Chuck,
    First do a high pass filter on your data with a corner frequency of say=
 0.005 Hz, to remove the secular trend in the record and eliminate the prob=
lem of Gibb effect.  Then take the FFT of the complete one-day record and l=
ook to see if there is a microseismic hump in the spectrum.  Note the frequ=
ency of the maximum in that hump and use its value to again do filtering, b=
ut this time both a high and a low pass at the same value, corresponding to=
 a bandpass filter at the hump frequency.  Then open the filtered signal wi=
th an x-scale of say 2000 s.  After doing this you will see that the micro'=
s 'come and go' according to what is called their correlation time, typical=
ly no more than 10 or twenty cycles.  For the hump having a period of 3 s (=
0.33 Hz), this means a given micro-signal persists for no more than about 3=
0 s before dying out, only to be followed by another short lived signal.  T=
hese approximately 30 s pieces comprise individually a set of very sharp sp=
ectral lines, separated from one another (closest neighbors) by what appear=
s to be values of the eigenmode frequencies, in mHz.  To see these individu=
al lines use the WinQuake FFT that is the rightmost one of the pair in Larr=
y's code.  They will be most vivid if displayed in linear rather than log s=
cale.  If you place the cursor on a given line you can read its frequency a=
nd then move on to an adjacent line, left or right; read it and then write =
down the difference value between them.  I am especially interested in seei=
ng how many of these difference frequency values comprise a distinct set of=
 values, as opposed to being randomly distributed group of numbers.  If so,=
 then the free modes must be modulating the microseisms.  The only way to k=
now for sure is to collect enough delta-f values in the manner I've just de=
scribed to generate a statistical base having reasonable confidence.  I did=
 a cursory inspection of some of my records this way, and the results look =
quite promising.  Those of us familiar with heterodyne processes in radio s=
hould not be surprised.  The process of modulation involves a carrier that =
is influenced by lower frequency signals by means of nonlinear processes.  =
In my thinking, the mircroseisms would be the carrier, and the eigenmodes b=
e "what our planet is trying to tell us if we have the demodulation means i=
n a receiver to hear what she is saying".=20
    Randall=
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