Doug,

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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