Gas-Rich Sediment And Coastal Wetland Loss In Louisiana

High rates of wetland loss in southern Louisiana provide the impetus for<br>examining the role that trapped, biogenic gases play in regulating subsidence of<br>coastal areas. A significant cause for wetland loss in this region is relative sea-level<br>rise produced by sediment-volume reduction. Dewatering, grain reorientation and<br>packing, and oxidation of organic-rich sediments are thought to be the main<br>processes for volume loss. It is argued that natural and anthropogenic causes for<br>sediment degasitication play a critical role in sediment-volume reduction.<br>Compressional wave velocities were measured at 34 sites in both the<br>abandoned (Holocene) and modem parts of the Mississippi Delta. A low-frequency<br>source (~200 Hz) was used to maximize sound-wave dispersion caused by<br>interstitial gas bubbles. Compressional wave velocities measured at low<br>frequencies relative to the gas-bubble resonant-frequency undergo maximum<br>change from the velocity for a gas-free sediment.<br>Seismic refraction studies and velocity measurements using standard<br>reverse-spread profiling indicate that the upper 40-70 m of the sedimentary section<br>has sound velocities significantly less than the speed of sound through water<br>(1,500 m/s), despite the fact that the water table is within 3 m of the ground<br>surface. The low-velocity zone has average velocities ranging from 800 to 1,150<br>m/s in the Holocene delta and from 900 to 1,300 m/s in the modem delta. Uphole<br>traveltimes obtained from approximately 90 km of continuous-coverage, seismic<br>reflection profiles yield velocity estimates that range from 900 to 1,300 m/s. First<br>and secondary arrivals determined from the reflection profiles are currently being<br>used to model the low-velocity zone.<br>Theoretical velocity modeling, using material properties consistent with<br>observed sediment types, suggests that approximately 6-20s of the available pore<br>space must be occupied by gas to account for the observed velocities. This gas is<br>believed to be trapped in unconnected pore spaces to exist in the form of methane<br>gas bubbles within the interstitial pore waters.