Acoustic Attenuation in Oceanic Gabbro

Compressional wave velocity and attenuation were measured in oceanic Layer 3 from nearly 0.5 km of recovered core at Ocean Drilling Program Site 735 in the southern Indian Ocean. High-frequency (400 kHz) acoustic transmission experiments were conducted on 117 water-saturated minicore samples under room conditions of temperature and pressure on board the drillship JOIDES Resolution. Although experimental errors were large in some deformed samples, the average of our measurements of attenuation, Q(p)-1, on these samples is 0.049 +/- 0.032. Agreement between spectral ratio and inversion estimates of Q(p)-1 indicates that acoustic noise in the data does not significantly reduce the measurement accuracy. Ultrasonic tests on a 1 per cent porosity sample at elevated effective pressures from 10 to 500 MPa showed an increase in V(p) from 6.8 to 7.0 km s-1 and a decrease in Q(p)-1 from 0.026 to 0.023. The small pressure dependence of Q(p)-1 in this low-porosity gabbro suggests that it is possible to use measurements at room conditions for relative interpretation as a function of depth.The measurements of Q(p)-1 show (1) an overall decrease as a function of depth, (2) a decrease in the average variance in the measurement with depth, and (3) an inverse relationship between average V(p) and average Q(p)-1, although Q(p)-1 shows greater variation in these rocks than V(p). Our explanation for these results is that rock fabric has a significant effect on V(p) and Q(p)-1. A simple deduction from these results is that seismic data will be strongly influenced by the degree of alteration and tectonism in similar crustal rocks. Particularly in fracture zones, seismic reflections from lithologic contacts may be attenuated, and deep crustal reflectors, identified as faults or as the Mohorovic discontinuity, may result from V(p) and Q(p)-1 contrasts caused by alteration and tectonism within Layer 3, not from the lithologic contact itself.