Hmmmmm. I think I answered this a few weeks ago......somewhere on this forum.
The higher the mineralization......the more attenuation takes place with higher Freq units. Lower Freq units work better in high mineralization; yet, PI's will trump all.
If you have low/no mineralization.............then Freq selection takes on a different meaning/function. Since attenuation becomes a moot point; it would then become a choice for finding low conductors (utilizing higher Freq units).........................or high conductors such as silver and copper; you would then utilize a lower Freq unit.
In general; multi-Freq units will handle bad dirt a bit better; yet, the T2/F75 platform will parallel the performance of a CZ. If the dirt is quite bad; the multi-Freq Explorer's would be the optimal choice.....if ID is a absolute requirement. If not................then................once again....the PI's will trump all.
Multi-Freq units do NOT operate with the same performance..... as mineralization becomes heavier. They too....will lose performance/depth/ID accuracy...................just not as detrimentally fast as single Freq units. Single Freq units (in general) will be the first units to suffer...as mineralization increases.