Ground thermal properties are the design basis of ground source heat pumps (GSHP). However, effective ground thermal properties cannot be obtained through the traditional thermal response test (TRT) method when it is used in the coaxial borehole heat exchanger (CBHE). In this paper, an improved TRT (ITRT) method for CBHE is proposed, and the field ITRT, based on the actual project, is carried out. The high accuracy of the new method is verified by laboratory experiments. Based on the results of the ITRT and laboratory experiment, the 3D numerical model for CBHE is established, in which the flow directions, sensitivity analysis of heat transfer characteristics, and optimization of circulation flow rate are studied, respectively. The results show that CBHE should adopt the anulus-in direction under the cooling condition, and the center-in direction under the heating condition. The influence of inlet temperature and flow rate on heat transfer rate is more significant than that of the backfill grout material, thermal conductivity of the inner pipe, and borehole depth. The circulating flow rate of CBHE between 0.3 m/s and 0.4 m/s can lead to better performance for the system.

Migration imaging is a key step in tunnel seismic data processing. Due to the limitation of tunnel space, tunnel seismic data are small-quantity, multi-component, and have a small offset. Kirchhoff migration based on the ray theory is limited to the migration aperture and has low migration imaging accuracy. Kirchhoff migration can no longer meet the requirements of high-precision migration imaging. The reverse time migration (RTM) method is used to realize cross-correlation imaging by reverse-time recursion principle of the wave equation. The 3-D RTM method cannot only overcome the effect of small offset, but also realize multi-component data imaging, which is the most accurate migration method for tunnel seismic data. In this paper, we will study the 3-D RTM method for multi-component tunnel seismic data. Combined with the modeled data and the measured data, the imaging accuracy of the 3-D Kirchhoff migration and 3-D RTM is analyzed in detail. By comparing single-component and multi-component Kirchhoff migration and RTM profile, the advantages of the multi-component RTM method are summarized. Compared with the Kirchhoff migration method, the 3-D RTM method has the following advantages: (1) it can overcome the effect of small offset and expand the range of migration imaging; (2) multi-component data can be realized to improve the energy of anomalous interface; (3) it can make full use of multiple waves to realize migration imaging and improve the resolution of the anomalous interface. The modeled data and the measured data prove the advantages of the 3-D multi-component RTM method.

(1) Rock bridge cracking is a key factor controlling toppling failure. Previous studies have suggested a potential relationship between fundamental natural frequency and rock bridge length, but frequency‐only identification may introduce systematic error when the damping effect becomes non‐negligible during rock bridge deterioration. (2) In this study, an improved dynamic analysis method was developed to quantify rock bridge length by jointly considering the fundamental natural frequency and damping ratio. The method was validated through indoor tests in the Y and Z vibration directions and was further examined using a dangerous toppling rock case in the Sichuan Province, China. (3) The experimental results show that the average error of the improved method is 6.28% and 6.36% in the Y and Z directions, respectively, which is lower than that of the original frequency‐only method (9.49% and 9.73%). The correction is most evident in the unstable stage, where the damping effect becomes more influential as the effective rock bridge shortens. (4) The field case provides preliminary engineering support for the proposed indirect identification framework. The method should be interpreted as an engineering approximation for evaluating the effective bonded portion of the structural plane, rather than as a direct measurement of the full three‐dimensional bonded area.