The minefields described on p. 48 of Ogre Miniatures are smart , and have no effect on friendly units. But other types of mines are possible. These cost only half as much as regular mines, and explode with the same effects. Compromised minefields are those whose precise type and location is known to the enemy. A defender can take a compromised minefield for half price, but must tell the enemy exactly where its borders are, and mark them clearly on the map, before the enemy even chooses his forces.
Effects are normal.
Dumb mines treat all units as hostile. Yes, this means that you can take six square inches of dumb, compromised mines for only 1 point. And, for balance, the referee should not allow objectives to be set up on the edge of the map and ringed with huge minefields. The referee can always limit the total number of square inches that can be mined. A referee may also set up interesting special effects.