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WO1994010585A1 - Procede de determination de la signature en champ lointain d'un groupe de sources sismiques marines - Google Patents

Procede de determination de la signature en champ lointain d'un groupe de sources sismiques marines Download PDF

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Publication number
WO1994010585A1
WO1994010585A1 PCT/NO1993/000156 NO9300156W WO9410585A1 WO 1994010585 A1 WO1994010585 A1 WO 1994010585A1 NO 9300156 W NO9300156 W NO 9300156W WO 9410585 A1 WO9410585 A1 WO 9410585A1
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WIPO (PCT)
Prior art keywords
field
pressure
far
parameters
measured
Prior art date
Application number
PCT/NO1993/000156
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English (en)
Inventor
Martin LANDRØ
Original Assignee
Den Norske Stats Oljeselskap A.S.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Den Norske Stats Oljeselskap A.S. filed Critical Den Norske Stats Oljeselskap A.S.
Priority to AU54347/94A priority Critical patent/AU5434794A/en
Publication of WO1994010585A1 publication Critical patent/WO1994010585A1/fr

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
    • G01V1/003—Seismic data acquisition in general, e.g. survey design
    • G01V1/006—Seismic data acquisition in general, e.g. survey design generating single signals by using more than one generator, e.g. beam steering or focusing arrays
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V13/00—Manufacturing, calibrating, cleaning, or repairing instruments or devices covered by groups G01V1/00 – G01V11/00

Definitions

  • a method for determining the far-field signature of a marine seismic source array is provided.
  • the invention relates to a method for determining the far-field signature of a marine seismic source array from near- field measurements of acoustic signals from the seismic sources, wherein the near-pressure field is measured by means of a plurality of sensors on a streamer means located beneath the source array.
  • the best known method for estimating or determining far-field signatures as a function of angle from a seismic source array is the so-called near-field to far-field extrapolation method which was first proposed by Ziolkowski et al. (see A.
  • This method has, however, some deficiencies, viz. that it is unstable for compact/large arrays, that it is sensitive for errors in source-hydrophone positioning, - that it is a problem that one does not know the movement of the air bubble, and the method is based on the assumption that the hydro ⁇ phones are situated in the linear zone around the source.
  • the most serious weakness of this method is that it becomes unstable for arrays having many air guns (see M. Landr ⁇ , S. Strandenes, S. Vaage, 1991, "Use of Near-field Measurements to compute Far-field marine source signatures - Evaluation of the Method", First Break Vol. 9, No. 8, p. 375-385).
  • Another method which -has been used for determining the far-field signatures of a seismic source array consists in s measuring the source response of a signature streamer (mini- streamer) just beneath the source array.
  • This method corresponds to the introductorily stated method and was proposed by Neil D. Hargreaves in 1984 (see said US patent and also N. Hargreaves, 1984, "Far-field Signatures by Wavefield Extrapolation", a paper o presented on the 46th EAEG-meeting in London, 1984).
  • the far- field signature in an arbitrary point is determined by extrapola ⁇ ting the measured response with a two-dimensional migration algorithm.
  • the drawback of this method is that it is based on a two-dimensional wave equation, and that aperture effects as a 5 result of a finite length of the streamer will deteriorate the result.
  • the above-mentioned object is achieved with a method of the introductorily stated type which, according to the invention, is characterized in that the pressure field from each single source is simulated by means of a computer on the basis of equations describing the pressure field, free adjusting para ⁇ meters being used to see to it that the simulated pressure becomes as equal as possible to the pressure measured by the sensors, the free parameters being adapted to the measured pressure field by means of an inversion algorithm, so that the parameters are updated gradually until optimum accordance between the measured and the simulated pressure field is obtained, whereafter the farfield signature of the source array is determined in that each single source is simulated with the best determined parameters and the contribution from each single source is superposed in the far field.
  • the method according to the invention is based on the fact that the physical parameters describing each single source in the array are calculated by means of an inversion algorithm. After the pressure field of the seismic sources has been measured on the streamer which is situated just beneath the source array, effective source signatures for each single source are determined by means of the inversion algorithm.
  • the inversion algorithm is based on the fact that the air bubble generated by each source can be modelled or simulated physically. Typical parameters which are determined by the inversion, are source depth, attenuation constants (which will be further described later), the reflection coefficient at the water surface and parameters describing the geometrical shape of the streamer. It is not necessary to know the exact depth of each single source, and neither the position of each single sensor or hydrophone in the streamer, the method also making possible a determination of these parameters.
  • Fig. 1 there is shown a vessel 1 towing a seismic source array 2 and a signature streamer 3 (mini-streamer) through a body of water 4 above a seabed 5.
  • the source array 2 consists of a plurality of seismic acoustic sources 6 towed at a depth z g beneath the ocean surface 7, and which for example may consist of air guns.
  • the streamer 3 comprises an array of a plurality of sensors 8 which for example may be hydrophones having a suitable spacing, in the way it will be known to a person skilled in the art.
  • the streamer 3 extends a distance ahead of and behind the source array 2 and is towed at a depth z 0 beneath the ocean surface, so that the streamer is situated just beneath the source array.
  • the sensors 8 then sense the near-pressure field of the sources 6 when these are fired when practising the method.
  • a description of the modelling or simulating theory underlying the invention will be given below.
  • R is the bubble radius
  • C is the sound velocity in the water
  • H is the enthalpy on the bubble wall
  • ⁇ is an empirical time constant influencing the bubble period. (Increased ⁇ gives an increased distance between two peaks in the signal) .
  • the main point of introducing the addi ⁇ tional members ⁇ and ⁇ is to enforce a best possible matching to the measured near-field signature. Without these members there will be large deviations between theoretically modelled and measured source signatures from air guns.
  • the attenuation constant ⁇ and the time constant ⁇ represent free adjusting parameters which are adapted to the measured pressure by means of the inversion algorithm.
  • the enthalpy H may be calculated from the so-called Tait equation which is a condition equation for the water and has the following form:
  • p(R) is the pressure in the water at the bubble wall.
  • m 0 is the initial air mass and ⁇ is the time needed for emptying the air chamber.
  • thermodynamics An equation for describing the temperature (T) within the air bubble may be derived, the starting-point being taken in a quasi-stationary process in an open thermodynamic system.
  • the first principle clause of the thermodynamics is modified to the following expression:
  • R g is ideal gas constant
  • p is the pressure within the bubble
  • V is the volume of the bubble
  • m is the air mass
  • c u is specific heat capacity with constant volume.
  • the far-field pressure generated by an oscillating air bubble may be expressed by the enthalpy on the bubble wall, the bubble wall velocity and the bubble radius.
  • Gilmore (1952) derived the following expression for the far-field pressure:
  • p is the density of the water and r is the distance from the centre of the bubble to the far-field point.
  • the far- field pressure may be estimated by a simple superposition of the pressure contribution from each single source.
  • This time-dependent hydrostatic pressure will affect the enthalpy (see above) which will in turn affect the bubble movement ( equation 1 ) .
  • the far-field signature (s n (t)) of a source n at a distance r n now can be expressed as follows:
  • ⁇ 0 is the reflection coefficient at the water surface
  • r' n is the propagation distance for the reflected wave from the water surface (see the drawing figure).
  • p n (t) is the farfield pressure given by equation (10), but with interaction effects. The pressure field at the streamer in the Figure now may be calculated from equation (12).
  • the pressure field from each single source is simulated by means of a computer on the basis of the above stated equations describing the pressure field.
  • free adjusting parameters are used to see to it that the simulated pressure becomes as equal as possible to the pressure measured by the sensors, the free parameters being adapted to the measured pressure field by means of an inversion algorithm.
  • the total pressure field of the array thereafter is determined by modelling each single source with the best determined adjusting parameters, and thereafter superposing the contribution from each single source in the far field.
  • the utilized inversion procedure is based on minimizing the following mismatching function: • t j ) )- ( 13 )
  • N r and N t are the number of receivers and the number of time steps.
  • H J T J is an approximate Hess matrix and J is the Jacobi matrix.
  • ⁇ is the Levenberg-Marquardt attenuation parameter.
  • the geometrical shape of the streamer in the water may be included in the inversion algorithm. This is done by assuming that the shape of the streamer can be expressed in the following manner:
  • z 0 is average streamer depth
  • L is the streamer length
  • K is the number of Fourier coefficients which are necessary for representing the shape of the streamer. It is envisaged that only 4-5 Fourier coefficients will be necessary to describe a usable streamer shape.
  • the reflection coefficient ( ⁇ 0 ) at the water surface may deviate from the ideal value (-1,0). This effect may be due to the fact that surface is rough, and that air all the time leaks out from the air guns. In order to takes this effect into account, also the reflection coefficient of the water surface may be included in the inversion procedure, the reflection coefficient being included in the above equation (12).

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Remote Sensing (AREA)
  • General Life Sciences & Earth Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Geophysics (AREA)
  • Acoustics & Sound (AREA)
  • Environmental & Geological Engineering (AREA)
  • Geology (AREA)
  • Manufacturing & Machinery (AREA)
  • Geophysics And Detection Of Objects (AREA)

Abstract

Procédé de détermination de la signature en champ lointain d'un groupe (2) de sources sismiques marines à partir de mesures en champ proche des signaux acoustiques émis par les sources sismiques (6). On mesure le champ proche de pression à l'aide d'une pluralité de capteurs (8) montés sur une flûte sismique (3) située sous le groupe (2) de sources. On crée une simulation du champ proche de chacune des sources (6) au moyen d'un ordinateur et en fonction d'équations décrivant le champ de pression, des paramètres libres de réglage étant utilisés pour rendre égales, dans la mesure du possible, la pression simulée et la pression mesurée par les capteurs (8). Les paramètres libres sont adaptés au champ de pression mesuré par un algorithme d'inversion afin que les paramètres soient actualisés progressivement jusqu'à obtention d'une conformité optimale des champs de pression simulé et mesuré. Par la suite, on peut déterminer la signature en champ lointain du groupe (2) de sources lorsque chaque source (6) est simulée au moyen des meilleurs paramètres déterminés, et que la contribution de chacune des sources est superposée dans le champ lointain.
PCT/NO1993/000156 1992-10-23 1993-10-22 Procede de determination de la signature en champ lointain d'un groupe de sources sismiques marines WO1994010585A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU54347/94A AU5434794A (en) 1992-10-23 1993-10-22 A method for determining the far-field signature of a marine seismic source array

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
NO924122A NO176227C (no) 1992-10-23 1992-10-23 Fremgangsmåte for bestemmelse av fjernfeltsignaturen for en marin, seismisk kildeoppstilling
NO924122 1992-10-23

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AU (1) AU5434794A (fr)
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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7218572B2 (en) * 2004-05-20 2007-05-15 Pgs Exploration (Uk) Limited Method of seismic source monitoring using modeled source signatures with calibration functions
GB2433594A (en) * 2005-12-23 2007-06-27 Westerngeco Seismic Holdings Determining source signatures for arrays of marine seismic sources
WO2008025944A1 (fr) * 2006-08-31 2008-03-06 Westerngeco Seismic Holdings Limited Construction de tracés sismiques à décalage d'origine
US7586810B2 (en) * 2003-01-30 2009-09-08 Westerngeco L.L.C. Directional de-signature for seismic signals
GB2468681A (en) * 2009-03-18 2010-09-22 Geco Technology Bv Determination of notional signatures using two sensors per source in the array
EP2322955A3 (fr) * 2002-03-14 2015-01-28 ION Geophysical Corporation Procédé et appareil de diagnostique de source marine et interface utilisateur graphique permettant de le faire fonctionner

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0166510A2 (fr) * 1984-06-15 1986-01-02 Western Atlas International, Inc. Procédé pour la détermination de la signature à distance d'une source sismique marine par des mesures proches
US4658384A (en) * 1985-01-07 1987-04-14 Western Geophysical Co. Of America Method for determining the far-field signature of an air gun array
US4908801A (en) * 1989-05-30 1990-03-13 Teledyne Exploration Real-time simulation of the far-field signature of a seismic sound source array

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0166510A2 (fr) * 1984-06-15 1986-01-02 Western Atlas International, Inc. Procédé pour la détermination de la signature à distance d'une source sismique marine par des mesures proches
US4658384A (en) * 1985-01-07 1987-04-14 Western Geophysical Co. Of America Method for determining the far-field signature of an air gun array
US4908801A (en) * 1989-05-30 1990-03-13 Teledyne Exploration Real-time simulation of the far-field signature of a seismic sound source array

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
FIRST BREAK, Volume 9, No. 8, August 1991, M. LANDROE et al., "Use of Near Field Measurements to Compute Far Field Marine Source Signatures...", page 375 - page 385. *
PROCEEDINGS OF THE IEEE, Volume 72, No. 10, October 1984, B.W. CAMERON et al., "Seismic Modeling and Inversion", page 1385 - page 1393. *

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP2322955A3 (fr) * 2002-03-14 2015-01-28 ION Geophysical Corporation Procédé et appareil de diagnostique de source marine et interface utilisateur graphique permettant de le faire fonctionner
US7586810B2 (en) * 2003-01-30 2009-09-08 Westerngeco L.L.C. Directional de-signature for seismic signals
US7218572B2 (en) * 2004-05-20 2007-05-15 Pgs Exploration (Uk) Limited Method of seismic source monitoring using modeled source signatures with calibration functions
GB2433594A (en) * 2005-12-23 2007-06-27 Westerngeco Seismic Holdings Determining source signatures for arrays of marine seismic sources
GB2433594B (en) * 2005-12-23 2008-08-13 Westerngeco Seismic Holdings Methods and systems for determining signatures for arrays of marine seismic sources for seismic analysis
US7440357B2 (en) 2005-12-23 2008-10-21 Westerngeco L.L.C. Methods and systems for determining signatures for arrays of marine seismic sources for seismic analysis
WO2008025944A1 (fr) * 2006-08-31 2008-03-06 Westerngeco Seismic Holdings Limited Construction de tracés sismiques à décalage d'origine
US8958266B2 (en) 2006-08-31 2015-02-17 Schlumberger Technology Corporation Zero-offset seismic trace construction
GB2468681A (en) * 2009-03-18 2010-09-22 Geco Technology Bv Determination of notional signatures using two sensors per source in the array
GB2468681B (en) * 2009-03-18 2012-09-12 Geco Technology Bv Determination of notional signatures

Also Published As

Publication number Publication date
NO176227C (no) 1995-02-22
NO924122L (no) 1994-04-25
NO176227B (no) 1994-11-14
AU5434794A (en) 1994-05-24
NO924122D0 (no) 1992-10-23

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