Large-Scale Electrical Resistivity and Bulk Porosity of the Upper Oceanic Crust at Hole 395A

Abstract

A long-spaced, DC electrical resistivity experiment was run during Leg 109, through over 400 m of the upper oceanic crust in Hole 395A. The same experiment had been run during Leg 83 in Hole 504B, and the results from the two holes are compared. This experiment responds to in-situ crustal resistivities over vertical and radial averaging scales of about 5-50 m, and is quite sensitive to the large-scale porosity of the crust. Apparent crustal resistivities are about 30 ohm-m through most of the pillow lavas cored in Hole 395A, and increase to about 300 ohm-m at the base of a thick unit of sealed pillows near the bottom of the hole. A cyclic downhole variation of resistivity with a wavelength of 20-100 m correlates with the main lithologic units; within each unit, resistivity increases and porosity decreases with depth. Bulk porosities in Hole 395A were estimated by applying Archie's Law, and range from 10%-15% in the uppermost pillows to about 4% in the resistive unit near the bottom of the hole. These porosity values are remarkably similar to the upper crustal porosities obtained in Hole 504B, although the porosity and sonic structure of the crust at Hole 395A are much less clearly defined than at Hole 504B.

Seismic attenuation in upper ocean crust at Hole 504B

“…The pulse lengthens at 1650-1700 mbsf, but the amount is small and there is no corresponding change in amplitude at this depth (Figures 7 and 8 There are no peaks in the number of fractures viewed on the borehole wails [Ayadi et al, 1998]. Resistivity measured with both the deep and shallow-penetrating laterologs is a local high, so the inferred porosity is low (Figure 1 l c, d) [Becker, 1985 Table 1 a. pores and vugs remaining from previous cycles of chemical deposition. While the circulation patterns indicated by the distribution of alteration minerals suggests that in the past this might have been a highly porous zone, the pore-filling nature of the zeolites and the low porosity inferred from the resistivity logs indicates that most open space has been filled.…”

Ocean floor volcanism: constraints from the integration of core and downhole logging measurements

“…Estimation of apparent porosity within ocean crust can be derived from resistivity measurements recorded with the longspacing resistivity device (Becker 1985) or by the dual laterolog ). The apparent porosity is a computed value which represents both electrolytic conduction processes in pores, fractures, cracks (including microcracks) and surface conduction mechanisms; the latter are particularly important when conductive alteration minerals (e.g.…”

“…The relationship between porosity and resistivity for ocean floor basalts has been extensively discussed by Becker (1985) and Lovell & Pezard (1990), who concluded that the use of Archie~)s formula with a = 1.0 and m = 2.0 approximates the relationship between porosity and resistivity. However, a different law with a = 10 and m close to 1.0 has been suggested for ocean floor basalts (Flovenz et al 1985;Broglia & Moos 1998), which reflects electrolytic conduction in elongated volumes filled with pore fluid (sea water).…”

“…The transition between these two volcanic settings correlates with the thick breccia unit, which may represent debris accumulated during a hiatus in volcanism as the crustal segment moved off-axis. Hydrothermal circulation in the ocean crust is strongly influenced by lithological anisotropy (Becker 1985;Pezard & Anderson 1990;Brewer et al 1991;Teagle et al 1996), such that the more massive impermeable flows serve to both focus and restrict fluid flow into the more permeable pillow lavas and breccia units (Alt et al 1993). As a result of this enhanced fluidrock interaction, the pillows and particularly the breccias, contain the most altered rocks in the drilled section (Teagle et al 1996), which ultimately controls the mass balances used to constrain chemical fluxes in the crustal segment.…”

Introduction, Background, and Principal Results of Leg 128 of the Ocean Drilling Program, Japan Sea

“…Similar electrical resistivity experiments have been previously performed in DSDP holes (Francis, 1982;Von Herzen et al, 1983;Becker, 1985); however, our experiment represents a novel two-vessel approach.…”