Small surveying and mapping blind area double-frequency synthetic aperture underwater acoustic imaging system and method

Abstract

The invention discloses a small surveying and mapping blind area double-frequency synthetic aperture underwater acoustic imaging system and method, and the system comprises a double-frequency acoustic array, a multi-channel transmitting and receiving machine, a double-frequency synthetic aperture imaging signal processing module, a double-frequency image fusion imaging module, and a high-frequency acoustic array and a low-frequency acoustic array for synchronous transmission. The high-frequency acoustic array transmits and receives high-frequency chirp signals to detect a short-range to medium-range area of a target area, the low-frequency acoustic array transmits and receives low-frequency chirp signals to detect a medium-range to remote area of the target area, and the beam opening angle of the high-frequency acoustic array is larger than that of the low-frequency acoustic array; and an overlapping region exists between the detection regions of the high-frequency acoustic array and the low-frequency acoustic array. According to the invention, the dual-frequency wave beams emitted by the dual-frequency acoustic array are used for cooperative detection to cover a larger surveying and mapping range, and the structural parameter optimization of the acoustic array is combined, so that the blind area is controlled within the minimum range acceptable by engineering on the premise of ensuring the remote high resolution, and the balance between the detection efficiency and the technical feasibility is realized.

Description

Technical Field

[0001] The present invention mainly relates to the technical field of underwater detection, and in particular to a dual-frequency synthetic aperture underwater acoustic imaging system and method with a small mapping blind area. Background Art

[0002] Traditional synthetic aperture sonar (SAS) systems operate in a side-looking mode, and there is a large blind area in their downward-looking region. Increasing the transmit beam width can expand the mapping range and reduce the blind area width, but the beam energy dispersion leads to a reduced imaging distance. This blind area problem seriously limits the mapping efficiency and imaging quality of traditional synthetic aperture sonar (SAS) systems, specifically manifested as follows: Due to the existence of a mapping blind area, traditional synthetic aperture sonar (SAS) systems cannot achieve efficient coverage of the entire mapping band, resulting in low mapping efficiency. In a complex marine environment, traditional synthetic aperture sonar (SAS) systems need to perform multiple repeated mappings to cover the target area, increasing the operation time, cost, and the risk of system damage during operation in dangerous scenarios.

[0003] In addition, traditional synthetic aperture sonar (SAS) systems are difficult to simultaneously meet the high-resolution imaging requirements of near-range and far-range regions in a single frequency band. Low-frequency sonars have low resolution in long-range imaging and cannot meet the requirements of high-precision target detection. Traditional synthetic aperture sonar (SAS) systems are usually designed for a single function and are difficult to adapt to complex environments with different water depths, bottom sediments, and sea conditions. The system integration level is low, and the hardware modular design is insufficient, resulting in high system deployment and maintenance costs. Summary of the Invention

[0004] Aiming at the problems of mapping blind areas, low mapping efficiency, insufficient resolution, and poor adaptability existing in traditional synthetic aperture sonar (SAS) systems, the present invention proposes a dual-frequency synthetic aperture underwater acoustic imaging system and method with a small mapping blind area.

[0005] To achieve the above object, the technical solutions adopted by the present invention are as follows: On the one hand, the present invention provides a dual-frequency synthetic aperture underwater acoustic imaging system, including: A dual-frequency acoustic array mounted on a carrier platform, including a high-frequency acoustic array and a low-frequency acoustic array. The high-frequency acoustic array and the low-frequency acoustic array are synchronously transmitted. Among them, the high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect the near-range to mid-range regions of the target area, and the low-frequency acoustic array transmits and receives low-frequency linear frequency modulation signals to detect the mid-range to far-range regions of the target area. The beam opening angle of the high-frequency acoustic array is greater than that of the low-frequency acoustic array, and there is an overlapping area between the detection areas of the high-frequency acoustic array and the low-frequency acoustic array; Multi-channel transmitting and receiving device, including a multi-channel signal transmitter and a multi-channel signal receiver. The multi-channel signal transmitter is used to transmit high- and low-frequency acoustic signals, and the multi-channel signal receiver is used to receive high- and low-frequency target echo signals; Dual-frequency synthetic aperture imaging signal processing module, which performs synthetic aperture imaging processing on the high-frequency echo signal and the low-frequency echo signal respectively to obtain corresponding target acoustic images; Dual-frequency image fusion imaging module, which generates a complete underwater acoustic image after fusion by adopting a weighted fusion strategy based on the target acoustic images corresponding to the high-frequency echo signal and the low-frequency echo signal.

[0006] Furthermore, the installation tilting angles of the high-frequency acoustic array and the low-frequency acoustic array are respectively and , then the overlapping angle corresponding to the overlapping area is: ; Wherein, is the difference between the installation tilting angles of the high-frequency acoustic array and the low-frequency acoustic array, reflecting the spatial offset of their detection directions; is the difference between the beam opening angles of the high-frequency acoustic array and the low-frequency acoustic array, reflecting the spatial offset of their energy distributions.

[0007] Furthermore, assume that the transmission loss of the high-frequency linear frequency modulation signal transmitted by the high-frequency acoustic array at the proximal end of the overlapping area is , and the transmission loss of the low-frequency linear frequency modulation signal transmitted by the low-frequency acoustic array at the distal end of the overlapping area is , then the combined echo energy in the overlapping area is expressed as: ; Wherein, and are respectively the transmitted acoustic energies of the high-frequency acoustic array and the low-frequency acoustic array; by adjusting the installation tilting angles , of the high-frequency acoustic array and the low-frequency acoustic array, and further adjusting the overlapping angle , so that the combined echo energy in the overlapping area reaches or exceeds the system detection threshold, and at the same time ensure that the resolution of the overlapping area satisfies: ; In the formula, is the speed of sound, and are respectively the signal bandwidths of the signals transmitted by the high-frequency acoustic array and the low-frequency acoustic array.

[0008] Furthermore, by optimizing the installation tilting angle and the beam opening angle of the high-frequency acoustic array , restricting the mapping blind area of the system within the range of the horizontal distance from the sonar carrier , where: ; wherein is the vertical height from the carrier platform to the bottom of the detection area, and the carrier platform is the carrier carrying the dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area.

[0009] Furthermore, the center frequency of the high-frequency linear frequency modulation signal is greater than the center frequency of the low-frequency linear frequency modulation signal, and the working frequency bands of the high-frequency linear frequency modulation signal transmitted by the high-frequency acoustic array and the low-frequency linear frequency modulation signal transmitted by the low-frequency acoustic array do not overlap. The bandwidths of the high-frequency linear frequency modulation signal and the low-frequency linear frequency modulation signal can be the same. Specifically, the working frequency ranges of the high-frequency linear frequency modulation signal and the low-frequency linear frequency modulation signal can be set according to actual needs, as long as there is no overlap in the working frequency ranges of the high-frequency linear frequency modulation signal and the low-frequency linear frequency modulation signal.

[0010] On the other hand, an imaging method based on the above-mentioned dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area is provided, including: Synchronously transmitting and receiving high-frequency linear frequency modulation signals and low-frequency linear frequency modulation signals by using the high-frequency acoustic array and the low-frequency acoustic array of the dual-frequency acoustic array carried on the carrier platform, wherein the high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect the short-range to medium-range area of the target area, the low-frequency acoustic array transmits and receives low-frequency linear frequency modulation signals to detect the medium-range to long-range area of the target area, the beam opening angle of the high-frequency acoustic array is greater than the beam opening angle of the low-frequency acoustic array, and there is an overlapping area between the detection areas of the high-frequency acoustic array and the low-frequency acoustic array; Using the multi-channel signal transmitter in the multi-channel transmitting and receiving machine to transmit high- and low-frequency acoustic signals, and using the multi-channel signal receiver in the multi-channel transmitting and receiving machine to receive high- and low-frequency target echo signals; Performing synthetic aperture imaging processing on the high-frequency echo signal and the low-frequency echo signal respectively to obtain corresponding target acoustic images; Based on the target acoustic images corresponding to the high-frequency echo signal and the low-frequency echo signal, adopting a weighted fusion strategy to generate a fused complete underwater acoustic image.

[0011] Furthermore, performing synthetic aperture imaging processing on the high-frequency echo signal and the low-frequency echo signal respectively to obtain corresponding target acoustic images, including: Performing space-time synchronization on the high-frequency echo signal and the low-frequency echo signal; Performing gain compensation, band-pass filtering, analog-to-digital conversion processing and motion compensation on the high-frequency echo signal and the low-frequency echo signal after space-time synchronization, wherein the motion compensation includes phase compensation and time delay compensation; The high-frequency data and low-frequency data after motion compensation are respectively subjected to sub-aperture division, synthesis, azimuth compression, and BP algorithm imaging to obtain the target acoustic image corresponding to the high-frequency data and the target acoustic image corresponding to the low-frequency data .

[0012] Furthermore, based on the target acoustic image corresponding to the high-frequency data and the target acoustic image corresponding to the low-frequency data the complete underwater acoustic image after fusion is obtained as follows: ; where the weight coefficients and adopt a gradual change manner in the range direction. Let be the horizontal length from directly below the carrier platform at the start of the overlapping area, be the horizontal length from directly below the platform at the end of the overlapping area. Then the horizontal length from directly below the carrier platform at The calculation formula for the weight coefficient is , , and it satisfies .

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention proposes a dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area, which has a dual-frequency acoustic array. The high-frequency acoustic array and the low-frequency acoustic array in the dual-frequency acoustic array transmit and receive synchronously. Among them, the high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect the short-range to medium-range areas of the target area, and the low-frequency acoustic array transmits and receives low-frequency linear frequency modulation signals to detect the medium-range to long-range areas of the target area. The beam opening angle of the high-frequency acoustic array is greater than that of the low-frequency acoustic array, and there is an overlapping area between the detection areas of the high-frequency acoustic array and the low-frequency acoustic array.

[0014] The present invention adopts a cooperative working mode of the high-frequency acoustic array and the low-frequency acoustic array, and cooperatively covers the non-blind area through dual-frequency beams, aiming to overcome the inherent contradiction between the detection range and the imaging resolution of the traditional single-frequency SAS system, so as to achieve wide coverage, high resolution, and imaging detection with a small mapping blind area of the underwater target area. Further, by optimizing parameters such as the transmitted signals, installation angles, and heights of the high-frequency acoustic array and the low-frequency acoustic array, on the premise of ensuring high resolution in the short range and wide coverage in the long range, the blind area is controlled within the minimum range acceptable in engineering, achieving the balance between detection efficiency and technical feasibility.

[0015] Furthermore, a high-frequency receiving acoustic array is used to receive high-frequency echo signals, and a low-frequency receiving acoustic array is used to receive low-frequency echo signals. The high-frequency echo signals and low-frequency echo signals are respectively subjected to signal processing to obtain corresponding images. Based on the images corresponding to the high-frequency echo signals and low-frequency echo signals, a weighted fusion strategy is adopted to generate a complete underwater acoustic image after fusion. For the images corresponding to the high-frequency echo signals and low-frequency echo signals, a weighted fusion strategy is adopted to generate a complete underwater acoustic image after fusion, where the weight coefficients of the images corresponding to the high-frequency echo signals and low-frequency echo signals and adopt a gradient method in the range direction. At close range, due to the high resolution of high-frequency data, the weight ratio is larger; at long range, due to the slow propagation attenuation of low-frequency data, the weight ratio is higher. Through this gradient weighting, both the high-resolution advantage of high-frequency signals in the near range and the wide coverage characteristic of low-frequency signals in the far range can be utilized. During the fusion process, the overlapping area (mid-range area) of high-frequency and low-frequency data enhances the signal intensity through energy superposition. At the same time, through sonar equation verification and parameter adjustment, it is ensured that the resolution of the overlapping area does not degrade. Finally, a complete underwater acoustic image after fusion is generated through the BP algorithm, realizing smooth connection and efficient imaging of the near-range, mid-range, and far-range areas, and controlling the mapping blind area within the minimum range acceptable to the project.

[0016] The present invention breaks through the inherent contradiction between detection range and resolution in traditional SAS, and is particularly suitable for scenarios such as deep-sea pipeline detection and seabed mineral exploration that require large-scale fine mapping. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on the structures shown in these drawings.

[0018] Figure 1 It is a schematic diagram of a carrier platform equipped with a dual-frequency acoustic array in an embodiment; Figure 2 is Figure 1 front view of; Figure 3 It is a schematic diagram of a carrier platform equipped with a dual-frequency acoustic array in another embodiment; Figure 4 is Figure 3 layout diagram of the dual-frequency acoustic array and the counterweight balance device in; Figure 5 is the detection principle diagram of the dual-frequency acoustic array in the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0020] In one embodiment, a dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area is provided, including: A dual-frequency acoustic array mounted on a carrier platform, including a high-frequency acoustic array and a low-frequency acoustic array. The high-frequency acoustic array and the low-frequency acoustic array are synchronously transmitted. Among them, the high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect the short-range to medium-range area of the target area, and the low-frequency acoustic array transmits and receives low-frequency linear frequency modulation signals to detect the medium-range to long-range area of the target area. The beam opening angle of the high-frequency acoustic array is greater than that of the low-frequency acoustic array, and there is an overlapping area between the detection areas of the high-frequency acoustic array and the low-frequency acoustic array; A multi-channel transmitting and receiving machine, including a multi-channel signal transmitter and a multi-channel signal receiver. The multi-channel signal transmitter is used to transmit high- and low-frequency acoustic signals, and the multi-channel signal receiver is used to receive high- and low-frequency target echo signals; A dual-frequency synthetic aperture imaging signal processing module that performs synthetic aperture imaging processing on the high-frequency echo signal and the low-frequency echo signal respectively to obtain corresponding target acoustic images; A dual-frequency image fusion imaging module that generates a fused complete underwater acoustic image based on the target acoustic images corresponding to the high-frequency echo signal and the low-frequency echo signal by using a weighted fusion strategy.

[0021] Referring to Figure 1 and Figure 2 , Figure 1 is a schematic diagram of a carrier platform equipped with a dual-frequency acoustic array in one embodiment, Figure 2 is Figure 1Front view. The carrier platform 1 is a sonar towed body, which includes a weight balancing device 2, a dual-frequency sonar array 3, a towing beam 4, a towing handle 5, a towing cable 6, an electronic cabin 7, a power supply and communication transmission watertight cable 8, etc. The sonar towed body is lifted into the water by a deployment device on the mother ship and recovered onto the mother ship by the deployment device. The mother ship connects the towing cable 6 to tow the sonar towed body to navigate in the water, and adjusts the length of the towing cable 6 to control the working depth of the sonar towed body in the water. The power supply and the upper computer are installed on the mother ship, and the sonar towed body is powered and communicated through the power supply and communication transmission watertight cable 8. The sonar towed body is based on surface ship towing, and can flexibly adjust the depth of the sonar towed body, and is less affected by the platform, which is a relatively mature application mode. The sonar towed body is a working mode in which the towed body is towed by the mother ship to navigate at different depths in the water to collect underwater signals. The sonar towed body is connected to the mother ship through the towing cable 6 connected to the towing handle 5, and is provided with flow stabilizing devices such as a fairing and a tail fin. The dual-frequency sonar array 3 is installed on the abdomen of the sonar towed body, including a low-frequency sonar array 3-1 and a high-frequency sonar array 3-2. The low-frequency sonar array 3-1 and the high-frequency sonar array 3-2 are installed on the abdomen of the sonar towed body through a sonar array mounting box. Further, a weight balancing device 2 is arranged on the sonar towed body to ensure the balance state of the sonar towed body in the water. By optimizing the installation angles of the low-frequency sonar array 3-1 and the high-frequency sonar array 3-2, the optimized overlap of the beam coverage area is realized. The detection range of the high-frequency sonar array 3-2 focuses on the short range, and the detection range of the low-frequency sonar array 3-1 covers the long range. The two form a beam superposition in the middle overlapping area. The high-frequency sonar array 3-2 and the low-frequency sonar array 3-1 work together in a way that the non-blind area is covered by the dual-frequency beam collaboration. Further, by optimizing parameters such as the transmitted signals, installation angles, and heights of the high-frequency sonar array 3-2 and the low-frequency sonar array 3-1, on the premise of ensuring high resolution in the short range and wide coverage in the long range, the blind area is controlled within the minimum range acceptable in the project, and the balance between detection efficiency and technical feasibility is achieved.

[0022] Figure 3 Figure of the carrier platform equipped with a dual-frequency sonar array in another embodiment; Figure 4 is Figure 3Layout diagram of the dual - frequency acoustic array and the weight balance device in it. In this embodiment, the carrier platform 1 is an autonomous underwater vehicle capable of operating in AUV mode. The autonomous underwater vehicle installs a dual - frequency acoustic array 3 on its abdomen, including a low - frequency acoustic array 3 - 1 and a high - frequency acoustic array 3 - 2. The low - frequency acoustic array 3 - 1 and the high - frequency acoustic array 3 - 2 are installed on the abdomen of the autonomous underwater vehicle through an acoustic array installation box. Further, a weight balance device 2 is arranged on the autonomous underwater vehicle to ensure the balance state of the autonomous underwater vehicle in water. By optimizing the installation angles of the low - frequency acoustic array 3 - 1 and the high - frequency acoustic array 3 - 2, the optimized overlap of the beam coverage area is achieved. The detection range of the high - frequency acoustic array 3 - 2 focuses on the near - range, and the detection range of the low - frequency acoustic array 3 - 1 covers the long - range. The two form a beam superposition in the middle overlapping area. In the way that the high - frequency acoustic array 3 - 2 and the low - frequency acoustic array 3 - 1 work together, the non - blind area is covered by the dual - frequency beam collaboration. Further, by optimizing parameters such as the transmitted signals, installation angles, and heights of the high - frequency acoustic array 3 - 2 and the low - frequency acoustic array 3 - 1, on the premise of ensuring high near - range resolution and wide long - range coverage, the blind area is controlled within the minimum range acceptable in engineering, achieving the balance between detection efficiency and technical feasibility.

[0023] Referring to Figure 5 , Figure 5 is the detection principle diagram of the dual - frequency acoustic array in the present invention, Figure 5 which clearly shows the division of labor and cooperation between the low - frequency acoustic array 3 - 1 and the high - frequency acoustic array 3 - 2 to jointly cover the entire detection strip.

[0024] The high - frequency acoustic array selects a relatively high operating frequency. The operating frequency of the low - frequency acoustic array is lower than that of the high - frequency acoustic array, and the operating frequency bands of the high - frequency acoustic array and the low - frequency acoustic array do not overlap. Specifically, the working frequency ranges of the high - frequency linear frequency - modulated signal and the low - frequency linear frequency - modulated signal emitted by the high - frequency acoustic array and the low - frequency acoustic array can be set according to actual needs, as long as the working frequency ranges of the high - frequency linear frequency - modulated signal and the low - frequency linear frequency - modulated signal do not overlap.

[0025] Specifically, in a preferred embodiment, the high - frequency acoustic array emits a linear frequency - modulated (LFM) signal with a center frequency of and a signal bandwidth of , that is, the frequency range of the signal covers 180 kHz to 220 kHz. This frequency band is selected based on the physical relationship between the acoustic wave frequency and the imaging resolution and attenuation characteristics: a higher frequency can provide a shorter wavelength, thereby achieving higher range - direction resolution and azimuth - direction resolution, which is crucial for precisely identifying and depicting underwater targets (such as small obstacles, sediment details, pipeline structures, etc.).

[0026] Detection Tasks and Range of High-Frequency Acoustic Array: The high-frequency acoustic array is designed to undertake high-resolution imaging tasks in the short-range to medium-range area (e.g., the slant range is in the range of about 50 meters to 150 meters). Within this distance range, the attenuation of high-frequency signals is still within an acceptable range, which can ensure sufficient echo signal-to-noise ratio to form clear images. Its main purpose is to make up for the inherent defect that the resolution of low-frequency sonar signals is relatively insufficient in this short-distance area. As Figure 5 shown, the effective detection area of the high-frequency acoustic array covers the blue area in the figure and the gray area overlapping with the effective detection area of the low-frequency acoustic array, ensuring high-precision mapping ability in the short-distance range starting from the edge of the blind area.

[0027] The low-frequency acoustic array selects a relatively low operating frequency. Specifically, a linear frequency modulation (LFM) signal with a center frequency of and a bandwidth of is adopted, and its signal frequency range is 130 kHz to 170 kHz. Importantly, the operating frequency band (130 kHz - 170 kHz) of the low-frequency acoustic array does not overlap with the operating frequency band (180 kHz - 220 kHz) of the high-frequency acoustic array, which is to effectively avoid possible signal crosstalk and mutual interference when the high-frequency acoustic array and the low-frequency acoustic array work simultaneously, and ensure the purity of their respective echo data.

[0028] The core advantage of the low-frequency acoustic array lies in taking advantage of the physical characteristics that low-frequency sound waves attenuate slowly and have relatively strong penetrability when propagating in water, and can effectively detect the medium-range to long-range area (e.g., the slant range is greater than 150 meters). Its main purpose is to overcome the detection distance limitation or the sharp decline in signal quality formed by high-frequency signals due to rapid attenuation in the long-range area, and avoid the resulting long-range mapping blind area. By using LF-SAS to cover the long-range area, the present invention ensures that effective acoustic images can also be obtained for the distal part of the entire detection strip. As Figure 5 shown, the effective detection area of the low-frequency acoustic array covers the Figure 5 red area in

[0029] In the design and performance evaluation of a synthetic aperture sonar (SAS) system, the transducer plays a crucial role. Its electro-acoustic conversion efficiency, power-carrying capacity, and the physical properties of the material jointly determine the maximum acoustic energy that the sonar system can effectively emit, which directly affects the detection distance performance of the sonar. Although increasing the transmission power is a direct means to improve the detection distance, in practical engineering applications, both the technological limitations of the transducer itself and the capacity constraints of the driving electronic equipment make the transmitted acoustic power of the sonar unable to be increased indefinitely, thereby restricting the maximum source level (Source Level, ).

[0030] Detection range of the transmitting acoustic array The physical basis can be described by the sonar equation. Its core idea is that the target echo signal must have sufficient energy at the receiving end to overcome the interference of background noise and reverberation, so as to be reliably detected, that is, to reach or exceed a certain detection threshold (Detection Threshold, ). During the round-trip propagation of the signal from the transmitting acoustic array to the target and then back to the receiving acoustic array, significant energy loss will occur, that is, transmission loss (Transmission Loss, ). Therefore, a simplified energy balance relationship can be conceptually expressed as: ; In this relationship, represents the one-way transmission loss, which is mainly composed of the spreading loss caused by geometric spreading with the detection range of the transmitting acoustic array and the absorption loss jointly acting with the transmitting signal frequency . Composition. represents the directivity index of the receiving transducer, which quantifies the ability of the transducer to concentrate the received acoustic energy in a specific direction and is directly related to the beam opening angle of the transmitting acoustic array.

[0031] For underwater acoustic detection, the absorption loss term is particularly crucial. The absorption coefficient strongly depends on the acoustic wave frequency . The higher the frequency, the larger the value of, and the more serious the acoustic energy loss. In the frequency band involved in the present invention, the absorption coefficient corresponding to the operating frequency of the high-frequency acoustic array (for example ) is significantly greater than the absorption coefficient corresponding to the operating frequency of the low-frequency acoustic array (for example ). This means that at the same propagation distance , the energy attenuation of the high-frequency linear frequency modulation signal transmitted by the high-frequency acoustic array is much more severe than that of the low-frequency linear frequency modulation signal transmitted by the low-frequency acoustic array.

[0032] In order to achieve effective coverage of a remote area (for example > 150 m), and overcome the detection blind area caused by severe signal attenuation, the low-frequency acoustic array of the present invention adopts an optimization strategy. At a limited transmitted sound source level Under these conditions, it is necessary to find a way to compensate for the high transmission loss caused by long-distance propagation. The present invention changes the beam opening angle of the low-frequency acoustic array Designed to be narrow. According to acoustic principles, the directivity index of the transducer With its beam angle Closely related, usually the narrower the beam, the more concentrated the energy. By using a narrower beam angle , which can improve the directivity index of the low-frequency subsystem , which is equivalent to increasing the signal gain at the receiving end in the sonar equation, helping to offset some of the transmission loss, thereby and Achieve a longer maximum detection distance under requirements This design allows limited acoustic energy to be more effectively focused and projected to the target remote area, such as Figure 5 As shown, the low-frequency beam (red area) is relatively concentrated and points far away.

[0033] In contrast, for short-range areas (such as detection distance For detection missions of 50-150 meters, the design goals of the high-frequency acoustic array are different. The absorption coefficient Large, but due to the high-frequency sound array detection distance Relatively short, the total transmission loss At this point, taking advantage of the high-frequency signal itself being able to provide high-resolution imaging, the main consideration in design turns to how to improve the efficiency of surveying and mapping operations, that is, to increase the coverage width of a single measurement. The high-frequency acoustic array of the present invention uses a wider beam opening angle. The coverage width of the high-frequency acoustic array on the seabed is directly related to its effective beam opening angle and installation geometric parameters (such as height from the bottom and installation inclination). Figure 5 As shown, given the height from the bottom and the installation angle of the high-frequency sound array After that, the beam angle of the high-frequency sound array in the elevation direction The larger the beam is, the wider the radial range of the seabed it can effectively illuminate. Although a wider beam will sacrifice a certain degree of directivity However, under the premise that the short-range signal strength is sufficient to ensure the imaging quality, this sacrifice is exchanged for a wider effective mapping strip width. , thereby reducing the number of survey lines required to complete area coverage and significantly improving work efficiency. Figure 5 The beams of the mid- and high-frequency acoustic arrays (blue and gray areas) show a relatively wider opening angle to meet the needs of short-range wide coverage.

[0034] In summary, the present invention determines the beam opening angle of the high-frequency sonar array to be greater than that of the low-frequency sonar array (i.e., ). This is a meticulous design based on a comprehensive trade-off of the physical properties of underwater sound propagation (especially the frequency-dependent absorption effect), the principles of the sonar equation, the transducer directivity theory, and the core optimization objectives at different detection distances (maximum distance for long-range, resolution and coverage efficiency for short-range). By matching different optimal beam opening angles for the high-frequency and low-frequency sonar arrays, an effective combination of high-resolution wide coverage in the short range and extended long-range detection capabilities is achieved under limited transducer performance and transmit power conditions, providing crucial support for ultimately achieving the overall goal of a small mapping blind area.

[0035] The beam energy distribution of the synthetic aperture sonar has significant directivity characteristics. The energy density is highest in the center pointing direction, and as the angle deviating from the center increases, the beam energy shows regular attenuation. This physical property results in weak echo signal intensities at the edges of the detection area in a single-frequency sonar system, easily leading to problems such as a decrease in imaging resolution and even data loss. To address this key pain point, there is an overlapping area between the detection areas of the high-frequency and low-frequency sonar arrays of the present invention, and the cooperative effect of the dual-frequency beams is used to make up for the energy deficiency of a single frequency band.

[0036] Specifically, although the high-frequency sonar array has the advantage of high resolution in the short-range area, the attenuation of the beam edge energy will lead to a decrease in the signal-to-noise ratio at the end of short-range detection; while the low-frequency sonar array can achieve long-range coverage through its low-attenuation characteristics in the long-range area, but the problem of insufficient resolution at the short-range edge is significant. By setting an overlapping area (i.e., the gray area in Figure 5 ) between the detection areas of the high-frequency and low-frequency sonar arrays in the mid-range area, the low-energy edges at the far end of the high-frequency beam and the low-energy edges at the near end of the low-frequency beam can form energy superposition. This superposition effect is achieved through the spatial complementarity of beam directivity: when the high-frequency sonar array covers the short-range to mid-range area with a wider beam, the edge energy attenuation area coincides exactly with the near-end energy attenuation area of the narrower beam of the low-frequency sonar array, and the sound energies of the two form cooperative enhancement in the overlapping area, thereby enhancing the echo signal intensity in this area.

[0037] Regarding the determination of the overlapping angle , it is necessary to comprehensively consider the energy balance relationship in the sonar equation and the imaging resolution requirements. According to the sound propagation theory, the energy enhancement effect in the beam overlapping area is closely related to the angle between the two beams and the directivity index. Let the beam opening angle of the high-frequency sonar array be , and the beam opening angle of the low-frequency sonar array be , ; The installation inclinations of the high-frequency acoustic array and the low-frequency acoustic array are respectively and , then the overlapping angle corresponding to the overlapping area is: ; Among them, is the installation inclination difference between the high-frequency acoustic array and the low-frequency acoustic array, reflecting the spatial offset of their detection directions; is the beam opening angle difference between the high-frequency acoustic array and the low-frequency acoustic array, reflecting the spatial offset of their energy distributions. The overlapping angle needs to ensure that the far edge of the high-frequency beam is accurately docked with the near edge of the low-frequency beam in space, avoiding both the energy gap caused by insufficient overlap and the signal aliasing interference caused by excessive overlap.

[0038] In a preferred embodiment, through the asymmetric inclination installation of the high-frequency acoustic array (such as the working frequency band is 180kHz - 220kHz) and the low-frequency acoustic array (such as the working frequency band is 130kHz - 170kHz), where the installation inclination of the high-frequency acoustic array is 68.1°, and the installation inclination of the low-frequency acoustic array is 33.1°, in this way, the optimized overlap of the beam coverage area can be further realized. The detection range of the high-frequency acoustic array focuses on the short range (such as within 50m), and the detection distance of the low-frequency acoustic array covers the long range (such as 270m). The two form a beam superposition in the middle overlapping area (such as 50m - 200m), thus eliminating the tomographic blind area of the traditional single-frequency system.

[0039] In the actual parameter design, it is necessary to verify whether the signal strength in the overlapping area meets the detection threshold requirements through the sonar equation. Let the transmission loss of the high-frequency linear frequency modulation signal emitted by the high-frequency acoustic array at the near end of the overlapping area be , and the transmission loss of the low-frequency linear frequency modulation signal emitted by the low-frequency acoustic array at the far end of the overlapping area be , then the synthetic echo energy in the overlapping area is expressed as: ; Among them, and are respectively the transmitted acoustic energies of the high-frequency acoustic array and the low-frequency acoustic array; by adjusting the installation inclinations , of the high-frequency acoustic array and the low-frequency acoustic array, and then adjusting the overlapping angle , so that the synthetic echo energy in the overlapping area reaches or exceeds the system detection threshold, and at the same time ensure that the resolution of the overlapping area meets: ; In the formula, is the speed of sound, and are the signal bandwidths of the high-frequency acoustic array and the low-frequency acoustic array for transmitting signals, respectively. This constraint ensures that the overlapping area can improve the signal quality through energy superposition and will not cause resolution degradation due to frequency band differences, thus realizing the smooth connection and efficient imaging of the dual-frequency system in the transition area.

[0040] In summary, the dual-frequency beam overlapping design effectively improves the signal quality at the edge of the detection blind area through the energy complementary mechanism, and the precise derivation of the overlapping angle is based on the quantitative balance of the acoustic propagation physical model and the system performance index, providing the key theoretical support and engineering design basis for realizing a small mapping blind area.

[0041] The core limitation of the present invention to achieve only small blind area detection rather than no blind area detection lies in the inherent multi-path effect physical characteristics in underwater acoustic propagation. The multi-path effect refers to the phenomenon that when the transmitted signal propagates underwater, due to factors such as sea surface reflection, seabed scattering, and complex terrain diffraction, the echo of the same target reaches the receiving end through multiple paths. When the transmitted signal approaches the area directly below it (i.e., the near-bottom distance range), the incident angle of the acoustic wave approaches perpendicular, and at this time, the propagation time delay difference between the direct wave and the multi-path signals such as the seabed reflection wave and the interface reflection wave is extremely small, making it difficult for the receiving end to effectively separate the signals of different paths through the time threshold. This phenomenon will cause coherent aliasing of the echo signal, manifested as the target position offset, edge blur, or the appearance of false scattering points (i.e., artifacts) in the image, seriously deteriorating the imaging quality.

[0042] Analyzed from the perspective of signal processing, the multi-path effect can be modeled as the received signal consisting of the direct wave and multiple multi-path reflected waves superimposed: ; where is the attenuation coefficient of the th path, and is the propagation time delay. In the near-bottom distance area, since the time delay difference between each path is less than the signal pulse width , it causes to be unable to be decomposed into independent multi-path components, and at this time, traditional time delay estimation and equalization algorithms (such as adaptive filtering, rake reception) are difficult to effectively suppress the aliasing.

[0043] Existing multi-path suppression techniques (such as beamforming weighting, matched field processing, deep learning denoising, etc.) can weaken multi-path interference to a certain extent, but due to the time-varying characteristics and spatial complexity of the underwater acoustic channel, they cannot completely eliminate the multi-path effect in the near-bottom distance area. For example, beamforming technology enhances the signal in the target direction through weighted summation, but in the directly below area, the main lobe width of the beam widens as the distance decreases, resulting in increased side lobe energy leakage; matched field processing depends on accurate environmental parameters (such as sound speed profile, seabed sediment), and the uncertainty of the actual ocean environment will reduce its suppression efficiency. Therefore, based on the objective limitations of the current technical level, the present invention adopts a "controllable blind area" strategy in the system design: by optimizing the installation inclination angle of the high-frequency acoustic array and the beam opening angle of the high-frequency acoustic array , the mapping blind area of the system is limited within the range of the horizontal distance from the sonar carrier, where: ; where is the vertical height of the carrier platform from the bottom of the detection area, and the carrier platform is the carrier carrying the dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area.

[0044] When increases (i.e., the high-frequency acoustic array tilts towards the directly below direction), decreases, but the long-range detection range will be compressed; conversely, when decreases increases, resulting in an enlarged blind area. The present invention selects the optimal value by weighing the near-range resolution requirement and the multi-path suppression effect, so that not only meets the imaging accuracy requirements (such as the resolution is better than ), but also does not exceed the processing limit of the current multi-path suppression technology.

[0045] In summary, the physical essence of the multi-path effect and the limitations of existing suppression techniques determine that the underwater acoustic imaging system cannot completely eliminate the blind area directly below. The present invention realizes the balance between detection efficiency and technical feasibility by using dual-frequency beam collaborative coverage of non-blind area regions and combining acoustic array attitude optimization, and controlling the blind area within the minimum range acceptable in engineering on the premise of ensuring high near-range resolution and wide long-range coverage.

[0046] To prevent the linear frequency modulation signals of the two sonars from interfering with each other, further, the operating frequency bands of the high-frequency linear frequency modulation signal transmitted by the high-frequency sonar array of the present invention and the low-frequency linear frequency modulation signal transmitted by the low-frequency sonar array do not overlap. The core principle of this design is to avoid the spectral aliasing effect of acoustic signals - when two high-frequency linear frequency modulation signals and the low-frequency sonar array work simultaneously, if the frequency bands overlap, the linear frequency modulation signals they transmit will overlap in the time domain and frequency domain, resulting in the mixing of each other's frequency components in the echo signal and forming crosstalk noise.

[0047] Specifically, in one embodiment, the high-frequency linear frequency modulation signal uses a linear frequency modulation signal with a center frequency of and a signal bandwidth of , and the frequency range covers to ; the low-frequency linear frequency modulation signal uses a linear frequency modulation signal with a center frequency of and a signal bandwidth of , and the frequency range is to .

[0048] Analyzing from the physical mechanism of signal propagation, the non-overlapping frequency band design can achieve interference suppression through the following paths: (1) Frequency domain orthogonality: According to the frequency domain convolution property of the Fourier transform, the spectral separation of the transmitted signals can avoid the mis-sampling of heterogenous frequency signals by the mixer at the receiving end, thus eliminating the frequency domain crosstalk. (2) Time domain synchrony: Although the dual-frequency sonar may transmit signals simultaneously, the non-overlapping frequency bands make the echo signals of both have unique identifiability in the frequency domain. The receiving end can use band-pass filters (the passband of the high-frequency channel is , and the passband of the low-frequency channel is ) to achieve physical isolation of the signals and avoid waveform distortion caused by time domain aliasing.

[0049] The non-overlapping frequency bands avoid the power competition of dual-frequency signals in the common frequency band. If the frequency bands overlap, the transmitted power needs to be reduced to suppress the intermodulation interference, which will lead to a shortened detection range. However, through independent frequency band allocation, the present invention allows each subsystem to work at its optimal power level. The high-frequency sonar can focus on short-time high-power pulses required for short-range high-resolution, while the low-frequency sonar can transmit long-time low-power pulses to cover the remote area, thereby maximizing the overall detection efficiency.

[0050] On the other hand, based on the dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area provided in any of the above embodiments, an imaging method is provided, including: The high-frequency acoustic array and the low-frequency acoustic array of the dual-frequency acoustic array mounted on the carrier platform synchronously transmit and receive high-frequency linear frequency modulation signals and low-frequency linear frequency modulation signals. The high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect the short-range to medium-range area of the target area, and the low-frequency acoustic array transmits and receives low-frequency linear frequency modulation signals to detect the medium-range to long-range area of the target area. The beam opening angle of the high-frequency acoustic array is greater than that of the low-frequency acoustic array, and there is an overlapping area between the detection areas of the high-frequency acoustic array and the low-frequency acoustic array; The multi-channel signal transmitter in the multi-channel transmitting and receiving machine is used to transmit high- and low-frequency acoustic signals, and the multi-channel signal receiver in the multi-channel transmitting and receiving machine is used to receive high- and low-frequency target echo signals; The high-frequency echo signal and the low-frequency echo signal are respectively subjected to synthetic aperture imaging processing to obtain corresponding target acoustic images; Based on the target acoustic images corresponding to the high-frequency echo signal and the low-frequency echo signal, a weighted fusion strategy is adopted to generate a complete underwater acoustic image after fusion.

[0051] It can be understood that the design of the high-frequency acoustic array and the low-frequency acoustic array in the dual-frequency acoustic array and the design of the transmitted signal are both based on the methods in any of the foregoing embodiments, and will not be elaborated here.

[0052] The high-frequency echo signal and the low-frequency echo signal are respectively subjected to signal processing to obtain corresponding target acoustic images. The single-frequency imaging methods of the existing technology can be respectively used to realize imaging based on the high-frequency echo signal and imaging based on the low-frequency echo signal, so as to obtain the target acoustic image corresponding to the high-frequency data and the target acoustic image corresponding to the low-frequency data .

[0053] Without loss of generality, the high-frequency echo signal and the low-frequency echo signal are respectively subjected to signal processing to obtain corresponding images, including: Performing spatio-temporal synchronization on the high-frequency echo signal and the low-frequency echo signal; Performing gain compensation, band-pass filtering, analog-to-digital conversion processing and motion compensation on the high-frequency echo signal and the low-frequency echo signal after spatio-temporal synchronization, where the motion compensation includes phase compensation and delay compensation; The high-frequency data and the low-frequency data after motion compensation are respectively subjected to sub-aperture division, synthesis, azimuth compression and BP algorithm imaging to obtain the target acoustic image corresponding to the high-frequency data and the target acoustic image corresponding to the low-frequency data .

[0054] In the dual-frequency image fusion stage of the present invention, that is, after obtaining the target acoustic image corresponding to the high-frequency data and the target acoustic image corresponding to the low-frequency data After that, based on the target acoustic image corresponding to the high-frequency data and the target acoustic image corresponding to the low-frequency data a complete underwater acoustic image after fusion is obtained, aiming to integrate the advantages of high-frequency and low-frequency data, reduce the mapping blind area and improve the overall imaging quality. The fusion formula is: ; where the weight coefficients and adopt a gradual change method in the range direction. By setting the detection distance, due to the high resolution of the high-frequency data, the weight ratio is larger than that of the low-frequency data; at long distances, due to the slow propagation attenuation of the low-frequency data, the weight ratio is higher than that of the high-frequency data.

[0055] When fusing the high-frequency and low-frequency images, the gradual change method adopted in the range direction is to combine the parameters in Figure 5 and set the weight coefficients and to change continuously with the detection distance. In the near-range non-overlapping area ( ), that is, from the near range to the front section of the middle range, the high-frequency signal attenuation is controllable and the resolution advantage is obvious in this area, and the weight ratio of the high-frequency data is larger, tends to 1, tends to 0, and the high-frequency high-resolution image data is preferably adopted. In the middle-range overlapping area ( ), this is the overlapping area between the end of the high-frequency detection and the start of the low-frequency detection. The weight coefficients are smoothly transitioned through linear interpolation. Let be the horizontal length from the platform directly below to the start of the middle-range overlapping area, be the horizontal length from the platform directly below to the end of the middle-range overlapping area, then the horizontal length from the platform directly below of the carrier is and the calculation formula for the weight coefficient at this position is , , and it satisfies , so as to ensure the superposition and enhancement of the high-frequency and low-frequency energies in this area. In the far-range non-overlapping area ( ), that is, from the back section of the middle range to the far range, the low-frequency signal has slow propagation attenuation and a far coverage range, and the weight ratio of the low-frequency data is higher, tends to 0, tends to 1, and the low-frequency wide-coverage data is preferably adopted.

[0056] Through this gradient weighting, both the high-resolution advantage of high-frequency signals in the short range and the wide coverage characteristic of low-frequency signals in the long range can be utilized. During the fusion process, in the overlapping region (medium-range region) of high-frequency and low-frequency data, the signal intensity is enhanced through energy superposition. At the same time, through sonar equation verification and parameter adjustment, it is ensured that the resolution in the overlapping region does not degrade. Finally, a complete underwater acoustic image after fusion is generated through the BP algorithm, realizing smooth connection and efficient imaging of the short-range, medium-range, and long-range regions, and controlling the mapping blind area within the minimum range acceptable to the project.

[0057] In the present invention, the high-frequency acoustic array and the low-frequency acoustic array of the dual-frequency acoustic array respectively transmit high-frequency linear frequency modulation signals and low-frequency linear frequency modulation signals. Preferably, the center frequency of the high-frequency linear frequency modulation signal is set to , and a rectangular pulse with a pulse width of 5 ms is adopted; the center frequency of the low-frequency linear frequency modulation signal is , and a trapezoidal pulse with a pulse width of 8 ms is adopted. The mathematical expressions of the transmitted signals (high-frequency linear frequency modulation signal, low-frequency linear frequency modulation signal) are: ; where is the amplitude of the transmitted signal, is the center frequency of the transmitted signal (for the center frequency of the high-frequency linear frequency modulation signal, and the center frequency of the low-frequency linear frequency modulation signal), is the frequency modulation slope of the transmitted signal (the frequency modulation slope of the high-frequency linear frequency modulation signal, and the frequency modulation slope of the low-frequency linear frequency modulation signal), is the pulse width of the transmitted signal (the pulse width of the high-frequency linear frequency modulation signal, and the pulse width of the low-frequency linear frequency modulation signal).

[0058] After the high-frequency echo signal and the low-frequency echo signal are collected by a multi-channel receiver (such as a 96-channel receiver), the high-frequency echo signal and the low-frequency echo signal need to be preprocessed for spatio-temporal synchronization first. Because in the actual marine environment, due to the influence of various factors, there may be a deviation in the sampling start point among the receivers of different channels. To eliminate this deviation, the multi-channel receiver realizes precise alignment of the sampling start points of each channel through the PPS second pulse, and strictly controls the synchronization error within 2.5 ns. Only by ensuring the temporal synchronization of the signals of each channel can the accuracy of subsequent signal processing be guaranteed.

[0059] During the propagation of sonar signals, due to the attenuation of sound waves during water propagation, the intensity of the echo signal will weaken as the propagation distance increases. To compensate for this attenuation, gain compensation needs to be performed on the echo signal. In the present invention, time-varying gain (TVG) compensation is respectively performed on the high-frequency echo signal and the low-frequency echo signal after spatio-temporal synchronization. The time-varying gain compensation function is: ; where is the linear gain coefficient of the echo signal (the linear gain coefficient of the high-frequency echo signal , and the linear gain coefficient of the low-frequency echo signal ), is the constant term (the constant term corresponding to the high-frequency echo signal , and the constant term corresponding to the low-frequency echo signal ). Through this time-varying gain compensation, the echo signals at different distances can reach a relatively balanced state in intensity, providing more favorable conditions for subsequent signal processing.

[0060] Band-pass filtering is respectively performed on the high-frequency echo signal and the low-frequency echo signal after gain compensation. The role of the band-pass filter is to remove noise and interference in the signal and only retain the useful signal components. For the high-frequency echo signal, the passband range of the band-pass filter is set to ; for the low-frequency echo signal, the passband range of the band-pass filter is .

[0061] The high-frequency echo signal and the low-frequency echo signal after band-pass filtering are respectively subjected to analog-to-digital conversion. After the analog-to-digital conversion is completed, motion compensation is respectively performed on the high-frequency echo signal and the low-frequency echo signal. Motion compensation can achieve high-precision correction of the dynamic disturbance of the carrier platform through multi-sensor data fusion. The specific implementation process is as follows: First, the system obtains the three-dimensional position offset (accuracy ) and attitude angle data (roll / pitch angle accuracy ) of the carrier from the integrated navigation system (INS) in real time, and at the same time combines the bottom velocity provided by the Doppler velocity log (DVL) ( ) and the sound speed gradient data measured by the sound velocity profiler (SVP). These data are spatio-temporally synchronized through the timestamp alignment module, and the 100Hz high-frequency data of the inertial navigation is accurately matched with the sonar sampling rate by using cubic spline interpolation to eliminate the time misalignment between the sensor data and the sonar echo.

[0062] Phase compensation, in essence, converts the platform displacement into the phase error of the echo signal through a mathematical model. For the high-frequency sonar array with an installation inclination angle of 68.1°, the influence of the platform lateral displacement on the slant range is amplified to: ; Combined wavelength Calculate the phase offset based on mm (corresponding to 200 kHz): ; For example, when the platform has a 2 cm lateral displacement due to ocean currents, the phase error of the high-frequency channel is as high as 33.5 rad (about 5.3 cycles), and dynamic correction needs to be performed in the frequency domain through the complex multiplication factor . For the low-frequency acoustic array (installation inclination ), its phase sensitivity is reduced by about , but the slant range change caused by the vertical displacement still needs to be compensated: ; Time delay compensation is for the time drift of the signal echo arrival time caused by platform movement. When the carrier moves forward at knots (about m / s), the time delay change caused by the displacement of the carrier platform within the adjacent pulse interval (PRI = 100 ms) is: ; where is the beam incident angle (high frequency corresponds to ). The sampled data is interpolated at the sub-pixel level through the Farrow structure fractional time delay filter, and its transfer function is: ; The coefficient is generated by fitting with a third-order polynomial to achieve a time delay resolution of 0.01 sampling intervals. After compensation, the time domain alignment error of the 96-channel data is reduced from 15 ns before compensation to 0.8 ns, significantly improving the coherence of the subsequent synthetic aperture processing.

[0063] The core idea of the sub-aperture synthesis technology is to decompose the received data of a large aperture into the data of multiple small sub-apertures, process the data of each sub-aperture separately, and then accumulate the processing results. This can improve the azimuth resolution without increasing the hardware cost. The specific steps are as follows: First, perform a fast Fourier transform (FFT) on the data of each sub-aperture to convert the time domain signal to the frequency domain. In the frequency domain, the signal is more convenient to process, and range migration correction (RCMC) can be more easily performed. Range migration refers to the change in the position of the target in the range direction during the synthetic aperture imaging process due to the movement of the target and the platform. Through range migration correction, the position of the target in the range direction can be corrected so that it is accurately displayed in the imaging result. After the correction is completed, the signal is converted back to the time domain through the inverse Fourier transform, and finally the results of each sub-aperture are accumulated.

[0064] The formula for azimuth resolution is as follows: ; Wherein, is the acoustic wavelength (high-frequency echo signal , low-frequency echo signal ), is the length of the receiving array ( ), is the speed of the carrier platform ( ), is the pulse repetition frequency ( ). Through the sub-aperture synthesis technology and azimuth compression processing, the resolution of near-range target imaging can be effectively improved, providing strong support for the fine detection of targets.

[0065] The BP imaging algorithm is a commonly used imaging method in this field, which coherently superimposes the compensated data along the virtual aperture. Specifically, with the target point as the center, the slant range of the echo signal of each array element reaching the target point is calculated, the signal is compensated for time delay and phase adjusted according to the slant range, and then the signals of all array elements are coherently superimposed. The calculation formula is (where is the echo signal of the th array element, is the slant range from the target point to the array element, is the speed of sound). The BP imaging algorithm forms the final imaging result by point-by-point focusing, can effectively process complex acoustic wave propagation paths, adapt to the time-varying characteristics and spatial complexity of the underwater environment, and thus improve the focusing quality and detail expressiveness of the image.

[0066] Matters not described in this invention are well-known technologies.

[0067] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0068] The above-described embodiments only represent several implementation manners of this application. Their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of this application, several modifications and improvements can still be made, and these all belong to the protection scope of this application.

[0069] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, various modifications and variations can be made to the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area, characterized in that Comprising: A dual - frequency acoustic array mounted on a carrier platform, including a high - frequency acoustic array and a low - frequency acoustic array. The high - frequency acoustic array and the low - frequency acoustic array transmit synchronously. The high - frequency acoustic array transmits and receives high - frequency linear frequency - modulated signals to detect the short - range to medium - range area of the target region, and the low - frequency acoustic array transmits and receives low - frequency linear frequency - modulated signals to detect the medium - range to long - range area of the target region. The beam opening angle of the high - frequency acoustic array is greater than that of the low - frequency acoustic array, and there is an overlapping area between the detection regions of the high - frequency acoustic array and the low - frequency acoustic array; A multi - channel transmitting and receiving machine, including a multi - channel signal transmitter and a multi - channel signal receiver. The multi - channel signal transmitter is used to transmit high - and low - frequency acoustic signals, and the multi - channel signal receiver is used to receive high - and low - frequency target echo signals; A dual - frequency synthetic aperture imaging signal processing module that performs synthetic aperture imaging processing on the high - frequency echo signal and the low - frequency echo signal respectively to obtain corresponding target acoustic images; A dual - frequency image fusion imaging module that generates a fused complete underwater acoustic image based on the target acoustic images corresponding to the high - frequency echo signal and the low - frequency echo signal using a weighted fusion strategy.

2. The dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area according to claim 1, characterized in that, The installation inclinations of the high-frequency acoustic array and the low-frequency acoustic array are respectively and , then the overlapping angle corresponding to the overlapping area is: Among them, is the installation inclination difference between the high-frequency acoustic array and the low-frequency acoustic array, reflecting the spatial offset of the detection directions of the two; is the beam opening angle difference between the high-frequency acoustic array and the low-frequency acoustic array, reflecting the spatial offset of the energy distributions of the two.

3. The dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area according to claim 2, characterized in that, assume The transmission loss of the high-frequency linear frequency modulation signal emitted by the high-frequency acoustic array at the proximal end of the overlapping region is , and the transmission loss of the low-frequency linear frequency modulation signal emitted by the low-frequency acoustic array at the distal end of the overlapping region is . Then the synthetic echo energy is expressed as: Among them, and are the transmitted sound energies of the high-frequency acoustic array and the low-frequency acoustic array respectively; by adjusting the installation angles and of the high-frequency acoustic array and the low-frequency acoustic array, and then adjusting the overlapping angle , so that the synthetic echo energy in the overlapping area reaches or exceeds the system detection threshold, and at the same time ensure that the resolution of the overlapping area satisfies: In the formula, is the speed of sound, and are the signal bandwidths of the emission signals of the high-frequency acoustic array and the low-frequency acoustic array, respectively.

4. The dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area according to claim 3, wherein By optimizing the installation inclination angle of the high-frequency acoustic array and the beam opening angle of the high-frequency acoustic array , the mapping blind area of the system is restricted within the range of the horizontal distance from the sonar carrier , where: Among them is the vertical height of the carrier platform from the bottom of the detection area. The carrier platform is the carrier equipped with the small mapping blind area dual-frequency synthetic aperture underwater acoustic imaging system.

5. The small mapping blind area dual - frequency synthetic aperture underwater acoustic imaging system according to claim 1 or 2 or 3 or 4, characterized in that, The center frequency of the high - frequency linear frequency - modulated signal is greater than that of the low - frequency linear frequency - modulated signal, and the working frequency bands of the high - frequency linear frequency - modulated signal transmitted by the high - frequency acoustic array and the low - frequency linear frequency - modulated signal transmitted by the low - frequency acoustic array do not overlap.

6. The dual-frequency synthetic aperture underwater acoustic imaging system with a small mapping blind area according to claim 5, characterized in that, The high-frequency chirp signal uses a chirp signal with a center frequency of , a signal bandwidth of , and the frequency range covers to ; The low-frequency chirp signal uses a chirp signal with a center frequency of , a signal bandwidth of , and a frequency range of to .

7. An imaging method for a small mapping blind area dual-frequency synthetic aperture underwater acoustic imaging system according to claim 1 or 2 or 3 or 4 or 6, characterized in that, Comprising: Using the high - frequency acoustic array and the low - frequency acoustic array of the dual - frequency acoustic array mounted on the carrier platform to synchronously transmit and receive high - frequency linear frequency - modulated signals and low - frequency linear frequency - modulated signals. The high - frequency acoustic array transmits and receives high - frequency linear frequency - modulated signals to detect the short - range to medium - range area of the target region, and the low - frequency acoustic array transmits and receives low - frequency linear frequency - modulated signals to detect the medium - range to long - range area of the target region. The beam opening angle of the high - frequency acoustic array is greater than that of the low - frequency acoustic array, and there is an overlapping area between the detection regions of the high - frequency acoustic array and the low - frequency acoustic array; Using the multi - channel signal transmitter in the multi - channel transmitting and receiving machine to transmit high - and low - frequency acoustic signals, and using the multi - channel signal receiver in the multi - channel transmitting and receiving machine to receive high - and low - frequency target echo signals; Performing synthetic aperture imaging processing on the high - frequency echo signal and the low - frequency echo signal respectively to obtain corresponding target acoustic images; Based on the target acoustic images corresponding to the high - frequency echo signal and the low - frequency echo signal, using a weighted fusion strategy to generate a fused complete underwater acoustic image.

8. The imaging method according to claim 7, wherein Performing synthetic aperture imaging processing on the high - frequency echo signal and the low - frequency echo signal respectively to obtain corresponding target acoustic images, including: Performing spatio - temporal synchronization on the high - frequency echo signal and the low - frequency echo signal; Performing gain compensation, band - pass filtering, analog - to - digital conversion processing, and motion compensation on the high - frequency echo signal and the low - frequency echo signal after spatio - temporal synchronization. The motion compensation includes phase compensation and time - delay compensation; The high-frequency data and low-frequency data after motion compensation are respectively subjected to sub-aperture division, synthesis, azimuth compression, and BP algorithm imaging to obtain the target acoustic image corresponding to the high-frequency data and the target acoustic image corresponding to the low-frequency data .

9. The imaging method according to claim 8, characterized in that Target acoustic image corresponding to high-frequency data and target acoustic image corresponding to low-frequency data The complete underwater acoustic image after fusion is as follows: Among them, the weight coefficient and adopt a gradual change method in the range direction. Let be the horizontal length from directly below the carrier platform at the start of the overlapping area, be the horizontal length from directly below the platform at the end of the overlapping area. Then, the horizontal length from directly below the carrier platform at is calculated by the weight coefficient formula , , and satisfies .

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