PSN-L Email List Message

Subject: Questions from a beginner
From: "Larry Conklin" lconklin@............
Date: Tue, 1 Aug 2000 10:45:52 -0400


I've been lurking on this list for a while and have some qestions for some
of the more experienced people.  I have build a Shackelford-Gunderson
seismometer, based on the electronics board that Larry Cochrane provides.
The thing has been running for a couple of months, and I managed to bag
several events.

My questions all relate to what my expectations should be for the
performance of the thing.  The sensor is sitting on my basement floor.  I
live in what is probably a noisier than average location, a residential
neighborhood about 500 yards from a very busy street.

First, what should I expect to be able to see as a function of distance and
magnitude of an event?  About the weakest events that I have seen were two
from Iceland, both about 6.5, both of which produced very nice and
unmistakeable records.  But there is a lot of activity that seems to be
below my sensititivity (or noise floor).  Should I be able to do better?

Second, I routinely see a very long period background noise.  If I run the
Winquake FFT routine on a "no event" record from my system, I see a broad
peak around 100 seconds with several spikes in the gram around 60 - 120
seconds.  I wouldn't be surprised if a lot of this is thermal noise, but it
would be nice to know if what I'm seeing is reasonably typical of seismic
noise.  The noise level does seem lower at night, when there isn't any
activity in the house.  I understand that there are continuout microseisms
with a period of around 6 seconds (?).  They aren't obvious in my records,
perhapse because I don't have the LF gain set high enough.  My LF noise
level produces peaks that average around 20 or so from a 12 bit A-D

My other problem is a basement floor that is about as stiff as a trampoline.
Haven't been able to find a spot that is immune to people walking on the
floor above.  Short of buying a new house or pouring concrete in the back
yard, I'm probably stuck with that one.

I'd appreciate anyone's comments or suggestions.  I'm having fun, but have a
lot to learn.

Larry Conklin


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Larry Cochrane <cochrane@..............>