A method of surveying the condition of an underground conduit by positioning a propelled carriage assembly within the underground conduit. The carriage assembly includes (i) at least one transmitter/receiver unit capable of transmitting a pulsed signal toward at least a portion of an inner wall of the conduit, and (ii) a secondary sensor positioned on the carriage assembly. The data derived from the pulsed signal at a given lateral location within an underground conduit is read as is a secondary sensor condition derived from secondary sensor data taken at the given lateral location. Then it is determined whether the secondary sensor condition indicates a basis for a false void detection by the data derived from the pulsed signal and if the basis for false void detection exists, providing an indication of such basis.