## 3 Results and Discussion
### Infrared Thermography (IRT)  
The results of IRT on both east and west sides in the morning (performed between 10:00 a.m. to 11:30 a.m.) and in the afternoon (performed between 2:00 p.m. to 3:30 p.m.) have been summarized in the Supplementary Material and Tables S1-S8. Figure 7 shows two thermograms as examples from these experiments. Note that the data in Tables S1-S8 were not necessarily collected on the same day; as a result, for most tests, there are considerable differences in the initial temperature between the morning tests and afternoon tests for a single heating-cooling combination. However, for each test, the IRT of the east and west sides was performed on the same day with a time difference of approximately 0.5 h. The defects are listed in each table in the Supplementary Material to show the defect detection improvement as the cycles increased to figure out the optimum or minimum number of heating-cooling cycles. The results shown in Tables S1-S8 indicate that, in most cases, the accuracy of completing five cycles is similar to six cycles. Therefore, performing five consequent heating-cooling cycles should be sufficient to detect the subsurface defects. The preferred heating-cooling methodology is the one that optimizes detection accuracy while minimizing the time taken to perform the test. The highest detection accuracy can be defined as the ability to detect the smallest aspect ratio (AR) and the largest ratio of defect depth (D) to the wall thickness (t) (D/t). For this reason, the plots of the smallest detected AR versus the maximum D/t for different heating-cooling combinations at the final cycle (cycle #6) are shown in Figures 8-11. The diameter of the bubbles in these figures represents the AR, and the AR values are printed next to each bubble. Therefore, the diameter of the bubbles can help to visually compare the performance of different heating-cooling combinations in detecting smaller defects. The results demonstrate that the maximum D/t is highly dependent on the initial \(\Delta\)T. As the \(\Delta\)T increases, regardless of being tested in the morning or afternoon, the maximum D/t increases. The reason is that, by having a high initial \(\Delta\)T and by depositing additional external heat on the surface, the final thermal gradient on the thickness is higher compared to the time when the initial \(\Delta\)T is negative. This is in good agreement with the previous experiments performed on the laboratory scale tank [8; 9]. Therefore, as was mentioned in Section 3.1, the best time for performing IRT should be when there is a high positive \(\Delta\)T, which can be achieved sometime between 9:00 a.m. to 6:00 p.m. The results indicate that the maximum D/t for the east side for all heating times is equal to 0.73 but, when the heating time was 20 s, it needed a larger positive \(\Delta\)T to overcome this D/t threshold. Therefore, achieving a D/t of 0.73 when the heating time is 20 s is not guaranteed all the time, and it requires certain  
Figure 7: Two examples of the thermograms obtained from (**a**) cycle #6 of the heating–cooling combination of 40–80 performed on the east side, and (**b**) cycle #6 of the heating–cooling combination of 60–60 performed on the west side. weather conditions; this indicates that 20 s of heating alone is not enough for deep heat penetration into the tank wall. Heating times greater than 30 s provide enough heat to penetrate deep enough for subsurface defect detection. By comparing the results of various heating durations on the east side of the tank, it is obvious that for any heating intervals larger than 30 s there is little improvement in the heat penetration depth in terms of the values of D/t. Therefore, for the east side, where the thickness of the wall was 12.7 mm (0.5 in), 30 s of heating should be sufficient to allow the heat to penetrate deeper into the tank wall. The smallest AR that could be detected for the east side when the heating times were 20 s, 30 s, 40 s, and 60 s were 0.86, 0.80, 0.80, and 0.73, respectively.