If there is something good under there, then the entire circle drive is a cash cow, cuz it sings positive VDIs all way around. It is the reinforcement, because my septic tanks ring good VDI also, and I am pretty sure there are no targets I'd want to have underneath THAT!
I actually got a reply elsewhere about the drive cement, and that there are wire ties in the reinforcing grid, so I understand the signals being different and positive now. Thanks. martin