it is possible that tobi used a shadow clone to plant the land mines, and his real body was in fact far away. far enough away to be out of the blast radius, and make it back to the hidden rain soon after the battle. This would make sense because for one, Tobi probably felt that Sasuke would win the battle, and that Deidra would be forced to use drastic measures. Also, due to Deidra's superiority complex, he possibly could not take losing again, and this could be predicted by Tobi. with this info, he possible knew it would be in his best interest to keep his actual body as far away from the battle as possible.