FR2822548A1 - Buried or submerged object acoustic image former, acoustic waves are transmitted or received at transducers through a non null thickness naturally occurring or artificially created water layer - Google Patents

Abstract

The faces of the transducers (201,202) transmit and receive in a non null water thickness layer covering the ground surface. When the non null water thickness layer (212) does not naturally exist on the ground, it is artificially created by injecting water under the acoustic transducers (201,202) via a pipe (211). A sealing skirt (203) laterally retains the water through which the sound passes. The acoustic pulses are preferably of the order of 20 micro-seconds, and their energy is spread over the frequency band between 10 to 100 kHertz. Acoustic imaging system which includes: (a) a group of acoustic emitting non parametric, non directive, transducers with relative pass band greater than 10 per cent. The directive term signifies the opening to -3 decibels of the directivity lobe is at least equal to 10 degree, and preferably greater than 60 degrees. The transducers (201) are linked in a single row or in a matrix so as to form a preferably plane transmitting antenna; a group of receiving acoustic transducers (202) with relative pass band greater than 10 per cent. The directive term signifies the opening to -3 decibels of the directivity lobe is at least equal to 10 degree, and preferably greater than 60 degrees. The transducers (202) are linked in a single row or in a matrix so as to form a preferably plane receiving antenna and are preferably distinct from the emitting transducers; an electronic transmitter (204) and receiver (208) for amplifying the transducer signals, an electronic system for focusing , conditioning and transmission interface (205) by means of which the transducers (201) emit focused acoustic pulses according to the technique known as delays in a point of space (M), the focusing point (M) is positioned by the set of focusing delays applied to signals emitted by the transducers at each emission in a soil volume (V) investigated; an electronic system for focusing , conditioning and reception interface (207) focusing the observation for the reception near to the points (M) where the reflected energy is emitted, in accordance with the technique known as delays and summation for signals from the receivers (202), electronic transmission units for the signals (206) and means for control, processing, archiving, display (101,201) for local or remote transmission and to record the acoustic signals in a two or three dimensional form.

Description

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The present invention relates to a device intended to detect, classify, and discriminate objects with respect to their environment by the use of acoustic waves.

The problem of detecting (i.e. the action of determining the presence or absence) of an object buried in a medium such as the ground by their ultrasonic echo, is a problem which has already been addressed: , we know how to detect objects buried in the bottom of the sea by means of sonars working in the field of low frequencies, which penetrate the ground well. On the other hand, the state of the art only includes very few classifying sonars or identifiers of buried objects, since these operations are more delicate: the sonar must create as fine an image as possible of the buried object in order to distinguish its details that will recognize it. There is no device for obtaining a precise image of objects buried in the ground.

une technique d'émission non linéaire dite'émission paramétrique', et de juxtaposer sur un support visuel les signaux reçus pour représenter l'objet. Toutefois, cette technique telle qu'exposée dans le brevet nO EP0965860 présente les inconvénients suivants : La technique d'émission paramétrique, décrite notamment dans la thèse de doctorat d'état ès sciences physiques de l'université Paris VI intitulée'Contribution théorique et expérimentale à l'étude de l'émission et de la réception paramétrique' (auteur : Pierre Cervehka) utilise un The closest known technique to the invention consists in projecting fine acoustic beams created by

a non-linear emission technique called 'parametric emission', and juxtaposing on a visual medium the signals received to represent the object. However, this technique as set out in patent No. EP0965860 has the following drawbacks: The parametric emission technique, described in particular in the thesis of doctorate of state in physical sciences of the university Paris VI entitled 'Theoretical and experimental contribution studying parametric emission and reception '(author: Pierre Cervehka) uses a

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 emitted radiation, called primary radiation, of carrier frequency greater than 100 kHz in the description of patent EP0965860 already cited, which generates by non-linear propagation in water low frequency radiation which penetrates the ground.

minimale D existe entre l'émetteur sonar et le sol où est mm enfoui l'objet. Pour permettre au rayonnement primaire d'engendrer au cours de sa propagation non linéaire à travers cette lame un rayonnement secondaire de niveau suffisant pour pénétrer le sol, cette lame d'eau doit typiquement atteindre une épaisseur minimale D de plusieurs dizaines à plusieurs centaines de centimètres. Le dispositif décrit dans le brevet EP0965860 déjà cité ne peut donc pas opérer directement en contact avec le sol, et par conséquent l'utilisation en est limitée aux zones où la profondeur d'eau est supérieure à Dmln. En particulier ce dispositif ne peut être utilisé là où il y a très peu ou pas d'eau notamment dans les rizières, les flaques, les marais, les terrains inondés par exemple, où peuvent se trouver des objets enfouis à investiguer. This emission technique has several drawbacks and limitations: First, it requires only a thick layer of water

minimum D exists between the sonar transmitter and the ground where the object is buried. To allow the primary radiation to generate, during its non-linear propagation through this plate, secondary radiation of a level sufficient to penetrate the ground, this layer of water must typically reach a minimum thickness D of several tens to several hundred centimeters . The device described in patent EP0965860 already cited can therefore not operate directly in contact with the ground, and therefore the use is limited to areas where the water depth is greater than Dmln. In particular, this device cannot be used where there is very little or no water, in particular in rice fields, puddles, marshes, flooded grounds for example, where there may be buried objects to be investigated.

The non-linear emission technique has, moreover, a very poor efficiency: the secondary radiation is typically attenuated by 30 to 40 decibels compared to the primary radiation emitted by the transmitter, which greatly limits the range of the sonar.

In addition, the small area of the transmitting antennas used to make a directional parametric acoustic transmitter constitutes a handicap. Indeed, the area of such an emission projector must be typically close to the area of the image resolution cell that one seeks to create, that is to say a few square centimeters. Taking into account the maximum sound power flows admissible by the transducers

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dans la bande considérée, typiquement de l'ordre de quelques Watts/cm2, le niveau sonore secondaire projeté sur la cible, déjà fortement atténué par le très mauvais rendement mentionné précédemment, est très faible, ce qui se traduit par une réduction supplémentaire de la profondeur de pénétration dans le sol.

in the band considered, typically of the order of a few Watts / cm 2, the secondary noise level projected onto the target, already greatly attenuated by the very poor performance mentioned previously, is very low, which results in a further reduction in the depth of penetration into the ground.

Another drawback comes from the way the image is made up. The device of patent EP0965860 already cited divides the object into voxels (volume elements) as fine as possible by the use of very directive beams on emission (typically 2 degrees). A drawback of this directivity is to limit the observation of the buried object to the only points of each of its slices where the directive sound beam cutting the slice is specularly reflected towards the sonar (so-called 'bright' points), the other points of the slice remaining little or not visible, hence a very incomplete observation of the object.

The object of the invention is precisely to provide a relatively simple and economical solution to the drawbacks of the previous embodiments, described above and to propose a device capable of giving three-dimensional images of objects buried in a medium such as the ground.

IV. DESCRIPTION OF THE INVENTION: The invention makes the following means referenced and presented cooperate with reference to the appended figures: transmission and reception means (100), signal processing means (101) and control means, representation, transmission and archiving (102).

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 The transmission and reception means consist of an antenna (200, 201, 202), electronic transmission and reception means (204 to 208), optional means of acoustic coupling between the antenna and the ground ( 203, 211, 212).

The transmitting and receiving antenna (200,201, 202) is preferably planar and includes one or more non-parametric, non-directional, broadband ultrasonic transmitters (201) and one or more broadband non-directive ultrasonic receivers (202) of the following types: known, arranged on the surface of the antenna preferably in a single row or in a matrix. The transmitters can be operated either simultaneously or sequentially. They emit a short pulse of duration T = 1 / B typically of the order of 20 microseconds, and of central frequency preferably comprised in the interval 10 kHz to 100 kHz. The pfd density emitted is of the order of 10 Watts per square centimeter of antenna. The transmission transducers (201) are in a preferred embodiment of the invention different from those of reception (202), and geometrically distributed between the reception transducers. However, a variant of the invention will use transducers which can both transmit and receive.

The transducers making up the antenna (201,202) have a directivity lobe as open as possible, preferably greater than 90 degrees, in order to be able to observe the entire volume located under the antenna. For this, the typical dimension of the speaking face of a transducer is half a wavelength, which can vary from 1 and 10 centimeters depending on the central frequency chosen in the preferred working band.

The largest dimension of the antenna (200) can therefore vary according to various embodiments of the invention from a few centimeters to a few meters, depending on the carrying capacities, the frequency chosen and the requirements for speed of coverage of the system, the dimension of the antenna being close

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 the size of the area of land that can be imaged under the antenna.

The electronic transmission and reception means (204 to 208) consist of transmission (204) and reception (208) amplifiers, electronic focusing, packaging and interface means (205,207), and signal exchange means (206).

The electronic module for focusing, conditioning and transmitting interface (205) distributes to the transmitting power transmitters (204) the signal transmitted with the appropriate delays so that the ultrasonic energy emitted by the transmitting transducers (201 ) is focused around the point (M) of the buried volume (V) that we are exploring. A variant of the invention uses only one and the same amplifier (204) to drive all the emitting transducers (201) in parallel, these transducers then all emit at the same time.

The electronic conditioning and reception interface unit (207) collects all the signals from the receivers (202) individually preamplified by the suitable low noise amplifiers (208), conditions them and transmits them to the signal processing means (101 ).

The optional means for acoustic coupling of the antenna and the ground (203, 211, 212) carry out the transmission of acoustic energy between the antenna and the medium such as the ground where the object is buried, when the latter is dry in particular, by a cushion of coupling material (212) which may advantageously be a blade of liquid. In this case, this liquid blade may advantageously be created by equipping the periphery of the antenna (200) with a flexible sealing skirt conforming to the shapes of the antenna (203) and by injecting the liquid under the antenna at using at least one liquid injection nozzle (211).

 The liquid injected through the nozzle (211) under the antenna is partially retained by the skirt (203), so that the antenna

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 (200) is partially supported by the retained liquid blade (212), in the manner of a hydrofoil.

When the antenna (200) is used in a liquid, at sea for example, the existence of a sheet of natural water between the antenna and the ground makes this means of acoustic coupling unnecessary.

An advantage of the invention is that it does not require that the thickness of the blade of coupling material (212) is significant: a thickness of the order of a millimeter is sufficient.

The signal processing means (101) collect the signals from the transducers (202) forming the reception antenna, suitably amplified and conditioned by the means (207,208), and focus the reception antenna at point (M). The focal point (M) is therefore simultaneously the focal point of the transmit antenna made up of all of the transducers (201) and the focal point of the receive antenna made up of all of the receive transducers (202).

Focusing the transmitting antenna at point (M) means that at each pulse emitted by the sonar, the delays applied to the instants of emission of the pulse by the transducers (201) are adjusted by the signal processing means. (101) so that the acoustic energy comes to concentrate in the vicinity of point (M). Focusing the reception antenna at point (M) means that the signal processing means (101) apply to the signals originating from the reception transducers (202) the known delay-summation-filtering algorithm. This known algorithm consists in compensating the delay of propagation back of the pulse reflected by the point (M) at the phase centers of the transducers (202) by delays individually applied to the signals received by said transducers (202), summing said received signals , filter the sum signal and detect the sound amplitude reflected by the point (M). The adjustment of said delays is

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 performed by means of a known autofocusing algorithm, for example of the type called “maximum contrast”.

In the case of the variant of the invention where all the transmitting transducers (201) transmit simultaneously, the focusing process of the reception antenna at point (M) is not modified. The advantage of this variant without focusing the transmitting antenna is to simplify the transmitting device and reduce the cost of the system, at the cost of reducing the contrast of the images produced.

The means of control, representation, transmission, archiving (102) control the entire device. They vary the coordinates (209,210) of the focal point (M) of the antennas within the volume (V) by varying the focal delays of the transmit and receive antennas to measure the amplitude reflected at any point (M ) of the volume (V) until (V) is fully observed.

They can thus create a three-dimensional image of the volume of soil (V) included under the antenna (200), or limit the investigation to two-dimensional sections of this volume.

pour pénétrer l'objet et/ou lorsque'enveloppe de cet objet est suffisamment perméable aux ondes acoustiques) la forme de ses constituants internes. This image data can be presented to an operator by visual means and / or transmitted to other users and / or recorded by the means (102). These images make it possible to identify various characteristics making it possible to identify the target, in particular:. the external shape of the object buried in the sediment. if applicable (when the incident energy is sufficient

to penetrate the object and / or when the envelope of this object is sufficiently permeable to acoustic waves) the shape of its internal constituents.

. the position and orientation of the object relative to the surface of the sediment. the presence, above the object, of a texture characteristic of the sediment where the object is buried. This measurement of the spatial variability of the signal reverberated by the sediment located above the object can, in known manner,

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 reveal whether this sediment has been freshly stirred, for example, or even if the nature of the sediment surrounding point (M) differs from the surrounding sediments, a fact which may mark the existence of a desired sedimentary structure, in particular gold or diamond.

The different means, the structure of which has been described above, are implemented as follows: A vehicle (104) or a man carries the antenna, its electronics, and moves it in space, in the immediate vicinity or in contact of the interface with the medium, to image the area where the object is buried. The antenna can be carried, for example, at the end of a pole of the 'frying pan' type (103), or by a terrestrial, marine or underwater vehicle such as a towed fish or as an underwater robot.

The advantages of the invention are as follows. The short distance separating the antenna from the object allows high transmission rates, of the order of several hundred hertz. Each sonar emission making it possible to examine, for certain embodiments of the invention, one to several tens of voxels (volume elements) per sonar emission, this results in an ability of the invention to deliver three-dimensional images at a rate of l of the order of a thousand to ten thousand voxels per second.

 The antenna emits acoustic waves in a conventional manner and no longer according to the parametric technique used in the state of the art, which has the advantage of increasing the noise level incident on the object buried at point (M) observed at least 30 decibels.

Image resolution no longer depends on the area of the transmit antenna, so you can use a transmit antenna

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 large area, which further increases the above noise level by at least 10 decibels.

In addition, the emission transducers project the acoustic energy onto the object in a non-directive manner, which makes it possible to observe all its parts, and not only its 'bright' points.

The advantages conferred on the invention by the gain in sound level and non-directive insonification thus obtained make more details of the object observable: in particular certain less reflective parts as well as the non-specular points. When the object is acoustically penetratable, its internal constitution is thus better contrasted. The range of the sonar, i.e. the maximum depth at which it is possible to examine a buried object, is also increased.

Claims (10)

 1. Device for detecting and acoustically imaging objects buried in an environment such as the ground, characterized in that its emission (201) and reception (202) transducers are arranged in contact with the surface of the environment in which is buried the object, or in the immediate vicinity of it at a distance of the order of a few centimeters. 2. Device according to claim 1, characterized in that it uses a blade (212) of coupling material ensuring the transmission of acoustic energy between the transducers (201,202) and the environment where the object is buried. 3. Device according to any one of the preceding claims, characterized in that the coupling blade (212) is produced by injecting a liquid such as water under the acoustic transducers (201,202) by means of at least one nozzle d injection (211) and at least one sealing skirt (203), which retains the liquid blade through which the sound passes. 4. Device according to any one of the preceding claims, characterized in that it comprises a set of acoustic emission transducers (201) broadband, non-parametric, non-directional, the latter term conventionally meaning that the opening to- 3 decibels of their directivity lobe is at least equal to 10 degrees, and preferably greater than 60 degrees, said transducers (201) being associated in a single row or in a matrix to form a preferably planar transmitting antenna. 5. Device according to any one of the preceding claims, characterized in that it comprises a set of wideband acoustic transducers (202), non-parametric, non-directional in the sense described above, associated in a single row or in a matrix for form a preferentially planar receiving antenna, these transducers <Desc / Clms Page number 11>  acoustic (202) being preferably distinct from the transmitting transducers (201). 6. Device according to any one of the preceding claims, characterized in that it comprises electronic transmission (204) and reception (208) means making it possible to amplify the signals transmitted and received from the transducers (201,202) and electronic means ensuring the focusing, conditioning and interfacing of emission (205) making it possible to cause the transducers (201) to transmit acoustic pulses focused according to the known technique of delays at a point (M) in space, the focusing point (M) being positioned by the set of focusing delays applied to the signals transmitted by the transducers (201) on each emission within a volume of soil (V) to be investigated. 7. Device according to any one of the preceding claims, characterized in that it comprises electronic means ensuring the focusing, the conditioning and the interfacing of reception (207) making it possible to focus the observation on reception in the vicinity of the points. (M) of the space where the emitted energy is reflected, according to the known technique “delays and summation” of the signals coming from the receivers (202). 8. Device according to any one of the preceding claims, characterized in that it comprises electronic means ensuring the transmission of the signals (206) and electronic means ensuring the control, processing, archiving and display (101,102 ) making it possible to represent or transmit to a local or remote operator, and to record the acoustic signals leaving the stage (207), in particular in the form of two or three-dimensional views of the energy reflected by the points (M) observed within the volume (V). 9. Device according to any one of the preceding claims, characterized in that the acoustic pulses emitted are preferably of short duration of the order of <Desc / Clms Page number 12>  20 microseconds, and their energy mainly distributed in the frequency band between 10 kHz and 100 kHz. 10. Device according to any one of the preceding claims, characterized in that the means used for transporting the invention in space is a sea or land vehicle (104) or a mechanical device suitable for transporting the vehicle on foot. invention (103).