How crack porosity and shape control seismic velocities in the upper oceanic crust: Modeling downhole logs from Holes 504B and 1256D

Mounting evidence from Holes 504B and 1256D suggests that porosity is the principal factor affecting velocities in the upper oceanic crust. Spheroidal inclusion and asperity compression models based on reprocessed sonic velocity logs and apparent fractional porosities estimated from deep resistivity logs reveal how both porosity and the geometry of the pore space affect seismic velocities in Layer 2. Models that best match the data indicate the following: First, there are three populations of cracks in Hole 504B; most of the transition zone and the dike section are populated by low concentrations of weak cracks, while the upper part of the transition zone and the extrusive pile contain a higher concentration of stiffer cracks. Similarly, the deepest part of the dike section in Hole 1256D can be modeled by a dilute concentration of thin or weak cracks, while the overlying dikes, the transition zone, and the extrusive pile contain a higher concentration of stiffer cracks. Second, in a striking confirmation that porosity controls velocities throughout Layer 2, 90% or more of the variance of sonic velocities logged at both sites is explained by porosity in these models, while third, the effect of pressure on the variation of sonic velocity is essentially negligible. Fourth, while there is no direct correlation between crack populations and igneous lithostratigraphy, some changes of crack population do correspond to metamorphic transitions, and fifth, velocities typical of Layer 3 are reached when the porosity falls to low values (0.2% in Hole 1256D and 0.6% in Hole 504B).