## 4 Applications to Field Data
### Data Processing
Figure 6a shows the dispersion energy of exploration point S33 obtained by the E-SPAC method. The dispersion energy is relatively concentrated within the frequency range of 1.2-16 Hz. The extracted dispersion curve is the line of black dots in the figure, which is smooth and continuous. The initial model is established mainly based on the site borehole data, and a low-velocity layer with a shear-wave velocity of 250 m/s is set at the depth of 50-70 m. An inversion of the extracted dispersion curve is conducted and the result is shown in the line of red triangles in Figure 6a, and the fitting matching degree is about 96.8%. The inversion apparent S-wave velocity model is shown in the red dotted line in Figure 6b. The apparent S-wave velocity varies between 185-202 m/s within the depth range of 0-31.9 m. As seen from the core photo of borehole zk10 in Figure 7, this layer is mainly composed of muddy silty clay mixed with silt (soft plastic) and silty clay (plastic). Below the layer are silty fine sand (slightly-moderately dense), medium coarse sand with gravel, and pebbles (moderately dense-dense), and the apparent S-wave velocity gradually increases to 359 m/s from 31.9 m to 57.6 m. There is a low-velocity interlayer between depth of 57.6 and 73.3 m and the inversion apparent S-wave velocity is about 298 m/s; the lithology revealed by the borehole is silt mixed with silty clay (plastic), and the depth is 57.8-73.3 m, with which the inversion model is highly consistent. The underlying layer is strongly-weathered argillaceous siltstone. The apparent S-wave velocity gradually increases to 498 m/s, and is greater than 770 m/s below the depth of 110.0 m. Thus, it is speculated to be moderately-weathered argillaceous siltstone. The overburden thickness is about 93.7 m according to the specifications. The field in situ test results are shown in  
Figure 5: Core sample photos of borehole zk43. Figure 6b, 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 consistent with the change trend of the inversion apparent S-wave velocity model. The wave velocity value is highly consistent with the stratification depth. Figure 6: Dispersion curves and inversion results of microtremor exploration points in study area II. (**a**) dispersion curves of S33; (**b**) inversion model of S33 and in situ test results of zk10; (**c**) dispersion curves of S95; (**d**) inversion model of S95 and in situ test results of zk11; (**e**) dispersion curves of S151; and (**f**) inversion model of S151 and in situ test results of zk12. Similarly, the measured and inversion dispersion curves of exploration points S95 and S151 are shown in Figure 6c,e, with the fitting matching degree of about 96.5% and 91.9%, and the overburden thickness of about 101.0 m and 107.6 m, respectively. The microtremor inversion velocity model reflects well the distribution of low-velocity interlayer, which, for exploration point S95, is at the depth of 54.6-73.3 m, and, for exploration point S151 point, is at 58.6-74.3 m, As shown in Figure 6d,f; for both, the apparent S-wave velocity is about 297 m/s, which is consistent with the depth range of the layer revealed by the nearby boreholes zk11 and zk12. These results fully prove that weak interlayer can be well identified by the method.