Change your selection mode to Edge Mode and switch to the front viewport.
Activate the Select Edge Ring tool.
Use the Select Edge Ring tool by double clicking on one of the verticle edges, it will select the ring associated with it. Double click and select the three verticle edges that are in the indent of the helmet.
Switch back to the normal selection tool (Keyboard shortcut Q). Holding Control draw a selection box around the edges outside of the helmet, deselecting them.