## 2 Experiment Methodology
### Infrared Thermography (IRT)  
Every material emits infrared (IR) radiation at a specific wavelength that is dependent upon its thermal properties (i.e., thermal conductivity and thermal capacity). IRT employs a thermal imager to capture the infrared radiation from a given surface, transforming it into electrical signals equivalent to the surface temperature profile. These signals are displayed in the form of colorful thermal images known as thermograms, in which each color corresponds to a temperature on the object's surface [8; 10; 11]. Usually, defects have different thermal properties compared to the intact original homogeneous solid [12], since the defects are often filled with air or a liquid. The difference in the thermophysical parameters between the original material and the defects allows for the effective application of IRT to detect subsurface defects [12]. In order to detect any defects or flaws, a thermal gradient must exist through the depth of the element, which can be generated through either passive or active methods. Deposited heat on the sample surface diffuses toward the interior side of the object and the presence of a subsurface flaw blocks the heat flow, resulting in defect detection in the thermograms. The crucial difference between the passive and active methods is that, in passive IRT, the ambient environment and sunlight are used to generate the thermal gradient on the component depth, but, in active IRT, the component is artificially thermally excited (e.g., using light, hot or cold air, direct contact) and observed with an IR camera as it responds to the thermal stimulus [12; 13]. According to the method and duration of the heating and thermographic data analysis procedures, the active thermography techniques fall into the following subcategories: pulsed thermography, step-heating thermography, modulated lock-in thermography, vibrothermography, pulse-compression thermography, and thermal wave imaging [12; 14; 15; 16; 17]. Moreover, active thermography can be classified into reflection and transmission methods, depending on the heating and measurement setups. If the imposed thermal gradient is generated from the same side of the object as the IR camera, then the technique is known as the reflection method and is used to locate superficial anomalies. If the heating is applied on the opposite side of the object from the IR camera, then the technique is known as the transmission method, which is used to identify deep anomalies. Since the goal of this study is to implement IRT in the field for the integrity evaluation of HDPE ASTs, and since only the outside of the AST is accessible when it is filled with brine solution, the reflection method is the primary focus of this research, as has been established in previous studies [8; 9]. In active IRT schemes, different external excitation sources can be used, such as optical lamps [18], microwaves [19], ultrasonic pulses [20], hot and cold air [21; 22], eddy currents [23], etc. First, during the preliminary investigations, this study tried to use the same experiment setup established in previous research [8], which utilized six 500 W halogen lamps with a total power of 3000 W at a distance of 0.4 m (15.75 in) to induce a thermal gradient on the object surface. The heating power used in the previous laboratory experiment was not sufficient to induce an adequate thermal gradient on the HDPE AST wall in the field. Therefore, the first correction for transferring the IRT from the laboratory  
Figure 1: The configuration of the tank under study and the arrangement of the installed thermocouples. to the field was changing the heating source. The preliminary investigations showed that utilizing two stacked 1500 W infrared heaters with a total power of 3000 W at a distance of 0.4 m (15.75 in) was effective for defect detection in the field for this study. Step heating involves heating the tank for a predetermined amount of time, and then letting the tank cool naturally for a predetermined amount of time.