PSN-L Email List Message

Subject: Re: Noise Reduction
From: Doug Crice dcrice@............
Date: Fri, 30 Jun 2000 06:37:17 -0700

I'll let one of our mathmaticians answer that one.  When you get to a
certain point in life, you remember the results but not the derivation,
especially in statistics.  I believe that the basic problem is that each
sensor picks up some random noise along with the signal.  So when you
add up the signal from N sensors you also add up N sets of random
noise.  When random noise signals are added they get bigger by square
root of N.  The signal gets bigger by N so signal-to-noise improves
N/(square root of N).

I don't know the answer in Winquake.


Dick Webb wrote:
> How does the square root of N fit in?  What is the rest of the
> equation.  Also, do you know if is it possible to sum inputs in
> Winquake.  Our export the Winquake data into, say Excel, and sum it results
> there.
> Dick
> At 09:22 PM 06/29/2000 -0700, you wrote:
> >Dick Webb wrote:
> > >
> > > PSN'ers,
> > >
> > > I have a question about noise reduction.  I have 3 short period
> > > seismometers.  If I place them several hundred feet apart can I sum the
> > > signals and expect that local noise will be cancelled?  Local noise could
> > > be due to trees, water, wind, local traffic, etc..  Since the seismometers
> > > are not closely spaced, the signals would arrive at each seismometer out of
> > > phase with respect to the other seismometers.  Regional signals would tend
> > > not to be cancelled out due to their longer period.  Is this hypothesis
> > > correct, or is it a sand castle in front of a wave?
> > >
> > > Dick
> > >
> >
> >Arrays of sensors are routine in exploration geophysics.  A string of
> >say 12 geophones are laid in a line to form an array.  Reflection
> >signals, which come pretty much straight up in petroleum surveys, hit
> >all the phones at the same time and sum.  Surface waves, noise, and
> >shallow refractions come in horizontally or at an angle and may cancel
> >or at least not stack.  The arrarys are often designed to form spatial
> >filters.
> >
> >So, the short answer is using three sensors will improve your ability to
> >see stuff coming from the bowels of the earth.  However, the signal to
> >noise improvement is square root of N, so the improvement may not be
> >worth the effort.
> >
> >Doug
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Doug Crice
19623 Via Escuela Drive		      phone 408-867-3792
Saratoga, California  95070  USA	fax 408-867-4900


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