PSN-L Email List Message

Subject: Re: FFT
From: "Barry" gbl@.......
Date: Sun, 11 Aug 2002 07:25:03 -0700

```Rolando
I'll contribute a little. As I understand it, any signal can be
representated by a sum of sine waves of different frequencies and
magnitudes. With the FFT, the math routine has selected multiples of 2 to
speed up the calculation process. You  don't get as much of the frequency
spectrum but it calculates faster. What you  see when you run an FFT is the
magnitude sum of the various sign waves ploted as a function of frequency
for that given quake and your sensor.  I, as an example,  usually filter
teleseismic events to remove frequencies below 0.003 hz and above 0.5 to 1
hz ( so my files won't be so large.) . You should save events at a sampling
frequency twice the value of the highest frequency  of interest (Nuquest ).
I think what happens if you save higher frequencies than the sampling rate,
the sampling picks  points on different signal cycles (like a beat) and a
higher frequency can look like a low one in the FFT.
Regards
Barry

> > 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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