GB2183834A

GB2183834A - Underwater seismic sources and method of operation

Info

Publication number: GB2183834A
Application number: GB08529604A
Authority: GB
Inventors: David Charles Newall Pratt
Original assignee: Decca Ltd
Current assignee: Decca Ltd
Priority date: 1985-12-02
Filing date: 1985-12-02
Publication date: 1987-06-10
Also published as: GB8529604D0

Abstract

An underwater seismic source comprises a subarray in which at least two water guns are of different volume selected so that their frequency spectra in the range 20-50 Hz mutually complement in the far field. Each gun is operated at its optimum depth for power output and the guns are fired so that their main energy pulses occur simultaneously in the far field. <IMAGE>

Description

SPECIFICATION Underwater seismic sources and method of operation The present invention relates to underwater seismic sources and particularly to a subarray comprising at least two implosive seismic devices, called hereinafter "water guns".

The frequency spectrum of a single water gun, though generally of braod bandwidth, has some undesirable characteristics. Guns with smaller volume chambers tend to have ppor low frequency content, making them unsatisfactory if deep penetration is required. Guns with larger volume chambers tend to have undesirable spectral notches, usually in or near the middle of the desired frequency range for seismic data. It has been proposed, for example in British Patent Application No.

2148503-A, to operate identical water guns at markedly different depths which are chosen such that reflections of the wave feilds of the two guns from the air-sea interface will substantially cancel each other. By this expedient some mutual cancellation of the notches in the far field may be obtained. Unfortunately, the power efficiency of a water gun diminishes markedly as the gun is moved from its optimum depth and accordingly the previously proposed method is not efficient in its use of the energy available from the water guns.

The object of the present invention is to provide improvement in the energy spectrum of an underwater seismic source, such as a subarray of water guns, while enabling (in contradistinction to the prior proposal mentioned) the operation of each water gun at substantially its optimum depth.

In one aspect, the invention provides an underwater seismic source comprising at least two water guns of substantially different volumes selected so that the guns provide in their far field different but mutually complementing energy spectra in a predetermined frequency range, the guns being arranged to be fired such that the main energy pulses that they respectively produce occur at substantially the same time in their far field. Typically the frequency range in which the spectrum is improved is from approximately 20 to 50 Hertz. The present invention includes within its scope an array of seismic sources comprising a plurality of subarrays each constituted by a source as set forth above.

According to another aspect of the invention, there is provided a method of operating an underwater seismic source which comprises at least two water guns of substantially different volume and which provide in a predetermined frequency range by virtue of their different volumes different but mutually complementing energy spectra in their far field, the method comprising disposing each gun at a depth at which it provides substantially its optimum power output and firing the guns so that their main energy pulses oocur at substantially the same time in the far field.

There follows a description, by way of example, of the invention in greater detail, reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a set of graphs illustrating the variation of power output with depth of water of guns of different volume for different frequency ranges; Figure 2 illustrates the far field signature of a first water gun; Figure 3 illustrates the spectrum of the first water gun; Figure 4 illustrates the far field signature of a second water gun; Figure 5 illustrates the spectrum of the second water gun; Figure 6 illustrates the far field signature of a subarray according to the invention; and Figure 7 shows the frequency spectrum of the subarray of which the signature is shown in Fig. 6.

DESCRIPTION OF A PREFERRED EMBODIMENT Fig. 1 is a graph showing power against depth below the sea surface for two sizes of water gun for different frequency ranges. The curve A shows the power output in the frequency range 0 to 125 Hertz of a water gun which has an effective volume of 80 cubic inches (1.3 litres). Curve B shows the variation in power output for the same volume gun but for the frequency range 0 to 180 Hertz. Curve C shows the variation with depth of power in the range of O to 125 Hertz for a gun of which the effective volume is 400 cubic inches (6.5 litres) and curve D shows the variation with depth of the power output of a 400 cubic inch gun in the frequency range 0 to 180 Hertz. As may be seen, the maximum power output of the gun occurs at similar (though not exactly the same) depth and the variation of power with depth is considerable.

For this reason the disposition of the guns in a given subarray at substantially different depths is not satisfactory from the point of view of efficient operation.

Fig. 2, which is a graph of amplitude (in decibels) against time, shows the far field signature of an 80 cubic inch water gun and Fig.

3 illustrates its spectrum. Fig. 4 illustrates the signature of a 400 cubic inch gun and Fig. 5 its amplitude spectrum. Fig. 5 shows that the spectrum has a marked notch, at about 40 Hertz, within the frequency range which is critical for seismic investigation. The notch has a depth of, typically, 12 to 20 decibels. By comparison, the spectrum shown in Fig. 3 illustrates a iow-frequency fall-off beginning at about 30 Hertz and it will be seen that a combination of the spectra in Figs. 3 and 5 will provide at least some substantial improvement of the frequency spectrum in the important frequency range and in particular substantially reduce the effect of the notch in the spectrum of the 400 cubic inch gun.

At present it is difficult to compute rigorously the frequency spectra of guns of different size. However, guns of different size may be tested and their spectrum measured so as to provide a basis from which guns of different size may be selected for a subarray.

Fig. 6 illustrates the far field signature of a subarray comprising a 400 cubic inch gun and an 80 cubic inch gun fired at depths such that each gun produces its optimum power output (as determined from Fig. 1). Fig. 7 illustrates the corresponding spectrum in which the amplitude of the notch in the spectrum of Fig. 5 has been substantially diminished.

Although the foregoing description refers mainly to the use of two guns in a subarray, a subarray may be constituted by three or more guns of different sizes selected so as to provide a desired resultant power spectrum notwithstanding the operation of each gun at the depth appropriate for maximising its power output.

It is presumed in the foregoing that the guns within the subarray are fired substantially simultaneously, that is to say in such a manner that the main energy pulses that they produce occur at the same time in the far field.

Claims

1. An underwater seismic source comprising at least two water guns of substantially different volume, arranged to be fired such that the main energy pulses that they respectively produce occur at substantially the same time in their far field, the guns providing in the far field different but mutually complementing energy spectra in a predetermined frequency range.

2. A source according to claim 1 in which the combined spectrum is improved within the frequency range from approximately 20 to 50 Hertz.

3. An array of seismic sources comprising a plurality of subarrays each constituted by a source according to claim 1 or claim 2.

4. A method of operating an underwater seismic source which comprises at least two water guns of substantially different volume and which provide in a predetermined frequency range different but mutually complementing energy spectra in their far field, comprising disposing each gun at a depth at which it provides substantially its optimum power output and firing the guns so that the main energy pulses that they produce occur at substantially the same time in the far field.