For this achievement you need 100 logs. There are a possible 151 logs. However there are 2 logs you obtain automatically. I give full credit to the user 360GameTV for this video. Note, that in the comments he/she posted hyperlinks to send the video to which point you need to go. The video gives you the location of all collectibles not just the logs. Hope this helps and be sure to like the video for the user. They did a great job!