Shallow acoustic velocity and attenuation profiles have been measured in different seafloor mass-flow features of the subaqueous Mississippi Delta. Velocities of less than 1000 ft/sec occur routinely in these gas-charged muds, and energy at frequencies above approximately 100 Hz is almost completely absorbed by only a few tens of feet of very gassy sediment.High resolution seismic profiles acquired on the seafloor show frequent contamination from seafloor "ground roll," "singing" phenomena, and other trapped energy events. These observed acoustic anomalies correlate well with the known local seafloor instability features.
Much of the seafloor in the immediate vicinity of the Mississippi Delta consists of sedimentary material containing high concentrations of free gas. Specialized techniques were developed for direct measurement of the acoustic properties of these gas-charged sediments, to sub-bottom depths of the order of 300 ft (100 m). These techniques included seafloor receiving arrays, modified checkshot profiles conducted in conventional engineering borings, and a seafloor-penetrating, hydrophone-instrumented rod used in the study of the shallowest sub-bottom layers. Collection of corresponding sediment samples was carefully integrated with these seismic experiments, to better determine the geologic nature and mechanical properties of the gas-charged materials. Final data acquisition practice was influenced both by the unusual physical properties of the materials being investigated, and the presence of strong and rapidly changing currents from the Mississippi River. Results of these in situ measurements demonstrate the widespread occurrence of anomalously low acoustic velocities (less than 1000 ft/s, or 300 m/s) and related extreme attenuation of higher frequency energy (greater than 100 Hz).
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