## 4 Results and Discussion
### Interpretation of Achievements  
The microtremor method is to divide the interface of the underground media according to the density and trend change in data points in the dispersion curve. Sparse data points indicate that the lithologic particle size is large, and dense data points indicate that the lithologic particle size is small. If the formation velocity suddenly changes or there is a low velocity interlayer, the dispersion curve at this position will have a zigzag inflection point [22]. Figure 9 shows the comparison between the dispersion data retrieved from some measuring points of the WD2 line and the actual observation data, and Figure 10 shows the change in the fitting mean square deviation of the dispersion curve with the number of iterations. It can be seen that the fitting effect of the dispersion curve was good. The initial model was automatically generated. Although it was somewhat different from the actual formation data, the initial value of the mean square error of dispersion curve fitting was small, and the target function converged quickly in the inversion process. The mean square error of 12-15 iterations was able to meet the inversion requirements. Figure 8: Two sets of parameters of WD1-1: (**a**) an example of the vertical-component records; (**b**) SPAC coefficients. Figure 9: Dispersion curve of some measuring points of WD2 line. (**a**) the dispersion curve of WD2-1; (**b**) the dispersion curve of WD2-4; (**c**) the dispersion curve of WD2-7; and (**d**) the dispersion curve of WD2-8. By calculating the D-V observation curves of 18 exploration points in the survey area (Figure 11), we found that the effective detection depth was approximately 50-500 m and the velocity range was 400-1600 m/s. At some measuring points, the formation velocity changed obviously, and the velocity curves of the measuring points all jumped between approximately 300 and 400 m buried depth, with a zigzag inflection point. In this experiment, data acquisition of 38 points was carried out, including 18 exploration points and 2 micro-measuring-lines. The following is the apparent shear wave velocity profile of the two survey lines (Figure 12). The black dotted line represents the  
Figure 11: D-V curves of each measurement point of WD2 line: **(a)** the DV curve of WD2-1; **(b)** the DV curve of WD2-4; **(c)** the DV curve of WD2-7; and **(d)** the DV curve of WD2-8. Figure 10: The change in fitting mean square deviation of dispersion curve of some measuring points of WD2 line with the number of iterations: **(a)** the RMS error of WD2-1; **(b)** the RMS error of WD2-4; **(c)** the RMS error of WD2-7; and **(d)** the RMS error of WD2-8. position of Coal Seam #2. The red oval represents the position of the goat. The white rectangular area represents the suspected subsidence area. It can be seen from the apparent shear wave velocity profile of the WD1 line that there were two obvious low-speed areas within a buried depth of approximately 400 m and a distance of 40-60 m. The apparent shear wave velocity was lower than 1250 m/s, which may be caused by the goat of Coal Seam #2. The WD1 line crossed two goat areas of Coal Seam #2, which are shown as two low-speed areas on the profile map. Above Coal Seam #2 and within the buried depth of 300-400 m, the apparent shear wave velocity was also relatively low. The two sides of the seam were slightly thin and the middle was relatively thick, which may be related to the collapse of Coal Seam #2 caused by goat ponding. The detection results of the above two survey lines are in good agreement with the known goat area of Coal Seam #2, and the resolution in the shallow part was fine. Especially in the transverse direction, the boundary of the goat was basically consistent with the boundary of the apparent shear wave velocity profile of the micromotion detection. However, the overlying strata collapse caused by coal mining and the geological structure in the area varied, making the thickness of the low velocity layer significantly greater than the thickness of the coal seam.