An indirect-coherent measurement method of the target strength of underwater objects by forward acoustic scattering

Abstract

The present application belongs to the technical field of underwater acoustic measurement, and relates to an indirect-coherent measurement method of forward acoustic scattering target strength of an underwater object. The method comprises: respectively performing background field measurement when the underwater object does not exist and target field measurement when the underwater object exists, and storing the received signals of an observation channel and a reference channel; based on the measurement signals of the background field and the target field, the forward acoustic scattering signal of the underwater object is extracted through time delay estimation and signal coherent subtraction processing; and based on sound propagation loss estimation and signal power, the forward acoustic scattering target strength of the underwater object is calculated by using the extracted forward acoustic scattering signal. After the forward acoustic scattering wave signal of the object is extracted, the definition formula of the target strength in the sonar equation is directly used to calculate the forward acoustic scattering target strength, so that the calculation links are less and the calculation efficiency is high.

Description

Technical Field

[0001] This invention belongs to the field of underwater acoustic measurement technology and relates to an indirect-coherent measurement method for the intensity of forward acoustic scattering from an underwater object. Background Technology

[0002] Forward scattering is considered one of the four basic detection modes of sonar. It mainly utilizes the forward scattered waves (also known as diffracted waves) formed by underwater objects blocking incident sound waves to detect and identify underwater objects intruding into the bistatic transceiver line. Although theoretical analysis and numerical calculations show that the intensity of forward-scattering targets is greater than that of backscattering and other side-scattering targets, and preliminary laboratory measurements have been obtained, in practical applications, forward-scattered waves are interfered with by direct-transmitting waves from the sound source. In forward scattering detection mode, because the forward-scattered waves and direct-transmitting waves arrive at the receiver almost simultaneously, coupled with the multipath effect of the ocean channel, it is almost impossible to directly separate these two sound waves using arrival delays. Furthermore, since the intensity of the forward-scattered waves is much smaller than that of the direct-transmitting waves, after interference and superposition at the receiver, the forward-scattered waves are almost completely masked by the interference of the direct-transmitting waves. Accurately separating forward-scattered acoustic waves from a background of strong direct-transmission interference, or effectively suppressing strong direct-transmission interference, is the basis for measuring the intensity of forward-scattering targets. This has significant academic and practical value for further verifying from an experimental measurement perspective that "the intensity of forward-scattering targets is greater than that of back-scattering and other side-scattering targets".

[0003] Publicly available references and technical data indicate that there are two different methods for measuring the forward scattering intensity of underwater objects in the laboratory. The first can be called the "direct-coherent" method, and the second can be called the "direct-incoherent" method. Both methods directly process and analyze the signal from the forward scattering receiving channel (or observation channel) with and without a scattering object to obtain the forward acoustic scattering signal of the object, but the signal processing approaches differ somewhat.

[0004] The key to the "direct-coherent" method is achieving clock synchronization between the transmitter and receiver through hardware triggering. This allows for accurate measurement of the time it takes for the sound wave to travel from the transmitter to the receiver during signal acquisition and recording, essentially measuring the absolute time delay relative to the transmitter. The significant advantage of the "direct-coherent" method is its ability to accurately measure the absolute transmission delay of the sound signal; the scattered signal from the target can be obtained by directly subtracting the recorded signals from those with and without a target. However, its disadvantages include high requirements for the transmitter-acquisition hardware system, and the fact that calculating target intensity using the far-field morphology function of the scattering is not intuitive and introduces additional calculation errors. When the hardware conditions of the measurement experiment cannot guarantee clock synchronization between the transmitter and receiver, an approximate method—the "direct-incoherent" method—can be used. Similar to the "direct-coherent" method, this method also measures the observation channel signal when an underwater object is present. and the observation channel signal when "no underwater object exists" Because clock synchronization between the transmitting and receiving ends cannot be guaranteed, the forward scattered waves of the object... It cannot be obtained directly by subtracting the two signals. An alternative approach is to calculate... and The cross-correlation function is calculated, and the time delay difference (or relative time delay) between the two acoustic signals arriving at the receiver is extracted based on the location of the maximum value of the cross-correlation function. Based on this relative time delay, the two signals are "aligned" in time and then subtracted to obtain the difference signal. That is, the forward acoustic scattering signal considered as the target (See Figure 1 The corresponding formula is or The subsequent steps are similar to those of the "direct-coherent" method, and are also handled by... Calculate the far-field scattering morphology function of the target Substituting the relationship between target intensity and far-field scattering morphology function, the forward acoustic scattering target intensity of the object is calculated.

[0005] When the amplitude of the forward scattered wave is much smaller than that of the direct transmitted wave, the "direct-incoherent" method can achieve good approximation results; however, as the amplitude of the forward scattered wave increases (for example, in a typical orthorhombic illumination condition), the approximation error of this method will also increase, and the approximation effect will deteriorate. Summary of the Invention

[0006] To address the shortcomings of current laboratory methods for measuring the intensity of forward acoustic scattering targets, this invention provides an indirect-coherent method for measuring the intensity of forward acoustic scattering targets of underwater objects. This method adds a reference receiving channel to the existing measurement system configuration to provide time delay difference assistance. It can accurately obtain the phase difference between the direct-transmitting wave and the interferometric superimposed wave without requiring the transmitter-acquisition hardware system to ensure clock synchronization between the transmitter and receiver. Based on the obtained forward-scattered wave, and combined with the calculated sound velocity and propagation loss, the intensity of the forward acoustic scattering target of the underwater object is directly calculated according to the standard defined formula.

[0007] The technical solution provided by this invention is: an indirect-coherent measurement method for the intensity of forward acoustic scattering from an underwater object, comprising:

[0008] (S1) Measurement and recording steps: Perform background field measurement when there are no underwater objects and target field measurement when there are underwater objects, and record the received signals of the observation channel and the reference channel;

[0009] (S2) Forward acoustic scattering signal extraction step: Based on the measured signals of the background field and the target field, the forward acoustic scattering signal caused by the underwater object is extracted by time delay estimation and signal coherent subtraction.

[0010] (S3) Target intensity calculation steps: Based on the estimation of acoustic propagation loss and signal power, the forward acoustic scattering target intensity of the underwater object is calculated using the extracted forward acoustic scattering signal.

[0011] Preferably, step (S1) includes:

[0012] (1) Under the condition that there are no underwater objects, control the underwater sound source to emit a predetermined signal and simultaneously record the signal of the observation channel. and reference channel signal ;

[0013] (2) In the presence of an underwater object, keep the positions of the sound source and the receiving channel unchanged. After placing the object to be measured, control the sound source to emit the same signal and synchronously record the signal of the observation channel. and reference channel signal .

[0014] Preferably, step (S2) includes estimating the speed of sound and propagation loss:

[0015] (1) Calculate the arrival time delay difference based on the cross-correlation function of any two channel signals in the background field measurement. Combined with channel spacing Estimating the speed of sound The arithmetic mean of multiple measurements was taken as the sound speed measurement value. ;

[0016] (2) Based on the sound pressure power of each channel signal and the sound source level of the transmitting transducer in the background field measurement, the sound propagation loss is estimated and compared with the theoretical model. Perform least-squares fitting to determine the coefficients of sound propagation spread. .

[0017] Preferably, the extraction of the forward acoustic scattering signal in step (S2) specifically includes:

[0018] (1) Calculate the reference channel signal under the background field and the target field. and The cross-correlation function is used to determine the relative time delay difference. ;

[0019] (2) Calculate the observation channel signal under the background field With reference channel signal The cross-correlation function is used to determine the relative time delay difference. ;

[0020] (3) Refer to the aforementioned time delay difference and Signal in the target field observation channel and background field observation channel signal The signals of the same duration are extracted and denoted as follows: and ;

[0021] (4) Subtract the two time window signals to obtain the forward acoustic scattering signal: .

[0022] Preferably, step (S3) includes:

[0023] (1) Establish a coordinate system with the target location as the origin, and determine the position coordinate vectors of the sound source, observation channel, and reference channel. , , ;

[0024] (2) Based on the established laws of sound propagation, calculate the propagation loss from the sound source to the object, the propagation loss from the object to the observation channel, and the propagation loss from the sound source to the reference channel;

[0025] (3) Calculate the forward acoustic scattering signal power level And calculate the reference channel time window signal under the background field. power level .

[0026] Preferably, in step (S3), the forward acoustic scattering target intensity Calculated using the following formula:

[0027] ;

[0028] in, To observe the sensitivity of the hydrophone in the channel, The hydrophone sensitivity is used as a reference channel; , , These are the propagation losses from the sound source to the object, from the object to the observation channel, and from the sound source to the reference channel, respectively.

[0029] Compared to the "direct-coherent" method, this invention offers several advantages: simpler operation and lower hardware requirements for experimental equipment. It only requires a reference channel to indirectly achieve phase synchronization of measurement signals under both "no underwater object" and "underwater object present" conditions, without needing a hardware system to ensure clock synchronization between transmission and acquisition, thus guaranteeing signal phase synchronization. Compared to the "direct-incoherent" method, it can more accurately estimate the phase difference between the direct-transmitted wave and the superimposed interferometric wave, thereby extracting the forward acoustic scattering signal of the object, rather than the projection of the forward acoustic scattering signal onto the direction of the superimposed interferometric wave extracted by the "direct-incoherent" method. Furthermore, both the "direct-coherent" and "direct-incoherent" methods estimate the target intensity after obtaining the object's forward acoustic scattering signal using the object's far-field morphology function. In contrast, this invention directly calculates the forward acoustic scattering target intensity using the definition of target intensity in the sonar equations after extracting the object's forward acoustic scattering signal, resulting in fewer computational steps and higher computational efficiency. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the extraction of forward acoustic scattering waves from underwater objects using the existing "direct-incoherent" method.

[0031] Figure 2 A schematic diagram illustrating the "indirect-coherent" method for extracting forward acoustic scattering waves from underwater objects provided by this invention;

[0032] Figure 3 Top view of the experimental layout of the anechoic water tank for measuring the target strength of a stainless steel cylindrical shell;

[0033] Figure 4 The results of the sound velocity measurement in water during the experiment;

[0034] Figure 5 The experimentally measured sound propagation spread loss results and their fitting curves are shown.

[0035] Figure 6 Example of signal phase alignment results for the reference channel (channel 6); where, (a) signal and (b) The residual signal after the two signals are aligned;

[0036] Figure 7 Here is an example of the signal phase alignment results for the observation channel (channel 2); where (a) signal and (b) Extracted forward acoustic scattering signal ;

[0037] Figure 8 The results show the intensity measurement of the forward acoustic scattering target in channel 2. Detailed Implementation

[0038] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] In this invention, the subscript "O" represents the "observation channel", and the subscripts "A" and "P" represent the two measurement states of "no underwater object" and "underwater object present", respectively; the reference receiving channel is identified by the subscript "R".

[0040] Example 1: This invention provides an indirect-coherent measurement method for the forward acoustic scattering intensity of underwater objects. It is suitable for measuring the forward acoustic scattering intensity of underwater objects under laboratory conditions (such as an anechoic pool). After the entire measurement system is built, the technical solution can be divided into three implementation steps: measurement recording and annotation, forward acoustic scattering signal extraction, and target intensity calculation. The extraction of the forward acoustic scattering signal from the underwater object is the core of the technical solution, and the principle is as follows: Figure 2 As shown, the complete implementation steps and operation process are as follows:

[0041] 1. Measurement, recording, and labeling

[0042] (1) Measurement and recording annotation when "no underwater object exists" (background field): The underwater sound source emits a pre-set signal, and the observation channel and reference channel synchronously receive and record the signal data. The signal of the observation channel is denoted as... The reference channel signal is denoted as .

[0043] (2) Measurement and recording annotation when "underwater object exists" (target field): Keeping the positions of the underwater sound source and each receiving channel unchanged, place the object to be measured in the preset underwater position; the sound source emits the same signal as when measuring "no underwater object exists", and the observation channel and reference channel synchronously receive and record the signal data, wherein the signal of the observation channel is recorded as The signal of the reference channel is denoted as .

[0044] 2. Forward Acoustic Scattering Signal Extraction

[0045] (3) Under the condition of "no underwater object", arbitrarily select two receiving channels (channels) i and j The arrival time delay difference between the two channels is calculated using the cross-correlation function. Using the distance between the two channels Dividing by this time delay difference yields the estimated speed of sound. The estimated sound velocity values ​​were obtained from multiple measurements under different channel combinations. The arithmetic mean of the values ​​is used as the measured value of the speed of sound under experimental conditions. .

[0046] (4) Under the condition of "no underwater object", calculate the electrical power of the measurement signal of each channel, and then calculate its sound pressure power in combination with the calibration value of the sensitivity level of the corresponding hydrophone. Then, based on the sound source level of the transmitting transducer at the signal frequency, estimate the sound propagation loss in the experiment, and compare it with the actual sound pressure level of the hydrophone. ( r To determine the propagation spread law of sound (i.e., to determine the propagation spread law coefficients), a least-squares fit is performed on the distance. (value).

[0047] (5) Reference channel received signal when calculating "no underwater object". Reference channel received signal when "underwater object is present" The cross-correlation function between the two signals is used to determine the relative time delay (or time delay difference) between them by identifying the location of the maximum cross-correlation value. .

[0048] (6) Calculate the reference channel received signal under the condition of "no underwater object". Signals from the observation channel The cross-correlation function between the two variables is used to determine the relative time delay (or time delay difference) between them based on the location of the maximum cross-correlation value. .

[0049] Reference delay difference and The relative time delay difference between them is used to extract the observation channel signal under the condition of "the presence of an underwater object" using time windows of the same length. and the observation channel signal when "no underwater object exists" ,get and The difference between the two signals is the forward acoustic scattering signal of the object. .

[0050] 3. Target Intensity Measurement

[0051] (7) Establish a three-dimensional Cartesian coordinate system with the target location as the origin, and convert the measured relative distance information of the sound source, reference channel, and observation channel into position coordinates (the coordinates of the sound source, reference channel, and observation channel are successively converted into vectors). , , (Represented). Based on the sound wave propagation law calculated in step (4), calculate the sound propagation loss value from the sound source to the underwater object, the sound propagation loss value from the underwater object to the observation channel, and the sound propagation loss value between the sound source and the reference channel.

[0052] (8) Further calculate the result obtained in step (6) Signal power level and used with Equal-length time windows are used to capture the reference channel signal. get Calculate the power level of the signal. .

[0053] (9) Substitute the propagation loss value calculated in step (7), the signal power level calculated in step (8), and the hydrophone sensitivity levels of the reference channel and the observation channel into the target strength calculation formula to obtain the forward scattering target strength. :

[0054] ;

[0055] in, To observe the sensitivity of the hydrophone in the channel, The hydrophone sensitivity is used as a reference channel; , , These are the propagation losses from the sound source to the object, from the object to the observation channel, and from the sound source to the reference channel, respectively.

[0056] Example 2: This invention demonstrates a specific implementation method by combining a target strength measurement experiment of a stainless steel cylindrical shell completed in a silencing water tank. (See attached document.) Figure 3The layout of the transmitter, cylindrical shell, and receiver during the experiment is presented, and the distance parameters between the transmitter and receiver, measured with the target location as a reference point, are labeled. During the experimental measurements, the underwater depth of the transmitter, cylindrical shell, and receiver was consistently 2.0 m, and their positions remained fixed. The target rotated half a revolution around its center in the horizontal plane, which is the target's attitude angle relative to the incident wave from the sound source. ψ It is variable, and the range is... Hydrophones 1-5 measure the target intensity in the forward-scattered far field. Hydrophone 6 is the reference channel in this invention. Hydrophone 8 measures the target intensity in the forward-scattered near field. Hydrophone 7 measures the target intensity in the backscattered far field, which is unrelated to the content of this invention. The sensitivity levels of the hydrophones in all receiving channels are... Test and measurement data are available for all channels: Channels 1, 6, 7, and 8. Values ​​are -160 dB, -163 dB, -160 dB, -161 dB; channels 2-5 The values ​​were all around -190 dB. In the experiment, a spherical transducer was used as the transmitter, and the transmitted signal was a CW pulse signal with a center frequency of 30 kHz and a pulse width of 20 carrier cycles, with a repetition period of 1.0 s.

[0057] (1) When conducting the "no underwater object" measurement, the signals received and recorded by channels 1-5 are labeled as follows: , , , , The signal received and stored by channel 6 is marked as follows: The signal received and stored by channel 8 is marked as... .

[0058] (2) When conducting measurements of "the presence of underwater objects", the signals received and recorded by channels 1-5 shall be labeled as follows: , , , , The signal received and stored by channel 6 is marked as follows: .

[0059] (3) Using the measurement data when "no underwater object exists", randomly select two receiving channels (e.g., channels 1 and 6) from channels 1-5, 6 and 8, and calculate the signal. and The cross-correlation function is used to determine the arrival time delay difference between the two channels by identifying the location of the maximum cross-correlation value. Then the estimated sound velocity is the distance between the two channels. The ratio of the delay difference to the time difference, i.e. Based on this, the sound velocity was measured and estimated under other channel combinations. The arithmetic mean of the sound velocity values ​​measured under all channel combinations was taken as the measured sound velocity under the experimental conditions. The results are attached. Figure 4 As shown. The measurement results show the sound velocity value under anechoic water tank conditions. It is approximately equal to 1486 m / s, which is close to the speed of sound in fresh water during summer and is consistent with objective reality.

[0060] (4) Then, according to the appendix Figure 3 The receiver-transmitter relationship shown directly calculates the distance from channel 6 to the sound source. Distance from channel 8 to the sound source Signals received by channels 6 and 8 , Calculate the corresponding sound power values ​​respectively and By combining the receiving sensitivity values ​​of the two channels, the sound pressure level at these two distances can be obtained. and Since the lateral aperture (0.9 m) covered by channels 1-5 is much smaller than its horizontal distance to the sound source (14.1 m), the arithmetic mean of the sound pressure levels of these five channels is taken as the sound pressure level at an observation distance of 14.1 m (channel 3). This yielded the sound pressure levels at three horizontal distances of 4.43 m, 7.13 m, and 14.10 m. , and The sound source level of the transmitting transducer at 30 kHz was then determined, and the estimated sound propagation loss under the experimental conditions is shown in the attached figure. Figure 5 As shown. The measurement results show that the sound propagation loss during the experiment follows a spherical propagation law as distance increases, i.e. .

[0061] (5) Calculation and The cross-correlation function between the two signals is used to determine the relative time delay between them by identifying the location of the maximum cross-correlation value. Based on this relative time delay, the start times of the two signals in the time domain are "aligned," and the results are shown in the appendix. Figure 6 As shown in the figure, the blue curve represents the residual signal after the two signals are aligned, and its amplitude is much smaller than that of the residual signal. and The amplitude indicates that it was determined by calculating the cross-correlation function. and The relative time delay between them is guaranteed to have a certain degree of accuracy.

[0062] (6) If channel 2 is selected as the observation channel, then calculate... and The cross-correlation function between the two is used to determine the relative time delay between them based on the location of the maximum cross-correlation value. .in accordance with and The relative time delay difference between them is used to extract the observation signal when "underwater object exists" using time windows of the same length. Observation signals when "no underwater objects exist" ,get and Subtracting the two signals gives the forward acoustic scattering signal of the object. Appendix Figure 7 The processing results for this step are shown, where the black solid line and the red solid line represent... and The blue solid line represents the extracted... The signal is used for subsequent calculations of the electrical power level of the forward acoustic scattering signal. .

[0063] (7) Attachment Figure 3 The location of the target is set as the origin, and a three-dimensional Cartesian coordinate system is established. Based on the measured relative distance information, the position coordinates of the sound source, reference channel, and observation channel are obtained, as shown in Table 1. The propagation loss from the sound source to the target is calculated based on the sound propagation extension law obtained in step (4). The acoustic propagation loss from the target to the second observation channel is... ; and the sound propagation loss value between the sound source and the reference channel is .

[0064] Table 1. List of location coordinates of the sound source and receiver

[0065] .

[0066] (8) For all target attitude angle values, calculate the value obtained in step (6). Signal power level (This value is due to) ψ It varies depending on the value taken, for example The result at that time was 0.0016 dB); used with Extracting time windows of the same duration get And calculate the power level of the signal. (This value is fixed).

[0067] (9) The propagation loss value calculated in step (7) TL S , TL O2 and TLRS The signal power level calculated in step (8) and Along with the sensitivity level of the hydrophone in channel 6 and the hydrophone sensitivity level of channel 2 Substitute these values ​​into the target intensity calculation formula to obtain an estimate of the forward-scattering target intensity. The results are attached. Figure 8 As shown in the figure, the red solid line represents the measurement results of the "indirect-coherent" method, the blue solid line represents the measurement results of the "indirect-incoherent" method, and the black dashed line represents the numerical results of the forward acoustic scattering target intensity of the stainless steel cylindrical shell calculated by the Kirchhoff integral approximation method.

[0068] The results show that: (1) the forward scattering target intensity values ​​of the "indirect-coherent" method are generally higher than those of the "direct-incoherent" method, which confirms that the forward acoustic scattering signal extracted by the "direct-incoherent" method is the projection of the real forward acoustic scattering signal onto the direction of the interferometric superimposed wave signal. (2) In Within the attitude angle range, the forward acoustic scattering target intensity measurement results obtained by the "indirect-coherent" method show a significantly better match with the typical numerical solution than the measurement results obtained by the "direct-incoherent" method. (3) In Within the attitude angle range, the deviation between the measurement results of the "indirect-coherent" method and the typical numerical results is greater than that of the "direct-incoherent" method. This phenomenon is caused by the fact that the hydrophone sensitivity level of the observation channel (channel 2) is approximately 27 dB lower than that of the reference channel (channel 6). It is no longer accurate enough. If the observation channel also uses a hydrophone with the same or similar sensitivity level as the reference channel, the measurement deviation in this attitude angle range will definitely decrease further as expected.

[0069] Examples show that the "indirect-coherent" method proposed in this invention can effectively measure the forward acoustic scattering target intensity of underwater objects, and the measurement results are generally better than those of the "direct-incoherent" method, especially at the incident attitude angle. Within the range. Although the forward scattering target intensity of a regular-shaped scatterer such as a finite-length stainless steel cylindrical shell was measured here only under laboratory conditions in an anechoic pool, the validity of the measurement results suggests that this "indirect-coherent" method is also applicable to stable or slowly changing field test environments, as well as scatterers with complex shapes or internal structures.

Claims

1. An indirect-coherent method for measuring the intensity of forward acoustic scattering from an underwater object, characterized in that, include: (S1) Measurement and recording steps: Perform background field measurement when there are no underwater objects and target field measurement when there are underwater objects, and record the received signals of the observation channel and the reference channel; specifically including: (1) Under the condition that there are no underwater objects, control the underwater sound source to emit a predetermined signal and synchronously record the observation channel signal. and reference channel signal (2) In the presence of an underwater object, keep the positions of the sound source and the receiving channel unchanged. After placing the object to be measured, control the sound source to emit the same signal and synchronously record the signal of the observation channel. and reference channel signal ; (S2) Forward acoustic scattering signal extraction steps: Based on the measured signals of the background field and the target field, the forward acoustic scattering signal caused by the underwater object is extracted by time delay estimation and signal coherent subtraction; specifically including: (1) calculating the reference channel signals under the background field and the target field. and The cross-correlation function is used to determine the relative time delay difference. (2) Calculate the observation channel signal under the background field. With reference channel signal The cross-correlation function is used to determine the relative time delay difference. (3) Refer to the aforementioned time delay difference and Signal observed in the target field channel and background field observation channel signal The signals of the same duration are extracted and denoted as follows: and (4) Subtract the two time window signals to obtain the forward acoustic scattering signal: ; (S3) Target intensity calculation steps: Based on the sound propagation loss estimation and signal power, the forward sound scattering target intensity of the underwater object is calculated using the extracted forward sound scattering signal; specifically including: (1) Establishing a coordinate system with the target position as the origin, and determining the position coordinate vectors of the sound source, observation channel and reference channel. , , (2) Based on the determined sound propagation spread law, calculate the propagation loss from the sound source to the object, the propagation loss from the object to the observation channel, and the propagation loss from the sound source to the reference channel; (3) Calculate the forward sound scattering signal. power level And calculate the reference channel time window signal under the background field. power level (4) Target intensity of forward acoustic scattering Calculated using the following formula: in, To observe the sensitivity of the hydrophone in the channel, The hydrophone sensitivity is used as a reference channel; , , These are the propagation losses from the sound source to the object, from the object to the observation channel, and from the sound source to the reference channel, respectively.

2. The method according to claim 1, characterized in that, The forward acoustic scattering signal extraction step (S2) includes sound velocity and propagation loss estimation: (1) Calculate the arrival time delay difference based on the cross-correlation function of any two channel signals in the background field measurement. Combined with channel spacing Estimating the speed of sound The arithmetic mean of multiple measurements was taken as the sound speed measurement value. ĉ ; (2) Based on the sound pressure power of each channel signal and the sound source level of the transmitting transducer in the background field measurement, the sound propagation loss is estimated and compared with the theoretical model. Perform least-squares fitting to determine the coefficients of sound propagation spread. .

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