Thanks to the generosity of Chief Scientist Dr. James Irish,
we were able
to execute a three-day cruise to test the SeaSoar configuration intended for use in the upcoming
Japan/East Sea Experiment. We planned to perform extensive engineering tests on SeaSoar
followed by a cross-bank section at the GLOBEC southern flank CTD section in support of the
GLOBEC Long-Term moored effort.
The SeaSoar was tested in the
configuration
for Japan/East Sea cruise, including the full
suite of physical and bio-optical instrumentation. New instrumentation added included a HiStar,
also called AC100, which is a visible-light spectrophotometer manufactured by WetLabs. It
measures light transmittance due to absorption and attenuation with approximately 3.3 nm
resolution from 400 to 726 nm. A second new instrument was the Hydroscat, manufactured by
HobiLabs, which measures light backscatter over six independent channels (wavelengths). To
accommodate the high data rate of the AC100, a new data acquisition system was developed
based on serial data transmission over optical fibers. The fiber-optic tow cable itself had already
been used in earlier SeaSoar experiments (GLOBEC) to transmit video signals from cameras
onboard SeaSoar. The large size of the added instrumentation required significant modification to
the vehicle, including a replacement nose cone, new stabilization weight etc. SeaSoar's hydraulic
unit, which turns its wings into dive and climb positions based on surface-supplied control
commands, had undergone major repairs before the cruise. On the shipboard end, new data
acquisition and display software for the optical sensors was to be tested as well. Since we had not
yet taken delivery on the winch purchased for the Japan/East Sea experiment, we used our
existing smaller winch with a shorter cable.
Following an approximately 12-hour steam to reach deep water, SeaSoar was initially
deployed in the morning hours of October 11th. Within the first hour, one of the
main objectives was achieved in that SeaSoar still "flew" well following all the modifications.
The data acquisition system worked satisfactory as well. To test different sensor
configurations (pumped versus unpumped CTD sensors, including the new fast-response oxygen
sensor) and placements (top cover versus stabilizer fins), SeaSoar was recovered and re-deployed
with slight modifications. This second, longer deployment provided further endurance testing of
the vehicle and the acquisition system. Previously, the fiber-optic sea cable termination inside
SeaSoar ("J-box") had been prone to flooding, likely due to the cyclical pressure changes inherent
in SeaSoar's undulating flight path. The re-designed J-box performed very well, and post cruise
inspection revealed no signs of leakage.
Good data was obtained on most systems. Some examples of the new optical sensor
results are shown in Figures 1 through 3. Figure 1
shows the HiStar attenuation "a" channel with four 15-minute averages. The wavenumber (400
to 730 nm), broken into 100 bands, is shown on the absissa and the depth on the ordinate. A
similar plot of the HiStar absorption "c" channel is shown in Figure 2.
The optical
backscattering measured by the Hydroscat in the bow of the SeaSoar is shown in Figure 3. The
six wavenumber bands are shown across the absissa for each 5-minute average.
Lessons were learned from failures as well. When recovering SeaSoar under the marginal
weather conditions of the cruise, the vehicle was prone to slamming into the stern of the ship,
potentially damaging the Hydroscat. A temporary
"bumper"
was attached during the cruise to the
vehicle frame to protect the sensors. A more permanent version will be incorporated into the
frame for the Japan Sea experiment. On the instrumentation side, the attenuation channels ("c")
of the AC100 started to fail early into the first deployment, rapidly getting worse (Figure 2).
After the cruise it was determined that an internal bulkhead connector had loosened. The new
oxygen sensor, which behaved poorly in pre-cruise laboratory tests, failed to function properly
and was returned to the manufacturer for repairs. Overall, the test cruise was extremely valuable
and will prove critical to our success in the Japan/East Sea. Unfortunately, deteriorating weather
prevented us from executing the section across the southern flank of Georges Bank.
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