Abstract: Defects in an underground channel, such as a sewer, are detected by an infrared scanning device (preferably an infrared video camera) moved internally along the channel. The scanning device senses temperature deviations which mark defects in the channel, as, for example, deviations due to the inflow of cool extraneous water into a sewer. The scanning device is in communication with an above-ground central unit which processes the scanning device output and displays and/or records thermal representations of the defect. The central unit also monitors the location (distance travelled) of the scanning device so that the locations of defects may be determined.