## 4 Applications to Field Data
### Data Processing  
For data processing of the He'an Line, 13 consecutive stations are selected for each exploration point to form a linear array, with an effective exploration depth of about 100 m. In order to fully illustrate the effectiveness of fine detection by the microtremor array method, the exploration points near the boreholes are used as an example. The spatial autocorrelation coefficients of some different frequencies obtained from the microtremor records at point H32 near borehole zk43 are shown in Figure 3.  
The frequency-velocity diagram can be obtained by fitting the first kind of zero-order Bessel function as shown in Figure 4a. The dispersion energy is relatively concentrated within the frequency range of 1.8-16 Hz. The extracted dispersion curve is the line of black dots in the figure, which is smooth and continuous. The initial model is established according to the site borehole data, and an inversion of the extracted dispersion curve is conducted. The inversion curve is shown in the line of red triangles in Figure 4a, and the fitting matching degree is about 93.1%. The inversion apparent S-wave velocity model is shown in the red dotted line in Figure 4b. According to the inversion result, the apparent S-wave velocity varies between 170-196 m/s within the depth range of 0-22.5 m. As seen from the core photo of borehole zk43 in Figure 5, this layer is mainly composed of silty clay (plastic) and silty fine sand (slightly dense). From 22.5 m to 49.6 m, the apparent S-wave velocity gradually increases to 314 m/s, and the main components are silty fine sand (moderately dense), medium sand (dense), and sand mixed with pebbles. The underlying bedrock is conglomerate. The apparent S-wave velocity of the stratum below 49.6 m depth is greater than 543 m/s. According to the _Code for Seismic Design of Buildings_ (GB 50011-2010), the shear-wave velocity of 500 m/s is defined as the boundary of soil and rock. Thus, it can be determined that the overburden thickness is about 55.8 m, which is of little difference from the depth of 51.8 m of the moderately-weathered conglomerate roof revealed by borehole zk43. The field in situ test result is shown in Figure 4b, where the blue dotted line is the shear-wave velocity curve per meter, and the black solid line is the average layer-wave velocity, which is basically consistent with the change trend of the inversion apparent S-wave velocity model. Figure 2: Schematic layout of the microtremor linear array. (**a**): photographs of field work; (**b**): diagram of the roll-along acquisition mode. Figure 4: Dispersion curves and inversion results of microtremor exploration points in study area I. (**a**) dispersion curves of H32; (**b**) inversion model of H32 and in situ test results of zk43; (**c**) dispersion curves of H110; and (**d**) inversion model of H110 and in situ test results of zk4. Figure 3: Autocorrelation coefficient of partial frequency at point H32. Similarly, the measured and inversion dispersion curves of exploration point H110 near borehole zk47 are shown in Figure 4c, and the fitting matching degree is about 97.2%. The apparent S-wave velocity below 53.5 m of the inversion model is greater than 645 m/s. Thus, it can be determined that the overburden thickness is about 57.2 m, which is of little difference from the depth of 55.0 m of the moderately-weathered roof revealed by the borehole. In Figure 4d, the change trend of the average layer-wave velocity obtained from the field in situ test is consistent with that of the inversion apparent S-wave velocity model, with high consistence especially in the depth range of 10.2-40.6 m. The application effect of the above conventional velocity model of increasing type further verifies the effectiveness of the microtremor array method in detecting shallow velocity structure. For data processing of the Shamao Line, 21 stations are selected to form a linear array, with an effective detection depth of about 150 m. The results are shown in Figure 6.  
Point S33 of the Shamao Line is located near borehole zk10.