## 6 Discussion  
To use ultrasonic technology for the non-destructive testing of a stone architectural heritage, sufficiently reliable data must be acquired and a customized evaluation of the integrity must be conducted by considering the actual conservation condition of the target heritage. Therefore, in this study, ultrasonic measurement conditions that were suitable for a stone architectural heritage were established based on previous studies (Figure 9). To achieve this objective, a 54-kHz exponential transducer and rubber clay couplant were used. The ultrasonic velocity acquired at a 200 mm distance using the indirect transmission method was converted to a direct ultrasonic velocity using the average correction factor for medium-to-coarse granite of 1.50, as proposed by [PERSON] and [PERSON] [29]. The existing evaluation method, which is extensively used, can be improved by proposing weathering standards based on the material characteristics and provenance site of the constituent stone materials of the stone block foundation of the Sungryemun Gate. In particular, the absolute weathering grade based on the ultrasonic velocity of universal hard rock and the relative weathering grade based on the ultrasonic velocity of fresh rock at the provenance site were suggested. The standard ultrasonic velocity of the absolute weathering grade was set to 5000 m/s, which corresponded to that of fresh rock, as proposed by studies such as those by [PERSON] et al. [41], [PERSON] [32], and [PERSON] [43]. The standard ultrasonic velocity of the relative weathering grade was measured from the provenance site of the constituent stone materials for the stone block foundation of the Sungryemun Gate, based on which the weathering standards for fresh pink feldspar granite (3829 m/s) and leucogranite (4036 m/s) were established. The difference in weathering between the original and new blocks exhibited that the absolute weathering grade underestimated the overall physical properties and strength compared with the relative weathering grade. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Weathering Classification**} & \multicolumn{4}{c}{**Relative Weathering Degree**} & \multicolumn{4}{c}{**Absolute Weathering Degree**} \\ \cline{2-9}  & **South** & **North** & **Total** & **South** & **North** & **Total** \\ \hline \multirow{4}{*}{Original block} & FR \(\leq\) 0 & 9.6 & 0.9 & 4.4 & 0.0 & 0.0 & 0.0 \\  & 0 \(<\) SW \(\leq\) 0.2 & 21.0 & 8.4 & 13.4 & 5.7 & 0.6 & 2.6 \\  & 0.2 \(<\) MW \(\leq\) 0.4 & 48.0 & 73.9 & 63.6 & 29.3 & 10.4 & 17.9 \\  & 0.4 \(<\) HW \(\leq\) 0.6 & 18.3 & 15.4 & 16.6 & 55.5 & 83.5 & 72.3 \\  & 0.6 \(<\) CW \(\leq\) 1.0 & 3.1 & 1.4 & 2.1 & 9.6 & 5.5 & 7.1 \\ \hline \multirow{4}{*}{New block} & FR \(\leq\) 0 & 52.5 & 60.7 & 55.2 & 0.0 & 0.0 & 0.0 \\  & 0 \(<\) SW \(<\) 0.2 & 44.1 & 35.7 & 41.4 & 40.7 & 46.4 & 42.5 \\  & 0.2 \(<\) MW \(\leq\) 0.4 & 3.4 & 3.6 & 3.4 & 55.9 & 50.0 & 54.0 \\  & 0.4 \(<\) HW \(\leq\) 0.6 & 0.0 & 0.0 & 0.0 & 3.4 & 3.6 & 3.4 \\  & 0.6 \(<\) CW \(\leq\) 1.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 & 0.0 \\ \hline \multirow{4}{*}{Total} & FR \(\leq\) 0 & 18.4 & 5.4 & 11.0 & 0.0 & 0.0 & 0.0 \\  & 0 \(<\) SW \(\leq\) 0.2 & 25.7 & 10.5 & 17.1 & 12.8 & 4.0 & 7.9 \\ \cline{1-1}  & 0.2 \(<\) MW \(\leq\) 0.4 & 38.9 & 68.6 & 55.7 & 34.7 & 13.4 & 22.7 \\ \cline{1-1}  & 0.4 \(<\) HW \(\leq\) 0.6 & 14.6 & 14.2 & 14.4 & 44.8 & 77.5 & 63.2 \\ \cline{1-1}  & 0.6 \(<\) CW \(\leq\) 1.0 & 2.4 & 1.3 & 1.8 & 7.6 & 5.1 & 6.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Ratios (%) of the relative and absolute weathering grades of the stone block foundation. In particular, the ultrasonic velocity of fresh rock that was acquired from the provenance site was used as a standard for determining not only the weathering degree of the stone block foundation of the Sungryemun Gate but also for the weathering trend.