A multiple scattering attenuation correction method for ocean lidar based on analytical model

Abstract

The present invention discloses a method for correcting multiple scattering attenuation of an ocean laser radar based on an analytical model, including: constructing an analytical model of radiation transmission of an ocean laser radar based on the quasi-single scattering small-angle approximation theory, simulating echo signals under different water environments and laser radar hardware conditions; constructing a numerical relationship between the laser radar effective attenuation coefficient and radar parameters and the inherent optical properties of the water body, and constructing a multiple scattering attenuation correction method using an empirical expression of the diffuse attenuation coefficient; according to the detection system of the ocean laser radar, selecting different inversion algorithms to calculate the laser radar effective attenuation coefficient distributed with depth, using the laser radar effective attenuation coefficient numerical model to invert the inherent optical properties of the water body, eliminating the influence of the multiple scattering effect on the depth, and obtaining a high-precision vertical profile of the optical properties of the water body. Using the present invention, the inversion accuracy of the components of the ocean water body can be greatly improved.

Description

Technical Field

[0001] The invention belongs to the technical field of ocean laser radar, and in particular relates to an ocean laser radar multiple scattering attenuation correction method based on an analytical model. Background Art

[0002] LiDAR technology can detect the vertical distribution of water optical properties (such as absorption and scattering characteristics), and is an important means of obtaining information on biological components of water. It plays an important role in the stereoscopic observation of global marine ecosystems, biogeochemistry, and research on the ocean carbon cycle.

[0003] For example, a satellite-borne ocean atmospheric parameter laser radar detection system with publication number CN113219496A includes: a laser radar information acquisition module, a detection test module, a water layer attenuation coefficient acquisition module, a backscattering coefficient acquisition module, a central control module, a total absorption coefficient acquisition module, an inversion parameter model construction module, a relative error model construction module, an optimal dual-wavelength determination module, and a laser radar detection module. The system uses a dual-wavelength ocean high-spectral resolution laser radar for detection, obtains laser radar information, and obtains water layer attenuation coefficient and backscattering coefficient through detection, thereby obtaining the total absorption coefficient; selects the two wavelengths of the ocean laser radar to improve the inversion accuracy of chlorophyll and CDOM absorption coefficients, and can achieve accurate acquisition of atmospheric temperature, humidity, and density parameter distribution information over a large range of detection over the ocean.

[0004] However, in seawater, the transmission of lasers is accompanied by a complex multiple scattering process. Under the influence of multiple scattering effects, the relationship between the effective attenuation coefficient of the lidar and the optical properties of the water body becomes complicated, which greatly affects the accuracy of the lidar inversion of the optical properties of the water body.

[0005] Existing studies have shown that, assuming that the effective attenuation coefficient of the lidar is a constant, when the product of the beam attenuation coefficient of the water body and the radius of the sea surface foot spot in the receiving field of view is greater than 5, the effective attenuation coefficient of the lidar will approach the diffuse attenuation coefficient. However, in actual detection, due to the influence of multiple scattering effects on the laser pulse, it will be broadened in time and space. Even in a uniform water body, the effective attenuation coefficient of the lidar at different water depths is not consistent. Simply equating it with the diffuse attenuation coefficient will bring non-negligible errors to the inversion of the vertical profile of the optical properties of the water body.

[0006] Therefore, there is an urgent need to develop a method that can correct the influence of multiple scattering on the effective attenuation coefficient of lidar at different depths, so as to achieve accurate inversion of the water optical property profile, improve the lidar's detection capability of ocean water, and enhance the understanding of the global ocean optical properties. Summary of the invention

[0007] In view of the problem that the multiple scattering effect in the existing ocean laser radar affects the accuracy of the vertical profile inversion of the optical properties of the water body, the present invention provides a multiple scattering attenuation correction method for the ocean laser radar based on an analytical model, which can greatly improve the accuracy of the water body optical property profile inversion by the shipborne laser radar.

[0008] A method for correcting multiple scattering attenuation of ocean lidar based on an analytical model, comprising:

[0009] (1) According to the laser radar system parameters and based on the quasi-single scattering small-angle approximation theory, a radiation transmission analytical model of the ocean laser radar is constructed to simulate the laser radar echo signal;

[0010] (2) setting multiple sets of different seawater intrinsic optical property IOPs parameters, fixing the absorption coefficient a and the scattering coefficient b respectively, and using the radiation transmission analytical model in step (1) to simulate and obtain the corresponding lidar echo signal;

[0011] (3) Solve the effective attenuation coefficient k of the lidar for different signals in step (2): lidar , analyze the influence of water parameters on the effective attenuation coefficient of lidar; fit the effective attenuation coefficient k of lidar lidar The numerical relationship between the intrinsic optical properties of water bodies IOPs and water depth z determines k lidar - Numerical model expression of IOPs-z;

[0012] (4) Using k lidar -IOPs-z numerical model fits the unknown parameters m1, m2, m3 of the model under different water optical properties; m1, m2, m3 are considered as backscattering coefficients b b Function to calculate m1, m2, m3 and b corresponding to different water bodies b the relationship between;

[0013] (5) Set up a non-uniform water body verification group to test k lidar -IOPs-z numerical model was used to verify the simulated attenuation coefficient and the model-fitted attenuation coefficient, and the correlation between the two was calculated to ensure k lidar -Validity of the IOPs-z numerical model;

[0014] (6) Invert the measured signals of lidars of different systems and calculate the effective attenuation coefficient k of the lidar lidar and water backscatter coefficient b b ; HSRL inversion method and Fernald inversion method are used for high spectral resolution lidar and Mie scattering lidar respectively;

[0015] (7) Using the effective attenuation coefficient k obtained by inversionlidar and the backscatter coefficient b b , substitute k lidar - Numerical model of IOPs-z, calculation of the absorption coefficient a of the water body;

[0016] (8) Based on the water absorption coefficient a and backscattering coefficient b obtained by inversion b , combining the inherent optical properties of water IOPs and the diffuse attenuation coefficient K d The empirical relationship between the diffuse attenuation coefficient K of the water body is calculated d .

[0017] In the present invention, an analytical model of ocean laser radar radiation transmission is constructed based on the quasi-single scattering small-angle approximation method to simulate the echo signals under different water environments and laser radar hardware conditions. The numerical relationship between the effective attenuation coefficient of the laser radar and the radar parameters and the inherent optical properties of the water body is constructed, and the empirical expression of the diffuse attenuation coefficient is used to construct a multiple scattering attenuation correction method. According to the detection system of the ocean laser radar, different inversion algorithms are selected to calculate the effective attenuation coefficient of the laser radar distributed with depth, and the inherent optical properties of the water body are inverted using the numerical model of the effective attenuation coefficient of the laser radar, eliminating the influence of the multiple scattering effect on the depth, and obtaining a high-precision vertical profile of the optical properties of the water body.

[0018] Furthermore, in step (1), the formula for simulating the laser radar echo signal is:

[0019]

[0020] Where P(z) represents the laser radar echo signal at depth z, W0 represents the emitted laser intensity, and b z represents the water scattering coefficient at depth z, v represents the speed of light in water, β b (z,|n'-n"|) represents the backscattering phase function, I src (z,r,n') and I rec (z,r,n”) represent the radiation angular spectrum distribution of the light source and the receiver at the point (z,r), respectively, where the receiver is a virtual light source.

[0021] In step (3), k lidar The numerical model expression of -IOPs-z is as follows:

[0022]

[0023] Among them, m1, m2, and m3 are parameter items, and z is the depth of the water body.

[0024] In step (4), k lidar -The coefficients m1, m2, m3 and backscattering coefficient b in the IOPs-z numerical modelb The relationship between them is as follows:

[0025] m1=f1(b b ); m2 = f2(b b ); m3 = f3 (b b ).

[0026] In step (6), for high spectral resolution lidar, the HSRL inversion method is used to solve the water backscattering coefficient b through the signals of the mixed channel and the molecular channel. b and the effective attenuation coefficient k of the lidar lidar , as shown below:

[0027]

[0028]

[0029] Where χ is the conversion factor, T B is the transmittance, P C (z) is the mixed channel signal of the laser radar, P M (z) is the molecular channel signal of the laser radar, b bw is the backscattering coefficient of water molecules.

[0030] For Mie scattering lidar, the Fernald method is used to solve the lidar effective attenuation coefficient k lidar and water backscatter coefficient b b , as shown below:

[0031]

[0032]

[0033] Among them, P(z) is the echo signal of the laser radar, k lidar,w is the effective attenuation coefficient of water molecules, k lidar,p is the effective attenuation coefficient of particles, R is the particle laser radar ratio R p Compared with water molecule lidar w The ratio of z c represents the boundary depth, P0(z) is the stable signal term, β π0 To stabilize the 180° volume scattering coefficient, Φ(z) is given by

[0034]

[0035] In step (7), the inversion formula of the absorption coefficient is as follows:

[0036]

[0037] Among them, a is the water absorption coefficient, b is b is the backscattering coefficient, m1, m2, m3 are k lidar -IOPs-z coefficient in the numerical model, and the backscatter coefficient b b Related to each other.

[0038] In step (8), the diffuse attenuation coefficient K of the water body is calculated d The formula is as follows:

[0039] K d =a+4.18(1-0.52e -10.8a )b b .

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] 1. The present invention uses the ocean laser radar radiation transmission equation and the quasi-single small-angle approximation theory to construct an analytical model of the laser radar echo signal. It can set different laser radar system parameters, simulate the echo signals of various water bodies, calculate the corresponding effective attenuation coefficient, and further invert the water body absorption coefficient a and backscattering coefficient b. b and the diffuse attenuation coefficient K d .

[0042] 2. The present invention establishes k lidar -IOPs-z numerical model can eliminate the influence of multiple scattering effect on the effective attenuation coefficient of lidar echo signal at depth, and improve the absorption coefficient a and diffuse attenuation coefficient K of ocean lidar to water body. d inversion accuracy.

[0043] 3. The present invention can be applied to ocean lidar systems with various detection mechanisms, and a suitable inversion algorithm can be selected according to the actual measurement situation to obtain a high-precision water optical property profile. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a flow chart of a method for correcting multiple scattering attenuation of ocean laser radar based on an analytical model according to the present invention;

[0045] Figure 2 It is a measurement site for shipborne laser radar data in an embodiment of the present invention;

[0046] Figure 3 is a distribution diagram of water absorption coefficient inverted in an embodiment of the present invention;

[0047] Figure 4 This is a distribution diagram of the water body diffuse attenuation coefficient inverted in an embodiment of the present invention. DETAILED DESCRIPTION

[0048] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention and do not have any limiting effect on the present invention.

[0049] A multiple scattering attenuation correction method for ocean lidar based on an analytical model is proposed. The klidar-IOPs-z model is used to remove the influence of multiple scattering on the effective attenuation coefficient of the lidar echo signal, and the accurate inversion of various water optical properties is achieved.

[0050] The specific process is as follows Figure 1 As shown in the figure, based on the quasi-single small-angle scattering approximation, a shipborne ocean lidar analytical model is built to simulate the lidar echo signal in different water environments, and a numerical model between the theoretical effective attenuation coefficient, the inherent optical properties of the water body, and the water depth is established, namely, k lidar -IOPs-z numerical model is used to eliminate the influence of multiple scattering effects on the effective attenuation coefficient, invert the inherent optical properties of water, and further invert the diffuse attenuation coefficient of water at different depths.

[0051] Figure 2 The detection site of the ship-borne lidar data is shown. The measurement height of the lidar is 5m, the field of view diameter and field of view angle of the receiver are 50.8mm and 200mrad respectively, and the tilt angle during detection is 40°.

[0052] The following steps are used to invert the optical properties of water bodies using an ocean lidar based on an analytical model:

[0053] The first step is to build an analytical model of the shipborne ocean lidar, calculate the corresponding radar echo signal strength, and approximate the lidar echo signal at the water depth z based on the quasi-single small-angle scattering, as shown in the following formula:

[0054]

[0055] Among them, z is the depth of the water body, W0 is the laser intensity of the lidar, b z is the scattering coefficient at z, v is the speed of light in water, β b (z,|n'-n"|) is the backscattering phase function, which is set as the FF phase function. src (z,r,n') and I rec (z,r,n”) is the angular spectrum distribution of the light source and receiver at point (z,r), n' and n” are the incident and scattered directions of the light beam, respectively. src (z,r,n') and I rec (z,r,n”) can be respectively obtained by the spatial angular distribution of the unit light source and the spatial angular distribution of the receiver (virtual light source) Perform calculations.

[0056] The second step is to simulate the laser radar echo signals corresponding to the optical characteristics of different water bodies. Considering the water body as a uniform water body, the absorption coefficient a and the scattering coefficient b are fixed as shown in Table 1. The effective attenuation coefficient of the laser radar corresponding to different water bodies is calculated using the simulated signal. The results show that the absorption coefficient a only affects the constant term of the effective attenuation coefficient of the laser radar, while the scattering coefficient b affects the shape and value of the effective attenuation coefficient of the laser radar at the same time. When the backscattering ratio B is fixed, the scattering coefficient b is converted to the backscattering coefficient b b Consider the backscatter coefficient b b It can be directly inverted through the lidar echo signal and is an intuitive and effective data parameter.

[0057] Table 1

[0058]

[0059] The third step is based on the effective attenuation coefficient k of the laser radar. lidar Analysis of the influence of the inherent optical properties of water, summary k lidar -IOPs-z model, found k lidar It shows a negative exponential distribution with depth, and the inherent optical properties of the water body determine the series parameters of the exponential relationship, as shown in the following formula:

[0060]

[0061] Among them, m1, m2, and m3 are related to the backscattering coefficient b. b The relevant parameters are: z is the depth of the water body and a is the absorption coefficient of the water body.

[0062] The fourth step is to use the formula in the third step to fit the effective attenuation coefficient of the lidar corresponding to the array water bodies in Table 1. The m1, m2, and m3 corresponding to Case 1-Case 7 are shown in Table 2.

[0063] Table 2

[0064]

[0065] It can be seen that the values ​​of m1, m2, and m3 are only related to b b Related, but not affected by the absorption coefficient. Summarize m1, m2, m3 and b b The relationship between them is shown in the following formula:

[0066] m1=-12.37×b b -0.0003383

[0067]

[0068] m3=11.85×b b +0.001378

[0069] The fifth step is to set the array verification group water body as shown in Table 3, use the analytical model to simulate the corresponding theoretical echo signal, calculate the corresponding effective attenuation coefficient, and fit the theoretical attenuation coefficient under the water body parameters according to the formula in the fourth step. For the shipborne ocean lidar system, the correlation between the simulated attenuation coefficient and the fitted attenuation coefficient reached 0.995, which proves that k lidar -The empirical model of IOPs-z has good reliability.

[0070] Table 3

[0071]

[0072] Step 6: According to the detection mechanism of the shipborne laser radar, invert the effective attenuation coefficient k of the laser radar lidar and water backscatter coefficient b b For the embodiment, the HSRL inversion method is adopted to calculate the backscattering coefficient b at different water depths using the following formula: b and the effective attenuation coefficient k of the lidar lidar .

[0073]

[0074]

[0075] Where χ is the conversion factor, T B is the transmittance, P C (z) is the mixed channel signal of the laser radar, P M (z) is the molecular channel signal of the laser radar, b bw is the backscattering coefficient of water molecules.

[0076] Step 7: Use k lidar -IOPs-z empirical model, effective attenuation coefficient k obtained by inversion lidar and the backscatter coefficient b b , calculate the absorption coefficient of the water body as shown below:

[0077]

[0078] The m1, m2, and m3 in the formula are based on the backscattering coefficient b obtained by inversion. b Calculate with the theoretical relationship in step 4, and the inverted absorption coefficient a is as follows Figure 3 shown.

[0079] Step 8: Use the water absorption coefficient a and backscattering coefficient b obtained by inversion b, calculate the diffuse attenuation coefficient K of the water body d , as shown below:

[0080] K d =a+4.18(1-0.52e -10.8a )b b

[0081] Example K d The calculation results are as follows Figure 4 As shown in Figure 2, compared to directly considering the effective attenuation coefficient of the lidar as K d , the relative error between the calculated result and the in-situ value can be reduced by 60%. This is of great significance for the analysis of the physical and chemical properties of seawater, and can help more accurately calculate the underwater light field distribution, deduce water body components, and understand ocean water bodies.

[0082] The present invention uses the quasi-single small-angle approximation theory to construct a simulation model of the shipborne ocean lidar echo signal and establishes the corresponding k lidar -IOPs-z numerical model, capable of inverting absorption coefficient a and backscattering coefficient b b and water diffuse attenuation coefficient K d , the inversion results are in good agreement with the in-situ values, indicating the reliability of the present invention.

[0083] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for correcting multiple scattering attenuation of ocean lidar based on an analytical model, characterized in that: include: (1) According to the laser radar system parameters and based on the quasi-single scattering small-angle approximation theory, the radiation transmission analytical model of the ocean laser radar is constructed to simulate the laser radar echo signal. The formula is: Where P(z) represents the laser radar echo signal at depth z, W0 represents the emitted laser intensity, and b z represents the water scattering coefficient at depth z, v represents the speed of light in water, β b (z,|n'-n"|) represents the backscattering phase function, I src (z,r,n') and I rec (z, r, n") represents the radiation angular spectrum distribution of the light source and the receiver at the point (z, r), where the receiver is a virtual light source; n' and n" represent the incident and scattered directions of the light beam, respectively; (2) setting multiple sets of different seawater intrinsic optical property IOPs parameters, fixing the absorption coefficient a and the scattering coefficient b respectively, and using the radiation transmission analytical model in step (1) to simulate and obtain the corresponding lidar echo signal; (3) Solve the effective attenuation coefficient k of the lidar for different signals in step (2): lidar , analyze the influence of water parameters on the effective attenuation coefficient of lidar; fit the effective attenuation coefficient k of lidar lidar The numerical relationship between the intrinsic optical properties of water bodies IOPs and water depth z determines k lidar - Numerical model expression of IOPs-z; (4) Using k lidar -IOPs-z numerical model fits the unknown parameters m1, m2, m3 of the model under different water optical properties; m1, m2, m3 are considered as backscattering coefficients b b Function to calculate m1, m2, m3 and b corresponding to different water bodies b the relationship between; (5) Set up a non-uniform water body verification group to test k lidar -IOPs-z numerical model was used to verify the simulated attenuation coefficient and the model-fitted attenuation coefficient, and the correlation between the two was calculated to ensure k lidar -Validity of the IOPs-z numerical model; (6) Invert the measured signals of lidars of different systems and calculate the effective attenuation coefficient k of the lidar lidar and water backscatter coefficient b b ; For high spectral resolution lidar, the HSRL inversion method is used to solve the water backscattering coefficient b through the signals of the mixed channel and the molecular channel. b and the effective attenuation coefficient k of the lidar lidar , as shown below: Where χ is the conversion factor, T B is the transmittance, P C (z) is the mixed channel signal of the laser radar, P M (z) is the molecular channel signal of the laser radar, b bw is the backscattering coefficient of water molecules; For Mie scattering lidar, the Fernald method is used to solve the lidar effective attenuation coefficient k lidar and water backscatter coefficient b b , as shown below: Among them, P(z) is the echo signal of the laser radar, k lidar,w is the effective attenuation coefficient of water molecules, k lidar,p is the effective attenuation coefficient of particles, R is the particle laser radar ratio R p Compared with water molecule lidar w The ratio of z c represents the boundary depth, P0(z) is the stable signal term, β π0 To stabilize the 180° volume scattering coefficient, Φ(z) is given by (7) Using the effective attenuation coefficient k obtained by inversion lidar and the backscatter coefficient b b , substitute k lidar - Numerical model of IOPs-z, calculation of the absorption coefficient a of the water body; (8) Based on the water absorption coefficient a and backscattering coefficient b obtained by inversion b , combining the inherent optical properties of water IOPs and the diffuse attenuation coefficient K d The empirical relationship between the diffuse attenuation coefficient K of the water body is calculated d .

2. The method for correcting multiple scattering attenuation of ocean laser radar based on analytical model according to claim 1, characterized in that: In step (3), k lidar The numerical model expression of -IOPs-z is as follows: Among them, m1, m2, and m3 are parameter items, and z is the depth of the water body.

3. The method for correcting multiple scattering attenuation of ocean laser radar based on analytical model according to claim 1, characterized in that: In step (4), k lidar -The coefficients m1, m2, m3 and backscattering coefficient b in the IOPs-z numerical model b The relationship between them is as follows: <h2 style=";text-align:left;direction:ltr">m1 = f1(b<h2 style=";text-align:left;direction:ltr"> b <h2 style=";text-align:left;direction:ltr"> );m2=f2(b<h2 style=";text-align:left;direction:ltr"> b <h2 style=";text-align:left;direction:ltr"> );m3=f3(b<h2 style=";text-align:left;direction:ltr"> b <h2 style=";text-align:left;direction:ltr"> )。 4. The method for correcting multiple scattering attenuation of ocean laser radar based on an analytical model according to claim 1, characterized in that: In step (7), the inversion formula of the absorption coefficient is as follows: Among them, a is the water absorption coefficient, b is b is the backscattering coefficient, m1, m2, m3 are k lidar -IOPs-z coefficient in the numerical model, and the backscatter coefficient b b Related to each other.

5. The method for correcting multiple scattering attenuation of ocean laser radar based on analytical model according to claim 1, characterized in that: In step (8), the diffuse attenuation coefficient K of the water body is calculated d The formula is as follows: K d =a+4.18(1-0.52e -10.8a )b b 。

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