Gas‐Rich Sediment and Coastal Wetland Loss in Louisiana

Abstract

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.