Acoustics – Geophysical or subsurface exploration – Seismic source and detector
Patent
1997-08-27
2000-07-11
Williams, Hezron
Acoustics
Geophysical or subsurface exploration
Seismic source and detector
181120, 367142, G01V 100, G01V 1145, H04R 144
Patent
active
060858621
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a drive assembly for acoustic sources having sound emitting surfaces adapted to be excited into vibrational motion, in particular for use in seismic prospecting.
TECHNICAL FIELD
Sources employed for generating sound waves in water can for example be sonar sources, flextensional sources or seismic transmitters or energy sources. Advantageously the invention can be employed for such types of sources, i.e. for emitting sound waves under water. Upon reflection from the sea bed and underlying geological formations, resulting echo signals can be detected by means of hydrophones or geophones of various types.
It is well known that low frequency sound waves can be transmitted over longer distances through water and geological structures than high frequency sound waves can. Within military applications as well as within the marine sector of oil and gas industry there has for a long time been a need for powerful low frequency sound sources which can operate under water. Sources of various constructions and designs for these purposes and fields of use, have been available for a long time. Such acoustic sources are for example described in Seismic Energy Sources 1968 Handbook, Bendix, United Geophysical Corporation 1968, and in Transducer Needs for Low-Frequency Sonar, Proceedings of the Second International Workshop on Power Transducers for Sonic and Ultrasonics, France, June 12-13, 1990.
Most of the acoustic sources employed today are of the impulsive type, in which efforts are made to have the sources emit as much energy as possible during as short time as possible. The frequency contents of such a source can be modified only to a very small degree, and different sources are selected for different surveying problems.
In recent time there have been developed seismic energy sources in the form of vibrators which can vibrate within various frequency bands, so-called "frequency sweep". To this group there belong vibrators which operate by employing hydraulic means and sources employing piezoelectric or magnetostrictive materials. In hydraulic vibrators a piston is controlled by a valve arrangement, and thereby it is possible to obtain large oscillation amplitudes. The piezoelectrical effect as known involves a change of length of a crystalline material when an electrical voltage is applied to the outer surfaces thereof, and conversely that an electrical voltage is generated when the material is subjected to a physical deformation. Magnetostriction means that a magnetic material being subjected to a magnetic field change will undergo a length change, and conversely that an applied length change of the material will give rise to a change of the magnetic field.
There are various manners of designing acoustic sources. For low frequency uses it is common to let the sources have a circular surface (in the form of a piston) when the hydraulic principle is employed, and a cylindrical shape with either a circular or elliptic cross-section when piezoelectric and magnetostrictive materials are used.
A concept where a hydraulic piston source is employed, is described in The Marine Vibrator Source, First Break Vol. 6 No. 9, September 1988/285.
The greatest problem with this type of controllable source is to obtain a well defined and sufficiently high amplitude of the oscillations. In order to obtain this there will be a need for either a large source surface or a small source surface having high oscillation amplitudes.
Vibrators based on the hydraulic principle (for example within marine seismic exploration) provide high amplitudes at low frequencies. The piston motions are controlled by a valve arrangement. The degree of control of these hydraulic piston sources as regards amplitude combined with frequency, is limited, however.
In Norwegian Patent 176.457 a drive assembly for acoustic sources is described based on a construction having a cylindrically shaped elastic shell or membrane with an elliptic cross section. The source has two beams at the ends of the main axis and the drive assembly is position
REFERENCES:
patent: 3701968 (1972-10-01), Broding
patent: 4941202 (1990-07-01), Upton
patent: 5329499 (1994-07-01), Molund et al.
patent: 5646380 (1997-07-01), Vaage
patent: 5786537 (1998-07-01), Anstey
Miller Rose M.
PGS Seres AS
Williams Hezron
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