I heard about this a long time ago too. I would think the petroleum industry has technology that could be used to map the rock layers underneath the island. I was hoping someone would have done something like that and had new information.
there actually was a survey done quite 'recently' (in relation to its now over 200 year history )
information about "The Woods Hole Oceanographic Institute Report" can be found here:
http://pws.prserv.net/djoltes/oakisland/whoi.htm
although it was done quite hastily, so it might have missed out on certain things or might not have taken all relevant facts into account