Patent ID: 8210256

Claim:
A method of heating at least a part of a subsurface hydro carbonaceous earth formation, comprising: forming a borehole into or adjacent to said formation, inserting an RF electric heater including two concentric tubular conductors into the borehole, the conductors including top and bottom portions and being electrically connected to each other near their bottom portions, at least a portion of at least one of the conductors comprising a ferromagnetic material, the conductors being connected at their top portions to an AC power supply, said AC power supply having an AC output having a selectable output frequency and current, wherein the two concentric tubular conductors include an inner conductor and an outer conductor, at least one longitudinal segment of at least one of said inner and outer conductors varies in at least one of geometry, chemical composition or heat treatment, and simultaneously using at least two frequencies in said AC output to preferentially heat a selected one of said longitudinal ferromagnetic segments.