Kilo Moana RDI 300kHz "Workhorse
Mariner" ADCP: preliminary report
April, 2005
During the Kilo Moana's April 2005 drydock period in Brisbane,
Australia, a 300kHz R.D.Instruments broadband "Workhorse Mariner" was
installed in the existing Doppler current profiler well in the port
side pod. This instrument was purchased to complement the lower
frequency 38kHz ADCP, which has been demostrated to be an excellent
deep (often below 1200m) current profiler on this ship. The 300kHz
Workhorse has higher resolution and profiles the top 100m.
Data were collected during the 8-day transit from Brisbane, Australia
to Pago Pago, American Samoa. Speeds were nominally 12kts, and weather
conditions were generally good. Although the range was not as deep as
we had hoped, data quality was excellent (
fig1,
fig2,
during this transit. The Workhorse accomplishes its primary goal of
profiling much closer to the surface, and provides a higher resolution
view of the top 100m.
ADCP data range is affected by acquisition settings, scattering
levels, and bubbles.
A variety of
acquisition parameters
were tested to see the effect on range.
Error velocity statistics,
indicate that the range stayed much the same during the time
of these these tests. A larger effect on range was the dirunal cycle
of scattering due to vertical migration of scatterers. Ranges
typically varied from 90m during the day (when the scatters have swum
deep to avoid predators) to 110m during the night (when the scatterers
are present in the upper hundred meters). The instrument's signal/noise
parameter showed the relative increase in scattering during the night
(compared to day) at
all depths measured. On one occasion, scattering was sufficiently low
to render the upper 100m of the 38kHz instrument nearly
useless. Although the Workhorse depth range was impaired during this
time (40-50m) it returned high-quality data in the
range it could reach.
A third effect on
data quality and range is bubbles and flow noise. There was no evidence
of bubbles, though that is not surprising since the transducer wells
on the Kilo Moana are ideal for acoustic surveying, and the weather
was calm.
Further experience will add to our understanding of
its performance in heavy weather, and the editing parameters to use
under those conditions.