## 4 Results and Analysis  
The first arrival time of the measured seismic wave signals was determined through PickWin software (OYO corporation, Tokyo, Japan), and the time-distance curve was obtained by using the SeisImager-Poltrefa program (OYO corporation, Tokyo, Japan). An interactive process was performed to increase resolution when carrying out the inversion process. Two groups, which are based on the picked first-arrival times and the results analyzed by the simultaneous iterative method, are plotted on a travel-time curve in Figure 4. Figure 4 shows that the two groups are nearly identical, reflecting that the quality of the measured data is high. The distribution of seismic wave velocity is plotted in Figure 5, and the soil layers are divided into various different layers according to the reference P-wave velocities suggested by [20]: 0-0.7 km/s (landfill and alluvial soil), 0.7-1.2 km/s(weathered soil), 1.2-1.9 km/s (weathered rock), and over 1.9 km/s (soft rock). The BTS, BS, MS, and TS lines were found to comprise four, four, three, and three layers, respectively based on [PERSON] (2003). Note that 0.7 km/s of primary wave velocity is an accepted criterion in South Korea for distinguishing soil layers from other geomaterials [21; 22]. Figure 4: Travel time-distance curves through seismic survey: (**a**) BTS; (**b**) BS; (**c**) MS; and (**d**) TS. Note 10, 20, 30, and 100 m distances indicate points 1, 2, 3, and 10, respectively, in Figure 3.  
The calculated DPI values from dynamic cone penetration tests are shown in Figure 6 for each stream. The BTS line shows high variation of DPI values according to the testing locations, which are described in Figure 3, because the line covers the bottom, middle, and top streams. Note that the high variation of DPI values means that the soil depth changes along the stream in the BTS line. Number 4, 8 and 9 holes in Figure 6a show high DPI values at shallow depth, reflecting the presence of weak soil layers. Being different from other testing holes, the DPI was measured at deeper depths at No. 7 and 9 holes, demonstrating that the soil depth near the top stream is deeper than that of other locations. Note that Figure 5a and the maximum extracted soil depth using a hand auger also support the presence of deep soil layer near the top stream in the BTS line. In the case of the BS line, high DPI values at the initial stage and high penetrated depth were observed at the right side of the stream. A similar DPI trend was observed in the MS line, and the distributions of soil depth in the No. 1, 2, and 3 holes look alike, reflecting similar soil depths along the MS line. Even though the DPI of the TS line shows a similar penetrated depth near the surface, the DPI was continuously measured at the only No. 2 hole. Therefore, it is predicted that the soil depth at the No. 2 hole, which is the center of the TS line, is high. The depth where the DPI value is close to zero is referred to as the soil depth of the testing site in this study, and the average soil depths based on DPI are calculated to be approximately 0.92, 0.75, 0.86 and 0.85 m for the BTS, BS, MS, and TS lines, respectively. Comparison between Figures 5 and 6 reveals that the soil depth of the testing site based on dynamic cone penetration test is generally shallower than 1 m; while, the depth based on seismic wave velocity (0.7 km/s) is around 2 m or more (Table 1), reflecting that the subsurface classification based on previous reference values of seismic wave velocity cannot precisely estimate the soil depth. Therefore, in this study, the graphical bilinear method is newly suggested for determining soil depth based on seismic wave velocity. Figure 5: Converted primary wave velocity profiles: (**a**) BTS; (**b**) BS; (**c**) MS; and (**d**) TS.