A method and system for distinguishing surface / underwater targets based on simple normal wave energy limitation in semi-channel in ice area

Abstract

The present application provides a method and system for distinguishing surface / underwater targets based on simple normal wave limited energy in ice zone semi-channel. The method includes: transforming the time domain signal received by the vertical array in the ice zone sea area into the frequency domain to obtain the frequency domain data of the array received signal; using the simple normal wave model to simulate and predict the simple normal wave modal depth function, horizontal wave number and phase velocity of the test sea area; extracting the simple normal wave modal amplitude that can be distinguished by the test measured data; by comparing the phase velocity with the sound velocity at the critical resolution depth, obtaining the order index of the limited mode; calculating the energy of the limited mode of the data extraction result as a binary detection statistic; setting the detection threshold according to the ROC curve, if the limited mode energy is less than the detection threshold, it is determined to be an underwater target. Otherwise, it is determined to be a surface target. Compared with the traditional matching field and matching mode methods, the method of the present invention has a small amount of calculation, high environmental tolerance, and is easy to promote in actual sonar platforms.

Description

Technical Field

[0001] This application belongs to the fields of underwater acoustic engineering, ocean engineering, and sonar technology. Specifically, it relates to a method and system for distinguishing surface / underwater targets based on the restricted energy of normal modes in an ice region semi-channel, which can be used for rapid discrimination of surface and underwater targets in the Arctic ice region waters. Background Art

[0002] The discrimination of surface / underwater targets is one of the hot and difficult issues in underwater acoustics and underwater acoustic signal processing research. In open waters, surface ships are the main noise interferences. To improve the detection and recognition capabilities of underwater sensitive targets, it is particularly important to first distinguish underwater / surface targets and eliminate strong surface interferences. The article "Source depth discrimination with a vertical line array" used a vertical array for mode separation, constructed a detection statistic by restricting the ratio of the restricted mode to the free mode, and achieved the discrimination of surface / underwater targets under a shallow sea negative gradient. This article was published in the 140th volume, issue 5 of "The Journal of the Acoustical Society of America" in 2016. However, this method is only limited to the shallow sea negative gradient sound speed structure and cannot be applied to the typical positive gradient sound speed structure in the Arctic.

[0003] In the Arctic ice region waters, it is a typical positive gradient sound speed structure. When sound waves propagate, they continuously refract and reflect upward and interact frequently with the ice surface. Ice source noises such as ice layer extrusion and collision are the main surface interference sources. Similarly, to achieve the detection of underwater targets, it is necessary to first distinguish the depths of surface / underwater targets and then perform further detection, positioning, tracking, etc. Summary of the Invention

[0004] The purpose of this application is to overcome the defect that the existing technology is only limited to the shallow sea negative gradient sound speed structure and cannot be applied to the typical positive gradient sound speed structure in the Arctic.

[0005] To achieve the above purpose, this application proposes a method for distinguishing surface / underwater targets based on the restricted energy of normal modes in an ice region semi-channel. The method includes:

[0006] Transform the time-domain signal received by the vertical array in the ice region waters into the frequency domain to obtain the frequency-domain data of the array received signal;

[0007] Use the normal mode model to simulate and predict the normal mode depth function, horizontal wave number, and phase velocity of the test sea area;

[0008] Extract the amplitudes of the normal modes that can be distinguished from the test measured data through the least squares normal mode separation method;

[0009] Set the critical resolution depth of the water surface / underwater, and obtain the order index of the limiting mode by comparing the phase velocity with the sound velocity at the critical resolution depth;

[0010] Calculate the energy of the restricted modes of the data extraction results as a binary detection statistic;

[0011] The detection threshold is set according to the ROC curve. If the restricted modal energy is less than the detection threshold, it is judged as an underwater target, otherwise, it is judged as a surface target.

[0012] As an improvement of the above method, the time domain signal received by the vertical array is Fourier transformed. The frequency domain sound pressure of the measured data can be expressed using a simple normal wave model as follows:

[0013]

[0014] Where p(r,z,ω) represents the received data in the frequency domain; r is the horizontal distance between the target and the vertical array; z is the receiving depth; ω is the angular frequency; i represents the imaginary unit; Ψ m (z) represents the modal depth function; k m represents the horizontal wave number; m represents the modal index; M is the total modal order; z s represents the depth of the target; ρ(z s ) represents the seawater density at the target depth.

[0015] As an improvement of the above method, the frequency receiving sound pressure of the vertical array is expressed as:

[0016] p=EA=[p(z 1 ),p(z 2 ),…p(z N )] H

[0017]

[0018] Among them, z 1 ,z 2 ,…z N represents the depth of different vertical array N elements; p∈C N represents the complex sound pressure vector of the vertical receiving array; E∈C N×M Represents the modal depth function matrix; A∈C M represents the modal amplitude function vector; N represents the number of vertical array elements; and the superscript H represents the conjugate transpose.

[0019] As an improvement of the above method, the simple normal wave modal amplitude that can be resolved by the actual measured data of the extraction test is expressed as:

[0020]

[0021] in, Represents the estimated result of the modal amplitude function; modal depth function matrix E∈C N×M The calculation is performed using a simple normal wave model.

[0022] As an improvement to the above method, the critical resolution depth z of the water surface / underwater is set lim , by comparing the phase velocity cp m The sound velocity c(z lim ), get the index of the restricted mode:

[0023]

[0024] Among them, cp m represents the phase velocity of the mth order normal wave, obtained through the normal wave model; K represents the phase velocity less than c(z lim ) and limits the mode index to 1 to K.

[0025] As an improvement of the above method, the energy R trapped It is expressed as:

[0026]

[0027] As an improvement of the above method, the detection threshold is set according to the ROC curve. If the restricted modal energy is less than the detection threshold, it is determined to be an underwater target; otherwise, it is determined to be a surface target, including:

[0028] According to the expected false alarm probability and detection probability, the energy detection threshold η is set to perform binary detection. The discrimination rule is:

[0029] H 0 :R trapped ≥η

[0030] H 1 :R trapped <η

[0031] Among them, H 0 Represents a surface target, H 1 Represents an underwater target.

[0032] The present application also provides a surface / underwater target discrimination system based on simple normal wave energy limitation in ice area semi-channel, which is implemented based on the above method, and the system includes:

[0033] The frequency domain data acquisition module is used to transform the time domain signal received by the vertical array in the ice sea area into the frequency domain to obtain the frequency domain data of the array receiving signal;

[0034] The simple normal wave mode simulation module is used to simulate and predict the simple normal wave mode depth function, horizontal wave number and phase velocity of the test sea area using the simple normal wave model;

[0035] The module for extracting the simple normal wave modal amplitude is used to extract the simple normal wave modal amplitude that can be distinguished by the test measured data through the least squares simple normal wave separation method;

[0036] The module for obtaining the order index of the limiting mode is used to set the critical resolution depth of the surface / underwater. The order index of the limiting mode is obtained by comparing the phase velocity with the sound velocity at the critical resolution depth.

[0037] An energy extraction module for limiting modes, for calculating the energy of limiting modes of the data extraction results as a binary detection statistic; and

[0038] The surface / underwater target determination module is used to set the detection threshold according to the ROC curve. If the restricted modal energy is less than the detection threshold, it is determined to be an underwater target; otherwise, it is determined to be a surface target.

[0039] Compared with the prior art, the advantages of this application are:

[0040] The present invention uses a simple normal wave mode separation method to separate the vertical array measured data. By constructing the energy of the mode confined in the waveguide, it is used as a detection statistic for surface / underwater target resolution, thereby achieving depth resolution in an ice area under a positive gradient waveguide. Compared with traditional matching field and matching mode methods, the method has a small amount of calculation and higher robustness and environmental tolerance. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 The figure shows the flow chart of the surface / underwater target discrimination method based on simple normal wave limited energy in the half-channel of ice area;

[0042] Figure 2 The figure shows the measured positive gradient sound velocity structure in the ice-covered sea area;

[0043] Figure 3 Shown is a schematic diagram of the vertical array placement position;

[0044] Figure 4 The figure shows the distribution of the simple normal wave mode depth function of different modes;

[0045] Figure 5 Shown is the simple normal wave mode diagram that the vertical array can resolve;

[0046] Figure 6 Shown is a graph of limiting modal energy versus target depth and distance;

[0047] Figure 7The figure shows the ROC curve of the surface / underwater target discrimination method based on simple normal wave energy limitation in the semi-channel in ice area. DETAILED DESCRIPTION

[0048] The technical solution of the present application is described in detail below with reference to the accompanying drawings.

[0049] The present application proposes a method and system for distinguishing surface / underwater targets based on simple normal wave limited energy in a half-channel in an ice area, so as to solve the problem of distinguishing surface / underwater targets in a positive gradient sound velocity structure in an ice area.

[0050] The applicable scope of this method / system is: the sound speed structure is an overall positive gradient (semi-sound channel), the sea depth range is 50-6000m; the horizontal distance range between the target sound source and each vertical linear array is 0-100km; the depth range of the target sound source is 0-500m.

[0051] Example 1

[0052] like Figure 1 As shown, the surface / underwater target discrimination method based on simple normal wave energy limitation in the half-channel of ice area includes:

[0053] Step 1) transforming the time domain signal received by the vertical array in the ice sea area into the frequency domain to obtain the frequency domain sound pressure data p(ω) of the array receiving signal;

[0054] The number of channels of the vertical array used is greater than 5 and the length is not less than 50m.

[0055] The frequency domain sound pressure of the measured data can be expressed using the simple normal wave model as follows:

[0056]

[0057] Where p(r,z,ω) represents the frequency domain received data, r is the horizontal distance between the target and the vertical array, z is the receiving depth, ω is the angular frequency, i represents the imaginary unit, Ψ m (z) represents the modal depth function, k m represents the horizontal wave number, m represents the modal index, M is the total modal order, z s represents the depth of the target, ρ(z s ) represents the seawater density at the target depth.

[0058] The frequency received sound pressure of the vertical array can be expressed as:

[0059] p=EA=[p(z 1 ),p(z 2 ),…p(z N )] H

[0060]

[0061] Among them, z 1 ,z 2 ,…z N Represents the depth of different vertical array N elements, p∈C N represents the complex sound pressure vector of the vertical receiving array, E∈C N×M Represents the modal depth function matrix, A∈C M represents the modal amplitude function vector, N represents the number of vertical array elements, and the superscript H represents the conjugate transpose.

[0062] Step 2) Using the simple normal wave model, the sound velocity structure and sea depth of the test sea area are used as model input to simulate and predict the simple normal wave modal depth function Ψ of the test sea area. m (z), horizontal wave number k m and the phase velocity cp m ;

[0063] When the sea depth is between 100 and 6000 m, the normal wave modal depth function Ψ m (z), horizontal wave number k m and the phase velocity cp m The order of can be 10 to 100.

[0064] Step 3) Extract the modal amplitude of the simple normal wave that can be distinguished by the experimental measured data through the least squares simple normal wave separation method

[0065] Modal amplitude functions estimated from measured data It can be expressed as:

[0066]

[0067] in, Represents the estimated result of the modal amplitude function. Modal depth function matrix E∈C N×M The calculation is performed using a simple normal wave model.

[0068] Step 4) Set the critical resolution depth z of the surface / underwater lim , by comparing the phase velocity cp m The sound velocity c(z lim ), and obtain the order index of the restricted mode 1~K;

[0069] The maximum order of the limiting mode can be expressed as:

[0070]

[0071] Among them, cp mrepresents the phase velocity of the mth order normal wave, obtained through the normal wave model; K represents the phase velocity less than c(z lim )’s maximum modal order; the restricted modal order index can be expressed as 1 to K.

[0072] Step 5) Calculate the energy R of the restricted mode of data extraction results trapped , as the binary detection statistic;

[0073] The energy of the restricted mode can be expressed as:

[0074]

[0075] Among them, R trapped It represents the sum of the energy of the modes confined in the waveguide. The deeper the sound source depth, the smaller the sum of the energy of the modes confined in the waveguide.

[0076] Step 6) Set the detection threshold according to the ROC curve, and if the restricted modal energy is less than the detection threshold, it is determined to be an underwater target. Otherwise, it is determined to be a surface target.

[0077] The energy detection threshold η can be set according to the expected false alarm probability and detection probability. 0 Represents a surface target, H 1 Represents underwater targets and performs binary detection. The discrimination rules are:

[0078] H 0 :R trapped ≥η

[0079] H 1 :R trapped <η

[0080] Figure 2 The positive gradient sound velocity structure of the central Arctic ice area measured by a sound velocity meter during China's Arctic scientific expedition was displayed. The sea depth at the test location was 2712m, the ice thickness was about 2m, and the sea ice density was 100%. Figure 3 The vertical array is shown with 30 elements and 10m spacing, covering a water depth of 10 to 300m. The target frequency is 750Hz, and surface / underwater target discrimination is performed by the following steps:

[0081] Step 1: The time domain data p of each depth element of the vertical receiving array n (t), n = 1, 2, ... N, N = 30. Sampling rate f s The data is 10kHz and 4s long, and Fourier transform is performed to convert it into frequency domain data p n (f) Select the 700 Hz frequency band and generate the frequency domain matrix p of the sound pressure of the measurement field.

[0082] Step 2: Using the simple normal wave KRAKEN model, Figure 2 The sound velocity structure and sea depth of the test sea area shown in the figure are used as model input to simulate and predict the simple normal wave modal depth function Ψ of the test sea area. m (z), horizontal wave number k m and the phase velocity cp m The obtained 500Hz, 600Hz and 700Hz modal depth function distributions are as follows Figure 4 The modal orders that the vertical array can resolve are as follows: Figure 5 As shown. Figure 4 It can be seen from the figure that as the order increases, the inversion depth of the modal depth function becomes larger. Figure 5 It can be seen that the vertical array can resolve 30 orders, and M=30 is set.

[0083] Step 3: The modal depth function Ψ calculated by the KRAKEN model m (z) Construct the matrix E∈C N×M The least squares simple normal wave separation method is used to extract the simple normal wave modal amplitude that can be distinguished by the test measured data.

[0084] Step 4: In the Arctic ice zone, underwater targets need to avoid icebergs and large ice ridges to ensure their own navigation safety and keep the safe navigation depth below 50m. Therefore, the critical resolution depth z between the surface and underwater is set. lim =50m, according to the measured sound velocity structure, we can get the channel c(z lim )=1437.58m / s. By comparing the phase velocity cp m The sound velocity c(z lim ), the order index of the restricted mode is obtained as 1 to 5;

[0085] Step 5: Take the 1st to 5th order modes as the limiting modes and calculate the energy of the limiting modes of the data extraction results The results are as follows Figure 6 shown.

[0086] Step 6: From Figure 6 As can be seen from the figure, as the target depth transitions from the water surface to underwater, R trapped It shows a monotonically decreasing trend, and R trapped (z lim ) as the detection threshold for depth resolution. If the restricted modal energy is less than the detection threshold, it is determined to be an underwater target. Otherwise, it is determined to be a surface target.

[0087] Step 7: To get R trappedCorresponding ROC curve. Assuming the signal-to-noise ratio is 5dB, the noise is Gaussian white noise, and 10,000 Monte Carlo simulations are performed, the ROC curve obtained is as follows Figure 7 The depth resolution method based on simple normal wave limited energy in the half-channel of the Arctic ice region proposed by the present invention has good detection performance.

[0088] Example 2

[0089] The present application also provides a surface / underwater target discrimination system based on simple normal wave energy limitation in ice area semi-channel, which is implemented based on the above method, and the system includes:

[0090] The frequency domain data acquisition module is used to transform the time domain signal received by the vertical array in the ice sea area into the frequency domain to obtain the frequency domain data of the array receiving signal;

[0091] The simple normal wave mode simulation module is used to simulate and predict the simple normal wave mode depth function, horizontal wave number and phase velocity of the test sea area using the simple normal wave model;

[0092] The module for extracting the simple normal wave modal amplitude is used to extract the simple normal wave modal amplitude that can be distinguished by the test measured data through the least squares simple normal wave separation method;

[0093] The module for obtaining the order index of the limiting mode is used to set the critical resolution depth of the surface / underwater. The order index of the limiting mode is obtained by comparing the phase velocity with the sound velocity at the critical resolution depth.

[0094] The energy module of the restricted mode extraction is used to calculate the energy of the restricted mode of the data extraction result as a binary detection statistic;

[0095] The surface / underwater target determination module is used to set the detection threshold according to the ROC curve. If the restricted modal energy is less than the detection threshold, it is determined to be an underwater target; otherwise, it is determined to be a surface target.

[0096] The present application may also provide a computer device, comprising: at least one processor, a memory, at least one network interface and a user interface. The various components in the device are coupled together through a bus system. It is understood that the bus system is used to achieve connection and communication between these components. In addition to the data bus, the bus system also includes a power bus, a control bus and a status signal bus.

[0097] The user interface may include a display, a keyboard or a pointing device, such as a mouse, a trackball, a touch pad or a touch screen.

[0098] It is understood that the memory in the disclosed embodiments of the present application may be a volatile memory or a non-volatile memory, or may include both volatile and non-volatile memories. Among them, the non-volatile memory may be a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), or a flash memory. The volatile memory may be a random access memory (RAM), which is used as an external cache. By way of example and not limitation, many forms of RAM are available, such as static random access memory (SRAM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), double data rate synchronous dynamic random access memory (DDRSDRAM), enhanced synchronous dynamic random access memory (ESDRAM), synchronous link dynamic random access memory (SLDRAM), and direct memory bus random access memory (DRRAM). The memories described herein are intended to include, but are not limited to, these and any other suitable types of memories.

[0099] In some embodiments, the memory stores the following elements, executable modules or data structures, or a subset thereof, or an extended set thereof: an operating system and applications.

[0100] The operating system includes various system programs, such as a framework layer, a core library layer, a driver layer, etc., which are used to implement various basic services and process hardware-based tasks. The application includes various application programs, such as a media player (Media Player), a browser (Browser), etc., which are used to implement various application services. The program for implementing the method of the embodiment of the present disclosure can be included in the application.

[0101] In the above embodiment, the processor may also call a program or instruction stored in the memory, specifically, a program or instruction stored in an application program, to:

[0102] Execute the steps of the above method.

[0103] The above method can be applied to a processor or implemented by a processor. The processor may be an integrated circuit chip with signal processing capabilities. In the implementation process, each step of the above method can be completed by an integrated logic circuit of hardware in the processor or an instruction in the form of software. The above processor may be a general processor, a digital signal processor (Digital Signal Processor, DSP), an application specific integrated circuit (Application Specific Integrated Circuit, ASIC), a field programmable gate array (Field Programmable Gate Array, FPGA) or other programmable logic devices, discrete gates or transistor logic devices, discrete hardware components. The above-disclosed methods, steps and logic block diagrams can be implemented or executed. The general processor may be a microprocessor or the processor may also be any conventional processor, etc. The steps of the above-disclosed method can be directly embodied as a hardware decoding processor to execute, or the hardware and software modules in the decoding processor are combined to execute. The software module can be located in a mature storage medium in the field such as a random access memory, a flash memory, a read-only memory, a programmable read-only memory or an electrically erasable programmable memory, a register, etc. The storage medium is located in the memory, and the processor reads the information in the memory and completes the steps of the above method in combination with its hardware.

[0104] It is understood that the embodiments described in the present application can be implemented by hardware, software, firmware, middleware, microcode or a combination thereof. For hardware implementation, the processing unit can be implemented in one or more application specific integrated circuits (ASIC), digital signal processors (DSP), digital signal processing devices (DSPD), programmable logic devices (PLD), field programmable gate arrays (FPGA), general-purpose processors, controllers, microcontrollers, microprocessors, other electronic units for performing the functions described in the present application or a combination thereof.

[0105] For software implementation, the technology of the present application can be implemented by executing the functional modules (such as procedures, functions, etc.) of the present application. The software code can be stored in a memory and executed by a processor. The memory can be implemented in the processor or outside the processor.

[0106] The present application may also provide a non-volatile storage medium for storing a computer program. When the computer program is executed by a processor, each step in the above method embodiment can be implemented.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present application and are not intended to limit it. Although the present application is described in detail with reference to the embodiments, a person skilled in the art should understand that any modification or equivalent replacement of the technical solution of the present application does not depart from the spirit and scope of the technical solution of the present application and should be included in the scope of the claims of the present application.

Claims

1. A method for distinguishing surface / underwater targets based on simple normal wave energy limitation in half-channel in ice areas, the method comprising: The time domain signal received by the vertical array in the ice sea area is transformed into the frequency domain to obtain the frequency domain data of the array receiving signal; Using the simple normal wave model, the simple normal wave modal depth function, horizontal wave number and phase velocity in the test sea area are simulated and predicted; The modal amplitude of the simple normal wave that can be distinguished by the experimental measured data is extracted by the least squares simple normal wave separation method. Set the critical resolution depth of the water surface / underwater, and obtain the order index of the limiting mode by comparing the phase velocity with the sound velocity at the critical resolution depth; Calculate the energy of the restricted modes of the data extraction results as a binary detection statistic; The detection threshold is set according to the ROC curve. If the restricted modal energy is less than the detection threshold, it is judged as an underwater target, otherwise, it is judged as a surface target.

2. The method for distinguishing surface / underwater targets based on simple normal wave energy limitation in ice area semi-channel according to claim 1 is characterized in that: The time domain signal received by the vertical array is Fourier transformed. The frequency domain sound pressure of the measured data can be expressed using the simple normal wave model as follows: Where p(r,z,ω) represents the received data in the frequency domain; r is the horizontal distance between the target and the vertical array; z is the receiving depth; ω is the angular frequency; i represents the imaginary unit; Ψ m (z) represents the modal depth function; k m represents the horizontal wave number; m represents the modal index; M is the total modal order; z s represents the depth of the target; ρ(z s ) represents the seawater density at the target depth.

3. The method for distinguishing surface / underwater targets based on simple normal wave energy limitation in ice area semi-channel according to claim 2 is characterized in that: The frequency received sound pressure of the vertical array is expressed as: p=EA=[p(z1),p(z2),…p(z N )] H Among them, z1,z2,…z N represents the depth of different vertical array N elements; p∈C N represents the complex sound pressure vector of the vertical receiving array; E∈C N×M Represents the modal depth function matrix; A∈C M represents the modal amplitude function vector; N represents the number of vertical array elements; and the superscript H represents the conjugate transpose.

4. The method for distinguishing surface / underwater targets based on simple normal wave energy limitation in ice area semi-channel according to claim 3 is characterized in that: The simple normal wave modal amplitude that can be resolved by the measured data of the extraction test is expressed as: in, Represents the estimated result of the modal amplitude function; modal depth function matrix E∈C N×M The calculation is performed using a simple normal wave model.

5. The method for distinguishing surface / underwater targets based on simple normal wave energy limitation in ice area semi-channel according to claim 4 is characterized in that: Set the critical resolution depth z of the surface / underwater lim , by comparing the phase velocity cp m The sound velocity c(z lim ), get the index of the restricted mode: Among them, cp m represents the phase velocity of the mth order normal wave, obtained through the normal wave model; K represents the phase velocity less than c(z lim ) and limits the mode index to 1 to K.

6. The method for distinguishing surface / underwater targets based on simple normal wave energy limitation in ice area semi-channel according to claim 5 is characterized in that: The energy of the limiting mode R trapped It is expressed as:

7. The method for distinguishing surface / underwater targets based on simple normal wave energy limitation in ice area semi-channel according to claim 6 is characterized in that: The detection threshold is set according to the ROC curve, and if the restricted modal energy is less than the detection threshold, it is determined to be an underwater target, otherwise, it is determined to be a surface target, including: According to the expected false alarm probability and detection probability, the energy detection threshold η is set to perform binary detection. The discrimination rule is: H0:R trapped ≥η H1:R trapped <the> Among them, H0 represents the surface target and H1 represents the underwater target.

8. A surface / underwater target discrimination system based on simple normal wave limited energy in ice area semi-channel, implemented based on any method described in claims 1-7, characterized in that: The system comprises: The frequency domain data acquisition module is used to transform the time domain signal received by the vertical array in the ice sea area into the frequency domain to obtain the frequency domain data of the array receiving signal; The simple normal wave mode simulation module is used to simulate and predict the simple normal wave mode depth function, horizontal wave number and phase velocity of the test sea area using the simple normal wave model; The module for extracting the simple normal wave modal amplitude is used to extract the simple normal wave modal amplitude that can be distinguished by the test measured data through the least squares simple normal wave separation method; The module for obtaining the order index of the limiting mode is used to set the critical resolution depth of the surface / underwater. The order index of the limiting mode is obtained by comparing the phase velocity with the sound velocity at the critical resolution depth. An energy extraction module for limiting modes, for calculating the energy of limiting modes of the data extraction results as a binary detection statistic; and The surface / underwater target determination module is used to set the detection threshold according to the ROC curve. If the restricted modal energy is less than the detection threshold, it is determined to be an underwater target; otherwise, it is determined to be a surface target.

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