Jack,

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
software project.

Larry Conklin
Liverpool, NY

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....
> 
> Jack
> 
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