PCT
`
`WORLD INTELLECTUAL PROPERTY ORGANIZATION
`International Bureau
`
`(51) International Patent Classification 3 :
`
`(11) International Publication Number:
`
`INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`wo 84/03153
`16 August 1984 (16.08.84)
`
`G01V 1/38
`
`A1
`
`(43) International Publicatimi Date:
`
`(21) InternationaJ Application Number:
`
`(22) International Filing Date.:
`
`PCTjN084f00007 Published
`With international search report.
`In English translation (filed in Norwegian).
`
`1 February 1984 (01.02.84)
`
`(31) Priority Application Number:
`
`830358
`
`(32) Priority Date:
`
`2 February 1983 (02.02.83)
`
`(33) Priority Country:
`
`NO
`
`(71) Applicant (for all designated States except US): AfS
`KONGSBERG VAPENFABRIKK [NO/NO]; Kirke(cid:173)
`gaardsveien, N-3600 Kongsberg (NO).
`
`(72) Inventor; and
`(75) Inventor/Applicant (for US only): ULRICHSEN, B0rre,
`B. [NO/NO]; Holmenkollveien 33B, N-Oslo 3 (NO).
`
`(74) Agent: ONSAGERS PATENTKONTOR AS; Camilla
`Colletts vei 4, N-Oslo 2 (NO).
`
`(81) Designated States: AU, BR, DE, DE (European patent),
`FR (European patent), GB, GB (European patent),
`JP, NL, NL (European patent), SU, US.
`
`(54) Title: DEVICE IN A HYDROPHONE CABLE FOR MARINE SEISMIC SURVEYS
`
`(57) Abstract
`
`-·
`
`A device in a hydrophone cable which in connection with seismic surveys is towed through the water behind a ves(cid:173)
`sel, the hydrophone cable comprising means for detecting echo signals which are reflected from the sea bed and various
`layers therebelow. For the purpose of improving the determination of the position of the hydrophone cable which can
`have a length of approx. 3000 meters, a transmission system is suggested, which comprises transmission elements arranged
`outside the hydrophone cable itself, the transmission elements serving to determine the position of the hydrophone cable
`in relation to the elements. In a simple embodiment of the device according to the invention the transmission elements are
`attached to or are constituted by a separate towing line (4) having a relatively small diameter, the towing line being
`equipped with stretching means (5) for achieving a relatively straight run. In an alternative embodiment the transmission
`elements can be implemented as reflectors (9a-9n) for preferably electromagnetic waves, for example in the form of light
`gas-filled balloons which can be attached to the hydrophone cable via thin, light lines, so that the balloons can be towed at
`surface positionor fairly high above the water surface. The transmission elements can also be included in a conventional
`radio and navigation system, possibly together with the system used by the towing vessel for its positioning, in addition to
`determination of distance and bearing by means of the radar system of the vessel. The transmission elements can also be
`included as elements in an adaptive control system for combined control of vessel and hydrophone cable.
`
`1
`
`ION 1007
`
`

`

`FOR THE PURPOSES OF INFORMATION ONLY
`
`Codes used to identify States party to the PCT on the front pages of pamphlets publishing international ap(cid:173)
`plications under the PCT.
`
`AT
`AU
`BE
`BG
`BR
`CF
`CG
`CH
`CM
`DE
`DK
`FI
`FR
`GA
`GB
`HU
`JP
`KP
`
`Austria
`Australia
`Belgium
`Bulgaria
`Brazil
`Central African Republic
`Congo
`Switzerland
`Cameroon
`Germany, Federal Republic of
`Denmark
`Finland
`France
`Gabon
`United Kingdom
`Hungary
`Japan
`Democratic People's Republic of Korea
`
`KR
`LI
`LK
`LU
`Me
`MG
`MR
`MW
`NL
`NO
`RO
`SD
`SE
`SN
`su
`TD
`TG
`us
`
`Republic of Corea
`Liechtenstein
`Sri Lanka
`Luxembourg
`Monaco
`Madagascar
`Mauritania
`Malawi
`Netherlands
`Norway
`Romania
`Sudan
`Sweden
`Senegal
`Soviet Union
`Chad
`Togo
`United States of America
`
`~
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`-;
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`Device in a hydrophone cable for marine seismic surveys
`
`The present invention relates to a device in a hydrophone
`cable which is adapted for marine seismic surveys and is
`towed through the water behind a vessel, the cable comprising
`means for detecting echo signals frqm the sea bed and various
`layers therebelow.
`Such hydrophone cables which are used in connection
`with seismic methods for mapping possible hydrocarbon sources
`below the sea bed, can be approx. 3000 meters long and be
`towed at a depth of approx. 10 meters.· At a distance of approx.
`100 meters behind the vessel there are also towed so-called
`air guns, the air guns firing shots according to an appropriate
`programme. The sound waves which are transmitted from the
`air guns, are reflected from the obstacles against which
`they may impinge below the water surface, as well as from
`the sea bed and various layers therebelow. The echo signals
`which return to the hydrophone cable, are detected by a series
`of hydrophones which are arranged along the cable, and which
`after a suitable conversion transfer the echo signals via
`the cable to an appropriate ~recessing device on the towing
`vessel.
`The seismic methods can be carried out by firing lines
`having a mutual distance of approx. 50 meters, and the inter(cid:173)
`vals between the shots from the air guns correspond to a
`distance of approx. 25 meters, for thereby achieving a very
`fine net of squares.
`Aside from comprising means for detecting echo signals
`from the water bottom and various layers therebelow, the
`hydrophone cable also comprises a plurality of compasses
`which indicate the form of the cable during the towing
`operation, and which thus constitute reference points for
`the line to which a sweep operation is to be referred. -However,
`such compasses suffer from some disadvantages, the direction
`indication of the compasses being apt to give significant
`errors, since the hydrophone cable has a substantial extension .
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`Besides, the compass section of the cable is often signi(cid:173)
`ficantly larger in diameter than the cable itself, and will
`therefore in itself be prone to generate noise. Further,
`the compasses necessitate a substantial number of surplus
`connections in the cable, which in itself is unfavourable.
`Further, the calibrating routine for the compasses is very
`sophisticated, and it is not unusual that several days are
`used for effecting the calibration and making all of the
`compass sections operable. In case magnet compasses are used,
`these may easily be disturbed by the magnetic fields occurring
`during the measuring work itself.
`In connection with such known magnet compass hydrophone
`cables no direct visual indication of the position of the
`cable exists other than an end buoy which is towed freely
`at an arbitrary position approx. 200-300 meters behind the
`terminal of the hydrophone cable.
`Other and more reliable and stabl~ direction references
`than magnet compasses have been evaluated, for example gyro
`compasses, but these have not been in commercial use, since
`it is expected that they will constitute a means which makes
`the hydrophone cable more expensive.
`The object of the present invention is to arrive at
`a device in a hydrophone cable which with simpler and less
`expensive means can determine the position of the hydrophone
`cable, the detection of the hydrophone cable's position being
`of importance not only during the sailing of the measuring
`lines itself, but also during the turning programmes after
`a terminated line, in connection with which significant extra
`distances have to be sailed before a new line is entered
`for thereby ensuring that the cable has a shape as straight
`as possible.
`The object is achieved according to the invention in
`a device which is characterized in that it comprises a trans(cid:173)
`mission system which is adapted to determine the position
`of the hydrophone cable, and which comprises transmission
`elements provided outside the hydrophone cable itself.
`
`~
`OMPI
`./1\1 WIPO
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`l'eRNATlO't'\~
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`By using such a transmission system it is possible to
`achieve a less expensive and more direct measuring method,
`a fact which includes significantly reduced equipment expenses,
`especially compared to the type of compass sections used
`today. In a transmission system in which the transmission
`elements are provided outside the hydrophone cable itself,
`it is also possible to achieve a substantially greater
`operational safety. By means of the transmission system it
`is, aside from achieving better informa~ion about the position
`and shape of the hydrophone cable at any time, also possible
`to include the position signals in the manoeuvring operations
`of the vessel during the turning operation. Finally, the
`suggested transmission system can be made to co-operate with
`means serving to aligne the shape of the cable both during
`the line sweeping and the turning operations.
`The transmission elements which are included in the
`proposed transmission system can either be stationarily
`anchored, or they may be provided on bodies floating more
`or less freely in the water. In the latter case the bodies
`carrying the transmission elements can then be connected
`to a continuous connection means facilitating the collection
`of the bodies after a measuring period.
`Possibly, the transmission elements serving to transfer
`the position signals to or from the hydrophone cable can
`be provided on one or more bodies which are towed behind
`the vessel, the bodies being towed separately or in groups,
`and the bodies substantially being arranged along a straight
`line.
`In an alternative embodiment the bodies carrying the
`transmission elements can be affixed to or be constituted
`by a separate towing line having a relatively small diameter,
`the towing line being provided with stretching means for
`achieving an approximately straight run.
`The towing line with the transmission element carrying
`bodies can then extend at least along the overall length
`of the hydrophone cable.
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`It is to be understood that the towing line itself can
`be adapted for transmission of substantially longitudinal
`acoustic waves which are received by the hydrophone cable.
`It is also to be understood that the transmission elements
`can comprise means for receiving position signals which are
`transmitted from signal elements in the hydrophone cableo
`It is further to be understood that the transmission
`of position pulses can take place to or from the vessel,
`for example via radio or radar, and it is thereby achieved
`an electromagnetic positioning system which works independently
`of the seismic hydrophone system. The transmission elements
`indicating the position of the cable can for example be con(cid:173)
`stituted by for example reflectors which are attached to
`the towing line, or with a suitable spacing are attached
`to the hydrophone cable and glide thereabove in or close
`to the surface by means of appropriate buoyancy means.
`The position of the reflectors can .then be determined
`by means of antennas mounted for example on the vessel itself
`or on paravans towed at a distance from the vessel.
`Possibly the position_of the reflectors can be determined
`by a system griding above the hydrophone cable, for example
`wire controlled from the vessel.
`The above described embodiments for determining the
`position of the hydrophone cable can in a simple manner be
`adapted to an adaptive regulating system for manoeuvring
`both the vessel and the hydrophone cable for thereby achieving
`a most favourable overall position at any time for covering
`the measuring area and complete measuring accuracy.
`The signals from the transmission system can suitably
`be used for influencing the manoeuvring of the vessel and/or
`influencing a means on the vessel which can move relative
`thereto, or influencing means which are provided along the
`hydrophone cable and in this way aligne the position of the
`cable relative to the vessel.
`If the means influencing the hydrophone cable are to
`constitute an as little a noise source as possible for the
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`hydrophone system, these means may appropriately be influenced
`during time intervals in which the hydrophone cable is close
`to inactive as regards the detection of echo signals.
`The invention will in the following be further described,
`reference being had to the drawing, which in diagrammatical
`form illustrates various embodiments of the present invention.
`Figure 1 illustrates diagrammatically a plurality of
`embodiments of the device according to the present invention.
`Figure 2 is similarly a sketch illustrating further
`embodiments of the device according to the present invention.
`Figure 3 illustrates diagrammatically further embodiments
`of the device according to the present invention.
`Figure 4 is a sketch illustrating further embodiments
`of the present invention.
`Figure 5 is a sketch illustrating how the hydrophone
`cable can be influenced in co-operation with the present
`device.
`Referring to Figure 1, a vessel which is designated
`by 1, moves along the surface of a larger body of water for
`surveying the bottom of the body of water and areas there(cid:173)
`below, the vessel 1 towing a hydrophone cable 2 which can
`have an overall extension of for example 3000 meters. The
`towing of the cable 2 takes place preferably at a depth of
`10 meters, and an even depth is sought maintained by means
`of for example active fins controlling the height direction
`of the cable, the specific weight of the cable being adjustable
`on the one hand by means of the paraffin type which is used
`for filling the cable, and on the other hand by ballast,
`for example in the form of lead plates arranged therearound.
`After the vessel there are also towed a couple of air
`guns 3a, 3b, said guns being adapted for firing in accordance
`with a predetermined programme for the transmission of sound
`waves, which are scattered towards the sea bed and are re(cid:173)
`flected therefrom and from various geological layers there(cid:173)
`below. The reflected sound waves or echo signals are received
`by the hydrophones which are mounted in the hydrophone cable
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`2, and the signals from the hydrophones are passed through
`the hydrophone cable to a combined storage and·computing
`machine on the vessel for further processing to appropriate
`values giving a picture of the sea bed and the formations
`thereof.
`In order to achieve an as accurate result as possible
`it is of greates importance to know where the various hydro(cid:173)
`phone positions of the hydrophone cables 2 are relative to
`the vessel and the air gun groups 3a, 3b which are found
`for example approx. 100 meters after the vessel 1. This
`accuracy is especially of great importance in the cases where(cid:173)
`in the lines over which the hydrophone cable 2 is to be passed,
`are arranged as close as 50 meters, and wherein the air guns
`are fired at 25 meters intervals while undertaking a so-called
`three-dimensional seismic survey.
`In Figure 1 there is as a first embodiment of a trans(cid:173)
`mission system for supervising the position of the hydrophone
`cable 2 depicted a relatively thin steel wire 4 which pre(cid:173)
`ferably has a somewhat longer extension than the hydrophone
`cable 2 itself. Appropriately, the steel wire 4 can be equipped
`-
`with a braking plate 5 or a suitable form of a controlled
`braking device serving to keep the wire in an as straight
`as possible shape during the towing operation._
`In a first utilization of the thin steel wire 4 this
`may constitute a carrier means for substantially longitudinal
`mechanical sound waves which are generated at the attachment
`points at the vessel 1, the acoustic waves or pulses which
`follow the wire 4 being registered by the hydrophones in
`the hydrophone cable 2, since the distance between the wire
`and the hydrophone cable in the utmost case usually runs
`to approx. 100 meters.
`Possibly the braking device or the plate 5 at the free
`end of the wire 4 can be controlled in such a way that it
`not necessarily finds itself in an extension of the centre
`line of the ship, but can be swung out in the proximity of
`the microphone cable, so that the signal communication between
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`the wire 4 and the hydrophone cable 2 is amplified. Since
`the wire 4 is relatively thin and is kept under tension,
`the drift of the wire can be made very small, but it should
`in connection with the use of acoustic transmission signals
`be towed in a position below the water surface, so that signal
`communication with the microphone cable is made as favourable
`as possible, while at the same time reducing wave noise.
`The distance between the wire 4 and the cable 2 should be
`adjusted so that no interference occurs between the means
`being included in the present transmission system and the
`hydrophone cable.
`As an alternative to the transmission of mechanical
`pulses along the wire 4 there might thereon be mounted small
`signal generators 6 which preferably can be initiated from
`the vessel, for example in those periods wherein the echo
`sound waves from the bottom are on a relatively inactive
`level, so that the distance between the signal generators
`6 on the wire 4 and the corresponding hydrophones on the
`hydrophone cable 2 can be detected.
`It is to be understood that the acoustic signal generators
`can also be arranged in the hydrophone cable at the same
`time as signals therefrom are registered in suitable receivers
`in elements provided on the wire 4. However, it might be
`appropriate to utilize existing hydrophone groups in the
`hydrophone cable 2, a fact which includes an advantage in
`connection with signal sources in systems outside the hydro(cid:173)
`phone cable 2.
`Still another embodiment of the device comprising a
`transmission system which is adapted for determining the
`position of the hydrophone cable and which comprises trans(cid:173)
`mission elements provided outside the hydrophone cable 2
`itself, is illustrated in Figure 1 and takes the form of
`transponders 7a, 7b, which are towed on paravans located
`approx. 200-300 meters behind the ship and defining an angle
`of approx. 45°. The signals from the transponders 7a, 7b
`will at suitable intervals be picked up by the hydrophones
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`in the hydrophone cable 2 and the relative strength and the
`shape of the signals received by the hydrophone cable 2,
`will give a picture of the shape and the position of the
`cable relative to the towing vessel 1. The length of the
`paravan lines must here be adjusted so as to achieve a best
`possible signal/noise ratio, since longer paravan lines
`can give a shorter signal path to the hydrophones in the
`hydrophone cable, but bring the sources of noise closer
`thereto.
`It is to be understood that the above discussed embodi(cid:173)
`ments of a transmission system in which the transmission
`elements are provided outside the hydrophone cable itself,
`is to be operated with signal frequencies and types thereof
`which make them easily recognizable in the registration
`pictures from the hydrophones.
`In Figure 2 there is illustrated an alternative embodiment
`of a device according to the present invention. As previously,
`1 designates a vessel which behind itself tows a hydrophone
`cable 2. The signal communication to the hydrophone cable
`2 is here suggested implemented by means of freely floating
`buoys 8a-8n, which aside from being equipped with hydro(cid:173)
`acoustic transponders, are also equipped with radar reflectors.
`The buoys are dropped from the vessel when this passes the
`area to be investigated, and the buoys will of course drift
`off by stream, wind and waves, but they will not give rise
`to noise. The transponders in the floating buoys can be adapted
`for transmission of hydroacoustic signals during given periods
`of t-imes, preferably during period of times in which the
`registration of the echo signals is not critical.
`If buoys are dropped at a distance of approx. 500 meters,
`a number of twenty buoys could cover a sailing line of approx.
`10 krn in a seismic surveying net.
`The buoys can preferably be connected by means of a
`rope 8' which appropriately can slide through an eye in the
`stern of the bouy until the hydrophone cable has passed by.
`Thereafter the buoys can be collected in a group and be hauled
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`in during the turning operation for another seismic line.
`In this period of turning the signal means in the buoys can
`possibly be reenergized if required.
`It is to be understood that a corresponding system can
`,.
`comprise transponders which instead of floating on the surface
`of the water are dropped to the sea bed to known bottom
`positions for therefrom transmitting signals to the hydrophone
`cable. After use the transponders can be collected by means
`of a line and be brought to the towing vessel.
`Still another variant of the transmission system according
`to the present invention is illustrated in Figure 3, in
`which reflectors for example in the form of light gass-filled
`balloons 9a-9n are attached to the hydrophone cable 2 which
`also in this case is towed behind a vessel 1. The balloons
`9a-9n are attached to the hydrophone cable 2 by means of
`thin, light lines, so that the balloons can be towed at surface
`posit~ons or at a fair height above the water surface, if
`required.
`Such a system including floating or gliding balloons
`can be made very economically and can be contemplated used
`as a supplemental system to another transmission and measuring
`system. By means of the radar equipment on the vessel the
`position of the various balloon reflectors can be detected,
`and the detected echo signals from the reflectors will form
`a picture of the shape and position of the cable behind the
`towing vessel 1.
`For further improving the detection of the floating
`reflectors radar antennas 10a, 10b can be arranged on towed
`paravans, as this appears from Figure 4. As previously, 9a-
`9n designate the floating reflecting elements which glide
`above the hydrophone cable 2, which on the other hand is
`towed by the vessel 1.
`Possibly, the radar antenna can be arranged gliding
`in the air above the .vessel 1, as this appears from Figure
`3, the radar antenna here being attached to a gliding drone
`11 which is located at a sui table distance and height. behind
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`the vessel 1 towing the hydrophone cable 2. The drone 11
`can suitably be controlled from the vessel 1. Further, it
`is to be understood that the reflector elements can co-operate
`with means forming a basis line established outside the vessel
`for thereby avoiding the uncertainty in the angular determi(cid:173)
`nation from a vessel in moving sea.
`The determination of the position of the buoys or reflec(cid:173)
`tors can also be carried out by means of for example con(cid:173)
`ventional radio and navigation systems, possibly by the system
`used by the ship itself for the positioning thereof. These
`systems can be used in addition to the distance and angular
`determination of the buoys by means of the radar system of
`the ship.
`On the basis of the information obtained by the above
`discussed embodiments as regards the position and shape of
`the hydrophone cable it is possible by suitable means to
`let the hydrophone cable be included in an adaptive control
`system which manoeuvres the cable in such a way that this
`will be positioned as favourably as possible in relation
`to the reference line from which data is wanted in the sailing
`programme.
`Via mathematical modelling of the hydrophone cable the
`vessel can be steered automatically in relation thereto,
`since this steering is also based on an adaptive control
`system. Such systems render dynamic compensation for wind,
`current and sea, as well as for the influences to which the
`vessel and the hydrophone cable ohterwise are subjected.
`Contrary to steering the vessel substantially along straight
`heading lines it is possible by co-operating the shape of
`the hydrophone cable and the heading of the vessel to obtain
`a most favourable shape and position of the hydrophone cable
`relative to the desired surveying line in the sailing pro(cid:173)
`gramme.
`When a survey line has been shot the vessel must be
`turned so that the hydrophone cable can enter another surveying
`line. This turning process is very time consuming, since
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`the turning must be carried out in such a way that the cable
`mu~t be sufficiently straight before starting a new line.
`By means of mathematical modelling and adaptive regulation
`technique based on the signals provided by means of the above
`discussed transmission system, such a turning programme can
`be put in as a completely controlled programme. In other
`words, the vessel can then be steered along a track which
`is as short as possible and renders an optimum shape of the
`cable prior to the commencement of another line. It is to
`be understood that the changing from one line to another
`not necessarily relates to two adjacent lines but lines which
`are located in various parts of the area in which the seismic
`surveys are to be carried out.
`In Figure 5 there are diagrammatically illustrated embodi(cid:173)
`ments wherein the vessel 1 towing the hydrophone cable 2
`therebeind, is equipped with a suspension 12 which is adapted
`to influence the cable 2 for thereby either cancelling or
`resisting the deflections which at any time can occur ·during
`the towing operation. Possibly, the hydrophon~ cable 2 can
`be equipped with actuators, for example in the form of a
`steerable end rudder 13 or steerable fins provided in the
`longitudinal direction of the cable.
`However, the use of steerable fins can pave the way
`for undesired acoustic noise, since in connection with seismic
`reflections ·one operates with signal levels in the magnitude
`range of + 5 microbar.
`However, the steering fins or the actuators can be in(cid:173)
`serted in such a way that they are active during given time
`intervals between the shots from the air guns, in which the
`accuracy, as regards the measuring technique, is of less
`importance. In other words, the control of the fins or the
`actuators will be excluded in the periods in which the feeble
`reflexes from the deep formations below the seabed are re(cid:173)
`ceived, since during these periods of time a strongest possible
`reduction of all possible sources of noise is desired for
`the achievement of a most favourable signal/noise ratio.
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`If the cable prior to the commencement of a l:Lne sailing
`is sufficiently aligned, short intervals of influence of
`some seconds' duration can be sufficient for the cable to
`maintain an approximately straight shape. The fins or actu(cid:173)
`ators should then be kept in a neutral noise-reduced position
`during the periods in which the seismic signals from the
`air guns are received.
`As regards the rudder device 13 illustrated in Figure
`5, such a towed steering device which either is located at
`or below the surface of the water, can be at a substantial
`distance from the hydrophone cable 2 for thereby constituting
`a noise source with minimum influence. The controlled rudder
`device 13 can suitably be equipped with a transmission element
`for determining the free end point of the cable in relation
`to a reference point on the vessel. Possibly, the controlled
`rudder device can comprise or constitute stretching means
`for the towing line.
`It is to be understood that the above discussed trans(cid:173)
`miss ion elements can be adapted for side detectio-n, i.e.
`detection of the side at which the elements are, in relation
`to the hydrophone cable. Further, the system can be adapted
`so as to detect whether certain reference elements are
`approaching or moving towards and apart from each other,
`respectively.
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`P a t e n t
`
`C 1 a
`
`i m s
`
`1. Device in a hydrophone cable (2) which is adapted for
`marine seismic surveys and is towed through the water behind
`a vessel (1) and comprises means for detecting echo signals
`which are reflected from the sea bottom and various layers
`therebelow,
`c h a r a c t e r
`i z e d
`i n
`that the
`device comprises one or more supporting bodies which are
`provided outside or at a distinct distance from the hydrophone
`cable itself and from the sea bed, and that the supporting
`bodies comprise transmission elements which are included
`in a reference system for determining the position of the
`hydrophone cable.
`
`2. Device as claimed in claim 1, ch a r a c t e r i z ed
`i n
`that the transmission elements which are included in
`the reference system and which serve to transfer positioning
`signals to or from the hydrophone cable (2), are arranged
`stationary ashore.
`
`Device as claimed in claim 1, ch a r a c t e r i z ed
`3.
`transmission elements arranged on supporting bodies
`b y
`(8a-8n) floating approximately freely in the water.
`
`4. Device as claimed in claim 1 or 3,
`t e -
`c h a r a c
`r
`i z e d
`i n
`that the bodies (8a-8n) supporting the
`transmission elements are connected to a continuous connection
`means (8') which facilitates the gathering of the bodies
`(8a-8n) after a measuring period, and which aids in aligning
`the transmission elements towards a straight line.
`
`5. Device as claimed in claim 1,
`i z e d
`t e r
`c h a r a c
`i n
`that the transmission elements are arranged on one
`or more bodies (6) which are towed behind the vessel (1),
`the bodies (6) being towed separately or in groups arranged
`substantially on a straight line.
`
`15
`
`

`

`wo 84/03153
`
`PCI' /N084/00007
`
`14
`
`c h a r a c t e -
`6. Device as claimed in claim 1 or 5,
`r
`i z e d
`i n
`that the bodies supporting the transmission
`elements are attached to or are constituted by·a separate
`towing line {4) having a relatively small diameter, the towing
`line {4) being equipped with possibly controllable stretching
`~·
`means (5) for achieving an approximately straight_run.
`
`7. Device as claimed in claim 5 or 6, char a c t e -
`that the towing line {4) with the trans(cid:173)
`r
`i z e d
`i n
`mission element supporting bodies (6) extend along at least
`the overall length of the hydrophone cable (2).
`
`.Device as claimed in claim 7, ch a r a c t e r i zed
`8.
`that the towing line (4) itself is adapted for trans(cid:173)
`i n
`mitting longitudinal acoustic waves which are picked up by
`the hydrophone cable (2).
`
`9. Device as claimed in any of the claims S-7,
`that the transmission
`c h a r a c
`t e r
`i z e d
`i n
`elements comprise means for receiving positioning signals
`transmitted from signalling elements in the hydrophone cable.
`
`10. Device as claimed in any of the claims 1-7 or 9,
`c h a r a c t e r
`i z e d
`i n
`that the transmission
`elements are adapted for co-operating with radio and navigation
`systems or the system which the ship itself utilizes for
`the positioning thereof, possibly in addition to distance
`and direction determination by means of the radar of the
`ship.
`
`.
`11. Device as claimed in any of the claims 5-7 or 10,
`c h a r a c t e r
`i z e d
`i n
`that the line formation
`of the transmission elements is determined by position de(cid:173)
`tection of a point, for example an end point, in relation
`to the position of the vessel or with reference to a navigation
`
`16
`
`

`

`wo 84/03153
`
`Per /N084/oooo7
`
`15
`
`system, the line (4) which connects the transmission elements,
`possibly comprising controlled manoeuvring means (5) serving
`to bring the line (4) in a favourable position relative to
`the hydrophone cable (2).
`
`c h a r a c t e -
`12. Device as claimed in claim 1 or 5,
`r
`i z e d
`i n
`that the transmission elements are supported
`by or are constituted by floating or gliding supporting bodies
`or reflectors, for example gass-filled reflector elements
`(9a-9n) which are attached to the hydrophone cable (2) and
`are towed thereby at or above the water surface.
`
`t e -
`c h a r a c
`13. Device as claimed in claim 10 or 12,
`that the reflector elements (9a-9n) co(cid:173)
`r
`i z e d
`i n
`operate with antennas (10a-10b) which are located in the
`proximity of the vessel (1), for example on paravans which
`are towed at a distance from the vessel or mounted station(cid:173)
`arily relative to the vessel.
`
`c h a r a c t e -
`14. Device as claimed in claim 12 or 13,
`that the reflector elements (9a-9n) are
`r
`i z e d
`i n
`adapted to co-operate with a detection system