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.

DISCLAIMERThe submitted manuscript has been authored by a contractor of the U.S. Government under contract No. W-31-1WENG-38. Accordingly, the U. S. Government retains a nonexclusive, royalty.free licensn to pvblish or reproduce the published form of this contribution. or allow others to do so, for U. S. Government purposes.This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recorn- ABSTRACTVelocity and attenuation data were used to test theoretical equations describing the frequency dependence of compressional wave velocity and attenuation through gas-rich sediments in coastal Louisiana. The cross-well data (obtained from a variable-frequency, cross-well seismic experiment using source frequencies of 1, 3, 5, and 7 kHz) were augmented with velocities derived from a nearby seismic refraction station using a lowfrequency (QO Hz) source. Velocities obtained h m the borehole-sonic tool (18 kHz)were not used, because it is unclear at this time what signal phase was being detected. Energy at 1 and 3 kJ3z was successfully transmitted over distances from 3.69 to 30 m; the 5-and 7-lcHz data were obtained only at distances up to 20 m.Velocity tomograms were constructed for one borehole pair and covered a depth interval of 10-50 m. Results from the tomographic modeling indicate that gas-induced low velocities are present to depths of greater than 4-0 m. Analysis of the velocity dispersion suggests that gas-bubble resonance must be greater than 7 kHz, which is above the range of frequencies used in the experiment. Washout of the boreholes at depths above 15 m resulted in a degassed zone containing velocities higher than those indicated in both nearby reitaction and reflection surveys. Velocity and attenuation information were obtained for a low-velocity zone centered at a depth of approximately 18 m. Measured attenuations of 1.57,2.95, and 3.24 dB/m for the 3-, 5-, and 7-m~ signals, respectively, were modeled along with the velocity data using a silt-clay sediment type, Density and porosity data for the model were obtained from the geophysical logs; the bulk and shear moduli were estimated from published relationships. Modeling results indicate that gas bubbles measuring 1 mm in diameter occupy at least 25% to 35% of the pore space.

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Detailed environmental studies associated with landfills, burial pits, vaults, underground storage tanks, contaminant plumes, and unidentified contaminant sources adjacent to buildings at Aberdeen Proving Ground, Maryland, are being conducted. Efficient and innovative data-acquisition procedures are imperative in order to provide complete coverage at a large number of small-sized sites. Because APG is a chemical weapons research and development facility, noninvasive geophysical techniques are a necessity. Real-time data processing and interpretation using computers in a field setting permit rapid changes in the design of the survey and in decision making.