EP0707307A1 - Self-contained acoustic source for ocean tomography - Google Patents

Abstract

A cylindrical container (2) holds the energy store and the electronics and is held in place vertically by axial lines to an anchor and a float. A toroidal emitter is fixed around the container (2). The motors (16) form a ring perpendicular to the axis (X) of the cylinder. They are held between counterweights (16), which are common to two motors, by pre-stressed rods (17) whose counterweights carry the acoustic apertures (15). The apertures are planar and form an angle between them marrying with the external cylindrical surface of the transducer. The transducer is enclosed in an elastic resin (18) to protect, seal and insulate the electrical circuits while transmitting the external pressure. The anchored line has hydrophones and a remote controlled release.

Description

The present invention relates to an autonomous acoustic source for ocean tomography.

The technical sector of the invention is the field of manufacturing electroacoustic transducers for the emission of acoustic waves when immersed in a liquid.

The main application of the invention is the possibility of emitting and / or receiving acoustic waves in horizontal planes to study the different properties, by layers or by slices, of the oceans, such as temperature, salinity, density, currents etc ..., both for understanding the phenomena and their fluctuation over time.

A publication of the review "Pour la Science" No. 158 of December 1990, pages 66 and following and presented by Messrs Robert SPINDEL and Peter WORCESTER describes such an application, the equipment and the measurements obtained to date in this field.

They teach us in particular "that it has long been known that the oceans soften the temperatures of the continents, influence the seasonal variations of the climate and transport dissolved chemicals, nutrients, pollutants and even plants and animals, d from one region of the Earth to another. The fluctuations of these enormous liquid masses are complex: their turbulence is manifested sometimes by permanent small-scale mixing and internal waves, sometimes by large ocean currents, such as the Gulf Stream, which vary seasonally or in the longer term at the scale of a basin; differences in scales make it difficult to study ocean structures accurately using conventional measuring instruments ".

It has therefore been developed a technique, called "acoustic tomography of the oceans" to generate a three-dimensional image of the area crossed by sound waves, as is done in medicine with beams of "X" rays, or in geology of the earth's crust with seismic waves in the oceanic domain, low frequency acoustic waves are used. These subject to known physical laws decrease in intensity as the distance from their emission source, can be accelerated, or deflected, or slowed down when they meet and move with the currents, be attenuated, reflected or refracted by variations in temperature, density, or salinity. Their behavior being predictable and interpretable with precision by physical laws, the disturbances of the acoustic field then provide the information necessary for the determination of certain properties of oceanic masses.

One of the important information is deduced from the variation in speed of the acoustic waves which increases with temperature and pressure: the sound thus travels faster and in warm surface water, and slower as the depth increases. due to the drop in temperature, up to about 1,000 meters on average, and sometimes more; and beyond this immersion, the pressure increase compensating for the temperature decrease effect, the speed increases again. These variations of a few percent of the average speed, therefore define a minimum speed depth, around which the waves are somehow trapped in a sound channel, because if the wave bends up or down around from this depth, the acceleration of its resulting speed, returns it either down or up respectively.

In this application, low frequency acoustic waves are used, and even at the lowest possible frequencies, because their energy is less attenuated with distance than for higher frequencies: thus waves of frequencies below 100 hertz travel thousands of kilometers without loss significant energy.

When these waves are channeled in the channel of minimum speed depth defined above, and are emitted in different directions, their multidirectional propagation, essential for tomography, samples the vertical plane between each pair of acoustic transmitting and receiving beacons.

Acoustic waves emitted in a direction practically parallel to the axis of the minimum speed channel, go along this one in a region where the sound thus propagates slowly.

On the other hand, the waves emitted either above or below this depth of the minimum speed channel, or upwards, or downwards, move near the surface or near the bottom of the ocean, and following a trajectory crossing regions of high speed; thus they reach receivers first in most regions of the ocean, because their speed is higher on average, and a very clear unique sound is received at a distant point in the form of several reproductions of the original sound.

Dozens of acoustic signal paths are possible and indeed exist for each pair of transmitter-receiver instruments. Each signal moving along a different path from the others, crosses a different part of the water column; thus, the paths close to the axis of the channel defined above at minimum speed, only provide data on the characteristics of the water at this depth. According to the path taken by each of the signals received by the receiver, and which is determined according to its moment of arrival, the part of the water column thus sampled is identified.

When we use several transmitters and receivers, we form a network of crossed trajectories, and the more we increase the number of these transmitters and / or receivers, the number of possible paths for these beams, increases faster than with conventional instruments, which only collect data in one direction.

One of the major advantages of tomography is the geometric progression of the amount of data collected, as it allows researchers to cover very large areas with relatively few instruments.

Oceanographers can also increase the amount of data by using transceivers on each anchor, rather than transmitters and receivers on separate moorings.

Thus, thanks to such surveys, we could realize that at the scale of a basin, the oceans were mainly dominated by what oceanographers call medium-scale fields: that is to say highly variable currents, very energetic, the speed of which can reach ten times the average speed of the most important currents, which being slow, then disturb the climatology in the short term less.

Thus, such phenomena discovered on so-called average scales, equivalent to meteorological phenomena, extend over a few hundred kilometers and persist for several tens of days weather phenomena generally cover thousands of kilometers, and last three to five days.

Thus, we can consider that more than 90% of the kinetic energy of all the oceans is transported by these medium-scale fields, which play a significant role in ocean mixing and ocean-atmosphere interactions, and, in some extremely important cases, an occasional role, in local climatological events, such as the El Nino in the Equatorial Pacific, whose atmospheric and oceanic disturbance affects the weather conditions of the whole Globe.

• la profondeur importante de mouillage dans les océans, l'immersion des transducteurs qu'il faut maintenir à une profondeur intermédiaire, donc souvent bien au-dessus du fond, et de toutes façons à de grandes immersions subissant des pressions importantes ;
• leur autonomie nécessaire pendant de longues périodes ;
• l'impératif de directivité dans le plan vertical pour bien maîtriser et définir un plan d'émission, et la multidirectionnalité de celle-ci dans ce plan pour éviter des zones d'ombre et limiter le nombre de balises.

However, despite all the interest indicated above and exposed even more precisely in the article cited above, from which some of the above data are extracted, the manufacture and implementation of the acoustic beacons necessary for such an application, pose many problems, in particular due to:

• the significant depth of mooring in the oceans, the immersion of the transducers which must be maintained at an intermediate depth, therefore often well above the bottom, and in any case at large immersions under significant pressure;
• their necessary autonomy for long periods;
• the imperative of directivity in the vertical plane to properly control and define an emission plan, and the multidirectionality of this in this plane to avoid gray areas and limit the number of beacons.

To date, we use assemblies composed of at least one low frequency acoustic transceiver coupled and fixed to a battery container ensuring its autonomy: these assemblies are retained at their lower part by an anchoring cable connected to the bottom with a release mechanism to ensure its recovery, and at their upper part to a cable pulled upwards by a set of floating buoys, and carrying hydrophones; their position in geographic coordinates is determined by any system, either surface or background. Such a tomographic beacon is described in the publication cited above by Messrs. SPINDEL and WORCESTER, with transducers of various known types, some of which have made it possible to carry out tests and research during oceanographic cruises, during the last five to six years.

In an attempt to ensure both vertical directivity and horizontal omnidirectionality of the transducers, these can be chosen from those of the type described in patent applications, such as FR. 2,600,227 published on December 18, 1987 and filed under German priority by the company HONEY WELL ELAK NAUTIK on a "tubular electroacoustic transducer", or FR. 2,674,717 published on October 2, 1992 and deposited by the company THOMSON CSF on a "directive low frequency acoustic antenna".

But in all the tomographic beacon equipment produced to date, with transducers such as above or others, the presence of the battery container, which cannot be too far from the transducer, disturbs the emission of the latter. by mask effect, except for having several transducers around in a ring, but weighing down the system and consuming more energy; moreover, if the transducers are moved away from the battery container, stability and therefore directivity in a determined plane is no longer ensured; some of these transducers also do not allow to be submerged at great depths, and their bandwidth is not in the ranges of very low frequencies desired; moreover, if cylindrical containers for autonomous energy storage are also used, which are small in diameter and quite high, on the one hand to improve stability and better resistance to pressure, and on the other hand less bulk to be more transparent to the waves and to facilitate their implementation, the envelope tubes can enter into vibration, because subjected strongly to the fields of emitting waves, if they are placed in the same plane as it is desired for questions of stability and of implementation.

Thus, the lack of specific equipment adapted to the requirements of this tomographic application as defined above and whose implementation criteria are very strict in order to be able to obtain reliable and interpretable results, results in the fact that the acoustic tomography of the oceans is not yet an operational tool for oceanography, despite the promising trials, research and tests carried out to date, whereas it is the only method which allows a real three-dimensional analysis of the oceans complementary to satellite measurements.

The objective of the present invention is to propose an autonomous source adapted to such an application to allow its development and respecting the constraints thereof.

The problem is therefore to be able to have such a source that can be hung and wetted on a cable anchored on the ocean floor several thousand meters away, and stretched upwards by any system of floats, and whose autonomy is sufficient to transmit and record using hydrophones for long periods, which source, which can be submerged at very great depth, must be able to transmit at very low frequencies and in a fairly wide bandwidth, in all radial directions in a given plane and without shadow area; this source of emission must therefore be both omnidirectional in a given horizontal plane of immersion, and directive in site in all vertical planes.

A solution to the problem posed is an autonomous source for the emission of acoustic waves immersed in a liquid, comprising an energy storage container and the electronic and electrical systems necessary for the operation of said source, at least one integral acoustic transmitter. of said container, and a device for holding along a determined axis said transmitter is a transducer of external cylindrical shape of symmetrical section with respect to its axis, hollow and open in its center, radiating the acoustic waves radially in all directions and in a plane perpendicular to its axis, and said container of cylindrical shape and compatible dimensions, threaded and fixed coaxially in the open interior part of the transducer. The determined axis of maintenance is of course preferably merged with that of the axis of the external cylindrical shape of the transducer.

Said holding device can be constituted on the one hand by a lower mooring line comprising a cable connected to an anchoring system placed at the bottom of the ocean, and on the other hand, an upward traction cable connected to any floating device, the buoyancy of which is greater than that of all of the elements which must remain at a given depth and less than the force of said anchoring system reduced by the hydrodynamic forces applied to the assembly. In this case of course, the determined axis of maintenance is a vertical axis.

In a preferred embodiment, said transducer is polygonal in shape and made up of at least six electroacoustic cylindrical motors, arranged in a ring along said plane perpendicular to the cylinder axis, each held between two countermasses, themselves common to two adjacent motors, by prestressed rods, which foremen bearing pavilions arranged parallel to the axis of the transducer and covering the outer periphery of the transducer.

The outside diameter of said polygonal transducer is preferably at least about one meter.

The result is a new autonomous acoustic source for ocean tomography that responds to the problem posed above, and to the various requirements as defined above for this application.

• d'assurer à la fois l'omnidirectionnalité de l'émission dans un plan horizontal, et la directivité en site dans tous les plans verticaux ;
• grâce à la possibilité d'avoir un transducteur de grand diamètre, les émissions d'ondes acoustiques sont de très basses fréquences, et l'existence d'une cavité interne, en plus de la structure vibrante elle-même, génère une émission de basse fréquence qui complète la largeur de bande de la vibration de structure, permettant d'obtenir globalement une bande passante assez large, dans les fréquences très basses.
• l'ouverture centrale du transducteur permet de pouvoir y enfiler le container portant la réserve d'énergie et l'électronique permettant de piloter l'ensemble, ce qui évite d'avoir tout obstacle extérieur au tore qui gênerait l'émission des ondes, alors que la disposition en tore permet effectivement un rayonnement continu, omnidirectionnellement et dans un plan ;
• la compacité de l'ensemble container et transducteur, permet d'une part, une très bonne stabilité de l'ensemble, ainsi qu'une possibilité de fixation de cet élément vertical à un ensemble de mouillage connu, sans encombrement vertical ou horizontal trop important, et d'autre part une connexion électrique entre ledit container et ledit transducteur d'une manière définitive et bien protégée ;
• la puissance des ondes acoustiques est bien utilisée dans les directions voulues suivant une directivité maîtrisée par la stabilité verticale du container.

Indeed, the choice of using a transducer in the form of an open cylinder at its center and preferably polygonal, brings many advantages such as:

• to ensure both the omnidirectionality of the emission in a horizontal plane, and the directivity in site in all the vertical planes;
• thanks to the possibility of having a large diameter transducer, the acoustic wave emissions are of very low frequencies, and the existence of an internal cavity, in addition to the vibrating structure itself, generates a low emission frequency which completes the bandwidth of the structure vibration, making it possible to obtain a fairly wide passband overall, in very low frequencies.
• the central opening of the transducer makes it possible to be able to thread there the container carrying the energy reserve and the electronics making it possible to control the assembly, which avoids having any obstacle external to the torus which would hamper the emission of the waves, then that the toroidal arrangement effectively allows continuous radiation, omnidirectionally and in a plane;
• the compactness of the container and transducer assembly, allows on the one hand, a very good stability of the assembly, as well as a possibility of fixing this vertical element to a known wetting assembly, without excessive vertical or horizontal dimensions , and on the other hand an electrical connection between said container and said transducer in a final and well protected manner;
• the power of the acoustic waves is well used in the desired directions according to a directivity controlled by the vertical stability of the container.

In addition, due to the arrangement of the transducer-container assembly, these can be made up of elements of small diameter, therefore able to withstand very strong immersions; they can also be put in equipression, then allowing an unlimited depth, such as for example for the parts constituting said transducer which can be coated in elastic resin, ensuring the protection, the sealing and the insulation of the electrical parts and transmitting then all of the outside pressure.

Such an autonomous source is then simple to implement, inexpensive, and of very good yield allowing acoustic measurements as defined above, to allow ocean tomography to develop in the best conditions.

We could cite other advantages of the present invention, but those mentioned above already show enough to demonstrate its novelty and interest.

The description and the figures below represent an exemplary embodiment of the invention, but have no limiting character: other embodiments are possible within the scope and scope of this invention, in particular in changing the wetting device and / or the assembly of the different electroacoustic motors in the transducer as defined below.

Figure 1 is an overall profile view of a device for wetting a source according to the invention.

Figure 2 is a side view of an autonomous source according to the invention.

Figure 3 is a horizontal sectional view of a transducer which can be used in the present invention.

FIG. 4 is a view in vertical section of a transducer of FIG. 3.

FIG. 1 represents an assembly making it possible to vertically maintain a container 2 for storing system energy electronic and electrical necessary for the operation of an acoustic transmitter 1, integral with said container, in a submerged ocean environment 10. A mooring line comprises, on the one hand downwards a cable 4 connected to an anchoring system 11 opposite the bottom 9 of ocean 10 and, on the other hand, a pull cable 3 upwards, and connected to any floating device not shown in the figure; it can be submerged or very close to the surface to be spotted, and its buoyancy must be greater than that of all the elements which must remain at a given depth such as all those represented in FIG. 1 above the bottom 9, and less than the force of the anchoring system 11 minus the hydrodynamic forces applied to the assembly.

Said lower mooring line 4 may include a release device 8 which can make it possible to recover all of the wetting, including the autonomous acoustic source, except of course the anchoring device 11, and this at the end of operations or in the event of technical problems with the equipment: any known system of this type can be used, and its method of implementation is known elsewhere, without it being necessary to describe it more precisely in the present description.

Likewise, this lower mooring line 4 may include various buoyancy devices and / or mooring stabilizer 12, so on the one hand not to pull too much on the attachment system to the container 2 located above and, d on the other hand, to stabilize the mooring line in an almost stable direction, so as to avoid gyration effects which could be detrimental to the mechanical systems and to the measurements.

Above the container 2 and fixed to the traction cable 3, a set of hydrophone networks 7 or listening flutes, makes it possible to collect acoustic waves which can be emitted by other beacon devices of the same type. If other transmitters 1 are arranged at different depths, the mooring line, 4 or 3, of this beacon may then have hydrophone networks 7 arranged at these same depths, as indicated above.

The whole of the holding device, constituted by the various elements of the mooring line 3, 4, therefore defines an axis determined xx 'vertical, and is fixed on the storage container 2 at its two ends, without subjecting any effort to the transmitter device 1 proper: indeed, according to Figures 2, 3 and 4, said transmitter 1 is a transducer external cylindrical shape 13 of symmetrical section with respect to its axis, hollow and open at its center, radiating the acoustic waves radially in all directions and in a plane perpendicular to the axis xx '; and said container 2 also of cylindrical shape, and of compatible diameter, is threaded and fixed coaxially in the interior part of said hollow cylinder 13.

In order to protect said transmitter and the container, at both ends thereof, an upper protective crown 5 and a lower protective crown 6 can be arranged which enclose the attachment devices to the parts of the mooring lines 3 and 4. .

Figure 3 is a horizontal sectional view along AA 'of Figure 2 or Figure 4 of the transducer alone, without the container 2, which is threaded inside the hollow cylinder 13 of the transducer and shown in dotted lines: said transducer 1 is in this embodiment of external polygonal shape, and consisting of at least six electroacoustic cylindrical motors 14 and, preferably eight as shown in Figure 3, arranged in a crown in a plane perpendicular to the axis xx ' which is both that of vertical retention and that of the cylinder 13, and each maintained between two counterweights 16, themselves and each common to two adjacent motors, by preload rods 17, which counterweights carry flags 15 arranged parallel to the axis xx ', and covering the external periphery of the transducer 1.

Said pavilions 15 can be planes and mounted two by two on each countermass 16, forming an angle matching the cylindrical external shape; these so-called pavilions can also be faceted or other shapes such as cylindrical themselves.

All the parts constituting said transducer are preferably coated in elastic resin 18 ensuring the protection, the sealing and the insulation of the electrical parts, and then undergoing external pressure, allowing an unlimited immersion depth of the transducer assembly. .

The outside diameter of said polygonal transducer is at least about one meter, and to increase its possible emission power, said transducer can comprise several superimposed rings of electroacoustic motors 14 and arranged one below the other.

Figure 4 is a sectional view along BB 'of Figure 3, showing in profile the space available between the interior of the pavilions 15 and the outer surface of this container defining an internal cavity 19, which can also be defined in top view in Figure 3. In this cavity can be arranged all types of transducer supports 1 fixed on this container 2, but not shown in the present figures.

Claims (9)

Autonomous source for the emission of acoustic waves immersed in a liquid (10), comprising a container (2) for storing energy and the electronic and electrical systems necessary for the operation of said source, at least one transmitter (1) acoustic integral with said container, and a holding device (3, 4) along a determined axis (xx '), characterized in that said transmitter (1) is a transducer of external cylindrical shape (13) of section symmetrical with respect to its axis, hollow and open in its center, radiating the acoustic waves radially in all directions and in a plane perpendicular to its axis, which coincides with that of maintenance (xx '), and said container (2) of cylindrical shape and compatible dimensions, threaded and fixed coaxially in the open interior part of the transducer. Autonomous source according to claim 1, characterized in that said transducer (1) is of polygonal shape and consists of at least six electroacoustic cylindrical motors (14), arranged in a ring along said plane perpendicular to the axis (xx '), each held between two counterweights (16), themselves common to two adjacent motors, by prestressing rods (17), which counterweights carrying pavilions (15) arranged parallel to the axis (xx ') and covering the outer periphery of the transducer (1). Autonomous source according to claim 2, characterized in that said transducer comprises several superimposed rings of cylindrical electroacoustic motors (14) and arranged one above the other. Autonomous source according to any one of claims 2 or 3, characterized in that said pavilions (15) are planar, mounted two by two on each counterweight (16) and forming an angle between them matching the external cylindrical shape of the transducer. Autonomous source according to any one of claims 1 to 4, characterized in that all the parts constituting said transducer (1) are coated in elastic resin (18), ensuring the protection, sealing and insulation of the parts electric, and transmitting the external pressure. Autonomous source according to any one of the claims 1 to 5, characterized in that said holding device (3, 4) consists firstly of a lower mooring line comprising a cable (4) connected to an anchoring system (11) placed at the bottom (9 ) from the ocean (10), and on the other hand, an upward pull cable connected to any floating device, the buoyancy of which is greater than that of all the elements which must remain at a given depth and less than the strength of said anchoring system reduced by the hydrodynamic forces applied to the assembly. Autonomous source according to claim 6, characterized in that the mooring line comprises a remote-controlled release device (8). Autonomous source according to any one of claims 1 to 7, characterized in that it comprises a network of hydrophones (7) carried by the holding device (3, 4). Autonomous source according to any one of claims 1 to 8, characterized in that the external diameter of said polygonal transducer (1) is at least about 1 meter.

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