Stefiod EM is when they over desired area with a plane or...

Stefiod EM is when they over desired area with a plane or helicopter fitted with eletromagnetic survey equipment this picks up the signals given though the ground from metals the EM records the anomalies and give the surveyor the positions of the anomaly readings
why it can't tell you how much. The strength of the readings give you an idication that these areas contain sufficent base metal to drill and see depth and with and quality. its certianly not cheap but saves heaps of time.
Also the levels of silver is excellent, with silver 17 to 18 dollars an ounce and rising this certain help in the feasability study especially when you are talking a couple of million tons of dirt 180 ounce equal $3400 us
x by a million pays for mining costs and the rest is cream, very nice.