Small waves appearing from two directions

Dear Torstein,

I received a dataset of a 600 kHz AWAC with AST in 11m of water. Wave heights are typically small as the site is shielded from the sea. During one day I see small 7 s waves which I assume come from the sea and also higher waves of 2.5 s from two directions almost 180 degrees apart. This could be wind waves which also reflect on the shore close by or shipping waves. We will have to look into that lateron.

My question is; can the AWAC 600 kHz distinguish these two 2.5 s wave fields properly ?

Best regards,

Sicco

Hello Sicco,

Sounds like interesting data.

You should be able to get a lot of high frequency information, since the AWAC is deployed in relatively shallow water.  I have attached a plot that shows the cut off frequencies for the AWAC - both for direction and nondirectional estimates.  For your 11 meter deployment the cut off frequency looks to be at 0.45 Hz  - or a period of 2.2 seconds.

You will be able to get all wave information, as long as you are interested in waves that are below this cut off frequency.  When I say "all" I mean that you will be able to see reflected waves or waves coming from two independent directions at the same frequency

kind regards,

Torstein

Hi Torstein,

Great graph. That helps a lot. 

Best regards,

Sicco

Dear Torstein,

Your graph is for the 600 kHz AWAC ? I  imagine the difference between 1 MHz and 600 kHz is not too large.

Best regards,

Sicco

Hi Sicco,

The graph is independent of transmit frequency.  The cut off frequency is a function depth only for both the directional and non-directional estimates.

Directional estimates are limited by the horizontal separation of the wave measurement cells and the AST measurement, and the horizontal separation is linearly related to deployment depth.

The non-directional, or energy density is limited by the footprint of the AST beam on the surface.  The relationship between cut off frequency and depth is the same for both the1 MHz and 600 kHz  Since the beam width is the same (1.7 degrees).  The effects of the "footprint" are quite mild, as one can see from the plot above.

These points are discussed also in the following forum thread (with illustrations):

Frequency/period Limits for the AWAC

-Torstein