What method is used to achieve this look?
I think a chainsaw was used to get this look. The grooves seem deeper in the middle, suggesting a curved shape. The middle parts could then easily have been chipped away by chisel or hammer, or even broken away by hand.

Lots of wood art is created this way, and these grooves usually serve as a starting point for getting the rough form out of a block of wood. Obviously, it's quite beautiful on its own, as well, exposing and emphasizing the natural texture and shapes of wood.