WO2018155788A1 - Method for measuring turbidity through bathymetric lidar waveform analysis - Google Patents

Abstract

A method for measuring turbidity, according to one embodiment of the present invention, comprises the steps of: providing, underwater and simultaneously, a laser pulsed beam having a first wavelength, which is reflected only from the sea surface, and a laser pulsed beam having a second wavelength, which transmits through the sea surface, reaches the seafloor and is reflected therefrom; collecting, via an optical receiving system, a first optical signal attributable to the water surface-scattering at the sea surface of the laser pulsed beam having the first wavelength, and a second optical signal attributable to the water surface-scattering, the underwater-scattering and the seafloor-scattering of the laser pulsed beam having the second wavelength, and detecting the first optical signal and the second optical signal, respectively, by time according to wavelength; transforming the first optical signal and the second optical signal into a first digital electrical signal and a second digital electrical signal, respectively; generating a first integrated signal by integrating the first digital electrical signal with respect to a predetermined time interval in which a water surface-scattering signal is generated; generating a second integrated signal by integrating the second digital electrical signal with respect to the predetermined time interval in which the water surface-scattering signal is generated; generating a standardized measured integrated signal by dividing the second integrated signal by the first integrated signal; and extracting a measured turbidity corresponding to the standardized measured integrated signal by using a relationship of a standardized theoretical integrated signal based on turbidity predicted using a theoretical value.

Description

Turbidity measurement method through depth lidar waveform analysis

The present invention relates to a method of obtaining turbidity of seawater by processing a signal of a depth lidar mounted on an aircraft and measuring a seabed topography.

LIDAR (light detection and Ranging) is a technique for measuring the distance to the target by irradiating the laser light on the target. The Airbone Bathymetry LIDAR is a lidar that can be mounted on an airplane and is a technology that can efficiently extract the topography and depth information of coastal areas. Other techniques for depth surveying include Sound Navigation And Ranging (SONAR) and Synthetic Aperture Radar (SAR). SONAR is the most widely used depth gauge, but it is difficult to access shallow coastal depths and is slow because it is mounted on board to measure seabed topography. SAR has a problem with resolution. On the other hand, depth survey aerial riders have the advantage of being able to measure large terrain quickly because they survey the seabed terrain at high altitudes.

The intensity of the reflected beam reaching the receiver of the LiDAR system is converted into a waveform over time, which is then analyzed to extract depth information. Receiving waveform is distorted by environmental variables such as surface wave height, turbidity of water, and seabed topography. Until now, research has been conducted on the effect of various variables on the Lidar signal.

LIDAR analyzes the wave shape of light reflected from the sea floor. When the water becomes cloudy by suspended matter in the sea, the signal strength is weakened. Therefore, the turbidity of the seawater limits the depth at which it can be measured. Since coastal turbidity changes continuously depending on the condition of the water flowing from the stream, it is necessary to know the turbidity information of the seawater currently measured when the terrain is measured by aerial lidar. Turbidity in seawater has a turbidity sensor that directly immerses in seawater to measure turbidity, but it is difficult to measure the field with these sensors during flight. Therefore, there is a need for a method for obtaining turbidity directly from an optical signal of a lidar.

One technical problem to be solved by the present invention is to provide a stable method for extracting the turbidity of seawater from the optical signal of the water depth lidar signal.

In the turbidity measurement method according to an embodiment of the present invention, providing a laser pulse beam of the first wavelength reflecting only at the sea surface and a laser pulse beam of the second wavelength passing through the sea surface to reach and reflect to the sea floor at the same time ; The laser pulse beam of the first wavelength receives a first optical signal due to surface scattering at the sea level and the laser pulse beam of the second wavelength receives a second optical signal due to surface scattering, underwater scattering, and bottom scattering. Collecting through the optical system and detecting the first optical signal and the second optical signal over time according to the wavelength; Converting the first optical signal and the second optical signal into a first digital electrical signal and a second digital electrical signal, respectively; Generating a first integrated signal by integrating the first digital electrical signal over a predetermined time interval at which a sleep scattering signal is generated; Generating a second integrated signal by integrating the second digital electrical signal over a predetermined time interval at which the sleep scattering signal is generated; Generating a normalized measurement integrated signal by dividing the second integrated signal into the first integrated signal; And extracting measurement turbidity corresponding to the normalized measurement integrated signal using the relationship of the standardized theoretical integrated signal according to the turbidity predicted using the theoretical value.

In one embodiment of the present invention, the first wavelength may be 1064 nm, the second wavelength may be 532 nm.

In one embodiment of the present invention, the relationship between the normalized theoretical integrated signal according to the turbidity predicted by using the theoretical value, the first theoretical signal is determined by the first sleep scattering signal at the first wavelength according to the predicted turbidity And a second theoretical signal is calculated by a surface scattering signal, an underwater scattering signal, and a bottom scattering signal at the second wavelength according to the predicted turbidity, and a first theoretical integral signal is used to convert the first theoretical signal to the first Integrating and calculating the predetermined time domain in which the first sleep scattering signal exists, calculating a second theoretical integral signal by integrating the second theoretical signal in the predetermined time domain in which the first sleep scattering signal exists, The normalized theoretical integrated signal is calculated by dividing the second theoretical integrated signal by the first theoretical integrated signal according to the predicted turbidity, and the predicted turbidity is normalized. To a linear function of the integral signal Ron can be fitted.

Turbidity measuring device according to an embodiment of the present invention, a laser pulse beam of the first wavelength that reflects only from the sea surface and a laser pulse beam of the second wavelength passing through the sea surface to reach and reflect to the sea bottom at the same time at the same time Pulsed light source; The laser pulse beam of the first wavelength receives a first optical signal due to surface scattering at the sea level and the laser pulse beam of the second wavelength receives a second optical signal due to surface scattering, underwater scattering, and bottom scattering. A first photodetector and a second photodetector, which collect through the optical system and detect the first optical signal and the second optical signal over time according to the wavelength, respectively; An analog-digital converter for converting the first optical signal and the second optical signal into a first digital electrical signal and a second digital electrical signal, respectively; Integrating the first digital electrical signal over a predetermined time interval during which a sleep scattering signal is generated to generate a first integrated signal, and integrating the second digital electrical signal over a predetermined time interval during which the sleep scattering signal is generated. Generate a second integrated signal, divide the second integrated signal into the first integrated signal to generate a normalized measurement integrated signal, and use the relationship between the normalized theoretical integrated signal according to the turbidity predicted using the theoretical value. And extracting a measurement turbidity corresponding to the normalized measurement integrated signal.

According to an embodiment of the present invention, the turbidity measurement method may provide stable turbidity information by analyzing a conventional rider signal.

1 is a view illustrating a turbidity measurement system according to an embodiment of the present invention.

Figure 2 is a form according to the time (t) of the surface scattering signal, the underwater scattering signal, the bottom surface scattering signal and the sum of the general LIDAR signal obtained from Equations 1 to 4 according to the pulse laser beam of 532nm wavelength that passes through the sea surface Indicates.

Figure 3 shows the shape of the LIDAR signal based on the time (t) of the LIDAR signal based on the surface scattering signal obtained from Equation 1, 4 according to the pulsed laser beam of 1064nm wavelength reflected from the sea surface.

FIG. 4 shows the relationship between the decay time and the turbidity K _d of the exponential function calculated through curve fitting in a section in which the scattering signal is generated through the simulation and attenuated by the exponential function.

FIG. 5 illustrates the relationship between the decay time of the exponential function and the turbidity K _d when noise is considered in the result of FIG. 4.

6 is a conceptual diagram illustrating an apparatus for measuring turbidity according to an embodiment of the present invention.

FIG. 7 is a flowchart illustrating a turbidity measurement method using the turbidity measurement device of FIG. 6.

8A and 8B show correlations between turbidity according to the presence or absence of noise and standardized integrated signals and turbidity between predetermined integral sections.

Hereinafter, with reference to the accompanying drawings will be described in detail the present invention. Hereinafter, the present invention will be described in more detail with reference to preferred examples. However, these examples are intended to illustrate the present invention in more detail, it will be apparent to those skilled in the art that the present invention is not limited or limited by experimental conditions, material types, and the like. The invention is not limited to the embodiments described herein but may be embodied in other forms. Rather, the embodiments introduced herein are provided so that the disclosure may be made thorough and complete, and to fully convey the spirit of the invention to those skilled in the art. In the drawings, the components are exaggerated for clarity. Portions denoted by like reference numerals denote like elements throughout the specification.

1 is a view illustrating a turbidity measurement system according to an embodiment of the present invention.

Referring to FIG. 1, LIDAR (LIght Detection And Ranging), which is mounted on an aircraft and measures underwater terrain, emits short pulsed light from the plane 1 to the sea level, and then measures the time difference between the reflected light and the depth of the sea bottom. It is a technique to measure.

In order to measure the seabed topography, it is necessary to penetrate the water and reach the bottom of the sea. Therefore, a wavelength of 532 nm with low absorption in the water is used. Since 1064 nm of light is mostly absorbed by water, it can be interpreted to reflect only at sea level.

Since the sea is always wave and blue due to various influences, the height of the surface of the water is constantly changing, so the measurement of the seabed topography requires the sea level height information at that time. Therefore, by sending two wavelengths of light at 532 nm and 1064 nm at the same time, information on the sea level and the sea floor can be obtained simultaneously.

The light reflected from the sea floor contains a combination of surface reflections, absorption and scattering in sea water, and reflections from the sea floor. The surface scattering signal P _s due to scattering and reflection at sea level, the underwater scattering signal P _c due to underwater scattering, and the sea bottom scattering signal P _b due to scattering and reflection at sea bottom are Is given together.

[Equation 1]

[Equation 2]

[Equation 3]

Where P _t is the transmitted peak power, τ is the pulse duration, c is the speed of light, L is the aircraft altitude, and H is the water depth).

l is the path length in air, h is the path length in water, k is the air attenuation coefficient, and R _s is the reflectance at the air-water interface. air-water interface, A _r is the aperture area of the receiver optics, and η is the receiver efficiency.

n _w is the refractive index of water, θ is the off-nadir transmit angle , β _π is the back scattering coefficient, a is the absorption coefficient, and b _b is the back scattering coefficient The backward-scattering coefficient of water, ρ is the bottom reflectance, φ is the refracted beam angle in water, and F is the FOV loss factor.

Scattering signal (P _signal ) can be obtained by the convolution of the response function (P _response ) of the equation (1) to 3 and the pulse signal (I _pulse ) incident on the sea water and is given as follows.

[Equation 4]

FIG. 2 is a time (t) of a general LIDAR signal consisting of the surface scattering signal, the underwater scattering signal, the bottom scattering signal, and the sum thereof obtained from Equations 1 to 4 according to a pulse laser beam of 532 nm wavelength passing through the sea surface. According to the form.

Referring to FIG. 2 and Equation 2, the variable H representing the water depth is a function of time and has a relationship of H = cos (φ) ct / (2n _w ).

In Equation 2, the term related to turbidity in water is represented by the sum K _d of the absorption coefficient and the rearward scattering coefficient b _b .

Figure 3 shows the shape of the LIDAR signal based on the time (t) of the LIDAR signal based on the surface scattering signal obtained from Equation 1,4 according to the pulsed laser beam of 1064nm wavelength reflected from the sea surface.

Referring to FIG. 3, a 1064 nm sleep scattering signal is generated in a time domain in which the 532 nm sleep scattering signal of FIG. 3 occurs. The inter-station where the sleep scattering signal is generated may be set as an integral section.

FIG. 4 shows the relationship between the decay time and the turbidity K _d of the exponential function calculated through curve fitting in the section in which the scattering signal is generated through the simulation and attenuated by the exponential function.

FIG. 5 illustrates the relationship between the decay time of the exponential function and the turbidity K _d when noise is considered in the result of FIG. 4.

Referring to FIGS. 4 and 5, in general, the underwater backscattering coefficient b _b , which is expressed as a coefficient of the exponential function, can be obtained by curve fitting the exponential function by the least-square method in a section where the signal is attenuated. However, in the seabed, the signal may be distorted by various causes such as seaweed or fish, and thus it may not be possible to obtain a signal that is attenuated by an exponential function. In addition, the higher the turbidity, the faster the attenuation, the shorter the signal range for curve fitting and the greater the error.

The K _d value is an index indicating turbidity as the sum of the absorption coefficient and the rearward scattering coefficient b _b .

Referring to FIG. 4, for a signal calculated under a noise-free condition for a given decay time, the K _d value obtained by curve fitting the decay section agrees well with the theoretical value.

Referring to FIG. 5, it is shown that a method of calculating attenuation coefficients by curve fitting in a time domain where an underwater scattering signal is attenuated by an exponential function is vulnerable to noise.

When the scattering signal is generated by considering the noise, the K _d value obtained by the curve fitting differs from the theoretical value. Therefore, a more reliable method of obtaining turbidity is needed.

The integrated signal near the sleep is insensitive to noise because it integrates the signals of the greatest intensity among the total signals as the sum of the signal strengths near the sleep. It also contains information about suspended solids near the surface of the water, making it easy to analyze turbidity.

6 is a conceptual diagram illustrating an apparatus for measuring turbidity according to an embodiment of the present invention.

FIG. 7 is a flowchart illustrating a turbidity measurement method using the turbidity measurement device of FIG. 6.

6 and 7, the turbidity measuring device 100 includes a laser pulse light source 110, a scanner 120, a reception optical system 130, a first photodetector 144, a second photodetector 146, and an analog. The digital converter 148, a signal processor 150, and a timing unit 149 may be included.

The laser pulse light source 110 underwater receives a first laser pulse beam D1 having a first wavelength reflecting only at sea level and a second laser pulse beam D2 having a second wavelength passing through the sea level and reaching and reflecting to the sea bottom. Provide at the same time. The laser pulse light source 110 may simultaneously output a first wavelength of 1064 nm and a second wavelength of 532 nm. The laser pulse light source 110 may include a second-harmonic generator to simultaneously generate a first wavelength and a second wavelength. The second-harmonic generator may use a crystal, an optical fiber, or a cavity structure such as KDP.

The scanner 120 may scan the laser pulse beam of the first wavelength and the laser pulse beam of the second wavelength on the surface of the water. The scanner 120 may include a reflection mirror and a rotating motor.

The receiving optical system 130 may include a telescope system. The receiving optical system may be a Gregorian or casearian reflective telescope. The receiving optical system 130 may include a primary mirror having a through hole at a center thereof, and a secondary mirror having a convex mirror or a concave mirror structure.

The reception optical system 130 is a laser pulse beam of the first wavelength due to the surface scattering at the sea level, the first optical signal and the laser pulse beam of the second wavelength is due to the surface scattering, underwater scattering, and bottom scattering A second optical signal is collected through the receiving optical system.

The first optical signal and the second optical signal collected by the reception optical system 130 are spatially separated according to wavelengths through the dichroic mirror 142.

The first photodetector 144 detects the first optical signal separated by the dichroic mirror 142 over time. The first photodetector may be a photodiode.

The second photodetector 146 detects the second optical signal separated by the dichroic mirror 142 over time. The second photodetector 146 may be a photo multiplier tube.

The analog-to-digital converter 148 converts the first optical signal and the second optical signal into a first digital electrical signal and a second digital electrical signal, respectively. The signal converted into a digital signal is stored in the memory. The analog-to-digital converter 148 is synchronized with the laser pulse light source 110 by the timing unit 149.

The timing unit 149 may provide a synchronization signal to the analog to digital converter and the laser pulse light source.

The signal processor 150 generates the first integrated signal by integrating the first digital electrical signal over a predetermined time interval during which the surface scattering signal is generated. In addition, the signal processor 150 generates a second integrated signal by integrating the second digital electrical signal over a predetermined time interval during which the sleep scattering signal is generated. The signal processor generates the normalized measurement integrated signal by dividing the second integrated signal into the first integrated optical signal. The signal processor 150 extracts the measurement turbidity corresponding to the standardized measurement integrated signal using the relationship of the standardized theoretical integrated signal according to the turbidity predicted using the theoretical value.

The turbidity measuring method may include: simultaneously providing a laser pulse beam of a first wavelength reflecting only at sea level and a laser pulse beam of second wavelength passing through the sea level and reaching and reflecting to the sea bottom (S110); The laser pulse beam of the first wavelength receives a first optical signal due to surface scattering at the sea level and the laser pulse beam of the second wavelength receives a second optical signal due to surface scattering, underwater scattering, and bottom scattering. Collecting through the optical system and detecting a first optical signal and a second optical signal according to wavelengths, respectively (S112); Converting the first optical signal and the second optical signal into a first digital electrical signal and a second digital electrical signal, respectively (S114); Generating a first integrated signal by integrating the first digital electrical signal over a predetermined time interval at which a sleep scattering signal is generated; Generating a second integrated signal by integrating the second digital electrical signal over a predetermined time interval at which the sleep scattering signal is generated; Generating a normalized measurement integrated signal by dividing the second integrated signal into the first integrated signal (S120); And extracting measurement turbidity corresponding to the normalized measurement integrated signal using the relationship of the standardized theoretical integrated signal according to the turbidity predicted using the theoretical value (S122).

The first photodetector 144 detects the first 1064 nm optical signal returned from the sea level through scattering and reflection. The second photodetector 146 detects a second light signal of 532 nm returned from the sea level and the sea bottom through scattering and reflection. The first optical signal is converted into a first digital electrical signal, and the second optical signal is converted into a second digital electrical signal.

The first 1064 nm optical signal is analyzed beforehand, and the integral section is set so that the signal is larger than the noise. The second digital electrical signal of 532 nm and the first digital electrical signal of 1064 nm are integrated for a predetermined integration period. The signal integration value of 532 nm is divided by the signal integration value of 1064 nm to generate a normalized measurement integral signal. Thereafter, the turbidity corresponding to the standardized measured integrated signal is determined by comparing the relationship between a predetermined turbidity and a standardized theoretical integrated signal through simulation.

Referring back to FIG. 2, in the second optical signal, the signal shape in the integration section is made up of the sum of the sleep scattering signal and the underwater scattering signal. Thus, the signal of the integral section is influenced by the state of the surface as well as the underwater float. That is, the signal strength may vary depending on the sleep state (tilt, surface state, etc.). The amount of light incident on the water is the total incident light intensity minus the intensity of the signal scattered at sea level. If the intensity of light incident on the water is different, the intensity of the scattering signal is different. The integral value of the near-surface signal type may vary. In addition, the output fluctuations or other environmental factors of the laser pulsed light source can change the integral value irrespective of turbidity. It is difficult to find the relationship between turbidity and signal type when the integral value is changed by a variable that is not related to turbidity. Therefore, to reduce this effect, integral value information that is not related to turbidity is needed.

Since the first optical signal of 1064 nm has a large absorption in water, only a scattering signal exists in the surface of the water. Therefore, the integrated value of the first optical signal is influenced by environmental factors such as the output change of laser light, the influence of wind or waves, and is hardly affected by turbidity. Therefore, if the integration value of the first optical signal of 1064 nm is normalized to the integral value of the second optical signal of 532 nm, the influence of environmental factors such as the output change of laser light, the influence of wind or waves can be reduced. Accordingly, the normalized integral signal can be represented as a function of turbidity only.

The relation of the normalized theoretical integrated signal according to the turbidity predicted using the theoretical value can be calculated as follows.

The first theoretical signal is calculated by the first sleep scattering signal at the first wavelength according to the turbidity K _d estimated using Equations 1 and 4.

The second theoretical signal is calculated by the surface scattering signal, the underwater scattering signal, and the bottom scattering signal at the second wavelength according to the turbidity K _d predicted using Equations 1 to 4.

The first theoretical integrated signal is calculated by integrating the first theoretical signal over a predetermined time region (integral section) in which the first sleep scattering signal is present.

The second theoretical integrated signal is calculated by integrating the second theoretical signal over a predetermined time domain (integral period) in which the first sleep scattering signal is present.

The normalized theoretical integrated signal is calculated by dividing the second theoretical integrated signal by the first theoretical integrated signal according to the predicted turbidity K _d .

The predicted haze K _d is fitted as a linear function of the normalized theoretical integral signal.

 In order to know the turbidity from the standardized integral signal, the relationship between the turbidity and the standardized integral signal must be found in advance.

8A and 8B show correlations between turbidity according to the presence or absence of noise and standardized integrated signals and turbidity between predetermined integral sections.

After generating signals for each turbidity value K _d using the theoretically derived equations 1 to 4, a first theoretical integrated signal integrated over an integral section is calculated for a signal of 1064 nm. Generate a second theoretical integral signal that is integrated over the same integral section for the 532 nm signal. The second theoretical integrated signal is divided by the first integrated signal, and a normalized theoretical integrated signal is calculated.

In the calculation of the signal, the noise-free (a) and the noise-added (b) are compared. Integrating area and turbidity assume a linear proportionality, and then curve fit. For (a), turbidity y is expressed as a function of normalized theoretical integral signal x as follows.

[Equation 5]

As an experiment, if the standardized measurement integrated signal of the signal is obtained, the turbidity can be obtained by substituting the equation (5).

While the invention has been shown and described with respect to certain preferred embodiments thereof, the invention is not limited to these embodiments, and has been claimed by those of ordinary skill in the art to which the invention pertains. It includes all the various forms of embodiments that can be implemented without departing from the spirit.

Claims (4)

1. Simultaneously providing a first laser pulse beam of a first wavelength reflecting only at sea level and a second laser pulse beam of second wavelength passing through the sea level and reaching and reflecting to the seabed;

   The first optical pulse beam of the first wavelength is due to the surface scattering at the sea level and the second laser pulse beam of the second wavelength is due to the surface scattering, underwater scattering, and bottom scattering Collecting an optical signal through a receiving optical system and detecting a first optical signal and a second optical signal according to a wavelength according to time, respectively;

   Converting the first optical signal and the second optical signal into a first digital electrical signal and a second digital electrical signal, respectively;

   Generating a first integrated signal by integrating the first digital electrical signal over a predetermined time interval at which a sleep scattering signal is generated;

   Generating a second integrated signal by integrating the second digital electrical signal over a predetermined time interval at which the sleep scattering signal is generated;

   Generating a normalized measurement integrated signal by dividing the second integrated signal into the first integrated signal; And

   And extracting a measurement turbidity corresponding to the normalized measurement integrated signal using the relationship of the standardized theoretical integrated signal according to the turbidity predicted using the theoretical value.
2. According to claim 1,

   The first wavelength is 1064 nm,

   The second wavelength is turbidity measuring method, characterized in that 532 nm.
3. According to claim 1,

   The relationship between the standardized theoretical integrated signal according to the turbidity predicted using the theoretical value is

   A first theoretical signal is calculated by the first sleep scattering signal at the first wavelength according to the predicted turbidity,

   A second theoretical signal is calculated by the surface scattering signal, the underwater scattering signal, and the bottom scattering signal at the second wavelength according to the predicted turbidity;

   A first theoretical integrated signal is calculated by integrating the first theoretical signal with respect to a predetermined time domain in which the first sleep scattering signal exists;

   A second theoretical integrated signal is calculated by integrating the second theoretical signal over a predetermined time domain in which the first sleep scattering signal is present;

   Calculating the normalized theoretical integrated signal by dividing the second theoretical integrated signal by the first theoretical integrated signal according to a predicted turbidity;

   And fitting a predicted haze to a linear function of the normalized theoretical integral signal.
4. A laser pulse light source for simultaneously providing underwater a first laser pulse beam having a first wavelength reflecting only at sea level and a second laser pulse beam having a second wavelength passing through the sea level and reaching and reflecting to the seabed;

   The first optical pulse beam of the first wavelength is due to the surface scattering at the sea level and the second laser pulse beam of the second wavelength is due to the surface scattering, underwater scattering, and bottom scattering A first photodetector and a second photodetector for collecting an optical signal through a receiving optical system and detecting a first optical signal and a second optical signal according to wavelengths, respectively;

   An analog-digital converter for converting the first optical signal and the second optical signal into a first digital electrical signal and a second digital electrical signal, respectively; And

   Integrating the first digital electrical signal over a predetermined time interval during which a sleep scattering signal is generated to generate a first integrated signal, and integrating the second digital electrical signal over a predetermined time interval during which the sleep scattering signal is generated. Generate a second integrated signal, divide the second integrated signal into the first integrated signal to generate a normalized measurement integrated signal, and use the relationship between the normalized theoretical integrated signal according to the turbidity predicted using the theoretical value. And a signal processor for extracting measurement turbidity corresponding to the normalized measurement integrated signal.

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