Did you get an answer to your question about locating an impact?

You can do this with geophones, but the solution is more complex than it
first seems because the average velocity will increase with distance
between the sensor and impact point.  The reason is that the "first
arrival" will take different travel paths to get there in the shortest
time. When close up, the waves will travel directly between the impact
and geophone.  At greater distances, the waves will go down to faster
materials (like bedrock), then sideways, then back up again.  There may
be several layers with different velocities.  All of them will need to
be measured in advance or computed by trial and error.  You can solve it
with three geophones, but more will give a better answer.

I would suggest that you use microphones instead of geophones and listen
for the sound of the impact.  Sound will travel about 1100 ft/sec and
will be reasonably constant (except for temperature variations) over
distance.  A bandpass filter could be used to look for the
characteristic sound.
-- 
Doug Crice			 http://www.georadar.com
19623 Via Escuela Drive		      phone 408-867-3792
Saratoga, California  95070  USA	fax 408-867-4900
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