`
`
`Ex. PGS 1055
`
`
`
`EX. PGS 1055
`
`
`
`
`
`

`

`United States Patent [191
`Langeland et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`‘4,992,990
`Feb. 12, 1991
`
`[54] METHOD FOR DETERMINING THE
`POSITION OF SEISMIC STREAMERS IN A
`REFLECTION SEISMIC MEASURING
`SYSTEM
`[75] Inventors: {an-Age Langeland, Games; Stein
`ASheim; Bjorn Nordmoen, both of
`Oslo; Erik Vigen, Drammen, all of
`Norway
`[73] Assignee: Geco A.S., Sandvika, Norway
`[21] Appl. No.:
`460,146
`[22] PCT Filed:
`Jun. 6, 1989
`[86] PCT No.:
`PCI/NO89/00055
`§ 371 Date:
`Jan° 30, 1990
`§ 102(e) Date:
`Jan. 30, 1990
`[87] PCT Pub. No.: W089/12236
`PCT Pub. Date: Dec, 14, 1989
`Foreign Application Priority Data
`[30]
`Jun. 6, 1988 [NO] Norway .................... .. 882495
`
`......................... .. GOIV 1/38
`[51] Int. Cl.5
`[52] US. Cl. ....................................... .. 367/19
`[58] Field of Search .................... .. 367/6, 19, 129, 130
`[56]
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,532,637 7/1985 Baeckel et a1. ..................... .. 367/19
`
`. . . . .. 367/19
`4,660,185 4/1987 French . . . . .
`.... .. 367/19
`4,669,067 5/1987 Roberts
`4,845,686 7/1989 Brac .................................... .. 367/19
`
`FOREIGN PATENT DOCUMENTS
`
`2393320 12/1978 France .
`2620536 3/1989 France .
`0831513 5/1984 Norway .
`
`WOO7732 12/1987 World Int. Prop. 0. _
`
`OTHER PUBLICATIONS
`Sonerdyne Limited Publication, “A Hydro-Acoustic
`System for Precision Tracking of Twin Seismic Hydro
`phone Streamers,” 7/87.
`“Improving the Accuracy of Merile 3D Seismic Sur
`veys," Ocean Industry, Jan. 1987.
`Primary Examiner-Jan J. Lobo
`Attorney, Agent, or Firm-Fleit, Jacobson, Cohn, Price,
`Holman & Stern
`ABSTRACT
`[57]
`A method for determining the position of at least two
`seismic streamers (S1, S2) in a re?ection seismic measur
`ing system, wherein hydroacoustic distance measure
`ments are used which are taken by means of acoustic
`transceivers provided in vessels (1), buoys (8), ?oats (5),
`seismic sources (2) and in the seismic streamers (S1, 82).
`Absolute reference positions are determined by position
`determining equipment provided in at least two loca~=
`tions, for instance, on a vessel (1) or a ?oat (5). The
`acoustic transceivers and the position determining
`equipment form a three-dimensional structure. The
`position determination takes place by trilateration be
`tween the acoustic transceivers and the determination
`of at least two reference positions so that there is no
`dependency on compass bearings or optical visibility,
`and high redundancy is obtained. The method is partic
`ularly suited for application in connection with three-di
`mensional marine seismic surveys. The method may be
`integrated with suitable surface navigation systems in
`order to ?nd the reference positions and provide abso
`lute positions at any point within a marginal error of 5
`to 10 m.
`
`18 Claims, 5 Drawing Sheets
`
`Ex. PGS 1055
`
`

`

`US. Patent
`US. Patent
`
`Feb. 12, 1991
`Feb. 12, 1991
`
`Sheet 1 of 5
`Sheet 1 of 5
`
`4,992,990
`4,992,990
`
`is
`
`.
`N
`.93
`LL
`
`Fig.1.
`
`“3‘
`J?
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`c;
`
`a,
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`
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`
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`“m
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`
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`
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`
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`
`<-
`
`Ex. PGS 1055
`
`Ex. PGS 1055
`
`

`

`US. Patent
`US. Patent
`
`Feb. 12, 1991
`Feb. 12, 1991
`
`Sheet 2 of 5
`Sheet 2 of 5
`
`4,992,990
`4,992,990
`
`
`
`Ex. PGS 1055
`
`Ex. PGS 1055
`
`

`

`US. Patent
`US. Patent
`
`Feb. 12, 1991
`Feb. 12, 1991
`
`Sheet 3 of 5
`Sheet 3 of 5
`
`4,992,990
`4,992,990
`
`
`
`Ex. PGS 1055
`
`Ex. PGS 1055
`
`

`

`US. Patent
`
`Feb. 12, 1991
`
`Sheet 4 of 5
`
`4,992,990
`
`
`
`Ex. PGS 1055
`
`

`

`
`
`Ex. PGS 1055
`
`

`

`1
`
`METHOD FOR DETERMINING THE POSITION
`OF SEISMIC STREAMERS IN A REFLECTION
`SEISMIC MEASURING SYSTEM
`
`4,992,990
`2
`cal estimation in connection with compass bearings,
`known distances and section lengths. As a rule, how
`ever, the error in the actual distance to a streamer sec
`tion will lie within the errors of the position determin
`ing system, referred to the navigation accuracy.
`At present a number of methods are employed or
`proposed to be employed, for determining the stream
`er’s position, based on distance measurements in con
`nection with compass bearings. One method is to deter
`mine supposed distances to the single streamer sections
`based on how much towing cable is being handed out
`from the towing vessel, at the same time as bearings are
`taken with the magnetic compasses in the streamer. The
`method, however, is encumbered with substantial er
`ror_s, since the stretch sections of the streamermay be
`stretched such that the distances may vary with about
`10 to 15 m, and there are small possibilities of determin—
`ing this discrepancy accurately. Changes in the course
`of the vessel will result in a poorer determination of
`bearing when the streamer with compasses starts to
`stretch as a result of the movement of the vessel.
`Norwegian patent application 83 1513 discloses a
`method for determining the position of a seismic
`streamer which is towed through the sea by a vessel.
`Herein azimuth and distance from the vessel to points
`on the seismic streamer are measured, and the coordi
`nates of the points are calculated by means of these
`values. Furthermore, a hydroacoustic measurement
`method based on a ultra-short base line system is used,
`which is integrated with the gyro-compass of the vessel
`for azimuth and distance measurements against tran
`sponders, responders or similar devices provided on or
`in the seismic streamer. This method gives no measure
`ment of redundancy and there are hence limited possi
`bilities of discovering errors in the system. Further, one
`is dependent on determining a reference bearing and
`this bearing must be taken with a gyro-compass having
`a limited accuracy.
`Another method which uses acoustic distance mea
`surement techniques for determining the streamer posi
`tion is described in the paper “Improving the accuracy
`of marine 3-D seismic surveys”, Ocean Industry, Jan.
`1987. Here acoustic transceivers provided on the fore
`end of the seismic streamer and at the seismic sources
`are used, while acoustic receivers are provided aboard
`the vessel. The determination of directions are made by
`compass bearings, but relatively acute intersecting an
`gles give fairly large errors in the angle readings and in
`addition too few measurements for achieving suf?cient
`redundancy and determination of measurement errors.
`A further hydroacoustic positioning system for two
`seismic streamers is disclosed by a report from Sonar
`dyne Ltd. with the title “A hydroacoustic system for
`precision tracking of twin hydrophonic streamers” (ref:
`C/87/363). This system provides more measurements
`so as to achieve a somewhat better redundancy than the
`aforementioned system. The determination of the direc
`tion takes place by compass bearings, but the bearings of
`the seismic streamer fore ends result in relatively acute
`angles having fairly large measurement errors. Addi
`tionally, the position of the seismic source is not deter
`mined.
`An acoustic distance measurement technique has also
`been applied to the problem of determining the horizon
`tal pro?le of a towed seismic streamer. To that end
`US-PS No. 4 532 617 (Baecker and Bijou) discloses a
`system and a method based on using a slave vessel in
`
`40
`
`45
`
`10
`
`35
`
`BACKGROUND OF THE INVENTION
`The invention relates to a method for determining the
`position of at least two seismic streamers in a reflection
`seismic measuring system, in connection with marine
`seismic surveys.
`Lately there have come into use three-dimensional
`exploration methods for marine seismic surveys. Such
`exploration methods place heavy demands on naviga
`tion and positioning and one is dependent on knowing
`the relative positions of seismic sources and the hydro
`phones of the seismic streamer with great precision. In
`modern seismic exploration methods several sources
`and several seismic streamers are usually employed, and
`the mutual distances between these must also be deter
`mined with relatively high precision. The seismic
`sources and the seismic streamers are usually towed by
`one or more exploration vessels, and the exploration
`vessels’ absolute position at any time is determined by
`means of surface navigation systems aboard the vessel
`or the vessels, these navigation systems preferably being
`land-based or satellite-based radio navigational systems
`which give a resolution below 0.5 m and a repeatable
`positioning accuracy of a few meters. It is then neces
`sary to determine the position of the seismic streamers
`or in reality the position of the hydrophones of the
`seismic streamers with as high accuracy as possible, the
`basis for this position determination being the absolute
`position found by the navigational system of the vessel.
`A seismic streamer comprises a plurality of active
`streamer sections with known length and equipped with
`hydrophones or hydrophone groups with known loca
`tion in the active sections. The seismic streamer is ar
`ranged between stretch sections, a fore stretch section
`being connected with a towing means on the towing
`vessel, whereas a tailbuoy with means for determining
`the position is provided at the end of the aft strech
`section. The means may for instance be an active navi
`gation system of the same type that is employed aboard
`the towing vessel or a microwave system, possibly com
`bined with a goniometer. The position of the tailbuoy
`may also be determined by passive distance measure
`ments between the towing vessel and the buoy, for
`instance by means of radar or laser re?ectors. Now
`knowing the positions of the vessel and the tailbuoy, the
`position of the hydrophones in the seismic streamer is
`determined on the basis of the known length of the
`streamer, the known location of the hydrophones in the
`streamer and the orientation of the separate sections of
`the vessel and the tailbuoy. This orientation may in
`principle be provided by taking a bearing between the
`vessel and the tailbuoy using compass devices aboard
`the vessel. However, due to swing of the seismic
`streamer caused by sea currents, an angular deviation
`between the streamer and the ship's bearing is gener
`ated. In practice the seismic streamer will hence have
`the shape of a plane or a spatial curve, but the said
`angular deviation may be determined by providing a
`plurality of magnetic compasses in the cable, typically
`for instance twelve compasses in a streamer of three
`kilometers length, and normally with a compass close to
`each end of the streamer. Compasses are also provided
`in the stretch section. The curve of the streamer may
`then be determined for instance by means of mathemati
`
`Ex. PGS 1055
`
`

`

`3
`addition to the towing vessel, acoustic transponders
`being provided in the vessels and along the seismic
`streamer. The'positions of _the vessels are determined
`and the distances between the vertices of a triangle
`formed by the vessels and a respective transponder on
`the streamers are found, allowing a representation of
`the pro?le of the streamer to be obtained. The method
`as taught by the publication thus employs per se known
`techniques, but the proposed system is not well suited
`for determining the position of several towed objects
`and would be hightly impractical to adopt for position
`ing tasks in a 3-D marine seismic surveying system em
`ploying more than one streamer.
`Systems wherein distance measurements are made by
`means of optical methods and microwave methods are
`also encumbered with a number of de?ciencies. Using a
`laser one is dependent on optical visibility and both
`laser measurements and radiogoniometry must be used
`in connection with compass bearings, which in view of
`20
`the measuring errors of the gyrocompass leads to inac
`curate direction determinations. Furthermore, it is only
`possible to determine the position of ?oats, rafts, para
`vanes etc., i.e. devices which ?oat on the surface of the
`sea. Thus, all the above-mentioned methods have cer
`tain disadvantages and de?ciencies. Even if distances
`and absolute positions may be found with suf?cient
`accuracy, these disadvantages, however, are of such a
`nature that the methods do not furnish a general, total
`measuring accuracy or suf?cient redundancy to achieve
`an accurate determination of the measuring errors.
`
`25
`
`4,992,990
`4
`comprising the vessel or the vessels, ?oats, buoys,
`points on the seismic streamers and the seismic sources.
`Then a triangle network may be established between
`every measuring point and the measuring points of the
`triangle network may be referred to absolute reference
`positions, for instance the position of a vessel or a buoy.
`Hence the position of all the acoustic transceivers may
`be absolutely determined. The acoustic distance mea
`surements may be integrated with the position determi
`nations for instance in a computer system located
`aboard the vessel. Another advantage is that the hy=
`droacoustic measuring system, i.e. the acoustic trans
`ceivers, may be based on standard units, for instance as
`provided by Navigation Technology, whereas the posi
`tion determining tailbuoys or ?oats may be of the Syle
`dis or Hyperfix type. By using a method for determining
`the position according to the above, one is wholly inde
`pendent of compass bearings in order to ?nd directions
`and orientations, and at the same time the method gives
`a better accuracy than other known systems, i.e. mea
`surement accuracies in the distances of 1:1 m non-nor
`malized repeatability within 2 to 3 m deviation and
`cross-line errors of 3 to 5 m in 300 at the most, which
`corresponds to that which is achieved with the best
`surface navigation systems. By providing a suf?cient
`number of acoustic transceivers, a very high redun
`dancy is achieved so that the measuring system will not
`go down or the positioning accuracy will not be in?u
`enced, even if local errors are caused by some of the
`acoustic transceivers falling out. The fact that the mea
`suring system is over-determined, i.e. measurements
`may be taken from a number of points to one and the
`same point and vice versa, so that a plurality of different
`triangle networks are generated, offers a possibility of
`performing a statistic analysis of the measurement er
`rors, and any suitable statistic optimizing method may
`be used, for instance the least squares method, in order
`to choose the best values for distances and positions.
`The method according to the invention may be ap
`plied when severel vessels are employed, and several
`seismic streamers and positions for seismic measuring
`equipment behind a plurality of vessels may be com
`bined in the same triangle network. Furthermore, a
`continuous monitoring of registered positions may be
`undertaken, while at the same time determining the
`position of the measuring devices, i.e. the acoustic trans
`ceivers, directly.
`The measuring system of the method according to the
`invention may advantageously be integrated'in a tele
`communication system which connects ship-based navi
`gation and computer equipment with further position
`determining devices, and functions such as synchroniz
`ing, controlling and monitoring may then advanta
`geously take place through this telecommunication
`system which also is used for transmitting measurement
`data. By means of suitable control software the measur
`ing system employed with the method may suitably be
`adapted to discover and compensate measuring errors
`automatically.
`Normally the acoustic distance measurements are
`taken within a measuring cycle with a duration of 5 to
`10 s and with regard to the seismic data collection. As
`the shot interval in seismic data surveys typically may
`be about 10 s, the acoustic distance measurements may
`advantageously lie within this interval, so that no prob
`lems with interference are encountered. But the acous
`tic transceivers operate typically in the frequency band
`25 to 40 kHz, i.e. 8 to 9 octaves above the frequency of
`
`60
`
`35
`
`BRIEF SUMMARY OF THE INVENTION
`The object of the present invention is to provide a
`method for determining the position of at least two
`seismic streamers in a re?ection seismic measuring sys
`tem, so as to avoid the above-mentioned drawbacks and
`the necessity of using relatively inaccurate compass
`bearings, while at the same time advantageously achiev
`ing a better measuring redundancy and a far better
`detection of the occurring measuring errors.
`According to the present invention a method based
`on acoustic trilateration is used, i.e. the use of hydroa
`coustic distance measurements between a plurality of
`acoustic transceivers arranged in a suitable manner. At
`the same time at least two points of the measuring sys=
`45
`tem are determined by means of surface navigation
`methods. These points may preferably be the explora
`tion or towing vessel, a tailbuoy on the seismic streamer
`or even more advantageous, a ?oat which is towed by
`the vessel in such a manner that it is located on the side
`of or near the beginning of the seismic streamer. Since
`all the units of the measuring system, whether being
`buoys, ?oats, seismic sources, vessels or seismic stream
`ers, are more or less submerged, it is a fairly simple
`matter to measure the distance between these units
`below the surface by means of hydroacoustic distance
`measurements. Therefore acoustic transceivers are pro
`vided at every point, whose mutual distances it is de»
`sired to determine, i.e. aboard the vessel or vessels on
`the seismic sources, on the end points of the seismic
`stretch sections and possibly also in the active sections
`of the seismic streamers and on the ?oat or ?oats towed
`by the vessels as well as in the tailbuoys. For instance a
`vessel and a ?oat or the vessel and a tailbuoy may now
`be suitably positioned by means of surface navigation
`65
`systems. By means of distance measurements between
`the hydroacoustic measuring devices, i.e. the acoustic
`transceivers, a triangle network may be established,
`
`Ex. PGS 1055
`
`

`

`5
`the seismic sources. Moreover, screw cavitation and air
`bubbles from the air gun may also cause errors in the
`hydroacoustic measurements, but with a suf?cient re
`dundancy While also the acoustic transceivers are given
`a favorable location in relation to such sources of error,
`these should in practice offer no problems.
`The above-mentioned objects and advantages are
`achieved by the method of the invention the features
`and advantages of which are described hereinafter.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The method according to the invention may be more
`easily understood from the following detailed descrip
`tion of some non-limiting examples of embodiments
`with reference to the accompanying drawing, wherein:
`FIG. 1 is a schematic top plan view of a towing vessel
`towing two seismic streamers, only the fore end of the
`streamers being shown;
`FIG. 2 is a schematic plan view of the aft end of the
`‘seismic streamer according to FIG. 1;
`FIG. 3 is a view similar to FIG. 1 which illustrates
`two towing vessels each towing two seismic streamers,
`only the fore part of each seismic streamer being shown;
`FIG. 4 is a view similar to FIG. 2 showing the aft part
`of the seismic streamers according to FIG. 3;
`FIG. 5 is a section diagram of a of a triangle network
`for a vessel towing two seismic streamers;
`FIG. 6 is a diagram of another section of the triangle
`network according to FIG. 5;
`FIG. 7 is a section diagram similar to FIGS. 5 and 6
`showing a of a triangle network for two vessels, each
`towing two seismic streamers;
`FIG. 8 is a diagram of another section of the triangle
`network according to FIG. 7; and,
`FIG. 9 is a diagram of a further second section of a
`triangle network for two seismic streamers.
`
`10
`
`25
`
`35
`
`4,992,990
`6
`.
`graphic orientation of all the triangles of the shown
`triangle network. It will also be understood that the
`triangle network may be generated by other triangles
`than those shown in FIG. 5, as in practice it is possible
`to measure all themutual distances between the acous
`tic transceivers. In FIG. 2 the aft part of the streamers
`in the system in FIG. 1 are shown, 5] and S2 denoting
`the two seismic streamers, 6 a last active section of the
`seismic streamers, 7 the aft stretch sections and 8 tail
`buoys connected to the aft stretch sections. FIG. 6
`shows a triangle network for the system of FIG. 2. The
`provision of acoustic transceivers at the end points of
`the last active section 6 is seen, in other words also at
`the beginning of each stretch section 7. Further, there
`are provided acoustic transceivers in the tailbuoys 8
`which are also provided with equipment for position
`determination. By means of these the distances between
`the tailbuoys 8, as shown by the punctuated line, are
`computed, whereas the other distances of the triangle
`network are determined by means of hydroacoustic
`measurements of the mutual distances between the
`acoustic transceivers, such as those distances shown by
`whole lines. The triangle network is direction deter
`mined by means of the known positions of the tailbuoys
`8. Also here it is evident that the triangle network may
`be generated in another way than the one shown,
`In FIG. 3 there are shown two towing vessels 1
`which respectively tows two seismic streamers S1, S2,
`S3, S4 and further a plurality of seismic sources 2. The
`seismic streamers are designed in the same way as the
`system in FIG. 1. In FIG. 7 there is shown a triangle
`network for the system in FIG. 3. All whole lines de
`note distances in the triangle network found by means
`of hydroacoustic measurements performed with the
`acoustic transceivers which are arranged as shown. i.e.
`at the end points of each stretch section 3 in each of the
`seismic streamers S1, S2, S3, S4, and at one of the seismic
`sources 2, as well as aboard the vessels 1. Two reference
`positions are determined by means of equipment for
`determining position aboard the two vessels 1 and pro
`vide in addition to the orientation of the triangle net
`work also the computed distance between the vessels 1,
`as shown by the punctuated line. Also in this case the
`triangle network may be generated in other ways than
`the one shown by measuring other mutual distances
`between the acoustic transceivers. Thus, it is easy to see
`that a high degree of measurement redundancy is
`achieved. FIG. 4 shows for the same system as in FIG.
`3, the end sections of each of the seismic streamers S1,
`S2, S3 and $4, the last active section 6 in each seismic
`streamer and the following aft stretch section 7 being
`shown, as well as tailbuoys 8 connected to each strech
`section 7. FIG. 8 shows a triangle network for that part
`of the system with two vessels and four seismic stream
`ers which is shown in FIG. 4. Two reference positions
`are determined with equipment in the two tailbuoys 8
`which are respectively connected to the seismic stream
`ers S1 and S3, and the mutual distance between these
`tailbuoys is computed. The other distances in the trian
`gle network are measured with the acoustic transceivers
`ehich are provided in the tailbuoys, as well as at the end
`points of the last active section 6 in each of the seismic
`streamers S1, S2, S3, S4. Also here other triangle net
`works than the one shown in the Figure may easily be
`generated.
`.
`Thus by the method according to the invention a
`triangle network is obtained, in which each of the mea
`suring points consists of hydroacoustic or acoustic
`
`40
`
`DETAILED DESCRIPTION
`In every Figure the same reference numbers denote
`similar parts, while in FIG. 5 to 9 a circle denotes an
`acoustic transceiver, a triangle a reference position and
`a circle in a triangle a reference position with anacous
`tic transceiver, a circle with a slanted arrow a compass,
`whole lines measured distances, double lines known
`distances and punctuated lines computed distances.
`45
`In FIG. 1 there is shown an exploration or towing
`vessel 1 which tows two seismic streamers S1 and S2 and
`seismic sources 2. To the extension of the towing cable
`of one of the seismic sources 2 a ?oat 5 is attached. Of
`the seismic streamers which are towed by a towing
`50
`cable a ?rst stretch section 3 is shown, followed by the
`fore active section 4. In FIG. 5 there is shown a section
`of a triangle network for the system in FIG. 1. The
`triangle network of FIG. 1 comprises the vessel 1, a
`seismic source 2, the fore stretch sections 3 of the seis
`mic streamers S1 and S2 as well as a ?oat 5. Aboard the
`vessel and the ?oat 5 equipment is provided for position
`determination, while acoustic transceivers are provided
`aboard the vessel, at the end points of each of the
`stretch sections, on the seismic source and on the ?oat,
`respectively, as shown. By using hydroacoustic mea
`surements the mutual distances between the acoustic
`transceivers are now determined, the distances, as men
`tioned, being shown by whole lines. At the same time
`the positions of the vessel 1 and the ?oat 5 are found and
`the distance between them, as shown by the punctuated
`line, is computed; The known positions of the vessel 1
`and the ?oat 5 are suf?cient to determine the geo
`
`60
`
`65
`
`Ex. PGS 1055
`
`

`

`transceivers arranged as shown above or of points, the
`absolute position of which is determined by means of
`surface navigation methods. The triangle network is
`thus completely determined by trilateration between
`the measuring points, i.e. the acoustic transceivers and
`two reference positions provided by means of equip
`ment for position determination aboard vessels, ?oats or
`tailbuoys. Thus it is not necessary to use compass mea=
`surements when determining the triangle network, since
`this, as mentioned, is completely directionally deter=
`mined by means of two reference positions.
`By arranging acoustic transceivers in a number of
`points and having the possibility of determining more
`reference positions than two by for instance providing
`each tailbuoy, each vessel, each ?oat etc. with position
`determining equipment, it will be seen that a large num
`ber of different triangle networks may be generated in
`order to determine speci?c positions in the measuring
`system. The positions are hence over-determined, and a
`substantial redundancy of the system is achieved. This
`provides the advantage of determining the position of
`the seismic streamers even if one or more measuring
`points should fall out or certain measurement values for
`one reason or other are subjected to inadmissible noise
`or other sources of error. If the redundancy comes into
`full effect because of a substantial degree of over-deter
`mination and several different triangle networks are
`generated, a statistical optimization of measuring errors
`may be performed and by means of statistical optimiza
`tion procedures the best values of positions and dis
`tances in the triangle network may be determined,
`which may lead to an improved accuracy in the deter
`mination of the position of the seismic streamers.
`If after all the equipment for determining the position
`should fail at one or more measuring points or the posi
`tion data for one or more measuring points for some
`reason or other should fall out, it is still possible to
`perform a position determination of the seismic stream
`ers by using for instance a single reference position
`combined with a compass bearing. Then it is possible to
`use compass bearings at an angle where the measure
`ment error may be relatively small, so that the error in
`the distance computed in this manner will also become
`relatively small. This means that the intersecting angle
`45
`should not be too acute. A practical example of a posi
`tion determination involving a compass angle and a
`reference position is shown if FIG. 9. Here 6 denotes
`the last active sections of the two seismic streamers S1,
`S2, 7 is the aft stretch section of the seismic streamer S1,
`8 is the tailbuoy connected to this stretch section which
`also is provided with a compass. The reference position
`is determined with the equipment for position determi
`nation provided in the tailbuoy 8, and the triangle net
`work is generated by distance measurements with
`acoustic transceivers provided in the tailbuoy 8 and on
`the end of the last active section 6, respectively. How
`ever, since one has only one reference position, the
`direction of the triangle network must be determined by
`the compass angle a which must be obtained by for
`instance a magnetic compass provided in the stretch
`section 7. However, the angle a has a value which gives
`a good relative measurement accuracy, for instance
`with a compass error of 0.5", it will be only 1.25% for an
`angle at equal to 40°. The bearing angles may be also be
`provided by compasses arranged in the last active sec
`tions 6 so that the measurement errors may be treated
`statistically and one still has redundancy when deter
`
`4,992,990
`8
`mining the position of the seismic streamers or the dis
`tance therebetween.
`By the method according to the invention position
`determination of the locations of the start points and
`end points of the seismic streamers is thus obtained, in
`practice the position of the ends of respectively the ?rst
`and last active section of the streamer, but it is of course
`also possible to use acoustic transceivers in other parts
`of the seismic streamer, possibly along the whole seis
`mic streamer. Hence, in theory, one may renounce the
`compasses provided in the streamer. However, the use
`of further acoustic transceivers other than those pro
`vided at the end of each active section of the seismic
`streamer would be a superfluous measure. The accuracy
`by using hydroacoustic measurements is so good that
`the error of the determination of the seismic streamer’s
`end points lies at about :1 m and in reality well within
`the statistical deviation of the position determination.
`However, since one knows the end point positions for
`the seismic streamer and furthermore the speed and
`bearing of the towing vessel and may estimate the cur
`rent conditions of the measurement location, it is possi
`ble to perform a mathematical estimation of the error
`transmission through the active sections of the seismic
`streamer, i.e. from the position of the first to the last end
`point, and thus obtain an estimate of the curve of the
`active section of the seismic streamer so that the esti
`mate will lie within the margins of error for the posi
`tions determined by means of surface navigation and
`acoustic trilateration. If the requirement of having the
`compass hearing as an additional possibility is disre~
`garded, one could by the method according to the in=
`vention completely renounce the magnetic compasses,
`at least in the active sections of the seismic streamer.
`We claim:
`'
`l
`1. A method for determining the position of at least
`two seismic streamers, each having a fore end and an aft
`end, in a re?ection seismic measurement system in 3
`dimensional marine seismic surveys, wherein each seis
`mic streamer is equipped with a plurality of compasses
`and comprises a plurality of active sections having
`known lengths inserted between a fore stretch section
`and an aft stretch section at each end, respectively, of
`the seismic streamers, said active sections being
`equipped with a plurality of hydrophones or hydro
`phone groups having known locations, wherein the fore
`stretch section is connected with a towing arrangement
`attached to a towing vessel and the end of the aft stretch
`section is provided with a tailbuoy having equipment
`for determining the position of the tailbuoy and an
`acoustic transceiver, and the measurement system fur
`ther includes at least one seismic source provided with
`an acoustic transceiver and at least one towing vessel
`each equipped with an onboard computer system for
`positioning and navigational purposes and at least one
`acoustic transceiver, the method comprising:
`towing a ?oat by at least one vessel;
`locating said ?oat off and between the fore ends of
`said seismic streamers;
`providing said ?oat with position determining means
`and an acoustic transceiver;
`disposing a plurality of acoustic transceivers at spe-=
`ci?c locations in each stretch section;
`providing further acoustic transceivers at certain
`locations in at least one of said active sections adja
`cent to a stretch section, so that said seismic
`streamers, vessels and seismic sources constitute a
`3-dimensional structure;
`
`65
`
`25
`
`35
`
`Ex. PGS 1055
`
`

`

`9
`connecting said seismic streamers, seismic sources
`and ?oat