What about adding a direction of probing? So instead of detecting ore within the radius (which could be huge) you can detect it and find it a bit easier. Plus this seems more realistic as a probe means like poking thru the ground to find something. Don't think theres any kind of multi-directional prober in the 1500's
I like how it is atm, it's hard to find ore, that's important, it makes it rare and it requires some deductive reasons. Finding our iron ore supply was one of the most fulfilling things I did so far