Z. and to add to that of Glorieux, NOT is using MT tecnonlogy and this was used at Voisy.

The high-frequency MT method, called audiomagnetotellurics
(AMT), has recently seen widespread application for
problems related to imaging the conductivity structure within
the accessible part of the crust, i.e., the top 3 km. In Canada
over the last few years, there have been more than 12 000AMT
soundings made for mineral exploration purposes, mostly
around Voisey’s Bay (Labrador), Sudbury (Ontario), and the
Thompson nickel belt (Manitoba), particularly for regional
mapping and for imaging structures at depths deeper than traditionally
mined (>500 m) (e.g., Balch et al., 1998; Stevens
and McNeice, 1998; Zhang et al., 1998). Controlled-sourceEM
methods, used effectively in the past (Boldy, 1981), have their
limitations for probing depths >»500 m, and theAMTmethod
offers greater depth penetration as well as a number of other
attractive features: easier logistics, more tractable mathematical
solutions for multidimensional targets, and the availability
of 2-D and 3-D modeling and inversion codes.

A link to the particulars:

http://www.pinemontgroup.com/87.pdf

Cheers.