Ex. PGS 1011
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`United States Patent [19J
`French
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`[11] Patent Number:
`[45] Date of Patent:
`
`4,486,863
`Dec. 4, 1984
`
`[54] CIRCULAR SEISMIC ACQUISITION
`SYSTEM
`
`Inventor: WilliamS. French, Covington, La.
`[75]
`[73] Assignee: Tensor Geophysical Service
`Corporation, Metairie, La.
`[21] Appl. No.: 522,329
`[22] Filed:
`Aug. 11, 1983
`Int. CJ.3 ............................................... GOlV 1/38
`[51]
`[52] u.s. Cl ....................................... 367/15; 367/130;
`367/19; 367/117; 367/16
`[58] Field of Search ....................... 367/15, 16, 19, 20,
`367/21, 17, 117, 106, 130, 14
`References Cited
`U.S. PATENT DOCUMENTS
`3,292,141 12/1966 Hines eta!. ........................... 367/21
`3,806,863 4/1974 Tilley et a!. ........................... 367/21
`3,831,136 8/1974 Sagoci ................................... 367/21
`3,840,845 10/1974 Brown ................................... 367/17
`3,890,593 6/1975 Davis .................................... 367/20
`3,921,124 11/1975 Payton .................................. 367/17
`4,033,278 7/1977 Waters .................................. 367/17
`4,068,208 1/1978 Rice, Jr. eta!. ...................... 367/19
`4,231,111 10/1980 Neeley .................................. 367/19
`4,301,523 11/1981 Meland eta!. ........................ 367/19
`
`[56]
`
`4,376,301 3/1983 Roberts ................................. 367/20
`4,404,664 9/1983 Zachariadis ........................... 367/19
`
`Primary Examiner-Nelson Moskowitz
`Assistant Examiner-Ian J. Lobo
`Attorney, Agent, or Firm-Vaden, Eickenroht,
`Thompson, Bednar & Jamison
`[57]
`ABSTRACT
`A marine seismic survey is disclosed in which a plural(cid:173)
`ity of steerage centers are selected for circular traversal
`thereabout by a marine vessel towing a streamer cable.
`While the vessel and the streamer are following a circu(cid:173)
`lar track about a steerage center, the feathering of the
`cable creates a separate concentric track line for each of
`the mid-points between the receivers included in the
`cable and the source. In this manner an areal coverage
`is assured which requires no reliance on unpredictable
`and uncontrollable currents for the desired feathering.
`An additional important feature of the current disclo(cid:173)
`sure is that there is little wasted time in which the sur(cid:173)
`vey vessel moves outside of the overall surveyed area.
`A preferred course tracking for the vessel is to move
`from the arc of one steerage course line to another in
`tangential fashion.
`
`6 Claims, 4 Drawing Figures
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`Ex. PGS 1011
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`

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`U.S. Patent Dec. 4, 1984
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`Sheet 1 of 2
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`4,486,863
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`Ex. PGS 1011
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`

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`U.S. Patent Dec. 4, 1984
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`Sheet 2 of 2
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`4,486,863
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`Ex. PGS 1011
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`1
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`4,486,863
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`2
`substantially equally spaced parallel vessel course lines.
`Usually, one-half of the lines are shot in one direction
`and one-half of the lines are shot in the opposite direc(cid:173)
`tion. Thus, upon finishing the shooting along one line,
`5 for example, west to east, the marine vessel comes about
`and a line is shot from east to west. The turns are made
`outside of the area of interest for the survey in order to
`acquire data from the entire area of interest. The time
`that the vessel is outside of the survey area represents
`wasted ship time as far as data acquisition is concerned.
`Therefore, it is a feature of the present invention to
`gather marine seismic data in an improved manner
`wherein feathering of the streamer cable is controlled
`independently of sea and wind currents to insure accu(cid:173)
`racy of areal coverage.
`It is another feature of the present invention to pro(cid:173)
`vide for the gathering of marine seismic data in an im(cid:173)
`proved manner wherein the marine vessel rarely tra-
`verses outside of the area of survey interest during the
`taking of the survey and, therefore, there is little waste
`time as with prior art surveys, when the vessel is com-
`ing about.
`
`CIRCULAR SEISMIC ACQUISITION SYSTEM
`
`25
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention pertains to marine seismic acquisition
`procedures employing a towed streamer cable having a
`plurality of data receivers spaced therealong.
`2. Description of the Prior Art
`Marine seismic reflection surveys are traditionally 10
`developed by steering a marine vessel in a straight line
`while periodically firing an acoustic source closely
`associated with the vessel, usually towed just behind the
`vessel and positioned slightly below the surface of the
`water. A streamer cable towed by the vessel is also 15
`played out behind the vessel. The cable includes along
`its length a plurality of suitable acoustic receivers
`known as hydrophones for receiving and detecting
`acoustic seismic reflections. The hydrophones are nor(cid:173)
`mally clustered or grouped together, the groups of 20
`receivers usually being evenly spaced along the cable.
`The data gathered by the receivers reveals the condi(cid:173)
`tion of the geophysical terrain between the source and
`the respective receivers as the source is fired and the
`vessel traverses its course.
`The processing of the developed data initially as(cid:173)
`sumed that the ship, the source and the respective re(cid:173)
`ceivers spaced along the streamer cable were all located
`in a straight line with respect to the sea floor.
`However, it was obvious that in practice, sea and 30
`wind currents caused the streamer cable to "feather" to
`one side or the other and not be towed directly behind
`the vessel. Therefore, the data line of tow for each of
`the data gathering elements had to be corrected for this
`feathering phenomenon or some erroneous data results 35
`would occur.
`It was recognized, however, that when feathering did
`occur, there was an effective areal coverage to the data,
`rather than just a linear coverage. That is, the receivers
`tracked along parallel lines, and the mid-points, (assum- 40
`ing horizontal reflection layers in the terrain) between
`the source and the respective receivers also tracked
`parallel lines. Hence, schemes were developed to select
`line spacing, ship heading and speed with respect to the
`currents to take advantage of the areal coverage caused 45
`by feathering. Complex positioning schemes were de(cid:173)
`veloped to determine the actual location of each "re(cid:173)
`ceiver", which is understood to apply herein to what is
`really a group of individual receiver detectors, of the
`streamer. The design of surveys (line spacing, shooting so
`direction and speed, number of lines, etc.) was made
`based upon assumptions of the currents.
`The details of the ocean currents and, hence, the true
`streamer configuration are unfortunately unpredictable.
`Many surveys have failed to provide adequate areal 55
`coverage when based on predicted currents because the
`actual feathering of the streamer cable during the sur(cid:173)
`vey departed significantly from what was expected
`before the survey. To overcome the bad experience
`resulting from erratic feathering, acquisition of data has 60
`been performed by positioning the receivers and by
`tracking adjacent shot line courses of the vessel much
`closer together than is actually required by data pro(cid:173)
`cessing. Of course, this significantly increases the cost
`of data acquisition in order to insure an adequate distri- 65
`bution of receiver locations over a surveyed area.
`As mentioned above, areal marine seismic reflection
`surveys are presently conducted using a number of
`
`SUMMARY OF THE INVENTION
`The preferred method of gathering marine seismic
`data disclosed herein employs a marine vessel having
`associated therein a marine seismic source and a
`streamer cable having hydrophone or similar seismic
`receivers spaced therealong, in conventional fashion.
`Steerage centers are determined throughout the survey
`area. The vessel then tracks in at least partial circular
`arcs about such centers while creating seismic signals
`and detecting or receiving seismic reflections at each of
`the receivers along the streamer cable. Assuming hori(cid:173)
`zontal geophysical reflecting properties, the mid-points
`between the source and each of the receivers track a
`circular or arcuate line about the steerage center, each
`mid-point tracking a slightly different concentric arc to
`thereby give areal coverage. The limits of the areal
`coverage about a single steerage center is determined
`by the line tracked by the mid-point between the source
`and the rearmost receiver carried by the cable.
`Unpredictable ocean and wind currents will produce
`variations in ship and streamer positioning from the
`ideal circular design, but those variations will have a
`minor effect. The actual position of ship and streamer
`are recorded during the survey using commercially
`available technology, and such position information is
`used to correct the recordea data during subsequent
`processing.
`Although it is acceptable to complete a circumferen(cid:173)
`tial circular coverage about a first steerage center and
`then the next until a complete areal survey has been
`taken, a preferred manner is to steer the vessel first
`partly around one center and then, in a tangential con(cid:173)
`tinuum around a successive steerage center in "S" like
`fashion and so forth until the entire area is covered. This
`is believed to be the quickest and easiest way to obtain
`comprehensive coverage of the entire survey area.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`So that the manner in which the above-recited fea(cid:173)
`tures, advantages and objects of the invention, as we11 as
`others which will become apparent, are attained and
`can be understood in detail, more particular description
`of the invention briefly summarized above may be had
`by reference to the embodiment thereof which is ill us-
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`Ex. PGS 1011
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`3
`trated in the drawings, which drawings form a part of
`this specification. It is to be noted, however, that the
`appended drawings illustrate only a typical embodiment
`of the invention and are therefore not to be considered
`limiting of its scope as the invention may admit to other
`equally effective embodiments.
`In the Drawings:
`FIG. 1 is a schematic representation of a marine ves(cid:173)
`sel equipped for seismic data survey gathering using a
`streamer cable.
`FIG. 2 is a simplified areal path of a marine vessel
`moving around a steerage center in accordance with the
`present invention.
`FIG. 3 is a simplified areal path of a marine vessel
`moving around a steerage center in accordance with
`another embodiment of the present invention.
`FIG. 4 is an areal survey plan for an entire area of
`interest in accordance with the present invention.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`Now referring to the drawings, and first to FIG. 1, a
`marine vessel 10 is shown moving in a path direction 12.
`The surface of the sea or ocean water is identified as
`surface 14 and the geophysical terrain is represented by 25
`reflecting surface 16. This surface is illustrated as being
`horizontal and is shown only as a single reflecting inter(cid:173)
`face for convenience of illustration. Of course, it is
`understood that in actual practice there will be many
`reflecting interfaces and one or more of them may not 30
`be predominantly horizontal. Marine vessel 10 tows a
`floating source 18, usually about 25 feet below the sur(cid:173)
`face of the water and close to the vessel, and a streamer
`cable 20, which is balanced and otherwise positioned by
`paravanes and the like so that it plays out behind the 35
`vessel also below the water surface, and therefore, free
`from most of the surface turbulence.
`Located along and integral with streamer cable 20 are
`a plurality of "receivers''. A "receiver" for purposes of
`this application is a seismic signal detection unit. In 40
`actual practice each unit is an array or group of closely
`associated hydrophones, but a unit can be a single hy(cid:173)
`drophone. The receivers are normally evenly spaced
`from one another. The streamer cable can be quite long,
`on the order of two miles long in many instances. For 45
`simplicity of illustration only a few of receivers 22a-22g
`are shown. The respective reflection paths from source
`18 to reflecting surface 16 to receivers 22a-22g are
`identified as paths 24-24g.
`In taking a survey, the source is periodically acti- 50
`vated to produce an acoustic signal emanating down(cid:173)
`ward into the water to be detected by the receivers after
`reflecting from the reflecting interfaces in the geophysi(cid:173)
`cal terrain. The acquired data is recorded and processed
`to reveal information about the geology of the forma- 55
`tion at the reflecting points, which are the mid-points
`between the source and the respective receivers when
`viewed from above and for a horizontal reflecting layer
`or interface. Although the reflecting layer may be other
`than horizontal, for convenience the reflecting points 60
`will be referred to herein as the "mid-points".
`Now referring to FIG. 2, an areal or top view of a
`vessel 10 is shown moving on a circular course 26 about
`a steerage center 28, streamer cable 20 feathering be(cid:173)
`hind vessel 10 and approximately overlying course 26. 65
`The radius of the circle or the arc of the circle of course
`26 is radius R1. A chord line 30 drawn from vessel 10,
`or more accurately from source 18 associated with ves-
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`4,486,863
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`4
`sel 10, to end receiver 22x, the one fartherest from ves(cid:173)
`sel 10, defines a circle 32 which is traversed by the
`mid-point associated with receiver 22x. This mid-point
`course 32 can be determined by drawing a perpendicu-
`5 Jar to steerage point 28 to chord line 30, that perpendic(cid:173)
`ular also being illustrated as radius R2.
`It may be seen that other chord lines drawn to the
`intermediate receivers located along the streamer cable
`define a locus of points illustrated by line 34. It is also
`10 apparent that the line tracked by the mid-points all
`slightly differ from each other because of the feathering
`action, but are all concentric with one another about
`steerage center 28. The knowledge of the relationships
`just described of source, receivers, course radius, mid-
`IS point radii and the locus of points of the mid-point radii
`make it quite easy to accurately design a survey to
`cover a given area and to process and interpret the
`seismic data collected. It will be apparent that the areal
`coverage for a complete traversal about steerage point
`20 28 is area 36, that area defined as being between course
`line 26 and mid-point course line 32.
`FIG. 3 is similar in all respects to FIG. 2 except that
`R1, the radius of the circle of course 46, is chosen so
`that the length of the streamer cable equals one-half of
`the circumference of circular course 46. In this case, the
`mid-point 32 associated with receiver 22x coincides
`with the steerage center 48. The areal coverage for a
`complete traversal about steerage point 48 is area 50,
`the area swept out by the locus of mid-points or arc 34.
`Area 50 coincides with the complete area within circu(cid:173)
`lar course 46.
`Now referring to FIG. 4, a planned area 40 for sur(cid:173)
`veying is illustrated as a square with sides 42 and 44.
`Although an area surveyed is commonly a square, the
`present invention is equally applicable to a rectangular
`area or even an irregularly shaped area.
`Areal coverage 36 between course line 26 and mid(cid:173)
`point course line 32 and areal coverage 50 between
`course line 46 and steerage center 48 are superimposed
`on survey area 40 in FIG. 4. It may be seen that com(cid:173)
`plete coverage of area 40, including repetitive data to
`give redundancy accuracy, is possible by selecting an
`appropriate radius R1 and by tracking similar courses
`about a multiplicity of steerage centers illustrated with
`the dot pattern shown. Each dot represents a steerage
`center. The desired redundancy achieved includes re(cid:173)
`flections from the same subsurface point from several
`source-receiver pairs with various distances between
`source and receiver and various azimuths of a line con(cid:173)
`necting source and receiver. There is some movement
`of the vessel outside of the boundries of the surveyed
`area, but nothing as compared with the prior art tech(cid:173)
`nique described above.
`ln order to most expeditiously cover or track area 40
`in accordance with the above individual procedure, it is
`convenient to establish the steerage centers and the
`radius of steerage so that the vessel can move from one
`steerage course to the next in "S" fashion. In other
`words, the course setting would be tangential to one
`another to permit the vessel to weave an overall tangen-
`tial course from an arc of one course line to the next
`until the complete area is covered.
`While a particular embodiment of the invention has
`been shown and described, it will be understood that
`the invention is not limited thereto, since many modifi(cid:173)
`cations may be made and will become apparent to those
`skilled in the art.
`
`Ex. PGS 1011
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`

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`4,486,863
`
`5
`1. The method of gathering marine seismic data em(cid:173)
`ploying a streamer cable towed by a marine vessel, said
`streamer cable having a plurality of seismic receivers
`spaced therealong, each of said receivers receiving seis(cid:173)
`mic reflections resulting from an acoustic source associ- 5
`ated with said marine vessel being received by said
`receiver after being reflected by the geophysical terrain
`located between said source and said receiver, which
`comprises
`traversing as closely as navigational techniques, wind 10
`and current conditions permit at least a partial
`circle with said marine vessel about a steerage
`center,
`creating acoustic source signals from said source, and
`receiving reflected signals from said source at at least 15
`one of said receivers, said receiver tracking a feath(cid:173)
`ered course about said steerage center, the data
`gathered therefrom being that which is associated
`with the midpoint of a subtending moving chord
`defined by said source and said receiver.
`2. The method of accordance with claim 1, and in(cid:173)
`cluding
`receiving reflected signals from said source at each of
`the other of said receivers, each of said receivers
`tracking a respectively feathered course about said
`steerage center, the data gathered being that which
`is associated with an areal coverage between the
`circle arc traversed by said marine vessel and the
`subtending moving chord defined by said source
`and the one of said receivers located fartherest 30
`therefrom along the streamer cable, the mid-points
`of each of the subtending moving chords defined
`by said source and said receivers traversing sepa(cid:173)
`rate circular arcs about said steerage center.
`
`20
`
`6
`3. The method in accordance with claim 2, and
`processing the received data from the receivers rec(cid:173)
`ognizing that said mid-points of each of the sub(cid:173)
`tending moving chords define a uniformly shaped
`locus of points.
`4. The method in accordance with claim 2, and in(cid:173)
`cluding
`traversing additional circle arcs with said marine
`vessel about additional steerage centers,
`creating acoustic source signals from said source, and
`receiving reflected signals from said source at each of
`said receivers, each of said receivers tracking a
`respectively feathered course about said additional
`steerage centers, the data gathered being that
`which is associated with additional areal coverages
`between the additional circle arcs traversed by said
`marine vessel and the subtending moving chords
`defined by said source and said receivers located
`along the streamer cables, the mid-points of each of
`the subtending moving chords defined by said
`source and said receivers traversing a circle arc
`about said additional steerage centers.
`5. The method in accordance with claim 4, wherein
`said areal coverages combine to provide data gathering
`25 from a complete survey area.
`6. The method in accordance with claim 4, and in(cid:173)
`cluding
`locating the additional steerage centers and the radii
`of said traversed arcs for tangential interfacing, and
`moving said marine vessel about one of said steerage
`centers on a first circle arc course and then con(cid:173)
`tinuing about a second steerage center on a second
`circle arc course.
`* * * * *
`
`35
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`40
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`50
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`55
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`60
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`65
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`Ex. PGS 1011