Treating even a moderate carpet contamination problem from the top of the carpet is not necessarily going to be enough to completely solve the problem. The contamination shown here was the work of just one cat over a period of several months. If the cat had been using only one area (which is normally the case), the contamination and resulting damage would have been considerably worse and the carpet backing would probably have been rotten and unrecoverable.
Please do not take this case study or its conclusion as an indictment of cats or pets in general. IT IS NOT MEANT TO BE ONE! I have a dog and 2 cats pictured on this site, and would not trade any of them. It is simply meant to demonstrates that what appears to be a small problem can be much larger than expected. Pet urine contamination of carpeting is a serious and often expensive problem that is very important to detect and correct as soon as possible.