Low-Cost High-Resolution Side-Scan Sonar Imaging Method

Abstract

In order to overcome the problems of high cost and low resolution of traditional side-scan sonar imaging methods, the present invention proposes a low-cost and high-resolution side-scan sonar imaging method. This method first designs a low-cost array that can be used for side-scan sonar imaging; then uses the designed low-cost array to perform high-resolution side-scan imaging on a single survey line; then continuously moves the side-scan sonar along the forward direction, and continuously repeats the transmission and reception processes to obtain K single-survey line imaging results, and then splices the K single-survey line imaging results to obtain the final side-scan sonar imaging result. Compared with traditional side-scan sonar imaging methods, the method proposed by the present invention can improve the imaging resolution of side-scan sonar while significantly reducing the number of receiving array elements, that is, significantly reducing the cost, by reasonably arranging the transmitting array and the receiving array.

Description

Technical Field

[0001] The present invention belongs to the field of sonar imaging methods, and particularly relates to a low-cost high-resolution side-scan sonar imaging method. Background Art

[0002] A side-scan sonar is a device that uses the echo measurement principle to map the seabed topography and detect underwater objects, and it is widely used in the fields of underwater search and underwater exploration.

[0003] Currently, during the working process, the side-scan sonar often uses a large-aperture linear array at the transmitting end to transmit pulsed signals, forming a single transmitting beam to irradiate the imaging scene; at the receiving end, a large-aperture linear array is used to form a single receiving beam to process the echoes, and the echo intensities are plotted in the order of time to obtain the side-scan imaging result of a single survey line; the side-scan sonar is moved for multiple transmissions and receptions to obtain imaging results of multiple survey lines; finally, the images of multiple single-survey-line imaging results are stitched together to obtain the image of the entire imaging area. Figure 1 Fig. shows the array schematic diagram in a side-scan sonar composed of a 128-element transmitting linear array and a 128-element receiving linear array.

[0004] The angular resolution of the traditional side-scan sonar imaging method is determined by the apertures of the transmitting array and the receiving array. In order to obtain higher imaging angular resolution, the traditional method generally uses a larger-aperture array to achieve this goal. However, a larger-aperture array will increase the number of receiving array elements, resulting in a sharp increase in sensor cost, hardware cost, etc. Therefore, how to obtain higher imaging angular resolution under the requirement of low cost has become one of the difficult problems in the development process of side-scan sonar technology. Summary of the Invention

[0005] In order to overcome the problems of too high cost and low resolution of the traditional side-scan sonar imaging method, the present invention proposes a low-cost high-resolution side-scan sonar imaging method. This method reduces the element cost of the side-scan sonar by reasonably arranging the transmitting array and the receiving array and significantly reducing the number of array elements, while obtaining higher angular resolution than the traditional side-scan sonar imaging method.

[0006] The technical solution of the present invention is as follows:

[0007] A low-cost high-resolution side-scan sonar imaging method, characterized by comprising the following steps:

[0008] Step 1: Design a low-cost array that can be used for side-scan sonar imaging;

[0009] Step ②: Use the designed low-cost array to perform high-resolution side-scan imaging of a single survey line;

[0010] Step 3: Continuously move the sidescan sonar along the forward direction, and continuously repeat the transmission and reception processes in Step 2 to obtain K single-line imaging results. Then, splice the K single-line imaging results to obtain the final sidescan sonar imaging result.

[0011] Further, in the said Step 1:

[0012] Define the transmitting array as an M-element horizontally uniform linear array, and the receiving array as N r sub-arrays with relatively small apertures. Then, the element coordinates (x t , y t , z t ) of the transmitting array are:

[0013]

[0014] y t = [0 0 … 0 0] 1×M

[0015] z t = [d tr d tr … d tr d tr 1×M

[0016] The element coordinates (x r , y r , z r ) of the receiving array are:

[0017]

[0018]

[0019]

[0020] Among them,

[0021] M is the number of elements of the transmitting array, and M ≥ 32;

[0022] d t is the element spacing of the transmitting array, and c / (2f t ) ≤ d t ≤ 5c / (2f t ), where c is the underwater sound speed and f t is the designed frequency of the transmitting array;

[0023] N is the number of elements of each receiving sub-array, and 1 ≤ N ≤ M / 2;

[0024] d r is the element spacing in each receiving sub-array, and c / (2f r ) ≤ dr ≤5c / (2f r ), where f r is the design frequency of a single receiving sub-array;

[0025] N r is the number of receiving sub-arrays;

[0026] d is the equivalent acoustic center spacing between N r sub-arrays, and its value is d = Md t ;

[0027] d tr is the spacing between the horizontal transmitting linear array and the horizontal receiving linear array in the z-axis direction in the same xOz plane, and c / (2f t ) ≤ d tr ≤ 5c / (2f t ).

[0028] Furthermore, step 2 includes:

[0029] Step 2-1: At the transmitting end, use an M-element horizontal linear array to transmit a pulse signal, and form a single transmitting beam in the normal direction of the transmitting linear array to illuminate a single survey line in the imaging area;

[0030] Step 2-2: At the receiving end, use N r sub-arrays to receive the echoes, sum the echoes on the N r sub-arrays, and form a single receiving beam in the normal direction of the receiving linear array;

[0031] Step 2-3: Calculate the intensity of the signals on the receiving beam in chronological order to obtain the single survey line imaging result.

[0032] Furthermore,

[0033] In step 2-2: Taking the two-dimensional side-scan sonar as a model, the z-axis coordinate does not need to be considered during the imaging process. Define P far-field targets as ideal scattering points and located at (x p , y p )(p = 1,.., P). The acoustic center of the receiving array of the side-scan sonar is located at (x0, y0) in the xOy plane. Then the angle θ p between the p-th target and the sonar normal is:

[0034]

[0035] N r The echo data X(t) collected by the sub-receiving arrays is expressed as:

[0036]

[0037] Where,

[0038] s(t0) is the synchronous transmission pulse signal of the M-element horizontal line array;

[0039] t0 is the time series of the transmitted pulse waveform;

[0040]

[0041]

[0042] t is the time series of the echoes collected by the N r sub-receiving arrays.

[0043] Optionally,

[0044] The conditions for the establishment of the said X(t) include: defining that the sonar is in a relatively stationary state between the instantaneous imaging moment and the target, ignoring the echo Doppler frequency shift; defining that the acoustic pulse does not undergo waveform distortion during propagation in the water medium, ignoring interface reverberation and volume reverberation, and ignoring the influence of noise on the echo.

[0045] Furthermore,

[0046] In the said step 2-3: The imaging result b(k,t) of the k-th single scan line is expressed as:

[0047]

[0048] Furthermore,

[0049] In the said step 3: The final side-scan sonar imaging result B(k,t) is expressed as:

[0050] B(k,t) = [b(1,t) … b(k,t) … b(K,t)].

[0051] The beneficial effects of the present invention are:

[0052] 1. The present invention proposes a side-scan sonar imaging method with low cost and high resolution, optimizes the receiving array of the traditional side-scan sonar, and realizes a higher resolution imaging effect by using the optimized side-scan sonar receiving array pattern.

[0053] 2. The basic principle and implementation scheme of the present invention have been verified by computer numerical simulation. The results show that: compared with the traditional side-scan sonar imaging method, the method proposed by the present invention can improve the side-scan sonar imaging resolution while significantly reducing the number of receiving array elements, that is, significantly reducing the cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 It is a schematic diagram of the traditional side-scan sonar array pattern with 128 elements for transmission and 128 elements for reception;

[0055] Figure 2 For the method proposed in the present invention, M = 128, N r = 2, N = 1, schematic diagram of the side-scan sonar array;

[0056] Figure 3 For the method proposed in the present invention, M = 128, N r = 2, N = 4, schematic diagram of the side-scan sonar array;

[0057] Figure 4 For the method proposed in the present invention, M = 64, N r = 4, N = 1, schematic diagram of the side-scan sonar array;

[0058] Figure 5 For the method proposed in the present invention, M = 64, N r = 4, N = 4, schematic diagram of the side-scan sonar array;

[0059] Figure 6 Schematic diagram of the double-scatterer target distribution in the implementation example of the present invention;

[0060] Figure 7 Imaging result diagram of the traditional side-scan sonar with 128 elements transmitting and 128 elements receiving;

[0061] Figure 8 For the method proposed in the present invention, M = 128, N r = 2, N = 1, imaging result diagram of the side-scan sonar;

[0062] Figure 9 For the method proposed in the present invention, M = 128, N r = 2, N = 4, imaging result diagram of the side-scan sonar;

[0063] Figure 10 For the method proposed in the present invention, M = 64, N r = 4, N = 1, imaging result diagram of the side-scan sonar;

[0064] Figure 11 For the method proposed in the present invention, M = 64, N r = 4, N = 4, imaging result diagram of the side-scan sonar;

[0065] Figure 12 Slice diagram of the five simulated imaging results sliced along y = 100m;

[0066] Figure 13 Slice diagram of the imaging result of the traditional side-scan sonar with 128 elements transmitting and 128 elements receiving;

[0067] Figure 14 For the method proposed in the present invention, M = 128, N r = 2, N = 1, slice diagram of the imaging result of the side-scan sonar;

[0068] Figure 15 For the method proposed by the present invention, M = 128, N r = 2, N = 4, the sliced view of the side-scan sonar imaging result;

[0069] Figure 16 For the method proposed by the present invention, M = 64, N r = 4, N = 1, the sliced view of the side-scan sonar imaging result;

[0070] Figure 17 For the method proposed by the present invention, M = 64, N r = 4, N = 4, the sliced view of the side-scan sonar imaging result;

[0071] Figure 18 The flow chart of the method proposed by the present invention. Detailed implementation manners

[0072] In order to overcome the problems of high cost and low resolution of the traditional side-scan sonar imaging method, the present invention proposes a low-cost and high-resolution side-scan sonar imaging method. By reasonably arranging the transmitting array and the receiving array, the number of array elements is significantly reduced to reduce the array element cost of the side-scan sonar, and at the same time, a higher angular resolution than the traditional side-scan sonar imaging method is obtained.

[0073] The technical solutions adopted by the present invention to solve the existing problems can be divided into the following 3 steps:

[0074] Step 1: Design a low-cost array for side-scan sonar imaging.

[0075] Design the transmitting array as an M (M≥32, M is the number of elements of the transmitting array) -element horizontal uniform linear array, and the element spacing is d t , and c / (2f t ) ≤ d t ≤ 5c / (2f t ), where c is the underwater sound speed and f t is the design frequency of the transmitting array.

[0076] Design the receiving array as N r sub-arrays with small apertures. Each sub-array is composed of an N (1≤N≤M / 2, N is the number of elements of each receiving sub-array) -element horizontal uniform linear array. The element spacing in each receiving sub-array is d r , and c / (2f r ) ≤ d r ≤ 5c / (2f r ), where f r is the design frequency of a single receiving sub-array. The equivalent acoustic center spacing between the N r sub-arrays is d, and its value is d = Md t, in order to reduce the number of array elements to lower the system cost, the number of sub-arrays N r is recommended to be 2, 3, 4, etc., and the number of array elements N of each receiving sub-array can take relatively small values, such as 1, 2, 4, etc.

[0077] The horizontal transmitting line array and the horizontal receiving line array are located in the same xOz plane. The horizontal transmitting line array and the horizontal receiving line array are parallel to each other, and the spacing in the z-axis direction is d tr , and c / (2f t ) ≤ d tr ≤ 5c / (2f t ), then the array element coordinates (x t , y t , z t ) of the transmitting array can be written as:

[0078]

[0079] The array element coordinates (x r , y r , z r ) of the receiving array can be written as:

[0080]

[0081] When the number of transmitting array elements M = 128, the number of receiving sub-arrays N r = 2, and the number of receiving array elements N of a single sub-array is 1, 4, and when the number of transmitting array elements M = 64, the number of receiving sub-arrays N r = 4, and the number of receiving array elements N of a single sub-array is 1, 4, the corresponding side-scan sonar formation schematic diagrams are respectively as Figure 2 , Figure 3 , Figure 4 , Figure 5 shown.

[0082] Step 2: Use the designed low-cost formation for single-line high-resolution side-scan imaging.

[0083] Step 2-1: At the transmitting end, use the M-element horizontal line array to synchronously transmit the pulse signal s(t0), where t0 is the time series of the transmitted pulse waveform. The M-element horizontal line array synchronously transmits to form a single narrow beam, illuminating a single survey line in the imaging area.

[0084] Step 2-2: After the pulse signal is reflected by the scatterers in the scene, at the receiving end, use N r sub-receiving arrays to collect the echoes. Taking the two-dimensional side-scan sonar as a model, the z-axis coordinate does not need to be considered during the imaging process. Assume that P far-field targets are modeled as ideal scatterers and are located at (x p , y pAt (p = 1, .., P), the acoustic center of the receiving array of the side-scan sonar is located at (x0, y0) in the xOy plane. Then the angle θ between the p-th target and the sonar normal p is:

[0085]

[0086] Assume that the sonar and the target are in a relatively static state at the instant of imaging, ignore the echo Doppler frequency shift, assume that the acoustic pulse does not undergo waveform distortion when propagating in the water medium, and ignore the interface reverberation and volume reverberation, and ignore the influence of noise on the echo. Then the N r echo data X(t) collected by the sub-receiving arrays can be expressed as:

[0087]

[0088] where

[0089]

[0090]

[0091] t is the time series of the echoes collected by the N r sub-receiving arrays;

[0092] Sum the echoes collected by all receiving array elements, and form a single receiving beam in the normal direction of the receiving linear array.

[0093] Step 2-3: Calculate the intensity of the signals on the receiving beam in chronological order, and the k-th single-line imaging result b(k, t) can be obtained:

[0094]

[0095] Step 3: Continuously move the side-scan sonar along the forward direction, and continuously repeat the transmission and reception processes in Step 2. Then K single-line imaging results can be obtained. Then splice the K single-line imaging results to obtain the final side-scan sonar imaging result B(k, t):

[0096] B(k, t) = [b(1, t) … b(k, t) … b(K, t)] (6)

[0097] Simulation example

[0098] Set the basic simulation parameters:

[0099] Suppose the true speed of sound wave propagation underwater is 1500 m / s. The side-scan sonar transmitting array is a horizontally uniform linear array. The designed frequency of the array is 450 kHz, the spacing between transmitting array elements is half a wavelength, i.e., 0.001667 m, the sampling frequency is 2 MHz, the transmitted signal is a CW pulse signal, the transmitted signal frequency is 450 kHz, and the signal pulse width is 0.05 ms.

[0100] Suppose the side-scan sonar moves from (-10 m, 0 m) along the positive x-axis to (10 m, 0 m) to image the first and second quadrants of the coordinate axis. Two equal-intensity scatterer targets are located at (0 m, 100 m) and (1.4 m, 100 m) respectively. The distribution schematic diagram is as Figure 6 shown.

[0101] The following gives a simulation embodiment of the present invention. Numerical simulation is carried out by computer simulation to verify the effect of the method proposed by the present invention:

[0102] Simulation 1: Image using the traditional side-scan sonar imaging method.

[0103] The number of elements in the side-scan sonar transmitting array is 128, the receiving array is a 128-element horizontally uniform linear array, the designed frequency of the array is 450 kHz, the spacing between array elements is half a wavelength, i.e., 0.001667 m. The schematic diagram of the array shape of the traditional side-scan sonar imaging method is as Figure 1 shown.

[0104] Simulation 2: Image using the low-cost high-resolution side-scan sonar imaging method of the present invention.

[0105] The number of elements in the side-scan sonar transmitting array is 128, the receiving array is composed of 2 sub-arrays. Both sub-arrays are horizontally uniform linear arrays. The number of elements in each sub-array is 1. The spacing between receiving hydrophone array elements within the sub-array is half a wavelength, i.e., 0.001667 m. The spacing between the acoustic centers of the sub-arrays is 128 times the spacing between transmitting array elements, i.e., 0.213333 m. The schematic diagram of the array shape is as Figure 2 shown.

[0106] Simulation 3: Image using the low-cost high-resolution side-scan sonar imaging method of the present invention.

[0107] The number of elements in the side-scan sonar transmitting array is 128, the receiving array is composed of 2 sub-arrays. Both sub-arrays are horizontally uniform linear arrays. The number of elements in each sub-array is 4. The spacing between receiving hydrophone array elements within the sub-array is half a wavelength, i.e., 0.001667 m. The spacing between the acoustic centers of the sub-arrays is 128 times the spacing between transmitting array elements, i.e., 0.213333 m. The schematic diagram of the array shape is as Figure 3 shown.

[0108] Simulation 4: Image using the low-cost high-resolution side-scan sonar imaging method of the present invention.

[0109] The side scan sonar transmitting array has 64 elements and the receiving array has 4 sub-arrays. The sub-arrays are all horizontal uniform linear arrays. The number of elements in each sub-array is 1. The spacing between the receiving hydrophone elements in the sub-array is half a wavelength of 0.001667m. The spacing between the sub-array acoustic centers is 64 times the spacing between the transmitting elements, 0.213333m. The array diagram is shown below. Figure 4 shown.

[0110] Simulation 5: Imaging using the low-cost, high-resolution side-scan sonar imaging method of the present invention.

[0111] The side scan sonar transmitting array has 64 elements and the receiving array has 4 sub-arrays. The sub-arrays are all horizontal uniform linear arrays. The number of elements in each sub-array is 4. The spacing between the receiving hydrophone elements in the sub-array is half a wavelength of 0.001667m. The spacing between the sub-array acoustic centers is 64 times the spacing between the transmitting elements, 0.213333m. The array diagram is shown below. Figure 5 shown.

[0112] Simulation 1 is imaged according to the traditional imaging process, and the two-dimensional imaging results of the traditional side-scan sonar imaging method are obtained as follows: Figure 7 As shown, according to the method proposed in the present invention, simulation 2, simulation 3, simulation 4, and simulation 5 are imaged respectively, and the imaging results of the low-cost and high-resolution side-scan sonar imaging method proposed in the present invention are respectively as shown in FIG. Figure 8 、 Figure 9 、 Figure 10 、 Figure 11 shown.

[0113] The imaging results of simulation 1, simulation 2, simulation 3, simulation 4, and simulation 5 are sliced along y = 100m. For the convenience of comparison, the slice results of the five simulation results are placed in the same figure, as shown in the figure below. Figure 12 As shown, the slice diagrams of the imaging results of simulation 1, simulation 2, simulation 3, simulation 4, and simulation 5 are shown separately as follows Figure 13 、 Figure 14 、 Figure 15 、 Figure 16 、 Figure 17 shown.

[0114] Comparing the imaging results of the traditional method with those of the proposed method, it can be seen that the traditional method cannot distinguish the two scatterers, while the low-cost and high-resolution method proposed in the present invention can clearly distinguish the two scatterers. Figure 1 and Figure 2 、 Figure 3 、 Figure 4 、 Figure 5, it can be seen that when the present invention is properly designed, the number of transmitting elements and the number of receiving elements can be optimized, resulting in a significant reduction in cost. Therefore, compared with the traditional method, the method proposed by the present invention can achieve higher resolution with fewer transmitting and receiving elements and lower cost.

Claims

1. A low-cost high-resolution side-scan sonar imaging method, characterized in that, Including the following steps: Step 1: Design a low-cost array for side-scan sonar imaging; Define the transmitting array as an M - element horizontally uniform linear array, and the receiving array as N r sub - arrays with smaller apertures. Then the element coordinates (x t , y t , z t ) of the transmitting array are as follows: y t ＝[0 0…0 0] 1×M z t = [d tr d tr …d tr d tr 1×M ​ The array element coordinates (x r , y r , z r ) of the receiving array are as follows: wherein, M is the number of transmitting array elements, and M≥32; d t is the element spacing of the transmitting array, and c / (2f t ) ≤ d t ≤ 5c / (2f t ), where c is the underwater sound speed, and f t is the designed frequency of the transmitting array; N is the number of elements in each receiving sub-array, and 1≤N≤M / 2; d r is the element spacing in each receiving subarray, and c / (2f r ) ≤ d r ≤ 5c / (2f r ), where f r is the design frequency of a single receiving subarray; N r is the number of receiving sub-arrays; d is N r the equivalent acoustic center spacing between sub-arrays, and its value is d = Md t ; d tr is the spacing between the horizontal transmitting line array and the horizontal receiving line array in the z-axis direction in the same xOz plane, and c / (2f t ) ≤ d tr ≤ 5c / (2f t ); Step 2: Use the designed low-cost array for single-line high-resolution side-scan imaging; Step 2-1: At the transmitting end, use an M-element horizontal linear array to transmit a pulse signal, and form a single transmitting beam in the normal direction of the transmitting linear array to illuminate a single line in the imaging area; Step 2-2: At the receiving end, use N r sub-arrays to receive the echoes, sum up the echoes on the N r sub-arrays, and form a single receiving beam in the normal direction of the receiving linear array; Step 2-3: Calculate the intensity of the signals on the receiving beam in chronological order to obtain the single-line imaging result; Step 3: Continuously move the side-scan sonar along the forward direction, and continuously repeat the transmitting and receiving processes in Step 2 to obtain K single-line imaging results, and then splice the K single-line imaging results to obtain the final side-scan sonar imaging result.

2. The low-cost high-resolution side-scan sonar imaging method according to claim 1, wherein, In Step 2-2: Taking a two-dimensional side-scan sonar as a model, the z-axis coordinate does not need to be considered during the imaging process. Define P far-field targets modeled as ideal scatterers and located at (x p , y p )(p = 1,.., P). The acoustic center of the receiving array of the side-scan sonar is located at (x0, y0) in the xOy plane. Then the angle θ p between the p-th target and the sonar normal is: N r The echo data X(t) collected by N sub-receiving arrays is expressed as: s(t0) is the synchronous transmission pulse signal of the M-element horizontal linear array; t0 is the time series of the transmitted pulse waveform; t is N r time series of echoes collected by 3. The low-cost high-resolution side-scan sonar imaging method according to claim 2, wherein, The conditions for the establishment of X(t) include: Define that the sonar is in a relatively stationary state between the instantaneous imaging moment and the target, and ignore the echo Doppler frequency shift; Define that the sound pulse does not undergo waveform distortion when propagating in the water medium, ignore the interface reverberation and volume reverberation, and ignore the influence of noise on the echo.

4. The low-cost high-resolution side-scan sonar imaging method according to claim 1, characterized in that, In Step 2-3: The k-th single-line imaging result b(k,t) is expressed as:

5. The low-cost high-resolution side-scan sonar imaging method according to claim 1, wherein In Step 3: The final side-scan sonar imaging result B(k,t) is expressed as: B(k,t) = [b(1,t)…b(k,t)…b(K,t)].

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