A method for profiling chlorophyll a concentration in open water bodies based on photon-counting lidar

Abstract

A method for profiling chlorophyll a concentration in open water bodies based on photon-counting lidar is proposed to accurately obtain the profile concentration distribution of chlorophyll a in water. Combining ICESat-2 photon-counting lidar data with MODIS satellite remote sensing imagery, Monte Carlo simulation is used to model the photon propagation process, calculate the energy attenuation and scattering characteristics of photons in the water, and further derive the extinction coefficient and backscattering coefficient. Based on these parameters, the chlorophyll a concentration distribution is retrieved. By accurately simulating the photon propagation process in water, considering the optical properties and environmental factors of the water body, high-precision chlorophyll a concentration profile data can be provided. The use of MODIS remote sensing data further improves the accuracy and reliability of the retrieval results, enabling precise chlorophyll concentration detection in different open water areas. This method is applicable to chlorophyll concentration monitoring in oceans, lakes, rivers, and other water bodies, and provides a scientific basis for aquatic ecological environment assessment.

Description

Technical Field

[0001] This invention belongs to the field of chlorophyll a concentration detection technology, and relates to a method for detecting chlorophyll a concentration profiles in open water bodies based on photon counting lidar. Background Technology

[0002] Chlorophyll a concentration is an important indicator reflecting the ecological health of aquatic bodies and is widely used in marine monitoring, fisheries resource management, and environmental protection. Particularly in oceans and lakes, changes in chlorophyll a concentration often reflect eutrophication levels, algal growth, and the overall health of the ecosystem. Traditional methods for monitoring chlorophyll a concentration mainly rely on on-site sampling and spectral remote sensing. However, these methods typically only provide two-dimensional information about the water surface and cannot accurately capture the vertical distribution within the water body. This is especially true in deeper or turbid waters, where the accuracy and spatial coverage of monitoring are significantly limited.

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

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

[0005] The technical problem this invention aims to solve is to provide a method for detecting chlorophyll a concentration profiles in open water bodies based on photon-counting lidar. This method utilizes photon point cloud data acquired by the ICESat-2 lidar system, combined with MODIS remote sensing data, and employs Monte Carlo simulation technology to accurately model the photon propagation process. The attenuation of photon energy in the water body is calculated using the photon propagation model, further deriving the extinction coefficient and backscattering coefficient, and inverting the chlorophyll a concentration distribution in the water body.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for detecting chlorophyll a concentration profile in open water bodies based on photon counting lidar, comprising the following steps:

[0007] Step 1: Acquire photon point cloud data. Use ICESat-2 satellite lidar to acquire photon point cloud data and estimate the energy of each photon using the known laser wavelength.

[0008] Step 2: Obtain MODIS chlorophyll concentration data. The MODIS remote sensing satellite was used to acquire chlorophyll concentration data of the study area for auxiliary purposes.

[0009] Step 3: Preliminary chlorophyll concentration estimation. Using MODIS data, a preliminary chlorophyll concentration distribution in the water body is simulated. And to provide hypothetical values ​​for subsequent inversion;

[0010] Step 4: Calculate the coefficients, including calculating the absorption coefficient, scattering coefficient, and extinction coefficient of the water body;

[0011] Step 5: Photon initialization, photon propagation and distance update, and photon energy decay;

[0012] Step 6, Data cleaning and filtering;

[0013] Step 7, photon scattering simulation: Each time a photon is scattered with a particle in the water, the photon's propagation direction will change according to the scattering phase function. Random updates;

[0014] Step 8: Calculate the total number of photons in the profile. And calculate the number of backscattered photons ;

[0015] Step 9, Calculation of backscattering coefficient. It is the number of photons backscattered per unit volume;

[0016] Step 10, chlorophyll concentration inversion.

[0017] In step 1, the ICESat-2's ATLAS lidar uses 532nm, i.e. The wavelength is used to calculate the energy of each photon using the following formula. :

[0018] ;

[0019] Where: h is Planck's constant ( c is the speed of light ( λ is the wavelength of the laser. ).

[0020] In step 4, the absorption coefficient is calculated using an empirical formula. : ;

[0021] in: ; It is the absorption coefficient of pure water, which is a constant.

[0022] Scattering coefficients are calculated using empirical formulas. :

[0023] ;in: ; The scattering coefficient of pure water is a constant; the extinction coefficient is the sum of the absorption coefficient and the scattering coefficient. .

[0024] In step 5, photon initialization is performed, emitting from the lidar. Each photon is initialized with its emission position and direction; the initial direction is fixed and they are uniformly distributed in a two-dimensional space.

[0025] Photon propagation and distance update: the distance each photon travels in the water. Extinction coefficient Decide:

[0026] Where R is a uniformly distributed random number. It is the extinction coefficient at depth z;

[0027] Photon energy decays; with each propagation, the photon's energy E decreases according to the extinction coefficient. attenuation: ;in: The photon energy before propagation; The photon energy after propagation.

[0028] In step 6, the laser radar emits... Each photon retains effective signal photon data from ocean waters: ;

[0029] Using the intensity threshold method, based on a pre-set intensity threshold... Photons below this threshold are considered background noise.

[0030] ;in: This represents the number of effective photons after noise removal. The initial number of photons; The number of photons represents the background noise.

[0031] In step 7, the scattering phase function is the Henyey-Greenstein scattering model:

[0032] Where: g is the asymmetry factor, representing the directional bias of the scattering; Let be the angle at which the i-th photon is scattered.

[0033] In step 8, the total number of photons in the profile is calculated. Water body profile division: First, an initial small-area profile is defined, with a track distance of 2km and an elevation distance of 1m. The small-area profile is updated using a sliding window method, with a step size of Δx=0.15km along the track and Δz=0.05m in the elevation direction. Photon count of the small-area profile: ;in: The depth reached by the i-th photon

[0034] Indicates whether a photon has reached that region; This indicates the orbital extent of the initial small-area profile; This represents the elevation range of the initial small-area profile; m and n represent the number of slides along the track and elevation directions, respectively.

[0035] In step 8, the number of backscattered photons is calculated. The number of backscattered photons refers to the number of photons scattered to a direction close to 180° during propagation. These photons are counted to obtain:

[0036] ;

[0037] in: It is a unit impulse function, when If the value returns 1, it indicates that the photon is a backscattered photon; It is the attenuated photon energy. It is the energy threshold that determines which photons are strong enough to contribute to the echo signal.

[0038] In step 9, the number of backscattered photons can be obtained by calculating the scattering angle and photon energy attenuation during photon propagation. Thus, the backscattering coefficient is obtained. : ;

[0039] in: It is the number of photons backscattered at depth z, which are scattered in a direction close to 180°. It is the total number of photons propagating along that profile.

[0040] In step 10, based on the backscattering coefficient obtained from the simulation... and extinction coefficient The concentration of chlorophyll in water was inverted using empirical formulas. : Where C and k are empirical coefficients obtained through experimental data or calibration, reflecting the relationship between the backscattering coefficient and chlorophyll concentration.

[0041] The main beneficial effects of this invention are as follows:

[0042] Photon point cloud data acquired using the ICESat-2 lidar system, combined with MODIS remote sensing data, was used to accurately model the photon propagation process using Monte Carlo simulation technology. The attenuation of photon energy in the water was calculated using the photon propagation model, and the extinction coefficient and backscattering coefficient were further derived. The concentration distribution of chlorophyll a in the water was then retrieved.

[0043] The acquired photon data is cleaned and filtered through data preprocessing steps to ensure that the photons originate from the water body and provide effective input for subsequent analysis. Then, by calculating the propagation distance and energy attenuation of photons in the water body, and combining this with the scattering characteristics of the water body, the interaction between photons and water particles is simulated to accurately obtain the scattering angle and direction of each photon. Using this information, the number of backscattered photons is counted and used to calculate the backscattering coefficient, ultimately achieving the inversion of chlorophyll a concentration.

[0044] By meticulously simulating photon propagation using the Monte Carlo method and combining it with environmental optical parameters, high-precision inversion of chlorophyll concentration at different depths was achieved. The use of MODIS data further improved the model's reliability and accuracy, ensuring the accuracy of chlorophyll concentration profile detection. Attached Figure Description

[0045] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0046] Figure 1 This is a flowchart of the present invention.

[0047] Figure 2 This is a cross-sectional view of the number of photons in the water body according to the present invention.

[0048] Figure 3 This is a schematic diagram of water background noise photons according to the present invention.

[0049] Figure 4 This is a Monte Carlo simulation of photon propagation according to the present invention. Detailed Implementation

[0050] like Figures 1-4 A method for detecting chlorophyll a concentration profiles in open water bodies based on photon-counting lidar includes the following steps:

[0051] Step 1: Acquire photon point cloud data. Use ICESat-2 satellite lidar to acquire photon point cloud data and estimate the energy of each photon using the known laser wavelength.

[0052] Step 2: Obtain MODIS chlorophyll concentration data. The MODIS remote sensing satellite was used to acquire chlorophyll concentration data of the study area for auxiliary purposes.

[0053] Step 3: Preliminary chlorophyll concentration estimation. Using MODIS data, a preliminary chlorophyll concentration distribution in the water body is simulated. And to provide hypothetical values ​​for subsequent inversion;

[0054] Step 4: Calculate the coefficients, including calculating the absorption coefficient, scattering coefficient, and extinction coefficient of the water body;

[0055] Step 5: Photon initialization, photon propagation and distance update, and photon energy decay;

[0056] Step 6, Data cleaning and filtering;

[0057] Step 7, photon scattering simulation: Each time a photon is scattered with a particle in the water, the photon's propagation direction will change according to the scattering phase function. Random updates;

[0058] Step 8: Calculate the total number of photons in the profile. And calculate the number of backscattered photons ;

[0059] Step 9, Calculation of backscattering coefficient. It is the number of photons backscattered per unit volume;

[0060] Step 10, chlorophyll concentration inversion.

[0061] In step 1, the ICESat-2's ATLAS lidar uses 532nm, i.e. The wavelength is used to calculate the energy of each photon using the following formula. :

[0062] ;

[0063] Where: h is Planck's constant ( c is the speed of light ( λ is the wavelength of the laser. ).

[0064] The aforementioned photon data not only includes spatial location but also provides information for each depth layer, which forms the basis for subsequent calculations of chlorophyll concentration profiles.

[0065] The final data format is: ;in: : Coordinates of the photon in the water profile being measured; Photon energy.

[0066] Preferably, in step 2, MODIS provides water information in different wavelengths by monitoring the reflectance data of the water body, and has high accuracy, especially in estimating chlorophyll concentration.

[0067] Preferably, in step 2, the acquired MODIS data will be used as a reference for subsequent calibration processes to provide a preliminary estimate of chlorophyll concentration for the photon counting lidar data.

[0068] Preferably, in step 2, during data processing, reasonable screening should be carried out based on the temporal and spatial characteristics of the study area to ensure data quality and applicability.

[0069] Preferably, in step 2, the chlorophyll concentration data from MODIS helps calibrate the water body properties in the lidar data, compensating for potential biases in lidar under different water conditions. Therefore, MODIS data not only provides effective reference values ​​but also supports the optimization of lidar data, further improving the accuracy and reliability of water body detection.

[0070] Preferably, in step 3, the initial chlorophyll concentration Used as an initial guess, it will affect the calculation of optical parameters and be calibrated through subsequent optimization.

[0071] In step 4, the absorption coefficient is calculated using an empirical formula. : ;

[0072] in: ; It is the absorption coefficient of pure water, which is a constant.

[0073] Scattering coefficients are calculated using empirical formulas. :

[0074] ;in: ; The scattering coefficient of pure water is a constant; the extinction coefficient is the sum of the absorption coefficient and the scattering coefficient. .

[0075] In step 5, photon initialization is performed, emitting from the lidar. Each photon is initialized with its emission position and direction; the initial direction is fixed and they are uniformly distributed in a two-dimensional space.

[0076] Photon propagation and distance update: the distance each photon travels in the water. Extinction coefficient Decide:

[0077] Where R is a uniformly distributed random number. It is the extinction coefficient at depth z;

[0078] Photon energy decays; with each propagation, the photon's energy E decreases according to the extinction coefficient. attenuation: ;in: The photon energy before propagation; The photon energy after propagation.

[0079] In step 6, the laser radar emits... Each photon retains effective signal photon data from ocean waters: ;

[0080] Using the intensity threshold method, based on a pre-set intensity threshold... Photons below this threshold are considered background noise.

[0081] ;in: This represents the number of effective photons after noise removal. The initial number of photons; The number of photons represents the background noise.

[0082] In step 7, the scattering phase function is the Henyey-Greenstein scattering model:

[0083] Where: g is the asymmetry factor, representing the directional bias of the scattering; Let be the angle at which the i-th photon is scattered.

[0084] Preferably, the scattering angle This will affect the calculation of the number of backscattered photons. The scattering angle is obtained by simulating the interaction between photons and water particles using the scattering phase function. This determines whether the photon is scattered backward.

[0085] In step 8, the total number of photons in the profile is calculated. Water body profile division: First, an initial small-area profile is defined, with a track distance of 2km and an elevation distance of 1m. The small-area profile is updated using a sliding window method, with a step size of Δx=0.15km along the track and Δz=0.05m in the elevation direction. Photon count of the small-area profile: ;in: The depth reached by the i-th photon

[0086] Indicates whether a photon has reached that region; This indicates the orbital extent of the initial small-area profile; This represents the elevation range of the initial small-area profile; m and n represent the number of slides along the track and elevation directions, respectively.

[0087] In step 8, the number of backscattered photons is calculated. The number of backscattered photons refers to the number of photons scattered to a direction close to 180° during propagation. These photons are counted to obtain:

[0088] ;

[0089] in: It is a unit impulse function, when If the value returns 1, it indicates that the photon is a backscattered photon; It is the attenuated photon energy. It is the energy threshold that determines which photons are strong enough to contribute to the echo signal.

[0090] In step 9, the number of backscattered photons can be obtained by calculating the scattering angle and photon energy attenuation during photon propagation. Thus, the backscattering coefficient is obtained. : ;

[0091] in: It is the number of photons backscattered at depth z, which are scattered in a direction close to 180°. It is the total number of photons propagating along that profile.

[0092] In step 10, based on the backscattering coefficient obtained from the simulation... and extinction coefficient The concentration of chlorophyll in water was inverted using empirical formulas. : Where C and k are empirical coefficients obtained through experimental data or calibration, reflecting the relationship between the backscattering coefficient and chlorophyll concentration.

[0093] In the above method, photon point cloud data acquired by the ICESat-2 lidar system is combined with MODIS remote sensing data, and Monte Carlo simulation technology is used to accurately model the photon propagation process. The attenuation of photon energy in the water body is calculated using 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.

[0094] This method first involves data preprocessing to clean and filter the acquired photon data, ensuring that the photons originate from a water body and providing effective input for subsequent analysis. Then, by calculating the propagation distance and energy attenuation of photons in the water, and combining this with the scattering characteristics of the water, the interaction between photons and water particles is simulated to accurately obtain the scattering angle and direction of each photon. Using this information, the number of backscattered photons is counted and used to calculate the backscattering coefficient, ultimately achieving the inversion of chlorophyll a concentration.

[0095] The innovation of this method lies in its meticulous simulation of photon propagation using the Monte Carlo method, combined with environmental optical parameters, to achieve high-precision inversion of chlorophyll concentration at different depths. The use of MODIS data further enhances the model's reliability and accuracy, ensuring the precision of chlorophyll concentration profile detection.

[0096] By simulating the propagation process of photons in water using Monte Carlo simulation and combining the calculation of extinction coefficient and backscattering coefficient, a new method for detecting chlorophyll a concentration is proposed. This method solves the problems of accuracy and spatial coverage in vertical profile monitoring of open water bodies. Furthermore, the use of MODIS remote sensing data enhances the reliability and accuracy of the inversion results.

[0097] The above embodiments are merely preferred technical solutions of the present invention and should not be considered as limitations on the present invention. The embodiments and features described in these embodiments can be arbitrarily combined without conflict. The scope of protection of the present invention should be limited to the technical solutions described in the claims, including equivalent substitutions of the technical features described in the claims. That is, equivalent substitutions and improvements within this scope are also within the scope of protection of the present invention.

Claims

1. A method for detecting chlorophyll a concentration profiles in open water bodies based on photon-counting lidar, characterized in that, Includes the following steps: Step 1: Acquire photon point cloud data. Use ICESat-2 satellite lidar to acquire photon point cloud data and estimate the energy of each photon using the known laser wavelength. Step 2: Obtain MODIS chlorophyll concentration data. The MODIS remote sensing satellite was used to acquire chlorophyll concentration data of the study area for auxiliary purposes. Step 3: Preliminary chlorophyll concentration estimation. Using MODIS data, a preliminary chlorophyll concentration distribution in the water body is simulated. And to provide hypothetical values ​​for subsequent inversion; Step 4: Calculate the coefficients, including calculating the absorption coefficient, scattering coefficient, and extinction coefficient of the water body; Step 5: Photon initialization, photon propagation and distance update, and photon energy decay; Step 6, Data cleaning and filtering; Step 7, photon scattering simulation: Each time a photon is scattered with a particle in the water, the photon's propagation direction will change according to the scattering phase function. Random updates; Step 8: Calculate the total number of photons in the profile. And calculate the number of backscattered photons ; Step 9, Calculation of backscattering coefficient. It is the number of photons backscattered per unit volume; Step 10, chlorophyll concentration inversion; In step 1, the ICESat-2's ATLAS lidar uses 532nm, i.e. The wavelength is used to calculate the energy of each photon using the following formula. : ； Where: h is Planck's constant, c is the speed of light. λ is the wavelength of the laser. ; In step 5, photon initialization is performed, emitting from the lidar. Each photon is initialized with its emission position and direction; the initial direction is fixed and they are uniformly distributed in a two-dimensional space. Photon propagation and distance update: the distance each photon travels in the water. Extinction coefficient Decide: Where R is a uniformly distributed random number, It is the extinction coefficient at depth z; Photon energy decays; with each propagation, the photon's energy E decreases according to the extinction coefficient. attenuation: ;in: The photon energy before propagation; The energy of the photon after propagation; In step 10, based on the backscattering coefficient obtained from the simulation... and extinction coefficient The concentration of chlorophyll in water was inverted using empirical formulas. : Where C and k are empirical coefficients obtained through experimental data or calibration, reflecting the relationship between the backscattering coefficient and chlorophyll concentration.

2. The method for detecting chlorophyll a concentration profile in open water bodies based on photon-counting lidar according to claim 1, characterized in that: in In step 4, the absorption coefficient is calculated using an empirical formula. : ; in: ; It is the absorption coefficient of pure water, which is a constant. Scattering coefficients are calculated using empirical formulas. : ;in: ; The scattering coefficient of pure water is a constant; the extinction coefficient is the sum of the absorption coefficient and the scattering coefficient. .

3. The method for detecting chlorophyll a concentration profiles in open water bodies based on photon-counting lidar according to claim 1, characterized in that: in In step 6, the laser radar emits... Each photon retains effective signal photon data from ocean waters: ; Using the intensity threshold method, based on a pre-set intensity threshold... Photons below this threshold are considered background noise. ;in: This represents the number of effective photons after noise removal. The initial number of photons; The number of background noise photons.

4. The method for detecting chlorophyll a concentration profile in open water bodies based on photon-counting lidar according to claim 1, characterized in that: in In step 7, the scattering phase function is the Henyey-Greenstein scattering model: Where: g is the asymmetry factor, representing the directional bias of the scattering; Let be the angle at which the i-th photon is scattered.

5. The method for detecting chlorophyll a concentration profile in open water bodies based on photon-counting lidar according to claim 1, characterized in that: in In step 8, the total number of photons in the profile is calculated. Water body profile division: First, an initial small-area profile is defined, with a track distance of 2km and an elevation distance of 1m. The small-area profile is updated using a sliding window method, with a step size of Δx=0.15km along the track and Δz=0.05m in the elevation direction. Photon count of the small-area profile: ; It is the depth reached by the i-th photon; Indicates whether a photon has reached that region; This indicates the orbital extent of the initial small-area profile; This represents the elevation range of the initial small-area profile; m and n represent the number of slides along the track and elevation directions, respectively.

6. The method for detecting chlorophyll a concentration profile in open water bodies based on photon-counting lidar according to claim 1, characterized in that: in In step 8, the number of backscattered photons is calculated. The number of backscattered photons refers to the number of photons scattered to a direction close to 180° during propagation. These photons are counted to obtain: ； in: It is a unit impulse function, when If the value returns 1, it indicates that the photon is a backscattered photon; It is the attenuated photon energy. It is the energy threshold that determines which photons are strong enough to contribute to the echo signal.

7. The method for detecting chlorophyll a concentration profile in open water bodies based on photon-counting lidar according to claim 1, characterized in that: in In step 9, the number of backscattered photons can be obtained by calculating the scattering angle and photon energy attenuation during photon propagation. Thus, the backscattering coefficient is obtained. : ; in: It is the number of photons backscattered at depth z, which are scattered in a direction close to 180°. It is the total number of photons propagating along this profile.

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