Post-Penetration Excess Pore-Pressure Evolution at Cold Seeps: Sustained Negative Responses at Active Vents

Submarine cold seeps are dynamic fluid-release systems; yet, in situ pore-pressure responses to penetration disturbance under different seep conditions remain poorly documented. To address this gap, we conducted in situ excess pore-pressure measurements using the Jiaolong manned submersible at six sites in the Qiongdongnan cold-seep area, northern South China Sea, supplemented by vane-shear tests on nearby sediments. Non-vent sites mainly exhibited positive excess pore-pressure dissipation, whereas transitions from positive to sustained negative excess pore pressure were observed only at the two active vent sites. These transitions occurred earlier at shallower depths, with zero-crossing times of 0.3~3.2 and 5.6~8.7 min at the two vents. Minimum excess pore pressures ranged from −22.1 to −19.8 kPa and from −27.3 to −22.0 kPa, respectively, and vertical pressure differences decreased over time. Short-term fluctuations included depth-localized variations and nearly synchronous multi-depth responses; the principal event showed a common positive excursion followed by a rapid negative drop and recovery, accompanied by concurrent spectral energy enhancement across all four depths. At the nearby A2 vane-test site, the sediment at 85 cm showed a higher peak shear strength and lower sensitivity than at 55 and 70 cm; longer pressure-decay times were independently observed at deeper pore-pressure sensors. These findings document contrasting post-penetration pressure pathways associated spatially with visible venting. A conceptual model integrates penetration disturbance, depth-dependent recovery, and subsequent negative-pressure evolution.