`
`
`Ex. PGS 1034
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`
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`EX. PGS 1034
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`
`
`
`
`
`
`United States Patent [19J
`Langeland et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,992,990
`Feb. 12, 1991
`
`[75]
`
`[54] METHOD FOR DETERMINING THE
`POSmON OF SEISMIC STREAMERS IN A
`REFLECTION SEISMIC MEASURING
`SYSTEM
`Inventors: Jan-Age Langeland, Garnes; 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] PCTNo.:
`PCT/N089/00055
`§ 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
`[51]
`Int. Cl.s ............................................... G01V 1/38
`[52] u.s. 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 eta!. ....................... 367/19
`4,660,185 4/1987 French .................................. 367/19
`4,669,067 5/1987 Roberts ................................. 367/19
`4,845,686 7/1989 Brae ...................................... 367/19
`
`FOREIGN PATENT DOCUMENTS
`2393320 12/1978 France .
`2620536 3/1989 France .
`0831513 5/1984 Norway .
`
`W007732 12/1987 World Int. Prop. 0 ..
`
`OTHER PUBLICATIONS
`Sonerdyne Limited Publication, "A Hydro-Acoustic
`System for Precision Tracking of Twin Seismic Hydro(cid:173)
`phone Streamers," 7/87.
`"Improving the Accuracy of Merile 3D Seismic Sur(cid:173)
`veys," Ocean Industry, Jan. 1987.
`Primary Examiner-Ian 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 (St, Sz) in a reflection seismic measur(cid:173)
`ing system, wherein hydroacoustic distance measure(cid:173)
`ments are used which are taken by means of acoustic
`transceivers provided in vessels (1), buoys (8), floats (5),
`seismic sources (2) and in the seismic streamers (St, Sz).
`Absolute reference positions are determined by position
`determining equipme"nt provided in at least two loca(cid:173)
`tions, for instance, on a vessel (1) or a float (5). The
`acoustic transceivers and the position determining
`equipment form a three-dimensional structure. The
`position determination takes place by trilateration be(cid:173)
`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(cid:173)
`ularly suited for application in connection with three-di(cid:173)
`mensional marine seismic surveys. The method may be
`integrated with suitable surface navigation systems in
`order to find the reference positions and provide abso(cid:173)
`lute positions at any point within a marginal error of 5
`to 10m.
`
`18 Claims, 5 Drawing Sheets
`
`Sz
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`3
`
`3
`
`Ex. PGS 1034
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`
`
`U.S. Patent
`
`Feb. 12, 1991
`
`Sheet 1 of 5
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`4,992,990
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`U.S. Patent
`
`Feb. 12, 1991
`
`Sheet 2 of 5
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`4,992,990
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`Ex. PGS 1034
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`
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`U.S. Patent
`
`Feb. 12, 1991
`
`Sheet 3 of 5
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`4,992,990
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`Ex. PGS 1034
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`
`
`U.S. Patent
`
`Feb. 12, 1991
`
`Sheet 4 of 5
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`4,992,990
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`
`
`Ex. PGS 1034
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`
`
`U.S. Patent
`
`Feb. 12, 1991
`
`Sheet 5 of 5
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`4,992,990
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`Fig. B.
`
`Sz
`
`6
`
`Fig.9.
`
`Ex. PGS 1034
`
`
`
`METHOD FOR DETERMINING THE POSITION
`OF SEISMIC STREAMERS IN A REFLECTION
`SEISMIC MEASURING SYSTEM
`
`10
`
`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(cid:173)
`tion and positioning and one is dependent on knowing
`the relative positions of seismic sources and the hydro- 15
`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(cid:173)
`mined with relatively high precision. The seismic 20
`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 25
`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(cid:173)
`sary to determine the position of the seismic streamers
`or in reality the position of the hydrophones of the 30
`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 35
`hydrophones or hydrophone groups with known loca(cid:173)
`tion in the active sections. The seismic streamer is ar(cid:173)
`ranged between stretch sections, a fore s,tretch section
`being connected with a towing means on the towing
`vessel, whereas a tailbuoy with means for determining 40
`the position is provided at the end of the aft strech
`section. The means may for instance be an active navi(cid:173)
`gation system of the same type that is employed aboard
`the towing vessel or a microwave system, possibly com(cid:173)
`bined with a goniometer. The position of the tailbuoy 45
`may also be determined by passive distance measure(cid:173)
`ments between the towing vessel and the buoy, for
`instance by means of radar or laser reflectors. Now
`knowing the positions of the vessel and the tailbuoy, the
`position of the hydrophones in the seismic streamer is 50
`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 55
`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(cid:173)
`ated. In practice the seismic streamer will hence have 60
`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 65
`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-
`
`1
`
`4,992,990
`
`2
`cal estimation in connection with compass ·bearings,
`known distances and section lengths. As a rule, how(cid:173)
`ever, the error in the actual distance to a streamer sec(cid:173)
`tion will lie within the errors of the position determin-
`5 ing system, referred to the navigation accuracy.
`At present a number of methods are employed or
`proposed to be employed, for determining the stream(cid:173)
`er's position, based on distance measurements in con(cid:173)
`nection with compass bearings. One method is to deter(cid:173)
`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(cid:173)
`ror~, since the stretch sections of the streamer may be
`stretched such that the distances may vary with about
`10 to 15m, and there are small possibilities of determin(cid:173)
`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(cid:173)
`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(cid:173)
`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(cid:173)
`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(cid:173)
`surement techniques for determining the streamer posi(cid:173)
`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(cid:173)
`gles give fairly large errors in the angle readings and in
`addition too few measurements for achieving sufficient
`redundancy and determination of measurement errors.
`A further hydroacoustic positioning system for two
`seismic streamers is disclosed by a report from Sonar(cid:173)
`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(cid:173)
`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(cid:173)
`mined.
`An acoustic distance measurement technique has also
`been applied to the problem of determining the horizon(cid:173)
`tal profile 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
`
`Ex. PGS 1034
`
`
`
`4,992,990
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`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 5
`the streamers are found, allowing a representation of
`the profile 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 10
`and would be hightly impractical to adopt for position(cid:173)
`ing tasks in a 3-D marine seismic surveying system em(cid:173)
`ploying more than one streamer.
`Systems wherein distance measurements are made by
`means of optical methods and microwave methods are 15
`also encumbered with a number of deficiencies. 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
`the measuring errors of the gyrocompass leads to inac- 20
`curate direction determinations. Furthermore, it is only
`possible to determine the position of floats, rafts, para(cid:173)
`vanes etc., i.e. devices which float on the surface of the
`sea. Thus, all the above-mentioned methods have cer(cid:173)
`tain disadvantages and deficiencies. Even if distances 25
`and absolute positions may be found with sufficient
`accuracy, these disadvantages, however, are of such a
`nature that the methods do not furnish a general, total
`measuring accuracy or sufficient redundancy to achieve
`an accurate determination of the measuring errors.
`
`4
`compnsmg the vessel or the vessels, floats, 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(cid:173)
`surements may be integrated with the position determi(cid:173)
`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(cid:173)
`ceivers, may be based on standard units, for instance as
`provided by Navigation Technology, whereas the posi(cid:173)
`tion determining tailbuoys or floats may be of the Syle(cid:173)
`dis or Hyperfix type. By using a method for determining
`the position according to the above, one is wholly inde(cid:173)
`pendent of compass bearings in order to find directions
`and orientations, and at the same time the method gives
`a better accuracy than other known systems, i.e. mea(cid:173)
`surement accuracies in the distances of ± 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 mcst, which
`corresponds to that which is achieved with the best
`surface navigation systems. By providing a sufficient
`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 influ(cid:173)
`enced, even if local errors are caused by some of the
`30 acoustic transceivers falling out. The fact that the mea(cid:173)
`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(cid:173)
`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 severe! vessels are employed, and several
`seismic streamers and positions for seismic measuring
`equipment behind a plurality of vessels may be com(cid:173)
`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(cid:173)
`communication system which connects ship-based navi(cid:173)
`gation and computer equipment with further position
`determining devices, and functions such as synchroniz(cid:173)
`ing, controlling and monitoring may then advanta(cid:173)
`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(cid:173)
`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
`
`40
`
`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 reflection seismic measuring sys- 35
`tern, so as to avoid the above-mentioned drawbacks and
`the necessity of using relatively inaccurate compass
`bearings, while at the same time advantageously achiev(cid:173)
`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(cid:173)
`caustic 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
`tern are determined by means of surface navigation
`methods. These points may preferably be the explora(cid:173)
`tion or towing vessel, a tailbuoy on the seismic streamer
`or even more advantageous, a float which is towed by
`the vessel in such a manner that it is located on the side 50
`of or near the beginning of the seismic streamer. Since
`all the units of the measuring system, whether being
`buoys, floats, seismic sources, vessels or seismic stream(cid:173)
`ers, are more or less submerged, it is a fairly simple
`matter to measure the distance between these units 55
`below the surface by means of hydroacoustic distance
`measurements. Therefore acoustic transceivers are pro(cid:173)
`vided at every point, whose mutual distances it is de(cid:173)
`sired to determine, i.e. aboard the vessel or vessels on
`the seismic sources, on the end points of the seismic 60
`stretch sections and possibly also in the active sections
`of the seismic streamers and on the float or floats towed
`by the vessels as well as in the tailbuoys. For instance a
`vessel and a float 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 1034
`
`
`
`4,992,990
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`5
`the seismic sources. Moreover, screw cavitation and air
`bubbles from the air gun may also cause errors in the
`hydroacoustic measurement~, but with a sufficient re(cid:173)
`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.
`
`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
`5 to measure all the. mutual distances between the acous(cid:173)
`tic transceivers. In FIG. 2 the aft part of the streamers
`in the system in FIG. 1 are shown, Stand Sz denoting
`the two seismic streamers, 6 a last active section of the
`seismic streamers, 7 the aft stretch sections and 8 tail-
`10 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
`15 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
`20 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 tail buoys
`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 St, Sz,
`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(cid:173)
`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 St. Sz, 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(cid:173)
`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 St,
`Sz, S3 and S4, 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(cid:173)
`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(cid:173)
`ers St and S3, and the mutual distance between these
`tailbuoys is computed. The other distances in the trian(cid:173)
`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 St, Sz, S3, S4. Also here other triangle net(cid:173)
`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(cid:173)
`suring points consists of hydroacoustic or acoustic
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The method according to the invention may be more
`easily understood from the following detailed descrip(cid:173)
`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 30
`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 35
`triangle network for two seismic streamers.
`
`25
`
`45
`
`DETAILED DESCRIPTION
`In every Figure the same reference numbers denote
`similar parts, while in FIG. 5 to 9 a circle denotes an 40
`acoustic transceiver, a triangle a reference position and
`a circle in a triangle a reference position with an.acous-
`tic transceiver, a circle with a slanted arrow a compass,
`whole lines measured distances, double lines known
`distances and punctuated lines computed distances.
`In FIG. 1 there is shown an exploration or towing
`vessel1 which tows two seismic streamers Stand Sz and
`seismic sources 2. To the extension of the towing cable
`of one of the seismic sources 2 a float 5 is attached. Of
`the seismic streamers which are towed by a towing 50
`cable a first 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- 55
`mic streamers Stand Sz as well as a float 5. Aboard the
`vessel and the float 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 float, 60
`respectively, as shown. By using hydroacoustic mea(cid:173)
`surements the mutual distances between the acoustic
`transceivers are now determined, the distances, as men(cid:173)
`tioned, being shown by whole lines. At the same time
`the positions of the vessel1 and the float 5 are found and 65
`the distance between them, as shown by the punctuated
`line, is computed: The known positions of the vessel 1
`and the float 5 are sufficient to determine the geo-
`
`Ex. PGS 1034
`
`
`
`25
`
`7
`transceivers arranged as shown above or of points, the
`absolute position of which is determined by means of
`surface navigation methoqs. The triangle network is
`thus completely determined by trilateration between
`the measuring points, i.e. the acoustic transceivers and 5
`two reference positions provided by means of equip(cid:173)
`ment for position determination aboard vessels, floats or
`tailbuoys. Thus it is not necessary to use compass mea(cid:173)
`surements when determining the triangle network, since
`this, as mentioned, is completely directionally deter- 10
`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 float etc. with position 15
`determining equipment, it will be seen that a large num(cid:173)
`ber of different triangle networks may be generated in
`order to determine specific positions in the measuring
`system. The positions are hence over-determined, and a
`substantial redundancy of the system is achieved. This 20
`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(cid:173)
`mination and several different triangle networks are
`generated, a statistical optimization of measuring errors
`may be performed and by means of statistical optimiza- 30
`tion procedures the best values of positions and dis(cid:173)
`tances in the triangle network may be determined,
`which may lead to an improved accuracy in the deter(cid:173)
`mination of the position of the seismic streamers.
`If after all the equipment for determining the position 35
`should fail at one or more measuring points or the posi(cid:173)
`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(cid:173)
`ers by using for instance a single reference position 40
`combined with a compass bearing. Then it is possible to
`use compass bearings at an angle where the measure(cid:173)
`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(cid:173)
`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 St.
`S2, 7 is the aft stretch section of the seismic streamer St. 50
`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(cid:173)
`nation provided in the tailbuoy 8, and the triangle net(cid:173)
`work is generated by distance measurements with 55
`acoustic transceivers provided in the tailbuoy 8 and on
`the end of the last active section 6, respectively. How(cid:173)
`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 60
`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 OS, it will be only 1.25% for an
`angle a equal to 40•. The bearing angles may be also be 65
`provided by compasses arranged in the last active sec(cid:173)
`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(cid:173)
`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 first
`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(cid:173)
`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(cid:173)
`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(cid:173)
`rent conditions of the measurement location, it is possi(cid:173)
`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(cid:173)
`mate will lie within the margins of error for the posi(cid:173)
`tions determined by means of surface navigation and
`acoustic trilateration. If the requirement of having the
`compass bearing as an additional possibility is disre(cid:173)
`garded, one could by the method according to the in(cid:173)
`vention completely renounce the magnetic compasses,
`at least in the active sections of the seismic streamer.
`We claim:
`1. A method for determining the position of at least
`two seismic streamers, each having a fore end and an aft
`end, in a reflection seismic measurement system in 3-
`dimensional marine seismic surveys, wherein each seis(cid:173)
`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(cid:173)
`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 s