ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

1aAO3. The effects of free-methane bubbles on the propagation and scattering of compressional and shear wave energy in muddy sediments.

Michael D. Richardson

Sean R. Griffin

Kevin B. Briggs

Naval Res. Lab., Stennis Space Center, MS 39529-5004

Aubrey L. Anderson

Anthony P. Lyons

Texas A&M Univ., College Station, TX 77843-3146

Free-methane bubbles cause significant scattering of acoustic energy in the soft sediments of Eckernfoerde Bay, Baltic Sea. In situ and laboratory measurement of sediment geoacoustic and physical properties were made in an attempt to understand the physical mechanisms responsible for this scattering. In situ shear wave velocities (at 100--500 Hz) increased from 5--7 near the surface to 15--20 m s[sup -1] at 2 m into the seafloor, whereas in situ compressional wave velocities (at 38 and 58 kHz) varied little (~1425 m s[sup -1]) with depth. Methane bubbles apparently caused significant attenuation of compressional waves at depths below 1 m, whereas shear wave attenuation was unaffected by gas and decreased with depth. Compressional waves (at 400 kHz) in cores (1%--5% free gas) maintained at in situ pressures were highly attenuated but show little evidence of velocity dispersion. Comparison of geoacoustic data with theory suggest that primary control of the interaction of acoustic profilers with this gassy seafloor is by bubbles larger than about 1 mm diameter and that the observed high-frequency scattering can be described by models of the seafloor bubbles as individual scatters of acoustic energy. Response of the seafloor as a medium with modified bulk propagation properties would occur at lower frequencies than those used in the Eckernfoerde experiments.