The ones for the GPZ, according to the Minelab ferrite white paper, need to be within a specific tolerance to be useful with it's ground balancing algorithms. Apparently you can't just use any old ferrite?
By using them you may likely upset the natural ground balance or even knock out specific signals. In saying that I do know that ferrites were used on VLF's to assist ground balancing & have read of at least one prospector using them on a GPX but that was for very high iron content/magnetic ground.
A lot of GPZ users, including myself now, don't routinely use the ferrite balancing procedure as it's not necessary in all ground types. I would only use it now if having issues in the above ground conditions & always carry it if needed. I find that the walk & sweep method minus the ferrite works well enough around here.
