I'm envisioning the opposite of a ferrite core, more like an exterior casing to concentrate flux such that the amount of magnetic field energy is maximized towards the ground and minimized towards open air. Since half of the field of a coil radiates upwards and thus is not particularly useful for detecting, it seems to make sense to put it where it's more useful - if possible?
Here is a cutaway diagram. Would this not have any benefit, at least for near surface flux density concentration? At the field strengths of PI machines I'd guess something like fairly thin 24 gauge steel would be more than enough, saturation is probably ok since the goal would just be some improvement, not capturing all the upwards flux.
Is there a way to use that other half of the "wasted" (using that term loosely) magnetic field energy more usefully downwards if this doesn't work? Sort of like the idea behind a Halbach Array, except whatever the EM version would be?
Weight alone doesn't have to be a deal breaker. For instance if a hypothetical 12" coil with flux shaping components weighed the same as a 20" standard coil, can it be made to obtain similar depths? If so, then it'd be worth it due to maneuverability with all other things being equal.