The influence of porosity and crack morphology on seismic velocity and permeability in the upper oceanic crust

The purpose of this study is to explore the relationships between porosity, crack morphology, seismic velocity, and permeability in the upper oceanic crust. In a theoretical model, velocity is a quadratic function of porosity , and the coefficients are functions of the fractional area of asperity contact A(f) across the cracks, a measure of crack morphology; thus, v=v(A(f), ). Fits to data sets from the lava and dike sections in Holes 504B and 1256D yield A(f)=0.096(8) and 0.045(3), with rms errors of 1.7 and 0.3%, respectively. Lower values of A(f) account for the high porosities and low velocities observed in Layer 2A. After 0.2 Ma, the porosities of both Layer 2A and the deeper part of the lava pile remain in the range 84%. The observed increase of Layer 2A velocities after 0.2 Ma is caused largely by changing crack morphology as opposed to decreasing porosity; at 8% porosity, an increase of A(f) from approximate to 0.003 to 0.1 accounts for an increase of velocity from approximate to 3 to >5 km/s. An empirical log-linear relationship between permeability and v suggests a relationship between and A(f) as well. This relationship between and v can be explained if the permeability coefficient (o) is a closely constrained quadratic function of the area of asperity contact A(f). A(f) is thus the primary variable affecting both seismic velocities and the permeability of the uppermost oceanic crust.