I suggest using “intensively” when referring to in-depth research involving intense effort narrowly focused on a specific issue and perhaps conducted by a small group of researchers. In contrast, use “extensively” when describing comprehensive research covering many aspects of an issue and perhaps conducted over a relatively long period of time by numerous research groups. - See more at: http://www.shearsoneditorial.com/2013/02/whats-the-difference-between-extensively-and-intensively/#sthash.qvfFd4gR.dpuf