A Technique for Suppressing Bubble Oscillations from an Air Gun during Shallow-Water Marine Seismic Surveying
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A Technique for Suppressing Bubble Oscillations from an Air Gun during Shallow-Water Marine Seismic Surveying M. V. Aleshkin* Department of Geology, Moscow State University, Moscow, 119991 Russia *e-mail: [email protected] Received September 9, 2019; revised October 16, 2019; accepted January 15, 2020
Abstract—Non-explosive sources have become widespread. Along with electric spark and electrodynamic sources, which are used mainly in works on the upper part of a section, these include pneumatic sources. This type of source meets all the requirements for geophysical equipment. The main problem of such sources is the presence of intense pulsations and the long duration of the emitted signals, as well as the poor reliability of some clusters. The problem of pulsations during shallow water acquisitions is especially topical. The depth of shallow water does not allow one to extract a signature that is acceptable for the construction of an effective operator from the data due to the interference of pulsations from the direct wave with the bottom, while the suppression operator that is extracted from the theoretical signature often gives an unacceptable result. To solve bubble oscillation problems in shallow-water acquisition it is proposed to use combined information from bubble oscillations extracted from a direct wave, extracted from bottom reflections, waveform predicting deconvolution, and adaptive subtraction algorithms. Keywords: air guns, marine seismic, signature deconvolution, bubble oscillations, ghost-wave DOI: 10.3103/S0145875220030023
INTRODUCTION The problem of the generation and suppression of bubble oscillations from sources of an expansion-center type (in particular, air guns) during marine works has been well studied theoretically. The processes of generation, propagation, and attenuation of waves have been described in publications on marine seismic surveying, e.g., in (Baum et al., 1975). However, in practice, geophysicists often face imperfections of both observation systems and instrumentation. Let us consider this problem in the case of an air gun. One of the popular approaches to suppression of bubble oscillations is the use of air guns with a special design of their pneumonic chambers, leading to secondary air inflow into the vapor-and-gas cavity that forms after operation of an air gun. Such an approach enables one to considerably reduce the rate of cavity collapse and, as a result, the oscillations weaken. Air guns of this type are designed with two chambers. After the first chamber (generator) operates and the cavity expands to reach its maximum radius, the second chamber (injector) operates. At properly adjusted volumes of these two chambers (I = Gn, n = 3.4) oscillations can be almost completely suppressed; however, the general efficiency of such a system will be proportionally smaller by n times (Gulenko, 2003).
THE HYDRODYNAMICS OF A SUBMARINE BLAST Bubble oscillations from an air gun occur as follows. When an air gun is fired, an underwater cavity filled with pressur
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