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



[1] 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 Φ, and the coefficients are functions of the fractional area of asperity contact Af across the cracks, a measure of crack morphology; thus, v = v(Af, Φ). Fits to data sets from the lava and dike sections in Holes 504B and 1256D yield Af = 0.096(8) and 0.045(3), with rms errors of 1.7 and 0.3%, respectively. Lower values of Af 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 8 ± 4%. 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 Af from ∼0.003 to 0.1 accounts for an increase of velocity from ∼3 to >5 km/s. An empirical log-linear relationship between permeability κ and v suggests a relationship between κ and Af as well. This relationship between κ and v can be explained if the permeability coefficient κo is a closely constrained quadratic function of the area of asperity contact Af. Af is thus the primary variable affecting both seismic velocities and the permeability of the uppermost oceanic crust.