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That's not true for N=1,2 or 3. If you mean as N approaches infinity, coverage approaches 97.8%, how do you figure that?

Only if you assume totally flat terrain and 100% probability of the Mine Kafon detonating each mine it passes over, neither of which are likely.

but because of the low cost we can create a lot of these and let them run over a long period of time.

Surely the trip to collect the thing after it passes out the minefield is an issue, it could take days to retrieve.

Piece of string on a reel?