How to make raycast do not ignore terrain when the start point is below terrain?

My case is : I have some turrets underground( y position <0, terrain y position = 0), I don't want them to fire anything on terrain. But terrain cannot block raycast when start point is below.

 

One quick fix is to reverse your raycast, so that instead of raycasting from turret to target, you raycast from target to turret. That way you'll hit the terrain.

 

Thx, I think I will add double raycast check on both direction, so the turrets above terrain can use the same check loop.