CN110441780B - Ultrasonic phased array correlation imaging method - Google Patents

Ultrasonic phased array correlation imaging method Download PDF

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CN110441780B
CN110441780B CN201910772193.5A CN201910772193A CN110441780B CN 110441780 B CN110441780 B CN 110441780B CN 201910772193 A CN201910772193 A CN 201910772193A CN 110441780 B CN110441780 B CN 110441780B
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CN110441780A (en
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任新敏
张钦伟
于永河
顾永建
李文东
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Ocean University of China
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Abstract

The invention provides an ultrasonic phased array correlation imaging method, which constructs an ultrasonic correlation imaging physical model, utilizes an ultrasonic phased array to generate a radiation sound field with certain directivity and certain space randomness, utilizes numerical calculation to analyze the relation between array parameters and the random radiation sound field, and finally utilizes a pseudo-inverse algorithm to perform correlation reconstruction on a scattering target so as to verify the feasibility and the effectiveness of the imaging method. The imaging method solves the technical problem that the resolution of the traditional sonar imaging is limited by the Rayleigh diffraction limit, and aims to realize the long-distance high-resolution target imaging in the underwater environment.

Description

Ultrasonic phased array correlation imaging method
Technical Field
The invention relates to the technical field of sonar imaging, in particular to an ultrasonic phased array correlation imaging method.
Background
Currently, underwater imaging technology is a common means for detecting underwater targets, and is divided into underwater optical imaging technology and underwater acoustic imaging technology. Due to the absorption and scattering effects of water, the underwater optical imaging technology can only detect the target at a short distance, and the detection distance is extremely limited. Compared with light waves, the energy of ultrasonic waves in water is attenuated more slowly, and the transmission distance is longer. Meanwhile, the ultrasonic wave has the characteristics of good directivity, energy concentration, strong reflectivity and the like.
The operating principle of the sonar detection system is that sound waves are radiated to a certain area at a certain sector and a certain pitching angle. The sound waves are scattered somewhat diffusely at the target surface, with a portion of the signals being received by the sonar system. The sonar transducer carries out corresponding signal processing processes such as compensation and filtering on the received echo signals to obtain the sound wave amplitude of different positions of the target field, and then acoustic imaging is achieved. A conventional ultrasound image map, as shown in figure 1. Traditional side scan sonar imaging and phased array multi-beam synthesis imaging rely on real beam point-by-point scanning detection to extract target information, and the imaging process is essentially to radiate the same sound wave signals to a target, and the target is recovered through coherent chromatography. The resolution of ultrasound imaging depends primarily on the arrayThe aperture produces a beam with a full width at half maximum and an angular resolution of theta-3dBAnd the value is approximately equal to 0.89 lambda/L, lambda is the ultrasonic wave length, and L is the array aperture length.
The resolution of traditional sonar imaging is limited by the rayleigh diffraction limit, and how to realize long-distance high-resolution target imaging in an underwater complex environment is a major subject of research at present. With the development of quantum information technology, correlation imaging for acquiring rich target information by using a second-order correlation function of a random light field is favored by researchers. The imaging technology has the characteristics of noise interference resistance, super-resolution imaging, non-local imaging and the like. The method is based on the idea of calculating associated imaging, and is introduced into the field of ultrasonic imaging to break through the limitation of array aperture and realize super-resolution imaging.
Disclosure of Invention
The application overcomes the problem that the resolution ratio of the traditional sonar imaging is limited by the Rayleigh diffraction limit by providing an ultrasonic phased array correlation imaging method, so as to realize the target imaging of long-distance high resolution ratio in the underwater environment.
In order to solve the technical problems, the application adopts the following technical scheme:
an ultrasonic phased array correlation imaging method comprises the following steps:
s1: the array sonar emits ultrasonic waves, constructs and generates a space random radiation sound field with certain directivity, radiates to a detection target, and generates sound pressure as follows:
Figure BDA0002173935540000021
in the formula, rsIs the position of the sound source s, | r-rsL is the distance from the sound source to the detection target, c is the sound wave propagation speed, t is the time, and after the sound pressure P (r, t) interacts with the detection target, a scattering sound field is generated, and the sound pressure of the scattering sound wave is: p is a radical ofscaWhere σ (r) is a target backscattering coefficient, and the sound pressure of an echo signal when the scattered sound wave propagates to a sonar receiving place is:
Figure BDA0002173935540000022
in the formula, s0Standing for receiving sonar r0A position for receiving sonar;
s2: obtaining Q times of samples by emitting Q times of random radiation sound fields, and performing first-order association on the Q times of samples of the radiation sound fields and the Q times of received scattering sound fields to obtain a target backscattering coefficient:
Figure BDA0002173935540000023
i.e. the imaging result of the correlation process, where P is the complex conjugate of P.
Further, the random radiation sound field at the detection target satisfies the temporal and spatial irrelevancy, namely: r (R)n,tn；rm,tm)＝∫Pn(rn,tn)Pm(rm,tm)dtdr＝δ(rn-rm)δ(tn-tm) In the formula, rm、rnAt two arbitrary points in the space within the beam, tm、tnAt any two times, Pm(rm,tm) As spatial spot r in the beammAt tmSound pressure magnitude at time, Pn(rn,tn) As spatial spot r in the beamnAt tnMagnitude of sound pressure at time, R (R)n,tn；rm,tm) Representing the correlation coefficient.
Further, the space random radiation sound field is generated by a two-dimensional plane ultrasonic phased array sonar structure. Further, the two-dimensional plane ultrasonic phased sonar far-field sound pressure is:
Figure BDA0002173935540000031
wherein B (x, y, z) is a vector at any position in space, ρ is the density of the liquid,
Figure BDA0002173935540000032
is the wave velocity, λBIs the liquid bulk modulus, v0(omega) is the average space distribution speed in sonar array element, k 02 pi/lambda is waveThe number of the first and second groups is,
Figure BDA0002173935540000033
is the angular frequency, /)xIs the length of the array element in the x direction, lyThe length of the array element in the y direction,
Figure BDA0002173935540000034
Figure BDA0002173935540000035
the direction of the single array element is shown,
Figure BDA0002173935540000036
the sound field deflection angle direction is
Figure BDA0002173935540000037
Figure BDA0002173935540000038
(theta, phi) is the deflection angle under the spherical coordinate system, delta phim,n(t) random phases of the additional superpositions of the subarrays which follow a uniform distribution of delta phim,n(t) to (0,2 π) and pitch of sonar units in x-direction is sxAnd the pitch of sonar units in the x direction is sy，sx＝lx+gx，sy＝ly+gy，gxSonar unit gap width in the x direction, gyThe sonar array has mkxnl sonar units in total for the y-direction sonar unit gap width, and is divided into mxn sub-arrays, each of which includes K × L array elements.
Further, the comprehensive directional directivity of the random radiation sound field is as follows:
Figure BDA0002173935540000039
further, a pseudo-inverse algorithm is adopted to carry out numerical simulation of the ultrasonic phased array correlation imaging so as to demonstrate feasibility of the ultrasonic phased array correlation imaging method.
Compared with the prior art, the technical scheme that this application provided, the technological effect or advantage that have are: the problem that the resolution ratio of the traditional sonar imaging is limited by the Rayleigh diffraction limit is solved, and the long-distance high-resolution target imaging in the underwater environment is expected to be realized.
Drawings
FIG. 1 is a diagram of conventional ultrasound imaging;
FIG. 2 is a diagram of a physical model of ultrasound correlation imaging;
FIG. 3 is a schematic diagram of arrangement of two-dimensional planar ultrasonic phased array sonar units;
FIG. 4 is a schematic view of the deflection angle direction of the sound field;
fig. 5(a) shows a random radiation sound field with an array element distribution M-N-10 and K-L-4;
FIG. 5(b) is a cross-sectional x-axis view of FIG. 5 (a);
fig. 5(c) shows a random radiation sound field with an array element distribution M-N-3 and K-L-4;
FIG. 5(d) is a cross-sectional x-axis view of FIG. 5 (c);
fig. 6(a) shows a random radiation sound field with an array element distribution M-N-20 and K-L-2;
FIG. 6(b) is a cross-sectional x-axis view of FIG. 6 (a);
fig. 6(c) shows a random radiation sound field with an array element distribution M-N-4 and K-L-10;
FIG. 6(d) is a cross-sectional x-axis view of FIG. 6 (c);
FIG. 7(a) shows a random radiated sound field with an array element spacing of 0.009 m;
FIG. 7(b) is a cross-sectional x-axis view of FIG. 7 (a);
FIG. 7(c) shows a random radiation sound field with an array element spacing of 0.005 m;
FIG. 7(d) is a cross-sectional x-axis view of FIG. 7 (c);
FIG. 8(a) is a randomly radiated acoustic field with a signal frequency of 150 kHz;
FIG. 8(b) is a cross-sectional x-axis view of FIG. 8 (a);
FIG. 8(c) shows a randomly radiated acoustic field with a signal frequency of 75 kHz;
FIG. 8(d) is a cross-sectional x-axis view of FIG. 8 (c);
FIG. 9(a) is a schematic diagram of an original distant target simulated under simulation conditions;
FIG. 9(b) is a schematic diagram of spatial distribution of a randomly radiated sound field under a simulation condition of one;
FIG. 9(c) is a schematic view of an imaging target reconstructed by pseudo-inverse correlation under a simulation condition one;
FIG. 9(d) is a schematic diagram of the spatial autocorrelation of a randomly radiated sound field under simulation condition one;
FIG. 10 is a plot of sample number versus imaging quality;
FIG. 11(a) is a schematic diagram of an original distant target simulated under simulation condition two;
FIG. 11(b) is a schematic diagram of the spatial distribution of the random radiation sound field under the second simulation condition;
FIG. 11(c) is a diagram of pseudo-inverse correlation reconstructed imaging target under simulation condition two
FIG. 11(d) is a schematic diagram of the spatial autocorrelation of a randomly radiated sound field under the simulation condition two
FIG. 12 is a diagram illustrating the relationship between the imaging SNR and the number of samples after reducing the randomness of the sound field.
Detailed Description
The embodiment of the application overcomes the problem that the resolution ratio of the traditional sonar imaging is limited by the Rayleigh diffraction limit by providing the ultrasonic phased array correlation imaging method, so that the long-distance high-resolution target imaging in the underwater environment is realized.
In order to better understand the technical solutions, the technical solutions will be described in detail below with reference to the drawings and specific embodiments.
Examples
An ultrasonic phased array correlation imaging method comprises the following steps:
s1: the array sonar emits ultrasonic waves, constructs and generates a space random radiation sound field with certain directivity, radiates to a detection target, and generates sound pressure as follows:
Figure BDA0002173935540000051
in the formula, rsIs the position of the sound source s, | r-rsI is the distance from the sound source to the detection target, c is the sound wave propagation speed, t is the time, the sound pressure P (r, t) and the detection targetAfter the interaction of the targets, a scattering sound field is generated, and the sound pressure of the scattering sound wave is as follows: p is a radical ofscaWhere σ (r) is a target backscattering coefficient, and the sound pressure of an echo signal when the scattered sound wave propagates to a sonar receiving place is:
Figure BDA0002173935540000052
in the formula, s0Standing for receiving sonar r0A position for receiving sonar;
s2: obtaining Q times of samples by emitting Q times of random radiation sound fields, and performing first-order association on the Q times of samples of the radiation sound fields and the Q times of received scattering sound fields to obtain a target backscattering coefficient:
Figure BDA0002173935540000061
i.e. the imaging result of the correlation process, where P*Is the complex conjugate of P.
Only enough space random sound fields are radiated on the target, the limitation of the traditional wave beam resolution ratio can be broken through, and the solution of the target scattering system in the wave beam is realized, therefore, the random radiation sound field at the position of the detection target must meet the irrelevance between time and space to realize the ultrasonic phased array correlated imaging, namely:
R(rn，tn；rm，tm)＝∫Pn(rn，tn)Pm(rm，tm)dtdr＝δ(rn-rm)δ(tn-tm)
in the formula, rm、rnAt two arbitrary points in the space within the beam, tm、tnAt any two times, Pm(rm,tm) As spatial spot r in the beammAt tmSound pressure magnitude at time, Pn(rn,tn) As spatial spot r in the beamnAt tnMagnitude of sound pressure at time, R (R)n,tn；rm,tm) Representing the correlation coefficient.
The basic physical model of ultrasound correlation imaging is shown in fig. 2, and comprises transmitting array elements and receiving elementsArray element, under far field condition, the acoustic wave that array sonar sent arrives detection target position department through propagating, and its acoustic pressure is the stack of all sound sources:
Figure BDA0002173935540000062
assuming that the backscattering coefficient distribution of the target in the plane of the target region is sigma (r), the backscattering sound wave of the target under the condition of first-order Bonn approximation and neglecting secondary scattering is as follows: p is a radical ofsca(r,t)＝p(r,t)σ(r)
The echo signals of the scattered sound waves propagated to the receiving sonar are as follows:
Figure BDA0002173935540000063
q times of samples are obtained by emitting Q times of space-time uncorrelated randomly radiated sound fields. Performing first-order association on the Q-time samples of the radiation sound field and the Q-time received scattering sound field to obtain the spatial distribution of the target scattering coefficient:
Figure BDA0002173935540000064
the conventional ultrasonic phased array sonar controls the phase of each array unit according to a rule to realize the scanning of beams in a space domain. However, the key to realizing the ultrasound phased array correlation imaging is to generate a radiated sound field with certain directivity and spatial randomness. In the application, a random radiation sound field with certain directivity is generated by adopting a two-dimensional plane ultrasonic phased array sonar, so that a random fluctuation speckle light field in optical correlation imaging is simulated.
Time-independent signal sampling can be achieved by multiple phase-switch modulations. As shown in fig. 3, the two-dimensional planar phased array sonar is divided into a plurality of signal sub-arrays with random initial phases, and array elements in each sub-array form beams with uniform directivity according to the conventional multi-beam synthesis technology; a fully independent phase is superposed among the sub-arrays, and the sound field of any point in the space is the incoherent superposition of the sound fields generated by all the sub-arrays, so that the radiation sound field with certain directivity and space random fluctuation is guaranteed to be constructed. In order to simplify the analysis, the forming process of a random radiation sound field of the phased array sonar distributed in a two-dimensional grid mode is studied by taking the phased array sonar as an example. The method for constructing the random radiation sound field can also be popularized to the case of other array configurations.
It is assumed that a two-dimensional array of elements consists of identical elements and radiates acoustic waves into a single liquid medium. As shown in FIG. 3, let the reference cell (0,0) be at O, and the length of the array element in X direction and Y direction be lxAnd lyThe gap width is gxAnd gyThe pitch of the sonar units is sxAnd syThen sx＝lx+gx，sy＝ly+gy. The sonar array has mkxnl sonar units in common, the array is divided into M × N sub-arrays, each sub-array contains K × L array elements, each unit is labeled with (q, i) coordinates, where q is the index of the array element in the X direction, i is the index of the array element in the Y direction, that is:
Figure BDA0002173935540000071
the coordinate position of the (q, i) th array element is R(q,i)Qx + iy, where X and Y represent unit vectors in the X and Y directions, respectively.
Vector B (x, y, z) at an arbitrary position in space, | R under far-field conditions(q,i)If R < 1, then the distance from the (q, i) th array element to position B (x, y, z) can be approximated as:
Figure BDA0002173935540000072
in the above formula
Figure BDA0002173935540000073
If the array element itself is smaller than λ in both the X direction and the Y direction, each array element can be regarded as a single point source model. In the case of a single-point source model of the array in liquid immersion, the sound pressure at the position of the spatial point B is:
Figure BDA0002173935540000081
in the above formula, the first and second carbon atoms are,
Figure BDA0002173935540000082
the directivity of the single array element is shown, rho is the liquid density,
Figure BDA0002173935540000083
is the wave velocity, λBIs the liquid bulk modulus, v0(omega) is the space average distribution speed in the sonar array element,
Figure BDA0002173935540000084
as a weighted value, Δ tq,iFor time delay, k 02 pi/lambda is the wave number,
Figure BDA0002173935540000085
is the angular frequency.
In far field conditions, ignoring different Rq,iThe sound pressure at any position B is
Figure BDA0002173935540000086
Assuming a sound field deflection angle direction of
Figure BDA0002173935540000087
As shown in fig. 4. Then the time delay of the (q, i) th array element is:
Figure BDA0002173935540000088
in FIG. 4, the deflection angles in the spherical coordinate system are (theta, phi), and the unit vector of the rotation direction
Figure BDA0002173935540000089
Assuming that the weighted values of the array elements are all 1, and through simplification, the far-field sound pressure of the two-dimensional plane ultrasonic phased array sonar is as follows:
Figure BDA00021739355400000810
the comprehensive directional directivity of the random radiation sound field under the far field condition is as follows:
Figure BDA0002173935540000091
next, the phased array synthesized deflection-free (Θ is 0, and Φ is 0) random radiated sound field will be analyzed by numerical calculation. And analyzing the relationship between the acoustic array factors such as the number of sub-arrays, the spacing between the sub-arrays, the array distribution, the signal frequency and the like and the randomness of the random radiation sound field.
1) Number of subarrays
FIG. 5 shows the random radiation sound field with different sub-array numbers, and the array parameters are as follows: the spacing between array elements is 0.009m, the length of the array elements is 0.005m, the transmission distance is 100m, the frequency is 150kHz (the wavelength is 0.01m), and the field range is 200m by 200 m. As shown in fig. 5, the phased array synthesis method produces a radiated sound field with random fluctuation in the main beam lobe and the side lobe, which is different from the approximate Sinc function form presented by the conventional multi-beam synthesis sound field. The number KL of sonar units in the subarray determines the width of a main lobe of a radiation sound field, the field range of the main lobe is determined, and the number MN of random subarrays determines the spatial fluctuation degree of the random radiation sound field. With the increase of the number of the subarrays MN, the randomness of a two-dimensional random radiation sound field is better, the amplitude fluctuation is more severe, the spatial cross correlation is worse, and the sound field radiated each time has independence, so that the reconstruction of a target is more facilitated. Therefore, the number of the sub-arrays can be increased properly, and a radiation sound field with strong space random fluctuation can be obtained. However, the subarrays cannot be increased one by one due to the fact that spatial randomness is sought. The increased subarrays cause system design complexity, which results in a large system and high cost.
2) Array arrangement
FIG. 6 shows the random radiation sound field of different array distributions, which has the following array parameters: the spacing between array elements is 0.009m, the length of the array elements is 0.005m, the transmission distance is 100m, the frequency is 150kHz (the wavelength is 0.01m), and the field range is 200m by 200 m. Fig. 6 shows the relationship between the geometrical arrangement of the array elements and the amplitude randomness of the two-dimensional random radiation sound field under the condition that the total number of the array elements is the same. When the number MN of the subarrays is reduced and the number KL of the single subarray elements is increased, the range of the main lobe view field of the two-dimensional random radiation sound field is reduced, and the detection of a large target is not facilitated. When the number MN of the subarrays is increased and the number KL of the single subarray elements is reduced, the main view field range of the two-dimensional random radiation sound field is enlarged, and the detection of a large target is facilitated. Therefore, an experimental system with a single subarray number KL can be designed according to the size of the target view field, and the main view field can be ensured to cover the target sufficiently. Meanwhile, the number MN of the sub-arrays is increased as much as possible, the randomness of the main lobe radiation sound field is increased, the cross correlation of the main lobe view field radiation sound field is reduced, and the independence of the single random radiation sound field is improved.
3) Spacing of array elements
FIG. 7 shows a random radiation sound field with different array element spacings, and the array parameters are as follows: the array element distribution M is 10, L is 4, the length of the array element is 0.005M, the transmission distance is 100M, the frequency is 150kHz (wavelength is 0.01M), and the field range is 200M by 200M. It can be found from fig. 7 that as the array element spacing increases, the fluctuation of the two-dimensional random radiation sound field is increased, the spatial cross-correlation is deteriorated, and the condition of spatial irrelevance is more satisfied. Therefore, the array element spacing can be properly enlarged, and the spatial irrelevance of the sound field in the range of the main lobe field of view is improved.
4) Frequency of signal
FIG. 8 shows the random radiation sound field with different signal frequencies, and the array parameters are as follows: the array element distribution M is 10, L is 4, the length of the array element is 0.005M, the spacing between the array elements is 0.009M, the transmission distance is 100M, the signal frequency is 150kHz (wavelength is 0.01M), and the field range is 200M by 200M. We analyzed the relationship of ultrasonic signal frequency to the random radiated sound field. As can be seen from fig. 8, when the ultrasonic frequency is high with the same array element parameters, the random fluctuation of the radiation sound field is severe, that is, the random fluctuation of the radiation sound field with shorter wavelength is good. Meanwhile, it should be noted that the high-frequency ultrasonic waves have large loss in the medium, and the transmission distance is reduced. Therefore, when an experimental system is designed, the detection distance needs to be comprehensively considered, the signal frequency needs to be properly increased, and the randomness of the random radiation sound field is improved.
In conclusion, the space random radiation sound field constructed by ultrasonic phased array synthesis meets the space-time independence. The number KL of array elements in the single subarray determines the field range of the main lobe, and the number MN of random subarrays determines the space randomness of a sound field. Meanwhile, the spatial randomness of a radiated sound field is seriously influenced by the array structure. With the increase of the number MN of random subarrays, the array element spacing is increased, the signal frequency is increased, the fluctuation of a two-dimensional random radiation sound field is enhanced, and the spatial cross-correlation is poor. The phase shifter of the sonar array is controlled through the phase switch, so that the time of a two-dimensional random radiation sound field can be guaranteed to be irrelevant, and the radiation sound field can be used for detecting a target in a view field in a certain direction.
Finally, the method and the device perform correlation reconstruction on the scattering target by using a pseudo-inverse algorithm, analyze the imaging resolution and the peak signal-to-noise ratio, and demonstrate the feasibility and the effect of ultrasonic phased array correlation imaging.
Reconstructing an ultrasonic phased array associated imaging target based on a pseudo-inverse algorithm:
according to the multi-beam synthesis technology and the Wheatstone Fresnel principle, as long as the spatial position of the transmitting array element and the modulation signal are known, the spatial distribution of the sound field radiated to the target scene is determined. Assuming that the scattering coefficient spatial distribution of the target at the far-field position B (x, y, z) is σ (B), Q sample signals are sampled by phase switch modulation, and the sampling result is obtained
Figure BDA0002173935540000111
P(Bi,ω,ti) Is the far field sound pressure, P (r), of a two-dimensional phased array sonar0,ω,ti) Refers to the sound pressure of the received sound field.
When Q is more than or equal to NxNyAnd by adopting a least square method to calculate the inverse operation of the radiation sound field and the receiving sound field, the target can be decoupled:
Figure BDA0002173935540000112
wherein the content of the first and second substances,
Figure BDA0002173935540000118
Figure BDA0002173935540000117
a first order correlation pseudo-inverse reconstruction algorithm is shown.
The Peak Signal-to-Noise Ratio (PSNR) is used for evaluating the target imaging quality of the correlation reconstruction, and the formula is as follows:
Figure BDA0002173935540000115
in the above equation, MSE is the mean square error between the reconstructed target and the original target,
Figure BDA0002173935540000116
in the above formula, Ii,jFor correlating the (i, j) th pixel of the reconstructed target image,
Figure BDA0002173935540000121
respectively, indicates the (i, j) th pixel associating the reconstructed target image with the original target image.
Ultrasonic phased array correlation imaging numerical simulation based on a pseudo-inverse algorithm:
the target is subjected to numerical simulation by using a random radiation sound field generated by the ultrasonic phased array synthesis structure and a pseudo-inverse correlation reconstruction algorithm, and the feasibility of ultrasonic phased array correlation imaging is demonstrated.
1) Resolution of imaging
FIG. 9 is a resolution diagram of pseudo-inverse correlation reconstructed object imaging. The array parameters are as follows: the frequency is 150kHz (wavelength is 0.01M), the length of the array element is 0.008M, the array arrangement M is N is 20, K is L is 2, the array element spacing is 0.008M, the transmission distance is 100M, and the field range is 200M by 200M. Simulation condition one: after a two-dimensional random radiation sound field generated by 40 × 40(20 × 20 subarrays) sonar units is transmitted for 100m, pseudo-inverse correlation reconstruction is carried out on targets at intervals of 2m within the range of 200m × 200m of the field of view. As shown in fig. 9, after 300 samples were collected, the scatter at 2m intervals could be clearly resolved; the half-height width of the wave beam of the traditional multi-beam synthetic imaging under the same condition is 3.5m, and the Rayleigh diffraction limit transverse resolution is 2.78m, which is identical with the advantage of improving the image resolution of the associated imaging.
2) Sample number versus imaging quality
On the basis of fig. 9, the PSNR versus the number of samples in fig. 9 was analyzed. It can be seen from fig. 10 that the signal-to-noise ratio of the associated imaging peak starts to rise rapidly, then increases slowly, and finally tends to be stable, and when the number of samples reaches 260, the number of samples tends to be substantially stable, and the increase of the number of samples has no great significance on the improvement of the image quality. The main reason for this is that the starting samples are not correlated spatio-temporally, each sample contains rich target information, and PSNR rises rapidly. As the number of samples increases, there is some correlation between the samples, and increasing samples provides the target information, but the information content decreases. When enough target information is provided by the sample, the target is completely reconstructed, and no additional target information is provided by adding the sample, and the PSNR is not increased. The number of samples can improve the quality of the image to a certain extent. Therefore, to reconstruct a clear object, a larger number of samples is required, which inevitably also results in complexity and temporal redundancy in object reconstruction. Therefore, the relationship between the time of target reconstruction and the imaging PSNR is selected as needed.
3) Relationship between sound field space randomness and imaging quality
FIG. 11 is a diagram of the relationship between spatial randomness of sound field and imaging quality for a target reconstruction, and the array parameters are: the frequency is 150kHz (wavelength is 0.01M), the array element length is 0.008M, the array element spacing is 0.008M, the array arrangement M is 15 equal to N, 2 equal to K, the transmission distance is 100M, and the field range is 200M. The randomness of the two-dimensional random radiation sound field is related to the number of sub-arrays, the spacing of the sub-arrays and the signal frequency. In order to observe the relationship between the space randomness and the imaging quality of the random radiation sound field, the analysis is performed by taking the number of sub-arrays as an example. For comparison with the case of fig. 9, condition two is simulated: the two-dimensional random radiation sound field generated by using 30 × 30(15 × 15 subarrays) sonar units still performs pseudo-inverse correlation reconstruction on scattering targets at intervals of 2m within a transmission distance of 100m and a field range of 200m × 200m, as shown in fig. 11. Under this condition, scattering targets at a distance of 2m cannot be resolved, and a scattering target of 2m cannot be resolved even if the number of samples is increased. Under the condition that array elements in the single subarrays are the same, namely the main lobe fields of view are the same, along with the reduction of the number of the random subarrays, the space randomness of the random radiation sound field is poor, namely the independence of the samples is poor. When the randomness of the random radiation sound field is poor, the imaging resolution is also reduced when the object is associated and reconstructed. The stronger the space randomness of the random radiation sound field is, the easier the target information can be obtained, and the high-resolution target detection can be realized.
The PSNR of the correlated imaging under this condition was further analyzed. As shown in fig. 12, when the number of samples is small, the samples are almost uncorrelated, and the PSNR rapidly increases as the number of samples increases. When the number of samples increases to a certain extent, only limited information is provided per sample, the PSNR increases slowly, and the PSNR stabilizes at about 6.8 at 300 samples. And the PSNR finally stabilizes around 7.8 at the sample number 300 in fig. 10. This also indicates that the stronger the spatial randomness of the randomly radiated acoustic field, the higher the imaging quality.
The relation between the associated imaging quality and the space randomness of a random sound field is explained only by taking the number of random sub-arrays as an example, the result is that along with the increase of the number of the random sub-arrays, the space autocorrelation is better, the random radiation sound field generated each time is almost irrelevant, each sample can provide more target information during target reconstruction, the target reconstruction is more facilitated, the quality of the reconstructed target image is better, and the imaging resolution is also improved. This conclusion is in full agreement with the principles of correlation imaging. Therefore, when the verification experiment is carried out, in addition to the increase of the signal frequency, the increase of the array element spacing, and the increase of the number of random sub-arrays mentioned in the above section, the randomness of the random radiation sound field space is also improved.
In fact, not all units are equipped with digital control phase shifters in the way of constructing a random radiation sound field by phased sub-array synthesis, and if the wave beams are not scanned electrically, the method can be realized as long as each sub-array shares one phase shifter, so that the system cost and complexity are greatly reduced. And the space random radiation sound field is formed by utilizing a plurality of subarray multi-beam synthesis by using the sonar parallel-serial phase shifter with the form, and the space random radiation sound field also has the characteristic of coexistence of directivity and randomness.
In the embodiments of the application, an ultrasonic phased array correlation imaging physical model is constructed by providing an ultrasonic phased array correlation imaging method, an ultrasonic phased array is used for generating a random radiation sound field with certain directivity and certain space, the relation between array parameters and the random radiation sound field is analyzed by using numerical calculation, and finally, a pseudo-inverse algorithm is used for performing correlation reconstruction on a scattering target so as to verify the feasibility and effectiveness of the imaging method. The imaging method solves the technical problem that the resolution of the traditional sonar imaging is limited by the Rayleigh diffraction limit, and aims to realize the long-distance high-resolution target imaging in the underwater environment.
It should be noted that the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make variations, modifications, additions or substitutions within the spirit and scope of the present invention.

Claims (2)

1. An ultrasonic phased array correlation imaging method is characterized by comprising the following steps:
s1: the sonar array emits ultrasonic waves, constructs and generates a space random radiation sound field with certain directivity, radiates to a detection target, and generates sound pressure as follows:
Figure FDA0003201468840000011
in the formula, rsIs the position of the sound source s, | r-rsL is the distance from the sound source to the detection target, c is the sound wave propagation speed, t is the time, and after the sound pressure P (r, t) interacts with the detection target, a scattering sound field is generated, and the sound pressure of the scattering sound wave is: p is a radical ofscaWhere σ (r) is a target backscattering coefficient, and the scattered sound waves reach an echo where sonar is received by propagationThe signal sound pressure is:
Figure FDA0003201468840000012
in the formula, s0Standing for receiving sonar r0A position for receiving sonar;
the space random radiation sound field is generated by a two-dimensional plane ultrasonic phased array sonar structure, the two-dimensional plane ultrasonic phased array sonar is divided into a plurality of signal sub-arrays with random initial phases, array elements in each sub-array form uniformly-directed beams according to a conventional multi-beam synthesis technology, a fully-independent phase is superposed between the sub-arrays, and the sound field of any point in the space is the incoherent superposition of the sound fields generated by all the sub-arrays, so that the radiation sound field with certain directivity and space random fluctuation is guaranteed to be constructed; the sonar array has MK multiplied by NL sonar units, the array is divided into M multiplied by N sub-arrays, each sub-array comprises K multiplied by L array elements, a reference unit (0,0) is arranged at the position O, and the lengths of the array elements in the X direction and the Y direction are respectively LxAnd lyThe gap width is gxAnd gyThe pitch of the sonar units is sxAnd syThen sx＝lx+gx，sy＝ly+gyLabeling each cell with (q, i) coordinates, where q is the index of the array element in the X direction and i is the index of the array element in the Y direction, i.e.:
Figure FDA0003201468840000021
the coordinate position of the (q, i) th array element is R(q,i)Qx + iy, whereinxAnd Y represent unit vectors in the X and Y directions, respectively;
s2: obtaining Q times of samples by emitting Q times of random radiation sound fields, and performing first-order association on the Q times of samples of the radiation sound fields and the Q times of received scattering sound fields to obtain a target backscattering coefficient:
Figure FDA0003201468840000022
i.e. processed for associationImaging results of wherein P*Is the complex conjugate of P, wherein the random radiated sound field at the detection target satisfies the temporal and spatial irrelevancy: r (R)n,tn；rm,tm)＝∫Pn(rn,tn)Pm(rm,tm)dtdr＝δ(rn-rm)δ(tn-tm) In the formula, rm、rnAt two arbitrary points in the space within the beam, tm、tnAt any two times, Pm(rm,tm) As spatial spot r in the beammAt tmSound pressure magnitude at time, Pn(rn,tn) As spatial spot r in the beamnAt tnMagnitude of sound pressure at time, R (R)n,tn；rm,tm) Representing a correlation coefficient;
the two-dimensional plane ultrasonic phase control sonar far-field sound pressure is as follows:
Figure FDA0003201468840000023
wherein B (x, y, z) is a vector at an arbitrary position in space, ρ is a liquid density, c is a wave velocity,
Figure FDA0003201468840000024
λBis the liquid bulk modulus, v0(omega) is the average space distribution speed in sonar array element, k02 pi/lambda is the wave number,
Figure FDA0003201468840000031
is the angular frequency, /)xIs the length of the array element in the x direction, lyThe length of the array element in the y direction,
Figure FDA0003201468840000032
Figure FDA0003201468840000033
representing single array elementsThe direction of the light beam is pointed,
Figure FDA0003201468840000034
the sound field deflection angle direction is
Figure FDA0003201468840000035
Figure FDA0003201468840000036
Figure FDA0003201468840000037
Is the deflection angle of delta phi under a spherical coordinate systemm,n(t) random phases of the additional superpositions of the subarrays which follow a uniform distribution of delta phim,n(t) to (0,2 π) and pitch of sonar units in x-direction is sxThe pitch of the sonar unit in the y direction is sy，sx＝lx+gx，sy＝ly+gy，gxSonar unit gap width in the x direction, gyThe sonar array has MK multiplied by NL sonar units which are the sonar unit gap width in the y direction, the array is divided into M multiplied by N sub-arrays, and each sub-array comprises K multiplied by L array elements;
the comprehensive directional directivity of the random radiation sound field is as follows:
Figure FDA0003201468840000038
2. the ultrasound phased array correlated imaging method according to claim 1, characterized in that a pseudo-inverse algorithm is used to perform numerical simulation of the ultrasound phased array correlated imaging to demonstrate feasibility of the ultrasound phased array correlated imaging method.
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