Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Patent
1992-02-27
1995-03-14
Eldred, J. Woodrow
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
367 15, 367142, 181110, H04R 2300
Patent
active
053982177
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a method of sea bottom prospecting through the generation of submarine acoustic pulses having time characteristic and spectrum adapted to a high-resolution prospecting, and a tuned array of paraboloidal electroacoustic transducers to carry out such method.
The described apparatus can be advantageously used for the prospecting of the sea bottom structure and the subsurface of the sea bottom both in civil engineering and archaeological research and in geophysical applications as well. Furthermore, it can be used for the prospecting of lakes or in any natural body of water. Generally the apparatus of this invention can be advantageously used in all of applications of submarine acoustics in which a high acoustic power with high resolution is needed in the low and medium frequency range (0.1 to 15 kHz) not covered by the traditional acoustic devices like SONAR.
Most of the traditional sources for the sea prospecting have only a few of the necessary requirements to generate high-resolution prospecting pulses because the frequency band thereof is rather limited and the high frequency harmonic content is poor. Moreover the cavitation pulse associated to the primary pulse further reduces the feasibility of a good performance of the sources. The criteria for judging of the quality of a seismic sea-source are essentially based upon the duration of the acoustic pulse, the frequency content and the repeatability thereof, and the primary/cavitation pulse ratio (see G. Parkes and L. Hatton, The Marime Seismic Source, D. Reidel edition 1986, Dordrecht, Holland).
As for the first two characteristics, i.e. duration of the acoustic pulse and frequency content, the ideal signal would be a socalled "spike" or pulsed signal which can be represented by a Dirac delta function, i.e. a very short pulse in which all frequencies are contained (see M. B. Dobrin, Introduction to Geophysical Prospecting, Mc Graw-Hill, 1960, New York). The cavitation pulse (see A. Prosperetti, Physics of Acoustic Cavitation, Rendiconti Societa Italiana Fisica, 1984, Varenna), is a drawback always present in seismic underwater prospecting which, however, can be eliminated in a digital signal processing, for example, by deconvolution techniques (see E. A. Robinson and S. Treitel, Geopnysical Signal Analysis, Prentice-Hall, Inc., 1980, Englewood Cliffs). Nevertheless a prospecting pulse is desired which is similar as far as possible to the ideal signal before any digital processing of the seismic signal. This is the necessary condition to obtain high-resolution "acoustic images", thus minimizing cost and waste of time during the signal processing.
SUMMARY OF THE INVANTION
The method and apparatus of the present invention have important features the above mentioned problem. They are based upon experimental examinations carried out by the inventors in underwater acoustic tests using an already known paraboloidal electroacoustic transducer which in the object of a previous patent of the same inventors (see G. B. Cannelli, E. D'Ottavi and S. Santoboni, Electroacoustic Pulse Source For High Resolution Seismic Prospecting, Italian Pat. No. 49127-A/84 filed on 23.10.84).
The inventors have preferred this type of transducer with suitable modifications as a basic component of the apparatus of the present invention because it is provided with such a versatility as to allow various experiments to be carried out under different conditions. Therefore, the electrical and mechanical parameters can be easily modified in such transducer in order to provide acoustic waves having different characteristics which cannot be provided by other known devices of sea bottom prospecting (see G. B. Cannelli, E. D'Ottavi, M. Gasperini, First Resuits of Sea Bottom Prospecting With Paraboloidal Acoustic Sources, VI national Meeting About Earth Geophysics, 1988, CNR Rome).
In the above mentioned paraboloidal transducer the acoustic wave is generated by an electrical high energy discharge which is provided between two electrodes whic
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Cannelli Giovanni B.
D'Ottavi Enrico
Consiglio Nazionale Delle Ricerche
Eldred J. Woodrow
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