Transcript Poster

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
Aim: Test sensor concept inspired by atmospheric THOR instrument (Cahalan et al., 2005), to
Applications: water quality assessment - turbidity measurements; oceanography and ecology -
simultaneously measure the absorption (a) and backscattering (bb) coefficients of seawater.
characterization of algal blooms and biogeochemical processes; calibration of satellite ocean color.
Emerging ideas
Idea
b0
b0
Top view
Side view
Hand-held in-water instrument
Long instrument mounted on AUVs
photon emitted
photon
detected
IOPs  observe the
Typical signal
3D profiling of a and bb
intensity and geometry
Plausible:
photon absorbed
of the backscattering
IOP change
Scattering
Absorption
b1
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.
R = 3.1 cm
photon scattered
Δr = 1 cm
Discussion
0.26
IF
D (Intensity)
7
α (Geometry)
Effect on
backscattering spot
Intensity
Lateral spread
Intensity
Lateral spread
THEN
≠
Scattering
Absorption
dGeometry/
dScattering
dGeometry/
dAbsorption
Intensity
Scattering
Geometry
Absorption
x 10
6
D
5
4
3
α
2
0.01
Model output
linear fit
0.015
0.02
0.025
0.03
Methodology
b [m-1]
10
10
-2
10
-1
10
10
+
-3
0
-2
10
 [m-1]
10
0
using the Fournier-Forand phase
function (Fournier & Jonasz, 1999)
References:
1.6
10
1.8
2
2.2
2.4
2.6
2.8
3
3.2
0.14
0.12
1.4
1.5
0
1.6
0.01
0.02
0.03
1.6
1.8
2
2.2
2.4
2.6
Scattering angle [rad]
2.8
3
3.2
• 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)
35
30
-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
-2
-3
10 -5
10
20
(1)
15
10
5
(2)
0
-0.2
-0.1
0
0.1
bb inversion relative errors
0.2
100
b
Invert relationships to obtain the
Low Bp values are typical of
algorithm that the actual instrument organic particles (e.g., phytoplankton);
would use to convert a measured high values are typical of inorganic
particles (e.g., suspended sediments).
signal into estimates of the water
Particle scattering is modeled
IOPs
0.14
0.12
1.4
bb =
bb
2
-0.074log
(D)+0.353log
(D)+0.656
10
10
=
10
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
b /a
Bp ≡bbp/bp= [0.5%,1%,1.5%,2%,2.5%]
10
0.16
0.16
1.048log
(α)+0.341
10
10
-2
-4
Identify robust mathematical
Figure 2. Particulate absorption and scattering
values chosen to drive the optical simulations.
relationships between the known,
imposed IOPs and some descriptors
of the modeled instrument signal
0.18
25
10 -4
10
ap [m ]
0.2
-1
0
-1
0.22
0.18
Results
-2
10
1.7
0.2
0
b
0
10 -3
10
0.24
0.22
r [m]
2
p
b [m-1]
10
x 10
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.
10
Use Monte Carlo modeling of light
propagation to simulate the
instrument response to different IOP
combinations
1.8
FF: Bp = 0.5%
FF: Bp = 1%
FF: Bp = 1.5%
FF: Bp = 2%
FF: Bp = 2.5%
Petzold (Bp = 1.83%)
• Test inversion on data
obtained using the
Petzold scattering
phase function
=> inversion algorithm
not sensitive to volume
scattering function (VSF),
Scattering angle [rad]
at least within the range Figure 6. Range of VSF shapes (in the
back direction) used in this study.
shown in Figure 6.
‘FF’ stands for ‘Fournier-Forand’.
-4
-4
1
0.005
10
10
0.26
0.24
r [m]
dIntensity/
dScattering
dIntensity/
dAbsorption
Long-term deployment
on dry platforms
bp
path length.
b1> b0
particulate VSF/b
beam attenuation with
a2> a1
(Detected/incident)
photon count
Interpret in terms of
a1
Out-of-water instrument
Cumulative
photon count
spot.
Adaptations to optical model
b
samples with different
a1> a0
VSF/b [sr-1]
Shine laser in water
a0
-3
10
D
10
-1
Figure 4. Water IOPs plotted against resulting signal
descriptors. bb - backscattering coef.; a - absorption
coef.; α - geometry parameter.; D - intensity.
These equations can be used to obtain a and bb from
80
a given measurement described by D and α. The maximum
60
relative errors when this inversion is applied to the original
modeled data set are 13.4% for the inversion of bb and
40
56.9% for a. 90% of the errors fall below 6.9% for bb and
20
below 29.7% for a.
The signal retrieved by the instrument is determined
0
-1
-0.5
0
0.5
1
a inversion relative errors
mainly by a and bb, with little or no added effect from the
Figure 5. Histograms of the relative errors in inverting
backscattering ratio, Bp.
bb and a from the modeled instrument response to
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
the full range of IOPs
Acknowledgements: