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

Abstract

[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 U, and the coefficients are functions of the fractional area of asperity contact A f across the cracks, a measure of crack morphology; thus, v 5 v(A f , U). Fits to data sets from the lava and dike sections in Holes 504B and 1256D yield A f 5 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 8 6 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 A f 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 j and v suggests a relationship between j and A f as well. This relationship between j and v can be explained if the permeability coefficient j 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.

Ocean Crustal Seismic Layer 2C

“…The apparent layer 2C velocities are 6.3 km/s at Site 504 and 6.2 km/s at Site 1256. In Holes 504B and 1256D these transitions mark changes in the porosity and the morphology of the cracks and voids that populate the rocks (Carlson, 2010(Carlson, , 2014a. Layer 2C is demonstrably present in both downhole logs and seismic profiles from Sites 504 and 1256.…”

Seismic Signatures of Hydrothermal Pathways Along the East Pacific Rise Between 9°16′ and 9°56′N

“…The discrepancy between the latter and our estimates most probably originates from assumptions in both approaches. In our case, permeability estimates are highly affected by uncertainties related to our modeling parameters and their effects on the absolute values of velocities (2‐D modeling of 3‐D space, underestimated magnitude of low‐velocity anomalies, assuming isotropic subsurface, just to name a few), along with the uncertainties in empirically derived velocity and porosity/permeability relationships (Carlson, ).…”

Receiver functions using OBS data: promises and limitations from numerical modelling and examples from the Cascadia Initiative

“…For example, V P /V S can be higher than 10 for unconsolidated sediments with more than 60% porosity and a P velocity approximately equal to 1.8 km s −1 , and V P /V S is ∼2-3 for compacted marine sediments with P velocity between 2.5 and 3 km s −1 (Hamilton, 1979). Note that near a mid-ocean ridge with no sedimentary cover, the seismic properties of the uppermost oceanic crust may be similar to those of marine sediments due to the high-porosity and low permeability of fractured rocks (Carlson, 2014). Table 2; model M4).…”