“BTW, I have yet to encounter dramatically mineralized ground here in the US. It may seem bad when using a VLF but is quite mild with the PI's I have used”
Terry… in reference to the above statement contributed by Reg to the magnetic viscosity thread… remember that those words are used in the context of the overall discussion that is about magnetic viscosity and not just about magnetic susceptibility. I doubt any of us have not encountered high magnetic susceptible ground conditions, for example black sands or mafic rocks for which either the TDI or Infinium can deal with effectively whereas VLF prospecting-capable units struggle by comparison.
In layman’s terms, magnetic susceptibility in a metal detecting context refers to the ability of a soil’s iron minerals to attract a magnetic field. The “follow the black sand” feature on the GMT, similar to the Fe3O4 bar graph on the F75 or other units, measures the strength of the ground’s magnetic susceptibility.
It is my understanding that magnetic viscosity refers the ease with which particles or atoms respond to a rapidly changing magnetic field. I think of it as a sort of friction or resistance by iron ground minerals to respond to an electromagnetic field.
I’m not suggesting for one moment that these two PI units are ideal for precious metals under roughly a gram or so and I’m also not suggesting that either unit performs, for example, with a depth / sens of a Minelab GPX unit. While either unit can and has detected nuggets down into four to five grain range on occasion, in my experience both these units have missed much larger pieces.
So for my purposes, the question comes down generally to size of nugget for which these two PI units are practically useful. In bad ground where VLF units struggle, probably something on the order of a gram or larger would be a reasonable gold nugget size to pursue IMO. Many factors affect detectability as you know… depth, ground mineral conditions, disturbed or undisturbed ground, soil moisture, electromagnetic (EMI) interference, detector settings / coil type and size, operator skill or awareness, and the physical / chemical characteristics of a given size nugget… particularly shape, character / structure.
I think it reasonable to conclude that operating over tougher magnetic susceptible ground, there will be nuggets that either VLF or PI units will overlook due to one or more of the above listed contributing factors.
Jim.