For those of you who want to try notch filtering, why not experiment with the FFT 
notch filter in WinQuake? If you use analog filtering before A/D conversion, you 
will never know what the raw data looks like. My recommendation is to use an octave bandwidth, with the geometric mean of the high pass and low pass filters at the 
frequency you want to suppress. Suppose the period of the microseisms is 6 seconds, 
corresponding to 0.167 Hz. The upper frequency corner of the notch will then be 
0.167 * 1.414 = 0.236 Hz. The lower frequency corner will be 0.167 / 1.414 = 0.118 Hz. 
I will leave it to you to experiment with the number of poles to use. The default 6 
poles seems OK.

  Personally, I would not use notch filtering, as it can mask or distort the phases of the event. 

Bob McClure 

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