QC and check factor values from directional buoys
Datawell directional buoys return 'check factors' for each frequency
band in the spectra they calculate. The ratio of horizontal displacements
to vertical displacements, these check factors are an important tool in
performing QC on the buoy data.
In general, the mid-frequency and high-frequency check factor values
should be very close to one. (The low-frequency bands often have
very low energy levels, so their check factor values are more
variable.) When the mid- or high-frequency check factors deviate
significantly from one, it indicates that there is likely an issue
with buoy. Below are some examples of these issues as highlighted
by the check factor values.
Monterey Canyon - fouling and cleaning (May 25, 2011)
|Although anti-fouling coatings are used on the buoys,
they sometimes accumulate considerable marine growth. When the
buoy and mooring are covered with barnacles and other organisms, the
high-frequency response can suffer. These changes are reflected in the
check factors. The plot below shows the Monterey Canyon buoy (right)
check factor values when heavily fouled and after cleaning the hull.
San Francisco Bar - tides and currents
|Buoys deployed in areas with strong currents often
have check factor values that track along with changes in the currents;
the force of the current can reduce buoy response.
The San Francisco Bar buoy experiences strong tidal-driven currents,
and the variablility in its check factor values can be largely
accounted for by the changing tidal cycles.
Cook Inlet - tides and currents
|Cook Inlet also experiences strong tidal shifts and
currents, and the check factors reflect this. Its check factors,
however, are far more variable than at SF Bar. Since the inlet is
quite sheltered, it seems that the currents have a far greater impact
on buoy motion than they do in locations which are more exposed to
Jeffreys Ledge - icing
|In very cold climates like in New Hampshire, the
buoys can ice up in the wintertime. When this happens, the buoy's
satellite communications often fail. Data from the buoys internal
loggers, however, show normal check factor values, indicating that
the icing doesn't significantly impact buoy motion.
Barbers Point - fishing
|When small craft tie up to a buoy's mooring, the
check factors can show changes in the buoy's response. In Hawaii,
fish-aggregating devices or FADs (as at right) are used and look very
similar to CDIP's buoys. Below the check factors for Barbers Point
show intermittent problems with high-frequency response, likely
caused by small fishing craft.
Majuro - fishing?
In the Marshall Islands, the check factors sometime look similar to those
from Barbers Point. These times of poor buoy response are often times
when the buoy's Iridium satellite transmissions also drop out.
Pt Loma South, buoy failure
|In September 2010, a rather heavily-fouled buoy at
Point Loma South was replaced. The check factors of the replacement
buoy, however, also looked off, with mid-frequency check factor
values lower than the higher frequency values. This was in fact
a buoy that had just experienced a sensor failure, perhaps when
in transit to the site. The pre- and post-deployment
calibration checks (right) help highlight such issues.
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