If neither search yields a clear site then the destination location and the preferred direction is used, the Y coordinate will be clamped such that it is between 70 and 118, and a small 3x2x3 volume will be constructed out of obsidian and air, such that the active area of the portal has an obsidian floor to step out on each side, and air to step out into. This replaces the original blocks in the original 3x2x3 volume.

More: Nether Portal Science