Thanks for that info, it gives me some useful detail to consider now.
I suspect a Caesium magnetometer may be capable of reasonably detecting the sort of thing you're looking for at that distance (125ft) but I don't have any, nor have I experience with them unfortunately.
That said I'll see what data I can find, and if it looks to be useful I'll pass it on. In the meantime I can do a little more with the fluxgate devices I have to get an idea of the range possible with them, given a similar target.
FP.