A method for locating the position of a flame front within a rubbled oil shale retort of known dimension and location during an in situ combustion of the retort involving initiating a seismic signal at a selected position on the earth's surface relative to the retort, detecting the reflected seismic energy, and determining the position of the flame front by virtue of a maximum in the received reflected/refracted seismic signal. Repeating the process at a plurality of positions relative to the burning retort establishes the inclination of the flame front.