I think the web site http://www.gns.cri.nz/earthact/probe/index.html
does an adequate job of explaining what a Stephenson probe is, so
probably the correct question is why?

It is well known that earthquakes cause much more damage to buildings
located on soil than on rock.  This is because the soil resonates and
amplifies the vibrations not unlike a bowl of jello will shake when you
bump it (not to mention the separate problem of liquefaction).  The
worse (and thicker) the soil, the worse the problem as evidenced by the
extensive damage in the Marina district from the 1989 Loma Prieta
Earthquake.  That soil was mud dredged up from the bay, and thus is
younger even than the city.

So, structural engineers take an interest in a factor called "ground
spectral amplification ratio", which is a measure of the increase in
acceleration between bedrock and the surface (and the dominant
frequencies caused by the resonating soil).  In some cases, buildings
are designed to resonate at different frequencies than the ground,
minimizing the destruction.

The usual investigation procedure is to measure the velocities of P
waves and shear waves on different layers in a soil structure. Those
parameters and the density are plugged into a program called "Shake",
which predicts the ground spectral amplification ratio. Anybody who
wants to learn more about the procedures for making these measurements
can download my paper on "Borehole Shearwave Velocity Measurements" from
http://www.georadar.com/Shearwaves2.pdf 

Now good scientists occasionally like to test their theories with actual
measurements. One way is to plant accelerometers in a soil foundation at
different depths, for example, one at the bottom in bedrock, one mid
depth, and one near the surface. Then, you simply wait for an earthquake
and record the actual vibrations which are readily converted into ground
spectral amplification ratio.  You can compare actual results with
predicted ones and present a paper at a scientific meeting showing the
comparisons.

The way this is usually implemented is to drill a borehole down to
"rocklike material" and install borehole accelerometers wired to a
surface recording system.  The sensors can be clamped in place ot
grouted in place, depending on whether you ever want to see them again.

The "Stephenson Probe" is merely a mechanism to circumvent the process
of drilling and preparing the borehole.  The accelerometer package is
pushed into the ground on the end of a rod by a standard vehicle widely
used in soils testing (called a penetrometer). Soils engineers believe
rightly that the harder it is to force a rod into the ground, the better
the foundation material. The method of installing accelerometers at
depth is more economical than the conventional method and Mr. Stephenson
has attached his name to the device.

Casey Crane wrote:
> 
> Hey y'all,
> What is a stephenson probe ?
> 
> Casey
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-- 
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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