(12)
`
`United States Patent
`Vaage
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,906,981 B2
`Jun. 14, 2005
`
`US006906981B2
`
`(54) METHOD AND SYSTEM FOR ACQUIRING
`MARINE SEISMIC DATA USING MULTIPLE
`SEISMIC SOURCES
`
`_
`-
`-
`(75) Inventor. Sveln Torlelf Vaage, Houston, TX (US)
`
`5,781,503 A
`7/1998 Kim
`5,793,702 A
`8/1998 PaffeIlhOlZ
`5,924,049 A
`7/1999 Beasley et al.
`5,940,788 A
`8/1999 Morman et al.
`6,021,379 A
`2/2000 Duren et al.
`6,151,556 A “0000 Allen
`
`.
`.
`(73) Ass1gnee: PGS Americas, Inc., Houston, TX (US)
`
`6,178,381 B1
`6,208,587 B1
`
`1/2001 P dh' t
`1.
`30001 Janine a
`
`( >1 ) Notice:
`
`Subject‘ to any disclaimer, the term of this
`patent is extended or adJusted under 35
`U.S.C. 154(1)) by 254 days.
`
`6,317,695 B1
`6,381,544 B1
`
`x311; a1_
`110001 Zhou et aL
`4/2002 Sallas et al.
`
`(21) Appl. N0.: 10/197,235
`(22) Filed.
`Jul_ 17 2002
`'
`’
`Prior Publication Data
`
`(65)
`
`* Cited by examiner
`Primary Examiner—Nelson MoskoWitZ
`(74) Attorney, Agent, or Firm—E. Eugene Thigpen;
`Richard A. Fagin
`
`US 2004/0013037 A1 Jan. 22, 2004
`
`(57)
`
`ABSTRACT
`
`(51) Int. Cl.7 ........................... .. GOIV 1/38; H04R 1/02
`(52) U-S- Cl- --------------------------- -- 367/21; 367/20; 367/23;
`367/57; 702/14; 181/110
`(58) Field of Search ............................ .. 702/14; 367/23,
`367/53, 56, 57, 20—21, 59, 142; 181/110,
`111
`
`(56)
`
`_
`References Clted
`
`U.S. PATENT DOCUMENTS
`
`3,351,899 A * 11/1967 Luhrmann et al. ........ .. 367/142
`4,300,653 A * 11/1981 Cao etal. ................. .. 181/111
`4,800,538 A * 1/1989 Passmore et al. ........... .. 367/55
`4,953,657 A
`9/1990 Edington
`4,955,952 A * 9/1990 Williams et al. .......... .. 181/111
`5,184,329 A * 2/1993 Regnault et al.
`5,281,773 A * 1/1994 Duren ...................... .. 181/111
`5,724,306 A
`3/1998 Barr
`
`A method for Seismic Surveying is disclosed Which includes
`toWing a ?rst seismic energy source and at least one seismic
`sensor system. A second seismic energy source is toWed at
`a selected distance from the ?rst source. The ?rst seismic
`energy source and the second seismic energy source are
`actuated in a plurality of ?ring sequences. Each of the ?ring
`sequences includes ?ring of the ?rst source, Waiting a
`selected time ?ring the second source and recording signals
`generated by the seismic sensor system. The selected time
`betWeen ?ring the ?rst source and the second source is
`varied betWeen successive ones of the ?ring sequences. The
`?ring times of the ?rst and second source are indexed so as
`to enable separate identi?cation of seismic events originat
`ing from the ?rst source and seismic events originating from
`the second source in detected seismic signals.
`
`38 Claims, 14 Drawing Sheets
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`5A1
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`2.
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`WesternGeco Ex. 1001, pg. 1
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 1 0f 14
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`US 6,906,981 B2
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`5 1/
`m
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`SEV /
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`8A1
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`23
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`2°
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`SA2
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`$81
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`I /5
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`FIG 1
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`$82
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`WesternGeco Ex. 1001, pg. 2
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 2 0f 14
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`US 6,906,981 B2
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`26
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`X.)*- 5000. 0 z
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`22
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`20
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`200
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`400
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`600
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`600
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`1000
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`1.200
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`FIG 2
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`RAY PATHS FOR SOURCE
`TOWED BY SEISMIC VESSEL
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`WesternGeco Ex. 1001, pg. 3
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 3 0f 14
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`26
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`22
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`my: 5000. 0
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`x.y: 5000. 1000
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`EuPur: 5 "Ga
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`FIG 3
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`RAY PATHS FOR SOURCE
`TOWED BY SOURCE VESSEL
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`WesternGeco Ex. 1001, pg. 4
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 4 0f 14
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`US 6,906,981 B2
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`l - Ill
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`.14].
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`[-201
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`1 .221.
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`FIG 4
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`SOURCE
`A
`B
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`FIRING TIME
`0
`0.1 SEC
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`WesternGeco Ex. 1001, pg. 5
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 5 0f 14
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`US 6,906,981 B2
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`WesternGeco Ex. 1001, pg. 6
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`Jun. 14,2005
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`Sheet 6 6f 14
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`US 6,906,981 B2
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`SHUT -TRCE
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`TIHE (SECS)
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`“ZOO
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`SOURCE
`A
`B
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`FIRING TIME
`0
`0.4 SEC
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`WesternGeco Ex. 1001, pg. 7
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`

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`Jun. 14, 2005
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`Sheet 7 0f 14
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`US 6,906,981 B2
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`SHOT JRCE
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`TIME I SECS]
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`SOURCE
`A
`B
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`FIRING TIME
`0
`0.2 SEC
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`WesternGeco Ex. 1001, pg. 8
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`Jun. 14,2005
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`Sheet 8 0f 14
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`US 6,906,981 B2
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`SHOT JRCE
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`TIHE (SECS)
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`FIG 8
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`SOURCE
`A
`B '
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`FIRING TIME
`O
`0.5 SEC
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`WesternGeco Ex. 1001, pg. 9
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`

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`Jun. 14,2005
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`Sheet 9 0f 14
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`SNOT-TRCE
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`TIME l?ECSI
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`SOURCE
`A
`B
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`FIRING TIME & DELAY
`0.4 SEC
`0.1
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`WesternGeco Ex. 1001, pg. 10
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 10 0f 14
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`US 6,906,981 B2
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`SHOT-TRCE
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`Tl?E [SECS]
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`h‘) A} N N M N N N N N N N N N . - - - - - - . ¢ 0 - I - .
`
`I") N N N N NI N . . . - . . .
`
`IQ N N N N N N
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`FIG 10
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`SOURCE
`A
`B
`
`FIRING TIME & DELAY
`042 SEC
`0.5
`
`WesternGeco Ex. 1001, pg. 11
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 11 0f 14
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`US 6,906,981 B2
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`SHIN-TREE
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`TIME (SECSI
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`3.1
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`3.11
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`3-21
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`3.31
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`3-51
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`3-311
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`FIG 11
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`SOURCE
`A
`B
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`FIRING TIME & DELAY
`0.1 SEC
`0.5
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`WesternGeco Ex. 1001, pg. 12
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 12 0f 14
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`US 6,906,981 B2
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`SHOT-TREE
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`TIME (SECSI
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`/'
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`002
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`on
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`FIG 12
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`SOURCE
`A
`B
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`FIRING TIME & DELAY
`0.3 SEC
`0.5
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`WesternGeco Ex. 1001, pg. 13
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 13 0f 14
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`US 6,906,981 B2
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`SHOT-TRCE
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`‘fl-‘1E (SECS)
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`.llll
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`FIG13
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`SOURCE
`A
`B
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`FIRING TIME & DELAY
`O
`0.5 SEC
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`WesternGeco Ex. 1001, pg. 14
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`

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`U.S. Patent
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`Jun. 14,2005
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`Sheet 14 0f 14
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`US 6,906,981 B2
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`SA
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`SB
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`CN
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`RN
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`COHERENT COI-IERENT COHERENT RANDOM
`
`TRACE
`(SINGLE SHOT)
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`CMPA
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`COHERENT RANDOM RANDOM RANDOM
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`CMP B
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`RANDOM COI-IERENT RANDOM RANDOM
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`FIG 14
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`WesternGeco Ex. 1001, pg. 15
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`

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`US 6,906,981 B2
`
`1
`METHOD AND SYSTEM FOR ACQUIRING
`MARINE SEISMIC DATA USING MULTIPLE
`SEISMIC SOURCES
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`Not applicable.
`
`STATEMENT REGARDING FEDERALLY
`SPONSORED RESEARCH OR DEVELOPMENT
`Not applicable.
`
`BACKGROUND OF INVENTION
`
`15
`
`1. Field of the Invention
`The invention relates generally to the ?eld of seismic
`exploration. More particularly, the invention relates to meth
`ods for acquiring marine seismic data using selected
`arrangements of sources and receivers.
`2. Background Art
`Seismic surveying is knoWn in the art for determining
`structures of rock formations beloW the earth’s surface.
`Seismic surveying generally includes deploying an array of
`seismic sensors at the surface of the earth in a selected
`pattern, and selectively actuating a seismic energy source
`positioned near the seismic sensors. The energy source may
`be an explosive, a vibrator, or in the case of seismic
`surveying performed in the ocean, one or more air guns or
`Water guns.
`Seismic energy Which emanates from the source travels
`through the earth formations until it reaches an acoustic
`impedance boundary in the formations. Acoustic impedance
`boundaries typically occur Where the composition and/or
`mechanical properties of the earth formation change. Such
`boundaries are typically referred to as “bed boundaries”. At
`an acoustic impedance boundary, some of the seismic energy
`is re?ected back toWard the earth’s surface, Where it may be
`detected by one or more of the seismic sensors deployed on
`the surface. Other portions of the energy are refracted and
`continue propagating in a generally doWnWard direction
`until another impedance boundary is reached. Seismic signal
`processing knoWn in the art has as an objective the deter
`mination of the depths and geographic locations of bed
`boundaries beloW the earth’s surface. The depth and location
`of the bed boundaries is inferred from the travel time of the
`seismic energy to the acoustic impedance boundaries and
`back to the sensors at the surface.
`Seismic surveying (marine seismic surveying) is per
`formed in the ocean to determine the structure of earth
`formations beloW the sea bed. Marine seismic surveying
`knoWn in the art includes having a vessel toW one or more
`seismic energy sources, and the same or a different vessel
`toW one or more “streamers”, Which are arrays of seismic
`sensors forming part of or otherWise af?Xed to a cable.
`Typically, a seismic vessel Will toW a plurality of such
`streamers arranged to be separated by a selected lateral
`distance from each other, in a pattern selected to enable
`relatively complete determination of geologic structures in
`three dimensions.
`The signals detected by the seismic sensors at the earth’s
`surface include components of seismic energy re?ected at
`the bed boundaries, as previously explained. In addition,
`both coherent noise (noise Which has a determinable pattern,
`such as may be caused by a ship propeller) and incoherent
`(random) noise may be present. The presence of such noise
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`in the signals received by the seismic sensors reduces the
`signal-to-noise ratio (“SNR”) of the seismic signals of
`interest. An objective of seismologists, therefore, is to seek
`methods of eliminating the effects of noise on the signals
`detected by the sensors Without appreciably reducing the
`true seismic signal component of the detected signals.
`Prior art methods Which have been used to reduce the
`effects of noise and acquire a higher quality seismic repre
`sentation of a particular subsurface structure include using
`multiple actuations of the seismic source (multiple “?rings”
`or “shots”) to record a plurality of sensor measurements
`from substantially the same subsurface structure, and then
`summing or “stacking” such measurements to enhance sig
`nal strength While substantially reducing the effects of
`random or incoherent noise.
`US. Pat. No. 5,818,795, Which is assigned to the assignee
`of the present invention provides a detailed summary of
`prior art methods and systems addressing the problem of
`noise elimination in seismic signals, and discloses a method
`of reducing the effect of “burst” noise in seismic signal
`recordings Without eliminating actual re?ection data.
`US. Pat. No. 5,761,152, Which is assigned to the assignee
`of the present invention describes a method and system for
`marine seismic surveying. The method disclosed in the ’152
`patent includes increasing the fold (number of recorded
`re?ections from a same re?ector), and hence the signal-to
`noise ratio of seismic signals, Without incurring the prob
`lems of drag, entanglement, complicated deck handling, and
`decreased signal-to-noise ratio associated With increased
`streamer length, increased number of streamers, and
`increased distance betWeen streamers. Source and streamer
`“offsets”, and time of ?ring of lead and trailing vessel
`sources in a time delay sequence are optimiZed to increase
`the fold While avoiding any in?uence by the seismic signals
`resulting from the source of one vessel on the seismic signals
`resulting from the source of the other vessel.
`
`SUMMARY OF INVENTION
`
`One aspect of the invention is a marine seismic acquisi
`tion system adapted to substantially remove the effects of
`noise from recorded seismic signals of interest. The system
`includes a seismic recording vessel adapted to toW a ?rst
`seismic source and to toW at least one seismic sensor system.
`The system includes a seismic source vessel adapted to track
`the seismic recording vessel and to toW a second seismic
`source at a selected distance from the ?rst source. The
`system includes a controller adapted to ?re the ?rst source,
`Wait a selected time, and to ?re the second source in
`predetermined ?ring sequences. The selected time betWeen
`the ?ring of ?rst source and the second source in each ?ring
`sequence varies from ?ring sequence to ?ring sequence. The
`?ring times of the ?rst and second source are indeXed so as
`to enable separate identi?cation of seismic events originat
`ing from the ?rst source and seismic events originating from
`the second source in detected seismic signals.
`In some embodiments, the selected time may be varied by
`about 100 milliseconds or more betWeen ?ring sequences. In
`other embodiments, the selected time may be varied in one
`of a systematic manner, a quasi-random manner, and a
`random manner. In some embodiments, the selected time is
`selected to be at least as long as a Wavelet time of the ?rst
`source.
`Another aspect of the invention is a method for seismic
`surveying. The method includes toWing a ?rst seismic
`energy source and at least one seismic sensor system. A
`second seismic energy source is toWed at a selected distance
`
`WesternGeco Ex. 1001, pg. 16
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`US 6,906,981 B2
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`3
`from the ?rst seismic energy source. The ?rst seismic energy
`source and the second seismic energy source are sequen
`tially actuated in a plurality of ?ring sequences. A time
`interval betWeen ?ring the ?rst source and the second source
`is varied betWeen successive ones of the ?ring sequences.
`The ?ring times of the ?rst and second source are indexed
`so as to enable separate identi?cation of seismic events
`originating from the ?rst source and seismic events origi
`nating from the second source in detected seismic signals.
`Another aspect of the invention is a method for determin
`ing signal components attributable to a ?rst seismic energy
`source and to a second seismic energy source in signals
`recorded from a seismic sensor array. The ?rst source and
`then the second source are ?red in a plurality of ?ring
`sequences. A delay betWeen ?ring the ?rst source and the
`second source in each ?ring sequence selected to be different
`than the corresponding delays in other ?ring sequences. The
`method according to this aspect comprises determining a
`?rst component of the recorded signals that is coherent from
`shot to shot and from trace to trace, then time aligning the
`recordings With respect to the ?ring time of the second
`source, and determining a second component of the signals
`that is coherent from shot to shot and from trace to trace. In
`some embodiments, determining the ?rst shot to shot coher
`ent component includes generating a common mid point
`gather With respect to the ?rst source. In some embodiments,
`determining the second shot to shot coherent component
`includes generating a common mid point gather With respect
`to the second source.
`Other aspects and advantages of the invention Will be
`apparent from the folloWing description and the appended
`claims.
`
`BRIEF DESCRIPTION OF DRAWINGS
`
`FIG. 1 is a diagram of one embodiment of a marine
`seismic acquisition system according to the invention.
`FIG. 2 shoWs an example of seismic energy paths (ray
`paths) from a source to a plurality of seismic receivers toWed
`by a vessel as the seismic energy re?ects from an acoustic
`impedance boundary.
`FIG. 3 shoWs an example of seismic ray paths for seismic
`energy from a source toWed by a source vessel to the seismic
`receivers toWed by the seismic recording vessel in FIG. 1.
`FIGS. 4 through 13 shoW example recordings of indi
`vidual receiver signals from the example “shots” shoWn in
`FIGS. 2 and 3 in order to explain a method according to one
`aspect of the invention.
`FIG. 14 is a table shoWing coherent and random compo
`nents of various types of trace gathers used in embodiments
`of the invention in order to identify a source to Which
`various signals may be attributable.
`
`DETAILED DESCRIPTION
`
`The invention relates generally to methods and systems
`for acquiring marine seismic data Which use more than one
`seismic energy source or source array disposed at spaced
`apart locations along or parallel to a survey line. Using
`spaced apart sources or source arrays enables increasing the
`effective subsurface coverage of a “line”, “string” or array of
`seismic receivers (sensors) With respect to What may be
`possible using only a single source or source array. The
`invention is also related to methods and systems for iden
`tifying Which one of the seismic sources caused particular
`events in signals detected by the seismic sensors. Identifying
`Which seismic source caused the particular events is impor
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`tant for determining subsurface structures from the seismic
`signals, and may be used to reduce the effects of coherent
`and random noise in the recorded seismic signals.
`In the description beloW, the term “seismic source” is used
`to describe a set of seismic energy sources such as air guns
`and Water guns Which are ?red substantially simultaneously.
`Such a seismic source Will normally include several air- or
`Water guns, but might also consist of only one such gun. A
`seismic vessel Will typically toW one, tWo or more seismic
`sources Which are ?red at separate times. In the folloWing
`description tWo sources is used. It should be clearly
`understood, hoWever, that a method and system according to
`the invention can use single sources or more than tWo
`sources on each vessel. Also, it is not necessary that all
`vessels operating together, toW the same number of sources.
`FIG. 1 shoWs an example of a marine seismic data
`acquisition arrangement Which may be used With the inven
`tion. Aseismic vessel (SEV) 1 toWs the ?rst seismic sources
`SA1, SA2, and one or more “streamers” or seismic sensor
`arrays as shoWn at 2a—2d.
`Each streamer 2a—2a' includes a plurality of seismic
`sensors (typically hydrophones) disposed thereon at spaced
`apart locations along each streamer 2a—2a'. The streamers
`2a—2a' are disposed along lines substantially parallel to the
`survey line 5. For purposes of the invention, only one
`streamer need be toWed by the SEV 1, hoWever, having a
`plurality of streamers as shoWn in FIG. 1 improves the
`ef?ciency and speed of data acquisition, as is knoWn in the
`art. Sensors (not shoWn) in the streamers 2a—2a' are opera
`tively coupled to a recording system disposed on the SEV 1.
`A source vessel (SOV) 4 trails the SEV 1 along the survey
`line 5. The SOV 4 toWs the second seismic sources
`SB1—SB2 The second sources SBl, SB2 are toWed at a
`selected distance from the ?rst sources SA1, SA2.
`The seismic recording system 6 may also include navi
`gation equipment (not shoWn separately) to enable precisely
`determining the position of the vessels 1, 4 and the indi
`vidual sensors (not shoWn separately) as seismic signals are
`recorded. The seismic recording system 6 may also include
`a source controller Which selectively controls actuation of
`the one or more sources toWed by the SEV 1 and by the SOV
`4. Timing of source actuation by the source controller (not
`shoWn separately) Will be further explained.
`Each of the seismic sources SA1, SA2, SBl, SB2 in this
`embodiment, as previously explained, Will typically include
`an array of air guns. Such arrays are used, for among other
`reasons as is knoWn in the art, to provide “Whiter” seismic
`energy (including a broader range of frequencies and having
`a more nearly constant amplitude for such frequencies). FIG.
`1 also shoWs the second sources SB1—SB2 toWed by the
`SOV 4 behind the seismic vessel 1. The second seismic
`sources may alternatively be toWed in front of the SEV 1 at
`a selected distance. In other embodiments, the seismic
`acquisition system may include additional source vessels,
`shoWn generally at 7 and 8 in FIG. 1. These additional
`source vessels 7, 8 may each toW one or more additional
`seismic sources, shoWn generally at SCI and SC2.
`The ?rst SA1, SA2 and second SBl, SB2 seismic energy
`sources are used in marine seismic surveying to increase the
`coverage area of the seismic data recorded by the recording
`system 6. Typically, each of the sources SA1, SA2, SB2,
`SB2 Will be actuated in a sequence Which reduces interfer
`ence in the recorded signals. For purposes of the description
`Which folloWs of methods according to the invention, a “?rst
`source” can be either one of the sources toWed by the SEV
`1, these being sources SA1 and SA2. A “second source”
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`WesternGeco Ex. 1001, pg. 17
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`referred to in the description can be either one of the sources
`toWed by the SOV 4, these sources being SBl and SB2.
`It should also be understood that for purposes of de?ning
`the scope of the invention, it is not necessary to have a
`separate source vessel, or source vessels, to toW the second
`source (or any additional sources) as shoWn in FIG. 1,
`although having such a separate source vessel provides
`practical bene?ts such as increasing the effective subsurface
`coverage of the streamers 2a—2a', as is knoWn in the art. For
`purposes of de?ning the scope of this invention, it is only
`necessary to have tWo seismic energy sources, Where the
`second seismic energy source (or source array) is toWed
`along (or parallel to) a survey line, such as 5 in FIG. 1, at a
`selected distance from the ?rst seismic source (or source
`array).
`During acquisition of seismic signals, the ?rst sources
`SAl, SA2 and the second sources SE1, SE2 are sequentially
`?red in a plurality of ?ring sequences, the timing of Which
`Will be further explained, and signals detected by the sensors
`(not shoWn) on the streamers 2a—2a' are recorded by the
`recording system 6.
`FIG. 2 shoWs an example of paths 21 (“ray paths”) of
`seismic energy as it travels from the ?rst sources or source
`arrays (SA1—SA2 in FIG. 1), the location along the survey
`line (5 in FIG. 1) of Which is shoWn at 20, doWnWard
`through the Water 26, to a subsurface acoustic impedance
`boundary (bed boundary) 24. Some of the seismic energy is
`re?ected from the bed boundary 24 and travels upWardly
`through the Water 26 Where it is detected by the sensors on
`each of the streamers (2a—2a' in FIG. 1), the locations of
`some of Which are shoWn at 22. The ray paths 21 shoWn in
`FIG. 2 correspond to the path traveled by the seismic energy
`to each tenth sensor in one of the streamers (2a—2a' in FIG.
`1), recordings of Which Will be shoWn and explained beloW
`With reference to FIGS. 4—13.
`FIG. 3 shoWs ray paths 31 for acoustic energy traveling
`from the second sources (SB1—SB2 as shoWn in FIG. 1), the
`position of Which is shoWn at 30 in FIG. 3. The sensor
`positions 22 are substantially the same as those shoWn in
`FIG. 2, because the second source (or array) is actuated at a
`time delay With respect to actuation of the ?rst source (or
`array) such that the seismic and source vessels, and thus the
`toWed sources and receivers, move only a very small dis
`tance along the Water 26 during the delay time. In FIG. 3, the
`position of the second source 30 With respect to the stream
`ers and ?rst source is typically selected such that the ray
`paths from 31 from the second source have different re?ec
`tion locations along the boundary 24 than do the ray paths
`from the ?rst source, such as shoWn in FIG. 2.
`Prior art methods for using tWo or more spaced apart
`sources in an arrangement such as shoWn in FIG. 1 include
`?ring the ?rst source, and Waiting before ?ring the second
`source a sufficient amount of time such that signals detected
`by the sensors resulting from ?ring the ?rst source have
`substantially attenuated. In methods according to the
`invention, the second source is ?red after a relatively small
`selected delay time after ?ring the ?rst source, such that
`signals from the ?rst source that have substantial amplitude
`are still being detected by the sensors.
`In a method according to one aspect of the invention, the
`?rst source is actuated or “?red”. Arecording is made of the
`signal detected by the sensors that is indexed to a knoWn
`time reference With respect to time of ?ring the ?rst source.
`The second source (or array) is then ?red at a knoWn,
`selected time delay after the ?ring of the ?rst source, While
`signal recording continues. Firing the ?rst source, waiting
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`the predetermined delay and ?ring the second source there
`after is referred to herein as a “?ring sequence.” This ?ring
`sequence, and contemporaneous signal recording, are
`repeated in a second ?ring sequence. The second ?ring
`sequence includes ?ring the ?rst source, Waiting a different
`selected time delay and then ?ring the second source, While
`recording seismic signals. The knoWn, selected time delay
`betWeen ?ring the ?rst source and ?ring the second source
`is different for each successive ?ring sequence. For purposes
`of the invention, seismic signals are recorded for a plurality
`of such ?ring sequences, typically three or more ?ring
`sequences, each having a different value of time delay.
`Although the time delay varies from sequence to
`sequence, the time delay betWeen ?ring the ?rst source and
`the second source in each ?ring sequence is preferably
`selected to be at least as long as the “Wavelet” time of the
`seismic energy generated by the ?rst source to avoid inter
`ference betWeen the ?rst and second sources. Typically,
`hoWever, the time delay is less than one second, but in some
`cases may be several seconds. In some embodiments, the
`time delay betWeen successive ?ring sequences may vary in
`a knoWn, but random manner. In other embodiments, the
`time delay may vary in a knoWn, but quasi-random manner.
`In still other embodiments, the time delay may be varied
`systematically. Examples of seismic signals as Will be
`explained beloW With reference to FIGS. 4—13 may include
`a time delay variation betWeen successive ?ring sequences
`of about 100 milliseconds.
`Firing the ?rst source and the second source in a plurality
`of ?ring sequences as described above, each having a
`different time delay, enables separating true seismic signals
`Which result from the ?rst source and from the second source
`as Will be explained beloW With respect to FIGS. 4—14.
`Various methods of separating the seismic signals may also
`substantially attenuate coherent noise and random noise, as
`Will be further explained.
`FIG. 4 shoWs a graphic display of amplitude With respect
`to time since source actuation of the signals as Would be
`detected by each of the sensors in one of the streamers
`(2a—2a' in FIG. 1) toWed by the seismic vessel (1 in FIG. 1).
`The signals shoWn in FIG. 4 Were synthesiZed for an
`example earth model such as the one shoWn in FIGS. 2 and
`3. The display in FIG. 4 shoWs signals resulting from a
`single ?ring of the ?rst source, folloWed by a single ?ring of
`the second source after a selected time delay. The display in
`FIG. 4 is arranged such that the signal from the sensor toWed
`closest to the seismic vessel is on the left hand side of the
`display. The sensor signal displays or “traces” displayed
`from left to right in FIG. 4 represent the individual sensor
`signals from successively more distant ones of the sensors.
`Re?ected seismic energy originating from the ?rst source (or
`array, the position of Which is shoWn at 20 in FIG. 2) appears
`as a high amplitude event that may be correlated in each
`successive trace, as shoWn at 40. Signals from the second
`source (or array, the position of Which is shoWn at 30 in FIG.
`3) that correspond to re?ected energy from the same sub
`surface boundary (shoWn at 24 in FIGS. 2 and 3) can be
`identi?ed by another event shoWn at 42. As Would be
`expected, the event 40 resulting from the ?rst source shoWs
`increased arrival time With respect to individual sensor
`distance from the ?rst source in a Well knoWn relationship
`called “moveout”. Correspondingly, the signals from the
`second source shoW moveout for event 42 in the opposite
`direction because of the placement of the second source With
`respect to the streamers (2a—2a' in FIG. 1).
`The table in FIG. 4 shoWs, for each source, a time of ?ring
`of each source With respect to a time index for signal
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`WesternGeco Ex. 1001, pg. 18
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`recording. For the sake of brevity of description that follows,
`the ?rst source (or source array) Will be referred to in
`corresponding tables in each Figure as “source A” and the
`second source (or source array) Will be referred to as “source
`B.” The time delay betWeen ?ring source A and source B
`identi?ed in FIG. 4 is 0.1 second (100 milliseconds).
`A display of synthesiZed signals resulting from a second
`?ring sequence of sources A and B, for the earth model of
`FIGS. 2 and 3, is shoWn in FIG. 5. The ?ring sequence for
`Which detected signals are shoWn in FIG. 5 is made at a
`selected time after recording the signals from the ?rst ?ring
`(corresponding signals for Which are shoWn in FIG. 4). This
`selected time depends on factors such as an approximate
`depth to Which seismic analysis is desired to be performed,
`length of the streamers (2a—2a' in FIG. 1), as is Well knoWn
`in the art, and typically is in a range of about 8 to 20 seconds.
`Arrival of re?ective events corresponding to the events
`shoWn at 40 and 42 in FIG. 4 is shoWn for source A at 50 in
`FIG. 5 and for source B at 52. As shoWn in the table in FIG.
`5, the selected time delay betWeen ?ring source A and source
`B is 0.3 seconds.
`FIG. 6 shoWs a display similar to the ones shoWn in FIGS.
`4 and 5, With corresponding re?ective events for source A
`shoWn at 60 and for source B and 62. The display in FIG. 6
`represents signals for a third ?ring sequences of the sources.
`And Wherein the time delay betWeen ?ring source A and
`source B is 0.4 seconds.
`FIG. 7 shoWs a display of signals for a fourth ?ring
`sequence of source A and source B, Wherein the selected
`time delay betWeen ?ring source A and source B is 0.2
`seconds. Corresponding re?ective events 70 and 72 are
`shoWn for source A signals and source B signals, respec
`tively.
`FIG. 8 shoWs a display of signals for a ?fth ?ring
`sequence of source A and source B, Wherein the selected
`time delay is 0.5 seconds. Corresponding re?ective events
`80 and 82 are shoWn for source A signals and source B
`signals, respectively.
`Re?ective events corresponding to signals from source A,
`shoWn at 40, 50, 60, 70 and 80, respectively, in FIGS. 4
`through 8, occur at very similar times With respect to the
`time of ?ring of source A. Differences in arrival time
`betWeen traces for each such event corresponding to source
`Amay depend on the actual position of the seismic vessel (1
`in FIG. 1) at the time of each source A ?ring, Which position
`depends on vessel speed and time betWeen ?ring sequences.
`The arrival time of the source A events may also depend on
`the subsurface structure of the earth, among other factors.
`Nonetheless, there is a very high degree of correspondence
`betWeen the source A re?ective events 40, 50, 60, 70, 80,
`respectively, in FIGS. 4 through 8.
`In some embodiments of a method according to the
`invention, a “true” seismic signal component corresponding
`to the ?ring of source A can be identi?ed in the traces by a
`tWo part procedure. The ?rst part includes determining
`coherence betWeen the traces Within an individual ?ring
`sequence. This part can be performed by selecting closely
`spaced subsets of all the traces (such as a subset of betWeen
`?ve and ten traces) such as shoWn in FIGS. 4 through 8, and
`determining coherence betWeen the selected traces Within
`selected-length time WindoWs. Coherence may be
`determined, for each subset of traces selected, by correlating
`the traces to each other over the selected-length time Win
`doWs. A result of the correlation is a curve or trace the
`amplitude of Which represents degree of correspondence
`from trace to trace With respect to time.
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`8
`The coherence betWeen traces determined in the ?rst part
`of the procedure includes components that are also coherent
`betWeen ?ring sequences With respect to the ?ring time of
`source A. These components represent the “true” signal
`corresponding to actuating source A. The trace correspon
`dence determined in the ?rst part of the method also includes
`coherent noise, such as Would result from signals caused by
`actuation of source