Dereferenced parametric echo sounder and method for characterizing a subbottom part of an underwater environment

Description

Technical field to which the invention relates

[0001] The present invention relates generally to the field of echo sounders, that is to say sonar-type depth measuring devices, using the reflection of acoustic waves.

[0002] It particularly concerns an echo sounder for exploring the contents of the subsoil of an underwater environment.

[0003] It concerns more particularly a parametric echo sounder configured for: emitting, in the underwater environment: a first acoustic wave varying at a first frequency and a second acoustic wave varying at a second frequency, or a modulated acoustic wave comprising a first component and a second component varying respectively at said first frequency and at said second frequency, and to, in response to said emission, capture an echo signal of frequency equal to the difference between said first frequency and said second frequency.

[0004] It also relates to an associated method for characterizing a part of the subsoil of an underwater environment. Technological background

[0005] Exploring the subsoil contents of an underwater environment is an important issue, particularly in the maritime domain.

[0006] Indeed, it is common to place facilities, such as pipelines and cables for telecommunications or electricity transmission, directly on the seabed. But these facilities can then become buried in a layer of sediment or sand, making them difficult to locate and characterize.

[0007] A particular echo sounder for probing the contents of such a sediment layer is described in the article "Feasibility of B-Scan Imaging in Sediment by Means of Parametric Transmission Technique" by J. Marchal and P. Cervenka (Acta Acustica united with Acustica, volume 90 (2004) pages 62 to 69).

[0008] The echo sounder described in this article comprises a large diameter, piston-type transmitter, driven to emit a first acoustic wave varying at a first frequency, as well as a second acoustic wave varying at a second frequency. Each of these two waves is emitted vertically, below the transmitter. They thus superimpose one on the other along the entire length of the water column located directly above the transmitter.

[0009] Since the response of water to a change in density is slightly non-linear, the superposition of the first and second acoustic waves generates, directly in the water, an additional acoustic wave, with a frequency equal to the difference between the first frequency and the second frequency. This non-linear mixing effect is generally called the "parametric effect". It corresponds in a way to a non-linear self-demodulation, directly in the water, of the modulated wave formed by the superposition of the first and second acoustic waves.

[0010] This additional acoustic wave, which is generated throughout the water column located below the transmitter, propagates vertically and reaches a point on the bottom located directly below the transmitter. It then makes it possible to probe the subsoil of the underwater environment, below the point in question.

[0011] The absorption coefficient of the additional acoustic wave in the sediment layer to be probed is lower than the absorption coefficients of the first and second acoustic waves, because the frequency of the additional "low frequency" acoustic wave is lower than those of the first and second acoustic waves. On the other hand, generating this additional acoustic wave directly in the water, by non-linear mixing, allows for a higher directivity than if the additional acoustic wave were emitted directly by the transmitter.

[0012] The echo sounder just described can be placed successively at different positions on the water surface in order to probe the sediment layer at different points on the bottom. This then makes it possible to generate a two-dimensional or three-dimensional image representative of the contents of this layer.

[0013] However, such a survey is long and inconvenient. Subject of the invention

[0014] In this context, the present invention proposes a new type of parametric echo sounder making it possible to generate a sound wave directly in the water, by parametric effect, the propagation axis of this wave being able to be modified, without displacement or pivoting of the echo sounder, by electronic control of the echo sounder transducers.

[0015] More particularly, the invention proposes a parametric echo sounder comprising: a transmitting antenna comprising several transducers, at least one receiver, and a control and acquisition system configured to carry out the following steps: a) control said transducers so that they transmit into an underwater environment a first acoustic wave having a first frequency and a second acoustic wave having a second frequency, the first wave and the second wave superimposing on each other along a superposition axis directed towards a point on the bottom of the underwater environment, each transducer being controlled by a transmission signal, the different transmission signals of the different transducers having predetermined delays relative to each other, and b) acquiring an echo signal, of frequency equal to the difference between said first frequency and said second frequency, picked up by said receiver in response to the transmission carried out in step a),the control and acquisition system being further configured to: execute all of steps a) and b) several times, by varying at least part of said delays from one execution of step a) to the next, so as to modify the orientation of said superposition axis from one execution of step a) to the next, and to c) determine, from the echo signals acquired during the different executions of step b), a two-dimensional or three-dimensional image representative of the contents of the subsoil of the underwater environment under each of said points on the bottom.

[0016] Following each execution of step a), an additional acoustic wave, of frequency equal to the difference between the first frequency and the second frequency, is generated directly in the water by non-linear mixing of the first acoustic wave with the second acoustic wave, along said superposition axis. This additional "low frequency" acoustic wave then propagates parallel to said superposition axis. The point on the bottom that it reaches, located in the extension of this axis, can therefore be selected by an appropriate choice of the delays between transmission signals.

[0017] The parametric echo sounder according to the invention therefore makes it possible, thanks to appropriate electronic control of the antenna transducers, to probe the contents of the subsoil of the underwater environment at different points on the bottom, with a "low frequency" acoustic wave penetrating well into this subsoil, and without having to move the echo sounder for this.

[0018] However, emitting high acoustic power proves to be significantly more difficult with the parametric echo sounder according to the invention than with an echo sounder which would comprise a single large transducer (such as in the aforementioned article by J. Marchal and P. Cervenka), mounted to pivot so as to be able to be pointed towards different points on the bottom.

[0019] Indeed, for a transmitting antenna consisting of a single transmitter occupying the entire surface of the antenna, each point on the surface of the antenna contributes to the emission of the acoustic waves produced.

[0020] On the other hand, for a transmitting antenna consisting, as here, of several distinct transducers distributed at different points on the surface of the antenna, only a part of the surface of the antenna, consisting of all the respective transmitting surfaces of these transducers, contributes to the emission of the acoustic waves produced.

[0021] However, the efficiency of the parametric non-linear mixing process in water is generally low, of the order of a few percent at most, and decreases sharply when the power of the acoustic waves to be mixed decreases. It is therefore crucial for a parametric echo sounder to emit very high acoustic powers PA, for example of the order of 230 decibels at 1 meter distance from the transducer(s) (the PA power value in decibels being equal to 10 log (PA / P Aref ), where the reference acoustic power P Aref is that of a pressure wave of 1 micropascal amplitude).

[0022] The construction of a parametric echo sounder with a transmitting antenna is therefore particularly technically difficult. The transducers used must be capable of transmitting very high power densities per unit area (typically greater than ten watts per square centimeter). And above all, these transducers must then be driven by transmission signals with high electrical power, which requires one or more power amplifiers that are generally bulky and which complicates the electronics for driving these transducers.

[0023] But despite these technical difficulties, a parametric echo sounder with such a transmitting antenna with several transducers is particularly interesting. Indeed, controlling the propagation direction of the acoustic wave, generated by a parametric effect, in a purely electronic manner makes it possible to change this direction very quickly.

[0024] This makes it possible, in particular, to capture a two-dimensional or three-dimensional image representative of the contents of a layer of sediment more quickly than by mechanically rotating a single large transmitter.

[0025] This also makes it possible to compensate "in real time" (with a very short latency time typically less than 0.1 seconds), upon transmission, for parasitic rolling or pitching movements of the echo sounder, so that the emitted acoustic waves point in well-defined directions, independently of these parasitic movements.

[0026] In addition, a multi-transducer transmitting antenna provides great flexibility in the choice of transmission sequences, thus allowing many different sounding configurations, possibly mixing parametric sounding and unmixed frequency sounding of the bottom of the underwater environment.

[0027] The echo sounder according to the invention comprises a receiving antenna which comprises the receiver mentioned above, as well as other receivers of the same type. Such a receiving antenna with several receivers allows selective reception from an angular point of view: it allows the control and acquisition system to select a given reception direction. The echo signal acquired in step b) is then representative of a low-frequency acoustic wave having a well-defined propagation direction, which is the reception direction in question, selected by the control and acquisition system (instead of this echo signal being representative of any acoustic wave received, regardless of its propagation direction).

[0028] Other advantageous characteristics of the echo sounder according to the invention are as follows: the control and acquisition system is configured to, in step a): control each transducer of a first group of said transducers by a first transmission signal varying over time at said first frequency, the transducers of this first group emitting said first acoustic wave, the different first transmission signals having first delays, predetermined, relative to each other, control each transducer of a second group of said transducers by a second transmission signal varying over time at said second frequency, the transducers of this second group emitting said second acoustic wave, the different second transmission signals having second delays, predetermined, relative to each other, and to vary said first delays and / or said second delays from one execution of step a) to the next,so as to modify the orientation of said superposition axis from one execution of step a) to the other; the control and acquisition system is configured so that: said transmission signals each comprise a first component and a second component varying respectively over time with said first frequency and with said second frequency, and so that the transducers of the antenna emit, in step a), a modulated acoustic wave propagating along said superposition axis, composed of said first acoustic wave and said second acoustic wave; the transmitting antenna has the shape of a two-armed cross, some of said transducers being arranged in line one after the other along the first arm of the antenna,the other transducers of the antenna being arranged in line one after the other along the second branch of the antenna; the transducers of the first group form the first branch of the antenna and the transducers of the second group form the second branch of the antenna, said first delays are such that the first acoustic wave propagates parallel to a first emission plane, in the form of a first beam having a narrow transverse section in a direction perpendicular to the first emission plane and elongated parallel to the first emission plane, said second delays are such that the second acoustic wave propagates parallel to a second emission plane, in the form of a second beam having a narrow transverse section in a direction perpendicular to the second emission plane and elongated parallel to the second emission plane,said superposition axis being located at the intersection between the first emission plane and the second emission plane, and the control system is configured to, from one execution of step a) to the next: vary said first delays so as to rotate the first emission plane around a first scanning axis perpendicular to the first branch of the antenna, and / or vary said second delays so as to rotate the second emission plane around a second scanning axis perpendicular to the second branch of the antenna; said transducers are arranged so as to form a matrix with several rows and several columns,the transducers being located respectively at the different intersections between the rows and columns of the matrix; the first and second delays are such that the first and second acoustic waves are emitted respectively in the form of a first collimated beam and a second collimated beam each centered on said superposition axis; the transducers of the first group occupy several distinct zones of said matrix, one or more of the transducers of the second group being interposed between any two of said zones; each transducer of the first group has transducers of the second group as nearest neighbors; several of said transducers each comprise an element made of piezoelectric material as well as a mechanical device applying a compressive stress to said element made of piezoelectric material; this compressive stress is greater than 7 bars,or even greater than 15 bars; each transducer is capable of emitting acoustic waves into the underwater environment at frequencies greater than 50 kilohertz; the receiver is capable of capturing and converting into electrical form acoustic waves whose frequency is included in a reception bandwidth,the reception bandwidth being between 0 and 30 kilohertz; the control and acquisition system is configured so that the first and second frequencies are each greater than 50 kilohertz; the control and acquisition system is configured so that the difference between the first frequency and the second frequency is included in said reception bandwidth; the control and acquisition system is configured so that the difference between the first frequency and the second frequency is less than 30 kilohertz; the control and acquisition system is configured to vary the difference between said first frequency and said second frequency, from one execution of step a) to the next; the control and acquisition system is further configured to, after each execution of step a), acquire an additional echo signal, of frequency equal to said first frequency or to said second frequency,captured by at least one of said transducers in response to the transmission carried out in step a); the control and acquisition system is configured to determine an image representative of a part of the underwater environment on the basis of said additional echo signals; the control and acquisition system is configured to determine depths of said points on the bottom of the seabed on the basis of said additional echo signals; the control and acquisition system is further configured to, in step a), control said transducers so that an angle of incidence, formed between said superposition axis and an axis perpendicular to the bottom of the underwater environment, is greater than a limit angle of incidence; the limit angle of incidence is equal to 15 degrees.

[0029] The invention also relates to a method for characterizing a part of the subsoil of an underwater environment, implemented by means of a parametric echo sounder as described above, during which the control and acquisition system executes the following steps: a) driving said transducers so that they emit into an underwater environment a first acoustic wave having a first frequency and a second acoustic wave having a second frequency, the first wave and the second wave superimposing on each other along a superposition axis directed towards a point on the bottom of the underwater environment, each transducer being driven by a transmission signal, the different transmission signals of the different transducers having predetermined delays relative to each other, and b) acquiring an echo signal, of frequency equal to the difference between said first frequency and said second frequency, picked up by said receiver in response to the transmission carried out in step a), the set of steps a) and b) being executed several times, by varying at least part of said delays from one execution of step a) to the next, so as to modify the orientation of said superposition axis from one execution of step a) to the next, the method further comprising the following step: c) determining, from the echo signals acquired during the different executions of step b), a two-dimensional or three-dimensional image representative of the contents of the subsoil of the underwater environment under each of said points on the bottom.

[0030] The optional features presented above in terms of device can also be applied to the method just described. Detailed description of examples of implementation

[0031] The description which follows with reference to the appended drawings, given as non-limiting examples, will make it clear what the invention consists of and how it can be implemented.

[0032] On the attached drawings: there Figure 1 schematically represents a vessel equipped with an echo sounder according to a first embodiment seen from the side, the Figure 2 schematically represents a vessel equipped with an echo sounder according to a first embodiment seen from below, the Figure 3 schematically represents different elements of the echo sounder of the Figures 1 and 2 , there Figure 4 schematically represents a way of controlling transducers of the echo sounder of the Figures 1 and 2 , there Figure 5 schematically represents beams of acoustic waves, emitted at a first instant by the echo sounder of the Figures 1 and 2 , there Figure 6 schematically represents beams of acoustic waves, emitted at a second instant by the echo sounder of the Figures 1 and 2 , there Figure 7 schematically represents beams of acoustic waves, emitted at a third instant by the echo sounder of the Figures 1 and 2 , there figure 8 schematically represents the steps of a process for characterizing a part of the subsoil of an underwater environment, implemented by the echo sounder of Figures 1 and 2 , there Figure 9 schematically represents the area occupied, at a given moment, by a first acoustic wave emitted by the echo sounder of the Figures 1 and 2 , front view, the Figure 10 schematically represents the area occupied, at a given moment, by a first acoustic wave emitted by the echo sounder of the Figures 1 and 2 , side view, the Figure 11 represents schematically, for the same instant as on the figures 9 and 10, the area occupied by a second acoustic wave emitted by the echo sounder, seen from the side; it also represents, at the same time, the areas occupied by additional second acoustic waves emitted by the echo sounder, the Figure 12 represents schematically, for the same instant as on the figures 9 and 10 , the area occupied by a second acoustic wave emitted by the echo sounder, seen from the front, the Figure 13 represents schematically, simultaneously, at the same time as for the Figures 9 and 11 , the areas occupied respectively by these first and second acoustic waves, seen from the front, the figure 14 represents the frequency spectra of the acoustic waves of the figures 9 to 13 , there Figure 15 schematically represents a vessel equipped with an echo sounder according to a second embodiment, seen from below, the figure 16 schematically represents beams of acoustic waves, emitted at a fourth instant by the echo sounder of the Figure 15 , there Figure 17schematically represents beams of acoustic waves, emitted at a fifth instant by the echo sounder of the Figure 15 , there figure 18 schematically represents beams of acoustic waves, emitted at a sixth instant by the echo sounder of the Figure 15 , and the figure 19 schematically represents the positions, relative to the ship of the Figure 1 , from different points of the bottom sounded by the echo sounder fitted to this vessel, seen from above. Overview

[0033] THE Figures 1 and 2 on the one hand, and the Figure 15 on the other hand schematically represent a first, and, respectively, a second embodiment of a particular parametric echo sounder 1; 1', making it possible to record a two-dimensional or three-dimensional image of the contents of the subsoil 40 of an underwater environment 3 (cf: figures 5 And 16 in particular), in a fast, precise and convenient manner.

[0034] The underwater environment 3 in question extends beneath the surface of the water, beneath and possibly around the echo sounder 1; 1'. Its subsoil 40, which is essentially solid, is composed, for example, of sediments, such as sand, or rocks. The bottom 4 of the underwater environment 3 is located at the boundary between the water and this subsoil 40.

[0035] This parametric echo sounder 1; 1' can equip a vessel 2, submersible or surface, accommodating a crew or configured to maneuver without anyone on board. It comprises a transmitting antenna 10; 10', for transmitting acoustic waves into the underwater environment 3 which surrounds it. This antenna 10; 10' can for example be installed in a housing provided for this purpose in the hull of the vessel 2, as shown in the figures.

[0036] Whatever the embodiment, the transmitting antenna 10; 10' comprises several electro-acoustic transducers 11, 12. These transducers can be controlled independently of one another. Each of them is capable of transmitting acoustic waves into the underwater environment 3, at frequencies greater than 50 kilohertz, or even greater than 100 or even 150 kilohertz. For example, these may be ultrasonic transducers 11, 12 capable of transmitting acoustic waves in a transmission bandwidth (-3 dB bandwidth) which is between 20 kilohertz and 1 megahertz.

[0037] At least three of the transducers 11, 12 of the antenna 10; 10' are non-aligned with respect to each other. In other words, the transducers 11, 12 of the antenna 10; 10' are not only distributed along the same line. The positions they occupy are distributed in a two-dimensional manner, over a given surface, for example so as to form together a cross (as in the first embodiment, of the Figure 2 ) or a rectangular matrix (as in the second embodiment, of the Figure 15 ).

[0038] The differences between the first and second embodiments of the echo sounder 1; 1', shown respectively in the figures 2 And 15, mainly concern the way of distributing these transducers 11, 12 relative to each other on the surface of the antenna 10; 10', and the number of transducers that the antenna 10; 10 comprises. These differences therefore also concern the shape of the beams W1, W2; W1', W2' emitted by the antenna, illustrated in the figures 5 And 16 ). It also concerns the details of the control of these transducers 11, 12.

[0039] These two embodiments nevertheless have many characteristics in common. Also, from one embodiment to another, the common elements are identified as much as possible by the same reference signs and are not necessarily described each time.

[0040] In these two embodiments, and moreover whatever the embodiment considered, the echo sounder 1; 1' further comprises at least one receiver 20 for capturing low-frequency acoustic waves. The receiver 20 is an electro-acoustic transducer of the hydrophone type. The receiver 20 is capable of capturing and converting into electrical form acoustic waves whose frequency is included in a reception bandwidth which extends in particular below 30 kilohertz. This reception bandwidth, which corresponds for example to the -3 decibel bandwidth of the receiver 20, may for example be between 0 and 20 kilohertz, between 0 and 30 kilohertz, or between 10 and 30 kilohertz. This receiver 20 is not necessarily directional.

[0041] As shown in the figures, the echo sounder 1; 1' comprises a single receiver 20 of this type. Alternatively, the echo sounder could however comprise several such receivers together forming a receiving antenna. Thus having several distinct receivers, located at different positions on the echo sounder, allows selective reception from an angular point of view. This in fact makes it possible, by electronic processing of all the signals picked up by these receivers, to obtain one or more echo signals each representative of an acoustic wave which, before reception by the echo sounder, propagated in the underwater environment with a given propagation direction, selected by the electronic processing in question.

[0042] For example, in a variant of the first embodiment of the Figure 2, the receiving antenna in question could comprise four receivers as described above, the four quadrants delimited by the two branches of the Mills cross-shaped transmitting antenna then each being occupied by one of these receivers, arranged in a rectangle or square.

[0043] In other variants, the different receivers of the receiving antenna could for example be located along the same line, to form a linear receiving antenna.

[0044] The echo sounder 1; 1' also comprises a control and acquisition system 30; 30' for controlling the transducers 11, 12 of the antenna 10; 10', acquiring the low-frequency echo signals E picked up by the receiver 20, and determining from these echo signals the two-dimensional or three-dimensional image of the contents of the subsoil 40 of the underwater environment 3, IM, mentioned above.

[0045] The control and acquisition system 30; 30' is configured more precisely to execute the following steps of a method for characterizing a part of the subsoil 40 of the underwater environment 3 (method shown in the figure 8 ) : a) driving the transducers 11, 12 of the transmitting antenna 10; 10' so that they transmit into the underwater environment 3 a first acoustic wave having a first frequency f1 and a second acoustic wave having a second frequency f2, the first wave and the second wave superimposing on each other along a superposition axis z S directed towards a given point P1, P2, P3 of the bottom 4 of the underwater environment ( figures 5 to 7 ), each transducer 11, 12 being driven by a transmission signal s 1,1 , s 1,2 , s 1,3 , s 2,1 , s 2,2 , s 2,3 ... which is associated with it, the transmission signals of the different transducers 11, 12 having predetermined delays relative to each other ( Figure 4), b) acquiring an echo signal s E , picked up by the receiver 20 in response to the transmission carried out in step a), and whose frequency f LF is equal to the absolute value of the difference between the first frequency f1 and the second frequency f2, all of steps a) and b) being executed several times, by varying at least part of said delays from one execution of step a) to the next, so as to modify the orientation of the superposition axis z S , from one execution of step a) to the next, and for c) determining, from the echo signals s E acquired during the different executions of step b), the two-dimensional or three-dimensional image IM, mentioned above, which is representative of the contents of the subsoil 40 of the underwater environment 3 under each of said points P1, P2, P3 of the bottom 4.

[0046] The control and acquisition system 30; 30' is configured so that the first and second frequencies f1 and f2 are greater than 50 kilohertz, or even greater than 100 or even, as here, greater than 150 kilohertz. It is further configured so that the difference between the first and second frequencies f1 and f2 is included in the reception bandwidth mentioned above. Here, the control and acquisition system 30; 30' is configured more precisely so that the difference between the first and second frequencies f1 and f2 is between 15 and 30 kilohertz. The transmission signals s 1,1 , s 1,2 , s 1,3 , s 2,1 , s 2,2 , s 2,3 ... which control the transducers 11, 12 are produced by the control and acquisition system 30; 30'.

[0047] Alternatively, the difference between the first and second frequencies f1 and f2 could, however, be smaller, such difference being, for example, less than 10 kilohertz. The values ​​of the first and second frequencies f1 and f2 could also be smaller than indicated above, each of these frequencies being, for example, between 20 and 50 kilohertz.

[0048] In any case, the difference |f1-f2| between the first and second frequencies f1 and f2 is preferably less than a quarter of the arithmetic mean (f1 +f2) / 2 of the first and second frequencies f1 and f2. Here, for example, this difference |f1-f2| is between a quarter and a sixth of the arithmetic mean (f1+f2) / 2 of the first and second frequencies f1 and f2. Such a ratio makes it possible to optimize the efficiency of the non-linear mixing of the first acoustic wave with the second acoustic wave.

[0049] Following each execution of step a), an additional acoustic wave, whose frequency f LF is equal to the absolute value of the difference between the first frequency f1 and the second frequency f2, is generated directly in the water, along the superposition axis z S , by non-linear mixing of the first acoustic wave with the second acoustic wave. This additional acoustic wave is called a "low-frequency wave" in the following. In the specialist literature, such an acoustic wave, produced directly in the underwater environment by non-linear mixing, is sometimes called a "parametric wave", or even a "secondary wave" (the first and second acoustic waves then being called "primary waves"). It propagates parallel to the superposition axis z S , and therefore reaches the point P1, P2, P3, ... of the bottom 4 towards which the superposition axis z S is directed (apart from possible acoustic refraction effects).

[0050] This low-frequency wave generally penetrates better into the subsoil 40 of the underwater environment 3 than the first and second acoustic waves, whose frequencies f1 and f2 are higher. It thus makes it possible to probe the contents of this subsoil more deeply, under the point P1, P2, P3 in question. In this subsoil 40, when the low-frequency acoustic wave encounters an element whose rigidity or density is different from the rest of the subsoil, such as a portion of pipeline buried under sediments, a portion of the low-frequency wave is reflected by this element in the manner of an echo. An instant of reception of this acoustic wave by the echo sounder then provides information on the distance between the echo sounder and the reflective element thus detected.

[0051] More generally, the acoustic wave of frequency f LF , reflected by the subsoil 40 of the underwater environment in response to the emission of the first and second acoustic waves, contains information which provides information on the presence of more or less reflective elements in a column 41, 42, 43 of this subsoil 40 which extends under the point P1, P2, P3 of the bottom 4 towards which the superposition axis z S is directed, and on the depth at which each of these elements are located, within this column. In other words, this reflected acoustic wave makes it possible to determine a sort of one-dimensional image of the contents of the subsoil 40, along this column 41, 42, 43 ( figures 5 to 7 ). It is this reflected acoustic wave, the frequency of which is equal to the absolute value of the difference between the first and second frequencies f1 and f2, which is captured in step b) by the receiver 20 and which is converted into electrical form to give the echo signal s E .

[0052] Since the directions of propagation of the first and second acoustic waves emitted in step a) depend directly on the values ​​of the delays between emission signals, the direction of the superposition axis z S , along which these first and second waves are superimposed, can be controlled in a purely electronic manner by adjusting these delays, in order to vary the orientation of the superposition axis z S from one execution of step a) to another.

[0053] The respective contents of several elementary columns 41, 42, 43, which extend under different points P1, P2, P3 of the bottom 4, are then probed, here successively, during the different executions of steps a) and b). In step c), the two- or three-dimensional image IM determined by the control and acquisition system 30; 30', can for example be obtained by placing side by side, column by column, the one-dimensional sonar images representative of the contents of these different columns 41, 42, 43.

[0054] For example, when the orientation of the superposition axis z S varies from one execution of step a) to another so that this axis points successively towards different points P1, P2, P3 distributed, on the bottom 4 of the underwater environment 3, along the same line, the echo sounder 1; 1' then records a two-dimensional image IM corresponding to a sectional view of the subsoil 40 of the underwater environment 3, according to a sectional plane passing through this line (and through the antenna 10; 10' of the echo sounder).

[0055] The parametric echo sounder 1; 1' according to the invention therefore makes it possible, in a purely electronic manner, thanks to appropriate control of the transducers 11, 12 of the antenna 10; 10', to record a two-dimensional or three-dimensional IM image representative of the contents of the subsoil 40 of the underwater environment. Advantageously, this image is obtained more quickly than by mechanically rotating a single large transmitter. This also makes it possible to compensate "in real time" (with a very short latency time typically less than 0.1 seconds), upon transmission, for the parasitic rolling or pitching movements of the echo sounder 1; 1', so that the emitted acoustic waves point in well-defined directions, independently of these parasitic movements.

[0056] After this presentation of the main characteristics of the echo sounder 1; 1', its structure and operation can now be described in more detail. Some characteristics of the electrical modules and transducers 11; 12 used will be presented first.

[0057] The structure of the transmitting antenna 10 of the first embodiment, with a “Mills cross” structure, and its control mode will be described in a second step, with reference to figures 2 And 5 à 7 .

[0058] The structure of the 10' transmitting antenna of the second embodiment and its control mode will be described next, with reference to figures 15 to 18 .

[0059] Transmission and acquisition techniques, making it possible to increase the acquisition rate of the two- or three-dimensional image of the subsurface 40 of the underwater environment by a type of multiplexing, are then described. A version of this technique, corresponding in some way to frequency multiplexing, is described with reference to figures 9 to 14 These techniques are applicable to both the first and second embodiments. Transducers and control electronics

[0060] As explained above, thanks to its antenna 10; 10' formed of several distinct transducers 11; 12, the parametric echo sounder 1; 1' according to the invention can be depointed in a purely electronic manner, in the manner of a non-parametric multi-beam echo sounder, which is particularly interesting.

[0061] But, since this is a parametric echo sounder, it is crucial that the emitted acoustic waves have very high acoustic powers. Indeed, the efficiency of the parametric non-linear mixing process in water is generally low, of the order of a few percent at most, and decreases sharply when the power of the acoustic waves to be mixed decreases.

[0062] To achieve the desired acoustic powers, even though only a portion of the surface of this antenna 10; 10' contributes to the emission of acoustic waves, each transducer 11, 12 of the antenna 10; 10' is made here from one or more elements made of piezoelectric material, subjected to a high compressive stress by a mechanical device, for example of the screw-nut type. In the absence of electrical excitation, this or these piezoelectric elements are therefore subjected to this high prestress. This then makes it possible to apply very high supply voltages to the piezoelectric elements (greater than several hundred volts), and therefore to generate very intense acoustic waves, while preventing these elements from working in extension, limiting the risks of breakage.In practice, the compressive stress applied to the piezoelectric elements by the mechanical device in question is at least greater than 7 bars (in the absence of electrical excitation), or even greater than 15 bars. And each transducer 11, 12 of the antenna 10; 10' is capable of emitting an acoustic wave having, in the immediate vicinity of the transducer, a power density per unit area greater than or equal to 20 watts per square centimeter.

[0063] In a variant of the echo sounder for which the first and second frequencies would be lower than here, for example of the order of 10 kilohertz instead of being greater than 150 kilohertz, the compressive stress applied to the piezoelectric elements mentioned above would be even greater, for example greater than 100 bars.

[0064] As for the control and acquisition system 30; 30' of the echo sounder 1; 1', it comprises here a logic unit 31, which comprises at least one processor and one memory, a conditioning module 32, which delivers the transmission signals s 1,1 , s 1,2 , s 1,3 , s 2,1 , s 2,2 , s 2,3 ... in a form suitable for driving the transducers 11, 12, and an acquisition module 33, for amplifying and converting to a digital format the echo signals s E captured by the receiver 20, before transmitting them to the logic unit 31.

[0065] The conditioning module 32 produces the transmission signals s 1,1 , s 1,2 , s 1,3 , s 2,1 , s 2,2 , s 2,3 ... from signals received from the logic unit 31. It can comprise one or more digital-to-analog converters, as well as filters for shaping the transmission signals or for carrying out impedance adaptations with transducers 11, 12. In any case, the conditioning module 32 comprises amplifiers, here one per transducer 11, 12, each capable of delivering an output electrical voltage of at least a hundred volts, to drive the corresponding transducer. These amplifiers, in this case of the pulse width modulation (PWM) type, allow the emission signals to be amplified so that their amplitudes are high enough to generate acoustic waves with a power density per unit area greater than 10 or even 20 watts per square centimeter.Here, the acoustic wave generated by each transducer, considered alone, has a power density per unit area, which, in the immediate vicinity of the antenna 10; 10' (for example at a distance from the antenna of between 10 and 50 cm), is more precisely between 10 and 40 watts per square centimeter.

[0066] The total power of the first, or second, ultrasonic wave emitted by the antenna 10 is here of the order of 218 dB. This transmission power is rather reduced compared to what is used in a usual parametric sonar. However, the image obtained representative of the contents of the subsoil 40 of the environment is of good quality, thus demonstrating a good signal-to-noise ratio. This good signal-to-noise ratio (despite the reduced transmission power) is explained for example by the fact that the buried objects (and here scanned) are buried under a few meters of water, and not a few tens of meters as is usually the case. In addition, here, the noise that could possibly interfere with the measurements comes only from the reverberation of the waves in these layers of sediment. Since the parametric beam is very fine, this reverberation is very reduced, and therefore the signal is usable.

[0067] For the first embodiment of the Figure 2, the total electrical power consumed by the control electronics, during a sequence of transmissions, is here between 2 and 5 kilowatts. Thus, at resonance, for each transducer 11, 12 of the antenna 10; 10' made up of piezoelectric elements, 90% of the applied energy is converted into acoustic energy.

[0068] Furthermore, in the embodiments described here, the transducers of the antenna 10; 10' are distributed into a first group 13; 13' of transducers 11 and a second group 14; 14' of transducers 12 ( figures 2 And 15 ), and the control and acquisition system 30; 30' is configured to: to drive each transducer 11 of the first group by a first transmission signal s 1,1 , s 1,2 , s 1,3 ... varying over time at the first frequency f1 (the first acoustic wave mentioned above then being emitted by this first group of transducers), and to drive each transducer 12 of the second group by a second transmission signal s 2,1 , s 2,2 , s 2,3 ... varying over time at the second frequency f2 (the first acoustic wave mentioned above then being emitted by this second group of transducers).

[0069] The first transmission signals s 1,1 , s 1,2 , s 1,3 ... have predetermined delays, relative to each other. More precisely, each of these first signals has a first delay Δt 1,1 , Δt 1,2 , Δt 1,3 ..., relative to a first reference signal s 1 (which varies at the frequency f1), as is schematically represented in the Figure 4. We can also predict that the different first transmission signals s 1,1 , s 1,2 , s 1,3 ... are each obtained from this same reference signal s1, by time shift. Modifying one of these first delays Δt 1,1 , Δt 1,2 , Δt 1,3 ... of course amounts to modifying the delay of the corresponding first transmission signal, relative to the other transmission signals.

[0070] In the same way, the second transmission signals s 2,1 , s 2,2 , s 2,3 ...... have predetermined delays, relative to each other. Here, each of these second signals has a second delay Δt 2,1 , Δt 2,2 , Δt 2,3 ... relative to a second reference signal s 2 , which varies at the frequency f2. Here too, the second transmission signals s 2,1 , s 2,2 , s 2,3 ... can for example be obtained from this same second reference signal s2, by time shifting. Mills cross transmitting antenna

[0071] As already indicated, in the first embodiment, the transmitting antenna 10 of the echo sounder 1 has the shape of a two-armed cross, usually called a Mills cross ( Figure 2 ).

[0072] In this case, the transducers 11 of the first group of transducers mentioned above, driven at the first frequency f1, form the first branch 13 of the antenna. They are arranged in line one after the other along this first branch 13.

[0073] And the transducers 12 of the second group of transducers, driven at the second frequency f2, form the second branch 14 of the antenna. They are also arranged in line one after the other, along this second branch 14.

[0074] In this first embodiment, in step a), the first acoustic wave is therefore emitted here by the transducers 11 of the first branch 13 of the antenna, while the second acoustic wave is emitted by the transducers 12 of the second branch 14 of the antenna.

[0075] The first and second branches 13, 14 form an angle between them of, for example, between 60 and 120 degrees. Here, these two branches are more precisely perpendicular to each other.

[0076] Such an arrangement of transducers, arranged in a cross, is known for conventional non-parametric echo sounders, for which one of the transducer branches is used for transmission while the other is used for reception. But the operation of echo sounder 1 is here very different from such a conventional echo sounder. Indeed the twobranches of the cross are here used in transmission, while reception is carried out, at a frequency different from those of transmission, by the receiver 20, distinct from the antenna 10, or as a variant, by the receiving antenna mentioned above.

[0077] Here, the antenna 10 is mounted so that the x-axis of the first branch 13 extends parallel to a longitudinal axis of the ship 2 which extends from the stern to the bow of the ship. And the y-axis of the second branch 14 of the antenna here extends transversely relative to the ship 2. The y-axis is thus perpendicular to the longitudinal axis of the ship and parallel to its deck. This arrangement of the antenna 10 relative to the ship makes electronic compensation for the parasitic pitching and rolling movements of the ship easy to implement. The z-axis, shown in the figures as a reference, is perpendicular to the antenna 10, that is to say perpendicular to its two branches 13 and 14.

[0078] Along each branch of the antenna 10, the transducers 11, 12 are distributed over a length which is here greater than 20 centimeters, and even greater than 50 centimeters. Due to its length, each of these branches is capable of emitting acoustic waves having, parallel to this branch, a reduced angular aperture.

[0079] In this first embodiment, the control and acquisition system 30 of the echo sounder 1 is configured to, in step a), give the first delays Δt 1,1 , Δt 1,2 , Δt 1,3 ..., which are applied to the different first emission signals s 1,1 , s 1,2 , s 1,3 ..., values ​​such that the first acoustic wave propagates parallel to a first emission plane Pl1, in the form of a first beam W1 having a narrow transverse section in a direction perpendicular to the first emission plane Pl1, and very elongated parallel to this plane ( Figure 5). In this emission plane, the first beam W1 has a large angular aperture, for example of several tens of degrees, while it has a reduced angular aperture perpendicular to this emission plane, for example less than 5 degrees (this angular aperture is defined for example as the angular width, at mid-height of the maximum of the peak representing the acoustic intensity of the first wave in a transverse section of the first beam W1). This first beam W1 thus has the shape of a thin, fan-shaped sheet, often called a "swath". This first beam W1 represents the area of ​​the underwater environment 3 swept by the first acoustic wave during its propagation.

[0080] To obtain that the first acoustic wave propagates in the form of this first beam W1, the control and acquisition system 30 gives for example to each of the first delays Δt 1,1 , Δt 1,2 , Δt 1,3 ... a value which is proportional to a position occupied by the corresponding transducer 11 (transducer n°1, n°2, n°3 etc...) along the first branch 13 (for example, if these first delays all have the same value, that is to say if the first transmission signals have no time shift with respect to each other, the first transmission plane Pl1, parallel to which the first acoustic wave propagates, extends perpendicular to the axis x of the first branch 13 of the antenna).The first emission plane Pl1 comprises an axis which is perpendicular to the first branch 13, in this case by the y axis, and pivots around this axis according to the values ​​given to the first delays (in this case, according to the coefficient of proportionality between the values ​​of these delays and the positions of the transducers 11 along the first branch 13 of the antenna).

[0081] In the same way, in step a), the control and acquisition system 30 gives the second delays Δt 2.1 , Δt 2.2 , Δt 2.3 ... values ​​such that the second acoustic wave propagates parallel to a second emission plane Pl2, in the form of a second beam W2 having a narrow transverse section in a direction perpendicular to the second emission plane P2, and very elongated parallel to this plane ( Figure 5). The second beam W2 has a shape similar to the first beam W1. In particular, the second beam W2 has a reduced angular aperture perpendicular to the second emission plane Pl2, for example less than 5 degrees (width at half-height of the maximum). The second emission plane Pl2, on which the second beam W2 is centered, comprises an axis perpendicular to the second branch 14 of the antenna, in this case by the x axis, around which it can pivot, depending on the values ​​given to the second delays.

[0082] The superposition axis z S is located at the intersection between the first emission plane Pl1 and the second emission plane Pl2 ( figures 5 to 7 ).

[0083] And the first and second waves are superimposed on each other mainly within a common beam Wo, formed by the intersection of the first beam W1 and the second beam W2, and centered on the superposition axis z S . In other words, it is only within this common beam Wo that the first and second waves have a substantial acoustic intensity. It is therefore within this common beam Wo that the low-frequency acoustic wave is efficiently generated.

[0084] The common beam Wo, approximately conical, has a reduced angular opening, for example less than 2 degrees, since each of the first and second beams W1, W2 forms the thin sheet described above. The transverse section of the low-frequency acoustic wave, generated by non-linear mixing in the water, is therefore not very extensive (especially since the mode of generation of this wave, non-linear, tends to further reduce its transverse extension). This makes it possible to probe the subsoil 40 of the underwater environment with good lateral resolution.

[0085] In order to probe the contents of this subsoil 40 under different points P1, P2, P3 of the bottom 4, the control and acquisition system 30 here varies the second delays Δt 2,1 , Δt 2,2 , Δt 2,3 ... from one execution of step a) to the next, so as to pivot the second emission plane Pl2 around the x axis. An angle of inclination, formed between the z axis and the second emission plane Pl2, thus varies, from one execution of step a) to the next ( figures 5 to 7 ). This pivoting of the second emission plane Pl2 makes it possible to vary the orientation of the superposition axis z S (which, during these successive executions, can then scan the entire fan formed by the first beam W1).

[0086] This way of scanning different points P1, P2, P3 of the background 4 is illustrated by the set of figures 5 to 7, which schematically represent the first and second beams W1 and W2 formed by the first and second acoustic waves, for three successive executions of the emission step a).

[0087] Furthermore, during this scanning, in the emission step a), the control and acquisition system 30 controls the transducers 11, 12 of the antenna 10 so that an angle of incidence, formed between the superposition axis z S and an axis perpendicular to the bottom of the underwater environment, z4, is greater than a limit angle of incidence. This limit angle of incidence is equal here to 15 degrees.

[0088] This prevents the low-frequency acoustic wave, generated along the superposition axis z S , from reaching the bottom 4 at normal incidence (i.e. with a zero angle of incidence), and this throughout the scanning of the bottom.

[0089] Deviating from normal incidence in this way prevents low-frequency acoustic waves, reflected specularly or quasi-specularly by the bottom 4 or by the interface between two different sedimentary layers, from being sent back towards echo sounder 1. This is interesting because the intensity of such waves, reflected specularly in this way, is generally much higher than the intensity of acoustic waves reflected by buried objects such as pipelines, which are reflected mainly diffusely. At normal incidence, the acoustic waves reflected specularly by the bottom, being much more intense, would therefore mask the acoustic waves coming from the buried objects to be detected, making the detection of such objects more difficult than here.

[0090] In the scanning mode described here, to probe the subsoil 40 with an angle of incidence greater than the limit angle of incidence, the control and acquisition system 30 adjusts the first delays Δt 1.1 , Δt 1.2 , Δt 1.3 ... so that the first emission plane Pl1 is inclined, by an angle α, relative to the axis perpendicular to the bottom z4 (the angle α is the angle formed between the plane Pl1 and the axis z4). The angle α is greater than the limit angle of incidence mentioned above. The angle α and the axis z4 are represented on the Figure 10 , which also has aspects relating to high-speed acquisition. The angle α is for example between 15 and 30 degrees.

[0091] As for the second delays Δt 2.1 , Δt 2.2 , Δt 2.3 ..., they vary from one execution of step a) to another, as described above, so as to rotate the second emission plane Pl2 around the x axis.

[0092] Thus, throughout this scan, the angle of incidence formed between the superposition axis z S and the axis perpendicular to the background z4 remains greater than the angle α, and therefore remains greater than the limit angle of incidence mentioned above.

[0093] There figure 19 schematically represents the position of the bottom points P1, P2, P3 sounded by the echo sounder, in a situation where the bottom 4 is horizontal as well as the antenna 10. In this situation, the first emission plane PI1 which is therefore inclined relative to the vertical (to avoid sounding at normal incidence), being in this case directed towards the front of the vessel 2. The bottom points P1, P2, P3 sounded by the echo sounder are then distributed along a line L, which extends over the bottom of the underwater environment, in front of the vessel 2 and perpendicular to the longitudinal axis of this vessel.

[0094] Compared to the scanning mode which has just been described, it could be provided as an alternative to vary the first delays Δt 1,1 , Δt 1,2 , Δt 1,3 ... from one execution of step a) to the next, so as to pivot the first emission plane Pl1 around the y axis, instead of varying the second delays Δt 2,1 , Δt 2,2 , Δt 2,3 ... .

[0095] It could also be envisaged to vary both the first delays Δt 1.1 , Δt 1.2 , Δt 1.3 ... and the second delays Δt 2.1 , Δt 2.2 , Δt 2.3 , ... during a series of several successive executions of step a), for example to record a three-dimensional image, representative of the contents of the subsoil 40 under several points of the bottom which are not all aligned with each other and the whole of which covers an entire surface, at the bottom of the underwater environment.

[0096] It is noted that the production of the echo sounder 1 of this first embodiment poses significant practical difficulties from the point of view of the acoustic intensity to be emitted. Indeed, the total emission surface of this Mills cross antenna 10 is significantly smaller than for a single “piston” type transmitter, or even than for the transmitter matrix antenna 10' of the second embodiment. In addition, only part of the first acoustic wave is superimposed on the second acoustic wave ( Figure 5 ). Thus, only part of the first wave, and only part of the second wave participate in the generation, in the water, of the low-frequency acoustic wave, further reducing the generation efficiency of this low-frequency wave.

[0097] But using such a Mills cross antenna (the two branches of which are driven in transmission, respectively at the first frequency f1, and at the second frequency f2) allows in return to significantly simplify certain aspects of the control electronics of the transducers (in particular in the conditioning module 32), compared to an echo sounder whose antenna is made up of a matrix of transducers.

[0098] Indeed, for a given extension of the antenna and for a given density of transducers on the surface of the antenna, the antenna 10' of the second embodiment comprises many more transducers than that of the first embodiment. For example, where the antenna 10 of the first embodiment will comprise 6×2 transducers (6 transducers per branch), that of the second embodiment will comprise 6×6. And reducing the number of transducers makes it possible to simplify their control electronics accordingly, which, it is recalled, may comprise, for each transducer, an amplifier capable of delivering an output electrical voltage of at least a hundred volts, varying over time in a fairly complex manner.Making this control electronics more compact also allows it to be housed in a head of the echo sounder 1, close to the antenna 10, thus considerably simplifying the cable system connecting the head of the echo sounder to the rest of the control and acquisition system.

[0099] Furthermore, this parametric echo sounder 1 with Mills cross antenna 10 makes it possible, during the series of successive executions of step a) described above, to record a “conventional”, non-parametric three-dimensional sonar image of the underwater environment 3 (obtained without mixing of frequencies between acoustic waves), representative of the content of an observation volume scanned by the second beam W2 during this series of executions of step a). Transducer array transmitting antenna

[0100] As shown in the Figure 15, in the second embodiment, the transducers 11, 12 of the transmitting antenna 10' are arranged so as to form a matrix with several rows and several columns, the transducers 11, 12 being located respectively at the different intersections between the rows and columns of the matrix.

[0101] In this case, it is a rectangular matrix, the columns of which are perpendicular to the rows. The transducers 11, 12 are therefore located at the nodes of a rectangular network. The axis parallel to the rows of this matrix is ​​denoted x, and the axis parallel to the columns is denoted y. The x axis is parallel to the longitudinal axis of the ship 2.

[0102] Along each line, and along each column of this matrix, the transducers 11, 12 are distributed over a length which is here greater than 20 centimeters, or even greater than 50 centimeters.

[0103] The way in which the transducers 11 of the first group 13' (driven at the first frequency f1) are arranged, relative to those 12 of the second group 14' (driven at the second frequency f2), is described below.

[0104] From the piloting point of view, in this second embodiment, the piloting and acquisition system 30' of the echo sounder 1' is configured to, in step a): give the first delays Δt 1.1 , Δt 1.2 , Δt 1.3 ... values ​​such that the first acoustic wave propagates in the form of a first collimated beam W1' ( figure 16 ), and to give the second delays Δt 2.1 , Δt 2.2 , Δt 2.3 ... values ​​such that the second acoustic wave propagates in the form of a second beam W2', also collimated.

[0105] These first and second beams W1', W2' are both centered on the same axis, which constitutes the superposition axis z S .

[0106] To emit the first acoustic wave in the form of this first collimated beam, the control and acquisition system 30' generates for example the first emission signals so that, for each transducer 11 of the first group 13', the first emission signal s 1,1 , s 1,2 , s 1,3 ... which feeds this transducer 11 has, with respect to the reference signal s1, a first delay Δt 1,1 , Δt 1,2 , Δt 1,3 ... having a value proportional to the scalar product of a direction vector u ZS by a vector r which identifies the position of the transducer 11 in question on the surface of the antenna 10'. The direction vector u ZS is a vector, for example unitary, whose direction is that of the superposition axis z S .

[0107] The second transmit signals are generated in the same way as the first transmit signals, but of course with a frequency equal to the second frequency f2.

[0108] In this second embodiment, the first and second beams W1', W2' are almost entirely superimposed on each other (they almost coincide with each other). The common beam Wo', within which the low-frequency wave is generated, then corresponds directly to one, or equivalently to the other of these two beams W1' and W2'. These different beams W1', W2' and Wo' are approximately conical. Each of the first and second beams W1' and W2' has a reduced angular aperture, for example less than 2 degrees both in a first plane which contains the superposition axis z S and the x axis and in a second plane which contains the superposition axis z S and the y axis (this angular aperture is the angular width, at mid-height of the maximum of the peak representing the acoustic intensity of the first or second acoustic wave, in a transverse section of the first or second beam).

[0109] In order to probe the contents of this subsoil 40 under different points P1, P2, P3 of the bottom 4, the control and acquisition system 30' here varies the first and second delays Δt 1.1 , Δt 1.2 , Δt 1.3 , Δt 2.1 , Δt 2.2 , Δt 2.3 ..., from one execution of step a) to the next, so as to jointly modify the orientation of the first beam W1' and that of the second beam W2', so that they remain superimposed on each other, but point towards another point of the bottom 4.

[0110] All of the figures 16 to 18 schematically represents the first and second beams W1' and W2' formed by the first and second acoustic waves for three successive executions of the emission step a). The axis z, represented in these figures as a reference and which is fixed, is perpendicular to the antenna 10'.

[0111] As in the first embodiment, the control and acquisition system 30' is configured to, in step a), control the transducers 11, 12 of the antenna 10' so that the angle of incidence, formed between the superposition axis z S and an axis perpendicular to the bottom 4 of the underwater environment, is greater than the limit angle of incidence mentioned above. As explained above, this facilitates the detection of buried objects. For this, the control and acquisition system 30' adjusts the first and second delays Δt 1,1 , Δt 1,2 , Δt 1,3 , Δt 2,1 , Δt 2,2 , Δt 2,3 ... so that the superposition axis z S is inclined relative to the axis perpendicular to the bottom.

[0112] In order for the first and second acoustic waves to overlap as much as possible, and this from the start of their propagation, it is planned to mix the transducers 11 of the first group 13' with the transducers 12 of the second group 14', on the surface of the antenna 10', rather than dividing them into two distinct zones (one of which would exclusively bring together the transducers 11 of the first group, and the other of which would exclusively bring together the transducers 12 of the second group).

[0113] On the antenna 10', the transducers 11 of the first group and those 12 of the second group are more precisely distributed alternately. Each transducer 11 of the first group 13' has as its nearest neighbors transducers 12 of the second group 14'. Here, the transducers 11, 12 are therefore distributed in the manner of a checkerboard, each white box being occupied by one of the transducers 11 of the first group, and each black box being occupied by one of the transducers 12 of the second group.

[0114] Other configurations, in which the transducers 11 of the first group would be, on the surface of the antenna 10', mixed with the transducers 12 of the second group, are conceivable. For example, the transducers could be distributed in the manner of a checkerboard, each white square being occupied by two transducers of the first group (instead of just one), and each black square being occupied by two transducers of the second group (instead of just one). Thus, it is more generally provided that the transducers 11 of the first group 13' occupy several distinct zones of the matrix formed by all the transducers, one or more of the transducers 12 of the second group 14' being interposed between any two of these zones. High-speed acquisition

[0115] When the bottom 4 of the underwater environment 3 is located at a significant depth relative to the echo sounder 1; 1', the point-to-point survey of the IM image representative of the contents of the subsoil 40 of this environment may prove particularly long if special arrangements intended to accelerate it are not implemented.

[0116] Indeed, in the absence of such provisions, it is necessary to wait a time greater than the round trip duration between the echo sounder 1; 1' and the bottom 4 (round trip at the speed of sound in water) between the sounding of the subsoil under a first point P1 of the bottom 4, and the sounding of the subsoil under a second point P2 of the bottom 4, to avoid a temporal superposition of the echo signals SE coming from these two points P1, P2, which would make them indistinguishable.

[0117] To overcome this difficulty and allow rapid acquisition of the IM image representative of the contents of the subsoil 40 of the underwater environment 3, even when this environment is deep, the control and acquisition system 30; 30' is configured here, both in the first and in the second embodiment, to vary the difference between said first frequency f1 and said second frequency f2, from one execution of step a) to the next.

[0118] Varying the difference between the first and second frequencies f1 and f2, from one execution of step a) to the next, allows in a way to encode in a frequency manner the orientation of the superposition axis z S (which varies from one execution of step a) to the next). Indeed, the different points P1, P2, P3 of the bottom 4 sounded by the echo sounder are then sounded by low-frequency waves having values ​​f LF1 , f LF2 , f LF3 , ... f LFn different from each other. The individual echo signals, s E , which come from these different points, can therefore be distinguished from each other even if they overlap each other from a temporal point of view.

[0119] In other words, even if all the individual echo signals s E, received following the different executions of the transmission step a), take the form of a single global echo signal (acquired for example continuously by the receiver 20 and the acquisition module 33), in which the different individual echo signals are temporally superimposed on each other (this global echo signal is then equal, at each instant, to the superposition, that is to say to the sum of the individual echo signals s E), each of these individual echo signals s E can be extracted from the global echo signal, in this case by frequency filtering.

[0120] The different executions of the emission step a) can then take place at short time intervals from each other, and even simultaneously, whatever the depth of the underwater environment, without risk of confusion between the individual echoes coming from the different points on the bottom. This considerably reduces the time required to record the IM image representative of the contents of the subsoil 40 of this environment.

[0121] Reducing the total duration of the series of emissions intended to probe the bottom in this way makes it possible to reduce the parasitic influence, on the IM image, of possible parasitic movements of the echo sounder 1 or possible fluctuations in the properties of the underwater environment 3.

[0122] THE figures 9 to 14illustrate certain characteristics of the first and second acoustic waves thus emitted during several successive executions of step a), during which the difference between the first frequency f1 and the second frequency f2 varies. These figures correspond to the case of the first embodiment of the echo sounder 1 (Mills cross antenna). This technique can be used in a similar manner to other embodiments, in particular to the second embodiment presented above.

[0123] In this case, during this series of emissions, the first frequency f1 maintains a constant value f1 0, while the second frequency varies. During successive executions of step a), the second frequency f2 successively presents different values ​​f2 1 , f2 2 , f2 3 , ... f2 n ( figures 11 And 14), each associated with one of these executions (i.e. associated respectively with execution number 1, execution number 2, etc.). These values ​​are different from each other. For example, they increase linearly during these successive executions of step a).

[0124] From one execution of step a) to the next, the frequency f LF of the low-frequency acoustic wave generated in the water therefore also varies, and successively presents, during these different executions, values ​​f LF1 , f LF2 , f LF3 , ... f LFn distinct from each other ( figure 14 ). These different values ​​f LF1 , f LF2 , f LF3 , ... f LFn can for example be separated two by two by 0.5 kilohertz, and be distributed between 15 and 25 kilohertz. This result could of course also be obtained by varying the first frequency f1 while keeping the second frequency f2 fixed.

[0125] To extract the individual echo signals s E , received in response to these different emissions, the control and acquisition system 30 carries out a frequency filtering of the global echo signal, which, here, is acquired continuously by the receiver 20 during the process of characterizing the subsoil 40. This global echo signal is filtered, by different bandpass filters, operating for example in parallel with each other, and centered respectively on the different frequency values ​​f LF1 , f LF2 , f LF3 , ... f LFn (equal to the different values ​​presented, during the successive executions of step a), by the difference between the first and the second frequency).The filtered signals delivered by these different filters then correspond to the different individual echo signals mentioned above, each of the individual echo signals making it possible, for example, to determine a one-dimensional image representative of the contents of the subsoil 40 under the point P1, P2, P3 of the bottom targeted during the corresponding execution of step a).

[0126] The spatial characteristics of the first and second acoustic waves emitted during this series of executions of step a) are now presented, with reference to figures 9 to 13 .

[0127] The first acoustic wave is here emitted continuously, without interruption from one execution of step a) to the next. It thus forms the same overall acoustic wave, the duration of which is greater than that of the second waves emitted by the second branch 14 of the antenna 10 during the different executions of step a).

[0128] On the figures 9 and 10, the zone Z1 of the underwater environment 3 occupied by this first wave has been represented, at a given instant, seen respectively from the front (that is to say in a plane perpendicular to the first branch 13 of the antenna 10), and from the side (in a plane parallel to this first branch 13).

[0129] And on the Figure 11 , we have represented the occupied zones Z2 1 , Z2 2 , ..., Z2 n, at the same time as for the Figure 9 , by the second acoustic waves emitted during the different executions of step a) mentioned above, seen from the front. These different zones are also shown seen from the side on the Figure 12 .

[0130] There Figure 13 corresponds to the superposition of the Figures 9 and 11 . It shows, at the same time as for the Figures 9 and 11 , the zones Z1 and Z2 1 , Z2 2 , ..., Z2 n occupied respectively by the first acoustic wave and by the second acoustic waves in question (front views). The Figure 13thus shows, from a spatial point of view, the variation of the frequency of the "low frequency" acoustic wave generated by mixing, as a function of the orientation of the superposition axis (identified by the reference signs z S1 , z S2 , ..., z Sn in this figure). It is noted in this regard that the figures 9 to 13 are instantaneous views of the areas occupied by the acoustic waves in question, and that in this sense they are very different from the views of the figures of the figures 5 to 7 which, for the first and second acoustic waves, show the entire area swept by this wave during its propagation (this swept area corresponding to beam W1, or W2). Non-parametric sonar image

[0131] The emission of the first and second acoustic waves carried out in step a) makes it possible, in addition to the low-frequency sounding of the subsoil 40 of the underwater environment 3 which has been described above, to record a “conventional” sonar image representative of at least part of the underwater environment 3. This additional image IM' is a conventional sonar image in the sense that it is obtained without mixing of frequencies in the water, unlike the image IM.

[0132] In the embodiments described here, the control and acquisition system 30; 30' advantageously takes advantage of this possibility. Indeed, it is configured here to ( figure 8 ) : after each execution of step a), execute a step b') of acquisition of an additional echo signal, of frequency equal to the first frequency f1 or to the second frequency f2, picked up by at least one of the transducers 11, 12 of the antenna in response to the transmission carried out in step a), and for (c) determining the additional image IM' mentioned above, on the basis of said additional echo signals.

[0133] This arrangement makes it possible, by carrying out a single series of emissions, that is to say by carrying out a single series of executions of step a), to record both the “parametric” image IM and the additional image IM', and thus to combine the advantages: of low-frequency parametric sonar imaging allowing good penetration into the subsoil 40, but for which the signal-to-noise ratio is generally lower than for conventional sonar imaging (and which is therefore less suitable than the latter for probing the rest of the underwater environment 3), and of conventional sonar imaging, for which the noise ratio is generally high, and which is well suited for probing the rest of the underwater environment 3, in particular for probing its depth.

[0134] The control and acquisition system 30; 30' is configured here to determine, on the basis of these additional echo signals, the depths, within the underwater environment 3, of the different points of the bottom P1, P2, P3 of the bottom 4 under which the subsoil 40 is probed by the low-frequency wave.

[0135] This makes it possible to combine a high-precision survey of the topography of a part of the bottom 4 of the underwater environment 3, by conventional sonar imaging, and a survey of the contents of the subsoil 40 located under this part of the bottom 4, by parametric imaging.

[0136] The control and acquisition system 30; 30' can also be configured to merge the data produced by processing the parametric echo signals E, with the data produced by processing the aforementioned additional echo signals. This fusion can for example comprise a recalibration of the depths deduced from the parametric echo signals E, on the basis of the depths of the points P1, P2, P3 of the bottom deduced, with more precision, from the additional, conventional echo signals.

[0137] For example, in the case of the first embodiment, when the orientation of the superposition axis z S is modified by varying the orientation of the second emission plane Pl2, then, in step b'), the echo sounder 1 acquires several additional echo signals, of frequency equal to the second frequency f2 (apart from any Doppler shifts). These additional echo signals are picked up by the different transducers 11 of the first branch 13 of the antenna, these transducers 11 then being used in reception. As is known, all of the additional echo signals acquired during this execution of step b') makes it possible to determine a two-dimensional image representative of the content of the swath W2, scanned by the second acoustic wave emitted in step a) during its propagation.As steps a) and b') are repeated for several different orientations of the second emission plane Pl2, the additional image IM' finally obtained is a three-dimensional sonar image, representative of the content of the underwater environment 3 in the observation volume scanned by the second beam W2 during this set of executions of step a).

[0138] Finally, the control and acquisition system 30; 30' can be configured to, on the basis of the additional echo signals (conventional, "non-parametric" echo signals), determine data representative of the composition of the surface layer of the subsoil 40 of the underwater environment, located under the bottom points P1, P2, P3 sounded by the echo sounder. This surface layer of the subsoil is the upper part of the subsoil, which comes into contact with the water. The data in question indicates in particular whether the surface layer of this subsoil is made up of sand, pebbles, fragmented rock or loose sediments, or whether it is made up on the contrary of a single-piece bedrock or a solidified sedimentary layer as a whole. In particular, the control and acquisition system 30; 30' can for example comprise a bottom classification module such as that included in the Seapix product developed by the applicant.Using a transverse swath, it is possible to measure a bottom backscatter index as a function of the grazing angle. These parameters are directly linked to the nature of the bottom. For more details, please refer to the document "Seafloor classification with a Multi-swath Multi-beam Echo Sounder" by Trung-Kien Nguyen, 2018. The method used in this case consists of observing a series of fixed areas on the bottom, insonified by the longitudinal swath. With the ship moving in a straight line, these areas are observed with a different viewing angle at each phase of the echo sounder imaging. By keeping in mind the brightness of each area as a function of the observation angle, a bottom backscatter profile is preserved for each of them, which is characteristic of the type of bottom observed (rock, type of sand, mud, algae, etc.).

[0139] The control and acquisition system 30; 30' can then be configured to determine a set speed for the movement of the ship 2, based on said data. This set speed is lower when the data in question indicates that the surface layer of the subsoil 40 is likely to contain buried objects, i.e. when this data indicates that the surface layer of the bottom is made up of sand, pebbles, fragmented rock or loose sediments, than when this data indicates that the surface layer of the subsoil 40 is not likely to contain buried objects, i.e. when it is made up of a single-piece bedrock or a solidified sedimentary layer as a whole. This set speed is then transmitted to a control system of the ship 2 which adjusts the movement speed of the ship to this set value.These arrangements make it possible to reduce the time required to explore a given area of ​​the underwater environment floor, by avoiding detailed sounding of parts of the floor which are not likely to contain buried objects.

[0140] Different variations can be made to the echo sounder and the process for characterizing a part of the subsoil of the underwater environment which have been presented above.

[0141] First of all, instead of the first acoustic wave being emitted by a first group of transducers, while the second acoustic wave is emitted by a second group of transducers distinct from the first, it could be provided that each transducer contributes to the emission of these two waves. In the context of such a variant, the control and acquisition system is configured so that the transmission signals, which control the transducers 11, 12, each comprise a first component and a second component varying respectively over time with the first frequency f1 and with the second frequency f2. The transducers 11, 12 of the antenna then emit, in step a), a modulated acoustic wave, in this case amplitude modulated, propagating along the superposition axis z S , composed of the first acoustic wave and the second acoustic wave.In other words, this modulated acoustic wave is broken down into the first acoustic wave and the second acoustic wave. The transmission signals are also amplitude-modulated signals. From the point of view of the electronics for controlling the transducers 11, 12, this variant is more restrictive to implement than transmission of the first and second acoustic waves by separate transducers, since it is then necessary to supply the transducers with oscillating, high-voltage signals of a more complex form.

[0142] Furthermore, a different arrangement of the transducers on the antenna surface than that described above could be used. For example, the transducers could be distributed so that together they form a spiral, instead of a cross or a matrix.

[0143] But in any case, the antenna transducers are not all distributed along the same line. As already indicated, their positions are distributed two-dimensionally, over an entire surface. More precisely, the assembly comprising the different transducers of the transmitting antenna extends over a surface which has, in two orthogonal directions, two dimensions each greater than 20 centimeters, or even greater than 50 centimeters. This makes it possible to emit the first and second acoustic waves with good directivity, which increases their power per unit area and therefore increases the efficiency of the non-linear process of generating the low-frequency acoustic wave. This makes it possible to probe the contents of the subsoil 40 of the underwater environment with good lateral resolution.

Claims

1. A parametric echosounder (1; 1') comprising: - a transmitting antenna (10; 10') comprising several transducers (11, 12), - at least one receiver (20), and - a driving and acquisition system (30; 30') configured to execute the following steps: a) driving said transducers (11, 12) so that they transmit in an underwater environment (3) a first acoustic wave having a first frequency (f1) and a second acoustic wave having a second frequency (f2), the first wave and the second wave superposing each other along a superposition axis (zS) directed towards a point (P1, P2, P3) of the bottom (4) of the underwater environment (3), each transducer (11, 12) being driven by a transmit signal (s1,1, s1,2, s1,3, s2,1, s2,2, s2,3), the different transmit signals of the different transducers (11, 12) having predetermined delays (Δt1,1 , Δt1,2 , Δt1,3 , Δt2,1 , Δt2,2 , Δt2,3) with respect to each other, and b) acquiring an echo signal (sE), of frequency equal to the difference between said first frequency (f1) and said second frequency (f2), picked up by said receiver (20) in response to the transmission made at step a), the driving and acquisition system (30; 30') being moreover configured to execute all steps a) and b) several times, by varying a part at least of said delays (Δt1,1, Δt1,2, Δt1,3, Δt2,1, Δt2,2, Δt2,3) from one execution of step a) to the next, in such a way as to modify the orientation of said superposition axis (zS) from one execution of step a) to the next, and to c) determine, based on the echo signals (sE) acquired during the different executions of step b), a two-dimensional or three-dimensional image (IM) representative of the content of the sub-bottom (40) of the underwater environment (3) below each of said points (P1, P2, P3) of the bottom, the driving and acquisition system (30; 30') being moreover configured to, at step a): - drive each transducer (11) of a first group (13; 13') of said transducers (11, 12) by a first transmit signal (s1,1, s1,2, s1,3) varying over time at said first frequency (f1), the transducers (11) of this first group transmitting said first acoustic wave, the different first transmit signals (s1,1, s1,2, s1,3) having first delays (Δt1,1, Δt1,2, Δt1,3), predetermined with respect to each other, - drive each transducer (12) of a second group (14; 14') of said transducers (11, 12) by a second transmit signal (s2,1, s2,2, s2,3) varying over time at said second frequency (f2), the transducers (12) of this second group transmitting said second acoustic wave, the different second transmit signals (s2,1, s2,2, s2,3) having second delays (Δt2,1, Δt2,2, Δt2,3), predetermined with respect to each other, - and vary said first delays (Δt1,1, Δt1,2, Δt1,3) and / or said second delays (Δt2,1, Δt2,2, Δt2,3) from one execution of step a) to the next, in such a way as to modify the orientation of said superposition axis (zS) from one execution of step a) to the next, the parametric echosounder being characterized in that said transducers (11, 12) being arranged in such a way as to form a matrix of several lines and several columns, the transducers (11, 12) being respectively located at the different intersections between lines and columns of the matrix, the first and second delays (Δt1,1, Δt1,2, Δt1,3, Δt2,1, Δt2,2, Δt2,3) being such that the first and second acoustic waves are transmitted as a first collimated beam (W1') and a second collimated beam (W2'), respectively, each centred to said superposition axis (zS), the transducers (11) of the first group (13') occupying several distinct areas of said matrix, one or several of the transducers (12) of the second group (14') being interposed between any two of said areas, each transducer (11) of the first group (13') having for nearest neighbours transducers (12) of the second group (14').

2. The parametric echosounder (1; 1') according to claim 1, wherein certain of said transducers (11, 12) each comprise an element made of a piezoelectric material as well as a mechanical device applying a compressive stress to said piezoelectric material element, said compressive stress being higher than 7 bars.

3. The parametric echosounder (1; 1') according to claim 1 or 2, wherein: - each transducer (11, 12) is adapted to transmit acoustic waves in the underwater environment (3) at frequencies higher than 50 kilohertz, - the receiver (20) is adapted to pick up and convert into electric form acoustic waves whose frequency is comprised between a reception bandwidth, the reception bandwidth being between 0 and 30 kilohertz, and wherein - the driving and acquisition system (30; 30') is configured in such a way that the first and second frequencies (f1, f2) are each higher than 50 kilohertz, and in such a way that the difference between the first frequency (f1) and the second frequency (f2) is comprised in said reception bandwidth and lower than 30 kilohertz.

4. The parametric echosounder (1; 1') according to one of claims 1 to 3, wherein the driving and acquisition system (30; 30') is configured to vary the difference between said first frequency (f1) and said second frequency (f2), from one execution of step a) to the next.

5. The parametric echosounder (1; 1') according to one of claims 1 to 4, wherein the driving and acquisition system (30; 30') is moreover configured to: after each execution of step a), acquire an additional echo signal, of frequency equal to said first frequency (f1) or to said second frequency (f2), picked up by one at least of said transducers (11, 12) in response to the transmission made at step a), and to - determine an image (IM') representative of a portion of the underwater environment (3) on the basis of said additional echo signals.

6. The parametric echosounder (1; 1') according to one of claims 1 to 5, wherein the driving and acquisition system (30; 30') is moreover configured to, at step a), drive said transducers (11, 12) in such a way that an incidence angle, formed between said superposition axis (zS) and an axis (z4) perpendicular to the underwater environment bottom (4), is greater than a limit incidence angle.

7. A parametric echosounder (1; 1') comprising: - a transmitting antenna (10; 10') comprising several transducers (11, 12), the transmitting antenna (10) having a two-arm cross shape, certain of said transducers (11) being arranged in a line one after the other along the first arm (13) of the antenna (10), the other transducers (12) of the antenna being arranged in a line one after the other along the second arm (14) of the antenna (10), - at least one receiver (20), and - a driving and acquisition system (30; 30') configured to execute the following steps: a) driving said transducers (11, 12) so that they transmit in an underwater environment (3) a first acoustic wave having a first frequency (f1) and a second acoustic wave having a second frequency (f2), the first wave and the second wave superposing each other along a superposition axis (zS) directed towards a point (P1, P2, P3) of the bottom (4) of the underwater environment (3), each transducer (11, 12) being driven by a transmit signal (s1,1, s1,2, s1,3, s2,1, s2,2, s2,3), the different transmit signals of the different transducers (11, 12) having predetermined delays (Δt1,1, Δt1,2, Δt1,3, Δt2,1, Δt2,2, Δt2,3) with respect to each other, and b) acquiring an echo signal (sE), of frequency equal to the difference between said first frequency (f1) and said second frequency (f2), picked up by said receiver (20) in response to the transmission made at step a), the driving and acquisition system (30; 30') being moreover configured to execute all steps a) and b) several times, by varying a part at least of said delays (Δt1,1, Δt1,2, Δt1,3, Δt2,1, Δt2,2, Δt2,3) from one execution of step a) to the next, in such a way as to modify the orientation of said superposition axis (zS) from one execution of step a) to the next, and to c) determine, based on the echo signals (sE) acquired during the different executions of step b), a two-dimensional or three-dimensional image (IM) representative of the content of the sub-bottom (40) of the underwater environment (3) below each of said points (P1, P2, P3) of the bottom, the driving and acquisition system (30; 30') being further configured to, at step a): - drive each transducer (11) of a first group (13; 13') of said transducers (11, 12) by a first transmit signal (s1,1, s1,2, s1,3) varying over time at said first frequency (f1), the transducers (11) of this first group transmitting said first acoustic wave, the different first transmit signals (s1,1, s1,2, s1,3) having first delays (Δt1,1, Δt1,2, Δt1,3), predetermined with respect to each other, - drive each transducer (12) of a second group (14; 14') of said transducers (11, 12) by a second transmit signal (s2,1, s2,2, s2,3) varying over time at said second frequency (f2), the transducers (12) of this second group transmitting said second acoustic wave, the different second transmit signals (s2,1, s2,2, s2,3) having second delays (Δt2,1, Δt2,2, Δt2,3), predetermined with respect to each other, and vary said first delays (Δt1,1, Δt1,2, Δt1,3) and / or said second delays (Δt2,1, Δt2,2, Δt2,3) from one execution of step a) to the next, in such a way as to modify the orientation of said superposition axis (zS) from one execution of step a) to the next, wherein: - the transducers (11) of the first group form the first arm (13) of the antenna (10) and the transducers (12) of the second group form the second arm (14) of the antenna (10), - said first delays (Δt1,1, Δt1,2, Δt1,3) are such that the first acoustic wave propagates parallel to a first transmission plane (PI1), as a first beam (W1) having a narrow cross-section in a direction perpendicular to the first transmission plane (PI1) and elongated parallel to the first transmission plane (PI1), - said second delays (Δt2,1, Δt2,2, Δt2,3) being such that the second acoustic wave propagates parallel to a second transmission plane (PI2), as a second beam (W2) having a narrow cross-section along a direction perpendicular to the second transmission plane (PI2) and elongated parallel to the second transmission plane (PI2), said superposition axis (zS) being located at the intersection between the first transmission plane (PI1) and the second transmission plane (PI2), and wherein the driving system (30) is configured to, from one execution of step a) to the next: - vary said first delays (Δt1,1, Δt1,2, Δt1,3) in order to pivot the first transmission plane (PI1) about a first scanning axis (y) perpendicular to the first arm (13) of the antenna (10), and / or - vary said second delays (Δt2,1, Δt2,2, Δt2,3) in order to pivot the second transmission plane (PI2) about a second scanning axis (x) perpendicular to the second arm (14) of the antenna (10).

8. The parametric echosounder (1; 1') according to claim 7, wherein certain of said transducers (11, 12) each comprises an element made of a piezoelectric material as well as a mechanical device applying a compressive stress to said piezoelectric material element, said compressive stress being higher than 7 bars.

9. The parametric echosounder (1; 1') according to one of claims 7 or 8, wherein: - each transducer (11, 12) is adapted to transmit acoustic waves in the underwater environment (3) at frequencies higher than 50 kilohertz, - the receiver (20) is adapted to pick up and convert into electric form acoustic waves whose frequency is comprised between a reception bandwidth, the reception bandwidth being between 0 and 30 kilohertz, and wherein - the driving and acquisition system (30; 30') is configured in such a way that the first and second frequencies (f1, f2) are each higher than 50 kilohertz, and in such a way that the difference between the first frequency (f1) and the second frequency (f2) is comprised in said reception bandwidth and lower than 30 kilohertz.

10. The parametric echosounder (1; 1') according to one of claims 7 to 9, wherein the driving and acquisition system (30; 30') is configured to vary the difference between said first frequency (f1) and said second frequency (f2), from one execution of step a) to the next.

11. The parametric echosounder (1; 1') according to one of claims 7 to 10, wherein the driving and acquisition system (30; 30') is moreover configured to: after each execution of step a), acquire an additional echo signal, of frequency equal to said first frequency (f1) or to said second frequency (f2), picked up by one at least of said transducers (11, 12) in response to the transmission made at step a), and to - determine an image (IM') representative of a portion of the underwater environment (3) on the basis of said additional echo signals.

12. The parametric echosounder (1; 1') according to one of claims 7 to 11, wherein the driving and acquisition system (30; 30') is moreover configured to, at step a), drive said transducers (11, 12) in such a way that an incidence angle, formed between said superposition axis (zS) and an axis (z4) perpendicular to the underwater environment bottom (4), is greater than a limit incidence angle.

13. A method for characterizing a portion of the sub-bottom (40) of an underwater environment (3), implemented by means of a parametric echosounder (1; 1') according to any one of claims 1 to 11, the method comprising the following steps, executed by the driving and acquisition system (30; 30'): a) driving said transducers (11, 12) so that they transmit in an underwater environment (3) a first acoustic wave having a first frequency (f1) and a second acoustic wave having a second frequency (f2), the first wave and the second wave superposing each other along a superposition axis (zS) directed towards a point (P1, P2, P3) of the underwater environment bottom (4), each transducer (11, 12) being driven by a transmit signal (s1,1, s1,2, s1,3, s2,1, s2,2, s2,3), the different transmit signals of the different transducers (11, 12) having predetermined delays (Δt1,1, Δt1,2, Δt1,3, Δt2,1, Δt2,2, Δt2,3) with respect to each other, and b) acquiring an echo signal (sE), of frequency equal to the difference between said first frequency (f1) and said second frequency (f2), picked up by said receiver (20) in response to the transmission made at step a), all steps a) and b) being executed several times, by varying a part at least of said delays (Δt1,1, Δt1,2, Δt1,3, Δt2,1, Δt2,2, Δt2,3) from one execution of step a) to the next, in such a way as to modify the orientation of said superposition axis (zS) from one execution of step a) to the next, the method further comprising the following step: c) determining, based on the echo signals (sE) acquired during the different executions of step b), a two-dimensional or three-dimensional image (IM) representative of the content of the sub-bottom (40) of the underwater environment (3) below each of said points (P1, P2, P3) of the bottom.