In situ electrical resistivity and bulk porosity of the oceanic crust Costa Rica Rift

Becker, Keir; Herzen, R. P. Von; Francis, T. J. G.; Anderson, Roger N.; Honnorez, J.; Adamson, A. C.; Alt, Jeffrey C; Emmermann, Rolf; Kempton, PD; Kinoshita, Hajimu; Laverne, Christine; Mottl, Michael J.; Newmark, R. L.

doi:10.1038/300594a0

Cited by 141 publications

(131 citation statements)

References 14 publications

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“…This is gives a measure of the bulk properties of the oceanic crust, which most probably varies from 1 S •m1 near the surface to perhaps 0.001 S•m-l at the base of the crust (Becker et al, 1982). The crustal conductivity decreases with depth due to crack closure and sealing.…”

Section: Tectonic Interpretationmentioning

confidence: 99%

Two-Dimensional Electrical Conductivity Structure across the Southern Coastline of Australia.

White

Heinson

1994

J. geomagn. geoelec

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Geomagnetic field measurements have been made at three new seafloor locations off the coast of southern Australia, to extend a magnetometer array deployed by White and Polatayko (1978). Geomagnetic depth sounding (GDS) and vertical gradient sounding (VGS) estimates provide a measure of both the TE and TM mode response of the continent-ocean boundary, which is relatively two-dimensional (2D) over hundreds of kilometres. The seafloor data from the continental shelf, mid-continental slope and at the edge of the abyssal plain show a strong geomagnetic coast effect which can largely be accounted for by the sea-water and a thick wedge of sediments in a rifted marginal basin on the continental shelf. A 2D inversion of the GDS and VGS estimates to determine the sub-seafloor conductivity structure suggests that (a) the oceanic crust and continental crust have conductivities of 0.05 and 0.005 S-m ' respectively to a depth of 10 km, (b) the upper mantle above 140 km has conductivity 0.01-0.003 S-m', (c) the upper mantle between 140 and 390 km has conductivity less than 0.003 S-m ', and (d) the lower mantle below 390 km has conductivity less than 0.3 S•m'. No distinct boundary between oceanic and continental lithosphere conductivity can be discerned. The conductivity structure beneath the coastline of southern Australia is somewhat less conductive than the preferred model ofKellett et al. (1991) for southeastern Australia.

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Section: Tectonic Interpretationmentioning

confidence: 99%

Two-Dimensional Electrical Conductivity Structure across the Southern Coastline of Australia.

White

Heinson

1994

J. geomagn. geoelec

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“…The bottom 514 m, cored during Leg 83, is characterized by a transition from pillow lavas, pillow breccias, hyaloclastites, and basalt flows to an underlying unit interpreted as a sheeted dike complex similar to that observed in many ophiolites. Geophysical logging and other downhole experiments have found significant physical property changes with depth Becker et al, 1982).…”

Section: Introductionmentioning

confidence: 99%

Seismic Velocities, Densities, and Porosities of Layer 2B and Layer 2C Basalts from Hole 504B

Christensen¹,

Salisbury²

1985

Initial Reports of the Deep Sea Drilling Project

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Compressional and shear wave velocities at confining pressures to 6 kb, densities, and porosities were measured for 32 samples obtained from 836 to 1350 m below seafloor (BSF) in Hole 504B, the section drilled on Leg 83 of the Deep Sea Drilling Project. These data in combination with similar measurements on 28 basalt samples from the section from 274.5 to 836 m, drilled on Legs 69 and 70, provide a comprehensive set of physical property data for over 1000 m of oceanic crust. The velocities, densities, and porosities measured in the laboratory exhibit greater variability in the upper portion of the hole. In general, compressional and shear wave velocities and densities increase with depth, reaching average values at 1 kbar of V p = 6.45 km/s, K s = 3.45 km/s and p = 2.94 g/cm 3 within the sheeted dike section. Porosities decrease with depth to values generally less than 1% near the bottom of the hole.

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“…The empirically determined expression relating fluid resistivity to temperature depends on a set of electrolyte-dependent coefficients (Quist & Marshall 1968). If the pore fluid has a salinity close to seawater then the fluid resistivity (ρ f ) depends on temperature as: Becker et al 1982) where T is the temperature in deg C. The relationship is approximately valid up to 300°C. If the predicted temperature profile for the Carnmenellis granite is used in this expression, the result indicates a factor of 10 decrease in the component of bulk resistivity determined by the fluid phase over the upper 10 km of the crustal section.…”

Section: Introductionmentioning

confidence: 90%

Deep geoelectric survey of the Carnmenellis granite

1991

International Journal of Rock Mechanics and Mining Sciences &am

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SUMMARYA magnetotelluric survey across the Carnmenellis granite, part of the Cornubian batholith of SW England, is described. The granite is being investigated with reference to the extraction of Hot Dry Rock geothermal energy. The sounding data are found to be influenced by near-surface static distortion which can be minimized by the construction of a granite-average sounding tensor. The anisotropy in the average tensor can be attributed to the spatial form of the granite. This is verified by 2-D modelling and the model results are used as control in the assessment of the vertical resistivity profile. The resistivity values obtained by the survey are found to correspond to a saturated 'wet' granite down to at least 10 km. Despite the high geothermal gradient the anticipated decrease of resistivity with increasing temperature does not take place. Below a depth of just over 1km, resistivity values are found to increase to a depth of about 6 km. The results are compared with the laboratory-observed response of granite to increasing applied pressure and stress. The resistivity/depth section appears consistent with the completion of joint and crack closure by a depth of 7 km and a transfer to a pore-dominated resistivity mechanism below this depth. An electrical base to the granite is resolved at a depth of 14 km in accord with the depth to base adopted by regional gravity models.2

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In situ electrical resistivity and bulk porosity of the oceanic crust Costa Rica Rift

Cited by 141 publications

References 14 publications

Two-Dimensional Electrical Conductivity Structure across the Southern Coastline of Australia.

Two-Dimensional Electrical Conductivity Structure across the Southern Coastline of Australia.

Seismic Velocities, Densities, and Porosities of Layer 2B and Layer 2C Basalts from Hole 504B

Deep geoelectric survey of the Carnmenellis granite

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