## 4 Field Test  
To verify the accuracy of the simulation response and to further investigate the mechanism of electromagnetic wave propagation underground, this study conducted a communication experiment targeting the helical loop antenna in an experimental well at a certain oilfield. As shown in Figure 14. The experimental design included two main parts: downhole transmission and reception. The experiment was validated using a specially designed external drill string component combined with a precise internal instrument assembly. The external assembly mainly consisted of a tool joint sub, a suspension sub, and the helical loop antenna, while the internal assembly was arranged in sequence with a top-end assembly, transmitter, power supply battery, and lower metal centralizer.  
In a cased well with a wide diameter of 150 mm and a depth of approximately 500 m, where the inner wall suffered from severe corrosion, the experimental setup was deployed. The water level in the well was measured at a depth of 10 m to 15 m, and the mud resistivity was determined to be 0.98 \(\Omega\cdot\Omega\); appropriate experimental parameters were set accordingly. The experiment utilized two sets of helical loop antennas, each with 100 turns and a cross-sectional area of 2.5 cm\({}^{2}\). The transmitter power was adjusted to 8 watts, and a frequency of 1000 Hz was used for signal transmission.  
At a depth of 100 m below ground level, the transmitting antenna was fixed, and the distance to the receiving antenna was gradually increased from this base point. The preliminary spacing between the antennas was set at 20 m, and it was subsequently  
Figure 13: The influence of helical loop position.  
increased to 30 m, 40 m, 50 m, and 60 m. After each adjustment, the experiment waited for the receiving antenna to stabilize and record the waveform before proceeding. To ensure the accuracy of the data and reduce the impact of random errors, signal recordings at each interval were collected multiple times and averaged.  
Such a rigorous experimental process ensured the controllability and reproducibility of the experimental conditions, providing quantitative data for assessing the communication efficiency and signal quality of the helical loop antenna in specific underground environments, as well as an experimental basis for validating and optimizing the simulation model. Figure 15 is a comparison of well logging results with the calculation results for the experimental well in the oilfield. It is evident from the figure that the calculated results show the same trend with the change in depth as the experimental measurements, and the calculated values at the same depth points are not significantly different from the measured values, verifying the correctness and applicability of the model. The deviations between the calculated values and the measured values at the same depth points are mainly caused by two factors: one is the model's simplified treatment of the formation, and the other is that the parameters in the model cannot be accurately obtained and only approximate values are used for the calculations.  
The simulation data and experimental data show good consistency, further validating the correctness of the theory. The theoretical research can be used to optimize and design electromagnetic wave transmission for water injection wells.  
Figure 14: Field construction status and instrumentation internals.  
Figure 15: Comparison chart of simulation results and field experiment data.