Long range ultrasonic testing (LRUT) is an advanced Non-destructive testing (NDT) technique that employs ultrasonic guided wave (UGW) for the inspection of large complex structures such as pipes, rods, cable, and rails. This technique is widely employed in recent years in field inspection which can monitor long lengths of pipelines rapidly and identify defects (e.g., corrosion, erosion) from a single test location. UGW operates at kHz range (20–100 kHz) to transmit the waves using a ring of dry-coupled transducers around the pipes. These waves propagate along the pipe and reflect back to the source of excitation (ring(s) of transducers) when interacting with discontinuities along the structure. The transducers then record these reflections and mode conversions in order to produce a rectified A-Scan in a PC/Laptop and locate the structure features of interest.
In those systems, signal interpretation can often be challenging due to multi-modal and dispersive propagation of UGWs. This results in degradation of the signals in terms of signal-to-noise ratio (SNR) and spatial resolution. The aim of LRUT is to generate an axisymmetric wave mode in order to promote non-dispersive propagation; however, the interaction of the UGWs with the non-axisymmetric features can cause mode conversions. These mode conversions along with waveguide geometric features result in generation of dispersive wave modes (DWMs). If a wave mode is dispersive, the different frequency components in the signal travel at different velocities so the signal duration increases which compromises the spatial resolution (the ability to distinguish echoes from closely spaced reflectors).
In this work an advanced signal processing technique, called split-spectrum processing (SSP) technique proposed in order to enhance the SNR and spatial resolution of UGW signals by minimizing the effect of the dispersive wave modes (DWMs). An investigation is provided to clarify the sensitivity of SSP performance to the filter bank parameter values such as processing bandwidth, filter bandwidth, and number of filters. As a result, the optimum values are estimated to significantly improve the SNR and spatial resolution. The proposed method is synthetically and experimentally compared with conventional approaches with the application of different SSP recombination algorithms. The proposed methods substantially improved the performances of SNR by an average of 27.5 dB. The outcome of this work paves the way to enhance the reliability of UGW inspections by increasing the SNR and spatial resolution of the UGW response that could lead to detect smaller defects and increase the inspection range.