PSN-L Email List Message

Subject: Re: Questions from a beginner
From: BOB BARNS roybar@........
Date: Tue, 01 Aug 2000 13:35:00 -0400

Larry Conklin,
  I also have my seismometer on a basement floor which is flexible.  As
a suggestion (I have'nt tried it):Find a contractor who can use a
diamond core drill to drill 3 holes, spaced like the feet on your
seismometer, thru the floor with a diameter of 2 to 3".  Drive smaller
diameter pipe thru the holes into the ground beneath but don't let the
pipe touch the basement floor.  Mount the seis on top of the pipes.
As a wild guess, the cost of the holes might be less than $200.
Let us know if this works as I'm sure that others have the same problem.
  About which events you should see:I get good recordings (with a Lehman
of about 20" period) of an event anywhere in the world which has a
magnitude of about 6 or higher unless the great circle route to the
event lies in the insensitive direction (along the boom).
Bob Barns
Berkeley Heights, NJ

Larry Conklin wrote:
> Hi
> 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
> converter.
> 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
> lconklin@............
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Larry Cochrane <cochrane@..............>