Expert: Robert Cummings, P.E. - 2/19/2008

Question
QUESTION: scaled distance of 100 to .." Your response was excellent! Do you bury the triaxial seismic geophone or comply with the ISEE and surface mount it in non-disturbed surface grade for accelerations below 0.2 Gs? www.nashvilleblastingvictimsunite.org - Tennessee

ANSWER: Some pracitioners bury the seismograph sensor which is hard to argue with, but I question the need and practicality of doing so unless it is a semipermanent installation. I have consulted on jobs where a third party came out to assess compliance, and buried its sensor when I had my array all on the ground surface.  There was no difference but then we were in a compliance situation -- the ppv values were designed to be below 0.25 ips.

The whole issue is whether the geophone will remain coupled to the ground throughout the entire duration of the ground motion.

Where I practice the ground is so variable that the focusing and inhomogeneities would mask the difference in most practical blasting situations. Plus, you can introduce a lot of disturbance burying the sensor.  

It always has made sense to me to spike the sensor tightly against the ground surface after removing loose leaves, loose soil, etc., when the accelerations are not large enough that decoupling is possible.  Where you run into problems, even in compliance situations, is if the sensor is not tightly spiked against the ground or where the spikes are in material that does not "grab" the spikes (real wet soil, ice, or loose sand).  In those cases, or if the sensor is likely to see more than 1 ips I like to use a LARGE sandbag (one with plenty of weight that covers the sensor and rests on the ground on all sides) just to be on the safe side.

If I thought >2ips likely I would consider burying the sensor.

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QUESTION: The ISEE Standard calls for surface mounting, as you practice and also placing a bag of sand on the surface mounted probe, as you practice. The Characteristic Resonant Frequency of the Triaxial Seismic Geophone "HEAD" is 5.5 HZ - 10.5 HZ. Is this a concern when burying it?

ANSWER: I don't know why the resonant frequency would make any difference whether the sensor was buried or not.

---------- FOLLOW-UP ----------

QUESTION: If the "Buried Seismic Probe Geophone Head - Electronics and Differential Magnetic Coil pickup instrument design" is vibrating at the same frequency of the subgrade Blast Impulse Wave energy - "Raleigh Wave" - then will it not be adversely affected in recording the Raleigh Wave IPS? Most probable a significant reduction of IPS PPV? DanFairfax@juno.com can email you a 2007 Research Paper if you send a personal email .. Dan Fairfax.

Answer
If the geophone is vibrating in resonance with the incoming ground motion then a prolonged exposure could enhance the geophone coil's motion amplitude.  How that affects the numbers reported by the machine would be specific to each machine's design.  Whether it occurs at all would also be specific to the geophone's design.

an e-mail where you could send your research paper would be sgsibob@aol.com