PSN-L Email List Message

Subject: Recording Micro Quakes
From: Roger Sparks rsparks@..........
Date: Sat, 18 Feb 2006 08:08:00 -0800


Hi Bryan,

You should find that your Lehmann would be more sensitive than a Geophone at the higher frequencies expected from micro quakes.   This would be because the longer period instruments would use less damping than the higher frequency units.

Think of it this way.   The shorter the period of a seismometer, the larger the damper will be to achieve critical damping.   The damper always reduces the motion induced in the seismometer by the passing wave.   Larger damper for a higher frequency sensor so less sensitivity (than the longer period sensor) at the higher frequencies.

You might consider if the damping and filters you are using on the Lehmann are seriously attenuating the higher frequency waves that could be expected from micro quakes.   You are probably sensing the micro quakes now, but losing the information in the filters and damper.

Several people are posting recordings of micro quakes on  The PSN web site .  A WinQuake FFT analysis of the posted micro quakes helps give a perspective of the bandpass requirements needed of the equipment.

Best wishes,   Roger

psn-l-digest-request@.............. wrote:
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From:    1goss@...........
Date:    Fri, 17 Feb 2006 08:54:25 +0000

Hi all I am trying to find a used geophone for recording micro earthquakes in the central US.
Mainly in the New Madrid Fault Zone. I have a Lehmann sensor, but I would like a Geophone. 
I am not sure what I need, Maybe something like Larry had on his website but is sold out of the GeoSpace HS 10-1 1 Hz Vertical Sensor. Where would I look to find one or does anyone have one they would part with for the right price.
Thanks Bryan S Goss
Corinth Ms


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