Underwater target resonant frequency analysis and fitting method

Abstract

The invention belongs to the technical field of underwater target resonant frequency analysis, and relates to an underwater target resonant frequency analysis and fitting method, which comprises the steps of constructing an ocean model, constructing an ocean sound field according to a formula, obtaining a distance-frequency spectrum, calculating resonant frequencies in different propagation modes, and fitting the resonant frequencies with the distance-frequency spectrum. According to the method, the resonance frequency of the target is predicted by using the surrounding waves generated by interaction of the sound waves and the underwater target, so that the problems of large calculation amount and insufficient accuracy of traditional resonance frequency detection are solved. According to the method, the resonance characteristic generated by interaction of sound waves and a target is utilized, and the correlation between a resonance structure in a frequency spectrum and a target scale and a material is analyzed, so that the influence of sound signal attenuation and distortion on the imaging quality is reduced, and the definition and reliability of a sonar image are improved. Meanwhile, the interference of the multipath effect on target detection is reduced, so that the resonant frequency of the target is extracted; and the stability and reliability of the underwater target resonance frequency fitting method are enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of underwater target resonance frequency analysis, and particularly relates to a new method for detecting the resonance frequency of underwater targets, and more particularly to an analysis and fitting method for the resonance frequency of underwater targets. Background Art

[0002] The resonance frequency of an underwater target refers to the vibration characteristics of an underwater object at a specific frequency, which is usually closely related to the geometric shape of the object, material properties, and the acoustic properties of the surrounding water environment. The analysis of the resonance frequency can help researchers understand the dynamic behavior of underwater targets, thus providing important information for detecting and identifying underwater objects. Early research mainly focused on the relationship between underwater acoustic properties and the physical properties of targets. Researchers explored the resonance phenomena of underwater targets with different materials and shapes when propagating sound waves in water through experiments and theoretical models. These studies laid the foundation for subsequent underwater detection technologies, especially in the design and optimization of sonar systems.

[0003] The analysis of the resonance frequency of underwater targets is of great significance in many fields. First, it provides basic data for underwater detection technologies, helping to improve the detection accuracy of sonar systems. Second, by understanding the resonance characteristics of targets, the design of underwater structures can be optimized to enhance their stability and reliability in complex environments. In addition, mastering the resonance frequency of underwater targets also has important practical value for military applications (such as submarine detection) and marine scientific research (such as ecological monitoring). Therefore, in-depth research on the resonance frequency of underwater targets not only contributes to technological progress but also provides important support for scientific research in related fields.

[0004] Spectrum analysis is a technique for decomposing complex signals into simpler signals. Its core task is to find the information of a signal at different frequencies, such as amplitude, power, intensity, or phase. This analysis process usually involves Fourier transform, which converts the time-domain signal to the frequency domain to reveal the distribution characteristics of the signal in terms of frequency. In the analysis of the resonance frequency of underwater targets, the spectrum analysis method can identify the resonance frequency by performing spectrum analysis on the sound waves emitted by underwater targets. This method relies on signal processing techniques such as Fourier transform to convert the time-domain signal to the frequency-domain signal, thereby extracting the resonance characteristics of the target. Spectrum analysis can convert the signal from the time domain to the frequency domain, intuitively reflecting the magnitude of the Fourier transform of the input signal, and is suitable for observing and measuring signal amplitude and distortion. The fast Fourier transform (FFT), as a type of spectrum analysis, is fast and accurate and is suitable for analyzing periodic and non-periodic signals.

[0005] Modal analysis is a technique used to determine the dynamic characteristics of structures, especially suitable for complex underwater structures. It involves applying excitation and measuring the response to identify the natural frequencies and modal shapes of the structure. In the analysis of the resonance frequencies of underwater targets, modal analysis can help us understand the vibration characteristics of underwater structures at different frequencies, which is crucial for evaluating and predicting the stability and safety of underwater structures. The results of modal analysis include the frequencies, vibration modes, relative stresses, and forces of the structure, and this information is of great significance for the design and optimization of underwater structures. Modal analysis is a method for studying the dynamic characteristics of structures, capable of identifying the natural frequencies, damping ratios, and modal vibration modes of a system, and is very important for the dynamic design of structures and the fault diagnosis of equipment. Through modal analysis, the actual vibration response of the structure under various external or internal vibration sources can be predicted, providing a basis for the vibration characteristic analysis, vibration fault diagnosis and prediction, and optimization design of the structural system.

[0006] The application of adaptive filtering algorithms in the analysis of the resonance frequencies of underwater targets is mainly reflected in their ability to adjust parameters in real time to adapt to the dynamic changes of the target, thereby improving the accuracy of frequency estimation. Adaptive filtering algorithms can change matrix A, B, or change the covariance matrix P(k|k - 1), and then change the gain matrix K, and further change the state matrix X and the covariance matrix P(k). The adaptability of this method gives it an advantage in dealing with non-Gaussian white noise and can better adapt to the colored noise that the actual system may encounter. For example, the adaptive unscented Kalman filter algorithm based on the fusion of azimuth and multi-line spectrum information (MFB-AUKF) is applied to the motion analysis of underwater maneuvering targets and can effectively handle the problem of resonance frequency estimation of maneuvering targets. Adaptive filtering algorithms can automatically iteratively adjust their own filter parameters to meet the requirements of a certain criterion and achieve optimal filtering, especially suitable for situations where the statistical characteristics of the input signal are unknown or changing. Adaptive filtering algorithms such as LMS and RLS algorithms are widely used in practice due to their small computational load and ease of implementation.

[0007] However, traditional spectrum analysis methods have limitations when processing non-stationary signals, and the time and frequency resolution is limited. For non-periodic signals, Fourier transform cannot be processed, and the analysis frequency bandwidth of fast Fourier transform (FFT) is limited by the ADC sampling rate. Modal analysis requires a large amount of test data and complex data processing, which is challenging for spatial modal measurement and analysis of complex structures. In practical applications, the accuracy and amount of information of modal analysis need to be further improved, especially in high-frequency modal detection and anti-noise interference. At the same time, the convergence speed, time-varying system tracking ability and steady-state misalignment of the adaptive filtering algorithm are key indicators to measure its performance, and the choice of step size factor has a significant impact on the performance of the algorithm. For the RLS algorithm, although the convergence speed is fast, its computational complexity is high and the required storage volume is extremely large, which is not conducive to real-time implementation; if the inverse of the estimated autocorrelation matrix loses its positive definite property, it may also cause the algorithm to diverge.

[0008] Based on this, it is urgent to develop an analysis and fitting method for the resonance frequency of underwater targets to effectively solve the problems of large amount of calculation and insufficient accuracy of traditional resonance frequency detection. Summary of the invention

[0009] The purpose of the present invention is to provide an analysis and fitting method of the resonance frequency of underwater targets based on the principles of elastic surround waves and multipath effects, and to use surround waves generated by the interaction between sound waves and underwater targets to predict the resonance frequency of the target, so as to solve the problems of large calculation amount and insufficient accuracy of traditional resonance frequency detection. This method uses the resonance characteristics generated by the interaction between sound waves and targets, analyzes the resonance structure in the spectrum, reduces the influence of sound signal attenuation and distortion on imaging quality, and reduces the interference of multipath effects on target detection, thereby extracting the resonance frequency of the target.

[0010] The objective of the present invention is achieved through the following technical solutions:

[0011] A method for analyzing and fitting the resonance frequency of an underwater target comprises the following steps:

[0012] A. Building an ocean model:

[0013] Assume that the ocean waveguide is an ideal waveguide, the target is a sphere, the water depth is H, the water surface with the center of the transmitter vertically upward is set as the origin, and the z-axis is vertically downward as positive. At this time, the acoustic signal propagation mode is divided into direct from the transmitter to the receiver and reflected from the target to the receiver; the distance from the transmitter to the target is r1, the distance from the target to the receiver is r2, and the depth of the transmitter is z s , the target depth is z t , the receiver depth is z r ;

[0014] B. Construct the ocean sound field according to the formula:

[0015] Construct the expression of the target scattering sound field in an ideal waveguide environment, obtain the scattering function of the target, and combine different propagation characteristics to predict the scattering characteristics of regular spherical targets in the ocean channel using the scattering function;

[0016] C. Obtain the range-frequency spectrum:

[0017] Obtain the range-frequency spectrum diagram of the backscattering sound pressure of an elastic sphere varying with the receiving position when the target is located in an ideal waveguide;

[0018] D. Calculate the resonance frequencies in different propagation modes:

[0019] According to different propagation paths, when the path difference between the Franz wave and the radiated echo is an integral multiple of the signal wavelength, resonance occurs between these two signals, and the corresponding position in the frequency spectrum diagram is the trough value of the horizontal stripe; the resonance frequency, as an inherent characteristic of the target, has a low correlation with the target material properties and a significant dependence on the geometric scale of the target; thus, under different depth conditions, the trough value points of the resonance frequencies remain constant; calculate the path difference between the two echoes, and then the resonance frequency formula for this propagation mode can be obtained;

[0020] E. Fit the resonance frequencies with the range-frequency spectrum:

[0021] Calculating the resonance frequency formula gives the resonance stripes in the range-frequency spectrum and the frequency-amplitude spectra intercepted at different ranges at the same depth.

[0022] Furthermore, in step B, the sound pressure field generated at the receiving end is the superposition of the incident sound field at the transmitting end and the target radiation sound field. The scattered sound pressure at the receiving end can be expressed as the result of the superposition of different normal modes; due to the ocean waveguide model, assuming the water body is an isovelocity layer, the sea surface is a pressure-release interface, and the seabed is an ideal rigid seabed, without considering the variation of ocean environment parameters with distance, the expression of the target scattering sound field in an ideal waveguide environment can be obtained as:

[0023]

[0024] In the formula, is the scattering function, θ m represents the incident angle of the m-th normal mode; represents the azimuth angle of the incident sound field; θ m′ represents the scattering angle of the m'-th normal mode; represents the azimuth angle of the scattered sound field; ω represents the frequency;

[0025] The overall formula can be interpreted as the radiation scattering field of a spherical center point source, whose amplitude is the product of the incident mode field at this point and the far-field scattering function. In the formula, For normalizing the sound pressure, k zn is the vertical wave number, related to the horizontal wave number k rn by the relationship:

[0026]

[0027] where,

[0028]

[0029] where, H is the seabed depth.

[0030] Further, in step C, the water area density is set to 1000 kg / m 3 , the elastic sphere target radius is 5 m, and the elastic sphere target density is 7900 kg / m 3 , the transmitting end signal frequency band is in 1000 Hz - 2000 Hz, the number of modes is taken as 40, and the target is located in an ideal waveguide at a depth of 30 m and 50 m.

[0031] Even further, in step C, when the target is located in the channel, the sound field at the underwater receiving point is generated by the superposition of the direct wave from the transmitting end and the reflected wave from the target. The direct wave and the reflected waves from the sea surface and the seabed constitute the typical interference phenomenon of the ocean channel; the underwater receiving point is composed of the superposition of various types of radiated sound waves and the direct wave, thus generating a complex interference structure in the frequency domain; in addition to the invariant interference structure, there is also a resonance structure in the channel for the elastic target; the reason for the generation of resonance fringes is that sound waves excite elastic waves in the elastic object, and the elastic waves interact with each other.

[0032] Define the sea depth as H, the transmitting and receiving field points and the target are at the same depth, and the distances from the transmitting end to the sea surface and the seabed are h1 and h2 respectively; the paths experienced by different components of sound waves mainly include: path Q0, that is, transmitting end - target - receiving point, path Q1, that is, transmitting end - target - Franz circumferential wave - receiving point, path Q2, that is, transmitting end - sea surface - target - Franz circumferential wave - receiving point, path Q3, that is, transmitting end - seabed - target - Franz circumferential wave - receiving point; path Q4, that is, transmitting end - seabed - sea surface - target - Franz circumferential wave - receiving point, and the sound wave propagation path diagram and the distances experienced by the propagation paths can be obtained.

[0033] Further, in step D, the resonance frequency formula in the propagation mode is as follows:

[0034]

[0035] where, The symbol represents rounding down, f represents the frequency of the received broadband signal, n1 represents the multiple relationship, dist lis the path difference between the propagation paths of the two echo types; it can be seen from formula (4) that the peak-to-valley value of the resonance frequency is related to dist l and different dist l have different contributions to the resonance fringes; in the above-mentioned shallow sea model, for the two types of echoes Q0 and Q1, the travel distance is the shortest, the energy attenuation is the least, and the path difference that contributes the most to the resonance fringes is the travel difference between Q0 and Q1, which is:

[0036] dist1 = Q1 - Q0 = 2a + πa (5).

[0037] Compared with the prior art, the beneficial effects of the present invention are:

[0038] 1. Reduce the influence of sound signal attenuation and distortion: The inhomogeneity of the seawater medium will cause sound signal attenuation and distortion, affecting the quality of sonar images. By analyzing the correlation between the resonance structure and the target scale and material, the present invention reduces the influence of sound signal attenuation and distortion on the imaging quality, and improves the clarity and reliability of sonar images;

[0039] 2. Enhance the stability and reliability of the underwater target resonance frequency fitting method: By simulating the elastic experiment test of the elastic target in the water tank of the ideal environment, the present invention verifies the correctness of the theoretical model and calculation formula, and enhances the stability and reliability of the underwater target resonance frequency fitting method. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required in the embodiments. It should be understood that the following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0041] Figure 1 is the geometric schematic diagram of the target in the ideal waveguide with depth H;

[0042] Figure 2 is the distance-frequency spectrum of elastic spheres at different depths, where Figure 2a is the distance-frequency spectrum of the elastic sphere at a depth of 30m, Figure 2b is the distance-frequency spectrum of the elastic sphere at a depth of 50m;

[0043] Figure 3 is the resonance fringe spectrum of different targets at different depths, where Figure 3a is the fitting diagram of the resonance fringes at different distances and the distance-frequency spectrum for the elastic sphere at a depth of 30m, Figure 3b is the fitting diagram of the resonance fringes at different distances and the distance-frequency spectrum for the elastic sphere at a depth of 50m;

[0044] Figure 4 It is a flowchart of steps for the analysis of the resonance frequency of underwater targets and its fitting method. Specific implementation mode

[0045] The present invention will be further described below in conjunction with embodiments:

[0046] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It can be understood that the specific embodiments described herein are only used to explain the present invention, rather than limiting the present invention. In addition, it should be noted that, for the sake of description, only parts related to the present invention rather than all structures are shown in the drawings.

[0047] It should be noted that: similar reference numerals and letters represent similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings. At the same time, in the description of the present invention, terms such as "first" and "second" are only used for distinguishing descriptions, and cannot be understood as indicating or implying relative importance.

[0048] The present invention provides a method for detecting and identifying underwater targets based on the analysis of acoustic resonance characteristics, which solves the limitations of the prior art in underwater imaging and sonar detection, and provides a new technical means for the detection and identification of underwater targets. Specifically, due to the color deviation problem caused by water absorption and scattering in the underwater environment, the image enhancement effect of traditional optical imaging technology is limited. The present invention aims at the color deviation correction problem and provides a color deviation correction mechanism by analyzing the resonance characteristics generated by the interaction between sound waves and targets, so as to improve the color accuracy of underwater images.

[0049] Aiming at the problem that the detection performance of small target objects by traditional optical imaging technology in the underwater environment is poor, the present invention will utilize the resonance characteristics of sound waves to enhance the detection ability of small targets and improve the identifiability of targets. In sonar technology, the non-uniformity of the seawater medium will cause attenuation and distortion of sound signals, affecting the quality of sonar images. The present invention aims at the problem of reducing sound signal attenuation and distortion, and reduces the influence of sound signal attenuation and distortion on imaging quality by analyzing the correlation between the resonance structure and the target scale and material. The multipath effect in the process of sound wave transmission increases the difficulty of target detection in sonar images. The present invention aims at the problem of reducing the multipath effect, and reduces the interference of the multipath effect on target detection and improves the target detection accuracy of sonar images by extracting resonance peak data and combining the radial distance and frequency of the hydrophone.

[0050] The method for analyzing the resonance frequency of underwater targets of the present invention and its fitting method include the following steps:

[0051] 1. Construct an ocean model:

[0052] Considering the ocean waveguide as an ideal waveguide, with the target being a sphere, the sound speed and density of the water layer medium are uniform and unchanged at this time. The water depth is H. Taking the water surface directly above the center of the transmitting end as the origin, with the z-axis perpendicular downward as positive, the sound signal propagation modes at this time are divided into direct arrival from the transmitting end to the receiving end and reflection from the target to the receiving end. The distance from the transmitting end to the target is r1, the distance from the target to the receiving end is r2, the depth of the transmitting end is z s , the depth of the target is z t , and the depth of the receiver is z r , as Figure 1 shown.

[0053] 2. Construct the ocean acoustic field according to the formula: At this time, the sound pressure field generated at the receiving end is the superposition of the incident acoustic field at the transmitting end and the radiation acoustic field of the target. The scattered sound pressure at the receiving end can be expressed as the result of the superposition of different normal modes. Considering a simple ocean waveguide model, with the water body being an isosound speed layer, the sea surface being a pressure release interface, and the seabed being an ideal rigid seabed, without considering the variation of ocean environmental parameters with distance, the expression of the target scattering acoustic field in an ideal waveguide environment can be obtained as follows:

[0054]

[0055] In formula (1) is the scattering function,

[0056] θ m : represents the incident angle of the m-th normal mode.

[0057] represents the azimuth angle of the incident acoustic field.

[0058] θ m′ : represents the scattering angle of the m'-th normal mode.

[0059] represents the azimuth angle of the scattered acoustic field.

[0060] ω: represents the frequency

[0061] The overall formula can be interpreted as the radiation scattering field of a point source at the center of the sphere, whose amplitude is the product of the incident mode field at this point and the far-field scattering function. In the formula, is used to normalize the sound pressure, k zn is the vertical wave number, and the relationship with the horizontal wave number k rn is as follows:

[0062]

[0063] where,

[0064]

[0065] Among them, H is the seabed depth. It can be seen from formula (1) that as long as the scattering function of the target can be obtained and combined with different propagation characteristics, the scattering characteristics of regular spherical targets in the ocean channel can be predicted using the scattering function.

[0066] 3. Obtain the range-frequency spectrum: Assume the water density is 1000 kg / m 3 , the radius of the elastic spherical target is 5 m, and the density of the elastic spherical target is 7900 kg / m 3 . The signal frequency band at the transmitting end is 1000 Hz - 2000 Hz, and the number of modes is taken as 40. Figure 2 shows the range-frequency spectrum diagram of the backscattered sound pressure of the elastic sphere changing with the receiving position when the target is located in an ideal waveguide at a depth of 30 m and 50 m.

[0067] When the target is located in the channel, the sound field at the underwater receiving point is generated by the superposition of the direct wave from the transmitting end and the reflected wave from the target. The direct wave and the reflected waves from the sea surface and the seabed constitute the typical interference phenomenon in the ocean channel. As mentioned above, the underwater receiving point is composed of the superposition of various types of radiated sound waves and the direct wave, thus generating a complex interference structure in the frequency domain. In addition to the invariant interference structure, there is also a resonance structure for elastic targets in the channel. The reason for the generation of resonance fringes is that sound waves excite elastic waves in elastic objects, and the elastic waves interact with each other. In the Figure 1 layout, define the sea depth as H. The transmitting, receiving field points, and the target are located at the same depth. The distances from the transmitting end to the sea surface and the seabed are h1 and h2 respectively. The paths experienced by sound waves of different components mainly include: path Q0 (transmitting end - target - specular reflection), path Q1 (transmitting end - target - Franz circumferential wave), path Q2 (transmitting end - sea surface - target - Franz circumferential wave), path Q3 (transmitting end - seabed - target - Franz circumferential wave); path Q4 (transmitting end - seabed - sea surface - target - Franz circumferential wave). The distances experienced by the propagation paths are shown in Table 1:

[0068] Table 1 Acoustic wave types of elastic targets and their propagation paths

[0069]

[0070] 4. Calculate the resonance frequencies in different propagation modes:

[0071] According to different propagation paths in Table 1, when the path difference between the Franz wave and the radiated echo is an integer multiple of the signal wavelength, resonance occurs between these two signals, and the corresponding position in the spectrogram is the trough of the horizontal stripe. As an inherent characteristic of the target, the resonance frequency has a low correlation with the target material properties and a significant dependence on the geometric scale of the target. Thus, under different depth conditions, the trough points of the resonance frequency remain constant, which reflects the dominant role of the target size in its resonance frequency. By calculating the path difference between the two echoes, the resonance frequency formula for this propagation mode can be obtained.

[0072]

[0073] Among them, The symbol represents rounding down, f represents the frequency of the received broadband signal, n1 represents the multiple relationship, and dist l is the path difference between the propagation paths of the two echo types. It can be seen from formula (4) that the peak and trough values of the resonance frequency are related to dist l , and different dist l contribute differently to the resonance stripes. In the above-mentioned shallow sea model, for the Q0 and Q1 types of echoes, the travel distance is the shortest and the energy attenuation is the least. Therefore, the path difference that contributes the most to the resonance stripes is the travel difference between Q0 and Q1, which is:

[0074] dist1 = Q1 - Q0 = 2a + πa (5)

[0075] 5. Fitting of resonance frequency with distance-frequency spectrum: Calculating the resonance frequency formula gives the resonance stripes in the distance-frequency spectrum and the frequency-amplitude spectra intercepted at different distances at the same depth, as shown in Figure 3. When estimating the size of the elastic sphere, the formula used shows high accuracy and applicability under different depth conditions. Specifically, when the elastic sphere is at a depth of 30m ( Figure 3a ) and 50m ( Figure 3b ), the estimated values obtained by the size estimation formula are all within the error tolerance range of the model radius. This result shows that despite the change in depth conditions, the formula can still effectively identify and accurately estimate the size of the sphere, demonstrating consistent estimation performance. Therefore, it can be confirmed that the size estimation formula has high reliability and wide applicability for estimating the size of the elastic sphere under different depth conditions.

[0076] Note that the above is only a preferred embodiment of the present invention and the technical principles applied. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein. Various obvious changes, re-adjustments, and substitutions can be made by those skilled in the art without departing from the protection scope of the present invention. Therefore, although the present invention has been described in more detail through the above embodiments, the present invention is not limited to the above embodiments. Without departing from the concept of the present invention, more other equivalent embodiments can be included, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. An analysis and fitting method of underwater target resonance frequency, characterized in that: The following steps are involved: A. Building an ocean model: Assume that the ocean waveguide is an ideal waveguide, the target is a sphere, the water depth is H, the water surface with the center of the transmitter vertically upward is set as the origin, and the z-axis is vertically downward as positive. At this time, the acoustic signal propagation mode is divided into direct from the transmitter to the receiver and reflected from the target to the receiver; the distance from the transmitter to the target is r1, the distance from the target to the receiver is r2, and the depth of the transmitter is z s , the target depth is z t , the receiver depth is z r ; B. Construct the ocean sound field according to the formula: Construct an expression of the target scattered sound field in an ideal waveguide environment, obtain the target scattering function, combine different propagation characteristics, and use the scattering function to predict the scattering characteristics of regular spherical targets in ocean channels; C. Obtain the distance-frequency spectrum: The target is located in an ideal waveguide, and the distance-frequency spectrum of the backscattered sound pressure of the elastic ball changes with the receiving position; D. Calculate the resonant frequency under different propagation modes: According to different propagation paths, when the distance difference between the Franz wave and the radiation echo is an integer multiple of the signal wavelength, the two signals resonate with each other, and the corresponding position in the spectrum diagram is the horizontal stripe valley value; the resonance frequency is an inherent characteristic of the target, and has a low correlation with the target material properties, but has a significant dependence on the geometric scale of the target; Therefore, under different depth conditions, the valley points of the resonance frequency remain constant; the path difference between the two echoes is calculated, and then the resonance frequency formula under this propagation mode can be obtained; E. Resonance frequency and distance-frequency spectrum fitting: The resonance frequency formula can be used to calculate the resonance fringes in the distance-frequency spectrum and the frequency-amplitude spectrum intercepted at different distances at the same depth.

2. The method for analyzing and fitting the resonance frequency of an underwater target according to claim 1, characterized in that: Step B, the sound pressure field generated at the receiving end is the superposition of the incident sound field at the transmitting end and the target radiated sound field. The scattered sound pressure at the receiving end can be expressed as the result of the superposition of different simple normal waves. Due to the ocean waveguide model, the water body is an isonic layer, the sea surface is a pressure release interface, and the seabed is an ideal rigid seabed. Without considering the change of ocean environmental parameters with distance, the expression of the target scattered sound field under the ideal waveguide environment can be obtained as follows: In the formula, is the scattering function, θ m represents the incident angle of the mth simple normal wave; Represents the azimuth of the incident sound field; θ m′ represents the scattering angle of the m′th normal wave; represents the azimuth of the scattered sound field; ω represents the frequency; The overall formula can be interpreted as the radiation scattering field of the point source at the center of the sphere, whose amplitude is the product of the incident mode field at that point and the far-field scattering function, where For normalized sound pressure, k zn is the vertical wave number, which is related to the horizontal wave number k rn The relationship between them is: in, Where H is the depth of the seafloor.

3. The method for analyzing and fitting the resonance frequency of an underwater target according to claim 1, characterized in that: Step C: Set the water density to 1000kg / m 3 , the radius of the elastic ball target is 5m, and the density of the elastic ball target is 7900kg / m 3 , the signal frequency band of the transmitting end is 1000Hz-2000Hz, the mode number is 40, and the target is located in an ideal waveguide at a depth of 30m and 50m.

4. The method for analyzing and fitting the resonance frequency of an underwater target according to claim 3, characterized in that: Step C, when the target is located in the channel, the sound field of the underwater receiving point is generated by the superposition of the direct wave from the transmitter and the reflected wave of the target, and the direct wave and the reflected wave from the sea surface and the seabed constitute a typical interference phenomenon of the ocean channel; the underwater receiving point is composed of multiple types of radiated sound waves and direct waves superimposed, thereby generating a complex interference structure in the frequency domain; in addition to the invariant interference structure, the elastic target also has a resonance structure in the channel; the resonance fringes are generated because the sound wave excites the elastic wave in the elastic object, and the elastic waves interact with each other; The sea depth is defined as H, the transmitting and receiving points and the target are located at the same depth, and the transmitting distances from the sea surface and the seabed are h1 and h2 respectively; the paths experienced by the sound waves of different components mainly include: path Q0, i.e. transmitting end-target-receiving point, path Q1, i.e. transmitting end-target-Franz surround wave-receiving point, path Q2, i.e. transmitting end-sea surface-target-Franz surround wave-receiving point, path Q3, i.e. transmitting end-seabed-target-Franz surround wave-receiving point; path Q4, i.e. transmitting end-seabed-sea surface-target-Franz surround wave-receiving point, the sound wave propagation path diagram and the distance experienced by the propagation path can be obtained.

5. The method for analyzing and fitting the resonance frequency of an underwater target according to claim 1, characterized in that: Step D, the formula for the resonant frequency in the propagation mode is as follows: in, The symbol represents rounding down, f represents the received broadband signal frequency, n1 represents the multiple relationship, and dist l is the distance difference between the two echo types; From formula (4), it can be seen that the peak and valley values ​​of the resonance frequency and dist l Related, different dist l The contribution to the resonance fringes is different; in the above shallow sea model, the two types of echoes, Q0 and Q1, have the shortest travel and the least energy attenuation. The distance difference that contributes the most to the resonance fringes is the distance difference between Q0 and Q1, which is: dist1=Q1-Q0=2a+πa (5).

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