Advanced Deployment planning for an AWAC in shallow water

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Advanced Deployment planning for an AWAC in shallow water

Posted by Camille Couzi at April 09. 2012

Hi Nortek!

This is a question concerning the panel "Waves - static mode" in the Advanced Deployment planning window.

We need to deploy an AWAC in a very shallow location. In low tide the depth is 2.5meters, in high tide, 5meters depth. What should I put in the following edit boxes?:

- Velocity cell size: the default is 2-4 meters. (is this the cell which is 2 to 4 meters below the surface?)

- AST window start

- AST window size

- Find first peak.

 

Thanks in advance for your answers!

Cheers!

Camille.

Re: Advanced Deployment planning for an AWAC in shallow water

Posted by Torstein Pedersen at April 10. 2012

Hi Camille,

 

Wave Static mode is for very special conditions where one knows where the surface will be and would like to specify a window about this surface - as well as the velocity cell positions.  If the surface position does not vary that much and one wants to draw in the limits of possible positions, this mode can help limit possible bad detects.

 

In practice, I find that I rarely use this mode or encourage the user to use this mode.   There are several reasons for this:

a) There is almost always tidal elevation changes present; this makes it hard to specify measurement positions.
b) Sea state is variable making surface position hard to predict.
c) Deployment depths often vary from what or where you intend to deploy.

 

The standard mode, where the measurement volumes and AST window are automatically specified works surprisingly well.  The secret to the success is that the AWAC always profiles for 60 seconds just prior to the wave burst, and here a good estimate of mean sea level is made as well as approximate sea state.

For your intended deployment with depths of 2.5-5.0 meters of water, I would simply use the standard mode and stay away from the static mode.  Given the infrequent need for Static Mode, Users should not be surprised in the future that Static mode is no longer found in the Deployment Planning ;)

 

-Torstein

 

Re: Advanced Deployment planning for an AWAC in shallow water

Posted by Camille Couzi at April 18. 2012

Hi Torstein,

 

Thanks a lot for your reply, we are then not going to use the "Waves static mode" function.

 

I need to ask you two more questions:

1. [URGENT] About the "sampling rate" in Waves panel (standard tab): 

In the AWAC's datasheet, it is marked in page 6, under Waves measurement section:

"Sampling rate: 2 Hz velocity, 4 Hz AST* (1MHz), 1 Hz velocity, 2Hz AST* (600kHz), 0.75 Hz velocity, 1.5Hz AST* (400kHz)."

 

We are using a 1MHz AWAC for this deployment. We thought, based on the information above, that with a 1MHz AWAC, we can measure waves using a sampling rate of maximum 4Hz.

However, when trying to select "4 Hz" in the corresponding list in the "sampling rate" of the deployment planning window, in fact we only have two options: 1Hz and 2Hz. Why?

Is the 4Hz an option we have to add to our instrument?

 


2. About the "estimated depth" in Advanced tab, deployment planning planel:
 

The depth in the location is 4meters, with -+1m (tide). We have keyed in 12 cells, and cell size 0.5m, so that we have 6meters covered (above the transducer and taking in account the blank size of course).

We are thus very curious of what's the use of the information entered in "estimated depth"... what does the software use it for?

Shall we enter our mean sea level, or a value above the sea level in high tide?

 

 

Thanks in advance for your answers,

Best regards,

 

Camille Couzi.

 

Re: Advanced Deployment planning for an AWAC in shallow water

Posted by Jonas Røstad at April 23. 2012

Dear Camille,

 

1) I know that you have been answered this by e-mail, but others might wonder.
The 4 Hz refers to the acoustic surface tracking (AST). The AWAC is constructed such that the surface measurements are done twice as fast as the specified sampling rate. Consequently, by selecting 2 Hz sampling rate you really choose 4 Hz surface tracking / non-directional wave measurements.

2) The estimated depth parameter is not really being used other than to calculate memory and battery consumption. The position of the wave cell and AST detection is chosen based upon most recent current profile.
The exception is if the pressure sensor should report malfunction; then the parameter is used. I have not seen any data set yet where this has been the case.

 

Best regards

Jonas Røstad

 

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