## 4 Discussion  
The present study provides a field-based validation of the All Materials Locator (AML) system's performance in detecting non-metallic pipelines under standardized testbed conditions. In contrast to previous studies focused on metallic utilities or shallow installations, the AML system demonstrated robust accuracy in locating deeply buried polyethylene (PE) pipelines at a 3 m depth--a scenario traditionally problematic for electromagnetic-based methods such as GPR. This outcome underscores the AML's potential as a regulatory-compliant alternative for subsurface utility detection in challenging environments. Unlike Ground-Penetrating Radar (GPR), which is sensitive to soil moisture and material conductivity, the AML system demonstrated stable signal acquisition through ultra-high-frequency signal processing based on subsurface density variations. Across 32 repeated trials, the system consistently detected all 10 designated points along the 3 m deep pipelines, with an average vertical depth of 2.524 m for PE and a horizontal reference distance averaging 5.75 m. The low root mean square errors (RMSE) of 0.0616 m for position and 0.1229 m for depth confirm the system's precision under favorable field conditions. Histograms and deviation analyses further showed that the measurement errors followed near-Gaussian distributions, indicating random noise characteristics rather than systematic bias. This supports the system's statistical stability and field reliability. Moreover, the compact, handheld nature of the AML device, combined with real-time visual feedback, enhances its usability in urban construction environments where space and time constraints are common. However, the AML system lacks a visual scanning interface such as the B-scan provided by GPR systems (see Figure 2). Although no false positives or spurious detections were observed during the trials, the absence of real-time visual feedback made it more challenging to identify the initial scanning area, particularly in zones without surface indicators. In addition, while the AML device includes GPS functionality, post-processing is required to convert the recorded measurements into usable geospatial formats compatible with GIS platforms. The ability of the AML system to detect non-metallic utilities at depths up to 3 m using a portable platform represents a substantial advancement in underground utility mapping. It offers a promising alternative or complement to GPR- and EMI-based technologies, particularly for infrastructure maintenance, excavation safety, and pre-construction planning. However, it is important to acknowledge that the field experiments in this study were conducted in decomposed granite soil, which is known to have favorable transmission characteristics. As such, the results represent an ideal-case scenario and may not fully generalize to more complex or adverse soil environments. In real-world scenarios, soils with higher clay content, water saturation, or heterogeneous backfill materials can significantly alter signal propagation, potentially reducing the system's accuracy and detection range. To address this limitation, future research should include systematic evaluations across a range of geotechnical conditions, including clayey, silty, and waterlogged soils. These expanded trials will be critical for assessing the AML system's robustness and validating its broader applicability in urban and non-urban contexts alike. While the present study focused on detection performance up to the 3 m depth specified by South Korean regulations, the testbed used does not contain buried utilities at depths greater than 3 m. As a result, a sensitivity analysis or success/failure rate assessment beyond this depth could not be performed. Future studies should include extended depth scenarios to evaluate the system's effectiveness in detecting deeply buried utilities, particularly in contexts where the infrastructure may exceed standard installation depths.