## 1 Introduction  
For more than half a century, ultrasonic testing (UT) and acoustic emission (AE) have played key roles in the field of nondestructive evaluation (NDE) [1, 2], and more recently contributed substantially to the successful development of structural health monitoring (SHM) [3]. Among the methods that are based on elastic wave propagation, those using guided waves (GW), e.g., Lamb and Rayleigh waves, have made significant advances in recent decades, benefitting from improved computational mechanics, signal generation and sensing methods, and signal processing techniques [4, 5, 6]. Two reviews provided detailed mathematical developments on guided waves from [PERSON]'s works to the present, covering more than a century [7, 8]. The first successful NDE application of guided waves occurred at Aerojet for the AE inspection of Polaris missile chambers in the early 1960s [9], followed by many more AE tests of rocket motorcases. Soon after, AE inspection was expanded to various pressurized installations, including pipelines, tanks, vessels, etc., of both metals and fiber-reinforced composites [2]. In 1967, buried pipelines were inspected using 300-m sensor spacing and detected corrosion pits at 100-mm location accuracy [2] (p. 166). By the 1980s, many industrial tanks and vessels in the USA were regularly inspected under the ASME Boiler and Pressure Vessels Codes and ASTM standards [10]. These AE tests were, however, conducted without benefiting from the theories of GW propagation, since field measurements of wave propagation velocities were successfully incorporated into the test routines. Up to 1990, only three AE papers were found to use the Lamb wave dispersion curves. Two were for detection frequency selection by [PERSON] [11] and [PERSON] and Alers [12]. The third was on long distance pipeline AE inspection by [PERSON] [13]. The GW attenuation was measured on long pipelines of up to 300 m [2,13], and the [PERSON] results at 30 kHz were analyzed in [14], separating signal reduction into geometrical spreading and steel absorption components. The GW attenuation was very small, or 0.17 dB/m, demonstrating the benefit of GW utilization on pipelines. In 1991, [PERSON] introduced mode-dependent velocities of GW, specifically Lamb waves, to properly conduct AE source location in thin structures [15]. In the 1990s, numerous computational studies started to make GW properties more accessible, even for anisotropic materials. A series of studies by [PERSON] and coworkers [16] used three-dimensional, dynamic finite element modeling and correlated source motion to displacements at sensor positions, accounting for GW propagation effects through a plate. These studies have been extended to fiber-reinforced composites [6]. GW-based AE studies (or GWAE) also profited from wavelet analysis as mode separation with time and frequency was visualized. [PERSON] and coworkers [17] developed wavelet analysis software based on Gabor wavelets and made available the source code written by [PERSON] This code was implemented by [PERSON] in 2003 into an easy-to-use free software, AGU-Vallen Wavelet [18]. With continual support of Vallen Systeme (Wolfrathausen, Germany), this wavelet software has assisted GW studies and NDE engineers everywhere. [PERSON] gathered main results of early studies of Lamb and Rayleigh waves [19]. In addition to velocity dispersion curves, parametric equations of attenuation of Lamb and Rayleigh waves were given. Around 1970, the GW dispersion curves of only a few materials (including Al, steel, and polymethyl methacrylate (PMMA)) were commonly accessible [20,21]. In the following 50 years, GW mechanics and NDE based on GW ultrasonic testing (or GWUT) have been refined, passing many milestones. Long-range inspection of pipelines has been practiced for two decades [22, 23, 24]. GW dispersion analysis software packages became available for GW velocities and attenuation [25, 26, 27].