Have you considered the field may not be uniform?
I think you will find that mass takes time to respond the field, and the field will not be be uniform as any emf field created will be affected by other emf's with equal and opposite forces.
Here is a good visual demonstration of how a rotating axial magnetic field affects a ferrofluid mass;
Note how the shape of the ferrofluid evolves over time. You will note many of the galactic shapes will be seen as it evovles. Now imagine that field if it had competing emf's to react with.
This MIT clip is very informative on many, many levels. From galaxy formation, to plasma geology, to even a possible theory on the origins of the blueprints for life.
Nature always has competition, even in the emf arena and all things respond to competition.
Just a thought. Your mining event has multiple levels of emf competition.Lorentz force law method
A charge q in the wire on the left side of the loop experiences a Lorentz force q v × B k = −q v B(xC − w / 2) j ( j, k unit vectors in the y- and z-directions; see vector cross product), leading to an EMF (work per unit charge) of v ℓ B(xC − w / 2) along the length of the left side of the loop. On the right side of the loop the same argument shows the EMF to be v ℓ B(xC + w / 2). The two EMF's oppose each other, both pushing positive charge toward the bottom of the loop. In the case where the B-field increases with increase in x, the force on the right side is largest, and the current will be clockwise: using the right-hand rule, the B-field generated by the current opposes the impressed field. The EMF driving the current must increase as we move counterclockwise (opposite to the current). Adding the EMF's in a counterclockwise tour of the loop we find