A link to area 51 (among other things) can be found here
The markings are northwest of the base. Interesting. If I lived near the base and had lots of land, I would have some fun with it and make a big ol' alien landing strip.
Then again, you can see a road going to the center of the circles. It may be a target of some kind. Easy to see from an altitude plus it has a bullseye and a big number "3".