Abstract: A passive method for locating the position and inclination of a flame front, within an oil-shale retort of known dimensions and location during an in situ combustion of the retort involving detecting the sound generated by the flame front, by two matched detectors separated by a fixed known distance. The pair of matched detectors are suspended vertically in a liquid-filled well which was drilled essentially parallel to the side wall of the retort. The outputs of the two detectors are fed directly to a differential amplifier and the resulting difference signal is monitored as a function of depth as the pair of detectors are raised and/or lowered in the well. The minimum in this signal corresponds to the position of the flame front within the retort. Repeated measurements in various observation wells establish the inclination of the flame front.