Ultrasonic P- and S-wave attenuation in oceanic basalt

Measurements of compressional wave attenuation are presented for 30 low-por osity (phi = 0.4-8.8 per cent) oceanic basalts collected from 10 oceanic dr ill holes. The first laboratory measurements of shear wave attenuation in o ceanic basalts are presented for 14 rocks from the test suite. For full sat uration, attenuation coefficients (alpha) range from 2.48 to 9.99 dB cm(-1) for shear propagation and 0.32 dB to 4.69 dB cm(-1) for compressional prop agation at 150 MPa. Q(p) and Q(s) values range from 14 to 167 and 8 to 37, respectively. Both Q and alpha show a significant confining pressure depend ence to 400 MPa. This pressure dependence is caused by the opening and clos ing of compliant microcracks. Q and alpha, both shear and compressional, ar e also shown to depend on porosity, with alpha increasing and Q decreasing with porosity. Q(s)/Q(p) values are reported for 14 samples from the test s uite and may be important in determining the degree of saturation when comb ined with V-p/V-s data, Q(s)/Q(p) values vary from 0.12 to 0.40 for fully s aturated samples. Saturated samples generally display low Q(s)/Q(p), (< 0.4 ) and high V-p/V-s (> 1.75), which is in good agreement with published sand stone Q(s)/Q(p) data. The mechanisms most likely to be responsible for the observed high P- and S-wave attenuation are viscous local or 'squirt' flow and to a lesser extent grain boundary frictional sliding. Laboratory data a gree well with field seismic measurements of oceanic layer 2A Q; however, t here is no clear explanation for this agreement, since no single attenuatio n mechanism has been proven to dominate at both high (MHz) and low (Hz) fre quencies. Nevertheless, the good agreement between laboratory and field dat a suggests that at seismic frequencies the shallow oceanic crust may behave similarly to laboratory samples. One possible explanation is the presence of a different fluid flow mechanism for each frequency scale.