I too had wondered about my Lehman in the past... I missed two quakes I felt 
sure I should have detected.  Both were about 100 miles from my location.  I 
reviewed the locations on a large map, then it became obvious to me.  The 
locations were almost exactly perpendicular to my boom!!  Armed with this, 
the P wave at least was visible, and a careful review indeed showed my 
background noise was way up.   Your situation may not be as simple, but I 
thought I'd mention it anyway.  Good Luck,  Mike
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