Communications – electrical: acoustic wave systems and devices – Seismic prospecting – Offshore prospecting
Patent
1984-02-28
1987-02-17
Moskowitz, Nelson
Communications, electrical: acoustic wave systems and devices
Seismic prospecting
Offshore prospecting
367 56, 367144, 181118, 181120, G01V 113, G01V 140
Patent
active
046445072
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the scaling of sound source signatures in underwater (e.g. marine) seismic exploration. The present invention extends and improves the inventions described and claimed in British Patent Applications Nos. 8013438 (Ser. No. 2048481) and 8013437 (Ser. No. 2048480) U.S. Pat. No. 4,500,978 and U.S. Pat. No. 4,326,271, respectively in the name of Antoni Marjan Ziolkowski. The present Application relates in particular to sound sources having the special feature of controllable initial firing pressure, e.g. air guns, which is necessary for the utilization of this invention.
In U.S. Pat. No. 4,500,978, the contents of which are incorporated herein by reference, it is proposed that the seismic method be applied twice in each place, with both the shot and receiver positions unchanged, but with the second shot a scaled version of the first shot. In the two received seismograms x.sub.1 (t) and x.sub.2 (t), the earth impulse response g(t) is the same, because the positions of shot and receiver are unchanged. However, the two source signatures s.sub.1 (t) and s.sub.2 (t) are different and are shown to be related to each other by a source scaling law. Thus there are three unknown quantities s.sub.1 (t), s.sub.2 (t) and g(t) which are related by the three equations: defined as the relation ##EQU1## Equations (1) and (2) neglect noise. These equations may be solved to obtain s.sub.1 (t), s.sub.2 (t) and g(t) from the measurements x.sub.1 (t) and x.sub.2 (t), by a robust method described in the above Application, which method is stable in the presence of noise. The point of the invention of that Application is that previous technology was able to provide only one equation, like equation (1) containing two unknowns. There has always been a chronic problem of determining g(t) precisely in the absence of accurate information about s.sub.1 (t). Repeating the experiment, as U.S. Pat. No. 4,500,978 proposes, would not make sense without the additional information provided by the scaling law, equation (3).
For this scaling law to apply, the only parameter which can be changed is the size of the point source. For example, if explosives are used at sea, the chemical composition must be the same for source 1 and source 2; the depths below the sea surface must also be the same; only the sizes are different and are related by .alpha. (thus the mass of the second explosive is .alpha..sup.3 times the mass of the first). If air guns are used, their volumes must differ by a factor .alpha..sup.3, but the depths and firing pressures must be the same.
In the U.S. Pat. No. 4,326,271, the idea was extended to two-dimensional arrays of point sources, with air gun arrays as an example. It was proposed that each point source element in one scaled array be simply a scaled version of the corresponding element in the other array. That is, the ratio of the energies of the corresponding elements of array 2 to array 1 is .alpha..sup.3 (obeying equation 4). The dimensions of the two arrays correspondingly scale by a factor .alpha.; thus the horizontal dimensions of array 2 are .alpha. times those of array 1. No other parameters are changed. The depths are the same; and the pressures (of air guns) or chemical compositions (of explosives) are the same, in these conditions equation (3) relates the waves s.sub.2 (t) and s.sub.1 (t) which would be generated at a given distance and in a given direction from the two scaled arrays. This is fully explained in U.S. Pat. No. 4,326,271, the contents of which are incorporated herein by reference.
Problems with the Two Previous Applications
There are two problems with the solutions of previous applications which both concern the "ghost reflection" from the sea surface.
Problem 1
The "ghost reflection" is the reflection of the sound wave by the surface. In the case of a marine seismic source the sea acts like a mirror which a reflection coefficient of almost exactly -1. It has the effect of creating a virtual seismic source whose sound wave is of opposite polarity to the real source, as shown i
REFERENCES:
patent: 3866161 (1975-02-01), Ban et al.
patent: 4326271 (1982-04-01), Ziolkawski et al.
patent: 4467459 (1984-08-01), Carrie
patent: 4476550 (1984-10-01), Ziolkawski
patent: 4476553 (1984-10-01), Ziolkawski et al.
patent: 4500978 (1985-02-01), Ziolkawski et al.
Ziolkawski, "A Method for Calculating . . . Air Gun", 1970, pp. 137-161, Geophys. J. R. Astr. Soc. 1, vol. 21.
Ziolkawski, "Design of a Marine Seismic . . . Sound Source", 1971, Geophys. J. R. Astr. Soc., vol. 23.
Ziolkawski, "Source Array Scaling . . . Decanvalution", 12/80, pp. 902-918, Geophys. Pros., vol. 28, #6; Abst. Provided.
Ziolkawski, "Wavelet Decanvalution . . . Scaling Law", pp. 872-901, 12/80, pp. 872-901, Geophys. Pros. vol. 28, #6 Abst. Prov.
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