PREDICTIONS OF THE ACOUSTIC SCATTERING RESPONSE OF FREE-METHANE BUBBLES IN MUDDY SEDIMENTS

The response of the sediments of Eckernfoerde Bay, Germany to acoustic remote sensing previously has been attributed qualitatively to gas fe atures which were postulated to exist in situ within the sediment. The existence of features as small as 0.5-mm equivalent spherical radius has now been confirmed by x-ray computed tomography of cores taken and scanned at in sial pressures. The interaction of an acoustic pulse fr om the acoustic sediment classification system (ASCS) with this type o f gassy sediment was modeled and the results are presented here. The b ubble scattering response model includes both the effects of nonspheri cal bubbles and of sediment resistance to shear deformation (as expres sed by the dynamic shear modulus). Model predictions made using the ob served gas bubble distribution agree with normal incidence ASCS data, exhibiting extended returns (i.e., greater than the source pulse lengt h) from the seafloor bubble layers as well as high attenuation within the bubble layers. Spectrograms of ASCS and modeled return pulses were also similar. These results suggest that the acoustic return characte ristics of the near surface sediments of Eckernfoerde Bay are dominate d by scattering from a distribution of gas bubbles with depth. (C) 199 6 Acoustical Society of America.