A method and apparatus for sensing conditions in a subsurface geologic formation heated for the extraction of hydrocarbons is disclosed. A time domain reflectometer in conjunction with an open wire transmission line is employed in real time to determine impedance discontinuities in the geologic formation. These impedance discontinuities correspond to physical conditions in the geologic formation. The open wire transmission line can include pipes running into the subsurface geologic formation or other conductors, including a split tube well casing. The method may operate in the low frequency window for subsurface electromagnetic propagation.