Observations on attenuation and shear-wave velocity in fine-grained, marine sediments

Compressional(P)-wave attenuation, shear(S)-wave attenuation, and shear(S)-wave velocity measurements, compiled for fine-grained, unconsolidated sediments, show trends that do not support empirical relationships [Hamilton, J. Acoust. Soc. Am. 68, 1313-1340 (1980); in Acoustics and Ocean Bottom (F. A. S. E. Specialize Conf., Madrid, 1987)] commonly used for geoacoustic modeling. The exception is compressional-wave attenuation data, from 10 Hz to 500 kHz, that essentially follow a frequency dependence of f(1) (+/-2 standard deviations). P-wave attenuation with depth is difficult, if not impossible, to predict given the variability in the data. Examination of several attenuation/depth profiles indicates that those of Mitchell and Focke [J. Acoust. Sec. Am. 67, 1582-1589 (1980)] may be the best choice for predicting P-wave attenuation with depth at the present time. The assumption that S-wave and F-wave attenuations are proportional [Hamilton, J. Acoust. Sec. Am. 60, 334-338 (1976c); in Acoustics and Ocean Bottom (F. A. S. E. Specialize Conf., Madrid, 1987)] is not supported by the data. S-wave attenuations calculated from effective stress mimmick the data, however, suggesting this as a better method for predicting S-wave attenuation with depth. S-wave velocity/depth profiles fall into high- and low-velocity groups that (1) reflect gross differences in sediment texture, and (2) demonstrate the importance of sediment characterization in order to select an appropriate velocity/depth function.