The unclassified EMP states up to 50,000 V/m from about 5kHz to 1GHz
The pulse rise time is about a nanosecond (that’s where the top end 1 GHz comes from)
“The officer wasn’t sure ANYTHING would be sufficient to shield against such a pulse.”
Steel will do it - low frequency permeability ducts magnetic flux around the shield. At higher frequencies it’s conductive enough to suffice.
“The brother set up an experiment where he created a Mu-metal cage around a detector on the theory that the cage would route the field around the detector.”
Not at the very low frequencies, low magnitudes involved in your described test scenario - the low-level magnetic pulse will blow through the shielding because there isn’t enough magnetization energy. This is a big problem for sensitive instruments such as newer electron microscopes.
At even slightly higher frequencies, passive eddy current shielding works very well for magnetic fields (such as 60Hz)
So really - what we each said doesn’t contradict each other ;-) You have a higher magnitude than my memory has stored in it - but we’re in the same ball park.
My statements are from memories at a ham club meeting I attended in 1982 - so please forgive the slight variance ;-) The officer had come down from Vandenburg AFB to Santa Barbara.