Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Patent
1997-09-17
2000-06-06
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
G01V 1137
Patent
active
060727500
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for producing seismic pulses and, more particularly, to a method for generating acoustic wave energy for use in seismic surveying employing, for example, a seismic gun or seismic source apparatus which conventionally utilises air and is commonly known in this field as an air gun.
2. Related Art
An air gun normally rapidly releases very high pressure air into, for example, water to generate an air bubble and associated acoustic pulse. The acoustic pulse generated in the water then propagates as a wave downwards through the water and, if generated in water offshore, down through the subwater or subsea strata, at the interfaces of which, in sedimentary layers, it is partially reflected back towards the surface. Reflected pulses, detected, for example, by receivers at the surface or in a nearby borehole, can usually be attributed to sedimentary layer structures. Seismic surveys employing this kind of technique are used, for example, in searches for hydrocarbon reserves to delineate the sedimentary structures below a seabed and identify potential structural `traps` for hydrocarbons.
The quality of an air gun is commonly specified in terms of the strength of the initial pulse (Pa) in bar-meters (the acoustic pressure in bars times the distance from the source in meters) together with the ratio of the strength of the initial pulse to the strength of the maximum bubble peak (Pa/Pb). This is illustrated in somewhat schematic form in FIG. 1.
Having regard to FIG. 1, the larger initial or primary pulse is the important pulse because this provides the initial impulse from which strongest reflections derive. The larger the amplitude the greater the pulse energy and hence the greater the penetration. The smaller subsequent bubble pulses that are generated are a problem because additional pulses complicate the reflection wavefield which must be removed in subsequent processing.
In order to increase both Pa and Pa/Pb, "tuned" arrays of guns are currently used. In the tuned arrays, guns of different volumes are placed at locations calculated to cause cancelling of the bubble pulses but to reinforce the initial or primary pulses.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method for facilitating the generation of seismic pulses having both increased Pa and increased Pa/Pb.
The invention consists in a method of operating acoustic wave generating apparatus which comprises a gas pressurisable chamber having an inlet for permitting a pressurised gas to be introduced into the chamber and a closed outlet which can be opened rapidly to release pressurised gas from the chamber into a liquid to generate acoustic wave energy, wherein the method comprises employing a single gas or gas mixture in the apparatus, the gas being substantially helium or substantially hydrogen, or the gas mixture comprising helium and/or hydrogen to the extent that the gas mixture has a notional molecular weight which is less than air, and introducing such helium, hydrogen or gas mixture under pressure into the chamber and then operating the apparatus to release the pressurised helium, hydrogen or gas mixture through the outlet to generate the acoustic wave energy. For example, the apparatus employed may be a so-called `Bolt air gun` or a so-called `sleeve gun` or a `GI-gas injector gun`.
The invention also consists in a seismic surveying method comprising producing acoustic wave energy in accordance with the method as defined above.
The gas mixture having a notional, average or mean molecular weight less than air may be 90% helium and 10% air.
Alternatively, the gas mixture may be helium and hydrogen.
Preferably, the gas used is helium alone or substantially alone. The use of helium is particularly convenient because it is practically completely inert and readily available in bottled form.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1, which was described above, is a schematic representation of a seismic signal illustrating important pa
REFERENCES:
patent: 4667766 (1987-05-01), Melvin
patent: 5091891 (1992-02-01), Thomas et al.
patent: 5270985 (1993-12-01), Thomas et al.
Arnold Gareth Christopher
Wood Stephen William
BG plc
Pihulic Daniel T.
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