A method and apparatus is applicable to in situ heating of oil shale or tar sand. The heating is by radio frequency that is applied down hole by a central conductor that extends beyond a coaxial shielding conductor to form the antenna or applicator. Encroachment by the heated formation is overcome by...http://www.google.com.au/patents/US4576231?utm_source=gb-gplus-sharePatent US4576231 - Method and apparatus for combating encroachment by in situ treated formations
