Method for determining the position and inclination of a flame front during in situ combustion of an oil shale retort

A method for determining the position of the flame front during an in situ combustion of a rubbled oil shale retort wherein said rubbled oil shale retort is constructed essentially vertical of known dimensions and known position having essentially vertical sidewalls and said flame front is intended to be an essentially horizontal plane passing vertically through said retort during combustion involving the steps of:

• (a) initiating a seismic signal on or near the surface of the earth at a position to one side of an essentially vertical sidewall of said retort such that the angle of incidence of the seismic signal to said sidewall inherently results in predominantly downward reflected seismic energy,
• (b) positioning at least one seismic detector means on or near the surface of the earth at a second position to the same side of said essentially vertical sidewall intended to reflect said seismic signal,
• (c) detecting the reflected seismic energy as a function of time by using said seismic detector means, and
• (d) determining the position of said flame front from the enhancement of the relative amount of seismic energy being reflected back to the surface of the earth associated with high temperature induced refraction of said initiated seismic signal which occurs in the vicinity of the flame front.