Rolando,

I'm far from an expert on FFTs. You should be able to fine more information
on the Internet if you do a search on Fast Fourier Transform. Many years ago
I found some code on the Net that does the FFT. I basically treat the FFT
function as a block box. How it works involves a lot of math that's above my
head.... What's amazing is it only takes a few lines of "C" code to do the
FFT function.

To filter the data in WinQuake you should use the Time Domain filtering
rather then using the filtering in the FFT Window. The time domain filtering
produces less errors or distortion then using the FFT filtering. The reason
is errors are introduced when you do the FFT and again when you do the
inverse FFT to get the data back into the time domain.

The reason for the filtering is to remove unwanted noise in the data. As an
example if you record a teleseismic event there is no need for data above
one or two Hertz.  Since the local man made (or is it person made in these
PC times?) noise is usually above one or two Hertz one can do a lowpass
filter to remove the local ground noise.

Hope this helps.

-Larry Cochrane
Redwood City, PSN

----- Original Message -----
From: "Rolando Benitez" 
To: "Psn-L@........... Com" 
Sent: Thursday, August 08, 2002 8:56 PM
Subject: FFT


> Larry et al,
> I am a newcomer -almost- in this list and an apprentice of the earthquakes
> sciences, I have been posting events at the PSN web page since late May,
> having the good fortune to live in the middle of a very active area, I
have
> posted a few. As a beginner I need to learn a lot about many things, I
have
> received wonderful help from several PSN members, thank to them I am
> enjoying this very much. I would like to address some questions to the
group
> looking for some light in the area of what the FFT module of WINQUAKE is,
> I've used the filters in a very practical and blind way, so I have a
slight
> idea of what it can be used for,  but have no clue in the relationship
among
> the period of sensor and the freq of the event and the proximity of quake,
> etc, and the FFT screen, so I really need further assistance. Can someone
> give me some light, please? How can I find some info in the Internet,
book,
> etc.,? Your help would be very appreciated.
> Best regards from Fraijanes, Guatemala.
> Rolando Benitez
>
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