Sand moisture critically determines buried gas pipeline leak patterns: insights from fiber-optic vibration–thermal signatures

Subsurface gas pipeline leaks present critical safety risks governed by soil moisture-regulated multiphase dynamics. This study employs laboratory experiments integrated with dual-modality distributed fiber-optic sensing, simultaneously acquiring vibration and temperature signatures, to establish moisture-dependent failure criteria. Subsurface gas leakage from pressurized pipelines generates two distinct failure modes through soil interaction: spewing leaks characterized by crater-forming gas jets, and diffusion leaks marked by gradual pore-scale migration. Experimental results demonstrate that these regimes are governed by soil moisture content: (1) Under arid conditions (<5% moisture content), spewing leaks manifest through violent gas ejection of particulate matter, producing intense vibration amplitudes, and rapid thermal transients. (2) Conversely, in moisture-saturated sands (5%–15% moisture content), diffusion leakage predominates, characterized by gas percolation through evolving cavity–fissure networks that induce dual vibration mechanisms—soil matrix deformation and gas–granular interactions—coupled with moderated cooling rates. The observed transition between spewing and diffusion leak patterns, governed by soil moisture levels, highlights the need for dynamic detection protocols in pipeline integrity management. These findings advance mechanistic understanding of subsurface gas transport phenomena while offering potentially actionable guidelines for optimizing fiber-optic monitoring systems in heterogeneous soil environments.