## 3 Results and Discussion
### Field Tests  
We also carried out field tests on the fully mechanized mine cave face. A detailed structure of the top coal is provided in Figure 7; the average thickness of the top coal is 12.36 m; the maximum number of gauge layers in the top coal is five; and the thickness of the gauge layer is from 0.1 to 4 m.  
Figure 6: Multi-frequency GPR data fusion results for top coal in laboratory environments: (**a**) 900 MHz GPR profile; (**b**) 1500 MHz GPR profile; (**c**) 900 MHz and 1500 MHz fused profile.  
In the field tests, we selected two groups of GPR central frequencies (100 MHz and 270 MHz) based on the thickness of the top coal (its average thickness was about 12 m) to detect the top-coal structure (see Figure 8). The GPR antenna was placed on a moving vehicle; in this experiment, the static measurement mode was adopted for multi-frequency GPR data acquisition, and 30 sampling channels of GPR signals were selected to analyze the data fusion performance. Also, we chose three representative borehole locations to verify our proposed method's effectiveness. Note that the detailed structures of the top coal in the borehole location were obtained via manual operation.  
The test results are provided in Figure 9, where the horizontal axis denotes the number of sampling channels, and the vertical axis represents the GPR signals' propagation time. Figure 9a,b denote the 100 MHz and 270 MHz GPR waveform accumulation profile, and Figure 9c represents the GPR-fused waveform accumulation profile. We can see that the  
Figure 8: The layout of the GPR in field tests.  
Figure 7: The structure of top coal in a fully mechanized cave face, where the horizontal axis denotes the horizontal direction of the mine face, and the vertical axis represents the thickness of the top coal.  
detection distance of the 100 MHz frequency GPR is relatively larger than the 270 MHz frequency GPR; in a range of 150-300 ns, the echo intensity and image resolution were higher. Conversely, the echo intensity and image resolution of the 270 MHz frequency GPR were outstanding, in a range of 0-150 ns, while for a range greater than 150 ns, the detection infection became worse. Figure 9c shows that the fused results not only possessed the merits of higher detection resolution in a range of 0-150 ns but also had larger penetration depth in a range of 150-300 ns.  
Figure 9: Multi-frequency GPR data fusion results for top coal in field environments: (**a**) 100 MHz GPR waveform accumulation profile; (**b**) 270 MHz GPR waveform accumulation profile; (**c**) 100 MHz and 270 MHz GPR fused waveform accumulation profile.  
Similar to Table 1, we also compared the genetic, time-varying and wavelet transform fusion methods with our proposed method to evaluate multi-frequency GPR data fusion's effectiveness in a field environment. The comparison results are provided in Table 2; we can see that the fusion results for the 100 MHz and 270 MHz GPR data using our proposed method possess the best performance; compared with the genetic, time-varying and wavelet transform fusion methods, the fusion results for the _IE_, _SF_ and _LG_ evaluation criterion improved by \"15.72%, 44.89% and 19.11%\", \"10.77%, 9.47% and 12.13%\"; and \"14.08%, 7.04% and 12.61%\", respectively. Moreover, the experimental results prove that multi-frequency GPR data fusing can increase the information content and profile clarity, and it also possesses both the merits of high-frequency GPR signals: higher-resolution and low-frequency signals and greater detection depth.