In 2005, Nortek patented a new method for processing wave data from current profilers with AST called the SUV method. Today, this method can be used with both AWACs and Signatures. The solution represents a hybrid of the PUV and AST measurements. Wave orbital velocity measurements are still made close to the surface, like in the array solution, but instead of using an array, the velocities are converted to collocated velocity components of U and V. The other difference is that pressure is no longer used, and the AST is used in its place.
The result is a solution that permits the AWAC and any Nortek ADCP with a vertical beam (e.g. most of the Signature ADPCs) to be mounted on a subsurface buoy. This means that when wave measurements are desired in waters where the total depth is too deep for the instrument to be mounted on the seabed and use the array solution, the instrument may be placed on a subsurface buoy and positioned closer to the surface. In contrast to the MLMST method, the SUV method accounts for and corrects for the motion of the buoy, making these measurements possible. For more details about how the SUV method works, please refer to the papers linked to under Additional Reading.
The SUV method may also be used for bottom-mounted deployments. The method is also better suited (than MLM) for deployments where the waves are exposed to large mean currents. Mean currents can present a Doppler shift on the wave field and introduce errors in the directional and non-directional estimates if not corrected because the transfer function used is wave number dependent. The SUV method does not require a correction for background currents because both the AST and the directional portion of the measurement do not have transfer functions, and so are not wave number dependent. Nortek's post-processing software packages recalculate the wavenumber when the MLMST method is used, but we recommend using the SUV method in places with known background currents to avoid any sources of error from the correction being introduced.
Limitations
Even though the introduction of AST enabled wave measurements at greater depths compared to pressure-only measurements, depth still limits the minimum wavelength that can be resolved. We distinguish between a directional and a non-directional frequency limit, where the non-directional limit is lower than the directional one. Both of these limitations should be considered when deploying an instrument for wave measurements and are described in detail below.
Non-directional limitations (AST footprint)
The ability to estimate wave parameters using AST is limited by the size of the surface area covered by the AST beam, referred to as the AST footprint. The footprint size is determined by the beam width and the distance between the instrument and the sea surface, and increases with both wider beam angles and greater distance to the surface.
The non-directional cutoff frequency (limit of the shortest measurable wave) is affected by the size of the footprint. As a rule, we follow a Nyquist-like reasoning; the frequency limit associated with the footprint is when half the wavelength is on the order of the diameter of the footprint (Equation 1). This clearly is the absolute shortest measurable wave, because shorter waves will have several crests and/or troughs within the AST footprint (Figure 1).
| \(\lambda_{non-directional\ minimum} = 2 \times AST\ Footprint \) | (1) |
\(AST\ Footprint = 2 \times tan(\frac{\theta}{2}) \times D\) |
(2) |
Where \(\lambda_{non-directional\ minimum}\) is the minimum resolvable wave length for non-directional parameters, \(\theta\) is the total opening angle of the AST beam and \(D\) is the deployment depth of the instrument. The beam width for a specific instrument can be found in the technical specifications.
Wave theory gives us the equation to relate the wave length found in Equation 1 to wave period (Equation 3)
| \( \lambda = \frac{gT^{2}}{2\pi} \) | (3) |
Where \( \lambda \) is wave length, \(g\) is the acceleration of gravity, and \(T\) is the wave period
Directional limitations
The shortest resolvable wavelengths are limited by the horizontal separation between measurement cells in the projected array. This separation depends on the beam geometry and the deployment depth, and increases with depth (Equation 4).
As the separation distance increases, so does the minimum wavelength that can be resolved directionally. A common rule of thumb is that waves must have a wavelength at least twice the separation distance to be resolved unambiguously. Shorter wavelengths lead to spatial aliasing, introducing a Nyquist-type limit beyond which wave direction cannot be determined.
| \(\lambda_{directional\ minimum} = 2 \times D_{velocity\ cell} \times tan(\alpha) \) | (4) |
Where \(\lambda_{directional\ minimum}\) is the minimum resolvable wave length for directional parameters, \(\alpha\) is angle of the slanted beam from the vertical and \(D_{velocity\ cell}\) is the distance from the instrument head to the location of the wave velocity cell. The angle of the slanted transducers for a specific instrument can be found in the general arrangement drawings.
Sampling limitations
In addition to instrument geometry, the sampling scheme can limit the shortest resolvable wavelength through temporal aliasing and sampling frequency. For further details, see Sampling and measurement principles
Additional reading
Pedersen, T., Lohrmann, A., Krogstad, H., 2005: Wave measurements from a subsurface platform (2005)
Pedersen, T. Siegel, E., 2008: Wave Measurements from a Subsurface Buoy (2008)
Pedersen, T., Horn, K., Wickström, K., 2009: Subsurface wave measurements taken to new depths (2009)
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