYSTEM
`
`
`
`
`
`
`
`10
`
`
`
`15
`
`
`
`FIELD OF THE INVENTION
`
`
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`
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`This invention relates to multiple sensor communica-
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`tion and, more particularly, to apparatus and methods
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`for achieving multi-channel communication with many
`sensor devices connected across a two-wire communi-
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`cation path in a seismic streamer cable.
`BACKGROUND OF TI-E‘. INVENTION
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`In a marine seismic survey, a surveying vessel tows
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`one or more seismic cables or streamers. Each streamer
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`is outfitted with distributed seismic transducers, namely
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`hydrophones, and position-control devices and posi-
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`tion-determining sensors, such as cable—leveling birds,
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`compasses, depth (pressure) sensors, and hydroacoustic
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`ranging transceivers. Data from the hydrophones are
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`sent to a controller on board the vessel via a high-speed
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`data link, which could be an optical-fiber link. Data
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`ed-pair line, each wire of the pair being no larger than
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`size 22 AWG. Sensors are connected along the twisted
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`pair in one of two ways. First, in-streamer sensors are
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`transceivers, are individually coupled to the twisted
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`across the twisted pair. Each outlying sensor has an
`individual, associated coil in the streamer.
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`The coupling coils in the streamer are conventionally
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`tuned to the same frequency and typically have a fairly
`high selectivity, or Q, giving the two-wire communica-
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`tion system a narrow bandwidth and a relatively low
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`data rate. The high Q further makes tuning of the trans-
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`mitting frequency critical for effective communication.
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`Because noise generated in the neighboring power sys-
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`tem for the high-speed hydrophone data link occurs at
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`a dominant frequency of about 2 kHz with harmonic
`level decreasing with frequency to beyond 100 kHz,
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`typical two-wire communication takes place at about 25
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`kHz. Conventional communication is achieved my
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`means of frequency-shift-keying (FSK) modulation.
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`Other variations of angle modulation, such as quadra-
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`ture-phase-shift keying (QPSK) and bipolarphase—shift
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`keying (BPSK), are also commonly used. Carrier fre-
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`quencies on the order of 20 kHz—30 kHz are common.
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`Proper tuning of the carrier frequency is critical to
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`achieve a signal-to-noise ratio adequate for effective
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`communication. Thus, present-day two-wire streamer
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`communication relies heavily on a properly tuned sys-
`tem.
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`Prior art two—wire communication with position sen—
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`sors on streamers has generally been realized by half-
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`duplex, single-channel communication schemes. Conse-
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`quently, only one sensor is allowed to send data at a
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`time. Likewise, no sensor may send data while the con-
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`troller is communicating. Such limitations have only
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`recently become important. Several developments
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`promise to make the half-duplex, single-channel com-
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`munication system inadequate to meet expected de-
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`mands in position-determining requirements. First, by-
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`droacoustic ranging systems are seeing more wide-
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`spread use. The positioning accuracy they provide,
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`particularly in multi-streamer applications, is necessary
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`to support the increased accuracy being demanded of
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`seismic surveys. Each hydroacoustic transceiver typi-
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`cally transmits much more data to the controller than
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`other sensors, such as compasses and depth sensors.
`Second, maximum streamer lengths of 10 km are ex-
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`pected to, become commonplace, in contrast to 6 km
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`today. Longer streamers accommodate more sensors on
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`a single twisted pair with the concomitant increase in
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`data traffic. Third, in the continuing quest for greater
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`accuracy, today’s typical spacings of every 300 m for
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`depth sensors and compasses may well be replaced by
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`spacings of 100 m, for a threefold increase in the number
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`of these sensing devices. Fourth, to avoid interference
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`with seismic measurement activity, availability of the
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`communication system for data traffic may be limited to
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`two seconds or less every seismic shot interval, which is
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`typically ten seconds. Thus, in view of the expected
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`expanded use of acoustics, longer streamers, closer sen.
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`sor spacing, and narrower data transmission intervals,
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`prior art two-wire streamer communication systems
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`will be inadequate to handle the increased data traffic.
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`Aside from being unable to handle the increased data
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`traffic, prior art two-wire communication systems do
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`not predict communication failures. Failure to termi-
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`nate the twisted-pair line properly causes standing
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`waves on the line that can null out the signals at sensor
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`positions along the line. Broken or shorted connections
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`are another source of faulty communications. Finally,
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`saltwater leakage causes deterioration of the communi-
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`cation link over time. Prior art communication systems
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`do not recognize deterioration of the communication
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`link until it is all but dead.
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`Therefore, one object of the invention is reliable, high
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`throughput data communication over existing twisted-
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`pair lines with many sensors distributed along marine
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`seismic streamers up to 10 km long. It is a further object
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`of the invention to permit early diagnosis of deteriorat-
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`ing communication so that prompt corrective action
`can be taken.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`For a further understanding of the nature and objects
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`of the present invention, reference should be made to
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`the following detailed description, including the accom-
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`panying drawings, in which:
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`FIG. 1 is side view of a seismic surveying vessel
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`towing a streamer outfitted with sensing and streamer
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`control devices in communication with a controller
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`aboard the vessel in accordance with the invention;
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`FIG. 2 is a partial cutaway schematic view of a sec-
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`tion of a conventional seismic streamer representing
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`two twisted-pair communication lines, one showing
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`parallel-connected coupling coils, in-line devices, and a
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`line termination;
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`FIG. 3A is a family of curves representing the signal
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`strength as a function of frequency for various distances
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`from a 20 dBV (10 V) signal source for a conventional
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`22 AWG twisted-pair communication line with a num-
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`ber of low Q coupling coils distributed therealong;
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`FIG. 3B is a family of curves as in FIG. 3A, but for
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`spectrum in the streamer environment;
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`FIG. 4A is a family of curves representing the group
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`from a signal source, corresponding to the line with low
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`45
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`Ex. PGS 1029
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`3
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`Q coupling coils having the signal strength characteris-
`tic of FIG. 3A;
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`FIG. 4B is a family of curves as in FIG. 4A, but
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`corresponding to the high Q coupling coil having the
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`signal strength characteristic of FIG. 3B;
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`FIG. 5 is a partial schematic block diagram of a pre-
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`the invention;
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`FIG. 6 is a schematic block diagram of another em-
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`nents in more detail than in FIG. 5;
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`FIG. 7 is a schematic diagram of a multiplexer circuit
`used in the invention to monitor electrical transmission
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`and reception parameters;
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`FIG. 8 is a timing diagram depicting an exemplary
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`polling and response sequence according to the inven-
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`tion; and
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`FIG. 9 is a chart showing the preferred channel as-
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`signment of the communication system of the invention.
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`SUMMARY OF THE INVENTION
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`5,200,930
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`DETAILED DESCRIPTION OF THE
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`INVENTION
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`A seismic surveying vessel 20 is depicted in FIG. 1
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`towing a seismic streamer 22 beneath the sea surface 21.
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`Distributed along the length of the streamer 22 are
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`iii-streamer sensors 24A-D, such as compasses and
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`depth sensors, and outboard devices, such as cable-lev-
`eling birds 26A—B and acoustic ranging transceivers
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`28A-B. For brevity, all such devices are hereinafter
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`referred to generally as sensors. The outboard sensors
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`are connected to the streamer 22 by means of collars 27
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`clamped around the streamer. The streamer includes a
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`front-end marker buoy 30 tethered to the streamer 22 by
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`a tether cable 32 and a tail-end buoy 34 tethered to the
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`' end of the streamer 22 by a tether cable 36. The sensors
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`24, 26, and 28 are all in communication with a central
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`controller 38 on board the vessel 20. Hydrophones (not
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`shown) are also distributed along the streamer 22 for
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`detecting seismic energy generated by a seismic source
`(also not shown) and reflected off geologic structures in
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`the earth’s surface. The birds 26A—B, such as the Model
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`5000 manufactured by DigiCOURSE, Inc., the subsid-
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`iary of the assignee of this invention, are used to control
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`the depth of the streamer 22. Outfltted with heading
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`sensors and depth sensors, a bird 26 can also communi-
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`cate heading and depth data to the on-board controller
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`38 for use in predicting the shape of the streamer 22.
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`The acoustic ranging transceivers 28A—B transmit tran-
`sit time information to the controller 38 also for use in
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`estimating the shape of the streamer 22. Of course, a
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`typical deployment would include many more of such
`sensors and more streamers than shown in FIG. 1.
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`Communication between the sensors and the on-
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`board controller is effected over one or more two-wire
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`lines running through the streamer as shown in FIG. 2.
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`The cutaway side view of a portion of a streamer 40
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`reveals, in this example, two twisted-pair hues 42A—B.
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`An outboard bird 44, clamped to the streamer 40 by a
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`collar (not shown), communicates with the on-board
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`controller by means of inductive coupling between an
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`in-streamer primary coil 46A and a secondary coil 48A
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`within the bird 44 or its collar. A capacitor 45A, in
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`series with the primary coil 46A, blocks direct current
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`Sused to power in-streamer sensors. Control signals are
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`received by the bird electronics 50 to control the wings
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`of the bird and, thereby, the depth of the streamer. The
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`bird electronics also measure various operating parame-
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`ters, such as depth, heading, wing angle, temperature,
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`and battery status, and send such data to the controller
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`upon request. In a similar manner, the controller com-
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`municates over the same line 42A with an acoustic
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`ranging transceiver 52 and its internal electronics pack-
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`age 54 by means ofa similar primary coil 463 and ca-
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`pacitor 45B and secondary coil 483. As can be seen,
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`each outboard device is put into communication with
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`the line 42A by means of a corresponding coil 46 con-
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`nected in parallel across the twisted-pair line.
`In-
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`streamer devices, such as a heading sensor 56, are con-
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`nected directly in parallel across the lines of the line
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`42B. To prevent line reflections that can cause nulls in
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`the communication signals, the line 42B is terminated
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`with its characteristic impedance 58. Thus, a twisted-
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`pair line over which cable-position sensors communi-
`cate with the on-board controller contains a number of
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`coupling coils or in-streamer devices all connected in
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`parallel across the line. Each sensor has a unique ad-
`dress or serial number identifier for communication
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`4
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`Ex. PGS 1029
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`40
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`These and other objects are achieved by the present
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`invention, which provides a multi-channel, two-wire
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`communication system for sending commands and data
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`requests to and receiving data rom many positioning
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`sensors and cable-leveling devices distributed along a
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`seismic streamer. The apparatus of the invention in-
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`cludes a central controller comprising an intelligent
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`modem that can scan the many streamer devices for
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`cable-positioning data each seismic shot interval. By
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`referring to an equipment table stored in memory, the
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`intelligent modem polls each device in an efficient and
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`orderly fashion by transmitting message signals over an
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`outbound channel. Responses from the polled devices in
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`the form of message signals are received by the modem
`over one or more inbound channels different from the
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`outbound channel. According to a preferred embodi-
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`ment, a modem has the capability of transmitting out-
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`bound commands or data request messages one of four
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`channels as frequency-modulated signals and of receiv-
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`ing frequency-modulated response messages on one of
`thirteen inbound channels.
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`My measurements and modeling ofthe loss and group4
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`delay characteristics of conventional 22 AWG and4
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`smaller twisted-pair, coupling-coil-laden streamer com-
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`munication hues revealed relatively flat loss and group
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`delay characteristics above conventional coil resonant
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`frequencies of 20 kHz—25 kHz up to about 100 kHz. My
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`further measurements of the power system interference
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`spectrum level revealed signal-tomoise ratios adequate
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`for successful communication at frequencies from about
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`200 kHz up to 100 kHz. The novel streamer communi-
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`cation system of the invention takes advantage of the
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`newly recognized loss and group delay characteristics
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`and the interference spectrum by operating over a mu]-
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`titude of channels between about 20 kHz and 100 kHz,
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`instead of a single channel as is conventionally done,
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`thereby permitting the much higher data throughput
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`rates necessary for advanced cable-positioning solu-
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`tions. The invention additionally provides an operator
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`on board the survey vessel with communication quality
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`and performance measures, such as inbound and out-
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`bound signal strengths and signal-to-noise ratios, ac line
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`impedance, dc line load, false carrier detections, and
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`message errors for detection of streamer problems ear-
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`lier than heretofore possible.
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`55
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`Ex. PGS 1029
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`addressing. An individual 10 km twisted-pair line could
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`include up to 377 parallel sensors. For long streamers
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`having more sensors than a single twisted-pair line can
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`handle, additional lines could be used as exemplified by
`the two twisted-pairs 42A and 428 in FIG. 2.
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`In a conventional twisted-pair communication sys-
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`tem, with a wire size of 22 AWG and having a number
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`of identical low Q coupling coils distributed therealong,
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`the signal strength characteristic is typified by FIG. 3A.
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`For convenience of comparison with the interference
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`spectrum, the signal strength of a signal 10 transmitted
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`at a level of 20 dBV, or 10 V, by a signal source is
`plotted as a function of frequency for various distances
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`from the source, instead of the reciprocal loss character-
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`istic. The signal strength characteristic for a line having
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`a number of high Q coils is shown in FIG. 3B. For
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`example, in FIG. 3A, at 50 kHz with low Q coils, the
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`signal strength (—50 dB) at a distance of 8 km from the
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`signal source is about 19 dB less than the signal strength
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`(—-31 dB) at a distance of 4 km. It is important to notice
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`that, although the signal strength decreases with fre-
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`quency, so does the interference spectrum level 60,
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`which decreases with frequency from 20 kHz to 100
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`kHz, as shown in FIG. 3B. Furthermore, the group
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`delay characteristic between 20 kHz and 100 kHz for
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`both high and low Q coils, shown in FIGS. 4A and 4B,
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`is relatively flat. A non-flat group delay characteristic
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`for which the delays of the upper and lower frequencies
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`of a frequency-modulated signal differ by more than
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`about 0.1 ms at 2400 baud degrades the performance of 30
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`conventional modems. Thus, a significant bandwidth is
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`available for frequency-modulated communication over
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`existing twisted-pair lines above the coil resonant fre-
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`quency of about 20 kHz.
`Instead of limiting communication to the relatively 3S
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`narrow bandwidth of the coil resonance characteristic,
`.
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`the present invention takes advantage of the wide band-
`width available above 20 kHz to communicate effi-
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`ciently with many sensors. The preferred communica-
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`tion scheme is a multi—channel approach, in which 17
`individual narrow-band channels from about 20 kHz to
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`100 kHz are used to permit full-duplex communication
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`with many sensors distributed along a streamer. Chan-
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`nels are limited to below 100 kHz, because, at higher
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`frequencies, the two-wire line behaves more like a dis-
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`tributed parameter transmission line than a lumped pa-
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`rameter circuit. Such a multi-channel approach permits
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`sensors operating on separate channels to communicate
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`simultaneously. Because of the signal strength charac-
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`teristics and interference spectrum shown in FIG. 3,
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`channels are assigned to sensors according their dis-
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`tances from the on.board controller for best signal-to-
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`interference ratios. Lower frequency channels are as-
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`signed to those sensors farthest from the controller. One
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`skilled in the art will recognize that the invention could
`likewise be used with communication lines other than
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`twisted-pair lines, as long as they exhibit similar signal
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`strength and group delay characteristics.
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`A block diagram of the multi-channel communication
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`system of the preferred embodiment of the invention is
`shown in FIG. 5. Communication with streamer sensors
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`is realized by a modem configured around a modem
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`processor 70 having a RAM scratchpad memory 74 and
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`non-volatile memory 72, such as ROM, for program
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`storage. The modem processor sends and receives sen-
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`sor commands and data from the other processing
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`equipment on board the survey vessel over a parallel
`system bus 76, such as a VME bus. Bus control circuits
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`45
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`65
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`5,200,930
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`78 interface the modem processor 70 with a communi-
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`cation processor 80 in communication with other on-
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`board processing equipment. (The central controller 38
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`in FIG. 1 includes the modem processor 70 and the
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`communication processor 80.) Preferably, data are
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`passed between the communication processor 80 and
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`the modem processor 70 through designated memory
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`areas in a dual-ported RAM 82 shared by the two pro-
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`cessors. Interrupts from the communication processor
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`80 signifying the start of a sensor scan cycle are also
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`passed to the modem processor 70 over the system bus
`76.
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`The modem processor 70 communicates with the
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`individual sensors over twisted-pair lines 84A—H each
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`containing a number of parallel-connected coils or in-
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`streamer sensors. For simplicity each twisted-pair line is
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`represented by a single line in FIG. 5. Each line is con-
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`nected to an individual modem physical layer 86A-H.
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`A typical physical layer comprises a transmit path, a
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`receive path, and an isolation transformer 88.
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`The transmit path includes a digital transmitter 90
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`controlled by the modem processor 70. The digital
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`transmitter 90 is programmed or preset to synthesize a
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`frequency-modulated digital signal at its output, the
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`modulation being a function of the data to be transmit-
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`ted. The digital signal is applied to a D/A converter 92
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`to produce an analog frequency-modulated signal,
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`which is filtered by a bandpass filter 94 to remove digi.
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`tal noise and out-of-channel signal, and amplified by a
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`power amplifier 96 before being coupled onto the line
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`84A via the transformer 88 for transmission to the sen-
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`sors. Data transmitted by a sensor are coupled into the
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`physical layer 86A through the transformer 88. A band-
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`pass filter 98 eliminates low-frequency interference,
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`such as seismic interference and power system interfer-
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`ence, and transmitter interference from the receiver.
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`The filtered signal is buffered in a pre—amp 100 before
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`being applied to an A/D converter 102, which converts
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`the analog receive signal into a digital signal to be de-
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`modulated by a digital receiver 104. The demodulated
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`data are then sent to the modem processor 70 from the
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`physical
`layers 86A-H over UART ports 106A-H.
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`Although only two lines 108A-H and 109A—H are
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`shown for each UART port 106A—H, a full RS-232C
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`handshaking link is implemented, as will be described
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`hereinafter. Although the block diagram of FIG. 5
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`shows eight physical layers, it should be recognized that
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`realizations having more physical layers are within the
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`scope of the invention.
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`In a preferred embodiment, the transmitter 90 and the
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`receiver 104 are realized by a multiplicity of digital-sig-
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`nal-processing (DSP) integrated circuits, such as the
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`Model TMS3ZOC40 manufactured by Texas Instru-
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`ments, Inc., Dallas, Tx. Such a device allows great
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`flexibility in selecting carrier frequencies and modula-
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`tion schemes. In this embodiment, however, a minimum
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`frequency-shift-keying modulation (MSK) scheme is
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`used whereby a logic low data bit causes a frequency of
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`1:400 Hz to be transmitted and a logic high data bit
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`causes a frequency of fc+600 Hz to be transmitted
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`where 1'; is the transmit channel center frequency. Data
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`bit rates of 2400 baud make the system compatible with
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`the group delay characteristic of the channel. As a
`transceiver, the DSP integrated circuit is capable of full
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`duplex Operation,
`i.e., simultaneous transmission and
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`reception, as well as simultaneous multi-channel recep-
`tion.
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`-
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`Ex. PGS 1029
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`Ex. PGS 1029
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`7
`In another embodiment, shown in the schematic
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`block diagram of FIG. 6, the transmit and receive func-
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`tions of the physical layer are performed by analog and
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