There is another method called "impact echo". The following link is a comercially available unit. Our firm has had some success with this unit provided field conditions are right.
http://www.olsoninstruments.com/ctg/ctg.htm
Good luck
sghvib
Is there any research/publications that address ground borne vibration attenuation with changes in grade elevation? For example say there is construction blasting on a hill. The hole depth is < 10 ft. there is a ravine next to the hill that goes down 25 ft and then grade rises back up 15 ft...