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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.

Oceanside Offshore - transient fouling

The buoys may also be exposed to transient sources of bio-fouling, such as when they are surrounded by kelp. For instance, in October 2014 the Oceanside Offshore buoy was entangled in kelp for more than three days, with the check factors indicating that motion was significantly restricted. After that the waters cleared around the buoy and the check factor values returned to normal. There was, however, some kelp left atop the buoy, as shown in the picture. This kelp covered the GPS antenna on the tophat and prevented GPS updates until the buoy was serviced.

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 wave energy.

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. Calibration history
Calibration history,
typical buoy

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