## 5 Indoor Experimental Verification  
To verify the effectiveness of water-soil interface identification based on the non-steady-state heat transfer laws of linear heat sources in water-soil coupled media, this paper relies on the national engineering research center for port water engineering technology of the tianjin water transport science research institute under the ministry of transport, and the key laboratory of port and waterway engineering structure detection, diagnosis and reinforcement technology in the transportation industry. Indoor verification experiments were conducted in the experimental soil tank of the port engineering structure at the port engineering structure test site. The water-soil identification system used in this experiment is shown in Figure 12.  
Figure 11: Temperature difference in water–soil coupled media with linear heat sources of different convective heat transfer coefficients. As shown in Figure 12, the water-soil identification system consists of a custom-made experimental tank (dual tank) with a single tank size of 2 m \(\times\) 1.2 m \(\times\) 1.369 m, a dynamic temperature data logger (NZS-DTS-A01, Nanzee Sensing, Suzhou, China), a computer, and a heating and temperature sensor. The experiment used kaolin clay, which was mixed with water and stirred into a slurry using a mixer, with a water-soil ratio of 1:1. Before the test, the surrounding environmental temperature was kept constant by using air conditioning. Then, the heating and temperature sensor was pushed into the kaolin clay under the condition that its own temperature was consistent with the ambient temperature. The temperature of the sensor was monitored continuously during the whole test. In the indoor experimental verification, the sensor dimensions were scaled at a 1:2 ratio compared to the prototype dimensions. The model sensor featured a diameter of 5 cm and a length of 70 cm, with a fiber optic pitch of 0.3 cm. The heating power of the linear heat source was configured at 500 W. The details of the sensor are shown in Figure 13. The error percentages between the theoretical and experimental temperatures are shown in Table 5.  
Figure 12: The water-soil identification system. Figure 13: The details of the heating and temperature sensor and the relevant temperature changes. (**a**) The details of the heating and temperature sensor. (**b**) The temperature changes in the testing and analytical solution. As shown in Figure 13a, the heating and temperature sensor consists of a linear heat source, encapsulation port and optic fiber twining along the trench on the surface of the encapsulation port. Regarding the linear heat source, it is used to lift the temperature of the whole sensor, and the encapsulation port is acting as a connector to combine the linear hear source and the optic fiber. Regarding the optic fiber, it is used to monitor the temperatures of the different positions on the encapsulation port. The differences between the theoretical values (real lines) and experimental values (dashed lines) are displayed in Figure 13b, and the results show that the theoretical values can capture the temperature changing trend and the interface between soil and water. As shown in Table 5, it cannot be denied that there is a certain degree of error between the theoretical calculation results and the experimental results (less than 14% in water and 19% in soil). This is because some details were simplified during the theoretical research process, such as neglecting the radial temperature variation in linear heat sources in the heat transfer process and assuming that linear heat sources are located in a semi-infinite medium. And also, the soil is a porous and discrete medium. In this experiment, the pores within the soil are filled with water, leading to certain changes in the temperature characteristics of the soil during the test. Further research needs to be conducted to update the theoretical calculation in real conditions.