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.