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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