Quickwave frequency range

(Posted by frazerhurst on Nov. 01 2007)

Hello
We are having trouble with Quickwave 2.03. An aquadop was deployed in 18m of water for three weeks. (let me know where and I'll ftp the .aqd file). We are able to get reasonable peak period and wave heights using the old WaveExtract software, but the only sensible information we have been able to get using Quickwave is the pressure data. What should the start, step and end frequencies be for the QuickaWave frequency range options?

(Posted by Torstein on Nov. 01 2007)

I am not sure why you would be having problems .  I think it would be best to place the raw data file up on our ftp server:
ftp://ftp.nortek-as.com/pub/incoming/

Once it is there, please send an email to inquiry@nortek-as.com telling us it is there and the filename.

The frequency configuration that is the most sensible for an Aquadopp is Start = 0.02, Step = 0.01, End = 0.49 Hz

-Torstein

(Posted by frazerhurst on Nov. 02 2007)

Hi Torstein
Thanks for the "typical" setup. I have used these figures and things look sweat-as. I think my confusion is that the Start and End seem sensible (T of 50s and 2s(aprox)) but the step, if interpreted as Hz translates to a step greater than the Start and End (100s). I presume that the 0.01 is the frequency range unit? Maybe this could be explained better. My apologies if this matter is clarified elsewhere and I could not find it.
Thank you Torstein

(Posted by Torstein on Nov. 02 2007)

Hi Frazerhust,

If I understand your question correctly, you are not sure about the frequency step.  It seems you are looking at this problem in terms of period and not frequency.  I think this is something you should avoid.

It is much better to view the processing as well as the spectra in terms of frequency.  If you try to look at this in terms of period, you will find that the scale is non-linear.  For example a step of 0.01 Hz starting at 0.02 Hz (ie 0.02, 0.03, 0.04, 0.05,...), would have a corresponding period increment step of 50s, 33s, 25s, 20s, ... as you can see this is not uniform and the 'period step' is not constanst (and not 100s!).  This is not particularly easy to work with.  You should really try to think in the Frequency Domain and not the time domain.

Wave estimates are reported as period (peak period), because it is more familiar and easy to count from crest to crest.

Hope this helps,
Torstein

PS did you sort out the the problems with Quickwave or should I expect to see something on our FTP?

Hi, I am Christopher Saez, Ocean Engineer, of OceanGreen Chile S.A.

since when acceding to the confguración to make the single filtrate it allows me to make it between the 0,01 to 1,99 Hertz. Additionally I can give like antecedent that the instrument midio to 1Hz every 35 minutes.

Christopher Saez A.

Ocean Engineer

Research & Development Departament

Hello Christopher,

I think that it is important to understand that the wave processing software available from Nortek is intended to process wave data that is commonly know as "wind waves", or waves generated by winds.  These are generally assumed to exist between 0.5 - 30 seconds (or 0.03 - 2 Hz).

Wave energy in bands below this are not generally classed with these waves and therefore not included.  This does not mean you can not measure them it just means that you can not process them with QuickWave. 

Since the wave frequency is so low, you can likely use the pressure sensor equally well as the AWAC's AST.  The pressure estimates are found in the one of the converted ASCII data files (depending on which instrument you have), which is output from the instrument's standard software; while the AWAC's AST file is output from the QuickWave software, where it is quality controlled.  From here you will have to use a analytical software such as MatLab to perform the spectral analysis.

I am glad to see that you are measuring for over 1/2 hour which is important for resolving wave events.  If the waves are wind generated then the energy content should be fairly consistent for several hours which would permit averaging several bursts together.  The averaging is straight forward by doing it in frequency domain.  Averaging several bursts together will reduce variability in your processed results.  I caution you however when averaging bursts together if long waves are generated by another mechanism other than wind; other generating mechanisms can lead to waves that are more "transient" or short lived in nature.

kind regards,

Torstein