Cross-medium laser acoustic imaging system and method based on step-by-step phase shift offset

Abstract

The invention discloses a cross-medium laser acoustic imaging system based on step-by-step phase shift offset, which is characterized in that a laser unit generates excitation laser, the excitation laser acts on the surface of a water body target through an optical transceiving unit, and spherical wave type laser acoustic waves generated by the water body target are transmitted to a microphone detection unit through the optical transceiving unit; the microphone detection unit scans point by point and acquires laser sound signals which are reflected by a water body target and propagate upwards, and transmits the acquired laser sound signals to the information acquisition and processing unit for amplification and analog-to-digital conversion processing; and cross-medium reconstruction of the underwater target object is realized through a signal processing / image reconstruction unit according to the first-step phase shift offset in the air and the second-step phase shift offset in the water. The problem of complex artifacts of laser sound wave cross-medium transmission can be effectively solved, and the process of the non-contact air detection field of underwater target objects can be promoted.

Description

Technical Field

[0001] The present invention belongs to the technical field of underwater laser acoustic remote sensing, and relates to a cross-medium laser acoustic imaging system and method based on step-by-step phase shift offset, which is used for accurately reconstructing the position and structure of an underwater target object. Background Art

[0002] Laser acoustic remote sensing technology can be divided into the "light-acoustic-light" mode and the "light-acoustic-acoustic" mode according to the different ways of receiving vibration echoes. In the "light-acoustic-acoustic" mode, the radiated sound waves generated by the interaction between laser and water generate reflected echoes after encountering the target object (such as reefs, riverbeds, etc.). The reflected echoes are mainly collected by acoustic receivers in the air (such as microphones, air-coupled ultrasonic transducers, etc.) for subsequent signal processing and underwater image reconstruction. The "light-acoustic-acoustic" mode of laser acoustic remote sensing technology mainly refers to "Laser Acoustic Remote Sensing Technology" edited by Li Rongfu et al. and "An Airborne Sonar System for Underwater Remote Sensing and Imaging" published by Aidan Fitzpatrick of Stanford University in the journal IEEE Access.

[0003] Due to the material properties of the transducer itself and platform limitations, the acoustic receiver is usually placed in the air, so the sound waves reflected by the underwater target object need to propagate through two media, water and air. However, the propagation speed of sound waves in liquids and gases is quite different (about 1500m / s in water and about 340m / s in air), which leads to large phase changes and energy losses in the propagation of sound waves at the water-air interface. At this time, if the delayed sum imaging method based on the assumption of constant sound speed is still used, the reconstructed image will contain displacement, distortion and artifacts, thereby reducing the image quality. The method adopted by Li Rongfu et al. is based on the total reflection vertical propagation approximation (that is, according to the law of total reflection of water surface sound waves, the underwater target echo is only contained in the cone angle of ±13.2° after passing through the water-air interface, and it can be approximately considered to propagate in the direction perpendicular to the water surface). However, this method produces large errors in experiments. For a standard reflector with a depth of 30 meters, the detection error reaches 5 milliseconds, and the equivalent depth error reaches 7.5 meters.

[0004] Aidan Fitzpatrick's team at Stanford University proposed using a piecewise synthetic aperture radar method to reconstruct the acoustic signal received by a capacitive micromachined ultrasonic transducer (CMUT) sensor. However, this method does not take into account the acoustic transmission of the air-water interface. In addition, since the piecewise synthetic aperture method uses a layer-by-layer iterative extrapolation mode, its computational efficiency is low and it is not applicable to laser acoustic remote sensing applications with high time accuracy requirements (such as image reconstruction of moving target objects). Therefore, a major breakthrough in the "light-sound-sound" mode of laser acoustic remote sensing technology is to achieve cross-media, high-precision, and low-frame-rate underwater target object detection and imaging. Summary of the invention

[0005] (I) Purpose of the invention

[0006] The purpose of the present invention is to provide a cross-medium laser acoustic imaging system and method based on step-by-step phase shift, so as to overcome the reconstruction distortion problem and the low imaging rate problem caused by the cross-medium transmission of sound waves in traditional laser acoustic remote sensing, and realize accurate and rapid reconstruction of laser acoustic remote sensing in cross-medium imaging scenarios.

[0007] (II) Technical solution

[0008] In order to solve the above technical problems, the present invention provides a cross-medium laser acoustic imaging system based on step-by-step phase shift, which includes: a laser unit, an optical transceiver unit, a microphone detection unit, an information acquisition and processing unit, and a signal processing / image reconstruction unit; wherein, the laser unit generates an excitation laser, which acts on the surface of a water target through the optical transceiver unit, and the laser acoustic wave in the form of a spherical wave generated by the water target is transmitted to the microphone detection unit through the optical transceiver unit, and the microphone detection unit scans and collects the laser acoustic signal reflected by the water target and propagated upward point by point, and transmits the collected laser acoustic signal to the information acquisition and processing unit for amplification and analog-to-digital conversion processing, and then realizes cross-medium reconstruction of the underwater target object through the signal processing / image reconstruction unit according to the first phase shift in the air and the second phase shift in the water.

[0009] Among them, the laser unit includes a laser driving circuit and a laser, and the laser driving circuit controls the laser to generate an excitation laser; the microphone detection unit includes a controller and a microphone, and the controller controls the microphone to perform point-by-point scanning; the information acquisition and processing unit includes an analog-to-digital conversion circuit and an amplification circuit; the optical transceiver unit includes a transmitting antenna and a receiving antenna.

[0010] Among them, the cross-medium laser acoustic imaging system also includes: a stepper motor, which is an aerial platform stepper motor, on which an omnidirectional single-beam microphone is arranged to scan and collect point by point the laser acoustic signal reflected by the water target and propagated upward.

[0011] Among them, the cross-media laser acoustic imaging system also includes: a system power supply, which supplies power to the laser unit, the microphone detection unit, and the information acquisition and processing unit.

[0012] The present invention also provides a trans-medium laser acoustic imaging method based on step-by-step phase shift, which comprises the following steps:

[0013] Step 1: Perform preliminary signal rough processing on the laser acoustic remote sensing data obtained by linear scanning;

[0014] Step 2: Perform a two-dimensional Fourier transform on the roughly processed photoacoustic data to obtain the photoacoustic field information in the frequency wavenumber domain

[0015] Step 3: Perform the first phase shift to reversely extrapolate the photoacoustic field information in the frequency wavenumber domain collected by the microphone to the water-air interface in the air medium through the phase shift method. Obtain the upper boundary photoacoustic field information in the frequency wavenumber domain of the water-air interface;

[0016] Step 4: Use Snell's law to correct the laser acoustic wave energy, calculate the projection coefficient τ of the air-water interface acoustic field, and obtain the lower boundary acoustic field at the air-water interface in the water medium

[0017]

[0018] Step 5: Execute the second phase shift offset—STOLT offset;

[0019] Step 6: Perform Hilbert transform on the reconstructed laser acoustic field information to obtain the envelope and complete the laser acoustic imaging of the underwater target object.

[0020] Wherein, step 1 includes the following sub-steps:

[0021] Step 1.1: Based on the prior imaging target information, estimate the flight time range of the effective laser acoustic signal and determine the interception range of the original signal;

[0022] Step 1.2: Filter the intercepted waveform through a bandpass filter with matching bandwidth to reduce the interference of the platform and surrounding environment on the laser acoustic signal.

[0023] Among them, in step 2, the formula of two-dimensional Fourier transform is as follows:

[0024]

[0025] Among them, in step 3, the formula for the first step of phase shift offset extrapolation is as follows:

[0026]

[0027] Among them, in step 3, the calculation formula of the lower boundary sound field is as follows:

[0028]

[0029] Wherein, step 5 includes the following sub-steps:

[0030] Step 5.1: Based on the horizontal wave number, calculate the vertical wave number in the water medium:

[0031]

[0032] Step 5.2: By replacing the variables of wave number and angular frequency, the differential transformation relationship between wave number and angular frequency is established, the STOLT migration operator is further calculated, and the laser acoustic field distribution after STOLT migration is obtained; the formula of STOLT migration is as follows:

[0033]

[0034] Step 5.3: Perform a two-dimensional inverse Fourier transform on the laser acoustic field after STOLT migration to obtain the reconstructed laser acoustic field information; the formula for the two-dimensional inverse Fourier transform is as follows:

[0035]

[0036] (III) Beneficial effects

[0037] The trans-medium laser acoustic imaging system and method based on step-by-step phase shift provided by the above technical solution have the following beneficial effects:

[0038] (1) The present invention is based on the requirements of laser acoustic remote sensing in cross-medium, high precision and low frame rate, and can realize the "acoustic-optical-acoustic" mode cross-medium photoacoustic imaging method through the step-by-step phase shift technology. It can effectively solve the complex artifact problem of laser acoustic wave cross-medium transmission and promote the progress of non-contact aerial detection of underwater target objects.

[0039] (2) The present invention utilizes a microphone in an air medium to collect underwater laser acoustic signals, thereby achieving cross-medium underwater detection and giving laser acoustic imaging flexibility in detection mode and scanning mode.

[0040] (3) The present invention can eliminate the phase deviation caused by the propagation of sound waves across the medium and eliminate reconstruction artifacts. When a 100kHz microphone is used for sound collection, the image resolution of laser acoustic imaging can reach 3cm.

[0041] (4) The present invention has low computational complexity and high reconstruction efficiency, and the imaging frame rate for target objects within a depth of 10m can reach 10Hz. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a general scheme diagram of the present invention.

[0043] Figure 2 It is a schematic diagram of the principle of the present invention.

[0044] Figure 3 It is a data processing flow chart of the present invention.

[0045] Figure 4 This is the first step phase shift migration result diagram for an underwater point source.

[0046] Figure 5 This is the result diagram of the second-step phase shift migration for an underwater point source.

[0047] Figure 6 This is the imaging result using iterative extrapolation imaging technology.

[0048] Figure 7 This is the imaging result using the step-by-step phase shift migration technique. DETAILED DESCRIPTION

[0049] In order to make the purpose, content and advantages of the present invention more clear, the specific implementation methods of the present invention are further described in detail below in conjunction with the drawings and examples.

[0050] See also Figure 1 The cross-medium laser acoustic imaging system based on step-by-step phase shift in this embodiment is a laser acoustic imaging system in an "acoustic-optical-acoustic" mode, including a laser unit, an optical transceiver unit, a microphone detection unit, an information acquisition and processing unit, and a signal processing / image reconstruction unit; wherein, the laser unit generates an excitation laser, which acts on the surface of a water target through the optical transceiver unit, and the laser acoustic wave in the form of a spherical wave generated by the water target is transmitted to the microphone detection unit through the optical transceiver unit, and the microphone detection unit scans and collects the laser acoustic signal reflected by the water target and propagated upward point by point, and transmits the collected laser acoustic signal to the information acquisition and processing unit for amplification and analog-to-digital conversion processing, and then the signal processing / image reconstruction unit realizes cross-medium reconstruction of the underwater target object according to the first phase shift in the air and the second phase shift in the water.

[0051] Among them, the laser unit includes a laser driving circuit and a laser, and the laser driving circuit controls the laser to generate an excitation laser; the microphone detection unit includes a controller and a microphone, and the controller controls the microphone to perform point-by-point scanning; the information acquisition and processing unit includes an analog-to-digital conversion circuit and an amplification circuit; the optical transceiver unit includes a transmitting antenna and a receiving antenna.

[0052] The cross-media laser acoustic imaging system also includes:

[0053] The stepper motor is an aerial platform stepper motor, on which an omnidirectional single-beam microphone is arranged to scan and collect the laser sound signal reflected by the water target and propagated upward point by point.

[0054] The system power supply provides power for the laser unit, microphone detection unit, and information acquisition and processing unit.

[0055] See also Figure 2 , which is a schematic diagram of the principle of the present invention, a high-energy excitation laser generated by a laser acts on the surface of a water body to generate a laser acoustic wave in the form of a spherical wave; an omnidirectional single-beam microphone is installed on a stepping motor of an aerial platform to scan and collect point by point the laser acoustic signal reflected by a water body target and propagated upward; the collected laser acoustic signal is sequentially offset according to the first phase shift in the air and the second phase shift in the water to realize cross-medium reconstruction of the underwater target object.

[0056] See also Figure 3 , is a data processing flow chart of the present invention. The specific steps include:

[0057] Step 1: Perform preliminary signal rough processing on the laser acoustic remote sensing data obtained by linear scanning, and the specific implementation includes the following sub-steps.

[0058] Step 1.1: Based on the prior imaging target information, estimate the flight time range of the effective laser acoustic signal and determine the interception range of the original signal to reduce the reconstruction calculation time.

[0059] Step 1.2: Filter the intercepted waveform by a bandpass filter with matching bandwidth to reduce the interference of the platform and the surrounding environment on the laser acoustic signal.

[0060] Step 2: Perform a two-dimensional Fourier transform on the roughly processed photoacoustic data to obtain the photoacoustic field information in the frequency wavenumber domain The formula for the two-dimensional Fourier transform is as follows:

[0061]

[0062] Step 3: Perform the first phase shift to reversely extrapolate the photoacoustic field information in the frequency wavenumber domain collected by the microphone to the water-air interface in the air medium through the phase shift method. Obtain the upper boundary photoacoustic field information of the water-air interface in the frequency wavenumber domain. Figure 4 , is the first step phase shift migration result diagram for underwater point source. The formula for the first step phase shift migration extrapolation is as follows:

[0063]

[0064] Step 4: Use Snell's law to correct the laser acoustic wave energy, calculate the projection coefficient τ of the air-water interface acoustic field, and obtain the lower boundary acoustic field at the air-water interface in the water medium The calculation formula of the lower boundary sound field is as follows:

[0065]

[0066] Step 5: Execute the second phase shift offset—STOLT offset, which is specifically implemented by the following sub-steps.

[0067] Step 5.1: Based on the horizontal wave number, calculate the vertical wave number in the water medium:

[0068]

[0069] Step 5.2: By replacing the variables of wave number and angular frequency, establish the differential transformation relationship between wave number and angular frequency, further calculate the STOLT migration operator, and obtain the laser acoustic field distribution after STOLT migration. Figure 5 , is the result of the second-step phase shift migration for underwater point sources. The formula for STOLT migration is as follows:

[0070]

[0071] Step 5.3: Perform a two-dimensional Fourier inverse transform on the laser acoustic field after STOLT migration to obtain the reconstructed laser acoustic field information. The formula for the two-dimensional Fourier inverse transform is as follows:

[0072]

[0073] Step 6: Perform Hilbert transform on the reconstructed laser acoustic field information to obtain the envelope and complete the laser acoustic imaging of the underwater target object.

[0074] See also Figure 6 and Figure 7 , which are the imaging result diagrams using the iterative extrapolation imaging technique and the imaging result diagram using the step-by-step phase shift migration technique, respectively.

[0075] It can be seen from the above technical scheme that the present invention proposes a cross-medium imaging method of step-by-step phase shift and applies it to "acoustic-optical-acoustic" mode laser acoustic remote sensing; the cross-medium imaging method of step-by-step phase shift refers to performing a first-step phase shift based on reverse extrapolation of phase shift in an air medium, and performing a second-step shift based on Stolt shift in a water medium; according to the characteristics of cross-medium transmission of laser acoustic signals in the "acoustic-optical-acoustic" mode, the present invention uses an omni-directional single-beam microphone to scan point by point and collect laser acoustic signals reflected by a water target and propagating upward; the collected laser acoustic signals are sequentially subjected to signal coarse processing, two-dimensional Fourier transform, the first-step phase shift in an air medium, air-water interface laser acoustic wave energy correction, the second-step phase shift in a water medium, and Hilbert transform to obtain signal envelope, thereby completing laser acoustic imaging of underwater target objects, successfully solving the problem of cross-medium transmission of laser acoustic waves and cross-medium reconstruction of laser acoustic remote sensing.

[0076] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A trans-medium laser acoustic imaging system based on step-by-step phase shift, characterized in that: include: A laser unit, an optical transceiver unit, a microphone detection unit, an information collection and processing unit, and a signal processing / image reconstruction unit; wherein the laser unit generates an excitation laser, which acts on the surface of a water target through the optical transceiver unit, and the laser sound wave in the form of a spherical wave generated by the water target is transmitted to the microphone detection unit through the optical transceiver unit. The microphone detection unit scans and collects point by point the laser sound signal reflected by the water target and propagated upward, and transmits the collected laser sound signal to the information collection and processing unit for amplification and analog-to-digital conversion. The signal processing / image reconstruction unit then realizes cross-medium reconstruction of the underwater target object according to the first phase shift offset in the air and the second phase shift offset in the water.

2. The trans-medium laser acoustic imaging system based on step-by-step phase shift migration according to claim 1, characterized in that: The laser unit includes a laser driving circuit and a laser, and the laser driving circuit controls the laser to generate an excitation laser; the microphone detection unit includes a controller and a microphone, and the controller controls the microphone to perform point-by-point scanning; the information acquisition and processing unit includes an analog-to-digital conversion circuit and an amplification circuit; the optical transceiver unit includes a transmitting antenna and a receiving antenna.

3. The trans-medium laser acoustic imaging system based on step-by-step phase shifting according to claim 2, characterized in that: Also includes: The stepper motor is an aerial platform stepper motor, on which an omnidirectional single-beam microphone is arranged to scan point by point and collect the laser sound signal reflected by the water target and propagated upward.

4. The trans-medium laser acoustic imaging system based on step-by-step phase shifting according to claim 3, characterized in that: Also includes: The system power supply provides power for the laser unit, microphone detection unit, and information acquisition and processing unit.

5. A trans-medium laser acoustic imaging method based on step-by-step phase shifting based on the trans-medium laser acoustic imaging system according to any one of claims 1 to 4, characterized in that: The following steps are involved: Step 1: Perform preliminary signal rough processing on the laser acoustic remote sensing data obtained by linear scanning; Step 2: Perform a two-dimensional Fourier transform on the roughly processed photoacoustic data to obtain the photoacoustic field information in the frequency wavenumber domain Step 3: Perform the first phase shift to reversely extrapolate the photoacoustic field information in the frequency wavenumber domain collected by the microphone to the water-air interface in the air medium through the phase shift method. Obtain the upper boundary photoacoustic field information in the frequency wavenumber domain of the water-air interface; Step 4: Use Snell's law to correct the laser acoustic wave energy, calculate the projection coefficient τ of the air-water interface acoustic field, and obtain the lower boundary acoustic field at the air-water interface in the water medium Step 5: Execute the second phase shift offset—STOLT offset; Step 6: Perform Hilbert transform on the reconstructed laser acoustic field information to obtain the envelope and complete the laser acoustic imaging of the underwater target object.

6. The method for trans-medium laser acoustic imaging based on step-by-step phase shifting according to claim 5, characterized in that: Step 1 includes the following sub-steps: Step 1.1: Based on the prior imaging target information, estimate the flight time range of the effective laser acoustic signal and determine the interception range of the original signal; Step 1.2: Filter the intercepted waveform through a bandpass filter with matching bandwidth to reduce the interference of the platform and surrounding environment on the laser acoustic signal.

7. The method for trans-medium laser acoustic imaging based on step-by-step phase shifting according to claim 6, characterized in that: In step 2, the formula for the two-dimensional Fourier transform is as follows:

8. The method for trans-medium laser acoustic imaging based on step-by-step phase shifting according to claim 7, characterized in that: In step 3, the first step of phase shift extrapolation is given by:

9. The method for trans-medium laser acoustic imaging based on step-by-step phase shifting according to claim 8, characterized in that: In step 3, the calculation formula of the lower boundary sound field is as follows:

10. The method for trans-medium laser acoustic imaging based on step-by-step phase shifting according to claim 9, characterized in that: Step 5 includes the following sub-steps: Step 5.1: Based on the horizontal wave number, calculate the vertical wave number in the water medium: Step 5.2: By replacing the variables of wave number and angular frequency, the differential transformation relationship between wave number and angular frequency is established, the STOLT migration operator is further calculated, and the laser acoustic field distribution after STOLT migration is obtained; the formula of STOLT migration is as follows: Step 5.3: Perform a two-dimensional inverse Fourier transform on the laser acoustic field after STOLT migration to obtain the reconstructed laser acoustic field information; the formula for the two-dimensional inverse Fourier transform is as follows:

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