Dual-frequency synthetic aperture underwater acoustic imaging system and method for small mapping blind area

Abstract

A dual-frequency synthetic aperture underwater acoustic imaging system and method for small mapping blind areas includes a dual-frequency acoustic array, a multi-channel transmitter-receiver, a dual-frequency synthetic aperture imaging signal processing module, and a dual-frequency image fusion imaging module. The high- and low-frequency acoustic arrays transmit synchronously. The high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect the short- 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- to long-range areas 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 and low-frequency acoustic arrays. The present invention uses dual-frequency beams emitted by the dual-frequency acoustic array to collaboratively detect to cover a larger mapping range. Combined with the optimization of the acoustic array structural parameters, the system controls the blind area within the minimum acceptable range for engineering while ensuring long-range high resolution, thus achieving a balance between detection efficiency and technical feasibility.

Description

Technical Field

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

[0002] Traditional synthetic aperture sonar (SAS) systems operate in a side-view mode, resulting in a large blind spot in their downward viewing area. Increasing the transmit beam width can expand the mapping range and reduce the blind spot width, but the dispersion of the beam energy reduces the imaging distance. This blind spot problem severely limits the mapping efficiency and imaging quality of traditional synthetic aperture sonar (SAS) systems. Specifically, due to the mapping blind spot, traditional synthetic aperture sonar (SAS) systems cannot achieve efficient coverage of the entire mapping swath, resulting in low mapping efficiency. In complex marine environments, traditional synthetic aperture sonar (SAS) systems require multiple repeated mappings to cover the target area, which increases operation time and cost, and also increases the risk of system damage when operating in dangerous scenarios.

[0003] Furthermore, traditional synthetic aperture sonar (SAS) systems, operating in a single frequency band, struggle to simultaneously meet the high-resolution imaging requirements for both short-range and long-range areas. Low-frequency sonars offer low resolution at long ranges and cannot meet the requirements for high-precision target detection. Traditional SAS systems are typically designed for a single function, making them difficult to adapt to complex environments with varying water depths, bottom conditions, and sea conditions. Low system integration and insufficient hardware modularity lead to high deployment and maintenance costs. Summary of the Invention

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

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] In one aspect, the present invention provides a dual-frequency synthetic aperture underwater acoustic imaging system for small mapping blind areas, comprising:

[0007] A dual-frequency acoustic array carried on a carrier platform includes a high-frequency acoustic array and a low-frequency acoustic array, which transmit synchronously. 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 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;

[0008] A multi-channel transmitter-receiver, comprising a multi-channel signal transmitter and a multi-channel signal receiver, wherein the multi-channel signal transmitter is used to transmit high-frequency and low-frequency sound signals, and the multi-channel signal receiver is used to receive high-frequency and low-frequency target echo signals;

[0009] The dual-frequency synthetic aperture imaging signal processing module performs synthetic aperture imaging on the high-frequency echo signal and the low-frequency echo signal to obtain the corresponding target acoustic image;

[0010] The dual-frequency image fusion imaging module uses a weighted fusion strategy to generate 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.

[0011] Furthermore, the installation inclination angles of the high-frequency sound array and the low-frequency sound array are respectively and , then the overlapping angle corresponding to the overlapping area is for:

[0012] ;

[0013] in, is the installation inclination angle 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 their energy distribution.

[0014] Furthermore, let the transmission loss of the high-frequency linear frequency modulation signal emitted by the high-frequency acoustic array near the overlapping area be , 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 is , then the synthetic echo energy in the overlapping area is Expressed as:

[0015] ;

[0016] in, and They are the emitted sound energy of the high-frequency sound array and the low-frequency sound array respectively; by adjusting the installation angles of the high-frequency sound array and the low-frequency sound array 、 Then adjust the overlap angle , so that the synthetic echo energy in the overlapping area Meet or exceed the system detection threshold while maintaining resolution in overlapping areas satisfy:

[0017] ;

[0018] Where, is the speed of sound, and are the signal bandwidths of the high-frequency acoustic array and the low-frequency acoustic array transmission signals respectively.

[0019] Furthermore, by optimizing the installation angle of the high-frequency sound array and the beam angle of the high-frequency acoustic array , limiting the system's mapping blind area to the horizontal distance from the sonar carrier within the range of:

[0020] ;

[0021] in It is the vertical height of the carrier platform from the bottom of the water in the detection area. The carrier platform is the carrier equipped with a small mapping blind area dual-frequency synthetic aperture underwater acoustic imaging system.

[0022] 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 operating frequency bands of the high-frequency linear frequency modulation signal emitted by the high-frequency acoustic array and the low-frequency linear frequency modulation signal emitted 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. The specific operating 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 the operating frequency ranges of the high-frequency linear frequency modulation signal and the low-frequency linear frequency modulation signal do not overlap.

[0023] On the other hand, an imaging method based on the above-mentioned small mapping blind area dual-frequency synthetic aperture underwater acoustic imaging system is provided, comprising:

[0024] A high-frequency acoustic array and a low-frequency acoustic array of a dual-frequency acoustic array carried on a carrier platform are used to synchronously transmit and receive high-frequency linear frequency modulation signals and low-frequency linear frequency modulation signals, wherein the high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect 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 medium-range to long-range areas 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;

[0025] The multi-channel signal transmitter in the multi-channel transmitter-receiver transmits high-frequency and low-frequency sound signals, and the multi-channel signal receiver in the multi-channel transmitter-receiver receives high-frequency and low-frequency target echo signals;

[0026] Perform synthetic aperture imaging on the high-frequency echo signal and the low-frequency echo signal to obtain the corresponding target acoustic image;

[0027] Based on the target acoustic images corresponding to the high-frequency echo signals and the low-frequency echo signals, a weighted fusion strategy is adopted to generate a fused complete underwater acoustic image.

[0028] Furthermore, synthetic aperture imaging is performed on the high-frequency echo signal and the low-frequency echo signal to obtain corresponding target acoustic images, including:

[0029] Performing spatiotemporal synchronization of high-frequency echo signals and low-frequency echo signals;

[0030] Perform gain compensation, bandpass filtering, analog-to-digital conversion, and motion compensation on the high-frequency echo signal and low-frequency echo signal after spatiotemporal synchronization, where the motion compensation includes phase compensation and delay compensation;

[0031] The high-frequency data and low-frequency data after motion compensation are subjected to sub-aperture division, synthesis, azimuth compression and BP algorithm imaging respectively to obtain the target acoustic image corresponding to the high-frequency data. And the target acoustic image corresponding to the low-frequency data .

[0032] 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:

[0033] ;

[0034] The weight coefficient and In the distance direction, a gradual change is adopted. is the horizontal length of the overlapped area from the bottom of the carrier platform. is the horizontal length from the end of the overlapping area to the bottom of the platform, then the horizontal length from the bottom of the carrier platform is The weight coefficient calculation formula is: , , and satisfies .

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention proposes a dual-frequency synthetic aperture underwater acoustic imaging system for small surveying and mapping blind areas, which includes a dual-frequency acoustic array. A high-frequency acoustic array and a low-frequency acoustic array in the dual-frequency acoustic array transmit and receive synchronously, wherein the high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect 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 medium-range to long-range areas 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.

[0037] This invention utilizes a high-frequency and low-frequency acoustic array working together to cover non-blind areas through dual-frequency beamforming. This approach aims to overcome the inherent conflict between detection range and imaging resolution in traditional single-frequency SAS systems, thereby achieving wide coverage, high resolution, and minimal blind spot imaging of underwater targets. By further optimizing parameters such as the transmission signal, mounting angle, and height of the high-frequency and low-frequency acoustic arrays, the blind spot is controlled to a minimum acceptable range while maintaining high resolution at short range and wide coverage at long range, achieving a balance between detection effectiveness and technical feasibility.

[0038] 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. Signal processing is performed on the high-frequency echo signals and the low-frequency echo signals respectively to obtain corresponding images. Based on the images corresponding to the high-frequency echo signals and the low-frequency echo signals, a weighted fusion strategy is used to generate a fused complete underwater acoustic image. For the images corresponding to the high-frequency echo signals and the low-frequency echo signals, a weighted fusion strategy is used to generate a fused complete underwater acoustic image, wherein the weight coefficients of the images corresponding to the high-frequency echo signals and the low-frequency echo signals are and A gradual approach is used in the distance dimension. At close range, high-frequency data has a greater weight due to its high resolution; at long distances, low-frequency data has a higher weight due to its slow propagation attenuation. This gradual weighting allows both the high-resolution advantage of high-frequency signals at short range and the wide coverage characteristics of low-frequency signals at long range to be utilized. During the fusion process, energy superposition is used to enhance signal strength in the overlapping areas of high- and low-frequency data (mid-range areas). Sonar equation verification and parameter adjustment are used to ensure that the resolution of the overlapping areas is not degraded. Finally, a complete fused underwater acoustic image is generated using the BP algorithm, achieving smooth connection and efficient imaging of the short-range, mid-range, and long-range areas, and controlling surveying and mapping blind spots to the minimum acceptable range for engineering.

[0039] This invention breaks through the inherent contradiction between detection distance and resolution of traditional SAS, and is particularly suitable for scenarios such as deep-sea pipeline inspection and seabed mineral exploration that require large-scale and detailed mapping. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0041] Figure 1 is a schematic diagram of a carrier platform equipped with a dual-frequency acoustic array in one embodiment;

[0042] Figure 2 for Figure 1 Front view of

[0043] Figure 3 is a schematic diagram of a carrier platform equipped with a dual-frequency acoustic array in another embodiment;

[0044] Figure 4 for Figure 3 The layout diagram of the dual-frequency acoustic array and the counterweight balancing device;

[0045] Figure 5 This is a diagram showing the detection principle of the dual-frequency acoustic array in the present invention. DETAILED DESCRIPTION

[0046] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0047] In one embodiment, a dual-frequency synthetic aperture underwater acoustic imaging system for small surveying and mapping blind areas is provided, comprising:

[0048] A dual-frequency acoustic array carried on a carrier platform includes a high-frequency acoustic array and a low-frequency acoustic array, which transmit synchronously. 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 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;

[0049] A multi-channel transmitter-receiver, comprising a multi-channel signal transmitter and a multi-channel signal receiver, wherein the multi-channel signal transmitter is used to transmit high-frequency and low-frequency sound signals, and the multi-channel signal receiver is used to receive high-frequency and low-frequency target echo signals;

[0050] The dual-frequency synthetic aperture imaging signal processing module performs synthetic aperture imaging on the high-frequency echo signal and the low-frequency echo signal to obtain the corresponding target acoustic image;

[0051] The dual-frequency image fusion imaging module uses a weighted fusion strategy to generate 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.

[0052] Reference Figure 1 and Figure 2 , Figure 1FIG. 1 is a schematic diagram of a carrier platform equipped with a dual-frequency acoustic array in one embodiment. Figure 2 yes Figure 1 The carrier platform 1 is the sonar towed body, which includes a counterweight balancing device 2, a dual-frequency acoustic array 3, a towing beam 4, a towing handle 5, a towing rope 6, an electronic cabin 7, a power supply and communication transmission watertight cable 8, etc. The sonar towed body is hoisted into the water by the deployment device on the mother ship and is recovered to the mother ship through the deployment device. The mother ship connects the towing rope 6 to tow the sonar towed body in the water and adjusts the length of the towing rope 6 to control the working depth of the sonar towed body in the water. The power supply and host computer are placed on the mother ship, and the power supply and communication are provided to the sonar towed body through the power supply and communication transmission watertight cable 8. The sonar towed body is based on the towing of the surface ship, and the depth of the sonar towed body can be flexibly adjusted. It is less affected by the platform and is a more mature application mode. The sonar towed body is a working mode in which the towed body is towed by the mother ship to sail at different depths in the water to collect underwater signals. The sonar towed body is connected to the mother ship through the towing rope 6 connected to the towing handle 5, and is equipped with flow stabilization devices such as diversion and tail fins. The sonar towed body has a dual-frequency acoustic array 3 mounted on its belly, comprising 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 mounted on the sonar towed body's belly via an acoustic array mounting box. Furthermore, a counterweight balancing device 2 is arranged on the sonar towed body to ensure its balance in the water. By optimizing the installation angles of the low-frequency acoustic array 3-1 and the high-frequency acoustic array 3-2, an optimal overlap of the beam coverage areas is achieved. The high-frequency acoustic array 3-2 focuses on the short range, while the low-frequency acoustic array 3-1 covers the long range. The two form a beam superposition in the intermediate overlapping area. The high-frequency acoustic array 3-2 and the low-frequency acoustic array 3-1 work in tandem, using dual-frequency beams to cover non-blind areas. By further optimizing the transmission signal, installation angle, height and other parameters of the high-frequency acoustic array 3-2 and the low-frequency acoustic array 3-1, the blind spots are controlled within the minimum range acceptable to the project while ensuring short-range high resolution and long-range wide coverage, thus achieving a balance between detection efficiency and technical feasibility.

[0053] Figure 3 A diagram of a carrier platform equipped with a dual-frequency acoustic array in another embodiment; Figure 4 for Figure 3The layout of the dual-frequency acoustic array and counterweight balancing device in the figure is shown. In this embodiment, the carrier platform 1 is an autonomous underwater vehicle (AUV) capable of operating in AUV mode. The AUV has a dual-frequency acoustic array 3 mounted on its belly. This array comprises a low-frequency acoustic array 3-1 and a high-frequency acoustic array 3-2. The low-frequency and high-frequency acoustic arrays 3-1 and 3-2 are mounted on the AUV's belly via an acoustic array mounting box. Furthermore, a counterweight balancing device 2 is deployed on the AUV to ensure the AUV's balance in the water. By optimizing the mounting angles of the low-frequency and high-frequency acoustic arrays 3-1 and 3-2, optimal overlap of beam coverage is achieved. The high-frequency acoustic array 3-2 focuses on short-range detection, while the low-frequency acoustic array 3-1 covers long-range detection. The two arrays form a superimposed beam in the overlapping area. The high-frequency and low-frequency acoustic arrays 3-2 and 3-1 work together to cover non-blind areas through dual-frequency beam coordination. By further optimizing the transmission signal, installation angle, height and other parameters of the high-frequency acoustic array 3-2 and the low-frequency acoustic array 3-1, the blind spots are controlled within the minimum range acceptable to the project while ensuring short-range high resolution and long-range wide coverage, thus achieving a balance between detection efficiency and technical feasibility.

[0054] Reference Figure 5 , Figure 5 This is a diagram showing the detection principle of the dual-frequency acoustic array in the present invention. Figure 5 It clearly shows that the low-frequency acoustic array 3-1 and the high-frequency acoustic array 3-2 work together to cover the entire detection strip.

[0055] The high-frequency acoustic array operates at a higher frequency, while the low-frequency acoustic array operates at a lower frequency. The operating frequency bands of the high-frequency and low-frequency acoustic arrays do not overlap. The operating frequency ranges of the high-frequency and low-frequency linear FM signals emitted by the high-frequency and low-frequency acoustic arrays can be set based on actual needs, as long as the operating frequency ranges of the high-frequency and low-frequency linear FM signals do not overlap.

[0056] Specifically, in a preferred embodiment, the high frequency acoustic array transmission center frequency is , the signal bandwidth is The linear frequency modulation (LFM) signal, which covers a frequency range of 180 kHz to 220 kHz, was selected based on the physical relationship between acoustic frequency, imaging resolution, and attenuation characteristics: higher frequencies provide shorter wavelengths, resulting in higher range and azimuth resolution, which is crucial for the detailed identification and delineation of underwater targets such as small obstacles, sediment details, and pipeline structures.

[0057] Detection mission and range of high-frequency acoustic arrays: High-frequency acoustic arrays are designed to undertake high-resolution imaging missions in short-range to medium-range areas (for example, slant ranges of approximately 50 to 150 meters). Within this distance range, the attenuation of high-frequency signals is still within an acceptable range, ensuring a sufficient echo signal-to-noise ratio to form a clear image. Its main purpose is to compensate for the inherent defect of low-frequency sonar signals in the relatively low resolution of this short-range area. Figure 5 As 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 capabilities in the close range starting from the edge of the blind spot.

[0058] The low-frequency sound array uses a relatively low operating frequency, specifically, the center frequency is , bandwidth is The linear frequency modulation (LFM) signal has a frequency range of 130 kHz to 170 kHz. Importantly, the operating frequency band of the low-frequency acoustic array (130kHz-170 kHz) does not overlap with the operating frequency band of the high-frequency acoustic array (180kHz-220kHz). This is to effectively avoid signal crosstalk and mutual interference that may occur when the high-frequency and low-frequency acoustic arrays operate simultaneously, ensuring the purity of their respective echo data.

[0059] The core advantage of the low-frequency acoustic array is that it utilizes the physical properties of low-frequency sound waves, which attenuate slowly and have relatively strong penetration when propagating in water. This allows for effective detection of medium to long-range areas (for example, a slant range greater than 150 meters). Its main purpose is to overcome the problem of detection range limitations or sharp declines in signal quality in long-range areas caused by the rapid attenuation of high-frequency signals, thereby avoiding the resulting blind spots in long-range mapping. By using LF-SAS to cover the long-range area, the present invention ensures that the far end of the entire detection strip can also obtain an effective acoustic image. Figure 5 As shown, the effective detection area of ​​the low-frequency acoustic array covers Figure 5 The red area in the figure and the gray area overlapping with the effective detection area of ​​the high-frequency acoustic array.

[0060] In the design and performance evaluation of synthetic aperture sonar (SAS) systems, transducers play a vital role. Their electroacoustic conversion efficiency, power carrying capacity, and the physical properties of the material together determine the maximum sound energy that the sonar system can effectively transmit, which directly affects the sonar's detection range performance. Although increasing the transmission power is a direct means to improve the detection range, in actual engineering applications, whether it is the process limitations of the transducer itself or the capacity constraints of the driving electronic equipment, the sonar's transmitted sound power is limited. It cannot be increased indefinitely, thus limiting the maximum sound source level of the system. ).

[0061] Detection range of the transmitting acoustic array The physical basis of the sonar equation can be described by the sonar equation. The core idea is that the target echo signal must have enough energy at the receiving end to overcome the interference of background noise and reverberation so that it can be reliably detected, that is, to reach or exceed a certain detection threshold (Detection Threshold, During the two-way propagation from the transmitting acoustic array to the target and then back to the receiving acoustic array, the signal will experience significant energy loss, namely transmission loss. ). Therefore, a simplified energy balance relationship can be conceptually expressed as:

[0062] ;

[0063] In this relationship, Represents the one-way transmission loss, which is mainly determined by the detection distance of the transmitting acoustic array. Expansion loss caused by geometric expansion and the detection range of the acoustic array and the frequency of the transmitted signal Combined absorption loss composition. It represents the directivity index of the receiving transducer, which quantifies the ability of the transducer to concentrate the received sound energy in a specific direction and is directly related to the beam angle of the transmitting acoustic array.

[0064] For underwater acoustic detection, the absorption loss term Absorption coefficient Strongly dependent on sound frequency , the higher the frequency, The larger the value of , the more serious the sound energy loss. In the frequency band involved in the present invention, the working frequency of the high-frequency sound array is (For example ) corresponding to the absorption coefficient It should be significantly greater than the operating frequency of the low-frequency acoustic array (For example ) corresponding to the absorption coefficient This means that at the same propagation distance Under these conditions, the energy attenuation of the high-frequency linear frequency modulation signal emitted by the high-frequency acoustic array is much more severe than that of the low-frequency linear frequency modulation signal emitted by the low-frequency acoustic array.

[0065] In order to achieve the remote area (such as >150 meters) to effectively cover the detection blind spots caused by severe signal attenuation. The low-frequency acoustic array of the present invention adopts an optimization strategy. 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 Its beam angle Closely related, generally 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, thus and Under the requirements, a longer maximum detection distance is achieved 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 to the distance.

[0066] In contrast, for short-range areas (such as detection distance For detection missions of 50-150 meters, the design objectives of the high-frequency acoustic array are different. Absorption coefficient Large, but due to the high-frequency sound array detection range Relatively short, total transmission loss At this point, the main consideration in the 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 adopts a wider beam opening angle. The coverage width of the high-frequency acoustic array's detection area on the seabed is directly related to its effective beam angle and installation geometric parameters (such as height from the bottom and installation inclination). Figure 5 As shown, the given height from the bottom and the installation angle of the high-frequency sound array After that, the beam angle of the high-frequency acoustic array in the elevation direction The larger the beam, the wider the radial range of the seabed that can be effectively illuminated. 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) exhibit a relatively wider opening angle to meet the needs of short-range wide coverage.

[0067] In summary, the present invention determines the beam angle of the high-frequency acoustic array The beam angle is larger than that of the low-frequency acoustic array (Right now ) is a meticulously designed system based on a comprehensive consideration of the physical properties of underwater acoustic propagation (particularly frequency-dependent absorption effects), sonar equations, transducer directivity theory, and core optimization objectives at different detection ranges (maximum range for long-range detection, and resolution and coverage efficiency for short-range detection). By matching differentiated optimal beam angles for high-frequency and low-frequency acoustic arrays, it effectively combines short-range, high-resolution, and wide coverage with extended long-range detection capabilities within limited transducer performance and transmit power, providing critical support for achieving the overall goal of minimizing surveying and mapping blind spots.

[0068] The beam energy distribution of synthetic aperture sonar has a significant directional characteristic, with the energy density in the direction pointing to the center being the highest. As the angle away from the center increases, the beam energy will show a regular attenuation. This physical characteristic causes the echo signal intensity at the edge of the detection area to be weak in a single-frequency sonar system, which can easily lead to problems such as reduced imaging resolution and even data loss. To address this key pain point, the high-frequency and low-frequency acoustic arrays of the present invention have overlapping detection areas, and the synergistic effect of the dual-frequency beam is used to compensate for the energy deficiency of a single frequency band.

[0069] Specifically, although the high-frequency acoustic array has the advantage of high resolution in the short-range area, its beam edge energy attenuation will lead to a decrease in the signal-to-noise ratio at the end of the short-range detection; while the low-frequency acoustic array can achieve long-range coverage in the long-range area through its low attenuation characteristics, its resolution at the short-range edge is significantly insufficient. By setting the detection areas of the high-frequency acoustic array and the low-frequency acoustic array to overlap in the mid-range area (i.e. Figure 5 The gray area in the image above (shown in the image above) allows the low-energy edge of the high-frequency beam at the far end to overlap with the low-energy edge of the low-frequency beam at the near end. This overlapping effect is achieved through the spatial complementarity of beam directivity: when the high-frequency acoustic array covers the short- to mid-range area with a wider beam, its edge energy attenuation coincides with the near-end energy attenuation of the narrower low-frequency beam. The acoustic energy of the two is synergistically enhanced in the overlapping area, thereby increasing the echo signal strength in that area.

[0070] About overlap angle The determination of needs 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 of the beam overlap area is closely related to the angle between the two beams and the directivity index. Assume that the beam opening angle of the high-frequency acoustic array is , the beam angle of the low-frequency acoustic array is , The installation angles of the high-frequency sound array and the low-frequency sound array are and , then the overlapping angle corresponding to the overlapping area is for:

[0071] ;

[0072] in, is the installation inclination angle difference between the high-frequency acoustic array and the low-frequency acoustic array, reflecting the spatial offset of the detection directions of the two; The difference in beam angle between the high-frequency and low-frequency acoustic arrays reflects the spatial offset in their energy distribution. The overlap angle ensures that the far edge of the high-frequency beam precisely aligns with the near edge of the low-frequency beam. This avoids both energy gaps caused by insufficient overlap and signal aliasing interference caused by excessive overlap.

[0073] In a preferred embodiment, a high-frequency acoustic array (e.g., operating in the 180kHz-220kHz band) and a low-frequency acoustic array (e.g., operating in the 130kHz-170kHz band) are installed at asymmetric angles—the high-frequency array at a 68.1° angle and the low-frequency array at a 33.1° angle—to further optimize the overlap of beam coverage areas. The high-frequency array's detection range is focused on short-range (e.g., within 50m), while the low-frequency array's detection range covers longer-range (e.g., 270m). The two arrays form a superimposed beam in the overlapping region (e.g., 50m-200m), thus eliminating the fault-prone blind spots of traditional single-frequency systems.

[0074] In 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. Assume that 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 is , 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 is , then the synthetic echo energy in the overlapping area is Expressed as:

[0075] ;

[0076] in, and They are the emitted sound energy of the high-frequency sound array and the low-frequency sound array respectively; by adjusting the installation angles of the high-frequency sound array and the low-frequency sound array 、 Then adjust the overlap angle , so that the synthetic echo energy in the overlapping area Meet or exceed the system detection threshold while maintaining resolution in overlapping areas satisfy:

[0077] ;

[0078] Where, is the speed of sound, and The signal bandwidths of the high-frequency and low-frequency acoustic arrays are respectively. This constraint ensures that the overlapping region can improve signal quality through energy superposition without degrading resolution due to frequency band differences, thus achieving smooth transition and efficient imaging in the dual-frequency system.

[0079] In summary, the dual-frequency beam overlapping design effectively improves the signal quality at the edge of the detection blind spot through the energy complementation mechanism. The precise derivation of the overlapping angle is based on the quantitative balance between the physical model of sound propagation and the system performance indicators, providing key theoretical support and engineering design basis for achieving small mapping blind spots.

[0080] The core limitation of the present invention, which only realizes small blind spot detection instead of no blind spot detection, comes from the inherent physical characteristics of the multipath effect in underwater acoustic propagation. The multipath effect refers to the fact that when the transmitted signal propagates underwater, due to factors such as seawater interface 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 range), the incident angle of the sound wave approaches vertical. At this time, the propagation delay difference between the direct wave and the multipath 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, which manifests as target position offset, blurred edges or false scattering points (i.e., artifacts) in the image, seriously deteriorating the imaging quality.

[0081] From the perspective of signal processing, the multipath effect can be modeled as the received signal By direct wave and Multipath reflected waves Superimposed:

[0082] ;

[0083] in, For the The attenuation coefficient of each path, is the propagation delay. In the near-bottom-distance area, due to the difference in delay between each path Smaller than the signal pulse width ,lead to It is impossible to decompose the multipath into independent components. In this case, traditional delay estimation and equalization algorithms (such as adaptive filtering and rake reception) are difficult to effectively suppress aliasing.

[0084] Although existing multipath suppression technologies (such as beamforming weighting, matched field processing, deep learning denoising, etc.) can weaken multipath interference to a certain extent, they are limited by the time-varying characteristics and spatial complexity of the underwater acoustic channel and cannot completely eliminate the multipath effect in the near-bottom range. For example, beamforming technology enhances the target direction signal through weighted summation, but in the area directly below, the width of the main lobe of the beam widens as the distance decreases, resulting in increased sidelobe energy leakage; matched field processing relies on precise environmental parameters (such as sound velocity 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 spot" strategy in system design: by optimizing the installation inclination angle of the high-frequency acoustic array and the beam angle of the high-frequency acoustic array , limiting the system's mapping blind area to the horizontal distance from the sonar carrier within the range of:

[0085] ;

[0086] in It is the vertical height of the carrier platform from the bottom of the water in the detection area. The carrier platform is the carrier equipped with a small mapping blind area dual-frequency synthetic aperture underwater acoustic imaging system.

[0087] when When it increases (i.e. the high-frequency sound array tilts toward the direction directly below), Reduce, but it will compress the long-range detection range; on the contrary, When decreasing The present invention balances the short-range resolution requirement and the multipath suppression effect to select the optimal value, make Both meet the imaging accuracy requirements (such as resolution better than ), without exceeding the processing limit of current multipath mitigation technology.

[0088] In summary, the physical nature of multipath effects and the limitations of existing mitigation technologies dictate that underwater acoustic imaging systems cannot completely eliminate the blind spot directly below. This invention utilizes dual-frequency beamforming to collaboratively cover non-blind zones, combined with acoustic array attitude optimization. While maintaining high resolution at short range and wide coverage at long range, this blind spot is controlled to a minimum acceptable for engineering purposes, achieving a balance between detection effectiveness and technical feasibility.

[0089] To prevent mutual interference between the chirp signals of the two sonars, the high-frequency chirp signals emitted by the high-frequency array and the low-frequency chirp signals emitted by the low-frequency array are designed to operate in mutually exclusive frequency bands. The core principle of this design is to avoid the spectral aliasing effect of acoustic signals. When two high-frequency chirp signals and a low-frequency array operate simultaneously, if their frequency bands overlap, the chirp signals they transmit will overlap in the time and frequency domains, causing the frequency components of the other to be mixed into the echo signal, forming crosstalk noise.

[0090] Specifically, in one embodiment, the high-frequency linear frequency modulation signal adopts a center frequency of , the signal bandwidth is Linear frequency modulation signal, the frequency range covers to ; The low frequency linear frequency modulation signal adopts the center frequency , the signal bandwidth is The linear frequency modulation signal has a frequency range of to .

[0091] From the analysis of 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 Fourier transform, the spectrum separation of the transmitted signal can avoid the mis-sampling of the different frequency signals by the mixer at the receiving end, thereby eliminating the frequency domain crosstalk. (2) Time domain synchronization: Although the dual-frequency sonar may transmit signals at the same time, the non-overlapping frequency bands make the echo signals of the two have unique identification in the frequency domain. The receiving end can use a bandpass filter (the high-frequency channel passband is , the low-frequency channel passband is ) to achieve physical isolation of signals and avoid waveform distortion caused by time domain aliasing.

[0092] Non-overlapping frequency bands avoid power competition between dual-frequency signals within the common frequency band. If the frequency bands overlap, transmit power must be reduced to suppress intermodulation interference, which reduces detection range. However, this invention allocates independent frequency bands, allowing each subsystem to operate at its optimal power level. The high-frequency sonar can focus on the short, high-power pulses required for short-range, high-resolution performance, while the low-frequency sonar can transmit long, low-power pulses to cover long-range areas, thereby maximizing overall detection efficiency.

[0093] On the other hand, based on the small mapping blind area dual-frequency synthetic aperture underwater acoustic imaging system provided in any of the above embodiments, an imaging method is provided, comprising:

[0094] A high-frequency acoustic array and a low-frequency acoustic array of a dual-frequency acoustic array carried on a carrier platform are used to synchronously transmit and receive high-frequency linear frequency modulation signals and low-frequency linear frequency modulation signals, wherein the high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect 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 medium-range to long-range areas 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;

[0095] The multi-channel signal transmitter in the multi-channel transmitter-receiver transmits high-frequency and low-frequency sound signals, and the multi-channel signal receiver in the multi-channel transmitter-receiver receives high-frequency and low-frequency target echo signals;

[0096] Perform synthetic aperture imaging on the high-frequency echo signal and the low-frequency echo signal to obtain the corresponding target acoustic image;

[0097] Based on the target acoustic images corresponding to the high-frequency echo signals and the low-frequency echo signals, a weighted fusion strategy is adopted to generate a fused complete underwater acoustic image.

[0098] 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 transmission signal are based on the method in any of the aforementioned embodiments, and will not be repeated here.

[0099] The high-frequency echo signal and the low-frequency echo signal are processed separately to obtain the corresponding target acoustic image. The imaging based on the high-frequency echo signal and the imaging based on the low-frequency echo signal can be realized by using the single-frequency imaging method of the existing technology respectively, 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 .

[0100] Without loss of generality, the high-frequency echo signal and the low-frequency echo signal are processed separately to obtain corresponding images, including:

[0101] Performing spatiotemporal synchronization of high-frequency echo signals and low-frequency echo signals;

[0102] Perform gain compensation, bandpass filtering, analog-to-digital conversion, and motion compensation on the high-frequency echo signal and low-frequency echo signal after spatiotemporal synchronization, where the motion compensation includes phase compensation and delay compensation;

[0103] The high-frequency data and low-frequency data after motion compensation are subjected to sub-aperture division, synthesis, azimuth compression and BP algorithm imaging respectively to obtain the target acoustic image corresponding to the high-frequency data. And the target acoustic image corresponding to the low-frequency data .

[0104] In the dual-frequency image fusion stage of the present invention, that is, when obtaining the target acoustic image corresponding to the high-frequency data And the target acoustic image corresponding to the low-frequency data Afterwards, the present invention is based on the target acoustic image corresponding to the high frequency data And the target acoustic image corresponding to the low-frequency data The fused complete underwater acoustic image is obtained to combine the advantages of high-frequency and low-frequency data, reduce the blind spots in mapping and improve the overall imaging quality. The fusion formula is:

[0105] ;

[0106] The weight coefficient and In the distance direction, a gradient method is used to set the detection distance. The high-frequency data has a higher weight than the low-frequency data due to its high resolution. At long distances, the low-frequency data has a higher weight than the high-frequency data due to its slow propagation attenuation.

[0107] When high-frequency and low-frequency images are fused, the gradient method used in the distance direction is to combine Figure 5 The parameters in the weight coefficient are set according to the characteristics of different regions. and Continuously changes with the detection distance. In the short-range non-overlapping area ( ), that is, from short range to the front of mid-range, the high-frequency signal attenuation in this area is controllable and the resolution advantage is obvious, and the high-frequency data weight accounts for a larger proportion. Approaching 1, Approaching 0, high-frequency and high-resolution image data are preferred. In the mid-range overlapping area ( ), which is the overlapping area between the end of high-frequency detection and the beginning of low-frequency detection, and the weight coefficient is smoothly transitioned by linear interpolation. is the horizontal length from the beginning of the mid-range overlapping area to the bottom of the platform, is the horizontal length from the end of the mid-range overlap area to the bottom of the platform, then the horizontal length from the bottom of the carrier platform is The weight coefficient calculation formula is: , , and satisfies , so as to ensure that high-frequency and low-frequency energy are superimposed and enhanced in this area. In the remote non-overlapping area ( ), that is, from the mid-range to the remote area, the low-frequency signal propagation attenuation is slow and the coverage range is long, so the low-frequency data weight accounts for a higher proportion. Approaching 0, When it approaches 1, low bandwidth coverage data is preferred.

[0108] This gradual weighting method leverages both the high-resolution advantages of high-frequency signals at short range and the wide coverage of low-frequency signals at long range. During the fusion process, energy is superimposed in the overlapping areas of high- and low-frequency data (mid-range regions) to enhance signal strength. Sonar equation verification and parameter adjustments are used to ensure that resolution in these overlapping areas is not degraded. Finally, a BP algorithm is used to generate a complete, fused underwater acoustic image, achieving smooth integration and efficient imaging of the short, mid, and long ranges, minimizing surveying and mapping blind spots to the minimum acceptable for engineering purposes.

[0109] The present invention transmits high-frequency linear frequency modulation signals and low-frequency linear frequency modulation signals respectively through the high-frequency acoustic array and the low-frequency acoustic array of the dual-frequency acoustic array. Preferably, the center frequency of the high-frequency linear frequency modulation signal is set to , a rectangular pulse with a pulse width of 5ms is used; the center frequency of the low-frequency linear frequency modulation signal is , using a trapezoidal pulse with a pulse width of 8ms. The mathematical expression of the transmitted signal (high-frequency linear frequency modulation signal, low-frequency linear frequency modulation signal) is:

[0110] ;

[0111] in, is the amplitude of the transmitted signal, is the center frequency of the transmitted signal (for high frequency linear frequency modulation signals , the center frequency of the low-frequency linear FM signal ), is the frequency modulation slope of the transmitted signal (the frequency modulation slope of the high-frequency linear frequency modulation signal , the frequency modulation slope of the low-frequency linear FM signal ), is the pulse width of the transmitted signal (the pulse width of the high-frequency linear frequency modulation signal , the pulse width of the low-frequency linear frequency modulation signal ).

[0112] After high-frequency and low-frequency echo signals are collected by a multi-channel receiver (such as a 96-channel receiver), they must first undergo spatiotemporal synchronization preprocessing. In real-world marine environments, various factors can cause sampling start points of receivers in different channels to deviate. To eliminate this deviation, multi-channel receivers use PPS pulses per second (PPS) to precisely align the sampling start points of each channel, strictly controlling synchronization error to within 2.5ns. Only by ensuring temporal synchronization of signals across each channel can the accuracy of subsequent signal processing be guaranteed.

[0113] During sonar signal propagation, the intensity of the echo signal decreases as the propagation distance increases due to the attenuation of sound waves in water. To compensate for this attenuation, gain compensation is required for the echo signal. The present invention performs time-varying gain (TVG) compensation on the high-frequency echo signal and the low-frequency echo signal after spatiotemporal synchronization. The time-varying gain compensation function is:

[0114] ;

[0115] in, is the linear gain coefficient of the echo signal (the linear gain coefficient of the high-frequency echo signal , the linear gain coefficient of the low-frequency echo signal ), is a constant term (the constant term corresponding to the high frequency echo signal , the constant term corresponding to the low-frequency echo signal Through this time-varying gain compensation, the echo signals at different distances can achieve a relatively balanced state in terms of intensity, providing more favorable conditions for subsequent signal processing.

[0116] The high-frequency echo signal and low-frequency echo signal after gain compensation are band-pass filtered respectively. The function of the band-pass filter is to remove noise and interference in the signal and retain only useful signal components. For the high-frequency echo signal, the passband range of the band-pass filter is set to ; For low-frequency echo signals, the passband range of the bandpass filter is .

[0117] The high-frequency echo signal and the low-frequency echo signal after bandpass filtering are converted into analog-to-digital respectively. After the conversion is completed, the high-frequency echo signal and the low-frequency echo signal are subjected to motion compensation respectively. 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) of the carrier in real time from the integrated inertial navigation (INS) ) and attitude angle data (roll / pitch angle accuracy ), combined with the bottom velocity provided by the Doppler velocimeter (DVL) ( ) and sound velocity gradient data measured by a sound velocity profiler (SVP). These data are synchronized in time and space using a timestamp alignment module. Cubic spline interpolation is used to precisely match the 100Hz high-frequency inertial navigation data with the sonar sampling rate, eliminating the time misalignment between the sensor data and the sonar echo.

[0118] The essence of phase compensation is to convert the platform displacement into the phase error of the echo signal through a mathematical model. For a high-frequency acoustic array with an installation angle of 68.1°, the lateral displacement of the platform is The effect on slant range is magnified as:

[0119] ;

[0120] Combined wavelength mm (corresponding to 200kHz) calculates the phase offset:

[0121] ;

[0122] For example, when the platform produces a 2cm lateral displacement due to the ocean current, the phase error of the high-frequency channel is as high as 33.5rad (about 5.3 cycles), which needs to be multiplied in the frequency domain by the complex multiplication factor For low frequency sound array (installation angle ), its phase sensitivity is reduced by about However, vertical displacement still needs to be compensated The slope distance change caused by:

[0123] ;

[0124] The delay compensation is aimed at the signal echo arrival time drift caused by platform movement. save m / s), the carrier platform displacement within the adjacent pulse interval (PRI=100ms) causes the delay change:

[0125] ;

[0126] in is the beam incident angle (high frequency correspond ). The sampled data is interpolated at the sub-pixel level through the Farrow structure fractional delay filter, and its transfer function is:

[0127] ;

[0128] coefficient Generated by a third-order polynomial fit, it achieves a delay resolution of 0.01 sampling interval. After compensation, the time domain alignment error of the 96-channel data is reduced from 15ns before compensation to 0.8ns, significantly improving the coherence of subsequent synthetic aperture processing.

[0129] The core concept of subaperture synthesis technology is to decompose the received data from a large aperture into data from multiple smaller subapertures, process the data from each subaperture separately, and then accumulate the processed results. This improves azimuth resolution without increasing hardware costs. The specific steps are as follows: First, a fast Fourier transform (FFT) is performed on each subaperture data to convert the time domain signal to the frequency domain. Signal processing in the frequency domain is more convenient, making it easier to perform range motion correction (RCMC). Range motion refers to the change in the target's range position during synthetic aperture imaging due to target motion and platform motion. Range motion correction can be used to correct the target's range position so that it is accurately displayed in the imaging result. After correction, the signal is converted back to the time domain through an inverse Fourier transform, and the results from each subaperture are finally accumulated.

[0130] The azimuth resolution calculation formula is:

[0131] ;

[0132] in, is the wavelength of the sound wave (high frequency echo signal , low-frequency echo signal ), is the receiving array length ( ), is the carrier platform speed ( ), is the pulse repetition frequency ( Through sub-aperture synthesis technology and azimuth compression processing, the resolution of close-range target imaging can be effectively improved, providing strong support for fine detection of targets.

[0133] The BP imaging algorithm is a commonly used imaging method in this field. It performs coherent superposition of compensated data along a virtual aperture. Specifically, with the target point as the center, the slant distance of each array element's echo signal to the target point is calculated, the signal is delayed and phase adjusted according to the slant distance, and then the signals of all array elements are coherently superimposed. The calculation formula is: (in For the The echo signal of each array element, Target point Slant distance to the array element, The BP imaging algorithm forms the final imaging result through point-by-point focusing. It can effectively process complex sound wave propagation paths and adapt to the time-varying characteristics and spatial complexity of the underwater environment, thereby improving the focus quality and detail expression of the image.

[0134] Matters not covered by the present invention are known technologies.

[0135] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, 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, they should be considered to be within the scope of this specification.

[0136] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that a person skilled in the art may make various modifications and improvements without departing from the spirit of the present application, and such modifications and improvements are all within the scope of protection of the present application.

[0137] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. Small mapping blind area dual-frequency synthetic aperture underwater acoustic imaging system, characterized by: include: A dual-frequency acoustic array carried on a carrier platform includes a high-frequency acoustic array and a low-frequency acoustic array, the high-frequency acoustic array and the low-frequency acoustic array transmit synchronously, 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, 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 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, wherein 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 operating frequency bands of the high-frequency linear frequency modulation signal emitted by the high-frequency acoustic array and the low-frequency linear frequency modulation signal emitted by the low-frequency acoustic array do not overlap with each other; A multi-channel transmitter-receiver, comprising a multi-channel signal transmitter and a multi-channel signal receiver, wherein the multi-channel signal transmitter is used to transmit high-frequency and low-frequency sound signals, and the multi-channel signal receiver is used to receive high-frequency and low-frequency target echo signals; The dual-frequency synthetic aperture imaging signal processing module performs synthetic aperture imaging on the high-frequency echo signal and the low-frequency echo signal to obtain the corresponding target acoustic image; The dual-frequency image fusion imaging module uses a weighted fusion strategy to generate 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.

2. The small blind area dual-frequency synthetic aperture underwater acoustic imaging system according to claim 1 is characterized in that: The installation angles of the high-frequency sound array and the low-frequency sound array are and , then the overlapping angle corresponding to the overlapping area is for: in, is the installation inclination angle 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 their energy distribution.

3. The small blind area dual-frequency synthetic aperture underwater acoustic imaging system according to claim 2 is characterized in that The transmission loss of the high-frequency linear frequency modulation signal emitted by the high-frequency acoustic array near the overlapping area is , 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 is , then the synthetic echo energy in the overlapping area is Expressed as: in, and They are the emitted sound energy of the high-frequency sound array and the low-frequency sound array respectively; by adjusting the installation angles of the high-frequency sound array and the low-frequency sound array 、 Then adjust the overlap angle , so that the synthetic echo energy in the overlapping area Meet or exceed the system detection threshold while maintaining resolution in overlapping areas satisfy: Where, is the speed of sound, and are the signal bandwidths of the high-frequency acoustic array and the low-frequency acoustic array transmission signals respectively.

4. The small blind area dual-frequency synthetic aperture underwater acoustic imaging system according to claim 3 is characterized in that: By optimizing the installation angle of the high-frequency sound array and the beam angle of the high-frequency acoustic array , limiting the system's mapping blind area to the horizontal distance from the sonar carrier within the range of in It is the vertical height of the carrier platform from the bottom of the water in the detection area. The carrier platform is the carrier equipped with a small mapping blind area dual-frequency synthetic aperture underwater acoustic imaging system.

5. The small blind area dual-frequency synthetic aperture underwater acoustic imaging system according to claim 1 is characterized in that: The high frequency linear frequency modulation signal adopts a center frequency of , the signal bandwidth is Linear frequency modulation signal, the frequency range covers to ; The low frequency linear frequency modulation signal adopts a center frequency of , the signal bandwidth is The linear frequency modulation signal has a frequency range of to .

6. The imaging method based on the dual-frequency synthetic aperture underwater acoustic imaging system for small surveying and mapping blind areas according to claim 1, 2, 3, 4, or 5, is characterized in that: include: A high-frequency acoustic array and a low-frequency acoustic array of a dual-frequency acoustic array carried on a carrier platform are used to synchronously transmit and receive high-frequency linear frequency modulation signals and low-frequency linear frequency modulation signals, wherein the high-frequency acoustic array transmits and receives high-frequency linear frequency modulation signals to detect 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 medium-range to long-range areas 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; The multi-channel signal transmitter in the multi-channel transmitter-receiver transmits high-frequency and low-frequency sound signals, and the multi-channel signal receiver in the multi-channel transmitter-receiver receives high-frequency and low-frequency target echo signals; Perform synthetic aperture imaging on the high-frequency echo signal and the low-frequency echo signal to obtain the corresponding target acoustic image; Based on the target acoustic images corresponding to the high-frequency echo signals and the low-frequency echo signals, a weighted fusion strategy is adopted to generate a fused complete underwater acoustic image.

7. The imaging method according to claim 6, characterized in that Perform synthetic aperture imaging on the high-frequency echo signal and the low-frequency echo signal to obtain the corresponding target acoustic image, including: Performing spatiotemporal synchronization of high-frequency echo signals and low-frequency echo signals; Perform gain compensation, bandpass filtering, analog-to-digital conversion, and motion compensation on the high-frequency echo signal and low-frequency echo signal after spatiotemporal synchronization, where the motion compensation includes phase compensation and delay compensation; The high-frequency data and low-frequency data after motion compensation are subjected to sub-aperture division, synthesis, azimuth compression and BP algorithm imaging respectively to obtain the target acoustic image corresponding to the high-frequency data. And the target acoustic image corresponding to the low-frequency data .

8. The imaging method according to claim 7, wherein: Target acoustic image based on high-frequency data And the target acoustic image corresponding to the low-frequency data The complete underwater acoustic image after fusion is: The weight coefficient and In the distance direction, a gradual change is adopted. is the horizontal length of the overlapped area from the bottom of the carrier platform. is the horizontal length from the end of the overlapping area to the bottom of the platform, then the horizontal length from the bottom of the carrier platform is The weight coefficient calculation formula is: , , and satisfies .

Images