## 1 Introduction  
Non-destructive testing (NDT) is an effective and compulsory method for the evaluation and testing of significant infrastructures in the civil engineering community. With the rapid development of the infrastructure construction, there is a growing need to monitor the internal environment and damage situation of the structures and their essential components for the assessment and prediction of the structure safety level [1; 2; 3; 4]. As the result of overcrowding in the modern international metropolis, the metro system has become a perfect alternative transportation to the ground one [5], while metro construction projects present significant challenges of nondestructive evaluation for the complex environments, e.g., under existing buildings or crossing under major municipal arteries and pipelines [6; 7; 8]. Thus, the safety level and durability of metro tunnel structures is of critical importance [9]. In China, after years of research and application, an innovative design method for underground structures, the steel tube slab (STS) structure method, has been extensively employed to succeed the traditional shallow tunneling method (STM) of the metro system, which was commonly used to construct urban subway tunnels [10; 11]. The STS structure dramatically improves the transverse load-carrying capacity by adding transverse high strength bolts and a flange plate between the tubes, which eliminate the drawbacks of the traditional pipe-roof. It serves as a primary support to resist the overburden earth pressure and ground overload with a maximum transverse span of 5 m during excavation. The STS structure was employed to construct the Olympic Center subway station in Shenyang Metro line 9 in super shallow buried (soil cover depth less than 5 m) Metro stations in downtown. The load-carrying capacity is an essential part of the entire project. The mechanical characteristics of the STS structure are different from those of a reinforced concrete structure due to the interaction among the steel tubes, the flange plate, and the concrete between tubes, which results in more complicated mechanics. The absence of concrete in the tubes is the weakest part of the STS structure, which may cause a major catastrophe with heavy losses of lives. Thus, each concrete-fill steel tube of the STS system should be strictly tested and inspected as per corresponding procedure. For the damage inspection part, ultrasonic computerized tomography (CT) is applied to interrogate the internal flaws and defects of the STS structures. Ultrasonic CT is a digital imaging technique consisting of a numerically reconstructed inverse problem, which is associated with the interaction of ultrasonic waves with the detected structures [12; 13; 14]. The characteristics that define the reconstruction of the quantitative tissue for the diagnostic application can be the speed of sound, mass density, and attenuation [15]. Ultrasonic CT has been extensively developed and applied in many areas such as medicine, material engineering, aerospace, civil engineering, etc. [PERSON] and [PERSON] developed an ultrasonic CT technique to diagnose cancer in the breast. The measured arrival time and changes in amplitude are used in a reconstruction algorithm to obtain estimates of the 2-D distribution of acoustic speed and attenuation of the breast. The quality of over 1000 scanned images demonstrating the sensitivity of the ultrasonic CT system approaches the work done by using the X-ray technique [12]. [PERSON] et al. obtained the mode tomography imaging for liquid/gas two-phase flow by using an ultrasonic CT technique [16]. [PERSON] et al. used Lamb waves ultrasonic computerized tomography to identify the material loss of real aircraft components by embedding piezoelectric sensors on the surface [17]. [PERSON] imaged the complex concrete structure via ultrasonic CT, indicating that this technique has accuracy capability with high resolution [18]. [PERSON] et al.