Ann: Stunning First Drill Results Lady Fanny - 27m @ 2.8% Copper, page-265

That would have been a high temperature squid probe... very sensitive gear. The probe area would have gone-up, gradient arrays of SQUIDS but basically the same tech if their trying to measure magnetic field change 1 part in 10⁸.

Must have been interesting Monie working with the gear?

The problem with the physics/maths is that there are an infinite number of solutions to an EM distribution measured this way. The numerical modelling of the ore body is only as good as the geophysicist, picking the solutions that match other boundary conditions, measurements and expect lithography. The actual computation has gotten alot faster, visualisations cooler. This type of EM work has become much cheaper and lighter, but I wouldn't say more sensitive or accurate, just that it can be done more routinely, and maybe not used on the tenements to best effect at the time.