To be a geologist, you can either go for M.Sc. or M.Tech in Geology. For M.Sc., one can take up the JAM entrance exam and for M.Tech, one will have to appear in GATE entrance exam. It is highly advisable to check the eligibility criteria for both the exams and then proceed further. Geology in M.Tech is offered in some of the IIT's like Kanpur and Roorkee.

Good Luck!