A novel concept for measuring seawater inherent optical properties in and out of the water
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A novel concept for measuring seawater inherent optical properties in and out of the water Alina Gainusa Bogdan and Emmanuel Boss School of Marine Sciences, University of Maine, Orono, ME, 04469, US IOP change Effect on backscattering spot Scattering Intensity Lateral spread Absorption Intensity Lateral spread Figure 3. Sketch of the proposed sensor, with typical modeled measurement. A collimated light beam is shone into the water and the backscattered light intensity is retrieved as a function of distance from the center of illumination by three concentric photodetector rings. The signal is described by the integrated detected photon count within each ring as a function of the distance from the center. The final signal descriptors are calculated on the basis of the cumulative photon count within each radius. The total detected photon count, D, is an indicator of the retrieved intensity; the geometry parameter, α, is a measure of the lateral spread of detected light. Top view R = 3.1 cm Δr = 1 cm Typical signal photon detect ed Side view photon emitted photon scattered photon absorbed 0 0.01 0.02 0.03 1.5 1.6 1.7 1.8 x 10 -4 (Detected/inciden t) photon count r [m] D (Intensity ) α (Geometry) Scatterin g Absorptio n 0.005 0.01 0.015 0.02 0.025 0.03 1 2 3 4 5 6 7 x 10 -4 Modeloutput linearfit r [m] Cumulative photon count α D References: R F Cahalan, M McGill, J Kolasinski, T Várnai, and K Yetzer. THOR – Cloud thickness from off beam LIDAR returns. Journal of Atmospheric and Oceanic Technology, 22:605-627, 2005 G R Fournier and M Jonasz. Computer-based underwater imaging analysis. Airborne and In-Water Underwater Imaging, 3761(1):62-70, July 1999 Acknowledgements : Aim: Test sensor concept inspired by atmospheric THOR instrument (Cahalan et al., 2005), to simultaneously measure the absorption (a) and backscattering (b b ) coefficients of seawater. Applications: water quality assessment - turbidity measurements; oceanography and ecology - characterization of algal blooms and biogeochemical processes; calibration of satellite ocean color. a 0 b 0 a 1 > a 0 b 0 a 2 > a 1 b 1 a 1 b 1 > b 0 Figure 1. Photographs of the backscattering spot for a series of solutions with increasing concentrations of absorbing (green die) and scattering (Maalox) agents. The camera sensitivity and exposure time were held constant. Idea Shine laser in water samples with different IOPs observe the intensity and geometry of the backscattering spot. Interpret in terms of beam attenuation with path length. I F dIntensity / dScattering dIntensity / dAbsorption dGeometry / dScattering dGeometry / dAbsorption ≠1 THEN Intensit y Geometry Scatterin g Absorptio n Methodology Use Monte Carlo modeling of light propagation to simulate the instrument response to different IOP combinations Identify robust mathematical relationships between the known, imposed IOPs and some descriptors of the modeled instrument signal Invert relationships to obtain the algorithm that the actual instrument would use to convert a measured signal into estimates of the water IOPs + B p ≡b bp /b p = [0.5%,1%,1.5%,2%,2.5%] Low B p values are typical of organic particles (e.g., phytoplankton); high values are typical of inorganic particles (e.g., suspended sediments). Particle scattering is modeled using the Fournier- Forand phase function (Fournier & Jonasz, 1999) 10 -3 10 -2 10 -1 10 0 10 -2 10 0 10 2 a p [m -1 ] b p [m -1 ] Figure 2. Particulate absorption and scattering values chosen to drive the optical simulations. Results 10 -4 10 -2 10 0 10 -4 10 -3 10 -2 10 -1 10 0 [m -1 ] b b [m -1 ] data forB p=2.5% data forB p=2% data forB p=1.5% data forB p=1% data forB p=0.5% fit 10 -5 10 -1 10 -3 10 -3 10 -2 10 -1 10 0 D b b /a data forB p=2.5% data forB p=2% data forB p=1.5% data forB p=1% data forB p=0.5% fit Figure 4. Water IOPs plotted against resulting signal descriptors. b b - backscattering coef.; a - absorption coef.; α - geometry parameter.; D - intensity. b b = 10 1.048log 10 (α)+0.341 (1) b b a = 10 -0.074log 10 2 (D)+0.353log 10 (D)+0.656 (2) -0.2 -0.1 0 0.1 0.2 0 5 10 15 20 25 30 35 b b inversion relative errors -1 -0.5 0 0.5 1 0 20 40 60 80 100 a inversion relative errors Figure 5. Histograms of the relative errors in inverting b b and a from the modeled instrument response to the full range of IOPs These equations can be used to obtain a and b b from a given measurement described by D and α. The maximum relative errors when this inversion is applied to the original modeled data set are 13.4% for the inversion of b b and 56.9% for a. 90% of the errors fall below 6.9% for b b and below 29.7% for a. The signal retrieved by the instrument is determined mainly by a and b b , with little or no added effect from the backscattering ratio, B p . Emerging ideas Hand-held in-water instrument 3D profiling of a and b b Long instrument mounted on AUVs Plausible : Adaptations to optical model Out-of-water instrument Long-term deployment on dry platforms Discussion 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 0.12 0.14 0.16 0.18 0.2 0.22 0.24 0.26 Scattering angle [rad] VSF/b b [sr -1 ] FF:B p = 0.5% FF:Bp = 1% FF:B p = 1.5% FF:Bp = 2% FF:B p = 2.5% P etzold (B p = 1.83% ) Figure 6. Range of VSF shapes (in the back direction) used in this study. ‘FF’ stands for ‘Fournier-Forand’. • Test inversion on data obtained using the Petzold scattering phase function => inversion algorithm not sensitive to volume scattering function (VSF), at least within the range shown in Figure 6. • Quality of b b inversion does not depend on the instrument size; larger instrument => better a inversion • Increasing the detector resolution does NOT improve the quality of IOP inversions • Radial symmetry of backscattering spot => other instrument shapes possible (keeping symmetry with respect to the center of illumination)
description
A novel concept for measuring seawater inherent optical properties in and out of the water. Alina Gainusa Bogdan and Emmanuel Boss School of Marine Sciences, University of Maine, Orono, ME, 04469, US . Intensity Geometry. Scattering Absorption. - PowerPoint PPT Presentation
Transcript of A novel concept for measuring seawater inherent optical properties in and out of the water
A novel concept for measuring seawater inherent optical properties in and out of the water
Alina Gainusa Bogdan and Emmanuel Boss School of Marine Sciences, University of Maine, Orono, ME, 04469, US
IOP change Effect on backscattering spot
Scattering Intensity
Lateral spread
AbsorptionIntensity
Lateral spread
Figure 3. Sketch of the proposed sensor, with typical modeled measurement. A collimated light beam is shone into the water and the backscattered light intensity is retrieved as a function of distance from the center of illumination by three concentric photodetector rings. The signal is described by the integrated detected photon count within each ring as a function of the distance from the center. The final signal descriptors are calculated on the basis of the cumulative photon count within each radius. The total detected photon count, D, is an indicator of the retrieved intensity; the geometry parameter, α, is a measure of the lateral spread of detected light.
Top view
R = 3.1 cm Δr = 1 cm
Typical signal
photon detected
Side view
photon emitted
photon scattered
photon absorbed
0 0.01 0.02 0.031.5
1.6
1.7
1.8 x 10-4
(Det
ecte
d/in
cide
nt)
phot
on c
ount
r [m]
D (Intensity)
α (Geometry)
Scattering
Absorption 0.005 0.01 0.015 0.02 0.025 0.031
2
3
4
5
6
7 x 10-4
Model outputlinear fit
r [m]
Cum
ulat
ive
phot
on c
ount
α
D
References: R F Cahalan, M McGill, J Kolasinski, T Várnai, and K Yetzer. THOR – Cloud thickness from off beam LIDAR returns. Journal of Atmospheric and Oceanic Technology, 22:605-627, 2005
G R Fournier and M Jonasz. Computer-based underwater imaging analysis. Airborne and In-Water Underwater Imaging, 3761(1):62-70, July 1999
Acknowledgements:
Aim: Test sensor concept inspired by atmospheric THOR instrument (Cahalan et al., 2005), to
simultaneously measure the absorption (a) and backscattering (bb) coefficients of seawater.
Applications: water quality assessment - turbidity measurements; oceanography and ecology -
characterization of algal blooms and biogeochemical processes; calibration of satellite ocean color.
a0b0
a1> a0b0
a2> a1b1
a1b1> b0
Figure 1. Photographs of the backscattering spot for a series of solutions with increasing concentrations of absorbing (green die) and scattering (Maalox) agents. The camera sensitivity and exposure time were held constant.
Idea Shine laser in water
samples with different
IOPs observe the
intensity and geometry
of the backscattering
spot.
Interpret in terms of
beam attenuation with
path length.
IFdIntensity/dScatteringdIntensity/dAbsorption
dGeometry/dScatteringdGeometry/dAbsorption
≠
THENIntensity
Geometry
Scattering
Absorption
Methodology Use Monte Carlo modeling of light propagation to simulate the instrument response to different IOP combinations
Identify robust mathematical relationships between the known, imposed IOPs and some descriptors of the modeled instrument signal
Invert relationships to obtain the algorithm that the actual instrument would use to convert a measured signal into estimates of the water IOPs
+Bp ≡bbp/bp= [0.5%,1%,1.5%,2%,2.5%]
Low Bp values are typical of organic particles (e.g., phytoplankton); high values are typical of inorganic particles (e.g., suspended sediments).
Particle scattering is modeled using the Fournier-Forand phase function (Fournier & Jonasz, 1999)
10-3 10-2 10-1 10010-2
100
102
ap [m-1]
b p [m-1
]
Figure 2. Particulate absorption and scattering values chosen to drive the optical simulations.
Results
10-4 10-2 10010-4
10-3
10-2
10-1
100
[m-1]
b b [m-1
]
data for Bp=2.5%data for Bp=2%data for Bp=1.5%data for Bp=1%data for Bp=0.5%fit
10-5 10-110-310-3
10-2
10-1
100
D
b b/a
data for Bp=2.5%data for Bp=2%data for Bp=1.5%data for Bp=1%data for Bp=0.5%fit
Figure 4. Water IOPs plotted against resulting signal descriptors. bb - backscattering coef.; a - absorption
coef.; α - geometry parameter.; D - intensity.
bb = 101.048log10(α)+0.341 (1)
bba = 10-0.074log10
2(D)+0.353log10(D)+0.656 (2) -0.2 -0.1 0 0.1 0.20
5
10
15
20
25
30
35
bb inversion relative errors
-1 -0.5 0 0.5 10
20
40
60
80
100
a inversion relative errorsFigure 5. Histograms of the relative errors in inverting
bb and a from the modeled instrument response to the full range of IOPs
These equations can be used to obtain a and bb from a given measurement described by D and α. The maximum relative errors when this inversion is applied to the original modeled data set are 13.4% for the inversion of bb and 56.9% for a. 90% of the errors fall below 6.9% for bb and below 29.7% for a.
The signal retrieved by the instrument is determined mainly by a and bb, with little or no added effect from the backscattering ratio, Bp.
Emerging ideasHand-held in-water instrument
3D profiling of a and bb
Long instrument mounted on AUVs
Plausible: Adaptations to optical model
Out-of-water instrument
Long-term deploymenton dry platforms
Discussion
1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.20.12
0.14
0.16
0.18
0.2
0.22
0.24
0.26
Scattering angle [rad]
VS
F/b b [s
r-1]
1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.20.12
0.14
0.16
0.18
0.2
0.22
0.24
0.26
Scattering angle [rad]
parti
cula
te V
SF/
b bp
FF: Bp = 0.5%FF: Bp = 1%FF: Bp = 1.5%FF: Bp = 2%FF: Bp = 2.5%Petzold (Bp = 1.83%)
Figure 6. Range of VSF shapes (in the back direction) used in this study. ‘FF’ stands for ‘Fournier-Forand’.
• Test inversion on data obtained using the Petzold scattering phase function => inversion algorithm not sensitive to volume scattering function (VSF), at least within the rangeshown in Figure 6.
• Quality of bb inversion does not depend on the instrument size; larger instrument => better a inversion
• Increasing the detector resolution does NOT improve the quality of IOP inversions
• Radial symmetry of backscattering spot => other instrument shapes possible (keeping symmetry
with respect to the center of illumination)
