# Research on Defect Detection of Bare Film in Landfills Based on a Temperature Spectrum Model
## 1 Introduction  
Currently, rapid economic development speeds up people's lifestyles and, as a result, the various wastes they generate are increasing [1, 2]. As an important destination for waste disposal, landfills also face a severe test [3, 4]. Once leakage occurs, they can seriously pollute the surrounding water and soil environment [5]. Therefore, strict standards are required from early-stage design, construction, and testing to later-stage operation monitoring. Laying an anti-seepage system is one of the important means to hinder diffusion of leachate from landfills. HDPE film is the main anti-seepage material used in landfills [6, 7] as it has the advantages of a high anti-seepage coefficient, stable chemical performance, high mechanical strength, long life, and low cost [8]. In the early stages of landfill anti-seepage systems construction, there are two main causes of surface damage to the HDPE film. First, irregular defects on the HDPE film surface can be caused by construction machinery and personnel activities. Second, during the welding of the HDPE film, incorrect welding operation and improper material selection can cause hole defects, missed or faulty welding, and uneven fusion between films. In addition, excessive welding pressure can cause welding seam fractures. The defects in HDPE film caused by these damage factors can cause potential safety hazards in the operation stage of the landfill [9; 10]. Methods for evaluating HDPE film defects during the construction stage are mainly divided into destructive [11] and nondestructive assessments. Destructive assessment involves testing the strength of geomembrane samples from the welding area through shear or tensile tests. The test results can be used to guide and set the relevant welding form and parameters, but the detection process may cause secondary damage to the HDPE. Nondestructive assessment is mainly achieved using an electrical method [12; 13] or an ultrasound testing method [14]. The electrical method involves installing electrodes beneath the HDPE film and applying a high-voltage power source above the film. When holes or defects exist, detectable current pathways are formed, enabling the identification of damaged locations. However, this electrical detection system features complex architecture and requires preinstalled electrodes, which significantly increases construction costs for landfill sites. The ultrasonic detection method is based on the characteristics of ultrasonic Lamb wave scattering through the damage point, resulting in energy attenuation. The method uses wavelet analysis to define the damage index and, combined with damage imaging, identifies the location of the leakage in the HDPE film. However, the Lamb wave signal is very easy to lose in the background noise. In addition, the method cannot effectively detect defects within the weld seam [15]. Compared with other methods, infrared thermal imaging inspection technology has the advantages of intuitive and easy-to-understand images, fast detection speed, and low cost; it can also achieve efficient and reliable detection of small defects. It has been widely used in the field of nondestructive testing [16; 17]. In wastewater treatment plants, defects in HDPE films can be detected by analyzing cloud-induced temperature changes on the film surface. This method requires that the cloud cover lasts for more than 1 min and that the solar radiation drops by more than 400 W/m\({}^{2}\). However, the duration of cloud cover is unpredictable in natural environments. This unpredictability limits the practical application of this method [18]. In this study, infrared technology was used to detect welding defects on the surface of HDPE film and inside the weld seam based on the characteristics of the simultaneous temperature difference between the defective area and the intact area of HDPE film in a landfill. HDPE film defect edges contain much information [19; 20; 21], and extracting the defect edges from infrared images can effectively identify the geometric features of the defects.