# Highway Tunnel Defect Detection Based on Mobile GPR Scanning
###### Abstract  
In order to overcome the difficulty in rapid detection for expressway tunnels, the coherence calculation of dual-frequency radar signal in the time domain is proposed to suppress the interference. A dual-frequency (400 MHz and 900 MHz) GPR and a manipulator are developed. In the horizontal direction, it has a \(-90^{\circ}\)-\(90^{\circ}\) range, a vertical rotation range from 0 to 90\({}^{\circ}\), a length range from 5-9.5 m, and an antenna rotation between \(-40^{\circ}\)-\(40^{\circ}\). The mobile scanning has been realized in the expressway tunnel, taking the vehicle as the mobile carrier. The research shows that: This equipment realizes the rapid detection of the tunnel without affecting the expressway's opening. The imaging algorithm recognizes the structural thickness, reinforcement distribution characteristics, and structural diseases. According to the detection carried out in Zhejiang, China, the spacing between reinforcement in the second liming is 4 cm, and the thickness of the structure is about 0.55 m. However, the reinforcement has deformed badly, and the defects are also discerned in the structure. Compared with the traditional handheld radar detection, the equipment dramatically reduces the labor demand, time, and cost and can meet engineering needs. With the proposed method, the detection time is reduced to 12 min/km from 0.5 d/km, and the cost is reduced by more than two times. Furthermore, during the detection, the traffic can be maintained normally. and severe safety problems for the mountainous highway tunnel with long mileage. At the same time, handheld detection is easy to causes data instability in space, which increases the difficulty of radar data interpretation. Conventional detection with the high cost and low efficiency has been difficult to meet the increasing demand for tunnel mileage. With the progress of technology and the rise of all kinds of automatic equipment, tunnel detection is gradually becoming intelligent and mechanized [6,7]. [PERSON] et al. studied the non-contact detection effect of GPR and proved the feasibility of non-contact measurement through model tests and numerical simulation [8]. Based on the non-contact detection of GPR, the research on mechanization and automation of detection has made significant progress. [PERSON] et al. Fixed the radar antenna behind the track detection vehicle in the form of fixed support to realize the rapid acquisition of radar data [4]. [PERSON] et al. installed GPR walking track on the shield machine to realize the circular detection of shield segments [9]. For non-contact measurement, high-frequency radar signals will form strong multiple reflected waves in the air layer, resulting in interference with the internal information of the structure [10]. Although the low-frequency radar signal has a great penetration ability, its resolution is low [11]. [PERSON] discussed the application of different frequency radar systems in structural detection, analyzed the relationship between GPR penetration ability and frequency, and analyzed the energy weakening effect of the tunnel reinforcement layer [12]. According to the stable reflection interface of the air layer, [PERSON] eliminated the influence of multiple reflected waves from the air layer through the background elimination method [13]. [PERSON] eliminated the multiples with regular shape by predictive deconvolution [14]. However, the reinforcement in the tunnel structure is densely distributed, resulting in the superposition of multiple reflected waves and other signals, which increases the difficulty of data processing. At the same time, the section form of the expressway tunnel is complex and changeable. The form of fixed support or track is difficult to meet the needs of different sections. There is no track in the expressway tunnel to ensure the stability of the detection operation. Consequently, a device that can flexibly adjust the shape is necessary.