1
Retrieval of ocean properties using multispectral methods
S. Ahmed, A. Gilerson, B. Gross, F. Moshary Students: J. Zhou, M. Vargas, A. Gill, B. Elmaanaoui, K. Aran
Spectral Algorithm Development for Sensing of Coastal Waters
Separation of Overlapping Elastic Scattering and Fluorescence from Algae in Seawater through Polarization Discrimination
2
Reflectance curves from the 2002 cruise in Peconic Bay, Long Island
Spectral Algorithm Development for Sensing of Coastal Waters
3
Ratio algorithm performance –Eastern Long Island
y = 0.3256e-0.0217x
R2 = 0.7879
0
0.1
0.2
0.3
0.4
0 5 10 15 20 25 30 35Chlorophyll, mg/m3
R44
0/R
550
690/670 = 2.0898*Chl + 99.549
R2 = 0.8726
80
100
120
140
160
180
200
0 10 20 30 40
Chlorophyll-a, mg/m3
690/ 67
0
Blue / Green NIR Spectral Ratio
In homogeneous waters where only Chlorophyll varies Blue / Green works only in Case I (see later) NIR Ratios work well in both Case I and Case II
but may be limited by small signals in open waters
4
1- Chlorophyll absorption can be probed effectively using 440-570 band ratios2- In presence of TSS and CDOM, Blue-Green ratios are contaminated. 3- Red-NIR algorithms are much less sensitive to TSS, CDOM.4- The 670-710 channels effectively probe the ChL absorption feature and the 730 channel effectively calculates the backscatter since water abs dominates
1 2 3Absorption/Backscatter features
5
Blue-Green Three Band NIR ratios
Very high spread in the Blue-Green Ratio due to CDOM and TSS randomized variability. This aspect is not relevant to the Red/NIR algorithms
Simulation
6
Multispectral versus Hyperspectral assessment of GOES-R Coastal Water Imager• Future sensors (GOES-R) need to decide
between multispectral or hyperspectral mode.
• Hyperspectral channels are very important for shallow water retrieval
• Preliminary tests compared multispectral vs hyperspectral sensing schemes based on Hydrolight Radiative transfer derived bio-optical model.
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Shallow Water Bio-Optical ModelBased on Hydrolight RT simulations
(Carder et al)
P Phytoplankton Absorption at 440nm Deep Shallow
G Gelbstoff Absorption at 440nm Deep Shallow
X Backscatter Amplitude at 440 nm Deep Shallow
Y Backscatter Power Exponent Deep Shallow
H Ocean Column Depth Shallow
B Bottom Surface Albedo Shallow
Parameterized Shallow Water Model Parameters
Remote Sensing Reflectance Spectra
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Inversion error versus measurement noise for all 6 parameters
0 2 4 6 8 10 12 14 16 18 200
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Bottom Albedo
6p hyperspectral6p multispectral
0 2 4 6 8 10 12 14 16 18 200
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Phytoplankton
6p hyperspectral6p multispectral
0 2 4 6 8 10 12 14 16 18 200
0.5
1
1.5
2
2.5
3
3.5
Gelbstoff
6p hyperspectral6p multispectral
0 2 4 6 8 10 12 14 16 18 200
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Phytoplankton
6p hyperspectral6p multispectral
0 2 4 6 8 10 12 14 16 18 200
0.05
0.1
0.15
0.2
0.25
Height
6p hyperspectral6p multispectral
0 2 4 6 8 10 12 14 16 18 200
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
Power Exponent
6p hyperspectral6p multispectral
Nor
mal
ized
Par
amet
er R
etri
eval
Err
or
Noise (%)
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Results• Hyperspectral channels are absolutely needed
to reduce errors in shallow bottom heights and bottom reflectance (Panels 1 and 5)
• Ocean column parameters are also much better retrieved using Hyperspectal configuration except for spectral slope of backscatter parameter which makes sense since this parameter caused only broad modification of the reflectance spectra. (Panel 6)
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• Chl retrieval in Productive Case I waters can be obtained by both conventional blue-green type algorithms as well as NIR ratio algorithms
• TSS and CDOM variability in case II waters makes blue/green ratios useless but three band NIR ratios are very insensitive to these parameters
• Ratio algorithms for case II waters need thorough testing with in-situ monitoring using a consistent field testing protocol.
• The effects of atmospheric correction to assess the sensitivity of the various two and three ratio algorithms need to be explored.
• Development and sensitivity analysis of simultaneous atmosphere /ocean parameter retrieval using both multispectral and hyperspectral algorithms
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Separation of Overlapping Elastic Scattering and Fluorescence from Algae in Seawater
through Polarization Discrimination
Objective: Separate overlapping fluorescence and elastic scattering spectra of algae excited by white light
Method: Utilize polarization properties of elastically scattered light and unpolarized nature of excited fluorescence to separate the two
Applications: Use fluorescence obtained as indication of Chl concentration even in turbid waters
Obtain elastic scattering spectra free of overlapping fluorescence for ocean color work
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Reflectance curves from the 2002 cruise in Peconic Bay, Long Island
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Fluorescence Height
670 685 745 Wavelength, nm
Fluorescence HeightR
efl
ecta
nce
Traditional method of the fluorescence height calculation over baseline
14
600 650 700 750 8000.01
0.02
0.03
0.04
0.05
746nm685nm
Ref
lect
ance
Wavelength, nm
Fluorescence
Reflectance peakat minimum absorption
Fluorescence heightover baseline
665nm
Reflectance
Reflectance +fluorescence
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Experimental SetupIlluminator
Nozzle
θ
Spectrometer
L
C
P2
FP
WL P1
i2i1
L – lens, FP – fiber probe, A – aperture, P1, P2 – polarizers, C – cuvette with algae, WL – water level.
Objects tested: algae Isochrysis sp., Tetraselmis striata, Thalassiosira weissflogii, “Pavlova”, concentrations up to 4x10^6 cells/mL,
algae with clays.
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Polarized Illumination
500 600 7000.0
0.2
0.4
0.6
0.8
1.0
1.2
FllaserRmin()
Rmax)
Re
fle
cta
nc
e,
a.u
.
Wavelength, nm
),(5.0)()(max FlRR
),(5.0)()(min FlRR ||
Near zero if no depolarization valid for spherical particles
)(2)( min RFl
Generally validated using laser induced fluorescence but significant error results due to scattering component
)()()(5.0)(
)(5.0)()()(
||||
minmax
RRFlR
FlRRR
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Extracted Fluorescence
500 600 7000.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
Rs
Fl
R
RD
Ref
lect
ance
, a.
u.
Wavelength, nm
500 600 7000.0
0.2
0.4
0.6
0.8
1.0
1.2
FlFllaser
Rs
RRD
Ref
lect
ance
, a.
u.
Wavelength, nm
Algae Isochrysis sp.
(brown algae spherical d ≈ 5 µm)
Algae Tetraselmis striata
(green algae slightly ellipsoidical d ≈ 12 µm)
Technique with polarized light
;)( || RRRD ;)( minmax RRR BRAR Ds *)(
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Unpolarized sourceLight scattered by the algae illuminated by unpolarized light has some degree of polarization and can be also analyzed using polarization discrimination withthe same linear regression approach
500 600 700
0.0
0.2
0.4
0.6
0.8
1.0
Fl
Rs
Rmin
R( Rmax
Ref
lect
ance
, a.
u.
Wavelength, nm
Algae Isochrysis sp. (brown algae spherical d ≈ 5 µm)
400 500 600 7000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Rmin
RminFl
RmaxFl
Ref
lect
ance
Wavelength, nm
Rmax
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Algae with clay
Clay – Na-Montmorillonite, particle size 2-4 µm
500 600 7000.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
Re
fle
cta
nc
e
Wavelength, nm
0 10 50 100 200
Clay conc Cs, mg/l
Reflectance curves for algae with clay, Cs = 0 - 200 mg/l
Fluorescence magnitude retrieved from algae with different concentrations of clay
50 100 150 2000.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
0.0014
0.0016
0.0018
0.0020
0.0022
0.0024
0.0026
0.0028
0.0030
0.0032
0.0034
Mag
nit
ud
e o
f fl
uo
resc
ence
Clay concentration, mg/l
unpolarized light polarized light
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Extraction of fluorescence in the waters with rough surface (lab experiments)
500 600 700
0.00
0.05
0.10
0.15
0.20
RD
Fl
R
Rs
Re
fle
cta
nc
e, a
.u.
Wavelength, nm
Unpolarized light
500 600 700-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Fl
RD
Rs
R
Re
fle
cta
nc
e, a
.u.
Wavelength, nm
Probe above the water, probe vertical
No wind Wind speed above the surface ≈ 9.5 m/s
Sample time increased to 10s from 1s
Algae Isochrysis. Concentration ~4.0 mln cells/ml.
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Extraction of fluorescence in the waters of Shinnecock Bay,
Long Island
Chl concentration about 8 µg/l
June 2004
Ratio between 2 polarization components is
close to linear
400 500 600 700 8000.0
0.2
0.4
0.6
0.8
1.0
1.2
Ratio of perp and par components Boat Hampton Bay 060904
Perp
/Par
Wavelengths, nm
500 600 700-0.02
0.00
0.02
0.04
0.06
0.08
0.10
Rmax
RD
Fl
R(
Rmin
Rs
Re
flect
an
ce, a
.u.
Wavelength, nm
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Simulation Model for Case 2 Waters
)((/((33.0)( bb babR
)()()()( min,min, bwbplbplb bNNb
)()()()()( min,min yaplw aNaaa
675
400
))(/)()(( daaEE pldFl
65.0* )(06.0)( Caa cpl
)](014.0exp[)()( 00 yy aa
]009.0exp[)()( 0minmin aa
- Reflectance
- Backscattering coefficient
- Absorption coefficient
- Absorption coefficient of phytoplankton
- Absorption coefficient of CDOM
- Absorption coefficient of minerals
- Energy of emitted fluorescence
Input
[Mobley, 1994]
[Bricaud, et al., 1981]
[Morel, 1991]
[Stramski, et al., 2001]
[Morel, 1977]
[Gower, et al., 1999]
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Half of fluorescence is superimposed on polarization components as a spectrum with Gaussian shape centered at 685 nm
Output
Polarization components of reflectance are calculated from Mie code for 45° illumination (30° in water) & vertical observation
Simulation model for case 2 waters
))(2)(/()(2*33.0( 150150 SaSR
))(2)(/()(2*33.0( 150||150|| SaSR
BARR )(min)(max
),(5.0)()(max FlRR )(5.0)(||)(min FlRR
Fluorescence is retrieved using polarization technique
)1/())()((2 maxmin ARBARFl
A and B are determined from fitting outside fluorescence zone
where
Polarization components of )(150 S were used for calculation of reflectance polarization components
)(150 S -scattering function at 150°, which was used as average value for calculating backscattering
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Simulation Model Results
400 500 600 700 800
0.000
0.005
0.010
0.015
0.020
0.025
0.030
Re
fle
cta
nc
e
Wavelength, nm
a
C=5mg/m3, Cs=10mg/l
400 500 600 700 800
0.00
0.02
0.04
0.06
0.08
0.10
C = 50 mg/m3
Cs = 10 mg/l
Cs = 40 mg/l
Cs = 100 mg/l
Ref
lect
ance
Wavelength, nm
b
Fluorescence retrieval from reflectance spectra for different concentrations of mineral particles: a) C = 5 mg/m3, b) C = 50
mg/m3.
25
Results of fluorescence retrieval, comparison with baseline method
Comparison of retrieved fluorescence peak to assumed values for a range of mineral particle concentrations using both
polarization discrimination and baseline subtraction
0 50 100 150 2000.004
0.006
0.008
0.010
0.012
0.014
0.016
0.018
0.020
0.022
Flretr
Fltheor
Ma
gn
itu
de
of
flu
ore
sc
en
ce
Concentration of particles, mg/l
Fl height
600 650 700 750 8000.01
0.02
0.03
0.04
0.05
746nm685nm
Ref
lect
ance
Wavelength, nm
Fluorescence
Reflectance peakat minimum absorption
Fluorescence heightover baseline
665nm
Reflectance
Reflectance +fluorescence
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Conclusions/Future Work• Separation of Chlorophyll Fluorescence from scattering using
polarization discrimination has been demonstrated for 4 types of algae with different shapes, sizes of particles
• Implementation of the technique using both white light and sun light sources has proven successful in the lab and in the field conditions
• Fluorescence extraction has been obtained even with the presence of high concentration of scattering medium
• Validation with laser induced fluorescence has been performed
• Extraction of fluorescence is successful for all illumination angles with polarized light, up to 50 deg for unpolarized light.
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Conclusions/Future Work• Magnitude of fluorescence peak extracted from reflectance
spectra through polarization technique does not change with the concentration of scattering medium up to 200 mg/l.
• Computer simulations show that fluorescence can be successfully retrieved for most water conditions typical for coastal zones with accuracy 7-11%.
• “Fluorescence height” over baseline strongly overestimates actual and retrieved fluorescence height and these values do not correlate with each other for different concentrations of mineral particles.
• Future simulations should include effects of multiple scattering and atmosphere on polarization components and fluorescence retrieval process.
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Long Island Field Measurements
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bb
b
Bu
cu
dprs
Bu
wB
Brs
cu
w
dprs
crs
Brs
crsrs
baba
bu
uDuDuur
HDr
HDrr
rrr
5.05.0 4.5104.14.2103.1170.084.0
cos
1exp
1
cos
1exp1
extinction totalrbackscatte total abb
waterBelow water Above5.11
5.0
rsRS
rs
rsRS rR
r
rR
Bio-Optical Model 1
Due to column and water floor respectively
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is the absorption coefficient due to phytoplankton
1 maaaa gwtotal
)()( bpbwtot bbb
wa
a
ga
is the absorption coefficient due to water
is the absorption coefficient due to gelbstoff
)(bpb
)(bwb is the backscattering of water
is the backscattering by particulate matters
Bio-Optical Model 2
31
taken from tabulated values in Lee et all.
is the phytoplankton absorption coefficient at 400 nm which varies with the CHLOROPHYLL concentration.
))400(exp( SGag
PPaaa ln10
G is the gelbstoff absorption at 440nm
10 aanda
0P
1P is dependent on0P
Bio-Optical Model 3015.0~S
32
y
bp Xb
440
)(
X is the backscattering coefficient of particulates at 440 nm
y gives an indication of the size particles.
Bio-Optical Model 4
The parameters in the reflectance model to be retrieved are:
BHYXGP ,,,,,
Particulate scatter
sdB BWater bottom (lambertian)Using sand based normalized spectralresponse
