One or two screen pixels per explosion. The best way to deal with minefields in TacOps is to go around them if possible. The current scenarios do not last long enough to justify having engineers in the game to do it right. If you feel you must go through, the best current way in TacOps to breach is to send an infantry unit through. Infantry draws a lot less booms per meter than vehicles.
Units mark a path as they progress through the minefield with or without booms.
I have no plans of adding foot engineers, but I will likely eventually add mine plows and vehicles that throw line charges. Arty does not degrade minefields in TacOps. Why can't I place artillery-delivered minefields and fire missions at the edge of the map. This is partly a needed program constraint. Mostly it is an arbitrary response to one of my early game opponents placing artillery delivered mines right up against the edge of the map in order to nail my units as soon as they came on the map - boring.