The first echo is located right beside the vending machines by the entrance. The second one is located in the encampment right before the bridge that leads to the main excavation site. Standing next to the Air Dome Generator while facing the bridge, it is in front of the small building to the right. The third is inside the room with a re-spawn point just after the bridge. It is sitting on top a crate. The fourth is inside a building directly ahead inside the gate into the fortress. It is sitting on one of the computer things in the back of the room.