Commandable parametric echo sounder and method for characterizing a sub-seafloor portion of an underwater environment - Patents.com

Abstract

The present invention provides a method for detecting a wavefront of an underwater environment (3) comprising: a) driving a transducer of the radiating antenna (10) to radiate first and second acoustic waves having first and second frequencies, the waves having overlapping axes (z) directed toward a point (P1) on the seabed (4) of the underwater environment (3). S a) acquiring echo signals of a first frequency and a second frequency, the echo sounder being configured to perform steps a) and b) a plurality of times while changing the direction of the superposition axis and to determine, based on the acquired echo signals, an image representative of the contents below the seafloor (40) of the underwater environment below each of said points. The present invention also relates to a related method for characterizing a portion of a sub-seafloor of an underwater environment.

Description

[Technical Field]

[0001] The present invention relates generally to the field of depth measurement devices, such as echo sounders or sonars, that use acoustic wave reflection.

[0002] In particular, it relates to echo sounders for exploring the contents below the seafloor of underwater environments.

[0003] especially, -In the aquatic environment, a first acoustic wave varying at a first frequency and a second acoustic wave varying at a second frequency, or transmitting a modulated acoustic wave comprising a first component and a second component varying at a first frequency and a second frequency, respectively; - in response to a transmission, pick up an echo signal at a frequency equal to the difference between a first frequency and a second frequency.

[0004] It also relates to related methods for characterizing the sub-seafloor portion of an underwater environment. [Background technology]

[0005] The exploration of the sub-seafloor contents of underwater environments is an important problem, especially in the maritime field.

[0006] Indeed, it is common practice to place equipment such as telecommunications or power transmission pipelines and cables directly on the seabed, however these equipment may subsequently become buried under layers of sediment or sand, making them difficult to locate and characterize.

[0007] A specific echo sounder for bathymetry of the contents of such sediment layers 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) pp. 62-69.

[0008] The echosounder described in this article includes a large-diameter piston-type transmitter that is actuated to transmit a first acoustic wave varying at a first frequency and a second acoustic wave varying at a second frequency. Each of these two waves is transmitted vertically below the transmitter. Thus, the two waves are superimposed on each other along the water column directly below the transmitter.

[0009] Because water's response to density changes is slightly nonlinear, the superposition of the first and second acoustic waves directly generates an additional acoustic wave in the water at a frequency equal to the difference between the first and second frequencies. This nonlinear mixing effect is commonly referred to as a "parametric effect." In a sense, it corresponds directly to the nonlinear self-demodulation in water of the modulated wave formed by the superposition of the first and second acoustic waves.

[0010] This additional acoustic wave generated along the water column below the transmitter therefore propagates vertically to reach a point on the seafloor directly below the transmitter, thereby enabling sub-bottom sounding of the underwater environment below the point of interest.

[0011] Since the frequency of the "low frequency" additional acoustic wave is lower than the frequency of the first and second acoustic waves, the absorption coefficient of the additional acoustic wave in the sediment layer being sounded is lower than the absorption coefficient of the first and second acoustic waves. On the other hand, generating this additional acoustic wave directly in the water by nonlinear mixing makes it possible to benefit from higher directivity than if the additional acoustic wave were directly transmitted by a transmitter.

[0012] The echosounders just described can be successively placed at various positions on the surface of the water to sound a sediment layer at different points on the seafloor, making it possible to generate two- or three-dimensional images representative of the contents of this layer.

[0013] However, such soundings are time consuming and inconvenient. Summary of the Invention [Problem to be solved by the invention]

[0014] In this context, the present invention proposes a new type of parametric echo sounder that allows, by parametric effects, to generate acoustic waves directly in water, where the propagation axis of this wave can be changed by electronic driving of the echo sounder's transducer, without moving or pivoting the echo sounder. [Means for solving the problem]

[0015] More specifically, according to the present invention, a parametric echo sounder is proposed, the parametric echo sounder comprising: a transmitting antenna including a plurality of transducers; at least one receiver; -The following steps, namely: a) driving transducers to transmit, in 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 being superimposed on one another along a superposition axis directed towards a point on the seabed of the underwater environment, each transducer being driven by a transmit signal, and different transmit signals of different transducers having a predetermined delay with respect to one another; b) obtaining an echo signal picked up by a receiver in response to the transmission made in step a) and having a frequency equal to the difference between the first frequency and the second frequency; Furthermore, - performing all of steps a) and b) several times by varying at least some of the delays between one execution of step a) so as to change the direction of the superposition axis from one execution of step a) to the next, and c) a drive and acquisition system configured to determine a two-dimensional or three-dimensional image representative of the sub-seabed contents of the underwater environment below each of the points on the seabed based on the echo signals acquired during the different executions of step b).

[0016] After each execution of step a), an additional acoustic wave whose frequency is equal to the absolute value of the difference between the first and second frequencies is generated directly in the water along the superposition axis by nonlinear mixing of the first and second acoustic waves. This "low-frequency" additional acoustic wave then propagates parallel to the superposition axis. The point on the seafloor that it reaches, located along the continuation of this axis, can therefore be selected by appropriately choosing the delay between the transmitted signals.

[0017] The parametric echo sounder according to the invention therefore makes it possible to sound the sub-seafloor contents of the underwater environment below different points on the seabed, thanks to suitable electronic driving of the antenna transducers, with "low frequency" acoustic waves having good penetration into this sub-seafloor and without the need to move the echo sounder for that purpose.

[0018] However, transmitting high acoustic powers has proven to be much more difficult with a parametric echo sounder according to the invention than with an echo sounder that includes a single large transducer (such as that in the article by J. Marchal and P. Cervenka mentioned above) that is pivotally mounted so that it can be aimed at different points on the seabed.

[0019] Indeed, in the case of a transmitting antenna consisting of a single transmitter occupying the entire antenna plane, each point in the antenna plane contributes to the transmission of the acoustic waves generated.

[0020] On the other hand, in the case of a transmitting antenna composed of several separate transducers distributed at different points on the antenna plane, as here, only the part of the antenna plane formed by the respective transmitting surfaces of all these transducers contributes to the transmission of the generated acoustic waves.

[0021] Note that the efficiency of the parametric process of nonlinear mixing in water is generally low, on the order of a few percent at most, and decreases significantly as the power of the acoustic waves being mixed decreases. Therefore, parametric echo sounders must be able to operate at very high acoustic powers P A It is important to emit, for example, a power of the order of 230 decibels (power value P in decibels) at a distance of 1 meter from the transducer. A is 10log(P A / P Aref )) where the reference sound power P Aref is the acoustic power of a pressure wave of 1 micropascal amplitude.

[0022] Therefore, creating a parametric echo sounder using a transmitting antenna is technically particularly challenging. The transducers used must be capable of transmitting very high power densities per surface unit (usually greater than 10 watts per square centimeter). Above all, these transducers must be driven by a high-power transmission signal, which usually requires one or more cumbersome power amplifiers, complicating the electronics used to drive these transducers.

[0023] Despite these technical difficulties, parametric echo sounders, including such a transmitting antenna with multiple transducers, are particularly interesting. Indeed, by driving the propagation direction of the acoustic waves generated by the parametric effect purely electronically, it becomes possible to change this direction very quickly.

[0024] This allows, among other things, two-dimensional or three-dimensional images of the contents of a sediment layer to be taken more quickly than would be possible with a single large transducer that is mechanically pivoted.

[0025] This also allows for the parasitic roll or pitch motions of the echo sounder to be corrected "in real time" (with a very short delay, typically less than 0.1 seconds) at the time of transmission, so that the transmitted acoustic waves point in a clearly determined direction independent of these parasitic motions.

[0026] Furthermore, a transmitting antenna with multiple transducers offers great flexibility in the selection of the transmission sequence, thus allowing many different bathymetry configurations and potentially mixing parametric and bathymetry without mixing the frequencies of the seafloor in the underwater environment.

[0027] An echo sounder according to the present invention may comprise a receiving antenna including the receiver described above, as well as other receivers of the same type. Such a receiving antenna with multiple receivers allows selective reception from angular viewpoints, thereby enabling the driving and acquisition system to select a given receiving direction. The echo signals acquired in step b) then represent low-frequency acoustic waves with a clearly determined propagation direction, which is the receiving direction of interest selected by the driving and acquisition system (instead of the echo signals representing any acoustic waves received regardless of their propagation direction).

[0028] Other non-limiting and advantageous characteristics of the echo sounder according to the invention, considered individually or according to all technically possible combinations, are: The drive and acquisition system in step a) - driving each transducer of a first group of transducers with a time-varying first transmit signal at a first frequency, the transducers of this first group transmitting first acoustic waves, the different first transmit signals having a predetermined first delay with respect to each other; - driving each transducer of a second group of transducers with a time-varying second transmit signal at a second frequency, the transducers of this second group transmitting second acoustic waves, the different second transmit signals having a second predetermined delay with respect to each other; varying the first delay and / or the second delay from one execution of step a) to the next so as to change the direction of the superposition axis from one execution of step a) to the next; -Drive and acquisition system - so that the transmitted signals each include a first component and a second component that vary over time at a first frequency and a second frequency, respectively; the transducer of the antenna is configured in step a) to transmit a modulated acoustic wave propagating along the superposition axis, the modulated acoustic wave being composed of the first acoustic wave and the second acoustic wave; the transmitting antenna has a cross shape with two arms, some of the transducers being arranged in a line one after the other along a first arm of the antenna and other transducers of the antenna being arranged in a line one after the other along a second arm of the antenna, - the first group of transducers forms a first arm of the antenna, the second group of transducers forms a second arm of the antenna, the first delay is such that when the first beam has a narrow cross section in a direction perpendicular to the first transmitting surface and extends parallel to the first transmitting surface, the first acoustic wave propagates parallel to the first transmitting surface, the second delay is such that when the second beam has a narrow cross section along a direction perpendicular to the second transmitting surface and extends parallel to the second transmitting surface, the second acoustic wave propagates parallel to the second transmitting surface, the superposition axis is located at the intersection between the first transmitting surface and the second transmitting surface, and the drive system is adapted to drive the drive system from one execution of step a) to the next execution. Varying the first delay to pivot the first transmit plane about a first scan axis perpendicular to the first arm of the antenna; - varying the second delay to pivot the second transmission plane about a second scan axis perpendicular to a second arm of the antenna; the transducers are arranged to form a matrix of several rows and several columns, the transducers being respectively arranged at 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 transmitted as first and second collimated beams, respectively, each centered on a superposition axis; the transducers of the first group occupy several distinct regions of the matrix, and one or more of the transducers of the second group are inserted between any two of the regions; each transducer of the first group has a transducer of the second group adjacent to it, some of the transducers each include an element made of piezoelectric material and a mechanical device that applies a compressive stress to the piezoelectric material element; - this compressive stress is higher than 7 bar and even higher than 15 bar, each transducer adapted to transmit acoustic waves at a frequency greater than 50 kilohertz in an underwater environment; - the receiver is adapted to pick up and convert into electrical form acoustic waves comprised in a reception bandwidth of 0 to 30 kilohertz; the drive and acquisition system is configured such that the first frequency and the second frequency are each greater than 50 kilohertz; the drive and acquisition system is configured such that the difference between the first frequency and the second frequency is included in the reception bandwidth; the drive and acquisition system is configured such that the difference between the first frequency and the second frequency is less than 30 kilohertz; the drive and acquisition system is configured to vary the difference between the first frequency and the second frequency from one execution of step a) to the next; the driving and acquisition system is further configured to acquire, after each execution of step a), additional echo signals picked up by the at least one transducer in response to the transmissions made in step a), at a frequency equal to the first frequency or the second frequency; the drive and acquisition system is configured to determine an image representative of a portion of the underwater environment based on the additional echo signals; the drive and acquisition system is configured to determine a depth of a point on the seabed based on the additional echo signal; the drive and acquisition system is further configured to drive the transducer in step a) such that an angle of incidence formed between the superposition axis and an axis perpendicular to the seabed of the underwater environment is greater than a limiting angle of incidence; - The limiting angle of incidence is equal to 15 degrees.

[0029] The invention also relates to a method for characterizing a sub-seafloor portion of an underwater environment, the method being carried out by a parametric echo sounder as described above, during which the driving and acquisition system performs the following steps: a) driving transducers to transmit a first acoustic wave having a first frequency and a second acoustic wave having a second frequency in an underwater environment, the first wave and the second wave being superimposed on one another along a superposition axis directed toward a point on the seabed of the underwater environment, each transducer being driven by a transmit signal, and different transmit signals of different transducers having a predetermined delay with respect to one another; b) obtaining, in response to the transmission made in step a), an echo signal picked up by a receiver, the echo signal having a frequency equal to the difference between the first frequency and the second frequency, and performing all of steps a) and b) several times by varying at least a portion of the delay from one execution of step a) to the next, so as to change the direction of the superposition axis from one execution of step a) to the next; c) determining a two-dimensional or three-dimensional image representative of the contents of the underwater environment below each point on the seabed based on the echo signals acquired during the different runs of step b).

[0030] The optional features presented above in relation to the device may also be applied to the method just described.

[0031] The following description, in conjunction with the accompanying drawings, given as non-limiting examples, will enable a full understanding of what the invention consists of and how it can be put into practice. [Brief explanation of the drawings]

[0032] [Figure 1] 1 is a diagram showing a schematic side view of a vessel equipped with an echo sounder according to a first embodiment; [Figure 2] 1 is a diagram showing a schematic view of a ship equipped with an echo sounder according to a first embodiment, seen from below; [Figure 3] FIG. 3 shows a schematic diagram of the different elements of the echo sounder of FIGS. 1 and 2. [Figure 4] FIG. 3 is a diagram illustrating a schematic of a method of driving the transducers of the echo sounder of FIGS. 1 and 2. [Figure 5] FIG. 3 shows a schematic diagram of an acoustic wave beam transmitted by the echo sounder of FIGS. 1 and 2 at a first instant in time. [Figure 6] FIG. 3 shows a schematic diagram of an acoustic wave beam transmitted by the echo sounder of FIGS. 1 and 2 at a second instant in time. [Figure 7] FIG. 3 shows a schematic diagram of an acoustic wave beam transmitted by the echo sounder of FIGS. 1 and 2 at a third instant in time. [Figure 8] 3A-3C show schematic diagrams of steps of a method for characterizing a sub-seafloor portion of an underwater environment performed by the echosounder of FIGS. 1 and 2; [Figure 9] FIG. 3 shows a schematic representation of the area occupied by the first acoustic wave transmitted by the echo sounder of FIGS. 1 and 2 at a given moment, seen from the front. [Figure 10] FIG. 3 shows a schematic view from the side of the area occupied by the first acoustic wave transmitted by the echo sounder of FIGS. 1 and 2 at a given moment; [Figure 11] FIG. 11 is a side view showing a schematic representation of the area occupied by the second acoustic wave transmitted by the echo sounder at the same instant as in FIGS. 9 and 10, and also showing the area occupied by an additional second acoustic wave transmitted by the echo sounder at the same time. [Figure 12] FIG. 11 is a diagram showing, from the front, a schematic representation of the area occupied by the second acoustic wave transmitted by the echo sounder at the same instant as in FIGS. 9 and 10; [Figure 13] 12 is a diagram showing, from a front view, a schematic representation of the areas occupied by these first and second acoustic waves, respectively, at the same instant and simultaneously as in FIGS. 9 and 11. FIG. [Figure 14] FIG. 14 is a diagram showing frequency spectra of the acoustic waves of FIGS. 9 to 13. [Figure 15] FIG. 10 is a diagram showing a schematic view of a vessel equipped with an echo sounder according to a second embodiment, seen from below. [Figure 16] FIG. 16 is a schematic diagram illustrating an acoustic wave beam transmitted by the echo sounder of FIG. 15 at a fourth instant in time. [Figure 17] FIG. 16 is a schematic diagram illustrating an acoustic wave beam transmitted by the echo sounder of FIG. 15 at a fifth instant in time. [Figure 18] FIG. 16 is a schematic diagram illustrating an acoustic wave beam transmitted by the echo sounder of FIG. 15 at a sixth instant in time. [Figure 19] 2 shows, from above, a schematic view of the vessel of FIG. 1 and the positions of different points of the seabed sounded by an echo sounder fitted to said vessel; DETAILED DESCRIPTION OF THE INVENTION

[0033] Figures 1 and 2, on the other hand Figure 15, show respectively a first and a second embodiment of a particular parametric echo sounder 1;1', which allows for fast, accurate and convenient acquisition of two-dimensional or three-dimensional images of the contents below the seabed 40 of an underwater environment 3 (see in particular Figures 5 and 16).

[0034] The underwater environment 3 of interest extends below the water surface, beneath and possibly around the echosounder 1;1'. The sub-seafloor 40 is essentially solid and consists of sediments such as sand or rock. The seabed 4 of the underwater environment 3 is located at the frontier between the water and the sub-seafloor 40.

[0035] This parametric echo sounder 1;1' may be fitted to a submersible or surface vessel 2, which is arranged or configured to operate without personnel on board, and which is equipped with a transmitting antenna 10;10' for transmitting acoustic waves in the surrounding underwater environment 3. This antenna 10;10' may be installed, for example, in a housing provided for this purpose within the hull of the vessel 2, as shown in the drawings.

[0036] Whatever the embodiment, the transmitting antenna 10; 10' comprises a plurality of electroacoustic transducers 11, 12, which may be driven independently of one another. Each of them is adapted to transmit acoustic waves in the underwater environment 3 at frequencies higher than 50 kHz, or higher than 100 kHz, or even higher than 150 kHz. They may be, for example, ultrasonic transducers 11, 12 adapted to transmit acoustic waves with a transmission bandwidth (-3 dB bandwidth) between 20 kHz and 1 MHz.

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

[0038] The differences between the first and second embodiments of the echo sounder 1; 1' shown in Figures 2 and 15 respectively relate mainly to the way these transducers 11, 12 are distributed relative to one another on the surface of the antenna 10; 10' and to the number of transducers included in the antenna 10; 10'. These differences therefore also relate to the shape of the beams W1, W2; W1', W2' transmitted by the antennas shown in Figures 5 and 16, as well as to the details of driving these transducers 11, 12.

[0039] Nevertheless, these two embodiments have many common features, and therefore, from one embodiment to another, common elements are marked with the same reference signs as far as possible and are not necessarily described in each case.

[0040] In these two embodiments, and in any embodiment considered in practice, the echo sounder 1; 1' further comprises at least one receiver 20 for picking up low-frequency acoustic waves. The receiver 20 is an electroacoustic transducer of the hydrophone type. The receiver 20 is capable of picking up and converting into electrical form acoustic waves whose frequencies are included in a reception bandwidth, in particular extending below 30 kilohertz. This reception bandwidth corresponds, for example, to the -3 dB bandwidth of the receiver 20 and may be, for example, 0 to 20 kilohertz, 0 to 30 kilohertz, or 10 to 30 kilohertz. This receiver 20 is not necessarily directional.

[0041] As shown in the drawings, the echo sounder 1;1' comprises a single receiver 20 of this type. However, alternatively, the echo sounder may comprise several such receivers which together form a receiving antenna. Thus, by arranging several separate receivers at different positions in the echo sounder, selective reception from angular viewpoints is possible. In fact, this makes it possible, by electronic processing of all the signals picked up by these receivers, to obtain one or more echo signals each representing an acoustic wave which has propagated in the underwater environment in a given propagation direction, selected by the electronic processing in question, before being received by the echo sounder.

[0042] By way of example, in a variation of the first embodiment of Figure 2, the target receive antenna may include four receivers as described above, with each of the four quadrants bounded by the two arms of the Mills Cross-shaped transmit antenna being occupied by one of these receivers arranged in a rectangular or square configuration.

[0043] In another variant, the different receivers of the receive antenna can be arranged, for example, along the same line to form a linear receive antenna.

[0044] The echo sounder 1;1' also comprises a drive and acquisition system 30;30' for driving the transducers 11,12 of the antennas 10;10' and for receiving the low frequency echo signals s picked up by the receiver 20. E and from these echo signals a two-dimensional or three-dimensional image IM of the contents below the seabed 40 of said underwater environment 3 is determined.

[0045] The drive and acquisition system 30; 30' is adapted to perform the following steps of a method for characterizing a portion below the seabed 40 of an underwater environment 3 (the method shown in FIG. 8 ): a) driving the transducers 11, 12 of the transmitting antenna 10; 10' in an underwater environment 3 to transmit 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 having a superposed axis z oriented towards given points P1, P2, P3 on the seabed 4 of the underwater environment; S 5 to 7, each transducer 11, 12 has its associated transmit signal s 1,1 , s 1,2 , s 1,3 , s 2,1 , s 2,2 , s 2,3 ..., the transmitted signals of the different transducers 11, 12 having a predetermined delay relative to each other (FIG. 4); b) the frequency f LF is equal to the absolute value of the difference between the first frequency f1 and the second frequency f2. E to obtain The overlapping axis z from one execution of step a) to the next execution S performing all of steps a) and b) several times by varying at least some of the delays between one execution of step a) to the next so as to change the direction of c) Echo signals s acquired during different runs of step b). E and more accurately determining a two-dimensional or three-dimensional image representing the contents below the seabed 40 of the underwater environment 3 below each point P1, P2, P3 of the seabed 4 based on the

[0046] The drive and acquisition system 30; 30' is configured so that the first frequency f1 and the second frequency f2 are higher than 50 kHz, or higher than 100, or even higher than 150 kHz in this case. Furthermore, the drive and acquisition system 30; 30' is configured so that the difference between the first frequency f1 and the second frequency f2 is included in the above-mentioned reception bandwidth. Here, the drive and acquisition system 30; 30' is more precisely configured so that the difference between the first frequency f1 and the second frequency f2 is 15 to 30 kHz. The transmission signal s for driving the transducers 11, 12 1,1 , s 1,2 , s 1,3 , s 2,1 , s 2,2 , s 2,3 ... are generated by the drive and acquisition system 30; 30'.

[0047] Alternatively, however, the difference between the first frequency f1 and the second frequency f2 can be small, for example less than 10 kilohertz. The values of the first frequency f1 and the second frequency f2 can also be lower than those shown above, for example, each of these frequencies being, in some cases, between 20 and 50 kilohertz.

[0048] In any case, the difference |f1-f2| between the first frequency f1 and the second frequency f2 is preferably less than one-fourth of the arithmetic mean (f1+f2) / 2 of the first frequency f1 and the second frequency f2. Here, for example, this difference |f1-f2| is one-fourth to one-sixth of the arithmetic mean (f1+f2) / 2 of the first frequency f1 and the second frequency f2. Such a ratio allows optimizing the efficiency of nonlinear mixing of the first acoustic wave and the second acoustic wave.

[0049] After each execution of step a), the frequency f LFAn additional acoustic wave, where f 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 zS by nonlinear mixing of the first and second acoustic waves. This additional acoustic wave is hereafter referred to as the "low-frequency wave." In the technical literature, such acoustic waves generated directly in the underwater environment by nonlinear mixing are sometimes also referred to as "parametric waves" or "second-order waves" (and therefore the first and second acoustic waves are referred to as "primary waves"). It is S and therefore (apart from the potential effects of acoustic refraction) S reaches points P1, P2, P3, ... on the seabed 4 towards which it is directed.

[0050] This low frequency wave generally penetrates better below the seafloor 40 of the underwater environment 3 than the first and second acoustic waves with higher frequencies f1 and f2, thus making it possible to probe deeper into the contents below this seafloor below points of interest P1, P2, P3. When this low frequency acoustic wave encounters an element below the seafloor 40 that has a different stiffness or density than the rest of the seafloor, such as a section of pipeline buried in sediment, part of the low frequency wave is reflected by this element like an echo. The time of reception of this acoustic wave by the echo sounder then provides information about the distance between the echo sounder and the reflecting element so detected.

[0051] More generally, the frequency f reflected by the sub-seafloor 40 of the underwater environment in response to the transmission of the first and second acoustic waves LF The acoustic wave of SThe reflected acoustic waves contain information about the presence of more or less reflective elements in a row 41, 42, 43 of this sub-seafloor 40 extending below points P1, P2, P3 of the seabed 4 towards which the acoustic wave is directed, and about the depth at which each of these elements is located within this row. In other words, the reflected acoustic waves make it possible to determine, along this row 41, 42, 43, a one-dimensional image of a kind of content of the sub-seafloor 40 (FIGS. 5 to 7). The reflected acoustic waves, whose frequency is equal to the absolute value of the difference between the first frequency f1 and the second frequency f2, are thus picked up by the receiver 20 in step b) and converted into electrical form to generate an echo signal s E Give.

[0052] The propagation direction of the first and second acoustic waves transmitted in step a) directly depends on the delay value between the transmitted signals, and therefore on the axis of superposition z along which these first and second waves overlap each other. S The direction of the superposition axis z from one execution of step a) to the next execution S can be controlled in a purely electronic way by adjusting these delays to change the direction of

[0053] The contents of each of several elementary contents 41, 42, 43 extending below different points P1, P2, P3 of the seabed 4 are then successively sounded during different executions of steps a) and b). In step c), the two-dimensional or three-dimensional image IM determined by the drive and acquisition system 30:30' can be obtained, for example, by arranging one-dimensional sonar images representing the contents of these different columns 41, 42, 43 side by side.

[0054] For example, the superposition axis z Schanges from one execution of step a) to the next, along the same line, so that this axis points successively to different points P1, P2, P3 distributed on the seabed 4 of the underwater environment 3, the echo sounder 1;1' then takes (and by means of the echo sounder antenna 10;10') a two-dimensional image IM according to a cross section through this line, corresponding to a cross section below the seabed 40 of the underwater environment 3.

[0055] The parametric echo sounder 1;1' according to the invention therefore makes it possible, thanks to suitable driving of the transducers 11, 12 of the antennas 10;10', to take in a purely electronic way a two-dimensional or three-dimensional image IM representing the contents below the seabed 40 of the underwater environment. Advantageously, this image is obtained more quickly than by mechanically pivoting a single large transducer. This also makes it possible to correct, during transmission, for parasitic roll or pitch movements of the echo sounder 1;1' "in real time" (with a very short delay time, typically less than 0.1 seconds), so that the transmitted acoustic waves point in an unambiguously determined direction, independent of these parasitic movements.

[0056] Following this presentation of the main features of the echo sounder 1;1', its construction and operation will now be described in more detail.

[0057] Some characteristics of the electrical modules and transducers 11; 12 used will first be presented.

[0058] The structure of the transmitting antenna 10 of the first embodiment having the "Mills Cross" structure and its driving mode will now be described with reference to FIGS. 2 and 5 to 7. FIG.

[0059] Next, the structure of a transmitting antenna 10' according to the second embodiment and its driving mode will be described with reference to FIGS.

[0060] In the following, transmission and acquisition techniques are described that allow for a kind of multiplexing to increase the acquisition rate of two-dimensional or three-dimensional images of the sub-seafloor 40 of an underwater environment. Versions of this technique that support frequency multiplexing are described with reference to Figures 9 to 14. These techniques can be applied to both the first and second embodiments.

[0061] Transducer and drive electronics As explained above, thanks to its antenna 10;10' formed by several separate transducers 11;12, the parametric echo sounder 1;1' according to the invention can be directed in a purely electronic way, like non-parametric multibeam echo sounders, which is of particular interest.

[0062] However, because it is a parametric echo sounder, it is important that the transmitted acoustic waves have very high acoustic power. In fact, the efficiency of the parametric process of nonlinear mixing in water is generally low, on the order of a few percent at best, and decreases significantly as the power of the acoustic waves being mixed decreases.

[0063] To achieve the desired acoustic power, only a portion of the surface of the antenna 10; 10' contributes to the transmission of acoustic waves. However, each transducer 11, 12 of the antenna 10; 10' is made of one or more elements made of piezoelectric material and subjected to a strong compressive stress by, for example, a screw-bolt type mechanical device. Therefore, in the absence of electrical excitation, the piezoelectric elements are subjected to this strong prestress. This subsequently allows very high power supply voltages (several hundred volts or more) to be applied to the piezoelectric elements, thus generating very strong acoustic waves while preventing these elements from functioning dilatedly and limiting the risk of damage. In practice, the compressive stress applied to the piezoelectric elements by the mechanical device in question (in the absence of electrical excitation) is at least 7 bar higher, or even 15 bar higher. Each transducer 11, 12 of the antenna 10; 10' can then transmit acoustic waves with a power density per surface unit of more than 20 watts per square centimeter in the immediate vicinity of the transducer.

[0064] In echo sounder variants in which the first and second frequencies are lower, for example in the order of 10 kilohertz instead of higher than 150 kilohertz, the compressive stress applied to the piezoelectric element is even higher, for example higher than 100 bar.

[0065] Here, the drive and acquisition system 30; 30' of the echo sounder 1; 1' is a logic unit 31 including at least a processor and a memory; - transmitting signals s in a form suitable for driving the transducers 11, 12 1,1 , s 1,2 , s 1,3 , s 2,1 , s 2,2 , s 2,3 a regulation module 32 that supplies... - the echo signal s picked up by the receiver 20 before sending it to the logic unit 31 E and an acquisition module 33 for amplifying and converting the signal into digital form.

[0066] The adjustment module 32 adjusts the transmitted signal s based on the signal received from the logic unit 31. 1,1 , s 1,2 , s 1,3 , s 2,1 , s 2,2 , s 2,3 .... It may comprise one or more digital-to-analog converters, as well as filters for adapting the transmitted signal or for performing impedance matching with the transducers 11, 12. In any case, the conditioning module 32 comprises amplifiers, here one for each transducer 11, 12, each capable of providing an output voltage of at least about 100 volts, to drive the corresponding transducer. These amplifiers are here of the "pulse width modulation" (PWM) type, and allow the amplification of the transmitted signal so that their amplitude is high enough to generate acoustic waves with a power density per surface unit higher than 10 watts, or even higher than 20 watts per square centimeter. Here, the acoustic waves generated by each transducer, considered alone, have a power density per surface unit, more precisely 10 to 40 watts per square centimeter, in the immediate vicinity of the antenna 10; 10' (for example, at a distance of 10 to 50 cm from the antenna).

[0067] The total power of the first or second ultrasonic wave transmitted by the antenna 10 is here of the order of 218 dB. This transmission power is significantly reduced compared to that used in conventional parametric sonars. Nevertheless, the acquired images, which represent the contents of the environmental seafloor 40, are of high quality and therefore exhibit 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 (scanned here) are buried at a depth of several meters, rather than tens of meters as is usually the case. Furthermore, here, the only noise that could interfere with the measurement arises from wave reverberations in these sediment layers. Because the parametric beam is very thin, this reverberation is greatly reduced, and therefore the signal is usable.

[0068] In the first embodiment of Figure 2, the total power consumed by the driving electronics during the transmit sequence is here between 2 and 5 kilowatts. Thus, at resonance, 90% of the applied energy is converted into acoustic power for each transducer 11, 12 of the antennas 10; 10', which are made up of piezoelectric elements.

[0069] Furthermore, in the embodiment described here, the transducers of the antenna 10; 10' are distributed in a first group 13; 13' of transducers 11 and a second group 14; 14' of transducers 12 (FIGS. 2 and 15), and the driving and acquisition system 30; 30' is a first transmitted signal s, varying over time at a first frequency f1; 1,1 , s 1,2 , s 1,3 ...(the first acoustic wave is then transmitted by the transducers of this first group), a second transmitted signal s, varying over time at a second frequency f2; 2,1 , s 2,2 , s 2,3 . . to drive each transducer 12 of the second group (the first acoustic wave is then transmitted by the transducers of this second group).

[0070] First transmitted signal s 1,1 , s 1,2 , s 1,3 ... have a predetermined delay relative to each other. More precisely, each of these first signals has a first delay Δt relative to a first reference signal s1 (varying with frequency f1), as shown diagrammatically in FIG. 1,1 , Δt 1,2 , Δt 1,3 .... Also, different first transmission signals s 1,1 , s 1,2 , s 1,3 ... can each be obtained by time shifting based on this same reference signal s1. These first delays Δt 1,1 , Δt1,2 , Δt 1,3 Changing one of these is of course equivalent to changing the delay of the corresponding first transmitted signal relative to the other transmitted signals.

[0071] Similarly, the second transmitted signal s 2,1 , s 2,2 , s 2,3 ... have a predetermined delay relative to each other, where each of these second signals has a second delay Δt relative to a second reference signal s2 that varies at a frequency f2. 2,1 , Δt 2,2 , Δt 2,3 …. Again, the second transmitted signal s 2,1 , s 2,2 , s 2,3 ... may be obtained, for example, by time shifting based on this same second reference signal s2.

[0072] Mills Cross Transmitting Antenna As already shown, in the first embodiment, the transmitting antenna 10 of the echo sounder 1 has a two-arm cross shape, usually called a Mills cross (FIG. 2).

[0073] Here, the transducers 11 of the first group of transducers, driven at a first frequency f1, form a first arm 13 of the antenna. They are arranged in a row, one after the other, along this first arm 13.

[0074] The transducers 12 of the second group of transducers, driven at a second frequency f2, form a second arm 14 of the antenna and are also arranged in a row, one after the other, along this second arm 14.

[0075] Thus, in this first embodiment, in step a), a first acoustic wave is now transmitted by the transducer 11 of the first arm 13 of the antenna, while a second acoustic wave is transmitted by the transducer 12 of the second arm 14 of the antenna.

[0076] The first arm 13 and the second arm 14 form an angle with each other, for example comprised between 60 degrees and 120 degrees, where these two arms are more exactly perpendicular to each other.

[0077] Such an arrangement of transducers in a cross configuration is known as a conventional non-parametric echo sounder, where one of the transducer arms is used for transmission and the other for reception. However, the operation of the echo sounder 1 differs significantly from such conventional echo sounders. Indeed, here both arms of the cross are used for transmission, but reception is performed at a frequency different from that of transmission, by a receiver 20 different from the antenna 10, or alternatively by a receiving antenna as described above.

[0078] Here, the antenna 10 is mounted so that the x-axis of the first arm 13 extends parallel to the longitudinal axis of the vessel 2, which extends from stern to bow. And the y-axis of the second arm 14 of the antenna here extends transversely to the vessel 2. The y-axis is therefore perpendicular to the longitudinal axis of the vessel and parallel to the vessel deck. This arrangement of the antenna 10 relative to the vessel makes it convenient to perform electronic compensation for the vessel's parasitic pitch and roll motions. The z-axis, shown in the drawings for reference, is perpendicular to the antenna 10, i.e., perpendicular to its two arms 13 and 14.

[0079] Along each arm of the antenna 10, the transducers 11, 12 are distributed over a length here greater than 20 centimeters, and even greater than 50 centimeters, such that each of these arms can transmit acoustic waves with a reduced angular aperture parallel to the arm.

[0080] In this first embodiment, the drive and acquisition system 30 of the echo sounder 1 in step a) generates different first transmission signals s 1 , the values of which are such that the first acoustic waves propagate parallel to the first transmission plane P11, as a first beam W1 having a narrow cross section perpendicular to the first transmission plane P11 and a very elongated cross section parallel to this plane. 1,1 , s 1,2 , s 1,3 The first delay Δt applied to 1,1 , Δt 1,2 , Δt 1,3 ...(FIG. 5). In the transmission plane, the first beam W1 has a large angular aperture, e.g., several tens of degrees, but a reduced angular aperture, e.g., less than 5 degrees, perpendicular to the transmission plane (this angular aperture is defined as the angular width representing the acoustic intensity of the first wave at the cross section of the first beam W1, e.g., at half its peak maximum). Thus, the first beam W1 resembles a shallow, fan-shaped layer, often referred to as a "scan swath." The first beam W1 represents the region of the underwater environment 3 scanned by the first acoustic wave during its propagation.

[0081] For the first acoustic wave to propagate as this first beam W1, the drive and acquisition system 30 may, for example, determine a first delay Δt along the first arm 13, the value of which is proportional to the position occupied by the corresponding transducer 11 (transducer n°1, n°2, n°3, etc.). 1,1 , Δt 1,2 , Δt 1,3 ... (for example, if these first delays all have the same value, i.e., if there is no time shift between the first transmit signals, the parallel first transmit plane Pl1 in which the first acoustic wave propagates extends perpendicular to the x-axis of the first arm 13 of the antenna). The first transmit plane Pl1 is here perpendicular to the first arm 13 by the y-axis and includes an axis that rotates about this axis as a function of the values given to the first delays (here as a function of the proportionality coefficient between the values of these delays and the position of the transducer 11 along the first arm 13 of the antenna).

[0082] Similarly, in step a), the driving and acquisition system 30 determines a second delay Δt , the value of which is such that the second acoustic wave propagates parallel to the second transmission plane P12, as a second beam W2 having a narrow cross section in a direction perpendicular to the second transmission plane P12 and being very elongated parallel to this plane. 2,1 , Δt 2,2 , Δt 2,3 ...(FIG. 5). The second beam W2 has a shape similar to that of the first beam W1. In particular, the second beam W2 has an angular aperture perpendicular to the second transmission plane P12, for example smaller than 5 degrees (maximum half-width). The second transmission plane P12, which is at the center of the second beam W2, includes an axis perpendicular to the second arm 14 of the antenna, here the x-axis, around which it can be rotated as a function of the value given to the second delay.

[0083] Superposition axis z S is located at the intersection of the first transmitting plane Pl1 and the second transmitting plane Pl2 (FIGS. 5 to 7).

[0084] The first and second waves are mainly formed by the intersection of the first beam W1 and the second beam W2, and are aligned along the overlap axis z S The first and second waves are superimposed on each other within a common beam Wo centered at W. In other words, it is only within this common beam Wo that the first and second waves have substantial acoustic intensity. Therefore, it is within the scope of this common beam Wo that low-frequency acoustic waves are efficiently generated.

[0085] The generally conical common beam Wo has a reduced angular aperture, e.g., less than 2 degrees, because each of the first and second beams W1, W2 forms the shallow layer described above. Therefore, the cross section of the low-frequency acoustic waves generated by nonlinear mixing in the water is less extended (especially since this nonlinear wave generation mode also tends to reduce its transverse extension). This allows bathymetry of the underwater environment below the seafloor 40 with good lateral resolution.

[0086] The drive and acquisition system 30 now performs a second delay Δt between one execution of step a) and the next, so as to pivot the second transmitting plane PI2 about the x-axis in order to bathymetry the contents below this seabed 40 below different points P1, P2, P3 of the seabed 4. 2,1 , Δt 2,2 , Δt 2,3 .... Thus, the tilt angle formed between the z axis and the second transmitting plane P12 changes from one execution of step a) to the next (FIGS. 5 to 7). This pivoting of the second transmitting plane P12 causes the rotation of the second transmitting plane P12 relative to the axis of superposition z S (During these successive runs, the entire fan formed by the first beam W1 can then be scanned).

[0087] This method of scanning different points P1, P2, P3 on the seabed 4 is illustrated by all of Figures 5 to 7, which show schematically the first and second beams W1 and W2 formed by the first and second acoustic waves for three successive executions of the transmitting step a).

[0088] Furthermore, during this scan, in the transmission step a), the drive and acquisition system 30 S The transducers 11, 12 of the antenna 10 are driven so that the angle of incidence formed between the axis z4 perpendicular to the seabed of the underwater environment is greater than a critical angle of incidence, which here is equal to 15 degrees.

[0089] Therefore, the superposition axis z S , the low frequency acoustic waves generated along the seabed 4 reach the seabed 4 at normal incidence (i.e., zero angle of incidence) and are avoided from reaching it during the entire scan of the seabed.

[0090] Such a movement away from normal incidence makes it possible to avoid sending back towards the echo sounder 1 low-frequency acoustic waves that are specularly or nearly specularly reflected by the seabed 4 or by the interface between two different sedimentary layers. This is interesting because the intensity of such specularly reflected waves is generally much higher than the intensity of acoustic waves reflected by buried objects such as pipelines, which are mainly reflected in a diffuse manner. At normal incidence, the acoustic waves specularly reflected by the seabed are much stronger and therefore mask the acoustic waves coming from buried objects to be detected, thus making the detection of such objects more difficult.

[0091] In the scan mode described herein, to bathymetry below the seafloor 40 at angles of incidence greater than the limiting angle of incidence, the drive and acquisition system 30 adjusts the first delay Δt so that the first transmitting plane P11 is tilted at an angle α with respect to an axis perpendicular to the seafloor z4 (angle α is the angle formed between the plane P11 and the axis z4). 1,1 , Δt 1,2 , Δt 1,3 ... is adjusted. This angle α is larger than the limiting incident angle mentioned above. The angle α and axis z4 are shown in FIG. 10, which also shows the aspect of high-speed acquisition. The angle α is, for example, 15 to 30 degrees.

[0092] Second delay Δt 2,1 , Δt 2,2 , Δt 2,3 ...is varied from one execution of step a) to the next in order to pivot the second transmitting plane P12 about the x-axis, as described above.

[0093] Therefore, along this scan, the superposition axis z S and the axis normal to the seabed z4 remains greater than the angle α and therefore remains greater than the limiting angle of incidence mentioned above.

[0094] 19 shows diagrammatically the positions of the points P1, P2, P3 of the seabed echo sounder in a situation where the seabed 4 is horizontal as is the antenna 10. In this situation, the first transmitting plane P11, which is thus inclined to the vertical (to avoid normal incidence soundings), is in this case directed towards the front of the vessel 2. The seabed points P1, P2, P3 sounded by the echo sounder are then distributed along a line L extending perpendicular to the seabed of the underwater environment, the front of the vessel 2 and the longitudinal axis of this vessel.

[0095] For the scan mode just described, an alternative is to add a second delay Δt 2,1 , Δt 2,2 , Δt 2,3 ..., from one execution of step a) to the next, the first delay Δt 1,1 , Δt 1,2 , Δt 1,3 ...can be changed.

[0096] Also, during a series of several successive executions of step a), a first delay Δt is used to capture three-dimensional images representing the contents below the seafloor 40 below several points on the seafloor, for example, not all aligned with each other, the entirety of which covers the entire surface of the seafloor of the underwater environment. 1,1 , Δt 1,2 , Δt 1,3 …and a second delay Δt 2,1 , Δt 2,2 , Δt 2,3 ...can change both.

[0097] It should be noted that this first embodiment of the echo sounder 1 presents significant practical difficulties in terms of the transmitted acoustic intensity. Indeed, the total transmitting surface of this Mills' cross antenna 10 is much smaller than that of a single "piston" type transmitter, or even that of the antenna 10' with the transmitter matrix of the second embodiment. Furthermore, only a portion of the first acoustic wave overlaps with the second acoustic wave (FIG. 5). Therefore, only a portion of the first wave and a portion of the second wave participate in the generation of a low-frequency acoustic wave in water, further reducing the efficiency of this generation.

[0098] However, the use of such a Mills cross antenna (whose two arms are driven with transmissions at a first frequency f1 and a second frequency f2, respectively) in return allows for a significant simplification of certain aspects of the transducer control electronics (in particular in the conditioning module 32) for echosounders whose antennas consist of a matrix of transducers.

[0099] In fact, for a given extension of the antenna and for a given density of transducers on its surface, the antenna 10' of the second embodiment includes many more transducers than that of the first embodiment. For example, if the antenna 10 of the first embodiment includes 6 x 2 transducers (6 transducers per arm), the antenna 10' of the second embodiment includes 6 x 6 of them. It is recalled that reducing the number of transducers makes it possible to correspondingly simplify their control electronics, thereby providing for each transducer an amplifier capable of providing an output voltage of at least about 100 volts, which varies over time in a fairly complex manner. This further compactness of the control electronics allows it to be accommodated in the head of the echo sounder 1 near the antenna 10, thereby significantly simplifying the system of cables connecting the echo sounder head to the rest of the drive and acquisition system.

[0100] Furthermore, this parametric echo sounder 1 equipped with a Mills cross antenna 10 makes it possible, during the successive execution of the above sequence of steps a), to take "traditional" non-parametric three-dimensional sonar images of the underwater environment 3 (obtained without frequency mixing between the acoustic waves) representing the contents of the observable scanned by the second beam W2 during the execution of this sequence of steps a).

[0101] Transmitting antenna with a matrix of transducers As shown in Figure 15, in the second embodiment, the transducers 11, 12 of the transmitting antenna 10' are arranged to form a matrix of several rows and several columns, with the transducers 11, 12 each being located at a different intersection between the rows and columns of the matrix.

[0102] Here, it is a rectangular matrix whose columns are perpendicular to the rows. The transducers 11, 12 are therefore arranged at the nodes of this rectangular network. The axis parallel to the rows of this matrix is designated x and the axis parallel to the columns is designated y. The x axis is parallel to the longitudinal axis of the vessel 2.

[0103] Along each row and along each column of this matrix, the transducers 11, 12 are distributed over a length here greater than 20 centimetres, or even greater than 50 centimetres.

[0104] The manner in which the transducers 11 of the first group 13' (driven at the first frequency f1) are arranged relative to the transducers 12 of the second group 14' (driven at the second frequency f2) is described below.

[0105] From the driving point of view, in this second embodiment, the driving and acquisition system 30′ of the echo sounder 1′ comprises, in step a), a first delay Δt, the value of which is such that the first acoustic wave propagates as a collimated first beam W1′; 1,1 , Δt 1,2 , Δt 1,3...(Figure 16) a second delay Δt, the value of which is such that the second acoustic wave also propagates as a collimated second beam W2′; 2,1 , Δt 2,2 , Δt 2,3 It is designed to give...

[0106] Both of these first and second beams W1′ and W2′ are aligned along the overlap axis z S are centered on the same axis that constitutes the

[0107] To transmit the first acoustic wave as this first collimated beam, the driving and acquisition system 30' may, for example, provide, for each transducer 11 of the first group 13', a first transmit signal s 1,1 , s 1,2 , s 1,3 ... is a steering vector u with respect to a vector r indicating the position of the transducer 11 of interest on the surface of the antenna 10' with respect to the reference signal s1. ZS a first delay Δt having a value proportional to the scalar product of 1,1 , Δt 1,2 , Δt 1,3 Generate a first transmit signal to have a steering vector u ZS is, for example, a unitary vector whose direction is the superposition axis z S This is the direction.

[0108] The second transmit signal is generated in the same way as the first transmit signal, but of course at a frequency equal to the second frequency f2.

[0109] In this second embodiment, the first and second beams W1', W2' overlap each other almost completely (they are almost coincident with each other). The common beam Wo', in which the low-frequency waves are generated, then corresponds directly to one of these two beams W1' and W2' or equally to the other. These different beams W1', W2', and Wo' are approximately conical. Each of the first and second beams W1' and W2' is, for example, aligned along the axis of overlap zS and a first plane including the x-axis, and a superposition axis z S and a second plane containing the y-axis, has a reduced angular aperture of less than 2 degrees (this angular aperture is an angular width of half the maximum value of the peaks representing the acoustic intensities of the first and second acoustic waves in the cross section of the first or second beam).

[0110] The driving and acquisition system 30' now adjusts the first and second delays Δt from one execution of step a) to the next, so as to change the direction of the first beam W1' and the second beam W2' together in order to bathymetry the contents below the seabed 40 below different points P1, P2, P3 of the seabed 4. 1,1 , Δt 1,2 , Δt 1,3 , Δt 2,1 , Δt 2,2 , Δt 2,3 ...and thus point to different points on the seafloor 4, while still overlapping each other.

[0111] 16 to 18 all show diagrammatically the first and second beams W1 and W2' formed by the first and second acoustic waves for three successive executions of the transmitting step a), with the z-axis, represented as a reference and fixed in these figures, being perpendicular to the antenna 10'.

[0112] As in the first embodiment, the drive and acquisition system 30' is configured in step a) to S The transducers 11, 12 of the antenna 10' are adapted to be driven such that the angle of incidence formed between the axis of superposition z and the axis perpendicular to the seabed 4 of the underwater environment is greater than the critical angle of incidence mentioned above. As explained above, this facilitates the detection of buried objects. To that end, the drive and acquisition system 30' is adapted to drive the transducers 11, 12 of the antenna 10' such that the angle of incidence formed between the axis of superposition z and the axis perpendicular to the seabed 4 of the underwater environment is greater than the critical angle of incidence mentioned above. S The first and second delays Δt are set so that 1,1 , Δt 1,2 , Δt 1,3 , Δt 2,1 , Δt 2,2 , Δt2,3 Adjust...

[0113] In order for the first and second acoustic waves to overlap as much as possible and at the beginning of their propagation, rather than dispersing them into two separate areas on the surface of the antenna 10', the transducers 11 of the first group 13' can be mixed with the transducers 12 of the second group 14' (one of which exclusively collects the transducers 11 of the first group and the other exclusively collects the transducers 12 of the second group).

[0114] On the antenna 10', the transducers 11 of the first group and the transducers 12 of the second group are alternately and more precisely distributed: each transducer 11 of the first group 13' has a transducer 12 of the second group 14' next to it. Here, the transducers 11, 12 are thus distributed as a checkerboard, each white square of which is occupied by one of the transducers 11 of the first group and each black square of which is occupied by one of the transducers 12 of the second group.

[0115] Other configurations can be envisaged in which the transducers 11 of the first group are intermixed with the transducers 12 of the second group on the surface of the antenna 10'. In this respect, the transducers can be distributed as a checkerboard, with each white square occupied by two transducers of the first group (rather than just one), and each black square occupied by two transducers of the second group (rather than just one). Thus, more generally, the transducers 11 of the first group 13' occupy several distinct regions of the matrix formed by all the transducers, with one or more of the transducers 12 of the second group 14' being inserted between any two of the regions.

[0116] fast acquisition If the seabed 4 of the underwater environment 3 is located at a considerable depth relative to the echosounder 1;1', the point-by-point acquisition of images IM representing the contents below the seabed 40 of this environment may be particularly long if specific equipment intended to accelerate it is not implemented.

[0117] In fact, without such equipment, the echo signals S coming from these two points P1, P2 would be indistinguishable, waiting for a time longer than the round trip time (at the speed of sound in water) between the echo sounder 1;1' and the seabed 4, between the sub-bottom sounding below the first point P1 on the seabed 4 and the sub-bottom sounding below the second point P2 on the seabed 4. E It is necessary to avoid overlapping of the time.

[0118] In order to overcome this difficulty and enable the rapid acquisition of images IM representing the contents below the seabed 40 of the underwater environment 3, even if this environment is deep, the drive and acquisition system 30; 30' is now configured, in both the first and second embodiments, to vary the difference between the first frequency f1 and the second frequency f2 from one execution of step a) to the next.

[0119] By varying the difference between the first frequency f1 and the second frequency f2 from one execution of step a) to the next, the superposition axis z (which varies from one execution of step a) to the next) S In fact, different points P1, P2, P3 on the seafloor 4 sounded by the echo sounder will then have different values of f LF1 , f LF2 , f LF3 , …f LFn Therefore, the individual echo signals s coming from these different points are E can be distinguished from one another even if they are superimposed on one another from a temporal point of view.

[0120] In other words, even if the individual echo signals s received after different executions of the transmission step a) EThe set of different individual echo signals is then superimposed on one another in time (the total echo signal is then superimposed, i.e., the individual echo signals s E These individual echo signals s E Each of the can now be extracted from the entire echo signal by frequent filtering.

[0121] The different executions of the transmission step a) can then be carried out at short intervals from one another and even simultaneously, regardless of the depth of the underwater environment, without risk of confusion between individual echoes coming from different points on the seabed, thereby significantly reducing the duration required to obtain an image IM representative of the contents below the seabed 40 of this environment.

[0122] In this way, by reducing the total duration of the series of transmissions intended to sound the seabed, it is possible to reduce parasitic influences on the image IM due to potential parasitic operations of the echo sounder 1 or potential variations in the characteristics of the underwater environment 3.

[0123] 9 to 14 show some characteristics of the first and second acoustic waves so transmitted during several successive executions of step a), during which the difference between the first frequency f1 and the second frequency f2 is varied. These figures correspond to the first embodiment of the echosounder 1 (Mills Cross antenna). This technique may be used in other embodiments as well, in particular the second embodiment described above.

[0124] Here, during this series of transmissions, the first frequency f1 maintains a constant value f10, while the second frequency f2 varies. During successive executions of step a), the second frequency f2 successively varies to different values f21, f22, f23, ... f2 n(FIGS. 11 and 14), each associated with one of these runs (i.e., associated with run n°1, run n°2, etc., respectively). These values are distinct from one another. They increase, for example, linearly over these successive runs of step a).

[0125] Therefore, from one execution of step a) to the next, the frequency fL of the low-frequency acoustic waves generated in water F also varies, and during these different runs, different values of f LF1 , f LF2 , f LF3 , …f LFn These different values of f LF1 , f LF2 , f LF3 , …f LFn For example, two frequencies f may be spaced 0.5 kHz apart and distributed over 15-25 kHz. Of course, this result can also be obtained by varying the first frequency f1 while keeping the second frequency f2 fixed.

[0126] The individual echo signals s received in response to these different transmissions E To extract f, the drive and acquisition system 30 performs frequent filtering of the entire echo signal, which is continuously acquired by the receiver 20 during the method for characterizing the sub-seafloor 40. This entire echo signal is filtered, for example, by different band-pass filters operating in parallel with each other and centered around different frequency values f, f, f, f, ... f (equal to the different values obtained by the difference between the first and second frequencies during the successive executions of step a). The filtered signals provided by these different filters then correspond to the above-mentioned different individual echo signals, each of which makes it possible to determine a one-dimensional image representative of the contents of the sub-seafloor 40 below, for example, a point P, P, P on the seafloor targeted during the corresponding execution of step a).

[0127] Now, with reference to Figures 9 to 13, the spatial characteristics of the first and second acoustic waves transmitted during this series of executions of step a) are presented.

[0128] The first acoustic wave is now transmitted seamlessly and continuously from one execution of step a) to the next, thus forming the same total acoustic wave, the duration of which is longer than the duration of the second wave transmitted by the second arm 14 of the antenna 10 during the different executions of step a).

[0129] Figures 9 and 10 show the area Z1 of the underwater environment 3 occupied by this first wave at a given moment in time, respectively in front view (i.e. in a plane perpendicular to the first arm 13 of the antenna 10) and in side view (in a plane parallel to this first arm 13).

[0130] FIG. 11 shows, at the same instant as FIG. 9, the regions Z21, Z22, ..., Z22 occupied by the second acoustic wave transmitted during different executions of step a) above. n These different regions are also shown in a side view in FIG.

[0131] 13 corresponds to the superposition of FIGS. 9 and 11. It shows the regions Z1 and Z21, Z22, ..., Z22 of the object occupied by the first and second acoustic waves, respectively, at the same instant in time as in FIGS. 9 and 11. n 13 shows, from a spatial perspective, the variation in frequency of the "low frequency" acoustic waves generated by mixing as a function of the direction of the superposition axis (reference symbol z S1 , z S2 , …, z Sn ) Note that Figures 9-13 are instantaneous views of the area occupied by the acoustic waves of interest, and in this sense they are quite different from the views of Figures 5-7, which show, for the first and second acoustic waves, the entire area scanned by this wave during its propagation (this scanned area corresponds to beam W1 or W2).

[0132] Non-parametric sonar imagery The transmission of the first and second acoustic waves performed in step a) makes it possible to take a "conventional" sonar image representing at least a portion of the underwater environment 3, in addition to the low-frequency bathymetry of the sub-seafloor 40 of the underwater environment 3 as described above. This additional image IM' is a conventional sonar image in the sense that, unlike image IM, it is acquired without mixing of frequencies underwater.

[0133] In the embodiment described here, the drive and acquisition system 30; 30' takes advantage of this possibility. - after each execution of step a), execute a step b') of obtaining additional echo signals of a frequency equal to the first frequency f1 or the second frequency f2 picked up by at least one of the transducers 11, 12 of the antenna in response to the transmissions made in step a), c') determining said additional image IM' based on the additional echo signals (FIG. 8).

[0134] This facility allows, by carrying out a single series of transmissions, i.e. by carrying out a single series of executions of step a), to obtain both the "parametric" image IM and the additional image IM', thus benefiting from their advantages, namely: - the advantage of low-frequency parametric sonar imaging, which allows good penetration below the seafloor 40 but generally results in a lower signal-to-noise ratio than conventional sonar imaging (and is therefore less adapted than the latter to bathymetry the rest of the underwater environment 3); It makes it possible to combine the advantages of conventional sonar imaging, which generally has a high signal-to-noise ratio and is particularly adapted to bathymetry the depth of the rest of the underwater environment 3.

[0135] The drive and acquisition system 30; 30' is now configured to determine, based on these additional echo signals, the depth of different points P1, P2, P3 of the seabed 4 within the underwater environment 3, where the sub-seafloor 40 is bathymetrically sounded by low-frequency waves.

[0136] This allows for a combination of high-precision surveying of the topography of a portion of the seabed 4 of the underwater environment 3 using conventional sonar imaging and surveying the contents of the sub-seafloor 40 located below this portion of the seabed 4 using parametric imaging.

[0137] Furthermore, the driving and acquisition system 30; 30' also generates the parametric echo signal s E The data generated by the processing of the parametric echo signal s may be merged with the data generated by the processing of the additional echo signals, for example, by merging the parametric echo signal s based on the depths of the seafloor points P1, P2, P3 more accurately estimated from the conventional additional echo signals. E This may include adjusting the depth estimated from

[0138] For example, in the first embodiment, the overlapping axis z SWhen the direction of the second transmission plane Pl2 is changed by changing the direction of the second transmission plane Pl2, on the other hand, in step b'), the echo sounder 1 acquires several additional echo signals at a frequency equal to the second frequency f2 (apart from a potential Doppler shift). These additional echo signals are picked up by different transducers 11 of the first arm 13 of the antenna, which are then used for reception. As is known, all the additional echo signals acquired during this execution of step b') make it possible to determine a two-dimensional image representative of the contents of the scanning swath W2 scanned by the second acoustic wave transmitted in step a) during its propagation. Since steps a) and b') are repeated for several different directions of the second transmission plane Pl2, the additional image IM' finally acquired is a three-dimensional sonar image representative of the contents of the underwater environment 3 in the observable scanned by the second beam W2 during this series of executions of step a).

[0139] Finally, the drive and acquisition system 30;30' may be configured to determine, based on additional echo signals (conventional "non-parametric" echo signals), data representative of the composition of the sub-seafloor 40 surface layer of the underwater environment located below the points P1, P2, and P3 of the seafloor sounded by the echo sounder. This sub-seafloor surface layer is the upper part of the sub-seafloor and in contact with the water. The data of interest may indicate, among other things, whether this sub-seafloor surface layer is composed of sand, pebbles, fragmented rock, or loose sediment, or whether it is composed entirely of a solid bedrock or consolidated sediment layer. In particular, the drive and acquisition system 30;30' may be equipped with a seafloor classification module, such as that included in the product Seapix developed by the applicant. Using transverse scan swaths, it is possible to measure the backscatter index of the seafloor as a function of grazing angle. These parameters are directly related to the properties of the seafloor. For more details, see the 2018 paper "Seafloor classification with a Multi-swath Multi-beam Echo Sounder" by Trung-Kien Nguyen. The method used in this case is to observe a series of fixed areas on the seafloor, ultrasonically processed by longitudinal scanning swaths. As the vessel moves in a straight line, these areas are observed at different viewing angles in each imaging phase of the sounder. By storing in memory the brightness of each area as a function of the observation angle, a backscattering profile of the seafloor is stored for each of them, characteristic of the observed seafloor type (rock, sand type, mud, seaweed...).

[0140] The drive and acquisition system 30; 30' may then be configured to determine a setpoint speed for movement of the vessel 2 as a function of the data. If the data of interest indicates that the surface layer of the sub-sea floor 40 is prone to contain buried material, i.e., the data indicates that the surface layer of the sea floor is composed of sand, pebbles, fragmented rock, or loose sediment, then the setpoint speed will be slower than if the data indicates that the surface layer of the sub-sea floor 40 is less prone to contain buried material, i.e., it is composed entirely of solid rock or consolidated sediment. This setpoint speed is then transmitted to the drive system of the vessel 2, which adjusts the seed of the vessel's movement to this setpoint value. These arrangements make it possible to reduce the time required to explore a given surface of the sea floor of an underwater environment by avoiding detailed bathymetric sounding of portions of the sea floor that are less prone to contain buried material.

[0141] Many variations of the echosounder and the method for characterizing the sub-seafloor portion of the underwater environment described above may be applied.

[0142] First, instead of transmitting a first acoustic wave by the transducers of the first group, a second acoustic wave is transmitted by a transducer of a second group different from the first group, but each transducer can contribute to the transmission of these two waves. Within the framework of such a variant, the drive and acquisition system is configured so that the transmission signals driving the transducers 11, 12 each contain a first component and a second component varying over time at a first frequency f1 and a second frequency f2, respectively. The transducers 11, 12 of the antenna then transmit, in step a), a signal of superposition axis z, here modulated in amplitude and composed of the first and second acoustic waves. SIn other words, this modulated acoustic wave is decomposed into a first acoustic wave and a second acoustic wave. Furthermore, the transmitted signal is also an amplitude modulated signal. In terms of the driving electronics of the transducers 11, 12, this variant is more difficult to implement than transmitting the first and second acoustic waves by separate transducers, since the transducers have to be supplied with a high-voltage vibration signal in a more complex form.

[0143] Furthermore, different arrangements of the transducers on the antenna plane than those described above can be used, in that the transducers can be distributed so that they together form a spiral instead of a cross or matrix.

[0144] In any case, however, the antenna transducers are not all distributed along the same line. As already indicated, their positions are distributed in two dimensions across the entire surface. More precisely, the entire transmitting antenna, including the different transducers, extends across a surface having two dimensions, each of which is longer than 20 centimeters, or even longer than 50 centimeters, along two orthogonal directions. This allows the first and second acoustic waves to be transmitted with good directionality, increasing their power per surface unit, thereby improving the efficiency of the nonlinear process of generating low-frequency acoustic waves. This allows bathymetry of the contents of the underwater environment below the seafloor 40 with good lateral resolution.

Claims

1. A parametric echo sounder (1; 1'), a transmitting antenna (10; 10') comprising a number of transducers (11, 12), said transmitting antenna (10) having the shape of a cross of two arms, some of said transducers (11) being arranged one after the other in a row along a first arm (13) of said antenna (10), and other transducers (12) of said antenna being arranged one after the other in a row along a second arm (14) of said antenna (10); at least one receiver (20); - the following steps: a) driving the transducers (11, 12) to 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 having a superposition axis (z 1 , 12 ) directed towards a point (P1, P2, P3) on the seabed (4) of the underwater environment (3); Ｓ ), and each transducer (11, 12) transmits a transmission signal (s １，１ , s １，２ , s １，３ , s ２，１ , s ２，２ , s ２，３ ), and the different transmission signals of the different transducers (11, 12) are driven by a predetermined delay (Δt １，１ , Δt １，２ , Δt １，３ , Δt ２，１ , Δt ２，２ , Δt ２，３ ) and b) receiving an echo signal (s) picked up by the receiver (20) in response to the transmission made in step a) at a frequency equal to the difference between the first frequency (f1) and the second frequency (f2); Ｅ ) and configured to execute Furthermore, the overlapping axis (z Ｓ ) from one execution of step a) to the next, so as to change the direction of the delay (Δt １，１ , Δt １，２ , Δt １，３ , Δt ２，１ , Δt ２，２ , Δt ２，３ performing all steps a) and b) several times by varying at least some of the c) the echo signals (s) acquired during different executions of step b). Ｅ and a drive and acquisition system (30; 30') configured to determine, based on the measured values of the seafloor depths, a two-dimensional or three-dimensional image (IM) representative of the sub-seafloor (40) contents of the underwater environment (3) below each of the points (P1, P2, P3) of the seafloor.

2. said drive and acquisition system (30; 30') in step a) - transmitting each transducer (11) of a first group (13; 13') of said transducers (11, 12) to a first transmission signal (s) varying over time at said first frequency (f1); １，１ , s １，２ , s １，３ ), and the transducers (11) of this first group transmit the first acoustic wave, and the different first transmit signals (s １，１ , s １，２ , s １，３ ) with respect to each other by a predetermined first delay (Δt １，１ , Δt １，２ , Δt １，３ ) - transmitting each transducer (12) of a second group (14; 14') of said transducers (11, 12) to a second time-varying transmission signal (s) at said second frequency (f2); ２，１ , s ２，２ , s ２，３ ), and the transducers (12) of this second group transmit the second acoustic wave, and the different second transmit signal (s ２，１ , s ２，２ , s ２，３ ) are separated by a predetermined second delay (Δt ２，１ , Δt ２，２ , Δt ２，３ ) The overlapping axis (z Ｓ ) from one execution of step a) to the next, so as to change the direction of the first delay (Δt １，１ , Δt １，２ , Δt １，３ ) and / or the second delay (Δt ２，１ , Δt ２，２ , Δt ２，３ 2. A parametric echo sounder (1; 1') according to claim 1, configured to vary the

3. - 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); - the first delay (Δt １，１ , Δt １，２ , Δt １，３ ) when a first beam (W1) has a narrow cross section in a direction perpendicular to a first transmission plane (P11) and extends parallel to said first transmission plane (P11), said first acoustic wave propagating parallel to said first transmission plane (P11), - the second delay (Δt ２，１ , Δt ２，２ , Δt ２，３ ) when the second beam (W2) has a narrow cross section along a direction perpendicular to the second transmission plane (P12) and extends parallel to the second transmission plane (P12), the second acoustic wave propagates parallel to the second transmission plane (P12), The overlapping axis (z Ｓ ) is located at the intersection between the first transmitting plane (P11) and the second transmitting plane (P12), The drive system (30) from one execution of step a) to the next execution of step a) - the first delay (Δt) for pivoting the first transmission plane (P11) about a first scanning axis (y) perpendicular to the first arm (13) of the antenna (10); １，１ , Δt １，２ , Δt １，３ ) and / or the second delay (Δt ２，１ , Δt ２，２ , Δt ２，３ 2. A parametric echo sounder (1) according to claim 1, configured to vary the

4. 4. A parametric echo sounder (1; 1') according to any one of claims 1 to 3, wherein some of the transducers (11, 12) each comprise an element made of piezoelectric material and a mechanical device applying a compressive stress to the piezoelectric material element, said compressive stress being higher than 7 bar.

5. - each transducer (11, 12) is adapted to transmit acoustic waves at frequencies above 50 kilohertz in said underwater environment (3); - said receiver (20) is adapted to pick up and convert into electrical form acoustic waves whose frequencies are constituted in a reception bandwidth of 0 to 30 kilohertz; A parametric echo sounder (1; 1') according to any one of claims 1 to 4, wherein the drive and acquisition system (30; 30') is configured such that the first and second frequencies (f1, f2) are each greater than 50 kilohertz, and such that the difference between the first frequency (f1) and the second frequency (f2) is included in the reception bandwidth and is less than 30 kilohertz.

6. 6. A parametric echo sounder (1; 1') according to any one of claims 1 to 5, wherein the drive and acquisition system (30; 30') is configured to vary the difference between the first frequency (f1) and the second frequency (f2) from one execution of step a) to the next.

7. The drive and acquisition system (30; 30') further comprises: After each execution of step a), obtaining an additional echo signal picked up by at least one of the transducers (11, 12) in response to the transmissions made in step a), at a frequency equal to the first frequency (f1) or the second frequency (f2); A parametric echo sounder (1; 1') according to any one of claims 1 to 6, configured to determine an image (IM') representative of a portion of the underwater environment (3) based on said additional echo signals.

8. The drive and acquisition system (30; 30') further comprises, in step a), a position sensor for detecting the position of the superposition axis (z Ｓ 8. The parametric echo sounder (1; 1') according to claim 1, configured to drive the transducers (11, 12) such that an angle of incidence formed between the transducer (11, 12) and an axis (z4) perpendicular to the seabed (4) of the underwater environment is greater than a limit angle of incidence.

9. The drive and acquisition system (30; 30') further comprises: - so that said transmitted signals each comprise a first and a second time-varying component at said first frequency (f1) and said second frequency (f2), respectively; the transducer of the antenna (10; 10') is, in step a), arranged along the axis of superposition (z Ｓ 2. A parametric echo sounder (1; 1') according to claim 1, configured to transmit modulated acoustic waves propagating along a

10. A method for characterizing a portion below the seabed (40) of an underwater environment (3) implemented by a parametric echo sounder (1; 1') according to any one of claims 1 to 9, comprising: The following steps are carried out by said drive and acquisition system (30; 30'): a) driving the transducers (11, 12) to 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 having a superposition axis (z 1 , 12 ) directed towards a point (P1, P2, P3) on the seabed (4) of the underwater environment; Ｓ ), and each transducer (11, 12) transmits a transmission signal (s １，１ , s １，２ , s １，３ , s ２，１ , s ２，２ , s ２，３ ), and the different transmission signals of the different transducers (11, 12) are driven by a predetermined delay (Δt １，１ , Δt １，２ , Δt １，３ , Δt ２，１ , Δt ２，２ , Δt ２，３ ) and b) receiving an echo signal (s) picked up by the receiver (20) in response to the transmission made in step a) at a frequency equal to the difference between the first frequency (f1) and the second frequency (f2); Ｅ ) to obtain The overlapping axis (z Ｓ ) from one execution of step a) to the next, so as to change the direction of the delay (Δt １，１ , Δt １，２ , Δt １，３ , Δt ２，１ , Δt ２，２ , Δt ２，３ performing all steps a) and b) several times by varying at least some of the steps a) and b); and further comprising the steps of: c) the echo signals (s) acquired during different executions of step b). Ｅ and determining a two-dimensional or three-dimensional image (IM) representing the contents below the seafloor (40) of the underwater environment (3) below each of the points (P1, P2, P3) of the seafloor based on the images of the seafloor.

Images