Equivalent sound velocity automatic inversion method and system based on multi-beam overlap region constraint

Abstract

The invention belongs to the technical field of marine surveying and mapping, and discloses an equivalent sound velocity automatic inversion method and system based on multi-beam overlap region constraint. The method comprises the following steps of: performing beam point location tracking on multi-beam original data by using surface sound velocity of multi-beam data to obtain an overlapping region of a multi-beam strip; gridding overlapped areas of the multi-beam strips, minimizing water depth deviation between overlapped grids by using a genetic algorithm, and performing inversion to obtain the optimal equivalent sound velocity of each measuring line; and performing beam point location tracking again on each measuring line by using the corresponding equivalent sound velocity obtained by inversion, weakening sound velocity deviation existing in beam point location tracking by using the surface sound velocity, weakening a curved terrain between the strips in the overlapping region, and obtaining a submarine topography with continuous splicing of the overlapping regions of the adjacent strips. According to the invention, the precision and reliability of submarine topography measurement can be ensured, and the speed of submarine topography measurement field operation is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of ocean surveying and mapping, and in particular relates to an equivalent sound speed automatic inversion method and system based on multi-beam overlapping area constraints. Background Art

[0002] The development and exploration of the ocean are increasing day by day. However, underwater topography measurement is the basic work of ocean surveying and mapping. It uses sound waves to measure the speed of sound, propagation time, attitude, water level and other information to obtain information such as water depth and location. It can provide services for the development of various marine projects, and can also provide basic data for nautical charts and marine geographic information applications. It is a vital basic work for people to develop and explore the ocean. Compared with the single-beam operation mode, multi-beam bathymetry technology has achieved an upgrade from linear measurement to surface coverage. A large amount of bathymetric point data can be obtained in the vertical direction of the track line, and the size, shape and fluctuation of underwater targets can be efficiently measured. This technology has the advantages of high efficiency, high precision, high resolution and full coverage. It is widely used in seabed resource exploration, channel mapping, island reef measurement, port monitoring and underwater archaeology, and has become the core means of obtaining seabed topography and geomorphology information from shallow sea to deep sea.

[0003] Multibeam bathymetry systems measure water depth by emitting sound waves and receiving signals reflected from the seafloor. However, the speed at which sound waves propagate in water changes with water depth, mainly due to the uneven distribution of temperature and salinity in the water column. Changes in the speed of sound in seawater affect the speed of sound wave propagation, which in turn affects the accuracy of the bathymetry results. Therefore, accurate sound velocity profile information is essential for converting the propagation time of sound waves into water depth. In some cases, such as an environment where freshwater and seawater are mixed in a certain area, the change in sound velocity may be very drastic, resulting in significant deviations from the traditional results based on measuring a small number of sound velocity profiles instead of the entire area, that is, the measured water depth is underestimated or overestimated in the outer part of the coverage area.

[0004] At present, there are towing systems for obtaining sound velocity profiles, such as mobile profilers or by measuring seawater conductivity, temperature, pressure, etc. to obtain sound velocity profile measurement data. However, they are not widely used because there is a risk of entanglement or grounding of towed instruments every time they are deployed. Instead, measurements are taken using sound velocity profilers and temperature-salinity-depth meters. In order to make such measurements, the ship needs to remain stationary. Therefore, obtaining sound velocity profile measurement data becomes a time-consuming process, which is of poor practicality and low efficiency in actual engineering applications. Summary of the invention

[0005] To overcome the problems existing in the related art, the disclosed embodiments of the present invention provide an equivalent sound velocity automatic inversion method and system based on multi-beam overlap area constraints. The present invention solves the problem of terrain distortion caused by inaccurate sound velocity measurement and improves the low efficiency of multi-beam field measurement.

[0006] The technical solution is as follows: an equivalent sound velocity automatic inversion method based on multi-beam overlap zone constraints comprises the following steps: S1, using the surface sound velocity of the multi-beam data to track the beam position of the multi-beam raw data, and obtain the overlapping area of ​​the multi-beam strips; S2, grid the overlapping areas of the multi-beam strips, use the genetic algorithm to minimize the water depth deviation between the overlapping grids, and invert the optimal equivalent sound speed for each survey line; S3, re-track the beam point using the corresponding equivalent sound speed obtained by inversion for each survey line, reduce the sound speed deviation existing in beam point tracking using surface sound speed, weaken the terrain distortion between strips in the overlapping area, and obtain the real seabed topography where adjacent strips overlap and are connected continuously, thus getting rid of the dependence of beam point tracking on the measured sound speed information.

[0007] In step S1, the overlapping area of ​​the multi-beam strips is obtained, including: S101, performing data analysis on the acquired multi-beam raw data, using the surface sound velocity information measured by the surface sound velocity meter in the data or assuming a sound velocity profile to perform sound line tracking on the data, and obtaining the initial water depth and position of all measuring points; S102, searching for overlapping areas between the survey lines along a direction perpendicular to the multi-beam heading.

[0008] In step S101 , the sound ray tracking includes the sound ray tracking based on the constant sound velocity within the layer and the sound ray tracking based on the constant gradient within the layer.

[0009] Furthermore, the sound ray tracking based on the constant sound velocity within the layer includes: The speed of sound is The layer propagates at a constant speed. The depths of the upper and lower interfaces are and , the layer thickness is , , Respectively The beam incident angle and sound speed of the layer, according to Snell's law, , then the horizontal displacement of the beam within the layer and propagation time They are: (1) The horizontal distance the beam travels through the water column and propagation time for: (2) In the formula, is the number of water layers; If the incident angle is 0, that is, single beam sounding, then: ; .

[0010] Furthermore, the sound ray tracking based on the constant gradient within the layer includes: Speed ​​of sound in layer Constant gradient The actual propagation trajectory of the beam in the layer is a continuous trajectory with a curvature radius of The arc segment is expressed as: (3) (4) In the formula, is the sound speed of the next water layer; The horizontal displacement experienced by the beam in this layer and arc length for: (5) In the formula, Deepen the shooting angle for the next layer of water; According to Snell's law: , then Horizontal displacement of sound rays within the layer and time for: (6) In the formula, For Layer The harmonic mean speed of sound within (7) If the incident angle is 0, the propagation trajectory of the sound wave in each layer is a straight line, vertically downward, and the propagation time in each layer is: (8).

[0011] In step S2, the water depth deviation between overlapping grids is minimized using a genetic algorithm, including: The overlapping area data of the gridded multi-beam strips are passed to the genetic algorithm, and the genetic algorithm is used for optimization; the genetic algorithm randomly generates a population within the range, and then performs crossover and mutation operations to calculate the fitness function. The fitness function for: ; In the formula, is the equivalent sound speed to be determined, indicating the area The sound velocity gradient corresponding to each strip; is the water depth at each grid node, is the inverse distance weighted average water depth of all measuring points in the grid, is the total number of grids in the selected area, For the region Grid The total number of measuring points of the measuring line; Determine the current fitness function Whether the calculated average water depth deviation of the overlapping area is less than 1% of the average water depth, if it is reached, the loop is exited and the optimal equivalent sound speed of each measuring line is returned. If it is not reached, the iterative calculation is repeated.

[0012] Furthermore, the genetic algorithm randomly generates a population within a range, and then performs crossover and mutation operations. The specific method for calculating the fitness function is as follows: S201, according to the range of initial value , randomly generated data volume The initial value of is called the generated initial value is the initial population; S202, respectively calculate the value of each seed of the generated initial population Function value, according to the size of each seed value, the initial population is screened to select The seeds with small function values ​​are retained; then the seeds with deviations exceeding the threshold are mutated and crossed according to the mutation probability and crossover probability to form a new population; S203, recalculate the respective Value, genetically retain excellent seed values, replace inferior individuals, and regenerate new population individuals; S204, performing a mutation crossover operation on the newly generated population individuals to generate a new population after mutation crossover, and then calculating the new population ; S205, repeating step S203 and step S204 until the optimal value is found or the maximum number of iterations is reached, thereby exiting and outputting the best equivalent sound speed value.

[0013] In step S2, the optimal equivalent sound velocity for each survey line is obtained by inversion, including: The equivalent sound speed homing process is: (9) In the formula, The sound velocity gradient is a constant, For depth, is the speed of sound, is the underlying sound velocity, is the surface sound velocity, is the bottom water depth, is the incident surface sound velocity, is a constant; The propagation of sound waves in water conforms to Snell's law, which is expressed as: (10) In the formula, , are the upper and lower beam incident angles of a single layer, is a constant obtained based on Snell; The propagation time is obtained according to the principle of equivalent sound velocity profile method for: (11) Combining equations (10) and (11), the propagation time is derived for: (12) Since the propagation time is a known quantity that is precisely measured, we can convert equation (12) into for: (13) In the formula, All are one-way propagation times; According to the theory of ray acoustics, the propagation trajectory of sound rays in a constant gradient layer is an arc, so the radius of the arc is for: (14) The displacement of the sound line in the depth direction is derived and the horizontal displacement They are: (15).

[0014] Another object of the present invention is to provide an equivalent sound velocity automatic inversion system based on multi-beam overlap area constraint, the system implements the equivalent sound velocity automatic inversion method based on multi-beam overlap area constraint, the system comprises: The multi-beam strip overlapping area acquisition module is used to track the beam points of the multi-beam raw data using the surface sound velocity of the multi-beam data to obtain the multi-beam strip overlapping area; The inversion equivalent sound velocity module is used to grid the overlapping areas of the multi-beam strips, minimize the water depth deviation between the overlapping grids using a genetic algorithm, and invert the optimal equivalent sound velocity for each survey line; The re-beam point tracking module is used to re-track the corresponding equivalent sound speed obtained by inversion for each survey line, reduce the sound speed deviation existing in beam point tracking using surface sound speed, weaken the terrain distortion between strips in the overlapping area, and obtain the real seabed topography of the overlapping area of ​​adjacent strips, thereby getting rid of the dependence of beam point tracking on the measured sound speed information.

[0015] Furthermore, the automatic inversion system for equivalent sound speed based on multi-beam overlapping area constraints is carried on a computer device, which includes: at least one processor, a memory, and a computer program stored in the memory and executable on the at least one processor. When the processor executes the computer program, the functions of the automatic inversion system for equivalent sound speed based on multi-beam overlapping area constraints are implemented.

[0016] In combination with all the above technical solutions, the beneficial effects of the present invention are as follows: The present invention uses the surface sound velocity of multi-beam data to perform beam point tracking on the multi-beam raw data to obtain the overlapping area of ​​the strips; the multi-beam strips are gridded, and the water depth deviation between the overlapping grids is minimized by using a genetic algorithm, and the optimal equivalent sound velocity of each survey line is inverted; the beam point tracking is re-performed on each survey line using the corresponding equivalent sound velocity obtained by inversion, thereby reducing the sound velocity deviation existing in the beam point tracking using the surface sound velocity, so that the crying and laughing face-shaped curved terrain between the strips in the overlapping area is weakened, and a more continuous seabed terrain is obtained in the overlapping area of ​​adjacent strips, thereby reducing the dependence of the beam point tracking on the measured sound velocity profile.

[0017] In actual measurements of multi-beam systems, adjacent strips will have a certain area of ​​overlap. The present invention will make full use of the overlapping areas between multi-beam measurement strips to re-track the sound line, thereby recalculating the measured depth. The present invention inverts the equivalent sound speed in seawater by minimizing the difference in water depth in the overlapping areas of different tracks as a constraint, thereby achieving re-tracking of the sound line. The advantage of this method is that it can use the redundant information in the multi-beam data to achieve accurate multi-beam measurement point position reduction without direct sound speed measurement. Through this method, the accuracy and reliability of seabed topography measurement can be guaranteed, and the speed of seabed topography field work can be improved.

[0018] The present invention can provide accurate sound velocity information for seabed detection by using the equivalent sound velocity automatic inversion method constrained by the multi-beam overlapping area, improve the efficiency of field acquisition, and provide more accurate basic data for scientific research and marine engineering. The present invention innovatively uses the multi-beam overlapping area as a constraint and combines the equivalent sound velocity principle to achieve the reduction of the sound velocity error of multi-beam bathymetry, get rid of the time-consuming and labor-intensive process of measuring dense sound velocity profiles in large-scale sea areas with drastic sound velocity changes, and objectively restore the real seabed topography. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings herein are incorporated in and constitute a part of the specification, illustrate embodiments consistent with the present disclosure, and together with the description, serve to explain the principles of the present disclosure; Figure 1 It is a flow chart of an equivalent sound velocity automatic inversion method based on multi-beam overlap area constraint provided by an embodiment of the present invention; Figure 2 It is a principle diagram of an equivalent sound velocity automatic inversion method based on multi-beam overlapping area constraint provided by an embodiment of the present invention; Figure 3 It is a schematic diagram of the gridding of the survey lines in the present invention; Figure 4 It is a schematic diagram of sound ray tracking based on the constant sound velocity in the layer in the present invention; Figure 5 It is a schematic diagram of the sound ray tracking based on the constant gradient within the layer in the present invention; Figure 6 Schematic diagram of the area difference of the sound velocity profile in the present invention; Figure 7 This is a schematic diagram of the equivalent sound velocity profile in the present invention; Figure 8 Schematic diagram of an equivalent sound velocity automatic inversion system based on multi-beam overlapping area constraints provided by an embodiment of the present invention; Fig. 9 is an experimental diagram of the accuracy of the inversion algorithm provided by an embodiment of the present invention; Fig.10 is a histogram of water depth deviation in the overlapping area provided by an embodiment of the present invention; In the figure: 1. Multi-beam strip overlapping area acquisition module; 2. Inversion equivalent sound speed module; 3. Heavy beam point tracking module. DETAILED DESCRIPTION

[0020] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific implementation disclosed below.

[0021] The innovation of the present invention lies in that the present invention utilizes the overlapping area between multi-beam bathymetric strips, minimizes the water depth difference in the overlapping area of ​​adjacent strips as a constraint, and inverts the equivalent sound speed through a genetic algorithm to achieve beam point tracking without measured sound speed profiles. The accuracy can reach the same level as that of sound line tracking using sound speed profiles, thereby getting rid of the need for sound speed collection in marine topography measurement.

[0022] Embodiment 1: The underwater measurement part of the multi-beam bathymetric system of the present invention uses a multi-beam sonar transducer installed on a survey vessel to obtain three-dimensional data of the detection target and the seabed topography as the detection carrier moves. The positioning and navigation part combines GNSS and attitude measurement equipment (including a compass) to obtain the precise position, attitude and heading information of the survey vessel. The surface sound velocity data is measured by a surface sound velocity meter and used for beam pointing correction of the multi-beam transducer. The water body sound velocity profile information is obtained by inversion of the present invention and does not require prior sound velocity information.

[0023] like Figure 1 As shown, the equivalent sound velocity automatic inversion method based on multi-beam overlap area constraint provided by the embodiment of the present invention includes the following steps: S1, using the surface sound velocity of the multi-beam data to track the beam position of the multi-beam raw data, and obtain the overlapping area of ​​the multi-beam strips; S2, grid the overlapping areas of the multi-beam strips, use the genetic algorithm to minimize the water depth deviation between the overlapping grids, and invert the optimal equivalent sound speed for each survey line; S3, re-track the beam point using the corresponding equivalent sound speed obtained by inversion for each survey line, reduce the sound speed deviation existing in beam point tracking using surface sound speed, weaken the terrain distortion between strips in the overlapping area, and obtain the real seabed topography where adjacent strips overlap and are connected continuously, thus getting rid of the dependence of beam point tracking on the measured sound speed information.

[0024] Exemplarily, in step S1, using the surface sound velocity of the multi-beam data to track the beam position of the multi-beam raw data, obtaining the overlapping area of ​​the multi-beam strips includes: S101, performing data analysis on the acquired multi-beam raw data, using the surface sound velocity information measured by the surface sound velocity meter in the data or assuming a sound velocity profile to perform sound line tracking on the data, and obtaining the initial water depth and position of all measuring points; When acquiring multi-beam raw data, it is necessary to ensure that there is a certain overlap area between adjacent survey lines.

[0025] S102, searching for overlapping areas between the survey lines along a direction perpendicular to the multi-beam heading.

[0026] Get the bounding box of the survey line, traverse all survey line pairs, and check whether the coverage areas of the two survey lines overlap. Use geometric algorithms to determine whether the coverage areas of the two survey lines overlap. If the coverage areas of the two survey lines overlap, calculate the boundary of the overlapping part and save it.

[0027] Exemplarily, in step S2, gridding the overlapping area of ​​the multi-beam strips includes: Due to the influence of factors such as ship speed, attitude and heading, the overlapping areas of the closest two Ping data in adjacent survey lines cannot completely overlap. Therefore, in order to evaluate the water depth consistency of the overlapping part, the overlapping area is gridded and divided into grids: according to the defined grid size, the boundary box of the overlapping area is divided into multiple grid cells. The size of each grid cell is the pre-set grid resolution.

[0028] For example, if the bounding box of the overlapping area is (x_min, y_min, x_max, y_max) and the size of the grid cell is (dx, dy), then the number of rows and columns of the grid can be calculated by the following formula: Number of rows = (y_max - y_min) / dy Number of columns = (x_max - x_min) / dx The bounding box of the overlapping area is divided into several small grids, represented by the row and column indexes of the grid, and the points in the grid are calculated; finally, the gridded result is saved and indexed by the rows and columns of the grid to avoid the process of matching the same-name points.

[0029] This can avoid the complex process of matching points with the same name and speed up the calculation. The grid resolution also needs to consider the density of the measuring points. The size of the grid can be set according to the actual water depth value to ensure that there are at least two measuring points from different measuring lines in a grid; Exemplarily, in step S2, the genetic algorithm is used to minimize the water depth deviation between overlapping grids, and the optimal equivalent sound velocity for each survey line is obtained by inversion, including: The overlapping area data of the gridded multi-beam strips are passed to the genetic algorithm, and the genetic algorithm is used for optimization. The genetic algorithm randomly generates a certain number of populations within the range, and then performs crossover and mutation operations, and then calculates the fitness function. The fitness function for: ; In the formula, is the equivalent sound speed to be determined, indicating the area The sound velocity gradient corresponding to each strip; is the water depth at each grid node, is the inverse distance weighted average water depth of all measuring points in the grid, is the total number of grids in the selected area, For the region Grid The total number of measuring points of the measuring line; Determine the current fitness function Whether the calculated average water depth deviation of the overlapping area is less than 1% of the average water depth, if it is reached, the loop is exited and the optimal equivalent sound speed of each measuring line is returned. If it is not reached, the iterative calculation is repeated.

[0030] Exemplarily, the genetic algorithm randomly generates a certain number of populations within a range, then performs crossover and mutation operations, and then calculates the fitness function. The specific method is as follows: S201, according to the range of initial value , randomly generated data volume The initial value of is called the generated initial value is the initial population; S202, respectively calculate the value of each seed of the generated initial population Function value, according to the size of each seed value, the initial population is screened to select For seeds with small function values, sort the function values ​​of the seed values ​​and select the top 80% to keep; sort the function values ​​of the seed values, and the last 20% are considered to be seeds with large deviations. Then, for seeds with deviations exceeding the threshold, perform mutation and crossover according to the mutation probability and crossover probability to form a new population; select the parent generation: select individuals with higher fitness as the parent generation according to the fitness function. The crossover operation generates new children by exchanging some genes of the parent generation. The goal of the crossover is to combine the excellent characteristics of the parent generation to generate more potential offspring. Mutation operation: perform mutation operations on offspring individuals and randomly change some genes of the individuals. The purpose of mutation is to increase the diversity of the population and prevent the algorithm from falling into a local optimum.

[0031] S203, recalculate the respective Value, genetically retain excellent seed values, replace inferior individuals, and regenerate new population individuals; S204, performing a mutation crossover operation on the newly generated population individuals to generate a new population after mutation crossover, and then calculating the new population ; S205, repeating step S203 and step S204 until the optimal value is found or the maximum number of iterations is reached, thereby exiting and outputting the best equivalent sound speed value.

[0032] Exemplarily, in step S3, it is assumed that the true surface sound velocity of the water body is The surface sound velocity measured by the surface sound velocity meter is , when the measured surface sound velocity is used When calculating the beam steering angle, according to the principle of beam steering, is smaller than the true value, so when the time delay of each array element is not changed, the angle of the beam record is When the attitude angle and installation deviation angle are ignored, the beam incident angle is Therefore, the water depth calculation formula at constant sound speed is as follows: ; In the formula, Represents the water depth value, represents the propagation vector corresponding to the beam. For any beam, the propagation vector is When the water depth decreases In this case, the water depth calculated by the edge beam will increase. Since the central beam will not be affected by the beam steering, the measured underwater terrain will appear like a "crying face". If it is greater than the true value, the situation will be the opposite of the above, and the terrain will appear like a "smiley face".

[0033] When there is an error in the sound velocity profile, such as The value is greater than the actual sound speed value, according to the following formula The value will be too small, and the propagation angle passed to the next layer It will also decrease accordingly, and the calculation formula is as follows: ; In the formula, is the sound velocity value of the next layer, due to If the water depth value is reduced, the water depth value will increase accordingly. Since the central beam is almost vertically incident, the central beam is not affected by refraction, and the water depth error is relatively small, so the terrain will appear to be a "crying face". When the value is smaller than the actual speed of sound, the terrain will appear like a "smiley face".

[0034] If there is no surface sound velocity error and only sound velocity profile error exists, the Snell constant corresponding to each beam is is the correct value. However, for a certain water layer For example, if Sound speed in the layer When it is greater than the true speed of sound, we know that the incident angle of the sound wave of the next layer is will increase, making the water depth value obtained by the subsequent sound ray tracking smaller, while the central beam is incident vertically and does not refract, so it is not affected by it. The water depth error is relatively small, which will cause the terrain to appear "smiley face". When the value is smaller than the actual speed of sound, the terrain will appear like a "crying face".

[0035] The optimal equivalent sound speed of each survey line obtained in step 2 is used to re-track the sound speed to obtain the seabed topography after the inverted sound speed. This completes the automatic inversion of the equivalent sound speed, gets rid of the dependence on the measured sound speed profile, and reduces the crying and laughing face-like topography of the seabed topography caused by inaccurate sound speed measurement.

[0036] For example, the principle of the automatic inversion method of equivalent sound velocity based on multi-beam overlap area constraint is as follows: Figure 2 shown.

[0037] Exemplary theoretical bases related to the present invention include: (1) Principle of multi-beam sounding ray tracking.

[0038] Ray tracking is a sound velocity correction method that uses the sound velocity profile to superimpose the positions of the sound rays layer by layer, thereby calculating the coordinates of the underwater projection point (also known as the beam footprint) of the sound ray in the hull coordinate system. Ray tracking usually divides the water layer between two adjacent sound velocity sampling points in the sound velocity profile N+1 sampling points into one layer. The entire water column that the sound ray propagates through can be regarded as a superposition of N water layers. If the vertical displacement and horizontal displacement of the sound ray in each layer are obtained, the vertical displacement and horizontal displacement of the beam through the entire water column can be obtained by superposition. The change of sound velocity in the layer is generally divided into two cases: when the sound velocity in the layer is assumed to be a constant value, the propagation trajectory of the sound ray is a straight line, and the calculation process of ray tracking is relatively simple, but the sound velocity at the junction of adjacent layers will change suddenly; when the sound velocity in the layer is assumed to change with a constant gradient, the propagation trajectory of the sound ray is an arc, which is more in line with the actual change of the sound ray underwater.

[0039] (1.1) Sound ray tracking based on constant sound velocity within the layer.

[0040] like Figure 3 As shown, the speed of sound is The layer propagates at a constant speed. The depths of the upper and lower interfaces are and , the layer thickness is , , Respectively The beam incident angle and sound speed of the layer, according to Snell's law, , then the horizontal displacement of the beam within the layer and propagation time They are: (1) The horizontal distance the beam travels through the water column and propagation time for: (2) In the formula, is the number of water layers; If the incident angle is 0, that is, single beam sounding, then: ; .

[0041] (1.2) Ray tracking based on constant gradient within the layer.

[0042] like Figure 4 As shown, assuming that the sound speed is in the layer Constant gradient The actual propagation trajectory of the beam in the layer is a continuous trajectory with a curvature radius of The arc segment is expressed as: (3) (4) In the formula, is the sound speed of the next water layer; The horizontal displacement experienced by the beam in this layer and arc length for: (5) In the formula, Deepen the shooting angle for the next layer of water; According to Snell's law: , then Horizontal displacement of sound rays within the layer and time for: (6) In the formula, For Layer The harmonic mean speed of sound within (7) If the incident angle is 0, the propagation trajectory of the sound wave in each layer is a straight line, vertically downward, and the propagation time in each layer is: (8).

[0043] (2) Principle of equivalent sound velocity profile method.

[0044] In the post-processing of multi-beam data, the ray tracing algorithm is widely used due to its high precision, but the model is relatively complex and the amount of calculation is large. In response to the above problems, Geng proposed the ESSP (Earth System Science Partnership) method, which converts the multi-layer ray tracing problem into a single-layer constant gradient ray tracing problem while keeping the beam incident angle and the surface sound velocity unchanged. It only needs to use an equivalent sound velocity profile with the same integral area as the actual complex sound velocity profile, which can greatly simplify the calculation. Therefore, the present invention adopts the inversion of equivalent sound velocity.

[0045] like Figure 5 As shown, in step S2, the optimal equivalent sound velocity of each survey line is obtained by inversion, including: The equivalent sound speed homing process is: (9) In the formula, The sound velocity gradient is a constant, For depth, is the speed of sound, is the underlying sound velocity, is the surface sound velocity, is the bottom water depth, is the incident surface sound velocity, is a constant; The propagation of sound waves in water conforms to Snell's law, which is expressed as: (10) In the formula, , are the upper and lower beam incident angles of a single layer, is a constant obtained based on Snell; The propagation time is obtained according to the principle of equivalent sound velocity profile method for: (11) Combining equations (10) and (11), the propagation time is derived for: (12) Since the propagation time is a known quantity that is precisely measured, we can convert equation (12) into for: (13) In the formula, All are one-way propagation times; According to the theory of ray acoustics, the propagation trajectory of sound rays in a constant gradient layer is an arc, so the radius of the arc is for: (14) The displacement of the sound line in the depth direction is derived and the horizontal displacement They are: (15).

[0046] It can be seen that the equivalent sound velocity profile method can be independent of the original sound velocity profile data to a certain extent, but the calculation accuracy of the equivalent sound velocity profile is closely related to the accuracy of the reference depth, and the reference depth in actual situations is the position number and needs to be obtained by other means; Figure 7 It can be seen that the equivalent sound velocity gradient It changes with the reference depth, that is, the equivalent sound velocity profile method is valid for a specific depth and is not equivalent to other depths.

[0047] It can be seen from the above embodiments that the present invention proposes an automatic inversion method of equivalent sound speed based on multi-beam overlapping area constraints, which first uses the surface sound speed of multi-beam data to track the beam points of multi-beam raw data to obtain the overlapping area of ​​the strips; then the multi-beam strips are gridded, and the water depth deviation between the overlapping grids is minimized by using a genetic algorithm, and the optimal equivalent sound speed of each survey line is obtained by inversion; finally, the beam point tracking is performed again for each survey line using the corresponding equivalent sound speed obtained by inversion, thereby reducing the sound speed deviation existing in beam point tracking using the surface sound speed, so that the crying and laughing face-shaped curved terrain between the strips in the overlapping area is weakened, and a more continuous seabed terrain is obtained in the overlapping area of ​​adjacent strips, thereby reducing the dependence of beam point tracking on the measured sound speed profile. Therefore, the method proposed in the present invention effectively fills the gap in the current automatic correction of multi-beam sound speed errors.

[0048] Embodiment 3, as Figure 8 As shown, the equivalent sound velocity automatic inversion system based on multi-beam overlapping area constraint provided by the embodiment of the present invention includes: The multi-beam strip overlapping area acquisition module 1 is used to track the beam points of the multi-beam raw data using the surface sound velocity of the multi-beam data to obtain the overlapping area of ​​the multi-beam strips; The inversion equivalent sound velocity module 2 is used to grid the overlapping areas of the multi-beam strips, minimize the water depth deviation between the overlapping grids using a genetic algorithm, and invert the optimal equivalent sound velocity for each survey line; The re-beam point tracking module 3 is used to re-track the beam points of each survey line using the corresponding equivalent sound speed obtained by inversion, reduce the sound speed deviation existing in beam point tracking using surface sound speed, weaken the curved terrain between strips in the overlapping area, and obtain a continuous seabed terrain in the overlapping area of ​​adjacent strips.

[0049] In order to verify the accuracy of the present invention in areas with drastic changes in sound speed, an experiment was conducted on a water area to verify the reliability of the present method. The experimental data contains four adjacent parallel survey lines. The experimental area data has terrain protrusions and relatively flat areas. This terrain undulation can better verify the accuracy of the inversion algorithm. The experimental area is as follows: Fig. 9 shown.

[0050] The data were operated according to the process of this method, and the processed results were compared with the internal longitude. The depth deviation histogram of the overlapping area is as follows: Fig.10 As shown, it can be seen that the water depth deviations in the overlapping areas after being processed by this method are small, no terrain distortion is generated, and the tolerances required by the hydrographic specification are met, and it is possible to get rid of the reliance on the measured sound velocity profile when performing beam point tracking. The above is only a preferred specific implementation of the present invention, but the protection scope of the present invention is not limited thereto. Any modification, equivalent substitution and improvement made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention shall be covered within the protection scope of the present invention.

Claims

1. An automatic inversion method of equivalent sound velocity based on multi-beam overlapping area constraint, characterized in that: The method comprises the following steps: S1, using the surface sound velocity of the multi-beam data to track the beam position of the multi-beam raw data, and obtain the overlapping area of ​​the multi-beam strips; S2, grid the overlapping areas of the multi-beam strips, use the genetic algorithm to minimize the water depth deviation between the overlapping grids, and invert the optimal equivalent sound speed for each survey line; S3, re-track the beam point using the corresponding equivalent sound speed obtained by inversion for each survey line, reduce the sound speed deviation existing in beam point tracking using surface sound speed, weaken the terrain distortion between strips in the overlapping area, and obtain the real seabed topography where adjacent strips overlap and are connected continuously, thus getting rid of the dependence of beam point tracking on the measured sound speed information.

2. The automatic inversion method of equivalent sound velocity based on multi-beam overlap area constraint according to claim 1 is characterized in that: In step S1, the overlapping area of ​​the multi-beam strips is obtained, including: S101, performing data analysis on the acquired multi-beam raw data, using the surface sound velocity information measured by the surface sound velocity meter in the data or assuming a sound velocity profile to perform sound line tracking on the data, and obtaining the initial water depth and position of all measuring points; S102, searching for overlapping areas between the survey lines along a direction perpendicular to the multi-beam heading.

3. The automatic inversion method of equivalent sound velocity based on multi-beam overlap area constraint according to claim 2 is characterized in that: In step S101 , the sound ray tracking includes the sound ray tracking based on the constant sound velocity within the layer and the sound ray tracking based on the constant gradient within the layer.

4. The automatic inversion method of equivalent sound velocity based on multi-beam overlap area constraint according to claim 3 is characterized in that: The sound ray tracking based on the constant sound velocity in the layer includes: The speed of sound is The layer propagates at a constant speed. The depths of the upper and lower interfaces are and , the layer thickness is , , Respectively The beam incident angle and sound speed of the layer, according to Snell's law, , then the horizontal displacement of the beam within the layer and propagation time They are: （1） The horizontal distance the beam travels through the water column and propagation time for: （2） In the formula, is the number of water layers; If the incident angle is 0, that is, single beam sounding, then: ； 。 5. The method for automatic inversion of equivalent sound velocity based on multi-beam overlap area constraint according to claim 4 is characterized in that: Ray tracking based on constant gradient within the layer, including: The speed of sound in the layer Constant gradient The actual propagation trajectory of the beam in the layer is a continuous trajectory with a curvature radius of The arc segment is expressed as: （3） （4） In the formula, is the sound speed of the next water layer; The horizontal displacement experienced by the beam in this layer and arc length for: （5） In the formula, Deepen the shooting angle for the next layer of water; According to Snell's law: , then Horizontal displacement of sound rays within the layer and time for: （6） In the formula, For Layer The harmonic mean speed of sound within （7） If the incident angle is 0, the propagation trajectory of the sound wave in each layer is a straight line, vertically downward, and the propagation time in each layer is: （8）。 6. The automatic inversion method of equivalent sound velocity based on multi-beam overlap area constraint according to claim 1 is characterized in that: In step S2, the water depth deviation between overlapping grids is minimized using a genetic algorithm, including: The overlapping area data of the gridded multi-beam strips are passed to the genetic algorithm, and the genetic algorithm is used for optimization; the genetic algorithm randomly generates a population within the range, and then performs crossover and mutation operations to calculate the fitness function. The fitness function for: ； In the formula, is the equivalent sound speed to be determined, indicating the area The sound velocity gradient corresponding to each strip; is the water depth at each grid node, is the inverse distance weighted average water depth of all measuring points in the grid, is the total number of grids in the selected area, For the region Grid The total number of measuring points of the measuring line; Determine the current fitness function Whether the calculated average water depth deviation of the overlapping area is less than 1% of the average water depth, if it is reached, the loop is exited and the optimal equivalent sound speed of each measuring line is returned. If it is not reached, the iterative calculation is repeated.

7. The method for automatic inversion of equivalent sound velocity based on multi-beam overlap zone constraint according to claim 6, characterized in that: The genetic algorithm randomly generates a population within a range, and then performs crossover and mutation operations. The specific method for calculating the fitness function is as follows: S201, according to the range of initial value , randomly generated data volume The initial value of is called the generated initial value is the initial population; S202, respectively calculate the value of each seed of the generated initial population Function value, according to the size of each seed value, the initial population is screened to select The seeds with small function values ​​are retained; then the seeds with deviations exceeding the threshold are mutated and crossed according to the mutation probability and crossover probability to form a new population; S203, recalculate the respective Value, genetically retain excellent seed values, replace inferior individuals, and regenerate new population individuals; S204, performing a mutation crossover operation on the newly generated population individuals to generate a new population after mutation crossover, and then calculating the new population ; S205, repeating step S203 and step S204 until the optimal value is found or the maximum number of iterations is reached, thereby exiting and outputting the best equivalent sound speed value.

8. The method for automatic inversion of equivalent sound velocity based on multi-beam overlap area constraint according to claim 1, characterized in that: In step S2, the optimal equivalent sound velocity for each survey line is obtained by inversion, including: The equivalent sound speed homing process is: （9） In the formula, The sound velocity gradient is a constant, For depth, is the speed of sound, is the underlying sound velocity, is the surface sound velocity, is the bottom water depth, is the incident surface sound velocity, is a constant value; The propagation of sound waves in water conforms to Snell's law, which is expressed as: （10） In the formula, , are the upper and lower beam incident angles of a single layer, is a constant obtained based on Snell; The propagation time is obtained according to the principle of equivalent sound velocity profile method for: （11） Combining equations (10) and (11), the propagation time is derived for: （12） Since the propagation time is a known quantity that is precisely measured, we can convert equation (12) into for: （13） In the formula, All are one-way propagation times; According to the theory of ray acoustics, the propagation trajectory of sound rays in a constant gradient layer is an arc, so the radius of the arc is for: （14） The displacement of the sound line in the depth direction is derived and the horizontal displacement They are: （15）。 9. An equivalent sound velocity automatic inversion system based on multi-beam overlapping area constraints, characterized in that: The system implements the equivalent sound velocity automatic inversion method based on multi-beam overlap zone constraint as claimed in any one of claims 1 to 8, and the system comprises: The multi-beam strip overlapping area acquisition module (1) is used to track the beam points of the multi-beam raw data using the surface sound velocity of the multi-beam data to obtain the multi-beam strip overlapping area; The inversion equivalent sound velocity module (2) is used to grid the overlapping area of ​​the multi-beam strips, minimize the water depth deviation between the overlapping grids using a genetic algorithm, and invert the optimal equivalent sound velocity for each survey line; The re-beam point tracking module (3) is used to re-track the beam point for each survey line using the corresponding equivalent sound speed obtained by inversion, reduce the sound speed deviation existing in beam point tracking using the surface sound speed, weaken the terrain distortion between strips in the overlapping area, and obtain the real seabed topography of the overlapping area of ​​adjacent strips, thereby getting rid of the dependence of beam point tracking on the measured sound speed information.

10. The equivalent sound velocity automatic inversion system based on multi-beam overlap area constraint according to claim 9, characterized in that: The automatic inversion system for equivalent sound speed based on multi-beam overlapping area constraints is carried on a computer device, which includes: at least one processor, a memory, and a computer program stored in the memory and running on the at least one processor. When the processor executes the computer program, the functions of the automatic inversion system for equivalent sound speed based on multi-beam overlapping area constraints are realized.

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