Field measurements of attenuation and permeability of shallow water sediments

Abstract

The velocity and attenuation of sands and clays in shallow oceans were measured by cross-hole tomography experiments using the pseudo-random binary sequence (PRBS) as source signals with the PRBS frequency of 1 and 3 kHz. Using measured velocity images, 2D velocity and density fluctuation spectra were calculated. The acoustic attenuation due to scattering by velocity and density fluctuations in the sediment volume was calculated using the measured velocity and density fluctuation spectra. The attenuation due to scattering occupies 30 to 98% of the total attenuation measured during the field experiments. The intrinsic attenuations of sands and clays were 0.3 to 1.5 dB/m/kHz and 0.01 to 0.02 dB/m/kHz, respectively. The permeability of the sands and the clays inverted by the Biot theory is 2.0 to 14.0 darcy and 0.1 to 120 md, respectively. The acoustically inverted permeability values agreed very well with the direct measurements of permeability of the sediments. The experimental results indicate that the acoustic attenuation measured in the fields is mainly due to volume scattering and that the intrinsic attenuation is due to the Biot mechanism of pore-fluid within the sediments. [Work supported by ONR Code 321OA and Kansai Power and Light Co.]