Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Patent
1999-07-30
2000-09-19
Lobo, Ian J.
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
367172, H04R 144, H04R 2300, G01H 900
Patent
active
061222259
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to hydrophones are used to determine ship movements on or in the sea using acoustic measurements to include measured reflections of acoustic waves brought about by explosions for seismic measurements.
BACKGROUND
Efforts are being made worldwide to develop a hydrophone having a high sensitivity which is influenced little (or not at all) by static pressure, i.e. the depth of the water in which the measurements are performed.
A hydrophone wherein use is made of an optical fibre is for instance known from the U.S. Pat. No. 5,247,490 and in European patent application No. 0,554,085.
In practice a glass fibre is arranged on a mandrel-like or disc-shaped support member, wherein pressure variations are measured by the deformation of the support member relative to a reference sensor element which undergoes no mechanical deformation, and wherein the change in length is measured by means of interference between light-beams transmitted through the fibres.
In order to compensate for the static pressure it is for instance known from the European patent application No. 0,554,085 to arrange a compensation chamber beneath a flexible disc-shaped membrane on which the optical sensor is arranged, which compensation chamber is then in open communication via an aperture of predetermined dimensions with the liquid medium in which measurement is taking place. The size of the aperture determines the dynamic behaviour of such sensors.
Such a sensor displays a drastic reduction in sensitivity relative to a sensor with a closed (air) chamber. Increasing the dimensions of the sensor, for instance by a factor of 10 to 20, in order to limit this reduction meets with practical drawbacks during use. In order to obtain sufficient resolution (i.e. the number of measurement points) the smallest possible dimensions are desirable.
The '377 patent discloses a hydrophone wherein a first chamber with a sensor fibre is filled with air and enclosed by water, and a second double-walled cylinder a with reference fibre is filled with air or a gas and enclosed by water. Under hydrostatic pressure both cylinders are deformed and the respective length of each of the fibres is thus changed.
SUMMARY OF THE INVENTION
The present invention has for its object to improve the known hydrophones and more particularly to provide a hydrophone with compensation for static pressure while retaining sufficient sensitivity.
The present invention provides a device for measuring pressure waves in a liquid medium having:
one or more sensor elements permeable to optical radiation;
one or more support members on which a sensor element is arranged in each case and which is at least slightly flexible;
a compensation chamber at least partly enclosed by a support member and filled with a liquid medium;
a second compensation chamber which is actively coupled to the first compensation chamber via an at least partially flexible wall, wherein the second compensation chamber is filled with gas; and
a detection device for detecting changes in the length of the sensor element.
According to the present invention the first compensation chamber filled with water is used to build up hydrostatic pressure. The volume herein changes of both the first compensation chamber and the second compensation chamber filled with air, with the result that the disc with the sensor fibre remains flat and there occurs no change in the length thereof. In the case of acoustic pressure the disc with the sensor fibre is bent and the length thereof changes. The deformation of the disc is made possible by volume changes in the second compensation chamber. The disc with the sensor fibre therefore remains flat in the case of hydrostatic pressure, in contrast to the bending when acoustic pressure occurs.
Further advantages, features and details of the present invention will be elucidated on the basis of the following description with reference to the annexed drawings, in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B respectively show views of
REFERENCES:
patent: 4518857 (1985-05-01), McMahon et al.
patent: 4530078 (1985-07-01), Lagakos et al.
patent: 4570248 (1986-02-01), Assard
patent: 5317929 (1994-06-01), Brown et al.
patent: 5363342 (1994-11-01), Layton et al.
patent: 5394377 (1995-02-01), Von Bieren
Bruijn Dick
Cheng Lun Kai
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