PSN-L Email List Message
Subject: Re: Microseisms and the need for PSN to look closer
From: Larry Conklin lconklin@............
Date: Fri, 25 Feb 2005 11:04:52 -0500
I would love to hear more about this idea. I worked for many years in
sonar development, so I have a pretty good idea
of the merits of the technique. I don't have Matlab or anything
comparable around here to try it. Sounds like a good
Jack Ivey wrote:
> There's a different way to look at seismic records that is particularly
> interesting for microseisms. For a long time I've been using the
> specgram function of Matlab to look at both microseisms and quakes.
> Specgram essentially divides the signal record into blocks of time
> and performs an FFT on each block. It then displays the FFT amplitude
> as a gray scale (or other color map). The Y axis of the display is
> increasing frequency, the X-axis is time, and brightness of each pixel
> corresponds to the amplitude of the signal at that time and frequency.
> Essentially you get an image showing how each frequency component changes
> with time.
> This type of display is frequently used in speech analysis, passive sonar,
> and probably other fields. This is not to be confused with the simple
> FFT function implemented by many of the data acquisition programs that
> gives a line of amplitude versus frequency, and which is useless
> by comparison.
> I was amazed at the different information available in this type
> of display compared with looking at a time series. You can see
> amplitude and frequency shifts of the microseisms (presumably as
> storms change location and intensity). You can see frequency shifts
> of the (dispersed) surface waves of a quake as it arrives.
> I have identified quakes by looking at the specgram display that I
> couldn't make out looking at the time series because they were buried
> in high-frequency noise.
> You can also see interesting higher-frequency signals, including line
> spectra that shift and come and go mysteriously (probably cultural noise
> of some type).
> The representation allows you to easily distinguish body and surface waves
> by their spectra, but because the FFT is done on blocks of data it is
> not useful for calculating very accurate arrival times.
> If anyone's interested I can dig out some old data and post a picture. It
> would be pretty easy to implement the algorithms in one of the data
> acquisition/display programs....
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