Low-frequency synthetic aperture imaging method and system based on multi-beam parametric array sonar

Abstract

The present invention provides a method and system for low-frequency synthetic aperture imaging based on multi-beam parametric array sonar, wherein the method comprises: step S1, emitting a beam and acquiring a time-delay-compensated echo signal; step S2, obtaining a beam output based on the echo signal; step S3, performing frequency domain processing and pulse compression on the beam output, and splicing the imaging result. The present invention aims to resolve the contradiction between low frequency and high resolution of buried small targets, as well as the problems of weak echo and difficult detection. When the parametric sonar sequence emits multiple adjacent scanning beams, the limitation of the low-frequency array size is resolved by a virtual array aperture. Combined with the low-frequency penetration advantage of parametric sonar, sequential multi-beam-synthetic aperture imaging is performed on buried small targets under the condition of a small-size receiving array, thereby obtaining high-resolution, high-quality images of underwater buried small targets.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater acoustic target imaging, and in particular to a method and system for low-frequency synthetic aperture imaging based on multi-beam parametric array sonar. Background Art

[0002] In the field of ocean exploration, high-resolution imaging of buried small targets has always been a major research challenge faced by scholars at home and abroad. The main reasons why it is difficult to achieve high-quality imaging are as follows: First, the geometric scale of the buried target itself is small, which directly leads to its weak scattered echo, making it difficult to capture and identify. Second, the echo of the buried target needs to undergo two-way attenuation of the sediment layer and two transmission losses at the seabed interface during propagation, which greatly reduces the echo intensity and greatly increases the difficulty of detection. Third, the complex marine environment, especially the multi-path conditions in shallow waters, causes the echo of the buried small target to be mixed with many other echoes, making it difficult to effectively distinguish them.

[0003] In order to effectively obtain high-quality images of buried small targets and realize the detection and identification of buried targets, scholars at home and abroad have carried out long-term research and exploration. The current buried small target detection technology mainly adopts side-scan low-frequency synthetic aperture and parametric shallow profiling. Although the side-scan low-frequency synthetic aperture imaging technology can penetrate the seabed to a certain extent, its equipment is bulky, which limits the use scenarios and flexibility, and the detection depth of buried targets is limited, which cannot meet the detection needs of deep buried targets. Although the parametric shallow profiling technology has the advantages of high directivity and concentrated energy of the difference frequency beam, it can effectively penetrate the sedimentary layer to detect deeply buried targets, but it has problems such as small detection coverage, low detection efficiency, and poor image quality.

[0004] Therefore, in actual buried small target detection operations, it is often necessary to carry a variety of different types of sonar detection equipment. This not only leads to a wide variety of equipment and complex technology, but also increases the difficulty and cost of operation and maintenance.

[0005] Through searching patent documents, it was found that the invention patent with publication number CN112505710B discloses a multi-beam synthetic aperture sonar three-dimensional imaging algorithm. The algorithm is based on the multi-beam synthetic aperture sonar transducer array structure. Through the movement of the carrier in the track direction, a virtual synthetic large aperture array is formed, and the echo arrival direction is resolved in the horizontal direction using a multi-receiving array element structure. Through the movement of the carrier, the detection area is illuminated at different track positions, and the received echo signals are coherently compensated and accumulated to obtain a three-dimensional sonar image output in the track-slant range-angle coordinate system. By using coordinate transformation, the sonar image is converted into a three-dimensional sonar image in the horizontal-track-depth coordinate system that is easier to observe. This patent lacks targeted technical means and processes in imaging buried small targets under the conditions of processing small-size and low-frequency receiving arrays.

[0006] In summary, there is an urgent need to explore new principles and methods for detecting and imaging buried small targets. Researching a low-frequency synthetic aperture imaging method and system based on multi-beam parametric array sonar has become a key task that needs to be solved urgently. Summary of the invention

[0007] In view of the defects in the prior art, the object of the present invention is to provide a low-frequency synthetic aperture imaging method and system based on multi-beam parametric array sonar.

[0008] According to the present invention, a multi-beam parametric array sonar low-frequency synthetic aperture imaging method is provided, comprising the following steps:

[0009] Step S1, transmitting a beam and acquiring a delay-compensated echo signal;

[0010] Step S2, obtaining a beam output based on the echo signal;

[0011] Step S3, performing frequency domain processing and pulse compression on the beam output, and splicing the imaging results.

[0012] Preferably, step S1 comprises: a parametric array sonar sequence transmits m adjacent beams, the time interval between two adjacent pulse beams transmitted by the transmitting array at each sampling position in azimuth is Δt, multi-array data of original target echo are obtained, and time delay compensation of Δt, 2Δt, …, (m-1)Δt is performed on the echo signals from the multiple transmitting beams respectively, to obtain the delay-compensated echo signals.

[0013] Preferably, step S1 includes the following sub-steps:

[0014] Step S1.1, the parametric array sonar sequence transmits m beams with basically the same beam width, then the sonar echo corresponding to the target will be divided into m segments in the heading direction, and the multi-segment sonar echo signal has a delay in the slant range direction. Suppose the time interval between two adjacent pulse beams emitted by the transmitting array at each sampling position in the azimuth direction is Δt;

[0015] Step S1.2, when the transmitting array is located at X T When the target is illuminated by the mth beam, the expression of the echo signal of the nth receiving array element is Among them, s m represents the received echo strength, t represents the time,

[0016] is the distance between the target and the transmitting array, Indicates the distance between the N+1 array elements in the receiving array and the target. The size of the receiving array element is D R , ΔL is the length of the transmitting array to the receiving array, r is the slant distance of the target from the route, d is the spacing between the receiving array elements, and h is the depth of the target from the route;

[0017] Step S1.3, echo signals e from multiple transmit beams mn (t) respectively perform delay compensation of Δt, 2Δt, …, (m-1)Δt to obtain the delay compensated echo signal and realize the time alignment of the sequence transmission beam.

[0018] Preferably, step S2 includes: based on the delay-compensated echo signal, according to a conventional multi-beam synthetic aperture receiving array signal processing method, beamforming processing is performed on the echo signal of each receiving array element at each sampling position after delay compensation to obtain beam outputs at different angles, namely, delay beamforming data.

[0019] Preferably, step S2 includes the following sub-steps:

[0020] Step S2.1, construct the coordinates of a certain point target P (x p ,y p ,0), the slope distance is r p At the time of the mth pulse emission, the transmitting transducer is located at coordinates B0,x m ,h p ), the echo signal received by the center element of the receiving array is expressed as D T is the size of the transmitting array.

[0021] Step S2.2, place the point target P in the polar coordinate system, determine the position of the target P relative to the receiving array at a certain sampling point at (r0, θ0), and express the received echo signal of each array element as in, n is the receiving array element number, r n0 is the slant distance from the target to the receiving array element n, l n is the distance from array element n to the center of the array.

[0022] Step S2.3, perform time delay beamforming on the echo signal of each array element according to spherical wave expansion to obtain beam output in, r n is the distance from the scanned point to the array element n, r is the slant distance of the scanned point, and θ is the angle of the scanned point. By searching for r and θ, the delay compensation amount (r n -r) / c is equal to the delay (r n0 -r0) / c, the received signals of each array element are superimposed in phase, and the beam output at a specific position angle is obtained as follows:

[0023] Preferably, step S3 includes: using the beam output as time-delay beamforming data, performing Fourier transform on each time-delay beamforming data, converting it to the frequency domain and then performing pulse compression, and then converting it to the time domain through inverse Fourier transform, and finally, splicing each beam imaging result to obtain the entire imaging result of the area illuminated by the transmit beam.

[0024] Preferably, step S3 includes the following sub-steps:

[0025] Step S3.1, Fourier transform each time-delay beamforming data, and convert it into the frequency domain, that is, y(ω)=FFT[y(t)], where ω represents frequency and FFT represents Fourier transform.

[0026] Step S3.2, pulse compression is performed on the frequency domain data to increase the image resolution in the distance dimension, i.e., Yy(ω)=y(ω)·S * (ω), where S * (ω) is the conjugate of the transmitted signal spectrum. After pulse compression, the inverse Fourier transform obtains the time domain result Yy(t)=IFFT[Yy(ω)].

[0027] Step S3.3, stitching the time domain imaging results of each beam to obtain the imaging result of the area illuminated by the transmitted beam.

[0028] The present invention also provides a low-frequency synthetic aperture imaging system based on a multi-beam parametric array sonar, comprising:

[0029] Module M1, transmits a beam and obtains a delay-compensated echo signal;

[0030] Module M2, obtaining beam output based on the echo signal;

[0031] Module M3 performs frequency domain processing and pulse compression on the beam output and splices the imaging results.

[0032] Preferably, module M1 includes: a parametric array sonar sequence transmits m adjacent beams, the time interval between two adjacent pulse beams transmitted by the transmitting array in azimuth at each sampling position is Δt, multi-array data of the original target echo are obtained, and the echo signals from the multiple transmitting beams are respectively compensated for delays of Δt, 2Δt, …, (m-1)Δt to obtain the delay-compensated echo signals.

[0033] Preferably, module M3 includes: using the beam output as time-delay beamforming data, performing Fourier transform on each time-delay beamforming data, converting it to the frequency domain and then performing pulse compression, and then converting it to the time domain through inverse Fourier transform, and finally, splicing each beam imaging result to obtain the entire imaging result of the area illuminated by the transmit beam.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. Compared with conventional multi-beam synthetic aperture sonar, the present invention combines the advantages of multi-beam parametric sonar's strong low-frequency penetration and innovatively proposes a new mode for underwater buried target detection, which can achieve deep buried target imaging under small-size array conditions.

[0036] 2. Compared with conventional multi-beam synthetic aperture imaging, the present invention uses a parametric sonar sequence to transmit multiple adjacent scanning beams to increase the detection range, and uses sequential multi-beam synthetic aperture two-dimensional shallow profiling technology to solve the problems of time delay compensation and two-dimensional high-resolution imaging, thereby realizing sequential multi-beam synthetic aperture imaging.

[0037] 3. The existing shallow profiling technology is based on single-beam transmission and single-beam reception, and can only perform one-dimensional detection on buried targets, with low detection rate and detection efficiency. The present invention uses multi-beam parametric sonar, which can achieve two-dimensional imaging of buried targets, expand the detection range to a certain extent, and improve the detection efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0039] Figure 1 This is a flow chart of a low-frequency synthetic aperture imaging method based on multi-beam parametric array sonar in an embodiment of the present invention;

[0040] Figure 2 The imaging result of the multi-beam parametric sonar according to conventional beamforming under the multi-bright spot simulation model data in the embodiment of the present invention;

[0041] Figure 3 It is the result of multi-beam parametric sonar synthetic aperture imaging under multi-bright spot simulation model data in an embodiment of the present invention. DETAILED DESCRIPTION

[0042] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0043] The present invention aims to solve the contradiction between low frequency and high resolution of buried small targets, as well as weak echoes and difficult detection problems, and further provide an imaging method and system based on low-frequency synthetic aperture of multi-beam parametric array sonar. When the parametric sonar sequence transmits multiple adjacent scanning beams, the limitation of low-frequency array size is solved by virtual array aperture. Combined with the low-frequency penetration advantage of parametric sonar, sequential multi-beam-synthetic aperture imaging is performed on buried small targets under the condition of a small-size receiving array, thereby obtaining high-resolution high-quality images of underwater buried small targets.

[0044] Embodiment 1:

[0045] This embodiment is completed in combination with target echo signal simulation data.

[0046] Figure 1 The present invention is a flowchart of a method for low-frequency synthetic aperture imaging based on multi-beam parametric array sonar in an embodiment of the present invention.

[0047] like Figure 1 As shown, this embodiment provides a low-frequency synthetic aperture imaging method based on multi-beam parametric array sonar, comprising the following steps:

[0048] Step S1, transmitting a beam and acquiring an echo signal after delay compensation.

[0049] Specifically, the parametric array sonar sequence transmits m adjacent beams, the time interval between two adjacent pulse beams transmitted by the transmitting array at each sampling position in azimuth is Δt, the multi-array data of the original target echo are obtained, and the echo signals from multiple transmitting beams are respectively compensated for time delays of Δt, 2Δt, …, (m-1)Δt to obtain the delay-compensated echo signals.

[0050] More specifically, step S1 includes the following sub-steps:

[0051] Step S1.1, the parametric array sonar sequence transmits m beams with basically the same beam width, then the sonar echo corresponding to the target will be divided into m segments in the heading direction, and there will be a delay in the slant range direction for the multi-segment sonar echo signal. Suppose the time interval between two adjacent pulse beams emitted by the transmitting array at each sampling position in the azimuth direction is Δt.

[0052] In this embodiment, m beams of the same width transmitted sequentially are constructed by simulation.

[0053] Step S1.2, when the transmitting array is located at X T When the target is illuminated by the mth beam, the expression of the echo signal of the nth receiving array element is Among them, s m represents the received echo strength, t represents the time, is the distance between the target and the transmitting array, Indicates the distance between the N+1 array elements in the receiving array and the target. The size of the receiving array element is D R , ΔL is the length of the transmitting array to the receiving array, r is the slant distance of the target from the route, d is the spacing between the receiving array elements, h is the depth of the target from the route, and c is the speed of sound.

[0054] Step S1.3, echo signals e from multiple transmit beams mn (t) respectively perform delay compensation of Δt, 2Δt, …, (m-1)Δt to obtain the delay compensated echo signal and realize the time alignment of the sequence transmission beam.

[0055] Step S2, obtaining a beam output based on the echo signal.

[0056] Specifically, based on the delay-compensated echo signal, according to the conventional multi-beam synthetic aperture receiving array signal processing method, beamforming processing is performed on the echo signal of each receiving array element at each sampling position after delay compensation to obtain beam outputs at different angles, namely, delay beamforming data.

[0057] More specifically, step S2 includes the following sub-steps:

[0058] Step S2.1, construct the coordinates of a certain point target P (x p ,y p ,0), the slope distance is r p At the time of the mth pulse emission, the transmitting transducer is located at coordinates B0,x m ,h p ), the echo signal received by the center element of the receiving array is expressed as D T is the size of the transmitting array.

[0059] Step S2.2, place the point target P in the polar coordinate system, determine the position of the target P relative to the receiving array at a certain sampling point at (r0, θ0), and express the received echo signal of each array element as in, n is the receiving array element number, r n0 is the slant distance from the target to the receiving array element n, l n is the distance from array element n to the center of the array.

[0060] Step S2.3, perform time delay beamforming on the echo signal of each array element according to spherical wave expansion to obtain beam output in, r n is the distance from the scanned point to the array element n, r is the slant distance of the scanned point, and θ is the angle of the scanned point. By searching for r and θ, the delay compensation amount (r n -r) / c is equal to the delay (r n0-r0) / c, the received signals of each array element are superimposed in phase, and the beam output at a specific position angle is obtained as follows:

[0061] Step S3, performing frequency domain processing and pulse compression on the beam output, and splicing the imaging results.

[0062] Specifically, the beam output is used as the time-delay beamforming data, each time-delay beamforming data is Fourier transformed, converted to the frequency domain and then pulse compressed, and then converted to the time domain through an inverse Fourier transform. Finally, the imaging results of each beam are spliced ​​to obtain the complete imaging results of the area illuminated by the transmitting beam.

[0063] More specifically, step S3 includes the following sub-steps:

[0064] Step S3.1, Fourier transform each time-delay beamforming data, and convert it into the frequency domain, that is, y(ω)=FFT[y(t)], where ω represents frequency and FFT represents Fourier transform.

[0065] Step S3.2, pulse compression is performed on the frequency domain data to increase the image resolution in the distance dimension, i.e., Yy(ω)=y(ω)@S * (ω), where S * (ω) is the conjugate of the transmitted signal spectrum. After pulse compression, the inverse Fourier transform is performed to obtain the time domain result Yy(t)=IFFT[Yy(ω)], where IFFT represents inverse Fourier transform.

[0066] Step S3.3, stitching the time domain imaging results of each beam to obtain the imaging result of the area illuminated by the transmitted beam.

[0067] Figure 2 The imaging result of the multi-beam parametric sonar according to conventional beamforming under the multi-bright spot simulation model data in the embodiment of the present invention; Figure 3 It is the result of multi-beam parametric sonar synthetic aperture imaging under multi-bright spot simulation model data in an embodiment of the present invention.

[0068] contrast Figure 2 and Figure 3 It can be clearly seen that the low-frequency synthetic aperture imaging scheme of multi-beam parametric sonar is used to solve the limitation of low-frequency array size through virtual array aperture, and the synthetic aperture imaging problem of sequential multi-beam parametric sonar is effectively solved through time delay compensation and two-dimensional synthetic aperture imaging, thus realizing high-resolution imaging of buried small targets of multi-beam parametric sonar under the conditions of small-size low-frequency receiving array.

[0069] Embodiment 2:

[0070] The present invention also provides a low-frequency synthetic aperture imaging system based on a multi-beam parametric array sonar. The low-frequency synthetic aperture imaging system based on a multi-beam parametric array sonar can be realized by executing the process steps of a low-frequency synthetic aperture imaging method based on a multi-beam parametric array sonar, that is, those skilled in the art can understand the low-frequency synthetic aperture imaging method based on a multi-beam parametric array sonar as a preferred implementation of the low-frequency synthetic aperture imaging system based on a multi-beam parametric array sonar.

[0071] The low-frequency synthetic aperture imaging system based on multi-beam parametric array sonar includes:

[0072] Module M1 transmits a beam and obtains a delay-compensated echo signal.

[0073] Specifically, module M1 includes: a parametric array sonar sequence transmits m adjacent beams, the transmitting array transmits two adjacent pulse beams at each sampling position in azimuth with a time interval of Δt, obtains multi-array data of the original target echo, and performs delay compensation of Δt, 2Δt, …, (m-1)Δt on the echo signals from multiple transmitting beams to obtain the delay-compensated echo signals.

[0074] Module M2 obtains beam output based on the echo signal.

[0075] Specifically, module M2 includes: based on the delay-compensated echo signal, according to the conventional multi-beam synthetic aperture receiving array signal processing method, beamforming processing is performed on the echo signal of each receiving array element at each sampling position after delay compensation to obtain beam outputs at different angles, namely, delay beamforming data.

[0076] Module M3 performs frequency domain processing and pulse compression on the beam output and splices the imaging results.

[0077] Specifically, module M3 includes: using the beam output as time-delay beamforming data, performing Fourier transform on each time-delay beamforming data, converting it to the frequency domain and then performing pulse compression, and then converting it to the time domain through inverse Fourier transform, and finally, splicing each beam imaging result to obtain the entire imaging result of the area illuminated by the transmit beam.

[0078] Those skilled in the art know that, in addition to realizing the system and its various devices, modules, and units provided by the present invention in a purely computer-readable program code, it is entirely possible to realize the same functions in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system and its various devices, modules, and units provided by the present invention can be considered as a hardware component, and the devices, modules, and units included therein for realizing various functions can also be regarded as structures within the hardware component; the devices, modules, and units for realizing various functions can also be regarded as both software modules for realizing the method and structures within the hardware component.

[0079] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A low-frequency synthetic aperture imaging method based on multi-beam parametric array sonar, characterized in that: The steps include: Step S1, transmitting a beam and acquiring a delay-compensated echo signal; Step S2, obtaining a beam output based on the echo signal; Step S3, performing frequency domain processing and pulse compression on the beam output, and splicing imaging results.

2. The method of low-frequency synthetic aperture imaging based on multi-beam parametric array sonar according to claim 1, characterized in that: The step S1 comprises: a parametric array sonar sequence transmits m adjacent beams, the time interval between two adjacent pulse beams transmitted by the transmitting array at each sampling position in azimuth is Δt, multi-array data of original target echo are obtained, and time delay compensation of Δt, 2Δt, ..., (m-1)Δt is performed on the echo signals from the multiple transmitting beams respectively, so as to obtain the delay-compensated echo signals.

3. The method for low-frequency synthetic aperture imaging based on multi-beam parametric array sonar according to claim 2, characterized in that: The step S1 includes the following sub-steps: Step S1.1, the parametric array sonar sequence transmits m beams with basically the same beam width, then the sonar echo corresponding to the target will be divided into m segments in the heading direction, and the multi-segment sonar echo signal has a delay in the slant range direction. Suppose the time interval between two adjacent pulse beams emitted by the transmitting array at each sampling position in the azimuth direction is Δt; Step S1.2, when the transmitting array is located at X T When the target is illuminated by the mth beam, the expression of the echo signal of the nth receiving array element is Among them, s m represents the received echo strength, t represents the time, is the distance between the target and the transmitting array, Indicates the distance between the N+1 array elements in the receiving array and the target. The size of the receiving array element is D R , ΔL is the length of the transmitting array to the receiving array, r is the slant distance of the target from the route, d is the spacing between the receiving array elements, and h is the depth of the target from the route; Step S1.3, echo signals e from multiple transmit beams mn (t) respectively perform delay compensation of Δt, 2Δt, …, (m-1)Δt to obtain the delay compensated echo signal and realize the time alignment of the sequence transmission beam.

4. The method for low-frequency synthetic aperture imaging based on multi-beam parametric array sonar according to claim 1, characterized in that: The step S2 comprises: based on the delay-compensated echo signal, according to a conventional multi-beam synthetic aperture receiving array signal processing method, beamforming processing is performed on the delay-compensated echo signal of each receiving array element at each sampling position to obtain beam outputs at different angles, namely, delay beamforming data.

5. The method for low-frequency synthetic aperture imaging based on multi-beam parametric array sonar according to claim 4, characterized in that: The step S2 includes the following sub-steps: Step S2.1, construct the coordinates of a certain point target P (x p ,y p ,0), the slope distance is r p At the time of the mth pulse emission, the transmitting transducer is located at the coordinate (0,x m ,h p ), the echo signal received by the center element of the receiving array is expressed as D T is the size of the transmitting array; Step S2.2, place the point target P in the polar coordinate system, determine the position of the target P relative to the receiving array at a certain sampling point at (r0, θ0), and express the received echo signal of each array element as in, n is the receiving array element number, r n0 is the slant distance from the target to the receiving array element n, l n is the distance from array element n to the center of the array; Step S2.3, perform time delay beamforming on the echo signal of each array element according to spherical wave expansion to obtain beam output in, r n is the distance from the scanned point to the array element n, r is the slant distance of the scanned point, and θ is the angle of the scanned point. By searching for r and θ, the delay compensation amount (r n -r) / c is equal to the delay (r n0 -r0) / c, the received signals of each array element are superimposed in phase, and the beam output at a specific position angle is obtained as follows:

6. The method for low-frequency synthetic aperture imaging based on multi-beam parametric array sonar according to claim 1, characterized in that: The step S3 includes: using the beam output as time-delay beamforming data, performing Fourier transform on each time-delay beamforming data, converting it to the frequency domain and then performing pulse compression, and then converting it to the time domain through inverse Fourier transform, and finally, splicing each beam imaging result to obtain the entire imaging result of the area illuminated by the transmit beam.

7. The method for low-frequency synthetic aperture imaging based on multi-beam parametric array sonar according to claim 6, characterized in that: The step S3 includes the following sub-steps: Step S3.1, Fourier transform each time-delay beamforming data, and convert it into the frequency domain, that is, y(ω)=FFT[y(t)], where ω represents frequency and FFT represents Fourier transform; Step S3.2, pulse compression is performed on the frequency domain data to increase the image resolution in the distance dimension, i.e., Yy(ω)=y(ω)·S * (ω), where S * (ω) is the conjugate of the transmitted signal spectrum. After pulse compression, the inverse Fourier transform obtains the time domain result Yy(t)=IFFT[Yy(ω)]; Step S3.3, stitching the time domain imaging results of each beam to obtain the imaging result of the area illuminated by the transmitted beam.

8. A low-frequency synthetic aperture imaging system based on multi-beam parametric array sonar, characterized in that: include: Module M1, transmits a beam and obtains a delay-compensated echo signal; Module M2, obtaining a beam output based on the echo signal; Module M3 performs frequency domain processing and pulse compression on the beam output and splices the imaging results.

9. The multi-beam parametric array sonar low-frequency synthetic aperture imaging system according to claim 8, characterized in that: The module M1 includes: a parametric array sonar sequence transmits m adjacent beams, the time interval between two adjacent pulse beams transmitted by the transmitting array at each sampling position in azimuth is Δt, multi-array data of original target echo are obtained, and time delay compensation of Δt, 2Δt, ..., (m-1)Δt is performed on the echo signals from multiple transmitting beams respectively, so as to obtain the echo signals after delay compensation.

10. The low-frequency synthetic aperture imaging system based on multi-beam parametric array sonar according to claim 8, characterized in that: The module M3 includes: using the beam output as time-delay beamforming data, performing Fourier transform on each time-delay beamforming data, converting it to the frequency domain and then performing pulse compression, and then converting it to the time domain through inverse Fourier transform, and finally, splicing each beam imaging result to obtain the entire imaging result of the area illuminated by the transmit beam.

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