Audio resonance metal detectors?

Tom, I understand your CZ owners - Salt training 101 experiment, as it pertains to eddy currents and the conductance of gold , I came across a
link to a fellow that is using audio resonance to detect counterfeit coins in Japan. (http://www.nature.com/news/2008/080122/full/news.2008.518.html). The reason I ask this question is because since ground penetrating radar is
using RF frequencies (around 1GHZ) , would it, could it be feasible to concoct a metal detector that uses sound energy directed into the ground and listen for the reflected audio, (much like a PI) , by having it tune up and down a given frequency band until it reaches a the "resonant" frequency of the unknown target. I cannot find any relevant information, even if it has EVER been tried. I do know that audio in the human hearing range can be passively amplified via a small parabolic dish. Anyone with relevant info, please respond.

Sincerely Tri

There are a bunch of factors that come into play .... IRT ultrasound. Ultrasound can be utilized to find large density 'shifts'......such as voids.....and underground rock formations. As far as finding something small ..... like a engagement ring.......... technology is 'not quite' far enough advanced.....for tiny, discrete items. At least...... not at a 'consumer affordable' level..... yet.