Ex. PGS 1002
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`EX. PGS 1002
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`IN THE UNITED STATES PATENT AND TRADEMARK OFFICE
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`BEFORE THE PATENT TRIAL AND APPEAL BOARD
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`PETROLEUM GEO-SERVICES INC.
`Petitioner
`v.
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`WESTERNGECO LLC
`Patent Owner
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`CASE IPR: Unassigned
`Patent 7,162,520 B2
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`DECLARATION OF DR. BRIAN EVANS, PhD.
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`TABLE OF CONTENTS
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`I.
`II.
`III.
`IV.
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`V.
`VI.
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`INTRODUCTION ...............................................................................................................1
`QUALIFICATIONS ............................................................................................................2
`COMPENSATION AND RELATIONSHIP TO THE PARTIES .......................................7
`LEGAL STANDARDS .......................................................................................................8
`A.
`Claim Construction ..................................................................................................8
`B.
`Anticipation..............................................................................................................8
`C.
`Obviousness .............................................................................................................9
`D.
`Person of Ordinary Skill in the Art ..........................................................................9
`SUMMARY OF OPINION ...............................................................................................10
`TECHNICAL BACKGROUND ........................................................................................11
`A.
`Overview of Marine Seismic Surveying ................................................................11
`1.
`Marine Seismic Surveying Equipment ......................................................11
`2.
`Collecting Data In a Marine Seismic Survey .............................................13
`3.
`Planning a Marine Seismic Survey ............................................................16
`Streamer Steering Overview ..................................................................................19
`1.
`Problems Encountered in Marine Seismic Data Acquisition .....................19
`a.
`In-filling .........................................................................................19
`b.
`Irregular Spatial Sampling .............................................................20
`c.
`Streamer Tangling ..........................................................................23
`d.
`Turning ...........................................................................................23
`e.
`Streamer Noise ...............................................................................28
`Keeping Streamers Straight and Parallel is the Optimal Solution to
`Marine Seismic Data Acquisition Problems ..............................................32
`Streamer Steering Can be Used to Keep Streamers Straight and
`Parallel .......................................................................................................33
`Streamer Positioning Devices to Implement Streamer Steering ................35
`Control Systems for Streamer Positioning Devices ...................................36
`Computerized Control Systems to Control Streamer Positioning
`Devices .......................................................................................................40
`Advances in Steamer Positioning in the 1980s and 1990s ....................................44
`1.
`Rouquette, U.S. Patent No. 5,200,930 .......................................................48
`2.
`Elholm, U.S. Patent No. 5,532,975 ............................................................49
`3.
`The ‘636 PCT, WO 98/28636 ....................................................................51
`4.
`Dolengowski, U.S. Patent No. 4,890,568 ..................................................53
`5.
`Workman, U.S. Patent No. 5,790,472........................................................56
`6.
`Bertheas, PCT/FR97/00263 & U.S. Patent No. 6,144,342 ........................58
`Four Dimensional Seismic Surveying ...................................................................61
`D.
`VII. THE ’520 PATENT ...........................................................................................................66
`A.
`Brief Description of the Relevant File History ......................................................66
`B.
`Relevant Time Frame for Analysis of the ’520 Patent ..........................................67
`C.
`The Specification of the ’520 Patent ......................................................................67
`D.
`Admitted Prior Art in the ’520 Patent ....................................................................69
`VIII. DETAILED OPINION ......................................................................................................71
`A.
`The Challenged Claims of the ’520 Patent ............................................................71
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`B.
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`C.
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`2.
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`3.
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`4.
`5.
`6.
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`b.
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`Apparatus Claims: Claims 18-20 and 22-34 ..............................................71
`1.
`Method Claims: Claims 1-3 and 5-17 ........................................................73
`2.
`Construction of Relevant Claim Terms .................................................................75
`1.
`“streamer positioning device” ....................................................................76
`2.
`“control system” .........................................................................................77
`3.
`“feather angle mode” .................................................................................78
`4.
`“turn control mode” ...................................................................................81
`5.
`“streamer separation mode” .......................................................................83
`6.
`“attempting to maximize distance between adjacent streamers” ...............89
`7.
`“a control system configured to use a control mode selected from a
`feather angle mode, a turn control mode, a streamer separation
`mode, and two or more of these modes” (Claim 18) .................................90
`The Feather Angle Mode Claims (Claims 3, 5, 20, and 22) of the ’520
`Patent are Obvious over Workman ........................................................................93
`1.
`Claims 18 and 19 .......................................................................................94
`“An apparatus comprising: (a) an array of streamers each
`a.
`having a plurality of streamer positioning devices there
`along;” ............................................................................................94
`“(b) a control system configured to use a control mode”
`“wherein the control mode is the feather angle mode, and
`the controlling comprises the control system attempting to
`keep each streamer in a straight line offset from a towing
`direction by a feather angle” ..........................................................96
`1)
`Workman Discloses a Control System ..............................97
`2)
`It would have been obvious to configure
`Workman’s control system to use a feather angle
`mode that “comprises the control system attempting
`to keep each streamer in a straight line offset from
`the towing direction by a feather angle” ............................98
`Motivation 1: Maintaining Data Quality.................................................104
`Motivation 2: Avoiding Obstacles ..........................................................110
`Motivation 3: 4D Surveying ...................................................................112
`Implementation of Feather Angle Modes By Using or Adapting
`Workman..................................................................................................113
`Claim 20 ...................................................................................................119
`6.
`Claim 22 ...................................................................................................121
`7.
`Claims 3 and 5 .........................................................................................123
`8.
`The Feather Angle Mode Claims (Claims 1, 2, 3, 5, 18, 19, 20, and 22) are
`Obvious Over Workman in View of Bertheas. ....................................................124
`1.
`Claims 18 and 19 .....................................................................................125
`2.
`Claims 20 and 22 .....................................................................................133
`3.
`Claims 1, 2, 3, and 5 ................................................................................134
`The Lateral Streamer Separation Mode Claims (Claims 13-14 and 30-31)
`are Anticipated By Workman ..............................................................................134
`Workman Discloses a Control System Configured to Use a
`1.
`Streamer Separation Mode. ......................................................................135
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`2.
`3.
`4.
`5.
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`B.
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`C.
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`D.
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`Workman’s Control System “Attempts to Minimize the Risk of
`Entanglement of the Streamers” and Anticipates Claims 13 and 30. ......140
`Workman’s Control System “Attempts to Maximize the Distance
`Between Adjacent Streamers” and Anticipates or Renders Obvious
`Claims 14 and 31. ....................................................................................143
`The Depth and Lateral Streamer Separation Mode Claims (Claims 15-17
`and 32-34) Are Obvious over Workman in view of Dolengowski. .....................146
`1.
`Claim 32 ...................................................................................................146
`2.
`Claims 33 and 34 .....................................................................................151
`3.
`Claims 15-17 ............................................................................................153
`The Turn Control Mode Claims (Claims 7-12 and 24-29) Are Obvious
`over the ’636 PCT in view of the ’153 PCT. .......................................................153
`1.
`Claim 23 ...................................................................................................154
`“An array of streamers each having a plurality of streamer
`a.
`positioning devices there along” ..................................................156
`“A Control System Configured to Use . . . a Turn Control
`Mode” ..........................................................................................158
`1)
`Control Systems of the ’636 PCT ....................................158
`2)
`Turn Control Mode ..........................................................161
`Claim 24 .......................................................................................170
`c.
`Claim 25 .......................................................................................171
`d.
`Claim 26 .......................................................................................172
`e.
`Claim 27 .......................................................................................173
`f.
`Claim 28 .......................................................................................175
`g.
`Claim 29 .......................................................................................176
`h.
`Claims 7-12 ..............................................................................................178
`2.
`The Turn Control Mode Claims (Claims 6-12 and 23-29) Are Obvious
`over Workman in view of the ’153 PCT. .............................................................179
`CONCLUSION ................................................................................................................181
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`F.
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`G.
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`H.
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`2.
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`3.
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`b.
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`IX.
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`I, Dr. Brian Evans, hereby state the following:
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`I.
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`INTRODUCTION
`1.
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`I have been retained by Petroleum Geo-Services, Inc. (“PGS”) to
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`provide technical assistance related to the filing of a Petition for Inter Partes
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`Review of U.S. Patent No. 7,293,520 B2 (“the ’520 Patent”) (Ex. 1001). I am
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`working as a private consultant on this matter and the opinions presented here are
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`my own.
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`2.
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`I have been asked to prepare a written report, including comments
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`related to whether certain claims of the ’520 Patent are unpatentable because they
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`are anticipated or would have been obvious to one of ordinary skill in view of the
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`prior art. I have reviewed the documents set forth in the attached Appendix of
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`Exhibits and relied on my decades of knowledge and experience in the field of
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`seismic marine surveys (detailed in Section II) in reaching my opinions regarding
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`validity. This report sets forth the bases and reasons for my opinions, including the
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`additional materials and information relied upon in forming those opinions and
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`conclusions.
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`3.
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`This report is based on information currently available to me. I reserve
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`the right to continue my investigation and analysis, which may include a review of
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`documents and information not yet produced. I further reserve the right to expand
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`or otherwise modify my opinions and conclusions as my investigation and study
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`continues, and to supplement my opinions and conclusions in response to any
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`additional information that becomes available to me.
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`II. QUALIFICATIONS
`4.
`I am a Professor of Geophysics in the Department of Petroleum
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`Engineering at Curtin University located in Bentley, Western Australia. I have
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`worked continuously in the field of marine seismic surveying for over 44 years,
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`since the 1970s. I have been involved in the design of dozens of marine seismic
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`surveys, and have been onboard seismic vessels as they were conducting a marine
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`seismic survey over one-hundred times.
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`5.
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`I authored a textbook devoted to marine seismic surveying and data
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`acquisition, entitled “A Handbook for Seismic Data Acquisition in Exploration.” I
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`began writing the textbook in 1985 for use in my “Seismic Acquisition” class, and
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`continued to update it over the years. It was first published in 1997 by the Society
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`of Exploration Geophysicists (SEG), the premier international organization for
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`seismic professionals and researchers, including marine seismic professionals. At
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`the time of its publication, it was considered the authoritative textbook in the field
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`of seismic data acquisition. Over the past 15 years, it has been used throughout the
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`world in seismic surveying courses and on seismic survey vessels.
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`6.
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`I obtained my Diploma of Electrical Engineering, the equivalent of a
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`bachelor’s degree, at the J.M. University of Liverpool in the United Kingdom in
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`1969. I took my first job in the marine seismic industry in 1971, working as an
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`instrument engineer for Geophysical Service, Inc. In that role, I monitored and
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`repaired the seismic recording and navigation instruments, including the equipment
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`that positioned marine seismic streamers and source arrays. As a qualified
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`electrical engineer, I also repaired electronic equipment on seismic vessels,
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`including on-board computers, and navigation/positioning systems. While with
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`Geophysical Services, Inc., I traveled the world working offshore West Africa,
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`South America, India, Vietnam, the Persian Gulf, Indonesia, the Philippines, the
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`South China Sea, and the Gulf of Thailand—all offshore oil exploration areas.
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`7.
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`After leaving Geophysical Service, Inc. in 1974, I joined Aquatronics,
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`a London-based seismic company, where I managed seismic survey ships used in
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`seismic surveys. In 1975, I joined Southern Geophysical Consultants of London as
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`a Seismic Acquisition and Surveying Consultant. In that capacity, I represented
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`many oil companies while onboard seismic survey ships to ensure the quality of
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`the acquired seismic data and that the seismic data was within the oil company’s
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`specifications. I was also involved in deep water operations and rig relocations for
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`different oil companies during my time at Aquatronics.
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`8.
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`In 1976, I established my own seismic-acquisition consulting
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`company in Perth, Australia, called “Offshore-Onshore Exploration Consultants
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`PTY LTD.” As an independent consultant, I participated in seismic surveys on
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`behalf of my oil company clients to ensure the quality of the seismic data acquired
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`and that the seismic data was within the oil company’s specifications. My
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`consulting company, which employed four other employees, was the only
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`company that did this type of work in Southeast Asia at the time. From 1980 to
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`1983, while at the peak of my consultancy operations, I also worked at Shell
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`Development Australia in Perth, Australia, as a Senior Operations Geophysicist.
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`My responsibilities at Shell Development included managing three marine-
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`seismic-survey ships and two land-seismic-survey crews.
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`9.
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`In 1983, I enrolled at Curtin University (known then as West
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`Australian Institute of Technology). From 1983 to 1985, as part of a Masters
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`program in Applied Physics, I wrote a thesis entitled “The Establishment of a
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`Digital Seismic Acquisition System and its Subsequent Application in the Field.” I
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`also designed and built a seismic recording system.
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`10. After receiving my Masters in Applied Physics in 1985, I enrolled in a
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`Geophysics Ph.D. program at Curtin University, focusing on 3D Seismic
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`Surveying Data Processing. As part of the Ph.D program, I taught seismic
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`acquisition, processing, and interpretation and lectured short-courses for industry
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`(including marine seismic companies) on conventional and 3D seismic acquisition
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`methods. While working on my Ph.D, I continued to consult on marine seismic
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`data acquisition. I also established the Department of Exploration Geophysics at
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`Curtin University. In 1997, I completed my Ph.D. program, and produced a Ph.D
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`thesis titled, “Advancements in the Techniques of Low-fold Three Dimensional
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`Seismic Reflection Surveying.”
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`11. After completing my Ph.D. in Geophysics in 1997, I continued to
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`teach seismic data acquisition, processing, and interpretation as an Associate
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`Professor at Curtin University. I also continued to teach short-courses to the
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`industry on marine seismic data acquisition. Over the years, I have supervised
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`twenty Master’s and Ph.D. students, many of whom have written theses pertinent
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`to the marine seismic industry. I continue to supervise four Ph.D. students today.
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`12.
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`I became a tenured Professor of Geophysics in 2002. I served as
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`Chair of the Department of Petroleum Engineering from 2007 to 2012. I then
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`became the Director of Curtin University’s Faculty of Science and Engineering’s
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`Oil and Gas Training and Research Project Initiatives in 2013. In that role, I
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`establish research projects with industry, establish teams to run projects, and
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`consult with industry and the research staff to ensure the projects stay on track.
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`13. Much of my research over the years has involved numerical and
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`physical modeling of the seismic data acquisition process, including in the context
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`of 3D and 4D seismic marine surveys. This has entailed both field and laboratory
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`research, in which I would frequently work onboard seismic survey ships during
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`marine seismic surveys and later attempt to improve on marine seismic data
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`acquisition techniques by testing in the laboratory. Building on my research to
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`optimize 3D and 4D data acquisition, I have built three seismic physical
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`acquisition simulation labs in Houston, Dhahran, and Rio de Janeiro. These labs
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`involved the use of physical models to simulate 3D marine seismic surveys. The
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`Houston lab was built in 1991 and later was moved and reconstructed at Curtin
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`University; the other labs were built in 2005 and are presently operated in Dhahran
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`and Rio de Janeiro. All of these labs are still in use today. I have also developed a
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`seismic numerical modeling lab at Curtin University, and a landmark seismic
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`interpretation lab, which oil companies use to train their employees and to interpret
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`3D marine seismic data.
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`14. Throughout the 1990s and 2000s, and through the present,, I have
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`continued to consult in the marine seismic survey field while working at Curtin
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`University. I have consulted with various marine seismic survey companies as part
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`of my job representing oil companies and in my independent consulting company.
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`In this role, I am typically asked to evaluate seismic survey plans and to advise
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`companies on their plans’ suitability for an optimal survey. This often requires me
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`to determine whether the seismic data acquisition and processing plans are
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`adequate to produce quality seismic data considering the survey area’s 3D geology.
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`To fulfill this role, I closely follow the literature and other available information
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`regarding the latest marine seismic acquisition technologies. I continue to do this
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`consulting work to this day. I have also consulted on a wide range of other issues
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`relating to marine seismic data acquisition, processing, and interpretation. For
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`instance, I have had an Independent Advisory Group since 2004 to review and
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`evaluate oil companies’ seismic data, drilling plans and proposed operations.
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`15.
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`I am currently a member of several professional organizations related
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`to the marine seismic industry, and the oil and gas industry in general. I have been
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`a member of the Australian Society of Exploration Geophysics since 1983 and the
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`Society of Exploration Geophysicists (“SEG”)—widely recognized as the principal
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`international society in the field—since 1993. I was President of the Australian
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`state chapter of the SEG twice, in 1986 and 1993. In addition to SEG, I have also
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`been a member of the Society of Petroleum Engineers (SPE) since 1994 and the
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`Petroleum Club of Western Australia since 2009, of which I am currently a Board
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`Member. From 2006 to 2012, I was a Board Member and Education Scholarship
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`Committee Chair of the West Australian State Government Minerals and Energy
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`Research Institute (MERIWA).
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`III. COMPENSATION AND RELATIONSHIP TO THE PARTIES
`16.
`I am being compensated at an hourly rate of three hundred and fifty
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`dollars ($350), plus expenses, for the time I spend in Australia studying materials
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`associated with this matter and providing testimony, and six hundred twenty five
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`euros (€625) for the time I spend on this matter outside Australia. This is my
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`standard consulting rate. I am an independent party and my compensation is not
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`contingent upon the outcome of this matter.
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`17.
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`It is my understanding that WesternGeco L.L.C. (“WesternGeco”), is
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`listed as the assignee of the ’520 Patent. Prior to this matter, I have not been
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`employed or retained by WesternGeco or PGS. I own no stock in WesternGeco or
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`PGS, and am aware of no other financial interest I have with those companies.
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`IV. LEGAL STANDARDS
`18. Although I am not an attorney and do not expect to offer any opinions
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`regarding the law, I have been informed of certain legal principles relating to
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`standards of patentability that I relied on in forming the opinions set forth in this
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`report.
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`A. Claim Construction
`19.
`I understand that for purposes of this matter, the terms in the ‘520
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`Patent claims are to be given their broadest reasonable interpretation in light of the
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`specification, as understood by one of ordinary skill in the art as of the priority date
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`of the ’520 Patent.
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`B. Anticipation
`20.
`I understand that for a claim to be anticipated, a single prior art
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`reference must disclose to a person of ordinary skill in the art, either expressly or
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`inherently, each and every limitation set forth in the claim. I understand that
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`claims are unpatentable if they are anticipated by the prior art.
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`C. Obviousness
`21.
`I understand that even if a claim is not anticipated, an invention that
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`would have been obvious to a person of ordinary skill at the time of the invention
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`is not patentable. I understand that obviousness is determined by considering
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`several factors, including: the state of the art at the time the invention was made;
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`the level of ordinary skill in the art; differences between what is described in the
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`art and the claims at issue; and objective evidence of nonobviousness (such as
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`commercial success, long-felt but unsolved needs, failure of others, and
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`unexpected results). I understand that claims are unpatentable if they would have
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`been obvious in view of the prior art.
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`D.
`22.
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`Person of Ordinary Skill in the Art
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`I have been informed that a person of ordinary skill in the art is a
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`hypothetical person who is presumed to have known all of the relevant art at the
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`time of the invention. I have been informed that a person of ordinary skill in the
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`art may possess the education, skills, and experience of multiple actual people who
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`would work together as a team to solve a problem in the field. I have been
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`informed that factors that may be considered in determining the level of ordinary
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`skill in the art may include: (1) the educational level of the inventor; (2) type of
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`problems encountered in the art; (3) prior art solutions to those problems; (4)
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`rapidity with which innovations are made; (5) sophistication of the technology; and
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`(6) educational level of active workers in the field.
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`23.
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` On the basis of my consideration of these factors and my
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`experience in solving problems in the area of marine seismic surveys for decades,
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`including my familiarity with the education, expertise, and experience of the teams
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`that devise solutions to those problems, I have been asked to opine as to the person
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`of ordinary skill in the art to which Claims 1-3, 5-20, and 22-34 of the ’520 Patent
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`are directed. In my opinion, such a person of ordinary skill in the art should have a
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`Master’s degree or Ph.D. in ocean engineering, mechanical engineering,
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`geophysics, applied physics, or a related area, who has preferably taken
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`coursework in hydrodynamics, advanced control systems, and other related fields.
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`Additionally, the person should have at least three years of experience designing
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`and/or operating seismic surveys, as well as significant experience aboard marine
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`seismic survey vessels during the course of several marine seismic surveys.
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`V.
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`SUMMARY OF OPINION
`24.
` It is my understanding that PGS (or “Petitioner”) requests Inter
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`Partes review of Claims 1-3, 5-20, and 22-34 of the ’520 Patent, titled “Control
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`System for Positioning of a Marine Seismic Streamers [sic],” which was issued to
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`Oyvind Hillesund and Simon Hastings Bittleston on November 13, 2007, and has
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`been assigned to WesternGeco. It is my opinion that all of Claims 1-3, 5-20, and
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`22-34 would have been well-known and obvious to a person of ordinary skill at the
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`time of the October 1, 1998 priority date.
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`VI. TECHNICAL BACKGROUND
`A. Overview of Marine Seismic Surveying
`25. The ’520 Patent is directed to marine seismic surveying technology.
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`Marine seismic surveys use reflected sound waves to determine geological
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`properties of the earth’s subsurface. Seismic surveying ships (also known as
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`vessels) tow equipment referred to in the industry as “seismic sources” or “guns”
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`to create small, controlled explosions underwater. The explosions generate
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`acoustic sound waves that travel down through the water, penetrate the ocean floor,
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`reflect off geological formations in the earth’s subsurface, and travel back towards
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`the seismic vessel. The reflected acoustic signals are recorded by seismic receivers
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`known as “hydrophones,” which are towed behind the vessel in long cables called
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`marine seismic “streamers.” Because recorded sound waves have different
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`properties depending on the geology of the ocean’s subsurface, the acoustic signals
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`recorded by the hydrophones provide information regarding characteristics of the
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`ocean’s subsurface, including evidence about the existence of oil and gas.
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`1. Marine Seismic Surveying Equipment
` In modern marine seismic surveys, a towing vessel will typically tow
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`26.
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`a plurality of streamers in a large areal spread known as an “array.” Each streamer
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`in the array contains groups of hydrophones located at pre-determined intervals
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`along the streamer. The acoustic data acquired by each hydrophone group is
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`recorded as a function of time and provides information about a two-dimensional
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`vertical slice of the earth’s surface below the area traversed by the streamer. By
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`towing a plurality of streamers, the seismic surveyor covers a large area and is able
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`to record reflected seismic signals at several locations simultaneously. This
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`technique results in seismic data from various locations that can be combined and
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`processed by computers to construct a three-dimensional image of the earth’s
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`subsurface.
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`27. Below is a graphical depiction of a modern marine seismic survey
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`system:
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`28. This figure depicts a survey vessel towing four streamers, each of
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`which contains hydrophones to record seismic data that reflects off the ocean’s
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`subsurface, and one air gun array (the acoustic source). This multiple-streamer
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`12
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`Ex. PGS 1002
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`

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`seismic surveying system became commonplace beginning in the late 1980s. See
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`Ex. 1038 (Brian J. Evans, A Handbook for Seismic Data Acquisition in
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`Exploration (David V. Fitterman & William H. Dragoset, Jr. eds., 1997)) (“Evans
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`Treatise”) at 250.
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`29. The figure depicted above shows all four streamers in a “straight-and-
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`parallel” configuration, meaning that the streamers are: (1) linear, and (2) parallel
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`to each other. In practice, maintaining the configuration depicted in the figure may
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`be complicated by a number of factors. For example, if there is a cross-current, the
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`streamers may drift and become offset from the towing direction of the vessel.
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`This phenomenon is known as “feathering.” The extent to which the streamers
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`become offset from the towing direction of the vessel is known as the “feather
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`angle.” The following illustration depicts an array in a straight-and-parallel
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`configuration with a slight feather angle.
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`2.
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`Collecting Data In a Marine Seismic Survey
`13
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`Ex. PGS 1002
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`30. Seismic data are recorded on a shot-by-shot basis. In a typical marine
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`seismic survey, the vessel will travel at approximately five nautical miles per hour
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`(5 knots) and fire a shot from one or more seismic sources approximately every ten
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`seconds. This is recognized as an ideal speed for a marine seismic survey. The
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`data recorded by each hydrophone group for each seismic shot is known as a
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`“trace.” With each “shot,” the seismic source emits acoustic signals (i.e., sound
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`waves) that are reflected at different points on the ocean’s subsurface. These
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`signals are received by the various hydrophones on the towed streamers, as
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`depicted below:
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`See Ex. 1038 (Evans Treatise) at 9.
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`31. As depicted above, when a shot is fired in a marine seismic survey,
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`the emanating acoustic signals travel in all directions, including downward. Ex.
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`1038 (Evans Treatise) at 28. When each acoustic signal reflects off the ocean’s
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`subsurface to a hydrophone, the point on the subsurface where it is reflected is
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`14
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`Ex. PGS 1002
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`halfway (the midpoint) between the air gun (i.e., the “source position”) and the
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`hydrophone. Id. The graphic depicts each hydrophone recording a seismic trace
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`from a different midpoint, because for each acoustic shot, the midpoint between
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`the air gun and the hydrophone is generally different for each hydrophone. Not
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`depicted in the figure above are the sound waves that continue to propagate upward
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`and reflect off the sea-air interface. This phenomenon, known as “ghosting,”
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`reduces data quality. Marine seismic surveyors account for ghosting by trying to
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`maintain the hydrophones at a common, pre-selected depth, which makes it easier
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`to discard unwanted “ghost” signals during data processing. See Ex. 1038 (Evans
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`Treatise) at 149-52; see also Ex. 1006 (International PCT Application No. WO
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`98/28636) (“’636 PCT”) at 1 (“The streamers are typically towed at a constant
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`depth of about ten metres, in order to facilitate the removal of undesired ‘ghost’
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`reflec