WO2012162208A2 - Systèmes et procédés d'optimisation de la puissance de sortie à basse fréquence émanant de réseaux de sources de type canon à air - Google Patents

Systèmes et procédés d'optimisation de la puissance de sortie à basse fréquence émanant de réseaux de sources de type canon à air Download PDF

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Publication number
WO2012162208A2
WO2012162208A2 PCT/US2012/038751 US2012038751W WO2012162208A2 WO 2012162208 A2 WO2012162208 A2 WO 2012162208A2 US 2012038751 W US2012038751 W US 2012038751W WO 2012162208 A2 WO2012162208 A2 WO 2012162208A2
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WIPO (PCT)
Prior art keywords
airguns
bubble
source
energy
largest
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PCT/US2012/038751
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English (en)
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WO2012162208A3 (fr
Inventor
Jon-Fredrik Hopperstad
Robert Laws
John Richard Tulett
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Schlumberger Technology B.V.
Schlumberger Canada Limited
Services Petroliers Schlumberger
Schlumberger Holdings Limited
Prad Research And Development Limited
Schlumberger Technology Corporation
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Priority claimed from US13/112,869 external-priority patent/US9025417B2/en
Priority claimed from US13/450,477 external-priority patent/US9360578B2/en
Application filed by Schlumberger Technology B.V., Schlumberger Canada Limited, Services Petroliers Schlumberger, Schlumberger Holdings Limited, Prad Research And Development Limited, Schlumberger Technology Corporation filed Critical Schlumberger Technology B.V.
Priority to EP12789555.5A priority Critical patent/EP2710410A4/fr
Priority to MX2013013383A priority patent/MX2013013383A/es
Priority to BR112013029774-3A priority patent/BR112013029774B1/pt
Publication of WO2012162208A2 publication Critical patent/WO2012162208A2/fr
Publication of WO2012162208A3 publication Critical patent/WO2012162208A3/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
    • G01V1/38—Seismology; Seismic or acoustic prospecting or detecting specially adapted for water-covered areas
    • G01V1/3861—Seismology; Seismic or acoustic prospecting or detecting specially adapted for water-covered areas control of source arrays, e.g. for far field control
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
    • G01V1/02—Generating seismic energy
    • G01V1/133—Generating seismic energy using fluidic driving means, e.g. highly pressurised fluids; using implosion
    • G01V1/137—Generating seismic energy using fluidic driving means, e.g. highly pressurised fluids; using implosion which fluid escapes from the generator in a pulsating manner, e.g. for generating bursts, airguns
    • 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
    • G01V2210/00—Details of seismic processing or analysis
    • G01V2210/10—Aspects of acoustic signal generation or detection
    • G01V2210/12—Signal generation
    • G01V2210/121—Active source
    • G01V2210/1212—Shot
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V2210/00—Details of seismic processing or analysis
    • G01V2210/10—Aspects of acoustic signal generation or detection
    • G01V2210/12—Signal generation
    • G01V2210/127—Cooperating multiple sources
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V2210/00—Details of seismic processing or analysis
    • G01V2210/10—Aspects of acoustic signal generation or detection
    • G01V2210/12—Signal generation
    • G01V2210/129—Source location
    • G01V2210/1293—Sea
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V2210/00—Details of seismic processing or analysis
    • G01V2210/10—Aspects of acoustic signal generation or detection
    • G01V2210/16—Survey configurations
    • G01V2210/161—Vertical seismic profiling [VSP]

Definitions

  • the present disclosure relates in general to seismic source arrays, and more particularly to systems and methods for optimizing low frequency output of such source arrays.
  • airgun source arrays are often used to generate acoustic output, which when reflected off of subsurface formations may be detected by associated seismic receivers. Such data may be used to build up an image of subsurface formations for assessing the likelihood of hydrocarbon production.
  • the low frequency output of marine airgun seismic sources is limited by the resonance frequency of the largest airgun bubble volume in the source array.
  • This oscillation frequency also referred to as the fundamental bubble frequency, is given by the well-known Rayleigh- Willis formula:
  • / is the bubble frequency measured in Hertz
  • d is the source depth in meters
  • P is the firing pressure in psi (pound per square inch)
  • V is the airgun chamber volume in cubic inches
  • the resulting bubble frequency substantially equals that of a single gun of the combined volume.
  • cluster design focused on maximizing the primary-to-bubble ratio of the resulting source signature.
  • the airgun cluster design in use today, where the clustered airguns are typically separated by less than one metre, and where the airgun bubbles coalesce into one non-spherical bubble.
  • Other airguns in the source array are only weakly interacting, and the volume of these guns is normally chosen to achieve maximum destructive interference of the bubble amplitude of the overall source signature. This is known as a 'tuned array'.
  • the present disclosure describes a source design and method for increasing low frequency output of a marine source array.
  • the approach comprises providing a plurality of airguns. At least some of the airguns are activated to generate an effective bubble energy that may be optimized through specific use, preparation and/or arrangement of the airguns.
  • Figure 1 is a schematic diagram of a marine seismic data acquisition system according to an embodiment of the disclosure
  • FIG. 2 is a schematic diagram of a VSP arrangement according to an embodiment of the disclosure.
  • Figure 3 is a graphical depiction of a source signature of conventional gun arrangements
  • Figure 4 is a schematic depiction of a tuned conventional source array
  • Figure 5 is a schematic depiction of a source array designed to optimize low frequency output according to the principles of the present disclosure
  • Figure 6 is a schematic depiction of another source array designed to optimize low frequency output according to the principles of the present disclosure
  • Figure 7 is a graphical depiction comparing the optimized low frequency output source arrays of Figures 5 and 6 with the conventional source array of Figure 4;
  • Figure 8 is a process flow chart for designing a source array to optimize low frequency output
  • Figure 9 is a schematic depiction of another source array designed to optimize low frequency output according to the principles of the present disclosure.
  • Figure 10 is a graphical depiction of an estimated far field spectrum of the source array of Figure 9;
  • Figures 11a and lib are a comparison of a conventional source array (Fig. 11a) and such source array redesigned to optimize low frequency output according to the principles of the present disclosure (Fig. l ib);
  • Figure 12 is a flowchart demonstrating a design application for a low frequency source array
  • Figure 13 is a schematic depiction of a source having a plurality of suspended airguns
  • Figure 14 is a graphical depiction of a measured far field spectrum of the source configuration of Figure 13;
  • Figure 15 is a graphical depiction of a measured far field spectrum of the source configuration of Figure 13 in which at least some of the airguns of the source array are operated at higher firing pressure;
  • Figure 16 is a graphical depiction of a measured far field spectrum of the source configuration of Figure 13 in which at least some of the airguns of the source array are charged to a higher pressure level than other airguns of the source array.
  • FIG. 1 depicts an embodiment 10 of a marine seismic data acquisition system in accordance with some embodiments of the disclosure.
  • a survey vessel 12 tows one or more seismic streamers 14 (one example of streamers 14 being depicted in Fig. 1).
  • the seismic streamers 14 may be several thousand meters long and may contain various support cables (not shown), as well as wiring and/or circuitry (not shown) that may be used to support communication along the streamers 14.
  • each streamer 14 includes a primary cable into which is mounted seismic sensors 16 that record seismic signals. It is to be appreciated that the sensors 16 are illustrated schematically for emphasis in Fig. 1, and that in practice, the sensors 16 may be disposed within the streamer cable 14.
  • the seismic sensors 16 may include pressure sensors (e.g., hydrophones) and/or particle motion sensors (e.g., accelerometers).
  • the marine seismic data acquisition system 10 includes a seismic source 20 that typically takes the form of an array of airguns.
  • the seismic source 20 may be towed by the survey vessel 12.
  • the seismic source 20 may operate independently of the survey vessel 12, in that the seismic source may be coupled to other vessels, buoys or rigs, as just a few examples.
  • acoustic signals 18 are produced by the seismic source 20 and are directed down through a water column 22 into strata 24 and 26 beneath a water bottom surface 28.
  • the acoustic signals 18 are reflected from the various subterranean geological formations, such as an exemplary formation 30 that is depicted in Figure 1.
  • the incident acoustic signals 18 produce corresponding reflected acoustic signals, or pressure waves 32, which are sensed by the seismic sensors 16.
  • the seismic acquisition enables the buildup of an image of a survey area for purposes of identifying subterranean geological formations, such as the exemplary geological formation 30.
  • Subsequent analysis of the representation may reveal probable locations of hydrocarbon deposits in subterranean geological formations.
  • portions of the analysis of the representation may be performed on the seismic survey vessel 12, such as by a signal processing unit 34.
  • VSP vertical seismic profile
  • a vertical seismic profile is a class of borehole seismic measurements used for correlation between surface seismic receivers and wireline logging data.
  • VSPs can be used to tie surface seismic data to well data, providing a useful tie to measured depths.
  • VSPs yield higher resolution data than surface seismic profiles provide.
  • VSPs enable converting seismic data to zero-phase data and enable distinguishing primary reflections from multiples.
  • a VSP often is used for analysis of portions of a formation ahead of the drill bit. Referring to Figure 2, a simplified view of an offshore rig 36 positioned over a subsea borehole 37 is shown.
  • the borehole 37 contains a plurality of spaced receivers 38 to facilitate, for example, a VSP acquisition.
  • the receivers 38 can be deployed in the borehole 37 with a variety of methods and systems, including a wireline cable; a downhole assembly, e.g. drill collars; permanent fixation to a side of the borehole; or with other suitable techniques.
  • the rig 36 is shown supporting a conventional seismic survey apparatus designated generally as 39.
  • the survey apparatus 39 includes a source 40, which takes the form of an air-gun or guns suspended below the surface by a float 41.
  • An analog hydrophone 42 is suspended below the air guns 40 and may provide information for correcting time break errors (errors attributable to time differences for swells, irregular source firings, etc.).
  • One or more analog lines 43 form part of an umbilical 44 that may also include an airline.
  • the analog lines 43 traverse a handling system, such as a crane 45.
  • the analog lines 43 provide an analog communications/control link between the guns 40, the hydrophone 42, a gun controller 46, and a computer processor 47.
  • seismic sources are normally comprised of airgun clusters; e.g. one to four clusters, wherein each cluster includes, for example, two to four airguns.
  • Airgun clusters also are prevalent in source arrays for towed marine, seabed seismic and some borehole seismic applications.
  • the teachings of the present disclosure may be utilized in any of the aforementioned settings.
  • the widespread use of airgun clusters can be attributed to two main characteristics. First, clustering of medium sized airguns can achieve the same bubble frequency as a large single airgun with the same total volume. Airgun clusters are considered more robust and easier to handle than large single airguns. Secondly, airgun clusters have a higher peak-to-bubble ratio than the equivalently sized single airgun, and are therefore well-suited for tuned arrays.
  • the higher peak-to-bubble ratio associated with airgun clusters is illustrated in the top panel of Figure 3, which compares a single airgun of about 600 in 3 with a three-gun cluster with the same total volume; the measurement bandwidth is 0-500 Hz.
  • the higher peak-to-bubble ratio manifests itself in a flatter spectrum in the bottom panel of Figure 3.
  • the spectral comparison also indicates that the output at the bubble frequency, i.e. 6.8 Hz, is substantially identical. It has been found that no matter how you arrange the cluster to release the air, the spectral level at the bubble frequency is substantially the same.
  • an initial embodiment of the present disclosure seeks to optimize low frequency output by implementing large airgun cluster(s), whose volume is larger than what is considered practical for a single airgun.
  • the low frequency output of airgun sources is limited by the resonance frequency of the largest bubble volume in the array.
  • the volume of the largest bubble is small compared with the total source volume. Accordingly, there is scope for increasing the low frequency output, without increasing the total gas volume, by shifting the bubble frequency towards zero.
  • the lower resonance frequency is achieved by releasing a larger amount of gas into one big bubble or multiple frequency locked bubbles.
  • Airgun bubbles can exhibit cluster-type oscillation frequency even at non-coalescing distances.
  • the bubbles from two closely spaced airguns of different volume may oscillate with the same frequency (the frequency of the combined volume), even though the bubbles are not coalescing. In fact, they may not even be touching.
  • the bubble interaction is entirely through the pressure field. This phenomenon is known as frequency locking. Bubbles are 'fully frequency locked' when their bubble frequency substantially equals that associated with the combined volume. Not fully interacting bubbles are commonly referred to as 'partially frequency locked', and as 'non-interacting' when the presence of the other bubbles do not affect the oscillation frequency.
  • Figure 4 depicts an example of a typical prior art airgun array 48 in which single and clustered airguns are spaced at 3 meters in the in-line direction.
  • the two cluster bubbles are fully frequency locked to each other, and partially frequency locked to an adjacent cluster 50, which has a volume of 2x195 in 3 . Accordingly, the 2x290 in 3 bubbles oscillate with a frequency corresponding to an effective volume of about 750 in 3 .
  • the array 48 further includes single airguns 51, 52, 53, 54 with volumes 280 in 3 , 195 in 3 , 145 in 3 , 105 in 3 , respectively, resulting in a total volume of each string/subarray of 1695 in 3 and a total source array volume of 5085 in 3 .
  • the largest bubble in array 48 uses only 750/1695 ⁇ 44% of the available subarray volume, and only 750/5085 ⁇ 15% of the total source volume available.
  • "Source volume” as used in the present disclosure means the total volume of airguns fired at each shot and excludes volumes that could be generated by guns (e.g., spare guns) not used during the shot. These numbers are quite typical for prior art source arrays.
  • the total high pressure gas capacity available is used to generate many weakly interacting bubbles, and the largest bubble is small compared with the total source volume.
  • source designs according to the present disclosure aim to optimize the use of available high pressure gas capacity for generating very low frequencies.
  • substantially all of the available high pressure gas should be released into one huge bubble oscillation. This can be achieved with an airgun cluster wherein some or all cluster bubbles are fully frequency locked, i.e., the cluster bubbles oscillate with the frequency associated with the total cluster volume.
  • Example 1 Source array wherein the largest bubble is one-third of the total source volume
  • the largest quantity of high pressure gas possible is released into one location to maximize the very low frequency output.
  • the largest bubble in an array e.g. the total source volume may be limited by the onboard compressor capacity and the seismic shot interval.
  • the total source volume also may be limited by the flow capacity of the conduits (e.g., hoses) connecting the source and the onboard compressor or onboard high pressure reservoir, or it may be limited by the number of high pressure hoses the ship can tow.
  • the largest bubble in the source array might be limited by how much of the total source volume can be released into substantially one location.
  • FIG. 5 depicts one embodiment of the present disclosure in which the largest bubble is limited by the flow capacity of the high pressure hoses that connect the source to the vessel.
  • a vessel 60 tows a source array 62, which includes a pair of substantially identical subarrays 64 disposed about another subarray 66 (which includes a pair of single airguns 67 disposed about a cluster 68 in the in-line direction).
  • the cross-line spacing between subarrays 64 and 66 is, for example, 6 meters and the in-line spacing between each of the single airguns 67 and the cluster 68 is, for example, 6 meters.
  • each subarray 64, 66 is charged by a shipborne high pressure reservoir via one high pressure hose, whose flow capacity is approximately 2100 in 3 per shot.
  • the source array 62 is further limited by how close one can place two hoses, and consequently it may not be possible to frequency lock the bubbles from different subarrays.
  • the largest bubble may be 2100 in 3 .
  • the bubble frequency associated with 2100 in 3 is achieved by fully frequency locking six 350 in 3 airgun bubbles, while the bubble frequency associated with 1050 in 3 is achieved by fully frequency locking two 525 in 3 airgun bubbles.
  • the dashed lines indicate the regions with full frequency locking. The interaction between the regions is negligible, and it is also negligible between any region and any of the single airgun bubbles.
  • the effective bubble volume of the subarrays 64 is substantially 2100 in 3
  • the subarray 66 has two distinct bubble frequencies: the frequencies associated with a bubble volumes of 1050 in 3 and 525 in 3 . These volumes result in bubble frequencies that are 1/3 and 2/3 of an octave higher than the lowest bubble frequency.
  • the outer subarrays 64 have been optimized according to the present disclosure, while the subarray 66 is used to fill-in the bubble notch frequencies in order to flatten the spectrum.
  • the largest effective bubble volume is approximately 33.3% of the total source volume, i.e. 2100 in 3 out of 6300 in 3 total volume.
  • Example 2 Source array wherein the largest bubble equals the total source volume
  • Figure 6 depicts a schematic view of another source somewhat different than that disclosed in Example 1 and wherein all of the released gas is frequency locked into one very large airgun bubble.
  • a source 70 comprises, for example, twenty - one 300 in 3 airgun bubbles arranged on a hexagonal grid in three layers.
  • the airguns are connected to a plurality of high pressure hoses 72, which extend from a vessel 74.
  • the high pressure hoses 72 terminate at substantially the same location, thus enabling the guns to be positioned within full frequency locking distance.
  • All twenty-one airgun bubbles are fully frequency locked and oscillate with the period associated with the combined volume, e.g. 6300 in 3 .
  • the volume of the largest effective bubble equals the total source volume.
  • Figure 7 compares the modeled spectra of the conventional source in Figure 4 (dotted curve) wherein the largest effective bubble volume is 15% of the total source volume; the new source illustrated in Figure 5 (dashed curve) wherein the largest effective bubble volume is approximately 33% of the total source volume; and the new source shown in Figure 6 (solid curve) wherein the effective bubble volume is 100% of the total source volume.
  • the comparison of Figure 7 demonstrates how the low frequency output increases with the size of the effective bubble volume as both the dashed and the solid curves have significantly more low frequency output than the conventional reference spectrum (dotted curve).
  • Figure 8 depicts a flow chart 80 showing how a source can be designed, or re-designed, to take this into account. That is, an optimum low frequency source may be designed, while maintaining some spectral flatness given design restrictions imposed by the available equipment.
  • the desired relative size of the largest effective bubble volume relative to the total source volume is determined.
  • This desired ratio of the volume of the largest bubble in the array to the total array volume affects the trade-off between maximizing the low frequency output and flattening the spectrum by introducing additional bubble frequencies to fill-in the spectral notches.
  • the desired size of the largest effective bubble is half of the total source volume available per shot. This ratio is 1/3 in Example 1 ; 1/1 in Example 2; and about 0.74 in Example 4.
  • the ratio illustrated in block 82 is equivalent to "the desired ratio Qeff/Qtotal" of block 137 in Figure 12.
  • Block 84 establishes the largest effective bubble volume that may be practically achieved given the restrictions imposed by the available equipment.
  • the total source volume per shot is limited by the flow capacity per high pressure gas hose and the number of hoses available.
  • Figure 9 which illustrates one implementation, only two gas hoses 90, 92 from a high pressure reservoir, e.g. the shipboard compressor, and a submerged source array 94 are available, with each hose having a flow capacity of, for example, 1680 in 3 per shot.
  • the largest effective bubble volume is further limited by how close the two strings can be positioned to avoid tangling of the equipment while towing, so it is not possible to frequency lock gas bubbles from different strings. Consequently, the actual effective volume of the largest bubble may be 1680 in 3 .
  • the largest bubble volume may be affected by the supply reservoir capacity. In other words, the output capacity of the compressor per shot may be increased to achieve maximum actual bubble volume.
  • the largest bubble volume may also be affected by the flow capacity of each hose between the supply and source.
  • the cross-sectional flow area of the hoses feeding the source may be increased to allow enough compressed air to be delivered to the source within a shot-recharge-cycle.
  • several gas hoses may be used in parallel so as to increase the total feed capacity.
  • Adjacent subarrays can be suitably designed and/or arranged within locking distance such that the air released from airguns that are located on different subarrays being fed by separate hoses may be frequency-locked so as to creat the maximum bubble volume.
  • the conduits between the high pressure supply and the airgun should contain no or minimum branched network of hoses and pipes so that there is no or insignificant restriction on the total volume of released air that can be frequency-locked.
  • block 86 contemplates design of an airgun cluster arrangement 96 ( Figure 9) to achieve the largest frequency-locked bubble, i.e. the one that results in the 1680 in 3 effective bubble volume, which corresponds to the total volume available from one gas hose.
  • the desired bubble frequency is achieved with six 280 in 3 airguns in close proximity.
  • Block 88 of the flow chart contemplates design of other bubble frequencies in the array so as to facilitate frequency-locking of other smaller bubbles in the array.
  • another airgun subarray 98 may be arranged to use the remaining gas to flatten the spectrum.
  • three additional bubble frequencies may be uniformly distributed, on a linear frequency scale, between the first bubble frequency and its first harmonic.
  • the second largest effective bubble volume, V2 is created by a three gun cluster having two 250 in 3 guns and one 360 in 3 gun.
  • V3 is created by a two gun cluster having two 250 in 3 guns
  • V4 is a single airgun of 310 in 3 .
  • the design of Figure 9 achieves a largest effective bubble volume that is 50% of the total source volume, while also achieving spectral flattening using three additional bubble frequencies that are uniformly distributed between the frequency of the largest bubble and its first harmonic. The spectral flattening of this array is illustrated in Figure 10.
  • Figure 1 1 A depicts the layout of a conventional cluster source deployed in dual mode as described in U.S. Patent No. 4956822.
  • the prior art cluster source has been designed for enhanced spectral flatness, i.e. high primary-to-bubble ratio, with little regard for the very low frequency source output.
  • the value of the effective bubble volume, V e ff can be calculated by solving the Rayleigh- Willis formula in Equation 1 for the bubble volume, V, and inputting values for air pressue (e.g., 2 i), source depth (e.g, 10 ft) and bubble frequency (e.g. 1/196.5 ms ⁇ 5.089 Hz).
  • such source can be redesigned to maximize the low frequency output while using the same total amount of compressed air and maintaining some spectral flatness.
  • the source depth is the same, i.e. 10 ft, and that the total volume cannot exceed 2400 in 3 and that there are no other restrictions on how the total volume can be distributed
  • the source can be redesigned to have two distinct bubble frequencies.
  • the first bubble frequency may be defined as low as possible and the second bubble frequency may be designed as being half an octave higher than the first bubble frequency, such that the second bubble frequency will coincide with e first bubble frequency. This restriction can be expressed as
  • the largest effective bubble may be approximately 74% of the total source volume. Consequently, the first bubble may oscillate with a frequency associated with 1773 in 3 and the second bubble may oscillate with a frequency associated with 627 in 3 . Similar to the other examples disclosed herein, such bubble oscillations are obtained by fully frequency locking multiple airgun bubbles in close proximity.
  • the redesigned low frequency source is depicted in Figure 1 IB as having a pair of cluster units 100, 102, including a 1773 in 3 bubble, which may be constructed from six 295.5 in 3 guns, and a 627 in 3 bubble, which may be constructed from two 313.5 in 3 guns.
  • the two cluster units are well separated such that the interaction between the cluster units is negligible.
  • Redesigning the prior art cluster according to the principles disclosed herein increases the effective bubble volume from 956 in 3 to 1773 in 3 , which at 10 ft depth corresponds to a bubble frequency of 5.1 and 4.1 Hz respectively. Accordingly, the bubble frequency has been shifted by 0.3 octave by redesigning the cluster source layout.
  • the source array can be selectively redesigned to emit more low frequency output by clustering several airguns to create a large bubble.
  • an approach is used to optimize low frequency output from airgun source arrays by, for example, increasing the firing pressure, thereby enabling an even lower bubble resonance frequency.
  • description is provided regarding how to create a bigger bubble by frequency locking the oscillation from multiple airgun bubbles. When airguns fire in close proximity the resulting bubble oscillates with the frequency associated with the total cluster volume.
  • the approach is generalized to also account for the firing pressure.
  • the embodiments also may be combined to utilize, for example, larger bubbles and/or more optimized firing pressure to further optimize low frequency output.
  • the largest quantity of high pressure gas possible may be released into one location.
  • the total source volume may be limited by the onboard compressor capacity and the seismic shot interval; the total source volume also may be limited by the flow capacity of the hoses connecting the source and the onboard compressor or onboard high pressure reservoir, or it may be limited by the number of high pressure hoses that a ship can tow.
  • an assumption was made that the supply pressure was already operating at maximum capacity, while here we also consider the case of increasing the supply pressure level.
  • / is the bubble frequency measured in Hertz and d is the source depth in metres.
  • Q is referred to as the "bubble energy" and measured in the awkward, yet convenient, unit psi-in 3 (pound-per-square-inch times cubic-inches); 1 psi-in 3 « 0.1 130 Joules.
  • the empirical constant k is the same as before.
  • This generalized version of the Rayleigh- Willis formula can be used to calculate the bubble frequency for fully frequency locked bubbles wherein the individual airguns have been charged with different firing pressure.
  • the combined bubble energy of the frequency locked bubbles is the sum of the product of the individual airgun volume and its firing pressure:
  • bubble frequency locking in a more general way: when airguns fire in close proximity the airgun bubbles are frequency locked when the resulting bubble(s) oscillates with the frequency associated with the total cluster bubble energy Q.
  • Equation 4 For the bubble energy, Q, we obtain a metric that describes the "effective bubble energy ", Q e g, of an array:
  • fbs is the observed bubble frequency from source signature measurements. This definition can be used to quantify the effective bubble energy of the largest bubble in any prior art source, thereby quantifying to what degree the individual bubbles frequency lock.
  • Example 5 Quantifying the effective bubble energy of a source
  • the effective bubble energy can be used to evaluate the low frequency utilization of any source.
  • a source configuration comprising multiple airguns.
  • the inter-gun distance is small enough for bubble frequency locking to occur, but it is not clear how many bubbles frequency lock and whether they fully frequency lock; i.e. oscillate with the frequency associated with the combined bubble energy.
  • the airguns have a total volume of 2400 in 3 , and that it has been tested at 3m source depth at two different pressure levels with the following results:
  • Equation 6 one can calculate the effective bubble energy and thereby quantify the degree of frequency locking of the source.
  • Example 6 Source configuration limited by the supply pressure
  • an example is provided for designing a source which is limited by a given firing pressure, e.g. 2000 psi firing pressure.
  • an initial setup procedure may be performed in which airgun source parameters are obtained, as indicated by block 110.
  • the setup may comprise obtaining initial hardware parameters, e.g. air guns (type, number, chamber volume, firing pressure) and source geometry (airgun to airgun spacings), as indicated by block 112.
  • initial hardware parameters e.g. air guns (type, number, chamber volume, firing pressure) and source geometry (airgun to airgun spacings)
  • the total bubble energy achievable may be calculated, as indicated by block 1 14.
  • a variety of processes may be employed to maximize the total possible bubble energy, as indicated by block 116.
  • a determination may be made whether airgun volume can be increased, as represented by decision block 1 18. If the volume can be increased, a larger chamber is selected, as indicated by block 120, and this updated information is used in updating the energy calculations (see block 1 14). Another determination may be whether the number of airguns can be increased, as represented by decision block 122. If the number of airguns can be increased, a larger number is selected as indicated by block 124. Again, this updated information may be used in updating the energy calculations at block 114.
  • Another example of a determination for maximizing total bubble energy may comprise evaluating whether the firing pressure can be increased, as indicated by decision block 126. If the firing pressure can be increased, the increased pressure parameters are determined, as indicated by block 128. This updated information also may be used to update the overall energy calculations at block 1 14.
  • the final selected parameter values may be used to facilitate assembly of a desired airgun source array, as indicated by block 130.
  • In-sea tests may then be conducted, as indicated by block 132.
  • the airgun source may be placed in-sea to enable firing of the airguns and obtaining of desired measurements, e.g. signatures, bubble frequencies, and depth.
  • desired measurements e.g. signatures, bubble frequencies, and depth.
  • the data is used to calculate an effective bubble energy, as indicated by block 134. This allows determination of whether the ratio of effective bubble energy to total bubble energy achievable is desirable, as indicated by decision block 136.
  • the desired ratio of effective bubble energy to total bubble energy may be input, as indicated by block 137. If the desired ratio is achieved, the design is validated, as indicated by block 138.
  • additional evaluations can be performed. For example, a determination may be made as to whether airgun spacings can be decreased, as indicated by decision block 139. If the airgun spacings can be decreased, another source geometry can then be determined, as indicated by block 140. The new source geometry may be used to facilitate assembly of another source array, as indicated by block 130. However, if the additional evaluations do not suggest a new source geometry, the specific design may be terminated, as indicated by block 142.
  • Figure 13 illustrates a source configuration that has been designed according to a present embodiment.
  • a source 144 (which may be part of a larger source array or may comprise the entire source array) has a total volume of 1500 in 3 , and comprises six 250 in 3 airguns 146 mounted in close proximity.
  • a group of three airguns 146 may be mounted on the first side of a framework 148 and a second group of three airguns 146 may be mounted on a second side of the framework 148.
  • the number and arrangement of airguns 146 may be adjusted/changed depending on the parameters of a given application.
  • the airguns 146 are suspended from a float 150 via suitable suspension lines 152. When operated with 2000 psi firing pressure at 7 metres depth, for example, and assuming the individual bubbles fully frequency lock, Equation 1 predicts the resulting bubble will oscillate at 5.5 Hz.
  • Equation 1 shows that the bubble frequency is inversely proportional to the cube-root of the product of the firing pressure and the airgun chamber volume.
  • Equation 5 shows that the bubble energy is proportional to the firing pressure and volume of the airgun source. Consequently, increasing the airgun chamber volumes from 250 in 3 to 350 in 3 is equivalent to increasing the total bubble energy by 40%.
  • Figure 14 shows measured far-field spectra of the 1500 in 3 source (illustrated in Figure
  • This low frequency source with an initial total volume of 1500 in 3 subsequently increased to 2100 in 3 , is the result of the disclosed design method described in figure 12.
  • the result of the design process is creation of the largest bubble possible using available airgun chambers to increase the low frequency output and to maximize the total bubble energy, i.e. to maximize Q tota i
  • This initial design example was limited by the total source volume (1500 in 3 ) and the maximum number of airguns (six) within the frame design.
  • total source bubble energy could be increased to 4200 ⁇ 10 3 psi-in 3 by fitting larger airgun chamber volumes.
  • the final design in this example is limited by the maximum number of airguns in the frame and the maximum safe firing pressure of the airguns.
  • Figure 13 may again be used to illustrate a configuration embodiment of the present disclosure.
  • the source 144 again has a total volume of 1500 in 3 and comprises six 250 in 3 airguns 146 mounted in close proximity.
  • Equation 1 predicts the resulting bubble will oscillate at 5.5 Hz.
  • a majority of seismic surveys operate airgun sources with about 2000 psi firing pressure.
  • Equation 1 shows that the bubble frequency is inversely proportional to the cube-root of the product of the firing pressure and the airgun chamber volume. Consequently, increasing the firing pressure from 2000 to 3000 psi is equivalent to increasing the total bubble energy by 50%, i.e. increasing P from 2000 to 3000 psi in Equation 5.
  • Figure 15 shows measured far field spectra of the 1500 in 3 source 144, illustrated in Figure 13, operated with 3000 and 2000 psi firing pressure at 7 metres depth.
  • the vertical dashed lines indicate the theoretical bubble resonance frequency calculated with Equation 4.
  • the figure confirms that the six bubbles fully frequency lock at both 2000 and 3000 psi, i.e. the bubbles oscillate with the frequency associated with the total bubble energy.
  • the figure shows that the bubble frequency is significantly lower by increasing the firing pressure from 2000 to 3000 psi.
  • This low frequency source with a total volume of 1500 in 3 and fired at 3000 psi, is the result of implementing the disclosed design method described in Figure 12. This design also creates the largest bubble possible within operational constraints to increase the low frequency output.
  • the inter-gun spacing and the distribution of bubble frequencies are normally chosen to achieve maximum spectral flatness, while in this case we aim to emit only one bubble frequency: the oscillation frequency associated with the total air supply capacity available.
  • the initial design was limited by the total source volume (1200 in 3 ) and the maximum operating pressure for the chosen airgun type (2000 psi).
  • total source bubble energy could be increased to 4500 ⁇ 10 3 psi-in 3 by replacing the airgun type with one that is designed to safely operate at 3000 psi and by increasing the airgun chamber volumes.
  • the final design may be limited by the number of airguns and the maximum airgun chamber volume considered.
  • the first part of the design process aims to maximize the total bubble energy available, while the second part of the design process aims to release all the bubble energy available into one bubble frequency. This is achieved by fully frequency locking the bubble oscillation for the air released from the six airguns 146 which may require compact source geometry.
  • the table below shows how the outer dimensions of the source shrunk following the second part of the design process outlined in Figure 12:
  • Example 8 Clustering airguns charged with different firing pressure
  • the airguns 146 illustrated in Figure 13 provide a special cluster configuration. It is the result of optimizing the low frequency output when only specific airguns are available and with limited high pressure air supply capacity.
  • the high pressure reservoir has a maximum output capacity of 1200 in 3 at 2000 psi per shot and that only six 250 in 3 airguns are available to use in our source design. Additionally, the desired optimization of low frequency output in this example is to maximize the low frequency output.
  • the largest possible bubble is created and this can be achieved by frequency locking as many of the 250 in 3 airguns as possible.
  • Figure 16 shows a measured farfield spectrum of the resulting source.
  • Figure 13 See Figure 13 and the resulting source example described in the preceding paragraph.
  • Figure 16 graphically illustrates measured farfield spectrum of the source configuration described in the preceding paragraph in which the source depth is 7 meters.
  • the fundamental bubble frequency of the measured spectrum coincides well with the theoretical prediction, which demonstrates that the source 144 is utilizing the full air flow capacity in the resulting bubble oscillation as desired.
  • Example 9 Source array wherein the largest bubble is one-third of the total source volume
  • Example 9 is similar to Example 1 discussed above, but the present example incorporates the effective bubble energy framework.
  • one aspect is release of the largest quantity of high pressure gas possible into one location to maximize the very low frequency output.
  • the total source volume may be limited by the onboard compressor capacity and the seismic shot interval.
  • the total source volume also may be limited by the flow capacity of the conduits (e.g., hoses) connecting the source and the onboard compressor or onboard high pressure reservoir, or it might be limited by the number of high pressure hoses the ship can tow.
  • FIG. 5 may again be referenced as depicting an embodiment of the present disclosure where the largest bubble is limited by the flow capacity of the high pressure hoses that connects the source to the vessel 60.
  • vessel 60 tows a source array 62, which includes the pair of substantially identical subarrays 64 disposed about another subarray 66 (which includes the pair of single airguns 67 disposed about cluster 68 in the in-line direction).
  • the cross-line spacing between subarrays 64 and 66 is, for example, 6 meters and the in-line spacing between each of the single airguns 67 and the cluster 68 is, for example, 6 meters.
  • each subarray 64, 66 is charged by a shipborne high pressure reservoir via one high pressure hose, whose flow capacity is approximately 2100 in 3 at 2000 psi per shot.
  • the subarrays 64, 66 are charged to the same pressure level, and the source array 62 may be further limited by how close one can place two hoses, and consequently it may not be possible to frequency lock the bubbles from different subarrays.
  • the largest bubble is 2100 in 3 .
  • the bubble frequency associated with 2100 in 3 is achieved by fully frequency locking six 350 in 3 airgun bubbles, while the bubble frequency associated with 1050 in 3 is achieved by fully frequency locking two 525 in 3 airgun bubbles.
  • the dashed lines indicate the regions with full frequency locking. The interaction between the regions is negligible, and it is also negligible between any region and any of the single airgun bubbles.
  • the subarray 66 has two distinct bubble frequencies: the frequencies associated with bubble volumes of 1050 in 3 and 525 in 3 . These volumes result in bubble frequencies that are 1/3 and 2/3 of an octave higher than the lowest bubble frequency.
  • the outer subarrays 64 have been optimized according to the present disclosure, while the subarray 66 is used to fill-in the bubble notch frequencies in order to flatten the spectrum in a conventional manner.
  • Example 10 Source array wherein the largest bubble equals the total source volume
  • Example 10 is similar to Example 2 discussed above, but the present example incorporates the effective bubble energy framework.
  • this figure depicts a schematic view of another type of source relative to that disclosed in Example 8, wherein all of the released gas is frequency locked into one very large airgun bubble.
  • the source 70 comprises twenty-one 300 in 3 airgun bubbles arranged on a hexagonal grid in three layers.
  • the airguns are again connected to the plurality of high pressure hoses 72, which extend from vessel 74. With this arrangement, the high pressure hoses 72 terminate at substantially the same location, thus enabling the guns to be positioned within full frequency locking distance. All twenty-one airgun bubbles are fully frequency locked and oscillate with the period associated with the combined volume, e.g.
  • the volume of the largest effective bubble equals the total source volume.
  • Figure 7 compares the modeled spectra of a conventional source and two of the new sources designed according to the present invention.
  • the firing pressure is 2000 psi for all three cases.
  • Figure 7 compares the conventional source illustrated in Figure 4 (dotted curve), in which the effective bubble energy of the largest bubble is 15% of the total bubble energy of the array; the new source illustrated in Figure 5 (dashed curve), in which the effective bubble energy of the largest bubble is approximately 33% of the total bubble energy of the array; and the new source shown in Figure 6 (solid curve), in which the effective bubble energy is 100% of the total bubble energy of the array.
  • Example 1 1 is similar to Example 3 discussed above, but the present example incorporates the effective bubble energy framework.
  • additional gas for tuning the source array by creating other distinct bubble frequencies may be desired.
  • the flowchart of Figure 12 shows how a source can be designed, or re-designed, to take this into account. That is, an optimum low frequency source may be designed, while maintaining some spectral flatness given design restrictions imposed by the available equipment.
  • the desired ratio of the effective bubble energy of the largest bubble to the total source bubble energy is determined. This ratio affects the tradeoff between maximizing the low frequency output and flattening the spectrum by introducing additional bubble frequencies to fill-in the spectral notches.
  • the desired effective bubble energy of the largest bubble is two-thirds of the total source bubble energy available per shot.
  • the design procedure establishes the largest effective bubble energy that may be practically achieved given the restrictions imposed by the available equipment.
  • the total source bubble energy per shot is limited by the flow capacity per high pressure gas hose and by the number of hoses available.
  • a high pressure reservoir e.g. the shipboard compressor
  • a submerged source array 94 are available.
  • Each hose has a flow capacity of, for example, 1680 in 3 at 2000 psi per shot.
  • the desired bubble frequency is achieved with six 280 in 3 airguns in close proximity.
  • Block 130 'Assembly' also can be used to design other bubble frequencies in the array.
  • another airgun subarray 98 may be arranged to use the remaining gas to flatten the spectrum.
  • three additional bubble frequencies may be uniformly distributed, on a linear frequency scale, between the first bubble frequency and its first harmonic.
  • the second largest bubble volume, V2 is created by a three gun cluster having two 250 in 3 guns and one 360 in 3 gun.
  • V 3 is created by a two gun cluster having two 250 in 3 guns
  • V4 is a single airgun of 310 in 3 .
  • each of the frequency locked bubbles can be designed via the second part of the flow chart by inputting the respective bubble energy ratio in block 137 'Desired ratio'.
  • the design of the layout in Figure 9 achieves a largest effective bubble energy that is 50% of the total bubble energy of the source, while also achieving spectral flattening using three additional bubble frequencies that are uniformly distributed between the frequency of the largest bubble and its first harmonic.
  • the spectral flattening of this array is illustrated in Figure 10.
  • Example 12 Redesign of a conventional cluster source
  • Example 12 is similar to Example 4 discussed above, but the present example again incorporates the effective bubble energy framework.
  • useful methods are provided for redesigning existing cluster arrays to optimize low frequency output.
  • Figure 1 1A depicts the layout of a conventional cluster source deployed in dual mode as described in U.S. Patent No. 4956822.
  • the prior art cluster source has been designed for enhanced spectral flatness, i.e. high primary-to-bubble ratio, with little regard for the very low frequency source output.
  • the value of the effective bubble energy, ⁇ ) can be calculated using the formula in Equation 6 and inputting values for air pressure (e.g., 2000 psi), source depth (e.g, 10 ft) and bubble frequency (e.g. 1/196.5 ms ⁇ 5.089 Hz).
  • such source can be redesigned to maximize the low frequency output while using the same total amount of compressed air and maintaining some spectral flatness.
  • the source depth is the same, i.e. 10 ft, and that the total volume cannot exceed 2400 in 3 and that there are no other restrictions on how the total volume can be distributed
  • the source can be redesigned to have two distinct bubble frequencies.
  • the first bubble frequency may be defined as low as possible and the second bubble frequency may be designed as being half an octave higher than the first bubble frequency, such that the second bubble frequency will coincide with the
  • the effective bubble energy of the largest bubble may be approximately 74% of the total bubble energy of the source.
  • the firing pressure in the present source embodiment may be similar to that of previous embodiments (e.g. 2000 psi). Consequently, at 2000 psi, the first bubble may oscillate with a frequency associated with 1773 in 3 (about 74% of 2400) and the second bubble may oscillate with a frequency associated with 627 in 3 (about 26% of 2400). Similar to the other examples disclosed herein, such bubble oscillations are obtained by fully frequency locking multiple airgun bubbles in close proximity.
  • the redesigned low frequency source is depicted in Figure 1 IB as having a pair of cluster units 100, 102, including: a 1773 in 3 bubble, which may be constructed from six 295.5 in 3 guns, and a 627 in 3 bubble, which may be constructed from two 313.5 in 3 guns.
  • the two cluster units are well separated such that the interaction between the cluster units is negligible.
  • sources and source arrays may be used, arranged, and/or positioned to optimize low frequency output.
  • the bubble size and/or pressures applied may be selected individually or in combination to optimize low frequency output from airgun source arrays.
  • the methodology comprises deploying a plurality of air guns and activating at least some of the airguns to generate an effective bubble energy as described in the embodiments above.
  • the airguns may be activated to generate an effective bubble energy of more than 4000 ⁇ 10 3 psi-in 3 .
  • at least some of the airguns may be activated to generate multiple frequency locked bubbles such that the largest effective bubble energy is greater than 10000 ⁇ 10 3 psi-in 3 .
  • the activating also may comprise generating effective bubble energy in which the largest bubble has effective bubble energy greater than 3000 ⁇ 10 3 psi-in 3 and in which the largest bubble comprises multiple frequency locked bubbles.
  • the effective bubble energy of the largest bubble is between 33% and 74% of the total potential bubble energy fired at each shot.
  • the largest bubble effective bubble energy and the ratio of the largest bubble effective bubble energy to the total potential bubble energy may vary depending on the parameters of a specific application.
  • the methodology comprises providing a plurality of air guns and identifying a largest effective bubble energy for a marine source array. At least some of the airguns may be arranged to achieve the largest effective bubble energy upon activation of those airguns. However, the effective bubble energy of at least some of the plurality of airguns may be optimized according to other parameters. Additionally, the airguns may be arranged in close proximity to one another and the airguns may be activated to fully frequency lock the bubbles emitted by the airguns. In any of these applications, the effective bubble energy and the ratio of effective bubble energy to potential bubble energy may be selected according to the needs/parameters of the seismic application.
  • the airguns may be activated to generate an effective bubble energy of more than 4000 ⁇ 10 3 psi-in 3 and/or those airguns may be activated to generate multiple frequency locked bubbles such that the largest effective bubble energy is greater than 10000 ⁇ 10 3 psi-in 3 as discussed above.
  • the activating also may comprise generating effective bubble energy where the largest bubble has effective bubble energy greater than, for example, 3000 ⁇ 10 3 psi-in 3 and in which the largest bubble comprises multiple frequency locked bubbles.
  • the effective bubble energy of the largest bubble in these latter examples may similarly be between 33% and 74% of the total potential bubble energy fired at each shot. Again, however, these values are provided as examples and the effective bubble energy values and ratio values can change depending on the parameters of a given seismic application.
  • VSP vertical seismic profiling

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Abstract

L'invention concerne une technique qui donne lieu à une conception de sources et à un procédé destinés à accroître la puissance de sortie à basse fréquence d'un réseau de sources marines. L'approche comporte une étape consistant à mettre en place une pluralité de canons à air. Au moins une partie des canons à air est activée pour générer une énergie efficace de bulles. Ladite énergie efficace des bulles peut être optimisée par le biais de l'utilisation, de la préparation et / ou de la disposition des canons à air.
PCT/US2012/038751 2011-05-20 2012-05-21 Systèmes et procédés d'optimisation de la puissance de sortie à basse fréquence émanant de réseaux de sources de type canon à air WO2012162208A2 (fr)

Priority Applications (3)

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EP12789555.5A EP2710410A4 (fr) 2011-05-20 2012-05-21 Systèmes et procédés d'optimisation de la puissance de sortie à basse fréquence émanant de réseaux de sources de type canon à air
MX2013013383A MX2013013383A (es) 2011-05-20 2012-05-21 Sistemas y metodos para optimizar la salida de baja frecuencia de arreglos de fuentes de pistolas neumáticas.
BR112013029774-3A BR112013029774B1 (pt) 2011-05-20 2012-05-21 Sistema para aumentar saída de baixa frequência de um conjunto de fonte marítima, e método para aumentar saída de frequência baixa de um conjunto de fonte marítima

Applications Claiming Priority (6)

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US13/112,869 2011-05-20
US13/112,869 US9025417B2 (en) 2010-08-24 2011-05-20 Systems and methods for optimizing low frequency output from airgun source arrays
US201161568655P 2011-12-09 2011-12-09
US61/568,655 2011-12-09
US13/450,477 2012-04-19
US13/450,477 US9360578B2 (en) 2010-08-24 2012-04-19 Systems and methods for optimizing low frequency output from airgun source arrays

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CN112180431A (zh) * 2019-07-04 2021-01-05 中石化海洋石油工程有限公司上海物探分公司 气枪震源设置方法、装置及机器可读存储介质

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Publication number Priority date Publication date Assignee Title
NL2016387B1 (en) * 2016-03-08 2017-09-27 Fugro N V Seismic source array and survey ship
CN112180431A (zh) * 2019-07-04 2021-01-05 中石化海洋石油工程有限公司上海物探分公司 气枪震源设置方法、装置及机器可读存储介质

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BR112013029774A2 (pt) 2017-01-24
EP2710410A2 (fr) 2014-03-26
WO2012162208A3 (fr) 2013-01-24
MX2013013383A (es) 2014-01-08

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