PSN-L Email List Message

Subject: Re: Modified sound card and datalogging and geophones
From: Gordon Couger gcouger@..........
Date: Thu, 21 Jul 2005 13:52:05 -0500

Hi Doug
 > Snip

 > UnSnip
 > This is not earthquake recording, this is exploration 
seismic.  The
 > frequencies you care about will be between 10 and 500 Hz and 
a perfectly
 > normal sound card will handle these signals. You may need 
some preamps like
 > you would with a dynamic phonograph cartridge, because the 
signals from
 > geophones are small.
 > You will need zero time from your shock source (a sledge 
hammer should be
 > adequate). The best choice is a small, cheap crystal 
phonograph cartridge
 > taped near the head connected to the second channel. You 
don't care about
 > absolute time, just time relative to the impact, so forget 
GPS timing.
I am using standard seismic sensors and I will probably use a 
..22 or .38 fired
into the ground or a slide hammer lifted up by a blank 
cartridge. My sledge hammer swinging days have been over for 20 
years due to multiple sclerosis.

I think I need a better clock than the computer has for relative 
measuring and I plan to divide down the 100 KHZ clock on the GPS 
to 1000 HZ and put on each channel of the sound card for a time 
base and reference. Each sensor will modulate an oscillator at a 
different frequency and all will be mixed together and recorded 
on the sound card and the 1000 HZ signal will be used to lock on 
to decoded the others as well as a time base that is 
automatically generated in the decoding process for each sensor.

There is just too much jitter in everthing I have tested in 
windows to trust it to keep time even on a the sound card that 
is supposed to run in real time. There also has to be some point 
to referenced everthing from in both time and frequency.



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