Field experiences in the direct measurement of anomalous acoustic behavior of gas-rich sediments

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).