Why AWAC with AST?
Wave measurements from bottom mounted instruments have classically been performed by instruments that measure pressure, wave-induced orbital velocity or a combination of the two. The most common is the approach known as the PUV method which measures the pressure (P), and the horizontal components of the orbital velocity (UV). Although a trusted approach, it is limited by the fact that both the pressure and velocity signals suffer from increasing attenuation as we move to greater depths. The long and short of this is that the PUV approach is limited to shallow waters or very long waves. The Nortek Aquadopp, and Aquadopp profiler and the Vector can be used to conduct PUV wave measurements
One clever solution of getting around this is to measure the orbital velocities further up in the water column using Doppler profilers. This has enabled bottom mounted instruments to move into deeper water and measure waves with shorter wavelengths. Unfortunately, this solution is still subject to similar constraints as the PUV and it does not represent a complete solution. Measurements are still limited to a particular depth and wave frequency due to this glass ceiling that the attenuation presents. Furthermore this approach of determining wave parameters is an inferred method, which means it is not directly measuring the changes in the water surface elevation but instead using a linearized transfer functions to estimate it.
For the majority of coastal wave measurement applications this is all well in fine, however this leaves several gaps in fully understanding a wave environment. For example, the inferred approach can not provide estimates for the maximum wave height, it can not properly perform ship wakes studies (transient waves), and nonlinear waves are not suited for this approach.
Clearly the best solution for a complete understanding of surface waves is to directly measure the free surface itself. Wave buoys can performed this task but of course they have their own limitations. These limitations are namely the limited frequency response associated with the buoys size, as well as the risk associated with the instrument existing on the surface (exposure to ships, fishing, and theft).
Therefore that which is necessary to measuring surface waves is an approach that measures the free surface directly and is unintrusive. The Acoustic Surface Tracking (AST) is Nortek’s answer to this challenge. This approach measures the distance to the free surface from the center transducer of the AWAC. The method is in essence an echo sounder, however pointed upwards and designed to track the changes in surface elevation. The advantages of the AST are numerous but here are a few of the primary advantages:
- Quantity measured does not attenuate with depth
- Possible to measure high frequency waves (up to 2 Hz!)
- Greater accuracy (i.e. elevation changes of less than a 1cm possible)
- Capable of measuring transient and nonlinear waves
The AST method in a nutshell
The operation of the AST is relatively straight forward. The process first begins by defining a receive window which is design to envelope the free surface. This is done by measuring the pressure just prior to the wave burst. Once the AST window is established, a short pulse is transmitted. The surface reflection of the transmitted pulse is searched for in the highly discretized AST window using a match filter. If this all seems confusing you could basically say that the AST sends a pulse and properly finds the return pulse by knowing when to listen and what to listen for.
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