Compressional- and shear-wave velocities and attenuation in deep-sea sediment during laboratory compaction

Velocities and attenuation of compressional (50 kHz) and shear waves (100 kHz) in two deep-sea sediment samples are investigated as a function of effective pressure up to 20 MPa with simultaneous measurement of porosity, density, and permeability. In both samples, the compressional-wave velocities show a strong increase with pressure, ranging from 1683 m/s at 1 MPa to 2128 m/s at 20 MPa in a silty clay and from 1840 m/s to 2462 m/s in a foraminiferal mud. Shear wave propagation is strongly restricted at pressures below 4 and 9 MPa in the respective sample, indicating a structural change in the sediment material at critical porosity values of 0.430 and 0.397, respectively. The compressional-wave attenuation, in terms of 1/Q(P), varies between 0.03 and 0.08, and 0.04 and 0.07, respectively. A result previously not reported in laboratory studies is a maximum in 1/Q(P) as a function of effective pressure observed in both samples. The shear-wave attenuation is nearly constant as a function of pressure, having average values of about 0.07. The maximum of compressional-wave attenuation occurs at the respective critical porosity for each sample. Velocities and attenuation from this experiment exhibit a systematic pattern in a Q(P)/Q(S) vs (V-P/V-S)(2) representation. (C) 2004 Acoustical Society of America.