## 3 Results and Analyses  
### Localization of AE Event Sources: Single-Stage AIC Result  
Figure 5a presents a typical set of the acquired AE signals, in which the onset times estimated by the single-stage AIC method are superimposed. Due to the high signal-to-noise ratio (SNR), the single-stage AIC picker estimated the onset time with a good level of accuracy. Figure 5b shows the estimated source location and the error contour plot estimated by the Simplex method. Our source localization result shows that the estimated source location was (29.3 cm, 18.8 cm) for the actual source location of (30 cm, 20 cm). Out of twenty trials, the average Euclidean distance error was 1.73 cm, and the zonation was correctly predicted for Zone 14 with Pile #14.  
Accordingly, the estimated source localization can be plotted for each AE event. Figure 6 shows the spatial distributions of the estimated source locations using the single-stage AIC picker and the Simplex method when the source was located in one of the central zones (Zone 14). The suggested algorithm resolved most of the trials for RT and RB cases in the central zones. The average accuracy for the RT and RB was recorded to be 93.3% and 100%, with the average Euclidean distance error (EDE) of 1.92 and 0.77 cm, respectively (Table 1). By contrast, the PM case showed poor accuracy of 66.67% with the EDE of 4.68 cm, mainly attributable to the reduced SNR because of the longer travel distance from the source to the accelerometers. This needs further refinement in the onset time determination.  
Figure 5: (**a**) The onset times determined through the single-stage AIC method, and (**b**) the spatial distribution of variances computed by using the Simplex method. Note that the AE source was generated at Zone 14 (30,20) on the raft top (RT) of the model PRF. The amplitude of each signal was normalized with its peak amplitude for comparison.  
Figure 6: Localized sources estimated using the single-stage AIC method when the hit sources were far within the sensor network (or central region): when a hit was applied (**a**) on the raft top, (**b**) under the raft bottom, and (**c**) at the middle of Pile #14. In (**c**), the middle of the pile at a distance 10 cm down from the top of the raft was struck.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{3}{*}{**Location**} & \multirow{3}{*}{**Zone\#**} & \multicolumn{3}{c}{**Single-Stage AIC**} & \multicolumn{3}{c}{**Two-Stage AIC**} \\ \cline{3-6}  & & **EDE** & **Zonation Accuracy** & **EDE** & **Zonation Accuracy** \\  & & **(cm)** & **(\%)** & **(cm)** & **(\%)** \\ \hline \multirow{8}{*}{**Ra  
Meanwhile, Figure 7 shows the spatial distributions of the estimated locations for the corner zones. It can be seen that the EDE became significantly larger when compared to the central zones. The average EDEs for the corner zones were 3.54 cm for RT case and 1.29 cm for RB case, both of which were greater than those of the central zones (Table 1).  
Figure 8 shows the average EDE and the average accuracy for the tested zones. The average EDE was the smallest in the RB case, which showed the average EDE of 1.08 cm and 99.5% accuracy for all the tested zones. On the other hand, in the PM cases, the average EDE increased more than four times to 4.78 cm, and the zoning accuracy was 81%. The result reveals the poor accuracy in the PM case, and less accuracy in central zones than that in corner zones.