Proximity Mines

From BioShock Wiki

Proximity mines are used by both the player and Rosie style Big Daddies. These mines are fired out of the Grenade Launcher. When fired they land on the ground and emit a red ring, when a enemy enters this ring it will explode causing damage. This ammuntion is similar to the Trap Bolt in use and can be paired with the Target Dummy Plasmid to lure your enemies into your traps. Another popular method that is more direct is to fire your mines onto a movable object, like a barrel, and flinging it at your enemies causing them to explode on contact.