A water body chlorophyll concentration profile inversion method and a laser radar detection method

Abstract

The application discloses a water body chlorophyll concentration profile inversion method and a laser radar detection method, wherein the inversion method comprises the following steps: obtaining laser radar detection echo signals at a specific depth in a target area, which comprises a fluorescence channel and a Raman channel; expressing a backscattering signal profile of the fluorescence channel as a function comprising a fluorescence backscattering coefficient; expressing a backscattering signal profile of the Raman channel as a function comprising a Raman backscattering coefficient; normalizing the backscattering signal profile function of the fluorescence channel with the backscattering signal profile function of the Raman channel to obtain a normalized signal; inverting the fluorescence backscattering coefficient based on a perturbation method; inverting a chlorophyll fluorescence absorption coefficient at a specific laser wavelength based on the inverted fluorescence backscattering coefficient; and inverting a chlorophyll concentration profile in the water body based on the inverted chlorophyll fluorescence absorption coefficient. The inversion method can invert the water body chlorophyll concentration profile from one measurement.

Description

Technical Field

[0001] This invention relates to the field of lidar detection, specifically to a method for inverting chlorophyll concentration profiles in water and a lidar detection method. Background Technology

[0002] Marine ecosystems play a vital role in global climate change and environmental protection. Phytoplankton are the primary producers in marine ecosystems, and their net photosynthetic carbon sequestration is approximately equal to the total carbon sequestration of all terrestrial plants. Lake ecosystems play a crucial role in regulating regional climate and maintaining regional ecological balance, and are also important freshwater resources. Eutrophication of lakes can lead to algal blooms, polluting the aquatic environment. Chlorophyll is the main pigment used in phytoplankton photosynthesis and is used to characterize phytoplankton biomass. Currently, methods for detecting chlorophyll mainly include biochemical methods, in-situ instrument measurements, passive remote sensing, and active remote sensing. Among these, lidar, as an active optical remote sensing device, can be used to detect the bio-optical profiles of water bodies, remotely sensing the vertical distribution structure of the upper ocean, and has advantages such as high spatiotemporal resolution, continuous day and night observation, and global-scale measurement.

[0003] However, existing fluorescent lidar systems have certain limitations: because the intensity of the backscattered signal is significantly weaker than that of the elastic scattering signal, even with high-power lasers, existing fluorescent lidar systems can only acquire information from the water surface. Fortunately, single-photon detection technology offers the possibility of obtaining fluorescence backscattered signal profiles. Based on single-photon detection technology, fluorescent lidar systems can achieve long-distance detection even with low-pulse-energy lasers and small-aperture telescopes, and can obtain fluorescence profile data accordingly.

[0004] However, after obtaining fluorescence profile data, existing inversion methods cannot deduce the two unknowns in the fluorescence lidar equation, namely the fluorescence lidar attenuation coefficient and the fluorescence lidar backscattering function, from a single measurement, which makes it difficult to calculate the bio-optical profile of aquatic bodies. Summary of the Invention

[0005] The purpose of this invention is to overcome the aforementioned defects or problems in the prior art and to provide a method for inverting the profile of chlorophyll concentration in water and a lidar detection method, which can invert the chlorophyll concentration in water from a single measurement.

[0006] To achieve the above objectives, the present invention and its preferred embodiments employ the following technical solutions, but the embodiments are not limited to the following solutions:

[0007] A method for inverting a chlorophyll concentration profile in water includes: acquiring lidar echo signals at a specific depth in a target area, including a fluorescence channel and a Raman channel; representing the backscattering signal profile of the fluorescence channel as a function including the fluorescence backscattering coefficient; representing the backscattering signal profile of the Raman channel as a function including the Raman backscattering coefficient; normalizing the backscattering signal profile function of the fluorescence channel using the Raman channel backscattering signal profile function to obtain a normalized signal; inverting the fluorescence backscattering coefficient based on a perturbation method; inverting the chlorophyll fluorescence absorption coefficient at a specific laser wavelength based on the inverted fluorescence backscattering coefficient; and inverting the chlorophyll concentration profile in the water body based on the inverted chlorophyll fluorescence absorption coefficient.

[0008] Furthermore, the backscattered signal profile of the fluorescence channel is represented as follows:

[0009]

[0010] Where: P f To emit laser wavelength λ L The fluorescence wavelength is λ f At depth z, the water fluorescence echo signal received by the lidar; H is the lidar's height above the water; B f Q is a constant in a lidar system that does not change with the detection distance, and it includes at least the lidar's emission power, the detector's quantum efficiency, and the transmittance of the transmitted and received signals of the optical system; f (z) is the geometric overlap factor; β f g(λ) is the backscattering coefficient of fluorescence scattering. f ,σ f Let be the transmittance function of the fluorescence filter, which coincides with a center wavelength of λ. f The bandwidth is σ f Gaussian function; For λ L The attenuation coefficient of the lidar beam at a given location; For λ f The attenuation coefficient of the lidar beam at a given location.

[0011] Furthermore, the backscattered signal profile of the Raman channel is represented as follows:

[0012]

[0013] The Raman backscattering coefficient is expressed as:

[0014]

[0015] Where: P r To emit laser wavelength λ L The Raman wavelength is λ rAt depth z, the water Raman echo signal received by the lidar; B r Q is a constant in a lidar system that does not change with the detection distance, and it includes at least the lidar's emission power, the detector's quantum efficiency, and the transmittance of the transmitted and received signals of the optical system; r (z) is the geometric overlap factor, Q r (z)=Q f (z); β t β is the Raman backscattering coefficient r With fluorescence backscattering coefficient B f The sum of, i.e., β t =β r +β f g(λ) r ,σ r Let be the transmittance function of the Raman filter, which coincides with a center wavelength of λ. r The bandwidth is σ r Gaussian function; For λ r The lidar beam attenuation coefficient at the location; b R To emit laser wavelength λ L The received Raman wavelength is λ r The Raman scattering coefficient of water molecules; f R The Raman wavelength distribution function; Let be the Raman scattering phase function.

[0016] Furthermore, the normalized signal is represented as:

[0017]

[0018] in:

[0019] Furthermore, the steps for retrieving fluorescence backscattering coefficients based on the perturbation method include:

[0020] The normalized signal is represented as:

[0021]

[0022] Where: β f (λ f ,z0)β t (λ r ,z0) β f β t and The portion of the matrix that does not change with depth, β f ′(λ f ,z)β t ′(λ r ,z) β f β t and The part that varies with depth, z0 is the depth of the first point in the lidar detection echo signal;

[0023] When the disturbance term is ignored, the normalized signal can be expressed as:

[0024]

[0025] Assumption The ratio B was determined through experimental calibration. f / B r , will β f Represented as:

[0026]

[0027] Where, β f The inversion result is expressed as the right-hand side of the equation for g(λ) f ,σ f Deconvolution of )

[0028]

[0029] Among them, F and F -1 These represent the Fourier transform and the inverse Fourier transform, respectively.

[0030] Define coefficients for:

[0031]

[0032] The inversion result of the fluorescence backscattering coefficient is:

[0033]

[0034] Furthermore, based on the fluorescence backscattering coefficient obtained by inversion, the steps for inverting the chlorophyll fluorescence absorption coefficient at a specific laser wavelength are as follows:

[0035] β f Represented as:

[0036]

[0037] in:

[0038] a ph For the laser wavelength λ L The chlorophyll fluorescence absorption coefficient below; Φ c The quantum yield of chlorophyll fluorescence; h c It is the emission wavelength function of chlorophyll fluorescence normalization;

[0039] Then a ph for:

[0040]

[0041] Furthermore, based on the chlorophyll fluorescence absorption coefficient obtained through inversion, the steps for inverting the chlorophyll concentration profile in the water body are as follows:

[0042] The chlorophyll concentration profile in water bodies can be determined using an empirical formula, which is:

[0043]

[0044] In addition, the present invention also provides a lidar detection method for chlorophyll concentration profile in water, which includes: emitting a laser of a specific wavelength toward a target area by lidar and acquiring Raman channel echo signal and fluorescence channel echo signal of the water; and using the water chlorophyll concentration profile inversion method as described in any one of the above claims to invert and obtain the chlorophyll concentration in the water.

[0045] Furthermore, in the lidar used, the wavelength λ of the emitted laser is... L The selectable wavelength range is 350nm to 540nm; the Raman filter bandwidth is selectable from 4nm to 20nm; the fluorescence filter bandwidth is selectable from 8nm to 20nm; and the wavelength λ of the Raman channel echo signal is [not specified]. r The wavelength λ of the fluorescence channel echo signal is the wavelength corresponding to the excitation wavelength of the water Raman scattering. f The range is from 650nm to 690nm.

[0046] Furthermore, in the lidar used, the wavelength λ of the emitted laser is... L The wavelength is 532nm, the Raman filter bandwidth is 6nm, the fluorescence filter bandwidth is 10nm, and the wavelength λ of the Raman channel echo signal is... r The wavelength λ of the fluorescence channel echo signal is 650nm. f It is 685nm.

[0047] As can be seen from the above description of the present invention, compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0048] This invention adds a Raman channel to the fluorescence channel of a lidar system. Based on the lidar detection echo signals including both the fluorescence and Raman channels, it establishes a backscattering signal profile function for the fluorescence channel (including the fluorescence backscattering coefficient) and a backscattering signal profile function for the Raman channel (including the Raman backscattering coefficient). After normalization, the fluorescence backscattering coefficient is retrieved using a perturbation method. Based on the retrieved fluorescence backscattering coefficient, the chlorophyll fluorescence absorption coefficient at a specific laser wavelength is obtained, thus yielding the chlorophyll concentration profile in the water. This invention solves the shortcomings of existing fluorescence lidar systems in detecting chlorophyll concentration in water, allowing for the retrieval of chlorophyll concentration in a single measurement. Attached Figure Description

[0049] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments are briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0050] Figure 1 This is a flowchart of the water chlorophyll concentration profile inversion method in Embodiment 1 of the present invention;

[0051] Figure 2 This is an analysis diagram of the Raman filter bandwidth selection in Embodiment 2 of the present invention;

[0052] Figure 3 This is a schematic diagram of the Monte Carlo simulation results in Embodiment 2 of the present invention;

[0053] Figure 4 For the detection depths of 1m, 5m, 10m and 20m in Embodiment 2 of the present invention, The error results resulting from the choice;

[0054] Figure 5 This is a theoretical inversion error distribution diagram of four different chlorophyll vertical distribution models in Embodiment 2 of the present invention;

[0055] Figure 6 This is a diagram showing the results of an outdoor experiment in Embodiment 2 of the present invention. Detailed Implementation

[0056] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are preferred embodiments of the present invention and should not be considered as excluding other embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0057] In the claims, description and accompanying drawings of this invention, the terms "comprising," "having," and variations thereof are used to mean "including but not limited to."

[0058] Example 1

[0059] Embodiment 1 of the present invention provides a method for inverting the chlorophyll concentration profile of water bodies, referring to... Figure 1 It includes the following steps:

[0060] S1. Acquire lidar detection echo signals at a specific depth in the target area, including fluorescence and Raman channels;

[0061] S2. Express the backscattering signal profile of the fluorescence channel as a function including the fluorescence backscattering coefficient;

[0062] S3. Express the backscattered signal profile of the Raman channel as a function including the Raman backscattering coefficient;

[0063] S4. Normalize the backscattering signal profile function of the fluorescence channel using the backscattering signal profile function of the Raman channel to obtain the normalized signal.

[0064] S5. Fluorescence backscattering coefficient inversion based on perturbation method;

[0065] S6. Based on the fluorescence backscattering coefficient obtained by inversion, invert the chlorophyll fluorescence absorption coefficient at a specific laser wavelength;

[0066] S7. Based on the chlorophyll fluorescence absorption coefficient obtained by inversion, the chlorophyll concentration profile in the water body is inverted.

[0067] In step S2, the backscattered signal profile of the fluorescence channel is expressed as the following function using equation (1):

[0068]

[0069] Where: P f To emit laser wavelength λ L The fluorescence wavelength is λ f At depth z, the water fluorescence echo signal received by the lidar; H is the lidar's height above the water; B f Q is a constant in a lidar system that does not change with the detection distance, and it includes at least the lidar's emission power, the detector's quantum efficiency, and the transmittance of the transmitted and received signals of the optical system; f (z) is the geometric overlap factor; β f g(λ) is the backscattering coefficient of fluorescence scattering. f ,σ fLet be the transmittance function of the fluorescence filter, which coincides with a center wavelength of λ. f The bandwidth is σ f Gaussian function; For λ L The attenuation coefficient of the lidar beam at a given location; For λ f The attenuation coefficient of the lidar beam at a given location.

[0070] In step S3, the backscattered signal profile of the Raman channel is expressed as the following function using equation (2):

[0071]

[0072] The Raman backscattering coefficient is expressed by equation (3):

[0073]

[0074] Where: P r To emit laser wavelength λ L The Raman wavelength is λ r At depth z, the water Raman echo signal received by the lidar; B r Q is a constant in a lidar system that does not change with the detection distance, and it includes at least the lidar's emission power, the detector's quantum efficiency, and the transmittance of the transmitted and received signals of the optical system; r (z) is the geometric overlap factor, Q r (z)=Q f (z); β t β is the Raman backscattering coefficient r With fluorescence backscattering coefficient B f The sum of, i.e., β t =B r +B f g(λ) r ,σ r Let be the transmittance function of the Raman filter, which coincides with a center wavelength of λ. r The bandwidth is σ r Gaussian function; For λ r The lidar beam attenuation coefficient at the location; b R To emit laser wavelength λ L The received Raman wavelength is λ r The Raman scattering coefficient of water molecules; f R The Raman wavelength distribution function; Let be the Raman scattering phase function.

[0075] In step S4, the normalized signal is expressed by equation (4):

[0076]

[0077] in,

[0078] In step S5, the normalized signal in equation (4) is represented as a part that varies with depth (i.e., the perturbation term) and a part that does not vary with depth, and is expressed by equation (5):

[0079]

[0080] Where: β f (λ f ,z0)β t (λ r ,z0) β f β t and The portion of the matrix that does not change with depth, β f ′(λ f ,z)β t ′(λ r ,z) β f β t and The part that varies with depth is z0, which is the depth of the first point in the lidar detection echo signal.

[0081] Equation (5) can be transformed into equation (6):

[0082]

[0083] The ratio B was calibrated through experiments. f / B r and will The value is expressed by equation (7):

[0084]

[0085] Among them, the ratio B was calibrated through experiments. f / B r This is achieved by attenuating a broadband continuous light source with a known spectral distribution to the single-photon level and coupling it into the optical collimator of the lidar system. The B signal can then be calibrated by measuring the ratio of the Raman and fluorescence channel detection signals. f / B r .

[0086] According to the perturbation method, assuming Based on equations (5) and (6), B f Equation (8) is used to express this as:

[0087]

[0088] After that, B can be... f The inversion result is expressed by equation (9) as the right-hand side of equation (8) for g(λ) f ,σ f Deconvolution of )

[0089]

[0090] Among them, F and F -1 These represent the Fourier transform and the inverse Fourier transform, respectively.

[0091] Next, define the coefficients. For equation (10):

[0092]

[0093] The inversion result of the fluorescence backscattering coefficient is expressed by equation (11):

[0094]

[0095] In step S6, ignoring fluorescence caused by substances in the water other than chlorophyll, the β... f Equation (12) can be expressed as:

[0096]

[0097] in:

[0098] a ph For the laser wavelength λ L The chlorophyll fluorescence absorption coefficient below; Φ c The quantum yield of chlorophyll fluorescence; h c It is the normalized emission wavelength function of chlorophyll fluorescence; among which, the quantum yield of chlorophyll fluorescence is affected by factors such as light, nutrients and temperature, and the normalized reflection wavelength function of chlorophyll fluorescence can be represented by a known model. By substituting the quantum yield of chlorophyll fluorescence and the waveform of the normalized reflection wavelength function of chlorophyll fluorescence into equation (12),

[0099] Then a ph Equation (13) is used to express:

[0100]

[0101] In step S7, the chlorophyll concentration profile in the water body can be determined using an empirical formula, which is Equation (14):

[0102]

[0103] Example 2

[0104] Furthermore, embodiments of the present invention also provide a lidar detection method for chlorophyll concentration profiles in water bodies, which includes the following steps:

[0105] Step 1: Emits a laser of a specific wavelength toward the target area using a lidar system and acquires the Raman channel echo signal and fluorescence channel echo signal of the water body.

[0106] Step 2: Using the water chlorophyll concentration profile inversion method provided in Example 1, the chlorophyll concentration in the water body is obtained by inversion.

[0107] In the lidar used, the wavelength λ of the emitted laser is... L The selectable wavelength range is 350nm to 540nm; the Raman filter bandwidth is selectable from 4nm to 20nm; the fluorescence filter bandwidth is selectable from 8nm to 20nm; and the wavelength λ of the Raman channel echo signal is [not specified]. r The wavelength λ of the fluorescence channel echo signal is the wavelength corresponding to the excitation wavelength of the water Raman scattering. f The range is from 650nm to 690nm.

[0108] In this embodiment, the laser radar used has a laser emission wavelength λ. L The wavelength is 532nm. The echo receiver of this lidar can receive echoes from both the Raman and fluorescence channels. The Raman filter bandwidth is 6nm, and the fluorescence filter bandwidth is 10nm. The wavelength λ of the Raman channel echo signal is... r The wavelength λ of the fluorescence channel echo signal is 650nm. f It is 685nm.

[0109] As can be seen from formula (11) in Example 1, in order to achieve β f The exact inversion requires minimizing β. * f0 Variation with depth. According to equation (10), the bandwidth of the fluorescence filter does not directly affect β. * f0 The value of the fluorescence filter is important. However, a larger bandwidth results in a stronger received fluorescence backscattered signal and a higher detection signal-to-noise ratio. Therefore, a fluorescence filter with a larger bandwidth is required. But in reality, a larger bandwidth of the fluorescence filter can also increase the contamination of the chlorophyll fluorescence backscattered signal by fluorescence signals from other substances. To balance the effects of both, this invention selects a fluorescence filter bandwidth of 10 nm.

[0110] Furthermore, equation (10) shows that β * f0 The bandwidth of the Raman filter has an impact. Similar to fluorescence filters, a wider bandwidth in a Raman filter results in a stronger Raman signal, a higher signal-to-noise ratio, and is more beneficial for detection. (Refer to...) Figure 2 In Figure a, the larger the bandwidth of the Raman filter, the higher the β value. tg The larger (of which) The stronger the returned Raman signal, the better. Specifically, increasing the bandwidth of the Raman filter from 6 nm to 10 nm can enhance the signal strength by about 1.5 times. However, a wider Raman filter bandwidth also increases the sensitivity of the Raman echo signal to changes in Chl, leading to a decrease in β in equation (10). tg The changes. (Refer to...) Figure 2 As shown in Figure b, a narrower Raman filter bandwidth can reduce β. th The stability of inversion is improved by varying Chl.

[0111] To further explain the β caused by changes in Chl tg Relative change, Error r Equation (15) is expressed as follows:

[0112]

[0113] Reference Figure 2 As shown in the diagram on the right, when the Raman filter bandwidth is selected as 6nm, Chl increases from 0.01mg / m 3 Change to 10 mg / m 3 Error r The variation range was only 4.3%.

[0114] Because in this case, β tg The variation with Chl is small, therefore it can be assumed that Approximately equal to Therefore, equation (10) can be rewritten in the form of equation (16) as follows:

[0115]

[0116] As can be seen from Equation 6 in Example 1, it is necessary to determine the following during the inversion process: The value of is influenced not only by the hardware parameters of the lidar system, but also by the inherent optical properties (IOP) of water and the multiple scattering of the laser light by particles in the water. To determine... The range of variation of the value was simulated using a Monte Carlo (MC) model in this embodiment, specifically a semi-analytical MC model. In the simulation, the Petzold phase function was used, with a sampling length of 20m and a sampling interval of 0.1m, totaling 200 sampling points. (Refer to...) Figure 3 In the simulation results, the fluorescence backscattering signal and the Raman backscattering signal exhibit exponential decay.

[0117] To mitigate the effects of multiple scattering in the lidar backscattered signal, a small-aperture telescope with a narrow field of view is used. (See reference...) Figure 3 When Chl is low, the percentage of multiple scattering (PMS), including secondary and higher-order scattering, is low, and the lidar signal is mainly single scattering. However, as Chl increases, PMS also increases. By selecting raw signals with PMS less than 100% and using the slope method, it is possible to obtain the results under different Chl values. and Specifically, for doses ranging from 0.01 to 10 mg / m² 3 The range of chlorophyll concentration, With c mf The relationship between them is as follows Figure 3 As shown in Figure b. With c mr The relationship between them is as follows Figure 3 As shown in Figure d. Second-order polynomial fitting was used for the fluorescence channel and the Raman channel, respectively. With c mf as well as With c mr The relationship between them, and the fitting results are as follows: Figure 3 Figure b shows and Figure 3 As shown in the middle d figure, the R of the two channels 2 A value of 0.99 indicates a high correlation. Therefore, the following conclusion can be drawn: when the lidar backscattering signal is mainly controlled by quasi-single scattering, K... lidar It tends to closely align with the beam attenuation coefficient (c), while when the backscattered signal is primarily controlled by quasi-single scattering, K... lidar The diffuse reflection attenuation coefficient (K) d As given, the signal is mainly affected by multiple scattering. Therefore, and The difference between them (called ),like Figure 3 As shown in Figures e and f, when Chl changes from 0.01 to 10 mg / m³... 3 hour, The value is between 0.10 and 0.13.

[0118] From equation (6), we can see that Deviation in the value will cause S fr0 The calculation error. Therefore, the following will evaluate... Errors introduced by deviations in the values ​​taken.

[0119] The inversion process Defined as S obtained based on this value fr0 and β f0 The inversion value can be expressed as S′ by equations (17) and (18), respectively. fr0 and β′ f :

[0120]

[0121]

[0122] Will The error introduced by the deviation of the value is defined as Error1, which can be expressed by equation (19) as follows:

[0123]

[0124] From equations (17) and (18), Error1 can be further expressed as equation (20):

[0125]

[0126] The MC simulation results above show that when chlorophyll is at 0.01 mg / m³, 3 Up to 10 mg / m 3 When the interval changes, The range is from 0.1 to 0.13. Substituting this range into equation (20) yields Error1 at different depths. For example... Figure 4 The figures shown represent the results at depths of 1m, 5m, 10m, and 20m, respectively. From the results, it can be observed that Error1 increases with... The larger the deviation in the value and the deeper the depth, the greater the increase. Within the detection range of the lidar system used in this embodiment (depth 10m), when... When set to 0.11, the error remains within 15%. Therefore, in the subsequent inversion process, The value was set to 0.11.

[0127] The following analysis addresses the errors caused by the aforementioned inversion method, focusing only on errors originating from the inversion algorithm and ignoring errors caused by the signal-to-noise ratio (SNR) of the lidar backscattered signal. Four typical chlorophyll vertical distribution models are selected for analysis, representing open ocean, mid-latitude Class I water bodies, lakes, and waters surrounding Europe. (Refer to...) Figure 5 It shows the chlorophyll vertical distribution profiles in these four chlorophyll vertical distribution models.

[0128] The process of calculating the above error is as follows: First, a lidar backscattering signal is constructed based on the vertical distribution profile of chlorophyll. Then, based on four vertical distribution models of chlorophyll, the beam attenuation coefficient c is calculated using a bio-optical model. m c m and c f Subsequently, the combined attenuation coefficient c was established using MC simulation. mf (c mf =cm +c f ) and lidar attenuation coefficient The relationship between them, The vertical profile. Then, β is calculated using Chl's vertical distribution model and equation (12). f Φ C The value is 0.06. Similarly, through application and calculation... The same method can be used to obtain The vertical cross-section. The coefficient β of the Raman channel. t This can be obtained by using equations (3) and (12). The above method is used to reconstruct the model. and β f ,as well as and β t In addition to B f and B r and Q r (z) and Q f Assuming (z), the radar signal P can be reconstructed based on equations (1) and (2). f and P r Subsequently, the inversion method detailed in the previous section was used to invert β. f and Chl, The value is set to 0.11. Finally, the deviation from the true value is calculated using the following formula, denoted as Error. β (inversion β) f Error) and Error Chl (The error of inverting Chl) is given by equations (21) and (22), respectively:

[0129]

[0130]

[0131] in, and Chl gt β in the model f And the true value of Chl.

[0132] Based on the above analysis, the error analysis process for the four different chlorophyll vertical distribution models... β and Error Chl like Figure 5 As shown. Figure 5 As shown in Figures a and b, when chlorophyll concentration changes monotonically with depth, the error increases with depth regardless of whether it increases or decreases monotonically. Within a depth of 10m, Error... β and Error ChlAll remained below 13%. In the other two cases, when chlorophyll concentration exhibited a stratified distribution with depth, such as... Figure 5 As shown in Figures c and d, even when the chlorophyll concentration ranges from 0.01 to 10 mg / m³, 3 Error β and Error Chl All remained below 8%. In summary, these results confirm the robustness and reliability of the inversion method.

[0133] Furthermore, to verify the effectiveness of the aforementioned lidar detection method in retrieving chlorophyll concentration in water bodies, this embodiment also conducted a field experiment. The experimental environment was a specific ocean surface at night, and the chlorophyll concentration of this specific ocean surface had been determined through actual exploration.

[0134] In this embodiment, the lidar used for detection is installed on the ship's deck at a vertical distance of approximately 15m from the water surface. The laser beam penetrates the water surface at an angle close to 0° at the zenith, and data is collected with a time resolution of 15s and a depth resolution of 0.5ns. f and P r The original data are as follows: Figure 6 As shown in Figures a and b, the detection depth of the backscattered signals from the fluorescence and Raman channels is approximately in the range of 5m to 10m.

[0135] Reference Figure 6 Figure c shows that β is... f The inversion value, as shown in the figure, indicates that the signal intensity fluctuations caused by the instability of the laser output power are eliminated. This is because the fluorescence backscattering signal is normalized using the Raman backscattering signal, thereby effectively reducing the impact of laser energy fluctuations. Furthermore, by sharing the transmitter-receiver devices of both channels and combining them with the normalization process, the need for correction of the geometric overlap factor in the backscattering signal is eliminated, significantly simplifying the inversion process. During the inversion process, The value is 0.11. Finally, a is calculated using equations (13) and (14). p h and Chl, where Φ c The value is set to 0.01. The result is as follows: Figure 6 As shown in Figures d and e, this illustrates that during the experiment, a p The distributions of h and Chl remain relatively stable near the water surface. Furthermore, with increasing depth, a... p Both h and Chl values ​​increased slightly, especially at depths of 8-10 meters. From Figure 6 As can be observed in Figure e, the surface chlorophyll concentration is approximately 0.03 mg / m². 3 This is consistent with previous findings from explorations in that specific ocean area.

[0136] This demonstrates that the lidar detection method provided by this invention adds a Raman channel to the lidar's fluorescence channel. Based on the lidar detection echo signals including both the fluorescence and Raman channels, it establishes a fluorescence channel backscattering signal profile function including the fluorescence backscattering coefficient and a Raman channel backscattering signal profile function including the Raman backscattering coefficient. After normalization to obtain a normalized signal, the fluorescence backscattering coefficient is inverted using a perturbation method. Based on the inverted fluorescence backscattering coefficient, the chlorophyll fluorescence absorption coefficient at a specific laser wavelength is obtained, thus yielding the chlorophyll concentration profile in the water body. The lidar detection method provided by this invention overcomes the shortcomings of existing fluorescence lidar in detecting chlorophyll concentration in water bodies, enabling the inversion of chlorophyll concentration in a single measurement.

[0137] The foregoing description of the specifications and embodiments is intended to explain the scope of protection of this invention, but does not constitute a limitation on the scope of protection of this invention. Modifications, equivalent substitutions, or other improvements to the embodiments of this invention or a portion thereof that can be obtained by those skilled in the art through logical analysis, reasoning, or limited experimentation, based on the teachings of this invention or the foregoing embodiments, in conjunction with common knowledge, general technical knowledge, and / or existing technology, should all be included within the scope of protection of this invention.

Claims

1. A method for inverting chlorophyll concentration profiles in water bodies, characterized in that, include: Acquire lidar detection echo signals at specific depths within the target area, including fluorescence and Raman channels; The backscattering signal profile of the fluorescence channel is expressed as a function including the fluorescence backscattering coefficient; The backscattered signal profile of the Raman channel is expressed as a function including the Raman backscattering coefficient; The backscattering signal profile function of the fluorescence channel is normalized using the backscattering signal profile function of the Raman channel to obtain the normalized signal. Fluorescence backscattering coefficients were inverted using the perturbation method. Based on the fluorescence backscattering coefficient obtained by inversion, the chlorophyll fluorescence absorption coefficient at a specific laser wavelength is inverted. Based on the chlorophyll fluorescence absorption coefficient obtained by inversion, the chlorophyll concentration profile in water body is retrieved. The backscattering signal profile of the fluorescence channel is represented as follows: in: To emit laser wavelength of The fluorescence wavelength is At time, depth The laser radar received the water fluorescence echo signal; The height of the lidar above the water; It is a constant in the lidar system that does not change with the detection distance, and it includes at least the lidar's transmission power, the detector's quantum efficiency, and the transmittance of the optical system's transmitted and received signals; The geometric overlap factor; is the backscattering coefficient of fluorescence scattering; Let be the transmittance function of the fluorescence filter, and let its coincident center wavelength be . bandwidth is Gaussian function; for The attenuation coefficient of the lidar beam at a given location; for The attenuation coefficient of the lidar beam at a given location; The backscattered signal profile of the Raman channel is represented as follows: The Raman backscattering coefficient is expressed as: in: To emit laser wavelength of Raman wavelength is At time, depth The received lidar Raman echo signal of the water body; It is a constant in the lidar system that does not change with the detection distance, and it includes at least the lidar's transmission power, the detector's quantum efficiency, and the transmittance of the optical system's transmitted and received signals; Geometric overlap factor = ; Raman backscattering coefficient With fluorescence backscattering coefficient The sum of ; Let be the transmittance function of the Raman filter, and let its coincident center wavelength be . bandwidth is Gaussian function; for The attenuation coefficient of the lidar beam at a given location; To emit laser wavelength of The received Raman wavelength is The Raman scattering coefficient of water molecules at that time; The Raman wavelength distribution function; Let be the Raman scattering phase function.

2. The method for inverting chlorophyll concentration profiles in water bodies as described in claim 1, characterized in that, The normalized signal is represented as: in: .

3. The method for inverting chlorophyll concentration profiles in water bodies as described in claim 2, characterized in that, The steps for retrieving fluorescence backscattering coefficients based on the perturbation method include: The normalized signal is represented as: in: , , They are respectively , and The portion that does not change with depth. , , They are respectively , and The portion that changes with depth, To determine the depth of the first point in the lidar echo signal; When the disturbance term is ignored, the normalized signal can be expressed as: Assumption The ratio was determined through experimental calibration. ,Will Represented as: in, The inversion result is expressed as the right-hand side of the equation. Deconvolution: in, and These represent the Fourier transform and the inverse Fourier transform, respectively. Define coefficients for: The inversion result of the fluorescence backscattering coefficient is: 。 4. The method for inverting chlorophyll concentration profiles in water bodies as described in claim 3, characterized in that, The steps for retrieving the chlorophyll fluorescence absorption coefficient at a specific laser wavelength based on the fluorescence backscattering coefficient obtained by inversion are as follows: Will Represented as: in: laser wavelength The chlorophyll fluorescence absorption coefficient under the following conditions; The quantum yield of chlorophyll fluorescence; It is the emission wavelength function of chlorophyll fluorescence normalization; but for: 。 5. The method for inverting chlorophyll concentration profiles in water bodies as described in claim 4, characterized in that, Based on the chlorophyll fluorescence absorption coefficient obtained by inversion, the steps for inverting the chlorophyll concentration profile in water are as follows: The chlorophyll concentration profile in water bodies can be determined using an empirical formula, which is: 。 6. A lidar detection method for chlorophyll concentration profiles in water bodies, characterized in that, include: The laser radar emits a laser of a specific wavelength toward the target area and acquires the Raman channel echo signal and the fluorescence channel echo signal of the water body. The chlorophyll concentration in the water body is obtained by inverting the water body chlorophyll concentration profile inversion method as described in any one of claims 1-5.

7. The lidar detection method for chlorophyll concentration profile in water as described in claim 6, characterized in that, The wavelength of the emitted laser in the lidar used The wavelength of the Raman channel echo signal can be selected from 350 nm to 540 nm, the Raman filter bandwidth can be selected from 4 nm to 20 nm, the fluorescence filter bandwidth can be selected from 8 nm to 20 nm, and the wavelength of the Raman channel echo signal is selectable. The wavelength of the fluorescence channel echo signal corresponding to the excitation wavelength of the water Raman scattering. The wavelength ranges from 650 nm to 690 nm.

8. The lidar detection method for chlorophyll concentration profile in water as described in claim 7, characterized in that, The wavelength of the emitted laser in the lidar used The wavelength is 532nm, the Raman filter bandwidth is 6nm, the fluorescence filter bandwidth is 10nm, and the wavelength of the Raman channel echo signal is... The wavelength of the fluorescence channel echo signal is 650nm. It is 685nm.

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