ULTRASONIC P- and S-WAVE ATTENUATION IN OCEANIC BASALT

Abstract

Measurements of compressional wave attenuation are presented for 30 low-porosity (φ = 0.4-8.8 per cent) oceanic basalts collected from 10 oceanic drill holes. The first laboratory measurements of shear wave attenuation in oceanic basalts are presented for 14 rocks from the test suite. For full saturation, attenuation coefficients (α) 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 propagation at 150 MPa. Qp and Qs values range from 14 to 167 and 8 to 27, respectively. Both Q and α show a significant confining pressure dependence to 400 MPa. This pressure dependence is caused by the opening and closing of compliant microcracks. Q and α, both shear and compressional, are also shown to depend on porosity, with α increasing and Q decreasing with porosity. Qs/Qp values are reported for 14 samples from the test suite and may be important in determining the degree of saturation when combined with Vp/Vs data. Qs/Qp values vary from 0.12 to 0.40 for fully saturated samples. Saturated samples generally display low Qs/Qp (< 0.4) and high Vp/Vs (> 1.75), which is in good agreement with published sandstone Qs/Qp 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 agree well with field seismic measurements of oceanic layer 2A Qp; however, there is no clear explanation for this agreement, since no single attenuation mechanism has been proven to dominate at both high (MHz) and low (Hz) frequencies. Nevertheless, the good agreement between laboratory and field data 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.