High rates of wetland loss in southern Louisiana provide the impetus for examining the role that trapped, biogenic gases play in regulating subsidence of coastal areas. A significant cause for wetland loss in this region is relative sea-level rise produced by sediment-volume reduction. Dewatering, grain reorientation and packing, and oxidation of organic-rich sediments are thought to be the main processes for volume loss. It is argued that natural and anthropogenic causes for sediment degasification play a critical role in sediment-volume reduction. Compressional wave velocities were measured at 34 sites in both the abandoned (Holocene) and modern parts of the Mississippi Delta. A low-frequency source (<200 Hz) was used to maximize sound-wave dispersion caused by interstitial gas bubbles. Compressional wave velocities measured at low frequencies relative to the gas-bubble resonant-frequency undergo maximum change from the velocity for a gas-free sediment. Seismic refraction studies and velocity measurements using standard reverse-spread profiling indicate that the upper 40-70 m of the sedimentary section has sound velocities significantly less than the speed of sound through water (1,500 m/s), despite the fact that the water table is within 3 m of the ground surface. The low-velocity zone has average velocities ranging from 800 to 1,150 rn/s in the Holocene delta and from 900 to 1,300 mJs in the modern delta. Uphole traveltimes obtained from approximately 90 km of continuous-coverage, seismic reflection profiles yield velocity estimates that range from 900 to 1,300 m/s. First and secondary arrivals determined from the reflection profiles are currently being used to model the low-velocity zone.