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
  • G01V1/387—Reducing secondary bubble pulse, i.e. reducing the detected signals resulting from the generation and release of gas bubbles after the primary explosion

Definitions

• This invention relates to a seismic airgun arrangement for attenuating unwanted oscillations in the emitted signal, whereby the airgun has one or more outlet ports for exhausting air in a generally radial direction from a central axis of the airgun, and whereby a perforated screen of plate material is provided around the airgun for the attenuation mentioned.
• the invention comprises a hydrodynamic screen being mounted externally of a seismic airgun.
• the purpose of the screen is in part to destroy and baffle the air bubble formed when a seismic airgun is fired.
• an airgun In marine seismic data acquisition an airgun is the mostly employed acoustic source. In such airguns the sound is generated by letting high pressure air (100-200 atmos ⁇ pheres) escape through port openings in the airgun.
• high pressure air 100-200 atmos ⁇ pheres
• One of the drawbacks of the airgun as a seismic source is the fact that the signal generated has an oscillating nature, i.e. the signal is not a pure transient pressure pulse. Because of the oscillating air bubble which is formed in the water, there will be emitted several subsequent pressure pulses in addition to the primary pulse or amplitude. As a consequence of this the sound signal being emitted down into the water and the subsurface formation will be long (in terms of time) , and this makes it difficult to interpret the seismic data being collected.
• the two first methods above involve the employment of interaction effects and the fact that different airgun volumes give different signals so as to compose a resulting signal being more optimal, i.e. wherein the bubble pulses will partially cancel each other.
• the GI airgun actually consists of two airguns in one and the same unit, where one airgun is used for generating the signal, whereas the other airgun is used for attenuating the bubble.
• the drawback of this airgun (which is the airgun emitting the most optimal signal) is that it is more expensive than conventional air- guns, as well as it requires twice or thrice as much high pressure air as conventional airguns.
• the "Waveshapekit” mentioned above consists of a plate with holes provided inside the actual pressure chamber in the airgun. This leads to an improved signal, but a disadvantage with this "Wave- shapekit", however, is that the primary signal is also weakened.
• An object of this invention is to provide a novel and improved arrangement of seismic airguns for attenuating the unwanted bubble oscillations.
• this according to the inven ⁇ tion, is obtained in an arrangement as stated initially above, thereby that the volume which is generally delimited within the screen is significantly smaller than the maximum bubble size which is normally generated by the airgun, that the screen has an open side or end at a part of the screen being adapted to face at least partially downwards in the normal operative position of the airgun, and that at least edge portions of the screen along the open side or end are located substantially directly opposite the outlet port or ports.
• Fig. 1 shows a schematic and axial section through a known type of airgun, for example having four outlet port Fig. 2 in two diagrams (2A resp. 2B) shows characteristic pressure signatures from an airgun being not provided with and being provided with a screen arrangement according to the invention.
• Fig. 3 in lateral perspective view shows a possible embodiment of a screen which can be involved in an arrangement according to the invention,
• Fig. 4 shows the cylindrical screen in Fig. 3 from above.
• Fig. 5 shows an axial section through the airgun of Fig.
• Fig. 6 in perspective view shows a conical embodiment of a screen for use in the arrangement according to the invention
• Fig. 7 shows the screen of Fig. 6 mounted on an airgun as shown in Fig. 1
• Fig. 8 shows the same airgun as in Fig. 1, intended for use in a horizontal position during operation, provided with a screen having a cylindrical main shape
• Fig. 9 shows an airgun in a corresponding position as in
• the airgun 10 in Fig. 1 is the kind of airgun which comprises an upper chamber 10B and a firing chamber IOC having a supply of high pressure air at 10A.
• a piston 8 is mounted centrally and axially between both chambers 10B and IOC, and upon firing is adapted to be displaced from the position shown with broken lines, i.e. upon release of the high pressure air so that the air can flow out through one or more ports 9 which are directed generally outwards from the longitudinal axis of the airgun. Firing release takes place by means of a solonoid valve at the top of the airgun 10.
• Fig. 2A shows the near field signature of such an airgun having a certain size of the firing chamber (1.6 ins.
• This near field signature which is the characteristic pressure signature from this and similar types of airguns, has a primary ampli- tude 1 and subsequent bubble pulses or amplitudes 2 which give the above discussed unfavourable effects during seismic measurements.
• the hydrodynamic screen or bubble attenuator according to the invention as shown in Figs. 3-5 of the drawings, in principle has a cylin ⁇ drical shape. However, many other main shapes can_ be employed, preferably having a rotational symmetry about an axis corresponding to the longitudinal axis of the airgun. In Fig. 5 it is not shown how the mounting means for the screen (bubble attenuator) on the airgun shall be designed, since there are many alternatives here, taking into account_ the various types of airguns on which the invention can be applied.
• an airgun of corresponding capacity will have a screen arrangement according to the invention of dimension 20-30 cm, possibly as much as 50 cm diameter or lateral dimension, i.e. significantly smaller than the maximum bubbl size normally being generated.
• the volume which is generall delimited within the screen is smaller than the half of the normally maximum bubble size from the airgun concerned.
• screen 20 has an open side or end adapted to face generally downwards in the normal operational position of the airgun.
• the airgun 10 is adapted to assume a vertical position during operation, and it is seen that screen 20 has a down ⁇ wardly directed open end which is defined by the circular plate edge 20A. At this part of the screen 20 therefore there exists an opening which has a quite significant size in relation to the total surface area of the screen 20.
• this large opening at 20A is located in relative close association with the port or ports 9 for air exhausted from airgun 10.
• This arrangement is optimal in order to avoid as much as possible an attenuation of the primary amplitude of the pressure signature, as shown at 1 and 1' resp. in Figs. 2A and 2B.
• the orientation described implying that the open side or end of the screen in principle shall face downwardly in the operative position of the air- gun, is related to the fact that the air bubble has a strong tendency to rise upwards in the water.
• the screen arrangement according to the invention to a substantial degree is adapted to "catch" the bubble by generally surrounding the bubble at its upper side and laterally.
• edge 20A of screen 20 around the open end thereof runs approximately flush with the lower or outer edge 9A of the outlet port or ports 9.
• This geometrical relationship between outlet port 9 and the bordering edge of the screen around the opening can be found at least partially also in the other embodiments according to the invention, to be discussed below with reference to Figs. 7, 8 and 9.
• the shape and dimensions of the screen arrangement are so adapted that edge portions of the screen along the opening lie substantially opposite the outlet port or ports, i.e. so that these portions just cover the port openings and therefore the air jet from these openings will impinge upon these portions of the screen wall immediately upon firing.
• the open side or end of the screen is located closely adjacent to the ports of the air- gun.
• Figs. 3 and 4 show that screen 20 has a somewhat more open perforation in the form of holes 5 at the edge portions adjacent to edge 20A, than over neighbouring areas of the screen, where the holes 4 are located. Moreover, it appears, in particular from Fig. 4, that a more open perforation, i.e. a more dense arrange- ment of holes 6, is to be found at the end part of the screen 20 opposite of the opening at 20A.
• holes 5 The more dense arrange ⁇ ment of holes 5 can be explained with reference to the higher intensity of air jets or exhaustion which impinge upon this part of screen 20, whereas the larger number of holes 6 at the top or end part around mounting opening 7, is related to the rising tendency of the air bubble. More particularly, water will flow in from the lower part of the screen, so that when the bubble rises, the air will be pressed upwardly towards the upper part and is gradually vented out through holes 6 at the top.
• the purpose of these holes and the arrangement shown is to enhance the process of disintegrating the bubble while it is rising.
• the regions within or at the screen to which the air bubble will move during the time after firing, depends on the buoyant effect in the water, and besides of water flow, possibly water flow or streams due to the towing of the airguns.
• the perforation as a whole i.e. the total assembly of holes 4, 5 and 6 in Figs. 3-5, constitute at least 5% of the total surface area of the screen. Because of the air bubble buoyancy the airgun will be subjected to a jumping movement upon firing. This effect must also be taken into account in optimizing the number and position of holes 4, 5, 6 and the size of these holes as well as the size and the weight of the screen.
• the screen is preferably weight loaded, possibly in the form of excess dimensions of the screen wall thickness, in particular at regions adjacent the open side or end thereof. Such a location will contribute to a better stability of the airgun with screen arrangement during operation.
• the screen 30 has a substantially conical main shape and is provided with a hole arrangement 14, 15, 16 being rather analogous to the perfor ⁇ ation .of the cylindrical screen 20 in Figs. 3-5.
• the conical embodiment is analogous to the cylindrical embodiment, and both are adapted to assume a vertical position in operation.
• the lower bordering edge 30A of the screen 30 as shown in Fig. 7, is located at a level somewhat lower than the lower or outer edge portion 9A of the air openings 9 in the airgun.
• the conical embodiment appar ⁇ ently will make it easier to obtain a large opening facing downwards from the assembled arrangement of airgun 10 and screen 30.
• the screen and the location of the holes must be modified so that a major portion of the bubble at any time is caught within the bubble attenuator.
• the screen must be extended and partially closed at some of the port openings so that for example there will be obtained a semi-cylindrical or possibly conical shape below the ports as shown in Fig. 8 and Fig. 9 respectively.
• the most impor ⁇ tant requirement with such a change of the airgun position with an accompanying modification of the shape of the bubble attenuator (the screen) is that the screen has an open side or end in a corresponding manner as in the preceding embodiments.
• FIG. 8 the airgun 10 when used in a horizonal position, is provided with a cylindrical screen 40 which at its (right hand) end part is attached to the airgun in a similar way as in the preceding embodiments.
• This screen 40 is only partially cylindrial, i.e. along the wall part 41, since a cut-out with bordering edges 44A and 44B provides for the necessary open side or end.
• the edge 44A runs in a radial plane and forms a semi- circle in relation to the central horizontal longitudinal axis of the airgun 10, whereas the edge 44B lies in an axial plane, as shown more or less coincident with the longitudinal axis.
• a semi-circular bottom part 42 which like the other wall parts of this screen 40 can be provided with perforation holes. Holes are shown at 24, 25 and 26 on the other wall parts, and these holes can be provided at a number and a distribution being based on corresponding principles as in the preceding embodiments.
• the horizontal conical embodiment in Fig. 9 comprises a screen 50 having a conical wall part 51, a top or end part which is attached to the airgun 10, and a bottom part 52.
• This bottom part is clearly larger than the bottom part 42 in Fig. 8.
• the open side or end which in Fig. 9 is delimited by edges 55A and 55B, will be larger than the corresponding opening in Fig. 8. This apparently is due to the conical shape, as also mentioned above.
• Fig. 9 there is shown holes 34, 35 and 36 in analogy with the preceding embodiments.
• the bordering edges 44A (Fig. 8) and 55A (Fig. 9) extend approximately radially flush with the corresponding outer edge 9A of outlet ports 9. Also in these two last embodi ⁇ ments the screen (40 and 50 resp.) will be able to catch most of the air mass being released immediately upon firing, at the same time as the free opening which is partially facing downwards, permits the primary pulse amplitude not to be significantly reduced.

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• Life Sciences & Earth Sciences (AREA)
• Physics & Mathematics (AREA)
• Geology (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Environmental & Geological Engineering (AREA)
• Oceanography (AREA)
• Remote Sensing (AREA)
• General Life Sciences & Earth Sciences (AREA)
• General Physics & Mathematics (AREA)
• Geophysics (AREA)
• Aeration Devices For Treatment Of Activated Polluted Sludge (AREA)
• Air-Flow Control Members (AREA)
• Buildings Adapted To Withstand Abnormal External Influences (AREA)

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Citations (3)

• 1992
  • 1992-08-25 NO NO923314A patent/NO176076C/no not_active IP Right Cessation
• 1993
  • 1993-08-23 CA CA002143196A patent/CA2143196C/en not_active Expired - Fee Related
  • 1993-08-23 EP EP93919710A patent/EP0657035B1/en not_active Expired - Lifetime
  • 1993-08-23 AU AU49855/93A patent/AU4985593A/en not_active Abandoned
  • 1993-08-23 WO PCT/NO1993/000128 patent/WO1994004945A1/en active IP Right Grant
  • 1993-08-23 DE DE69308310T patent/DE69308310D1/de not_active Expired - Lifetime
• 1995
  • 1995-04-13 US US08/367,314 patent/US5572486A/en not_active Expired - Fee Related

Patent Citations (3)

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