Large-Scale Electrical Resistivity and Bulk Porosity of the Oceanic Crust, Deep Sea Drilling Project Hole 504B, Costa Rica Rift

Abstract

A long-spaced, DC electrical resistivity experiment was run successfully during Leg 83, throughout over 1 km of Layer 2 of the oceanic crust at Hole 504B. This experiment responds to in situ crustal resistivities over vertical and radial averaging scales of about 5 to 50 m. Apparent crustal resistivities are about 10 ohm-m in about 600 m of pillow lavas, sharply increasing to nearly 500 ohm-m in the underlying dikes. Bulk crustal porosities were approximated by applying Archie's Law to the apparent resistivities. Apparent porosities decrease sharply from about 10% in the pillow lavas to about 2% in the dikes, and the results indicate three porosity layers which seem to correspond to seismic Layers 2A, 2B, and 2C. Comparison of the apparent bulk porosities to shipboard porosity measurements on recovered basalt samples suggests that Layers 2A and 2B have significant open fracture porosity, whereas the dikes of Layer 2C have very little fracture porosity. The sharp increase with depth of apparent resistivity and the order-of-magnitude decrease in apparent bulk porosity are highly consistent with the steep reduction with depth of bulk crustal permeability measured in Hole 504B.

A short electromagnetic profile across the Kane Oceanic Core Complex

“…This allows, to first order, an interpretation of bulk resistivity in terms of porosity [e.g., Becker, 1985]. Yet, there are additional compositional variations that influence conductivity, as well as differences in rock strength that might control degrees of cracking and porosity and therefore allow inference of lithology from resistivity measurements.…”

Electrical Resistivity Structure of the Snail Site at the Southern Mariana Trough Spreading Center

“…19.8a). The porosity of the oceanic crust of the study area could be higher than this profile because less compression is expected for a younger oceanic crust (the seafloor age of the study area is almost 0 Ma, while that of Hole 504B is approximately 6 Ma (Becker 1985)). We consider a hypothetical porosity profile for the newborn seafloor of the study area by assuming that 34 AE 16 % Evans et al (1998), and a dotted line represents an estimation with the hypothetical doubled porosity profile.…”

“…To determine a t value in Archie's law, two representative exponents of 1.2 and 2.0 are examined, which were extensively used in prior studies (e.g., Becker 1985;Evans 1994). In general, a larger t implies a less interconnected conductive fluid network through the rock (e.g., Evans 1994).…”

The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers

“…Large-scale electrical resistivities measured in this borehole (e.g. Becker, 1985) have been used to calculate apparent bulk porosities using Archie's law (Figure 3b). Bulk porosities are high (10- Becker eta!., 1985), theoretical velocity-porosity relationships based on extended Walsh and Kuster-Toksoz theories (from , and an exponential decrease in bulk porosity with depth ($ 0 =34.5%; A.= 3.3*1Q-3 m-1).…”