Long-Distance Inspection of Pipes Using Electromagnetic Acoustic Transducer for Cylindrical Wave

A nondestructive inspection of gas pipelines is urgently required. However, inspection is impossible in many cases, because the pipes are underground. Although a cylindrical wave is suitable for such inspections, because it can travel long distances along a pipe without distance attenuation, conventional transmitting and receiving technology for the cylindrical wave has not yet been established. Therefore, we have attempted to develop an electromagnetic acoustic transducer (EMAT) that can transmit and receive cylindrical waves using an electromagnetic force. A cylindrical wave is a type of guided wave with several modes (L-, F-, and T-modes) determined by the diameter, the wall thickness and the driving frequency. The L-mode has also the vertical components of oscillation which are axi-symmetric. Therefore, we considered that the L-mode cylindrical wave could travel long distances in the axis direction of a pipe, without ultrasonic attenuation. Finally, we have optimized the number of turns in the sensor coil and the drive frequency in the single-channel-type EMAT and the multi-channel-type EMAT. In addition, we evaluated the ultrasonic velocity and the ultrasonic distance attenuation coefficients of the L-mode cylindrical wave. As a result, we obtained the best received signal using the 2-channel-type EMAT with 5 turns in the sensor coil for the transmitter, and 30 turns for the receiver.