## PSN-L Email List Message

Subject: Re: Filtering local noise
From: "Tom Schmitt" tschmitt@..............
Date: Fri, 21 Apr 2000 09:00:22 -0400

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----- Original Message -----
From:
To:
Sent: Thursday, April 20, 2000 8:04 PM
Subject: Re: Filtering local noise

Dear Chris

this to make it easier to read.  It is thus long and I apologize for that.

> Dear Tom Schmitt,
>
>     You wrote :-
>
> >  In exploration seismic work geophone arrays were used to filter local
> noise.
> >  The signals from several geophones would be summed.  The noise would be
out
> >  of phase at the geophones but the signal would be in phase. In
reflection
> >  work that assumption is more justified than in earthquake or refraction
> >  seismology as in reflection the signal is coming more or less straight
up.
> >  Has this ever been used in earthquake seismology?
>
>     Your letter raises several questions. How many and how widely spaced
> locations were you proposing for your recording stations? The wavelength
of
> earthquake waves is likely to be VERY long when compared to explosive
> sounding signals / environmental noise and what you actually record at a
> particular site is effected by the wave path, which may have different
> properties from different directions.

Well first I was hoping that one of the academic seismologists would answer
with (1) that's impossable
or (2) here is how to do it.  Lacking that clear guidance :-) I will attempt
some speculations.

The period of the earthuakes is very long but the period of local noise is
very short.  If I remember Sean-Thomas was talking about 100 HZ notch
filters to remove the local noise.   At 5 km/sec for surface material and a
frequency of 100hz the wavelength is 50 meters.   Considering a wave
propigating only in the x-z plane, and a seismometer located at 0,0 a second
seismometer located at 25,0  would be 180 degrees out of phase for a 100 hz
signal.  Summing the two signals would result in zero ( minus the
attenuation over 25 meters and local effects).

>
> >  The usual answer to noise is a quiet location but with
> > educational seismometers rather than research
> >  seismometers, location near the classroom is important.
>
>     If you can use your many observers to identify the various man made
noise
> sources, there is no reason in principle why you should not place an
> auxiliary sensor(s) closer to sources and feed the relatively strong
> interfering signals back to provide compensation, but don't expect a
perfect
> match. It is easier to do this using digital delays, rather than analogue
> delay lines. Can you record an environmental noise channel to help in
> identification and the seismograph channel? Could the identification /
> characterisation / avoidance of local sources be a worthwhile project? If
the
> local noise is terrible, could you get on line data from some quiet
location
> for your 'real earthquakes' and use your local set-up as a teaching /
backup
> aid?
>

The noise is usually diffuse in origin, thus having sensor near the noise
probably will not work.    The idea is to have enough sesors out there that
the sum of them at any given time covers an intergal number of  wave lengths
of the noise.  The sum is then zero.  Since the signal is a much longer wave
length it is not reduced by the summing.

It would be prohibitively expensive to do this with long periond
seismometers but your idea of noise monitors is good.  One might be able to
use exploration geopones to get the "noise",  high pass those signals and
sum them with the seismometer and reduce the noise.  The news broadcsasters
often use a second noise canceling microphone in noisey environments.

The only reason for this is cosmetic or to look at local events.   Also
exploration work is much simpler in that one knows the source and noise
characteristics better.   Also that was an old technique in exploration,
with vibroseis@ they may not have to do it any more.   Also a quiet beats
filtering any day.

> >  Second.  There is a new chip that is up to a 6 pole analogue filter.
The
> >  configuration of the filter is set in a device similar  to an EPROM
burner.
> >  The software that comes with it has full design features such as
simulation
> >  of amplitude and phase.
>
>     Filters of this type are produced for use in hearing aids, to vary the
> audio characteristics over a wide band. In your application, it is
probably
> better to use four pole Bessel filters, which are relatively easy and
cheap
> to make with a couple of OPAs. When you go to six pole filters, the delay
can
> get rather long. You may find that shifting the corner frequency downwards
> just a bit is actually preferable to adding more filter stages. If you
have a
> two channel recording system, could you use it to compare the simultaneous
> results of various filter choices? If you initially use 'plug in'
> (with silver plated contacts), you can easily change the filter
> characteristics.
>
>     Regards,
>
>     Chris Chapman
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I am going to send this whole thing to one of my academic friends to see
what he says.

I suspect some of these tricks might have been used in the good old days
when we were monitoring the nuclear detonations of the former Soviet Union.
There were some very long dedicated arrays focused to specific points in
Asia.

I do not have a machine right now, I hope to have one by the end of the
Summer as I want that as a Summer science project for my daughters.  I am
working on a computer interface now.  We live in a noisy environment which
is why Sean Thomas' comments hit home.

Thank you

Tom Schmitt

tschmitt@..............

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