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Three-Dimensional Mapping of Fluorescent Dye Using a Scanning,
Depth-Resolving Airborne LIDAR
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PIs: |
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M. A. Sundermeyer, E. A. Terray
and J. R. Ledwell |
Grant Title: |
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Airborne LIDAR Dye Mapping for Upper Ocean
Mixing and Dispersion Studies |
Funding Agency: |
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The Cecil H. and Ida M. Green Technology Innovation Fund
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Grant Title:
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A Pilot Study Using Airborne LIDAR to
Survey Dye-Release Experiments:
Supplement to ONR Grant Number: N00014-01-1-0984 |
Funding Agency:
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Office of Naval Research
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Objectives
We conducted a pilot study in summer 2004 using airborne LIght Detection
And Ranging (LIDAR) to survey dye release experiments in the upper ocean.
Two releases of 5 kg Rhodamine dye were performed off the east coast of
Florida under fair weather conditions, and surveyed for 0.5-1.5 hr after
release. Airborne surveys used the SHOALS bathymetric LIDAR, a scanning
pulsed laser system, manufactured by Optech International, and operated
by the U.S. Army Corps of Engineers (USACE) Joint Airborne LIDAR Technical
Center of Expertise (JALBTCX). Ship-based in situ fluorometer
observations were used as ground truth for the airborne surveys. Results
are used to examine the feasibility of using airborne LIDAR to study
horizontal and vertical dispersion in the upper ocean on space scales of
2-100 m horizontally and 1-10 m vertically, and time scales of minutes
to hours.
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Sea surface chlorophyll at the 2004 study site.
Inset shows the ship tract during the June 3 experiment, with
different line colors indicating successive transects through
the dye patch.
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Description of Field Experiments
Two dye releases were conducted in the near surface
waters approximately 5 km due east of Ft. Lauderdale, Florida, one on
June 2, and the other on June 3, 2004. On each of the two days, 5 kg of
Rhodamine WT were injected in a single streak, and subsequently mapped
over a period of 1-2 hrs using a fluorometer/CTD system towed from the
ship, as well as airborne LIDAR. We focus here on results from the
second of these two releases.
The dye was injected in a stair-step profile
approximately 1.5 km in length, starting at a depth of 2.5 m
followed by a deeper segment at 5 m, followed by an even deep segment
at 10 m, then returning to the surface, and repeating.
This resulted in a series of surface segments of the patch,
interspersed with progressively deeper segments at discrete depths.
Ship-based sampling of the dye patch commenced immediately after injection,
and continued for 1-2 hrs thereafter. Surveys of the patch consisted
of a single line transect along the major axis of the dye streak, followed
by a zig-zag survey, followed by another line transect.
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Dye streak and boat as viewed from plane.
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View of fantail from lab during dye injection.
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SHOALS-1000T LIDAR System Specifications
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Laser |
Nd:YAG (532, 1064 nm) |
Power |
5-6 mJ |
Pulse Duration |
6 nsec |
Pulse Rate |
1 kHz |
Depth Penetration |
1-50 m |
Horizontal Accuracy |
2.5 m |
Aircraft Speed |
125-175 kts |
Operating Altitude |
200-400 m |
Swath Width |
Variable, up to 0.58 x altitude |
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Description of the SHOALS Airborne LIDAR
Overflights of the dye patches were conducted using
a SHOALS-1000T LIDAR manufactured by Optech Incorporated, and operated
by JALBTCX. Specifications of the SHOALS-1000T are listed in the table
to the right. For the present experiments, a slight modification of the
SHOALS-1000T bathymetric configuration was required. Namely, we replaced
the receiver optics of the existing Raman channel with a narrow-band
filter centered around the peak emission wavelength of the dye, i.e.,
approximately 580 nm. This receive channel was then routed to the high
gain electronics of the shallow green channel. The latter spanned a wider
range of depths than the Raman channel, typically down to about 20 m,
compared to only 5-8 m in the Raman. The reconfiguration took about
20 minutes on the ground.
We use the measured profiles of backscatter at 532 nm
and fluorescence at 580 nm to estimate dye concentration as a function
of depth. Details of the inversion approach are given in Terray
et al. (2005). Briefly, we reduce the problem to two dimensions
using an exact inversion of our fluorescence signal.
This allows us to parameterize the concentration profile
in terms of an unknown constant of proportionality, P, which is
related to the incident intensity just below the surface, and the dye
concentration Cr = C(zr) at some reference
depth zr. These parameters can then be determined by
a joint least-squares fit to the fluorescence intensity profile, and to
the backscatter-derived concentration profile over a limited region
where the signal-to-noise ratio is high.
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LIDAR Results
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Raw infrared, green and fluorescence backscatter signals observed
from the LIDAR during a pass over the near-surface segment of the
dye patch, showing a mix of background (without dye) and profiles
within the dye patch.
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A total of 21 flight lines were conducted over
two hours, 12 north-south (along the axis of the streak) and 9
east-west (across the axis of the streak). As expected, raw LIDAR returns
(right) for the green channel showed strong returns at the ocean surface, then
decaying with depth. Enhanced attenuation was clearly evident in profiles
within the dye patch. In addition, clear peaks in the dye channel were also
evident, distinct from profiles in which no dye was detected.
Plan views of the raw lidar signal for each of the
north-south flight lines are shown below. The most striking feature of
the image is the
appearance of distinct bands in the near-surface dye patch on time scales
of order tens of minutes. These features have wavelengths in the range
30-50 m. Although their dominant orientation (south-east to north-west)
is roughly in the direction of the wind (and the current in the surface
stratified layer), the mechanism for their formation is not clear.
Langmuir circulation can be ruled out based on the observed wind speed,
wave height and stratification. Given the strong shear in the
stratified surface layer, we suspect that the observed features may be
associated with shear instability in the upper portion of the water column.
However, this is still under investigation.
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Plan views of the raw LIDAR signal from the north-south flight lines,
with successive overflights are offset in the x direction for
clarity.
Times of the overflights (hh:mm:ss) are shown below each flight line.
The three surface segments yielded the strongest returns, followed
by the 5 m segments. A faint signal from the 10 m segments could
also be seen at early times. Note, here the weaker signal at depth
is not necessarily due to a decrease in dye concentration, but rather
to natural attenuation of the laser light in seawater.
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Dye concentration from a ship-based transect taken along
the major axis of the dye streak (left), as well as results from a
corresponding LIDAR overflight (right) are shown in the figure below.
The results show a clear signal in both channels, although minor differences
between the two channels and the in situ results are also evident.
The surface segments of the dye streak are clearly visible in both the
green and fluorescence channel inversions. Also visible in the green channel
inversion are the two southern-most 5 m segments of the dye patch. The
northern-most 5 m segment and the two 10 m segments do not appear, possibly
because they had already been advected westward out of the field of view of
the LIDAR.
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(Click on image to enlarge)
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(Left) Dye concentration as measured by ship during
a tow-yo transect along the major axis of the dye streak.
(Right) Horizontal slices of dye concentration through the
tracer patch as measured by airborne LIDAR - (top) concentration
inferred from the green channel and (bottom) dye channels,
respectively. Vertical slices are approximately every 2 m, from
the surface to 12 m.
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(Click on image to enlarge)
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Conclusions
We conducted a pilot study using airborne LIDAR to survey
dye release experiments in the upper ocean on spatial scales of meters to
kilometers, and temporal scales of minutes to days.
Results show qualitative as well as quantitative agreement between dye
distributions inferred from airborne LIDAR and ship-based observations
using a towed fluorometer.
While the LIDAR observations are subject to greater noise than
in situ measurements, the very large number of observations made
by the LIDAR (of order 30,000 profiles during a single overflight compared
to 200 in situ profiles for the entire experiment) provide a wealth
of information about overall distribution of the dye patch, which would
otherwise not be obtainable from in situ measurements alone.
Given the very high resolution, both temporally and spatially, provided by
airborne LIDAR, we believe measurements such as those presented here hold
great promise for our understanding of small-scale dispersion in the
upper ocean.
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Publications
Sundermeyer, M. A., E. A. Terray, J. R. Ledwell,
A. G. Cunningham, P. E. LaRoque, J. Banic, and W. J. Lillycrop.
Three-Dimensional Mapping of Fluorescent Dye Using a Scanning,
Depth-Resolving Airborne Lidar.
J. Atmos. Ocean. Technol. , 24, 1,050-1,065, 2007.
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Abstract
PDF
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Terray, E. A., J. R. Ledwell, M. A. Sundermeyer,
T. Donoghue, S. Bohra, A. G. Cunningham, P. E. LaRoque, J. Banic,
W. J. Lillycrop, and C. E. Wiggins.
Airborne Fluorescence Imaging of the Ocean Mixed Layer.
In:
Proc. of the IEEE/OES Eighth Working Conference on Current
Measurement Technology , June, 2005, Southampton, U.K.
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Abstract
PDF
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