Lara Croft Guardian of Light - Flooded Passage?

In the level "flooded passage" when you have left the artefact room (which is to the left of a larger area with lots of green poison plants) and go up stairs there are four wooden poles which block the way. I think you have to shoot the target which is underneath you on the wall. I've used the grapple to get down there but I can't aim at the target. I have no idea how to get past these wooden poles... help anyone?