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/

670

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

12

Reflectance curves from the 2002 cruise in Peconic Bay, Long Island

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Fluorescence Height

670 685 745 Wavelength, nm

Fluorescence HeightR

efle

ctan

ce

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)

Ref

lect

ance

, 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

RRFlRFlRRR

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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, nm500 600 700

0.0

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

Ref

lect

ance

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 2000.00000.00020.00040.00060.00080.00100.00120.00140.00160.00180.00200.00220.00240.00260.00280.00300.00320.0034

Mag

nitu

de o

f flu

ores

cenc

e

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

Ref

lect

ance

, 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

Ref

lect

ance

, 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 1sAlgae 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

Per

p/Par

Wavelengths, nm500 600 700

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

Rmax

RD

Fl

R(

Rmin

Rs

Ref

lect

ance

, 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

OutputPolarization 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

Ref

lect

ance

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.

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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

Mag

nitu

de o

f flu

ores

cenc

eConcentration 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

babab

u

uDuDuur

HDr

HDrr

rrr

5.05.0 4.5104.14.2103.1170.084.0

cos1exp1

cos1exp1

extinction totalrbackscatte total abb

waterBelow water Above5.11

5.0

rsRS

rs

rsRS rR

rr

R

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

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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 on 0P

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