## 1 Introduction  
The CRTS-II-slab ballastless track in the China high-speed railway (HSR) track system is a new type of track structure that is developed on the basis of Germany's Burger-slab ballastless track system. The foundation of the CRTS-II-slab ballastless track in China takes the forms of viaducts or subgrades, of which viaducts account for a large proportion of the rail line. For example, the viaducts of Beijing-Tianjin intercity railway and Beijing-Shanghai HSR account for 93% and 91.8% of the whole line, respectively [1]. The CRTS-II-slab ballastless track on the viaduct consists of rail, track slab, cement emulsified asphalt mortar (CA mortar) layer, concrete supporting layer, and bridge deck. The track slab and the concrete supporting layer are connected by a cast-in-place CA mortar layer, and the concrete supporting layer and the bridge deck are connected by a so-called \"two cloth and one film sliding layer\", forming a monolithic layering structure vertically. The adjacent track slabs are connected by cast-in-place joints in the longitudinal direction, constituting a seamless structure. This type of track structure is widely advocated for its low vibration and noise, good smoothness, and lower maintenance. Nonetheless, during the long-termservice life, the track structure may suffer from various problems, such as cracking of track slab, debonding between track slab and CA mortar layer, and slab upwarping [2; 3; 4; 5]. Under the double effects of operational and environmental loads, the debonding between track slab and CA mortar layer tends to develop into a void of CA mortar layer and slab upwarping. Interlayer debonding is the root cause of the void defect of CA mortar layer and upwarping defect of the track slab, so it has attracted the focused attention of scholars and engineers, and it is also the focus of this study. Investigations into nondestructive testing (NDT) and structural health monitoring (SHM) of ballastless rail track and concrete slabs are globally existent and extensive. A group of scholars from Taiwan and Hong Kong studied the detection of delaminated cracks inside the CA mortar layer by a nondestructive method and validated the method by using a sandwiched slab specimen [6]. They used amplitude spectra of impact-echo (IE) waveform to detect the delaminated flaw inside the CA mortar layer of the HSR track, and the results showed that the method could identify different lateral sizes of cracks in the CA mortar layer of a concrete-CA mortar-concrete sandwiched slab specimen. [PERSON] et al. in the United States investigated the assessing sensitivity of impact-echo and ultrasonic-surface-wave (USW) methods for nondestructive evaluation of concrete structures [7]. They employed the dominant frequency of the IE waveform, and the USW dispersion curves to identify the artificial defects (delaminated zones, air void, or water void) embedded in the center of the slabs at different depths. [PERSON] et al. in Korea applied three different processing methods for analyzing data acquired from the IE test of a concrete slab with simulated delamination and void defects, namely the classical impact-echo method, the peak frequency map, and the 4D spectrum technique [8]. In addition, several other approaches, including hammer test [9; 10], full-wavefield imaging [11], and ground-penetrating radar (GPR) [12; 13; 14] methods, have been employed by scholars in different countries for the damage detection of the track or concrete slab. Guidelines for track maintenance are commonly seen in both eastern and western areas [15; 16]. Regarding the NDT of CRTS-II-slab ballastless track for debonding detection, [PERSON] [17] employed on-board sensors on a vehicle, and [PERSON] et al. [14] used a GPR method to detect debonding. [PERSON] et al. [18] adopted the IE method and took advantage of elastic waves to identify interlayer defects.