Multi-frequency machine in the hot Virginia dirt

Thoughts on whether a multi-frequency machine would work better in the hot Virginia dirt

Minelab E-Trac or Exp would be my recommendation. And if you are REALLY brave..... a good PI will trump all....and by a huge margin. But..... you must understand that no ID is the 'norm' for PI units.

Is there anything inherent in the technology of PI detectors that prevents discrimination?

Yes. They can not see phase shift angle. (The component needed for ID).