Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Patent
1998-01-16
1999-11-30
Lobo, Ian J.
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
367144, 367147, 181111, 181120, G01V 102, G01V 104, G01V 1157, G01S 1500
Patent
active
059954523
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
The present invention relates to systems for generating acoustic or seismic signals in underwater environments, and particularly to air gun acoustic signal generating and transmitting arrays.
Air guns are well known in the field of marine seismic exploration. Such guns are commonly formed into large horizontal arrays that are streamed from a tow vessel at a fixed depth of 15 feet or so. Such arrays are used for projecting a coherent, time-aligned impulsive signal, of high energy content and broad spectral bandwidth, downwardly into the earth for the delineation of oil bearing strata.
The acoustic signature of typical air guns, such as shown in Harrison, U.S. Pat. Nos. 4,623,033 of Nov. 18, 1986 and/or 5,654,493 of Nov. 15, 1994, mounted near a water-air interface with the measurement made in the far field below the gun, consists of an impulsive event followed by a rapidly decaying wave train, as illustrated in FIGS. 1 A & B. The positive-going impulse, produced by the explosive release of compressed gas, is quickly followed by a mirror-like negative impulse arising from the pressure-release surface reflection. This impulsive-pair is followed by a decaying oscillatory tail which arises from the resonant, decaying oscillation of the residual gas bubble.
The resonant frequency of this gas bubble oscillation is given by: ##EQU1## where:
f.sub.c =Bubble resonant frequency (Hz)
d =Depth in feet
P.sub.c =Gun cavity pressure
V.sub.c =Charge cavity volume relatively narrow spectral peak associated with the fundamental bubble resonant frequency followed by lower level harmonic contributions.
This spectrum by itself is inadequate to yield the resolution required for seismic exploration. Accordingly, investigators in the field have developed long arrays of air guns with each gun (or gun group) having a different volume of compressed air so as to distribute the resonant frequencies of the guns over a wide frequency range. The peaks and valleys of the spectra of the multiplicity of guns are then distributed by design to help flatten the composite spectrum of the array of guns. Since guns of different volumes have different spectral levels, groups of guns are incorporated to establish the final flattened spectrum. This flattened spectrum is associated with a singular impulse cycle, as shown in FIGS. 3 A & B, and becomes useful for broadband high resolution geophysical prospecting.
Reference may be had to the following U.S. Patents for information concerning such horizontal arrays: Huizer, 4,727,956 of Mar. 1, 1988; Williams, 4,955,952 of Sep. 11, 1990; French, 4,254,480 of Mar. 3, 1981; Ray, 4,441,174 of Apr. 3, 1984; Brock, 3,437,170 of Apr. 8, 1969; Mott-Smith, 3,893,539 of Jul. 8, 1975; White, 4,372,420 of Feb. 8, 1983; Ruehle, 4.382,486 of May 10, 1983; Ongkiehong, 4,486,864 of Dec. 4, 1984; Desler, 4,718,045 of Jan. 5, 1988; Dunbar, 4,868,793 of Sep. 19, 1989; Garrotta, 4,914,636 of Apr. 3, 1990; and Duren, 5,281,773 of Jan. 25, 1994.
In the field of underwater detection of submerged objects at long horizontal ranges in a deep water column, it has been found useful to deploy a vertical line array of sources, long enough to possess vertical directivity to assist in the delineation of the submerged object from surface and bottom reflections (reverberation).
It has been discovered, in accordance with the invention, that one can use a vertical line array of air guns for the purpose of flattening the spectrum and that, instead of requiring a distribution of air gun volumes to distribute the resonant frequencies of the individual guns as is done in horizontal arrays for seismic exploration, the desired distribution of resonances can be obtained from the distribution in depth of guns of the same or essentially similar volumes.
It has further been found through model studies and measurements that the peak output of a properly designed air gun of a given, fixed volume is relatively constant over a sizable depth range (e.g., 100 to 400 feet), even though its bubble resonant frequency varies with depth. As stated
REFERENCES:
patent: 4170002 (1979-10-01), Strange
patent: 4497044 (1985-01-01), Silverman
patent: 4956822 (1990-09-01), Barber et al.
patent: 5228011 (1993-07-01), Owen
patent: 5535176 (1996-07-01), Yang
Dragoset, B., "A Comprehensive Method for Evaluating the design of Airguns and Airgun Arrays," Paper presented at the 1984 Offshore Technology Conference in Houston, Texas, May 1984.
Hydroacoustics Inc.
Lobo Ian J.
Lukacher K.
Lukacher M.
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