The Affects of Alteration and Porosity on Seismic Velocities in Oceanic Basalts (and Diabases) Based on Logging Results and Laboratory Studies of Samples Recovered by Drilling

Abstract:

Seismic velocities in the lavas that cap normal oceanic crust are affected by both crack porosity and alteration of the primary mineral phases, chiefly to clays. Porosity accounts for 75-80% of the velocity variation in sonic log velocities in the lava sections of Holes 504B and 1256D, but the effect of alteration on the properties of the basalts has not been assessed. In this analysis, the grain velocities in basalt and diabase samples have been estimated from an empirical linear relationship between grain density and the P-wave modulus. The estimated velocity in fresh, zero-porosity basalt is 6.96±0.07 km/sec. In basalts, alteration reduces the average grain velocity to 6.74±0.02 km/sec; cracks at the sample scale further reduce the velocity to 5.86±0.03 km/sec, and large-scale cracks in the lava pile reduce the average in situ velocity to 5.2±0.3 km/sec. Cracks thus account for nearly 90% of the difference between seismic (in situ) velocities and the intrinsic velocity in the unaltered solid material. Alteration also accounts for a small, but statistically significant increase of velocity with depth in the lavas. Grain velocities in the diabase samples are statistically indistinguishable from the intrinsic velocity, and show no variation with depth; alteration does not significantly affect the velocities in the diabase samples from Hole 504B. This result is consistent with previous analyses, which have demonstrated that velocities in the dikes are controlled by crack porosity. Except in the uppermost part, velocities in diabase samples measured at 20 MPa match sonic log velocities, indicating that velocities in the dike section are controlled by cracks at the scale of lab samples, as opposed to large-scale cracks in the formation.