BP will have all kinds of 3D seismic data from surveys that they used to discover this reservoir in first place. They can use that data to compare with new data - a fault or a new fault would have been obvious.
Somebody linked to a site that contains some good information and some great graphs.
It takes a while to wade through, and sort out the wheat from the chaff, but it is worth it.
http://mentaljudo.blogspot.com/2010/07/whats-inside-doughnut.html