## 5 Results  
The refraction exploration data collected in this paper are from the highway tunnel exploration project, which is located in Northeast China. The purpose of this project is to determine the weak weathering interface at the entrance of the tunnel and reduce the potential risk of the exit as the entrance of the tunnel. The geological composition of the area is as follows: the surface layer is quaternary system, the lower part is bedrock, and the bedrock is not exposed in the area. The inclination of the ground is about 15\({}^{\circ}\). Data acquisition was carried out using the seismic refraction method and an S-Land seismograph produced in the United States. Field measurement was carried out by hammering the source, which met the requirement of an interchange time difference less than 5 milliseconds. The length of the measuring line was about 240 m, the sampling rate was 0.5 milliseconds, the sampling time was 512 milliseconds, and the source spacing was 60 m with 5 sources; each source had 24 seismic receivers, and the trace spacing was 10 m.  
In the first step, we picked up the first arrival wave, as shown in Figure 16a. According to the far-shot travel time data, the t0 difference method is used for preliminary interpretation, and the basic formation interface results are obtained, as shown in Figure 16b. Then, the inverse distance gradient weighted interpolation of the velocity interface between the layers is carried out, and the model of Figure 16c is obtained. Finally, after the inversion calculation, the elevation data is introduced to get the results, as shown in Figure 16d.  
Figure 16: Inversion results of the measured data. (**a**) Time-distance curve; (**b**) t0 difference method result chart; (**c**) prior model picture; (**d**) inversion result chart.  
Figure 15: Error fitting curves using different constraints.  
The elevation line and geological interpretation map of the tunnel are shown in Figure 17. The green line represents the upper and lower interfaces of the tunnel. According to the inversion results and geological data, the velocity range of the first layer, which is quaternary sediment, is 400-600 m/s. The low velocity layer is very obvious. Generally, the P-wave velocity of quaternary sediments is generally between 200-800 m/s. The depth of quaternary sediments is about 20-25 m. The velocity interface of the second layer, which is supposed to be strong weathered Granite, is about 1000-2500 m/s, and the lowest layer, which is supposed to be weak weathered Granite, is greater than 3000 m/s. The change rate of P-wave velocity of strong weathered granite is relatively large. The P-wave velocity of weak weathered granite is relatively high. In conclusion, according to the inversion results, the stratigraphic structure can be effectively divided.  
The entrance of the tunnel in 230 m in the horizontal direction. It can be seen from Figure 15 that the top of the entrance is near the interface between the quaternary sediments and strong weathered granite. Therefore, attention should be paid to the construction in those areas. The types of lithology have guiding significance for tunnel excavation. According to the actual situation, it is a better choice for tunnel excavation to avoid a low-speed zone of strong weathering and select a hard and complete high-speed zone of rock. Thus, this method has guiding significance for tunnel engineering, can reduce the blindness of construction, and has the potential to improve the safety and reliability of tunnel engineering. The exploration of the seismic refraction method is limited by the site conditions, and the geological situation of the whole tunnel can be understood through very little exploration work. This method is fast and accurate, and is the first choice of shallow geological investigation work such as highway route investigation and hidden danger detection.