ASA 127th Meeting M.I.T. 1994 June 6-10

1pAO2. Porosity, permeability, and shear strength images of the seabed measured by acoustic cross-well tomography experiments.

Tokuo Yamamoto

Appl. Marine Phys. Div., RSMAS, Univ. of Miami, Miami, FL 33149

Cross-well tomography of the sedimentary seabeds below man-made islands in Tokyo Bay and Florida Bay was affected by very high attenuation and very high background noise; nevertheless, high resolution (0.2 to 1 m) velocity images were obtained between wells separated by long distances (50 to 250 m). A piezoelectric source in a 60-m-deep well-used long sequences (4095 cycles) of phase-modulated pseudo-random binary codes at high carrier frequencies (up to 10 kHz). A 24-channel hydrophone array in another well received the signal. Inversion of first arrival times by damped least-squares imaged the compressional wave velocities. Assuming the normal consolidation condition, the porosity and shear strength images were predicted from the compressional wave velocity image. The differences in the compressional wave velocity images obtained using different carrier frequencies were used to determine the permeability image of sediments based on the Biot theory. The direct measurements of porosity and shear strength conducted on the cores and the standard penetration test boreholes were used to verify the tomographic predictions, resulting in good agreements. The direct measurements of permeability from pumping tests confirmed the tomographically determined permeability images. [Work supported by KSC and ONR.]