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.
Retrieval of ocean properties using multispectral methodsS. 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 WatersSeparation of Overlapping Elastic Scattering and Fluorescence from Algae in Seawater through Polarization Discrimination
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
1- Chlorophyll absorption can be probed effectively using 440-570 band ratios
2- 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
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
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.Multispectral versus Hyperspectral assessment of GOES-R Coastal Water Imager
Parameterized Shallow Water Model Parameters
Remote Sensing Reflectance Spectra
Normalized Parameter Retrieval Error
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
Fluorescence Height Island
Traditional method of the fluorescence height calculation over baseline
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.
Near zero if no depolarization valid for spherical particles
Generally validated using laser induced fluorescence but significant
error results due to scattering component
Algae Isochrysis sp.
(brown algae spherical d ≈ 5 µm)
Algae Tetraselmis striata
(green algae slightly ellipsoidical d ≈ 12 µm)
Technique with polarized light
Light scattered by the algae illuminated by unpolarized light has some degree of polarization and can be also analyzed using polarization discrimination with
the same linear regression approach
Algae Isochrysis sp.(brown algae spherical d ≈ 5 µm)
Fluorescence magnitude retrieved from algae with different concentrations of clay
Reflectance curves for algae with clay, Cs = 0 - 200 mg/l
Clay – Na-Montmorillonite, particle size 2-4 µm
Probe above the water, probe vertical
Wind speed above the surface ≈ 9.5 m/s
Sample time increased to 10s from 1s
Algae Isochrysis. Concentration ~4.0 mln cells/ml.
Extraction of fluorescence Islandin the waters of Shinnecock Bay, Long Island
Ratio between 2 polarization components is close to linear
Chl concentration about 8 µg/l
-Absorption coefficient of phytoplankton
-Absorption coefficient of CDOM
[Bricaud, et al., 1981]
-Absorption coefficient of minerals
[Stramski, et al., 2001]
- Energy of emitted fluorescence
[Gower, et al., 1999]
Polarization components of reflectance are calculated from Mie code for 45° illumination (30° in water) & vertical observation
-scattering function at 150°, which was used as average value for calculating backscattering
Polarization components of
were used for calculation of reflectancepolarization components
Half of fluorescence is superimposed on polarization components as a spectrum with Gaussian shape centered at 685 nm
Fluorescence is retrieved using polarization technique
A and B are determined from fitting
outside fluorescence zone
Fluorescence retrieval from reflectance spectra for different concentrations of mineral particles: a) C = 5 mg/m3, b) C = 50 mg/m3.
Comparison of retrieved fluorescence peak to assumed values for a range of mineral particle concentrations using both polarization discrimination and baseline subtraction
Due to column and water floor respectively
is the absorption coefficient due to gelbstoff
is the backscattering of water
is the backscattering by particulate mattersBio-Optical Model 2
is the absorption coefficient due to phytoplankton
G method is the gelbstoff absorption at 440nm
taken from tabulated values in Lee et all.
is the phytoplankton absorption coefficient at 400 nm
which varies with the CHLOROPHYLL concentration.
is dependent onBio-Optical Model 3
is the backscattering coefficient of particulates at 440 nm
gives an indication of the size particles.
Water bottom (lambertian)
Using sand based normalized spectral
The parameters in the reflectance model to be retrieved are: