PSN-L Email List Message

Subject: Re: distortion on geophones
From: Karl Cunningham karlc@..........
Date: Sun, 27 Jun 2004 08:20:52 -0700

Don't go for the shovel just yet...

It looks to me like an asymmetry somewhere in the electronics.  The mine
blasts seem to be much higher signal level than the earthquakes, and the
signal could be exceeding the voltage range of one of the stages.  Likely
in a filter stage or before it.  What makes me think so is that in "local
event vertical geophone3.GIF", after the distortion the signal takes a
relatively long time to return to the original zero level (10 seconds or
more).  I don't think the geophone itself would take that long to return,
which makes me think it's an electronic artifact.

This distortion could be explained by the following:  There is an
asymetrical limit to the signal level in a stage in or prior to a low-pass
filter.  By asymetrical, I mean that the signal limit is greater negative
than positive.  Geophones, by their nature, put out fairly symmetrical
signals -- the amount of electical energy on the plus side of zero is close
to the amount on the minus side.  But if there is an asymmetrical limit
somewhere in the electronics and the signal is large enough to hit that
limit, the resulting signal is no longer symmetrical about zero.  Then the
signal goes through a stage which filters out most of the high-frequency
parts of the signal, so what you have left is the average which is no
longer near zero.

Obviously, I'm guessing at what it might be.  Asymmetries in signal limits
are often caused by assymetrical power supplies (if you have + and -
supplies).  They can also be caused by a large offset somewhere in the
electronics such that the average is no longer centered bewteen the limits.
You might not see such an offset because it is filtered out by Winquake or
in the electronics.

Good luck with it.

Karl Cunningham

--On Sunday, June 27, 2004 09:22 -0500 tdick 



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