Open water body chlorophyll concentration profile detection method based on photon counting laser radar

Abstract

A photon counting laser radar-based open water chlorophyll a concentration profile detection method is used for accurately acquiring profile concentration distribution of chlorophyll a in a water body. According to the method, ICESat-2 photon counting laser radar data and MODIS satellite remote sensing images are combined, a photon propagation process is modeled by using a Monte Carlo simulation technology, energy attenuation and scattering characteristics of photons in a water body are calculated, an extinction coefficient and a backscattering coefficient are further derived, and chlorophyll a concentration distribution is inversed according to the parameters. High-precision water chlorophyll a concentration profile data can be provided by accurately simulating the propagation process of photons in a water body and considering the optical characteristics and environmental factors of the water body. MODIS remote sensing data is used for assistance, the accuracy and reliability of an inversion result are further improved, and accurate chlorophyll concentration detection can be achieved in different open water body areas. The method is suitable for monitoring the chlorophyll concentration of water bodies such as oceans, lakes and rivers, and provides a scientific basis for water body ecological environment evaluation.

Description

Technical Field

[0001] The invention belongs to the technical field of chlorophyll a concentration detection, and relates to a method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar. Background Art

[0002] Chlorophyll a concentration is an important indicator of the ecological health of water bodies and is widely used in the fields of marine monitoring, fishery resource management and environmental protection. Especially in water bodies such as oceans and lakes, changes in chlorophyll a concentration often reflect the degree of eutrophication of water bodies, algae growth and the health of the ecosystem. Traditional chlorophyll a concentration monitoring methods mainly rely on on-site sampling and spectral remote sensing, but these methods can usually only provide two-dimensional information on the surface of the water body and cannot accurately obtain the vertical distribution inside the water body. Especially in deep or turbid water bodies, the monitoring accuracy and spatial coverage are greatly limited.

[0003] With the rapid development of photon counting lidar technology, the application of this technology has greatly enhanced the depth resolution capability of water body detection. The water body detection method based on lidar can provide information at different depths in the water body. In particular, the emergence of photon counting lidar (such as ICESat-2) provides a new technical path for obtaining vertical profile information of water bodies. However, due to factors such as water surface reflection, multiple scattering of photons in water bodies, and refraction effects in water bodies, accurately extracting effective signals and calculating the chlorophyll a concentration of water bodies still faces great challenges.

[0004] Therefore, how to use photon counting lidar data to accurately simulate the propagation process of photons and achieve high-precision chlorophyll a concentration profile detection on this basis has become a research hotspot in the current field of water ecological monitoring. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a method for detecting the chlorophyll a concentration profile in an open water body based on a photon counting laser radar, using the photon point cloud data obtained by the ICESat-2 laser radar system, combined with MODIS remote sensing data, and using Monte Carlo simulation technology to accurately model the photon propagation process. The attenuation of photon energy in the water body is calculated through the photon propagation model, and the extinction coefficient and backscattering coefficient are further derived, and the concentration distribution of chlorophyll a in the water body is inverted.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar, comprising the following steps: Step 1: Obtain photon point cloud data using ICESat-2 satellite laser radar, and estimate the energy of each photon by using the known laser wavelength; Step 2, obtaining MODIS chlorophyll concentration data, using MODIS remote sensing satellite to obtain chlorophyll concentration data of the study area for assistance; Step 3: Preliminary chlorophyll concentration estimation, using MODIS data to simulate preliminary water chlorophyll concentration distribution , and provide hypothetical values ​​for subsequent inversion; Step 4, calculating coefficients, including calculating the absorption coefficient of the water body, calculating the scattering coefficient of the water body and calculating the extinction coefficient; Step 5, photon initialization, photon propagation and distance update, and photon energy decay; Step 6: Data cleaning and screening; Step 7, photon scattering simulation, each time a photon scatters with particles in the water, the propagation direction of the photon will be based on the scattering phase function Random updates; Step 8: Calculate the total number of photons in the cross section and calculate the number of backscattered photons ; Step 9, calculation of backscatter coefficient, backscatter coefficient is the number of backscattered photons per unit volume; Step 10, chlorophyll concentration inversion.

[0007] In step 1, the ATLAS lidar of ICESat-2 uses 532nm, i.e. The wavelength of each photon is calculated by the following formula : ; Where: h is Planck's constant ( ); c is the speed of light ( );λ is the wavelength of the laser ( ).

[0008] In step 4, the absorption coefficient is calculated using the empirical formula : ; in: ; is the absorption coefficient of pure water, which is a constant; Calculate the scattering coefficient using the empirical formula : ;in: ; is the scattering coefficient of pure water, which is a constant; the extinction coefficient is the sum of the absorption coefficient and the scattering coefficient: .

[0009] In step 5, photons are initialized and emitted from the lidar photons, initialize the emission position and emission direction of each photon; the initial direction is fixed and evenly distributed in two-dimensional space; Photon propagation and distance update: when each photon propagates in the water, the propagation distance By extinction coefficient Decide: ; where R is a uniformly distributed random number, is the extinction coefficient at depth z; Photon energy decays. Each time a photon propagates, the energy E of the photon changes according to the extinction coefficient. attenuation: ;in: is the photon energy before propagation; is the energy of the photon after propagation.

[0010] In step 6, the laser radar is emitted photons, retaining the valid signal photon data in the ocean water: ; Using the intensity threshold method, according to the pre-set intensity threshold , photons below this threshold are considered background noise: ;in: Represents the effective number of photons after removing noise; is the initial photon number; is the number of background noise photons.

[0011] In step 7, the scattering phase function is the Henyey-Greenstein scattering model: ; Where: g is the asymmetric factor, which indicates the bias of the scattering in the front and back directions; is the angle at which the i-th photon is scattered.

[0012] In step 8, calculate the total number of photons in the profile , Water body profile division: First, calibrate the initial small area profile, the distance along the track is 2km, the elevation distance is 1m, and update the small area profile by sliding window. The step length along the track direction is Δx=0.15km, and the step length in the elevation direction is Δz=0.05m. The photon number statistics of the small area profile: ;in: is the depth reached by the ith photon Indicates whether the photon reaches the area; represents the along-track extent of the initial small-area profile; represents the elevation range of the initial small area profile; m and n represent the number of sliding along the track and in the elevation direction, respectively.

[0013] In step 8, the number of backscattered photons is calculated The number of backscattered photons refers to the photons that are scattered to a direction close to 180° during propagation. By counting these photons, we get: ; in: is the unit pulse function, when When , it returns 1, indicating that the photon is a backscattered photon; is the energy of the photon after attenuation, It is the energy threshold that determines which photons are strong enough to contribute to the echo signal.

[0014] In step 9, the number of backscattered photons can be obtained by calculating the scattering angle and photon energy attenuation during the photon propagation process. , thus obtaining the backscattering coefficient : ; in: is the number of backscattered photons at depth z that are scattered in directions close to 180°; is the total number of photons propagating in this section.

[0015] In step 10, the backscattering coefficient obtained by simulation is and extinction coefficient , invert the chlorophyll concentration in the water body through the empirical formula : ; Where: C and k are empirical coefficients obtained through experimental data or calibration, reflecting the relationship between backscattering coefficient and chlorophyll concentration.

[0016] The main beneficial effects of the present invention are: The photon point cloud data obtained by the ICESat-2 lidar system is combined with MODIS remote sensing data, and the Monte Carlo simulation technology is used to accurately model the photon propagation process. The photon propagation model is used to calculate the attenuation of photon energy in the water body, further derive the extinction coefficient and backscattering coefficient, and invert the concentration distribution of chlorophyll a in the water body.

[0017] The acquired photon data is cleaned and screened through the data preprocessing step to ensure that the photons come from the water area and provide effective input for subsequent analysis. Then, by calculating the propagation distance and energy attenuation of photons in the water body, combined with the scattering characteristics of the water body, the interaction between photons and water particles is simulated to accurately obtain the scattering angle and scattering direction of each photon. With this information, the number of backscattered photons is counted and used to calculate the backscattering coefficient, and finally the inversion of chlorophyll a concentration is achieved.

[0018] The Monte Carlo method is used to simulate photon propagation in detail, combined with environmental optical parameters, to achieve high-precision inversion of chlorophyll concentrations at different depths. The use of MODIS data for assistance further improves the reliability and accuracy of the model, ensuring the accuracy of chlorophyll concentration profile detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0020] Figure 1 It is a flow chart of the present invention.

[0021] Figure 2 This is a cross-sectional diagram of the number of photons in a water body according to the present invention.

[0022] Figure 3 Schematic diagram of water background noise photons of the present invention.

[0023] Figure 4 This is the Monte Carlo simulation photon propagation diagram of the present invention. DETAILED DESCRIPTION

[0024] like Figure 1~Figure 4 A method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar comprises the following steps: Step 1: Obtain photon point cloud data using ICESat-2 satellite laser radar, and estimate the energy of each photon by using the known laser wavelength; Step 2, obtaining MODIS chlorophyll concentration data, using MODIS remote sensing satellite to obtain chlorophyll concentration data of the study area for assistance; Step 3: Preliminary chlorophyll concentration estimation, using MODIS data to simulate preliminary water chlorophyll concentration distribution , and provide hypothetical values ​​for subsequent inversion; Step 4, calculating coefficients, including calculating the absorption coefficient of the water body, calculating the scattering coefficient of the water body and calculating the extinction coefficient; Step 5, photon initialization, photon propagation and distance update, and photon energy decay; Step 6: Data cleaning and screening; Step 7, photon scattering simulation, each time a photon scatters with particles in the water, the propagation direction of the photon will be based on the scattering phase function Random updates; Step 8: Calculate the total number of photons in the cross section and calculate the number of backscattered photons ; Step 9, calculation of backscatter coefficient, backscatter coefficient is the number of backscattered photons per unit volume; Step 10, chlorophyll concentration inversion.

[0025] In step 1, the ATLAS lidar of ICESat-2 uses 532nm, i.e. The wavelength of each photon is calculated by the following formula : ; Where: h is Planck's constant ( ); c is the speed of light ( );λ is the wavelength of the laser ( ).

[0026] The above photon data not only contains the spatial position, but also provides information for each depth layer, which is the basis for the subsequent calculation of the chlorophyll concentration profile.

[0027] The final data format is: ;in: : The coordinates of the photon in the water section to be measured; : Photon energy.

[0028] Preferably, in step 2, MODIS provides water body information in different bands by monitoring the reflectance data of the water body, and has high accuracy in chlorophyll concentration estimation in particular.

[0029] Preferably, in step 2, the acquired MODIS data will be used as a reference for the subsequent calibration process to provide a preliminary estimate of chlorophyll concentration for the photon counting lidar data.

[0030] Preferably, in step 2, when processing data, reasonable screening is required based on the temporal and spatial characteristics of the study area to ensure data quality and applicability.

[0031] Preferably, in step 2, the chlorophyll concentration data of MODIS helps to calibrate the water attributes in the lidar data and compensate for the possible deviation of the lidar under different water conditions. Therefore, MODIS data not only provides an effective reference value, but also provides support for the optimization of lidar data, further improving the accuracy and reliability of water detection.

[0032] Preferably, in step 3, the initial chlorophyll concentration Used as an initial guess, it will influence the calculation of optical parameters and be calibrated by subsequent optimization.

[0033] In step 4, the absorption coefficient is calculated using the empirical formula : ; in: ; is the absorption coefficient of pure water, which is a constant; Calculate the scattering coefficient using the empirical formula : ;in: ; is the scattering coefficient of pure water, which is a constant; the extinction coefficient is the sum of the absorption coefficient and the scattering coefficient: .

[0034] In step 5, photons are initialized and emitted from the lidar photons, initialize the emission position and emission direction of each photon; the initial direction is fixed and evenly distributed in two-dimensional space; Photon propagation and distance update: when each photon propagates in the water, the propagation distance By extinction coefficient Decide: ; where R is a uniformly distributed random number, is the extinction coefficient at depth z; Photon energy decays. Each time a photon propagates, the energy E of the photon changes according to the extinction coefficient. attenuation: ;in: is the photon energy before propagation; is the energy of the photon after propagation.

[0035] In step 6, the laser radar is emitted photons, retaining the valid signal photon data in the ocean water: ; Using the intensity threshold method, according to the pre-set intensity threshold , photons below this threshold are considered background noise: ;in: Represents the effective number of photons after removing noise; is the initial photon number; is the number of background noise photons.

[0036] In step 7, the scattering phase function is the Henyey-Greenstein scattering model: ; Where: g is the asymmetric factor, which indicates the bias of the scattering in the front and back directions; is the angle at which the i-th photon is scattered.

[0037] Preferably, the scattering angle The scattering angle is obtained by simulating the interaction between photons and water particles through the scattering phase function. , which in turn determines whether the photon is backscattered.

[0038] In step 8, calculate the total number of photons in the profile , Water body profile division: First, calibrate the initial small area profile, the distance along the track is 2km, the elevation distance is 1m, and update the small area profile by sliding window. The step length along the track direction is Δx=0.15km, and the step length in the elevation direction is Δz=0.05m. The photon number statistics of the small area profile: ;in: is the depth reached by the ith photon Indicates whether the photon reaches the area; represents the along-track extent of the initial small-area profile; represents the elevation range of the initial small area profile; m and n represent the number of sliding along the track and in the elevation direction, respectively.

[0039] In step 8, the number of backscattered photons is calculated The number of backscattered photons refers to the photons that are scattered to a direction close to 180° during propagation. By counting these photons, we get: ; in: is the unit pulse function, when When , it returns 1, indicating that the photon is a backscattered photon; is the energy of the photon after attenuation, It is the energy threshold that determines which photons are strong enough to contribute to the echo signal.

[0040] In step 9, the number of backscattered photons can be obtained by calculating the scattering angle and photon energy attenuation during the photon propagation process. , thus obtaining the backscattering coefficient : ; in: is the number of backscattered photons at depth z that are scattered in directions close to 180°; is the total number of photons propagating in this section.

[0041] In step 10, the backscattering coefficient obtained by simulation is and extinction coefficient , invert the chlorophyll concentration in the water body through the empirical formula : ; Where: C and k are empirical coefficients obtained through experimental data or calibration, reflecting the relationship between backscattering coefficient and chlorophyll concentration.

[0042] In the above method, the photon point cloud data obtained by the ICESat-2 lidar system is combined with MODIS remote sensing data, and the Monte Carlo simulation technology is used to accurately model the photon propagation process. The photon propagation model is used to calculate the attenuation of photon energy in the water body, further derive the extinction coefficient and backscattering coefficient, and invert the concentration distribution of chlorophyll a in the water body.

[0043] In this method, the acquired photon data is first cleaned and screened through the data preprocessing step to ensure that the photons come from the water area and provide effective input for subsequent analysis. Then, by calculating the propagation distance and energy attenuation of photons in the water body, combined with the scattering characteristics of the water body, the interaction between photons and water particles is simulated to accurately obtain the scattering angle and scattering direction of each photon. With this information, the number of backscattered photons is counted and used to calculate the backscattering coefficient, and finally the inversion of chlorophyll a concentration is achieved.

[0044] The innovation of this method is to simulate photon propagation in detail through the Monte Carlo method, combined with environmental optical parameters, to achieve high-precision inversion of chlorophyll concentration at different depths. By using MODIS data for assistance, the reliability and accuracy of the model are further improved, ensuring the accuracy of chlorophyll concentration profile detection.

[0045] A new chlorophyll a concentration detection method was proposed by Monte Carlo simulation of the propagation process of photons in water bodies and combined with the calculation of extinction coefficient and backscattering coefficient. It solved the accuracy and spatial coverage problems of vertical profile monitoring of open water bodies, and used MODIS remote sensing data to assist in improving the reliability and accuracy of the inversion results.

[0046] The above-mentioned embodiments are only preferred technical solutions of the present invention and should not be regarded as limitations of the present invention. The embodiments and features in the embodiments of the present application can be arbitrarily combined with each other without conflict. The protection scope of the present invention shall be the technical solutions recorded in the claims, including the equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A method for detecting chlorophyll a concentration profile in open water based on photon counting laser radar, characterized in that: The steps include: Step 1: Obtain photon point cloud data using ICESat-2 satellite laser radar, and estimate the energy of each photon by using the known laser wavelength; Step 2, obtaining MODIS chlorophyll concentration data, using MODIS remote sensing satellite to obtain chlorophyll concentration data of the study area for assistance; Step 3: Preliminary chlorophyll concentration estimation, using MODIS data to simulate preliminary water chlorophyll concentration distribution , and provide hypothetical values ​​for subsequent inversion; Step 4, calculating coefficients, including calculating the absorption coefficient of the water body, calculating the scattering coefficient of the water body and calculating the extinction coefficient; Step 5, photon initialization, photon propagation and distance update, and photon energy decay; Step 6: Data cleaning and screening; Step 7, photon scattering simulation, each time a photon scatters with particles in the water, the propagation direction of the photon will be based on the scattering phase function Random updates; Step 8: Calculate the total number of photons in the cross section and calculate the number of backscattered photons ; Step 9, calculation of backscatter coefficient, backscatter coefficient is the number of backscattered photons per unit volume; Step 10, chlorophyll concentration inversion.

2. The method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar according to claim 1, characterized in that: In step 1, the ATLAS lidar of ICESat-2 uses 532nm, that is, The wavelength of each photon is calculated by the following formula : ； Where: h is Planck's constant ( ); c is the speed of light ( );λ is the wavelength of the laser ( ).

3. The method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar according to claim 1, characterized in that: In step 4, the absorption coefficient is calculated using the empirical formula : ; in: ; is the absorption coefficient of pure water, which is a constant; Calculate the scattering coefficient using the empirical formula : ;in: ; is the scattering coefficient of pure water, which is a constant; the extinction coefficient is the sum of the absorption coefficient and the scattering coefficient: .

4. The method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar according to claim 1, characterized in that: In step 5, photons are initialized and emitted from the lidar photons, initialize the emission position and emission direction of each photon; the initial direction is fixed and evenly distributed in two-dimensional space; Photon propagation and distance update: when each photon propagates in the water, the propagation distance By extinction coefficient Decide: ; where R is a uniformly distributed random number, is the extinction coefficient at depth z; Photon energy decays. Each time a photon propagates, the energy E of the photon changes according to the extinction coefficient. attenuation: ;in: is the photon energy before propagation; is the energy of the photon after propagation.

5. The method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar according to claim 1, characterized in that: In step 6, the laser radar emits photons, retaining the valid signal photon data in the ocean water: ; Using the intensity threshold method, according to the pre-set intensity threshold , photons below this threshold are considered background noise: ;in: Represents the effective number of photons after removing noise; is the initial photon number; is the number of background noise photons.

6. The method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar according to claim 1, characterized in that: In step 7, the scattering phase function is the Henyey-Greenstein scattering model: ; Where: g is the asymmetric factor, which indicates the bias of the scattering in the front and back directions; is the angle at which the i-th photon is scattered.

7. The method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar according to claim 1, characterized in that: In step 8, calculate the total number of photons in the profile , Water body profile division: First, calibrate the initial small area profile, the distance along the track is 2km, the elevation distance is 1m, and update the small area profile by sliding window. The step length along the track direction is Δx=0.15km, and the step length in the elevation direction is Δz=0.05m. The photon number statistics of the small area profile: ;in: is the depth reached by the ith photon Indicates whether the photon reaches the area; represents the along-track extent of the initial small-area profile; represents the elevation range of the initial small area profile; m and n represent the number of sliding along the track and in the elevation direction, respectively.

8. The method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar according to claim 1, characterized in that: In step 8, calculate the number of backscattered photons The number of backscattered photons refers to the photons that are scattered to a direction close to 180° during propagation. By counting these photons, we get: ； in: is the unit pulse function, when When , it returns 1, indicating that the photon is a backscattered photon; is the energy of the photon after attenuation, It is the energy threshold that determines which photons are strong enough to contribute to the echo signal.

9. The method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar according to claim 1, characterized in that: In step 9, the number of backscattered photons can be obtained by calculating the scattering angle and photon energy attenuation during photon propagation. , thus obtaining the backscattering coefficient : ; in: is the number of backscattered photons at depth z that are scattered in directions close to 180°; is the total number of photons propagating in this section.

10. The method for detecting chlorophyll a concentration profile in an open water body based on a photon counting laser radar according to claim 1, characterized in that: In step 10, the backscattering coefficient obtained by simulation and extinction coefficient , invert the chlorophyll concentration in the water body through the empirical formula : ; Where: C and k are empirical coefficients obtained through experimental data or calibration, reflecting the relationship between backscattering coefficient and chlorophyll concentration.

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