Set your env_explosion (give it a name like explosion01) on the ground, then draw a brush around it with a trigger texture. Tie the brush to trigger_once (call it trigger_explosion01). In the outputs, set OnTrigger to Explode your explosion01. Your flags should be okay by default, as you want just players to set it off...but you can always set it to Everything, if you want people to be able to throw stuff across the field to set off the mines.