Abstract: Apparatus is disclosed for providing one hundred percent inspection of natural gas mains under operating flow conditions. The apparatus includes a scan unit assembly which is inserted into the gas main to collect data for the production of graphic images of the wall of the gas main. The scan unit includes a plurality of scan rollers, each containing an ultrasonic transducer, which traverse the surface of the wall defining the inner diameter of the gas main. The ultrasonic transducers transmit interrogating pulses into the wall of the gas main at a pre-determined spaced apart interval resulting in a helical scan pattern of interrogating pulses. The return pulses from the surfaces defining the inner diameter and outer diameter of the gas main and any imperfections and/or flaws within the wall of the main are received by the transducer and transmitted to a display unit for a visual indication (a tomograph) of the wall of the gas main.