That is going to be the most difficult hurdle to overcome. Presumably the area of the buried coil is the area covered by several vehicles? So any cone-sized coil will have an extremely low coupling coefficient with the buried coil.
I like Ronsimpson's idea of working this backwards. Disconnect a coil from the controller and inject into it a (legal) RF signal. Use a portable radio to pick this up at the roadside. There's a beneficial side-effect: the guy with the radio gets some healthy exercise going from one loop to another.