Acoustics – Geophysical or subsurface exploration – Seismic source and detector
Reexamination Certificate
2000-05-15
2001-11-20
Nappi, Robert E. (Department: 2837)
Acoustics
Geophysical or subsurface exploration
Seismic source and detector
C084S111000, C367S141000, C310S337000
Reexamination Certificate
active
06318497
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a pressure-sensitive switch and to a hydrophone assembly incorporating such a switch. The switch is especially, though not exclusively, intended for use in hydrophone cables such as those used in underwater seismic surveying.
BACKGROUND OF THE INVENTION AND PRIOR ART
Pressure-sensitive switches are used in a variety of applications where it is desired to switch apparatus on or off at predetermined pressures. Switching may be desirable, for example, because the electrical circuitry controlled by the switch may exceed its design limits, might be damaged, or give inaccurate and misleading readings when operated at, extreme pressures. Pressure-sensitive switches are also required by certain government regulations in commercial forms of apparatus capable of both commercial and military uses to prevent commercial forms of the apparatus from being converted to military applications.
One important application of pressure-sensitive switches is in hydrophone streamer cable arrays used in underwater surveying. In such surveying, a survey ship tows a plurality of submerged cables extending substantially perpendicular to the ship's direction of travel. Each of the plurality (typically 4-10) of hydrophone cables is secured to one of a series of laterally spaced apart drums located on the ship stem to keep them laterally separated so that they extend parallel to each other and to the ship's direction of travel. Additional lateral control is provided by paravanes associated with each cable to steer them as necessary. These hydrophone cables are of substantial length, up to 5000 meters. Each cable comprises a waterproof hollow elongate prismatic sheath, typically a hollow, flexible polymeric tube and at least one tensile member fixedly associated with the sheath; this tensile member providing structural integrity to the cable so that it will be not damaged by the substantial drag forces exerted upon the lengthy cable as it is towed through the water at speeds of several kilometers per hour. Commercial cables usually have three tensile members in the form of steel cables secured within the plastic tube at intervals of 120°. Hydrophones are secured within the plastic tube, inside the cables and lying on the axis of the tube at regular intervals, typically about 1 m; these hydrophones incorporate pressure detectors, normally piezo-electric detectors, capable of detecting sound pressure in the water caused by the explosions used in seismic surveying. The hollow interior of the tube is filled with oil so that vibrations in the water surrounding the cable are efficiently transmitted to the hydrophones. Electrical conductors extend the full length of the hydrophone cable to supply power to the detectors and to carry signal from the detectors back to recording and/or analysis equipment carried on the ship. Signal conditioning modules are usually included approximately every 300 m for amplification and signal conditioning such as filtering, if required.
Although commercial hydrophone cables are normally towed at depths of about 6 to about 25 meters during seismic surveying, the hydrophones they carry may operate down to 100 meters or more. As will be apparent to those knowledgeable in anti-submarine warfare, in the absence of any special precautions, a commercial hydrophone cable of the type already described would make an excellent submarine-hunting device, and international sales of such cables would have to be regulated under munitions control regulations. To permit international sales of commercial hydrophone cables and certain other dual-use technologies without cumbersome regulations, the United States and thirty-two other countries have concluded the Wassenaar Agreement on Export Controls for Conventional Arms and Dual-Use Goods and Technologies. This Wassenaar Agreement, and the U.S. government regulations promulgated thereunder (see Commerce Control List, Part 774, Supplement No. 1, Category 6 - Sensors and Lasers) provide that hydrophone cables may be freely sold provided they are equipped with pressure-sensitive switches such that the hydrophones will cease to operate at depths exceeding 35 meters. This somewhat arbitrary limit is the average value of the depth of the thermocline present in deep ocean waters; to be useful in anti-submarine warfare, hydrophones must be capable of operating below the thermocline. Further, the commerce control list states that the pressure switches should not be adjustable once installed in the tube.
Providing a suitable form of pressure-sensitive switch to meet this “cut-out” requirement of the Wassenaar Agreement has proved difficult. Such a switch must be inexpensive. In practice, each of the thousands of individual hydrophones in an array needs its own switch (commercial users prefer to buy the hydrophone and the switch as an integrated unit, since installing separate hydrophones and switches in a cable is complicated and too expensive), and since the price for the integrated unit cannot exceed about $12, the cost of the switch must be very low. The pressure at which the switch closes cannot deviate substantially from the desired 35 meter setting, since in practice the hydrophones within each cable are arranged in sections of (typically) 96 further arranged in groups of 8 (typically), and premature closing of any one switch deactivates the entire group of hydrophones, so that premature closing of a few switches among the thousands in an array may deactivate so many hydrophones that the value of the survey may be greatly reduced, or the survey may even have to be suspended while the affected groups of hydrophones are replaced. With the costs of survey ships running into thousands of dollars per hour, such downtime is highly undesirable.
Cables are sometimes also immersed, accidentally or otherwise, more than 35 meters deep, and if the cable is no longer operational after such deep immersion, its replacement is costly, so the switch should also tolerate substantial over-pressure (i.e., it should be capable of being submerged substantially below 35, for example, 150 or more meters) without such over-pressure affecting the pressure at which the switch thereafter closes.
Vibrations from the water flowing past the cables are always a problem in seismic surveying. Since such vibrations appear as “noise” in the detected acoustic signals, it is undesirable for this noise problem to be compounded by vibrations caused by structures within the cable, and thus the in switch should, so far as possible, not transmit vibrations to the hydrophone.
In addition, it is desirable for any switch used with a hydrophone to not appreciably add to the overall volume of the combination since limited space is allocated for each hydrophone in an array assembly. Moreover, it is important to keep the hydrophone sensitive detection areas as far as possible from the noisy boundary layer at the external surface of the cable to enhance signal to noise ratios. Therefore, the switch should not alter any optimized hydrophone design that achieves this feature, and it is desirable for the switch to be acoustically isolated from the hydrophone and not alter its acoustic response characteristics.
Finally, although the cable is designed to surround the hydrophones with a non-corrosive oil, in practice sea water often leaks into a cable during extended commercial use, so the switch should be capable of resisting corrosion by salt water.
A typical prior art pressure-sensitive switch (generally designated
1
) is illustrated in schematic cross-section in
FIG. 1
of the accompanying drawings. This switch, which is of the so-called “dome” type, comprises a lower diaphragm
2
which is shaped to provide a circular elevated portion
3
. The periphery of the lower diaphragm
2
is fixed within an annular insulating washer
4
, through which passes an electrical conductor
5
extending from the lower diaphragm
2
to external circuitry (not shown). The switch
1
also comprises a dome-shaped upper diaphragm
6
, the periphery of which is fixed with
de Groot Thomas J.
Ferguson Glen
Hulsman William H.
Prescott Robert C.
Smith Richard D.
Benthos, Inc.
Caufield Francis J.
Lockett Kim
Nappi Robert E.
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