Make a gamelogic in the center of your minefield, and name it mine_logic1. Now create the little script below and name it qmine.sqs :
_pos = getpos player
_gren = "handgrenade" camcreate _pos
exit
Now, in your trigger's Condition field, add ?((player distance mine_logic1) <= 250) . Now put this exec "qmine.sqs" in the On Activation field.
Note that you could do all of this by trigger, but I prefer to keep my triggers clean and use short scripts whenever possible. Besides, using scripts makes it easier to go back later and add complex explosion effects ;D .