Proactive evaluation of cavities and leakages near buried pipelines is essential for preventing accidents caused by pipeline damage. Although various detection techniques have been investigated, methods that enable continuous monitoring over extended areas are few. This study proposes a method for detecting cavities and leakages using transmission lines attached to pipelines, which offers the potential for wide-area and long-term subsurface monitoring. Laboratory experiments are conducted in a soil chamber partitioned into three segments, in which eight model pipelines and multiple electrical wires are installed. The soil chamber is filled with air, soil, and water to simulate ground conditions representing cavities, intact ground, and leakage, respectively. Electromagnetic waves are measured using electrical wires arranged at seven different spacings (2.6, 6.0, 13.7, 20.0, 27.4, 40.0, and 67.4 mm) in a soil chamber. Experimental results indicate that the electromagnetic wave velocity increases and decreases in the presence of cavities and leakage, respectively. Velocity is particularly sensitive to changes in spacing up to 13.7 mm, beyond which variations are minimal. These trends are explained by considering the influence range and permittivities of the materials. Based on the permittivity and influence range, the optimal spacing of the transmission lines is estimated to be 20.0 mm. This study demonstrates that optimally spaced transmission lines enable the effective evaluation of cavities and leakages in buried pipelines while offering the potential for continuous monitoring over extended areas.
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