Directional Waves for ADPs/ADCPs
Frequently Asked Questions
Q. How do SonTek/YSI ADPs measure directional waves?
A. Our systems measure time series of pressure (P) and horizontal
velocities (UV). A combination of auto and co-spectra yields the wave height, direction,
and spreading.
Q. I thought that Doppler profilers could not measure velocity
fast enough for wave work.
A. This is essentially true. But, we do not need to measure the full profile
to get directional waves, because all we need is the velocity in the vicinity of the
pressure sensor. We apply a Doppler technique known as "Pulse Coherent
Processing" to measure bursts of velocity at short range. This technique is the same
highly-accurate technique that we use for our ADV, Hydra, and PC-ADP systems. Using this
technique allows the ADP to periodically gather bursts of highly-accurate PUV data at
rates of up to 4 Hz -- which is generally the maximum rate of interest for wave work.
Q. Is this technique new?
A. Yes and no. The idea of setting a profiler to periodically measure bursts
of high-resolution velocity in a single layer to combine with a pressure measurement is
new, but the PUV technique for measuring directional waves has been firmly established
over the past decades as a very robust measurement technique that is preferred by most
researchers.
Q. I have been told that the PUV technique is unreliable
because recent research indicates that the mean currents must be considered in these
analyses.
A. There has indeed been recent research that indicates that mean currents
above a certain size can influence wave measurements. This is not a problem, because the
ADP also measures the mean current profile in addition to the PUV bursts.
Q. So how is your technique different from that used by other
ADCP manufacturers?
A. The techniques are vastly different. The beam technique is a newly
developed way to treat the beam velocities of a profile as a spatial array. The great
advantage of PUV measurements is that they all take place in a single location: as soon as
you go to an array, you must assume spatially homogenous flow over the entire array. It
might be worth noting that many of the early deployments of profilers using the beam
technique required the use of nearby SonTek sensors (measuring PUV) to help calibrate
their results.
Q. Since the beam technique allows me to take my measurements
near the surface even though my instrument is on the bottom, it seems to me that I will be
able to gather wave data from significantly greater depths.
A. Not really. The Nyquist limit on wavelengths is that an array can resolve
waves no shorter than twice the spacing between measurements. Most profilers have an
off-vertical angle of 20-30 degrees, which means that a profiler deployed 50-m deep will
create an array with measurement points a minimum of 73-115 m apart, making the
minimum measurable wavelength 146-230 m. For reference, 10-second waves have
wavelengths of 150 m.
Q. I understand that I do not need a pressure sensor to use
the beam technique.
A. Using acoustics to measure the distance to the surface is tricky, at
best. First of all, the sampling bandwidth available for surface measurement decreases
with range. Second, the fact that the beams intersect the surface at an angle allows for
significant contamination from leaking side-lobe energy. Last, changes in the density of
the water column (e.g., a thermocline) will affect the speed of sound, and therefore the
associated level measurement. In summary, while it is possible to make the measurements
without using a pressure sensor, it is much less reliable to do so.
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