Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Patent
1996-11-01
1998-05-26
Eldred, J. Woodrow
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
367163, 367174, 181110, 310337, H04R 1700
Patent
active
057577269
DESCRIPTION:
BRIEF SUMMARY
This invention relates in general to flextensional acoustic sources for emitting sound waves in water, in particular low-frequency sound waves for use in offshore seismic exploration. Such sources, being employed for generation of sound waves in water, are referred to under various terms, such as seismic transmitters, flextensional transmitters or acoustic pingers, cf. also Sonar.
The acoustic source according to the invention can with advantage find uses in connection in such sound or tone transmitters, whereby the soundwaves emitted into the water can be reflected from the sea bottom and underlying geological formations, with subsequent listening or detection by means of hydrophones or geophones of various types.
The invention is particularly directed to a special spring arrangement for the purpose of being able to influence the oscillating system in such an acoustic source, i.e. the resonance relationships therein and an improved impedance matching when generating low-frequency sound waves in water.
It is well known that low-frequency sound waves can propagate over longer distances through water and geological structures, than what is the case with high frequency sound waves. For military uses and in offshore oil and gas industry there has for a long time been a need for powerful, low-frequency sound sources which can operate under water. Transmitters of various designs and embodiments for these purposes and uses have been available for a long time. Such acoustic sources are described for example in Seismic Energy Sources 1968 Handbook, Bendix, United Geophysical Corporation USA 1968, and in Transducer Needs for Low-frequency Sonar, Proceedings of the Second International Workshop on Power Transducers for Sonic and Ultrasonics, France, June 1990.
Most of the acoustic sources being presently in use, are of the impulse type, where it is intended that the transmitter or source shall emit as much energy as possible during the shortest possible time. The frequency content of such an acoustic source can be changed only to a small degree, and different types of sources are selected for the various surveying tasks.
In recent time there have been developed seismic energy sources in the form of vibrators being able to vibrate within different frequency bands, i.e. with frequency sweep. To this group belong vibrators operating with hydraulic drive medium and sources utilizing the piezo-electric effect or the magnetostrictive effect. The hydraulic vibrators control a piston through the intermediary of valve means, and thereby can provide large vibration amplitudes. As known, the piezo-electric effect involves a length variation of a crystaline material when an electric voltage is applied to its end fases, and conversely there will be generated an electric voltage when the material is subjected to a physical deformation. Magnetostriction means that a magnetic material being subjected to a change of magnetic flux, will undergo a change of length, and conversely an externally applied length variation will result in a change of the magnetic flux.
There are a variety of designs of acoustic sources. In low-frequency uses it is common that they have a circular surface in the form of a piston in case of hydraulic operation, or a cylindrical shape with either a circular or an elliptic cross section when based upon piezo-electric or magnetostrictive material. A hydraulic concept related to piston sources is described in The Marine Vibrator source, First Break Vol. 6, No. 9, September 1988, pp 285-294.
A big problem with this type of source is that the efficiency is low when the source operates outside its resonance frequency. Moreover the signal will be phase shifted and distorted depending on the location of the resonance frequency in relation to the applied or driving frequency. This will also restrict the ability to control a well-defined signal from the source. By bringing the source to operate close to its resonance frequency within a broad frequency range, the amplitude will normally be higher than when resonance is no
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Molund Gunnar
Tenghamn Rune
Zetterlund Magnus
Arnold Gordon T.
Eldred J. Woodrow
Petroleum Geo-Services ASA-Norway
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