Comprehensive Manual56
© 2018 Nortek AS
Burst. Burst sampling mode takes rapid sampling over a specified time interval. Burst sampling is of
particular use when there is an interest in sampling a specific part of the energy spectra.
Basic Sampling Requirements: Waves are random and therefore measuring waves requires
sampling over a period of time that will best “capture” or represent the complete sea state
statistically. The objective is to measure the wave event over a long enough time so that the random
event can be properly characterized.
Sampling theory suggests, as a rough rule of thumb that the measurement duration is long enough
to capture a minimum of 100 cycles of an event (waves, in this context). When we measure ocean
waves we have to consider what the longest expected wave period is going to be; 100 cycles of this
longest wave will be the minimum wave burst duration.
Number of samples: This is the number of data samples collected during a wave burst. The
optimum choice for number of samples is used in conjunction with the sampling rate where the total
burst length in time is considered. As with all measurements there is a trade off between accuracy of
the estimates and the consumption of resources (battery power and memory). The consumption is
considerably more for wave measurements than current profiles.
Example: If waves are to be measured in the Mediterranean sea, we would expect the waves to be
10 seconds or shorter. Therefore 10 seconds x 100 cycles is 1000 seconds. Sampling rate and
number of samples defines the sampling length. A corresponding configuration could be either
1024 samples at 1 Hz or 2048 samples at 2 Hz.
In order to use Nortek developed waves post-processing software the instrument must be set in burst
mode. The number of samples should be set to 1024, 2048, or a number consistent with the chosen
sampling rate to ensure over 15 minutes of data is collected.
Sampling Interval: The sampling interval specifies how often the instrument will collect wave data
ensembles. This should be specified so that changes in the wave climate can be properly detected.
Since wave events are relatively slowly developing events, choosing a relatively long interval make
sense. A Typical interval is every hour but may range from 0.5-3 hours depending on length of
deployment time and available resources (battery and memory).
Sampling Rate: This is the sample rate for the wave data measurements and it applies to the
pressure and velocity measurements. The sample rate represents the absolute upper limit on the
resolvable waves. The upper limit is half the sample rate; this is also known as the Nyquist Limit.
Example: Waves sampled at 1 Hz can only resolve waves up to 0.5 Hz, and waves sampled at 2
Hz can only resolve waves as short as 1 Hz. Note that there are other mechanisms, such as
deployment depth, that can have a stronger influence on the frequency limits of resolvable waves.
Choosing the best setting is often used in conjunction with the number of samples. The Vector can
sample at 1, 2, 4, 8 Hz, etc. However, note that if one is only interested in measuring waves then it
is probably not necessary to sample any higher than 2 Hz. Therefore the upper limit for the length of
the data record is 2048 samples. This is due to the fact that the PUV approach can not detect waves
much higher than 1 second in period due to the attenuation effects associated with the deployment
depth and wave frequency.
If the sampling rate is too slow compared to the time variation of the motions, under-sampling is the
result and the data is said to be "aliased" with respect to the wave motion, and the waves cannot be
well resolved.
Velocity Range: It is also suggested to set the nominal velocity to no more than +/- 1.0 m/s; where
the associated horizontal velocity range is 2.1 m/s (the horizontal and vertical ranges are provided in
the Deployment Planning dialogue box). The orbital velocity associated with large waves can reach
this speed in shallow water. This may be adjusted downward if waves are expected to be small, or
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