No need for cracked software.

Field strength is a bad term in an engineering sense. They should call
it signal strength. In  any event, DC fields don't propagate, but AC
waves do. See the difference.

Self resonant? That would imply it only works at one frequency.

If you were over a tunnel and there was nothing in the tunnel, I don't
see how you would detect radio waves. The notion of use VLF is higher
frequencies don't travel well through earth. If you look up radio
systems for caving, they are VLF based.

Now by your theory, if I point the ferrite bar downward, I have to be
in the forbidden territory or the tunnel has to be in free territory.