I have the same thing around my 7 acres.  I used pivot corner system wire and laid it on top of the grass.  I've had to fix it a few times when I forgot about it but the system has worked very good.  My dogs (year old female lab and 5 year old hound mutt) did get out last week for some unknown reason but it was when my boy was running around the edge of the yard so they had some "motivation".  I change the collar batteries every three months and so far use stimulation 2 on the older dog and 3 on the lab.  My older dog is a wimp.  The collars have 5 levels of stimulation.  1 is audible only and 2-5 are audible with shock.

FYI....the lab often runs around with the fence collar and an additional training collar.  I tried with only the training collar for a while but then you are constantly wondering where the dog is.....this way I don't worry when I leave the yard or go to bed.