A numerical simulation method of broadband ambient noise in horizontally layered medium

Abstract

The present application relates to a kind of wideband marine ambient noise numerical simulation methods in horizontal layered medium, comprising the following steps, step one: according to the environmental information of target sea area, set marine environmental parameter, calculate the spatial correlation function of marine ambient noise in horizontal layered medium;Step two: using the sea surface noise source level empirical formula of wind-generated noise and the spatial correlation function of wind-generated noise to calculate noise intensity;Step three: the spatial correlation function of marine ambient noise is normalized, obtains the spatial correlation coefficient of marine ambient noise.The present application can effectively solve the problem of marine ambient noise numerical simulation in horizontal layered medium, and provide more real environmental noise for sonar system design and simulation.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of underwater acoustic signal processing, and particularly relates to a numerical simulation method for wideband ocean ambient noise in horizontal layered medium. BACKGROUND

[0002] Ocean ambient noise is an interference background field in the underwater acoustic channel, and any sonar system will be affected by ocean ambient noise in the process of design and use. Therefore, fully understanding the characteristics of the ocean ambient noise field is very important for the design of a hydrophone array and the improvement of the performance of the array. In the design of underwater acoustic array signal processing methods, it is necessary to generate a simulated noise field to verify the effectiveness of the method, but at present, although people have conducted quite in-depth research on the spatial characteristics of the ocean ambient noise field, various noise models have been proposed, including volume noise models and surface noise models, K / I models considering the influence of seawater sound velocity distribution, seabed acoustic characteristics and other factors, and ray models, but in actual application, in order to more realistically simulate the actual ocean environment, it is necessary to construct a simulated noise time sequence that meets part of the characteristics of the ocean ambient noise.

[0003] The existing numerical simulation method for ocean ambient noise field of underwater acoustic array is mainly to construct an ocean ambient noise time sequence by superimposing independent noise sources, and to realize high-precision calculation of the arrival time delay of different noise sources by designing an accurate time delay filter and a FIR digital filter. The above method is only applicable to the numerical simulation of ocean ambient noise in a homogeneous medium, but in reality, the homogeneous medium environment is too ideal, and the constructed noise time sequence also has differences with the actual result. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a numerical simulation method for wideband ocean ambient noise in horizontal layered medium, which can effectively solve the problem of numerical simulation of ocean ambient noise in horizontal layered medium and provide a more realistic environmental noise for sonar system design and simulation.

[0005] The technical solution of the present application is to provide a numerical simulation method for wideband ocean ambient noise in horizontal layered medium, comprising the following steps,

[0006] Step 1: According to the environmental information of the target sea area, set the ocean environmental parameters, and calculate the spatial correlation function of the ocean ambient noise in the horizontal layered medium;

[0007] Step 2: Calculate the noise intensity using the sea surface noise source level empirical formula of wind-generated noise and the spatial correlation function of wind-generated noise;

[0008] Step 3: Normalize the spatial correlation function of the ocean ambient noise to obtain the spatial correlation coefficient of the ocean ambient noise, and construct the ocean ambient noise covariance matrix C m×m;

[0009] Step four: generate m-dimensional uncorrelated standard normal distribution random sampling sequence, m represents the mth array element, the rows of the sampling sequence are uncorrelated with each other, each row is a random complex number vector, corresponding to l frequency domain shots, and the root mean square value is 1, which is represented by a matrix as follows: X m×l =(X1; X2; …; X m );

[0010] Step five: singular value decomposition is performed on the covariance matrix C m×m to obtain the eigenvalue diagonal matrix Λ and the eigenvector matrix V;

[0011] Step six: linear transformation is performed on X m×l to obtain the frequency domain noise sequence Y m×m with the correlation characteristics of the covariance matrix C m×l ;

[0012] Step seven: inverse Fourier transform is performed on each frequency domain shot of Y m×l , Hanning window is used to weight the time sequence calculated for a single frequency domain shot, different shots of noise time sequence are superimposed using the overlap-add method with 50% overlap rate to obtain a single frequency noise time sequence of a single array element, and the noise amplitude is corrected according to the noise intensity calculation result;

[0013] Step eight: directly superimpose the different frequency single frequency noise time sequences in the frequency band to obtain a wideband noise time sequence as the result output.

[0014] As preferred, in step one, the surface noise spatial correlation function formula in the horizontally layered medium is

[0015]

[0016] where θ s is the outgoing grazing angle of the sound ray at the sea surface, θ r is the receiving grazing angle of the sound ray reaching the hydrophone, γ represents the pitch angle of the line connecting the two hydrophones, d is the hydrophone spacing, which is assumed to satisfy the far-field condition, m is the noise source directivity index, θ0 is the arrival grazing angle of the sound ray horizontally emitted from the depth of the noise source to the receiving point, R s , R b represent the sound intensity reflection coefficients of the sea surface and the sea bottom respectively, θ b is the sea bottom grazing angle. a represents the absorption coefficient in seawater, which changes with the frequency of the sound wave, S c represents the length of a complete span of the sound ray propagation, and S p is the length of the partial span of the sound ray directly reaching the receiving point from the sea surface.

[0017] As preferred, in step two, the wind-generated sea surface noise source level SLW is directly related to the wind speed, and can be expressed according to an empirical formula as

[0018] SLW = 55 - 6lg[(f / 400) 2 + 1] + (18 + v2.06)lg(v5.14)

[0019] According to the ray theory, the noise intensity NL at a receiving point in a horizontally layered medium can be expressed as

[0020]

[0021] As preferred, in step three, C m×m is a symmetric matrix, wherein each element is a spatial correlation coefficient of the received noise corresponding to a hydrophone pair.

[0022] Compared with the prior art, the present application has the following advantages:

[0023] The present application aims at the numerical simulation of ocean ambient noise in a horizontally layered medium, and utilizes a ray theory ocean ambient noise model to obtain spatial correlation coefficients of the ocean ambient noise field. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a flow chart for implementing the present application.

[0025] Figure 2 is a simulated sound velocity profile.

[0026] Figure 3 is a spatial correlation coefficient of an embodiment of the present application.

[0027] Figure 4 is a simulation result of 300Hz ocean ambient noise of an embodiment of the present application.

[0028] Figure 5 is a simulation result of 400Hz ocean ambient noise of an embodiment of the present application.

[0029] Figure 6 is a simulation result of 500Hz ocean ambient noise of an embodiment of the present application.

[0030] Figure 7 is a simulation result of 300Hz-500Hz wideband ocean ambient noise of an embodiment of the present application. DETAILED DESCRIPTION

[0031] The application will be further described in the specific embodiments in combination with the drawings:

[0032] In order to obtain a noise time sequence more in line with the actual marine environment, the application proposes a numerical simulation method for wideband marine environmental noise in horizontal layered medium based on the ray theory, which uses the ray model to calculate the marine environmental noise spatial correlation coefficient and the marine environmental noise intensity, and generates the marine environmental noise time sequence based on the simulation.

[0033] As shown in the specific implementation process as follows: Figure 1

[0034] 1) According to the environmental information of the target sea area, set the marine environmental parameters, and calculate the marine environmental noise spatial correlation function in the horizontal layered medium.

[0035] Assuming that the noise sources are uniformly distributed on an infinite surface, and the noise sources are not correlated in space, according to the ray theory, the surface noise spatial correlation function formula in the horizontal layered medium can be obtained as

[0036]

[0037] Where θ s is the outgoing grazing angle of the sound ray at the sea surface, θ r is the receiving grazing angle of the sound ray reaching the hydrophone, γ represents the pitch angle of the line connecting the two hydrophones, d is the hydrophone spacing, which is assumed to satisfy the far-field condition in the application, m is the directivity index of the noise source, which is taken as 1 in the application, θ0 is the arrival grazing angle of the sound ray horizontally emitted from the depth of the noise source to the receiving point, R s , R b represent the sound intensity reflection coefficients of the sea surface and the seabed respectively, θ b is the seabed grazing angle. a represents the absorption coefficient in seawater, which changes with the frequency of the sound wave, S c represents the length of the sound ray propagation for one complete span, S p is the partial span length of the sound ray directly reaching the receiving point from the sea surface, S c , S p , θ r are calculated by the Bellhop model.

[0038] 2) Calculate the noise intensity.

[0039] The wind-generated sea surface noise source level SLW is directly related to the wind speed, which can be expressed according to the empirical formula as

[0040] SLW = 55-6lg[(f / 400) 2 +1]+(18+v2.06)lg(v5.14)

[0041] ​According to the ray theory, the noise intensity NL at the receiving point in the horizontally layered medium can be expressed as

[0042]

[0043] 3) Constructing the ocean ambient noise covariance matrix.

[0044] Suppose that the spatial correlation function between the ocean ambient noise received by the ith array element and the jth array element is ρ(d ij ), and the spatial correlation coefficient c ij between the ocean ambient noise received by the ith array element and the jth array element is obtained by normalizing ρ(d ij ):

[0045]

[0046] After the above processing is performed on all the hydrophone pairs, the ocean ambient noise covariance matrix C m×m is constructed:

[0047]

[0048] The elements in the matrix C m×m are the spatial correlation coefficients of the noise received by the corresponding hydrophone pairs, and the matrix C m×m is a symmetric matrix in the present application.

[0049] 4) Generating an m-dimensional uncorrelated standard normal distribution random sampling sequence, m representing the mth array element, the rows of the sampling sequence being uncorrelated with each other, each row being a random complex number vector corresponding to l frequency domain shots, and the matrix being represented as: X m×l =(X1; X2; …; X m ), and the root mean square value of each dimension of the random complex number vector being 1 in the present application.

[0050] 5) Singular value decomposition is performed on the covariance matrix C m×m ,

[0051] C m×m =VΛV T

[0052] The eigenvalue diagonal matrix Λ and the eigenvector matrix V are obtained.

[0053] 6) Linear transformation is performed on the uncorrelated random sampling sequence X m×l , and the linear transformation process is as follows:

[0054] Y m×l =VΛ 12 X m×l

[0055] Y m×l obtained is the noise with the covariance matrix C m×mFrequency domain noise sequence with relevant characteristics, Λ 12 It is a diagonal matrix whose elements are the square roots of the eigenvalues.

[0056] 7) Regarding Y m×l Each frequency domain snapshot undergoes an inverse Fourier transform. The time series calculated by a single frequency domain snapshot is weighted using a Hanning window. The noise time series of different snapshots are superimposed with a 50% overlap rate using an overlapping summation method to obtain the single-frequency noise time series of a single array element. The noise amplitude is then corrected based on the noise intensity calculation results.

[0057] 8) The broadband noise time series is obtained by directly superimposing the time series of single-frequency noise at different frequencies within the frequency band, and the result is output as a reliable and reasonable marine environmental noise data for the design and simulation of sonar systems.

[0058] Simulation and sea trial data test results

[0059] Based on the above implementation process, the processing results of this invention are given through simulation. The simulation processing is as follows: Figures 3-7 As shown.

[0060] Computer Simulation 1: The spatial correlation coefficient of marine environmental noise was calculated based on the ray-based marine environmental noise model. The simulation parameters are as follows: sea depth 200m, sound velocity profile as shown. Figure 2 As shown. Seawater density is 1024 kg / m³. 3 The medium attenuation coefficient in seawater is obtained using Thorp's empirical formula, and the sea surface is considered an absolutely soft plane. The seabed is a homogeneous liquid half-space, and the seabed sediment is silty sand. The sound velocity and density on the seabed are 1617 m / s and 1800 kg / m³, respectively. 3 The longitudinal wave attenuation coefficient is determined by the frequency from (0.25±0.01)f (2.16 ±0.04) The noise source was located on an infinitely large plane at a depth of 0.1m below the sea surface, and the receiving point was at a water depth of 40m. The coordinates and initial grazing angles of each sound ray in the sound field were calculated using the Bellhop model. The arrival grazing angle at the receiving position was calculated using Snell's law, and S was calculated using the sound ray trajectory. c S p The length of the noise. Under the above environmental parameters, the vertical spatial correlation coefficients for noise frequencies of 300Hz, 400Hz, and 500Hz. (From...) Figure 3 It can be seen that the spatial correlation coefficient of marine environmental noise oscillates with frequency. The correlation radius and oscillation amplitude of the noise spatial correlation coefficients obtained at different frequencies are all different. The reason for this phenomenon is not only the difference between wavelengths, but also the different environmental parameters corresponding to different frequencies.

[0061] Computer simulation 2: According to the noise spatial correlation coefficient results given by the ocean ambient noise model, the ocean ambient noise time series of each frequency can be simulated. The array element spacing is 0.5 m, the number of array elements is 50, the target frequency band center frequency is 400 Hz, and the bandwidth is 200 Hz. The sea surface wind speed is 5 m / s, which is used to calculate the ocean ambient noise level. A total of 500 frequency domain shots are set, and the time series length corresponding to each frequency domain shot after inverse Fourier transform is 0.1 s, and the sampling rate is 5000 Hz. The obtained noise time series and spatial correlation coefficients of three frequencies of 300 Hz, 400 Hz and 500 Hz are compared with the model calculation results as shown in Figures 4-6 , wherein, Figures 4-6 The left (a) figure is the time domain noise data construction result, and the right (b) figure is the correlation coefficient. The time domain noise data gives the time domain noise received by three different array elements with a time length of 20 s. It can be seen from Figures 4-6 that the generated spatial correlation coefficients of each frequency noise are in good agreement with the model calculation results, which shows that the generated ocean ambient noise time series meets the preset spatial correlation characteristics, and proves the effectiveness of the method.

[0062] Computer simulation 3: The time series of single frequency noise at different frequencies in the frequency band is directly superimposed, and the wideband noise time series is obtained. The wideband noise time series received by three different array elements with a bandwidth of 300-500 Hz is shown in Figure 7 (a), and the time length is 20 s. The corresponding noise spectrum level is shown in Figure 7 (b), and the obtained noise spectrum level is basically consistent with the set wind noise empirical value, and the constructed ocean ambient noise sequence meets the requirements.

[0063] The simulation analysis results show that the wideband ocean ambient noise numerical simulation method in the horizontal layered medium can realize the construction of ocean ambient noise time series for the real ocean waveguide environment, and the constructed ocean ambient noise time series is consistent with the actual ocean ambient noise in terms of spatial correlation characteristics and spectrum level characteristics. The method provides a reasonable and real background noise for sonar system design and simulation, and has good application prospect.

[0064] The method utilizes a ray ocean environmental noise model and a wind-generated ocean environmental noise source level empirical formula to obtain ocean environmental noise spatial correlation coefficients and noise spectrum levels and the like, and then utilizes singular value decomposition and inverse Fourier transform to construct a series of single-frequency noise time series with prescribed correlation and intensity, and finally directly superimposes the single-frequency noise time series of each frequency to obtain a shallow sea broadband ocean environmental noise time series. Simulation results show that the ocean environmental noise time series constructed by the application meets requirements in terms of spatial correlation characteristics and spectrum level characteristics, proving that the method can realize generation of broadband ocean environmental noise time series in a horizontally layered medium, and provides effective ocean environmental noise data for sonar system design and simulation.

[0065] The above only describes the preferred embodiments of the present application, but should not be understood as limiting the claims. Any equivalent process transformation made by using the present application specification is included in the patent protection scope of the present application.

Claims

1. A method for numerical modeling of broadband ambient noise in horizontally stratified media, characterized in that: The method comprises the following steps, Step one: setting marine environmental parameters according to environmental information of a target sea area, and calculating a spatial correlation function of marine environmental noise in a horizontally layered medium; Step two: calculating noise intensity by using an empirical formula of a sea surface noise source level of wind-generated noise and a spatial correlation function of wind-generated noise; Step three: normalize the marine ambient noise spatial correlation function to obtain the marine ambient noise spatial correlation coefficient and construct the marine ambient noise covariance matrix C m×m ; Step four: generate m-dimensional uncorrelated standard normal distribution random sampling sequence, m represents the mth array element, the rows of the sampling sequence are uncorrelated with each other, each row is a random complex number vector, corresponding to l frequency domain shots, and the root mean square value is 1, which is expressed by a matrix as follows: X m×l = (X1; X2;...; X m ); Step five: Singular Value Decomposition of the Covariance Matrix C m×m yields the eigenvalue diagonal matrix Λ and the eigenvector matrix V. Step six: Perform a linear transformation on X using the eigenvalue diagonal matrix Λ and the eigenvector matrix V to obtain a frequency domain noise sequence Y with covariance properties m×l m×m m×l m×l 1 / 2 m×l ;​​​​​ Step seven: inverse Fourier transform is performed on each frequency domain snapshot of Y m×l , the time series calculated for a single frequency domain snapshot is weighted using a Hanning window, different snapshots of the noise time series are superimposed using an overlap-add method with an overlap rate of 50% to obtain a single-frequency noise time series of a single array element, and the noise amplitude is corrected according to the noise intensity calculation result; Step eight: directly superimposing time series of different frequency single-frequency noises in a frequency band to obtain time series of wideband noise as a result output; In step one, a spatial correlation function formula of surface noise in the horizontally layered medium is where θ s is the outgoing grazing angle of the sound ray at the sea surface, θ r is the receiving grazing angle of the sound ray at the hydrophone, γ represents the pitch angle of the line connecting the two hydrophones, d is the distance between the hydrophones, m is the directivity index of the noise source, θ0is the arriving grazing angle of the sound ray from the level of the noise source to the receiving point, R s , R b represent the sound intensity reflection coefficients of the sea surface and the sea bottom respectively, θ b is the grazing angle at the sea bottom, a represents the absorption coefficient in the seawater, which varies with the frequency of the sound wave, S c represents the length of a complete span of the sound ray, S p is the length of the partial span of the sound ray from the sea surface to the receiving point.

2. The horizontally stratified medium wideband numerical simulation method of ocean ambient noise according to claim 1, characterized in that: In step two, the wind-generated sea surface noise source level SLW is directly related to wind speed, and can be expressed according to an empirical formula SLW = 55 - 6lg[(f / 400) 2 + 1] + (18 + v / 2.06)lg(v / 5.14) According to the ray theory, noise intensity NL at a receiving point in the horizontally layered medium is expressed as 3. The horizontally layered medium wideband numerical simulation of ocean ambient noise method of claim 1, wherein: In step three, C m×m is a symmetric matrix, where each element is the spatial correlation coefficient of the noise received by the corresponding hydrophone pair.

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