SonWave-Pro wave spectra software package uses the proven PUV method for directional wave measurement. This method requires accurate measurement of pressure (P) and horizontal velocities (UV) at data rates high enough to resolve wave energy (typically 2 Hz or higher).
Most of our instruments (ADP, ADV, Hydra, PC-ADP, Triton-ADV) allow you to collect the data needed for directional wave measurements. In some cases, upgrading the capabilities of an existing system is fairly simple and does not require the return of the system to SonTek.
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These instruments are already capable of the appropriate measurement rates. As long as the system has a pressure sensor, all that is needed is the PUV software package. |
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To gather accurate velocity data at rates sufficient for directional wave analysis, the SonTek ADP uses a modified measurement strategy. Burst sampling of highly accurate short-range measurements for waves is interspersed with the standard current profiles. |
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The Triton-ADV is a directional wave, tide, and current measurement system that combines the power of SonTek's ADV technology with the proven PUV wave method. The Triton-ADV is the most value-packed system available for currents, directional waves, and tides. |
The SonWave-Pro software package offers flexible processing schemes, custom plots (surface, polar, mesh), time-series displays, spectral displays, and wave statistics. The software can create publication quality graphics. Most of our instruments (ADP, ADV, Hydra, PC-ADP, Triton-ADV) allow you to collect the data needed for directional wave measurements.
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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.
Directional Wave Measurement Overview
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Directional Waves for ADPs/ADCPs - Frequently Asked Questions