## 2 Materials and Methods
### AML System  
The All Materials Locator (AML) is a handheld subsurface utility detection device that utilizes modulated ultra-high radio frequencies (UHRF) to detect variations in subsurface density. This technology is particularly effective in identifying underground structures by analyzing contrasts in material composition and density, making it well suited for detecting linear features such as pipelines, cables, and other buried infrastructure [27]. According to the manufacturer, the AML \(\text{Pro}^{\text{TM}}\), SubSurface Instruments, Inc. (De Pere, WI, USA) employs ultra-high radio frequency technology to locate buried PVC and other plastic pipes using GPS and advanced patented digital signal processing. Additionally, the AML \(\text{Pro}^{\text{TM}}\) is capable of detecting subsurface materials of various compositions--plastic, metal, wood, cable, or pipe--and functions effectively under challenging field conditions, including clay, wet soil, snow, and even standing water [27; 28; 29]. These capabilities and application ranges of the AML device are consistently documented in the manufacturer's specifications and product manuals, as well as in third-party technical reports that assess its performance under various field conditions [27; 28; 29]. One major advantage of ultrasonic-based detection systems like the AML is that they do not require direct physical contact with the target. Instead, they rely on wave propagation and reflection to detect and localize subsurface objects. Unlike traditional electromagnetic or Ground-Penetrating Radar (GPR) systems, ultrasonic-based methods are less sensitive to the electromagnetic properties of surrounding materials, allowing for greater adaptability across varying soil types and moisture conditions [30; 31]. The detection process of the All Materials Locator (AML) begins when the device emits high-frequency acoustic- or radio-frequency waves into the ground. These waves propagate through multiple subsurface layers and interact with embedded materials or voids that differ in density. When such discontinuities are encountered, a portion of the wave energy is reflected, while the remaining energy continues to propagate deeper. The reflected signals are received and analyzed to calculate the time delay between emission and reception, which is subsequently used to determine the depth and location of buried objects. The acquired data are then processed using computational algorithms to generate visual representations of the subsurface structure [11; 23]. The fundamental principle of this method--based on time-of-flight measurement and reflection of wave energy--has been widely adopted in recent ultrasonic detection research. For instance, ref. [32] developed a positioning algorithm for buried polyethylene pipelines by considering acoustic attenuation properties in heterogeneous soil environments. Similarly, ref. [33] proposed an ultrasonic array tomography approach to accurately reconstruct subsurface images and detect cracks within reinforced concrete structures. Ref. [34] demonstrated that real-time signal processing techniques embedded in ultrasonic visualization frameworks can significantly improve defect localization accuracy. In another study, ref. [35] numerically compared physics-based and machine learning approaches applied to Ground-Penetrating Radar (GPR) data for underground utility network characterization. Their results confirmed the effectiveness of signal interpretation methods, particularly travel time analysis, for determining the depth and geometry of buried infrastructure. These findings are consistent with the operational principles of All Materials Locator (AML) devices and further support their applicability in various geotechnical and infrastructure monitoring contexts. As illustrated in Figure 3, the fundamental principle of this method relies on the time-of-flight of transmitted and reflected waves, a concept similar to that used in ultrasonic detection and widely applied in recent subsurface sensing studies.