I'm planning on using a peizo disc element for the mic.That seems to be what the expensive ,purpose built units use. A 43mm fixed into an aluminum puck,drilled and tapped for a probe rod and surrounded by a thick silicon cup to block above ground noise. A 43mm disc is the largest I've found,I guess bigger is better?
I REALLY appreciate the input but, please keep in mind that while I'm comfortable building such a unit , The terminology can get over my head quickly. I've been to Wikapedia several times already. So, go slow with me.
Thanks again , Mark