Electrical conductivity of dry lower crustal rocks

Abstract

This study provides values of electrical conductivity of possible lower crustal materials to assist interpretation of lower crustal magnetotelluric soundings. We present mean values of conductivity measurements collected from the literature for dry mafic and silicic rocks in the temperature range of 500°C to 1000°C. We observe statistically significant differences between rock types: mafic rocks are better conductors than granites by about half an order of magnitude and within the mafic group, aphanitic (fine‐grained) rocks have higher conductivity than phaneritic (coarse‐grained) ones. “Best‐fitting” curves of log conductivity versus temperature are presented for each rock type to show mean log conductivity values together with standard deviations so that most probable temperature ranges can be inferred from conductivity. Because the laboratory rocks are dry, their conductivities are lower at a given temperature than they would be if fluids or volatiles were present; hence any temperatures inferred from magnetotelluric (MT) contivities are upper bounds.

Simultaneous measurements of elastic wave velocities and electrical conductivity in a brine-saturated granitic rock under confining pressures and their implication for interpretation of geophysical observations

“…For the crack density parameter of 0-0.01, a large increase in the normalized conductivity is expected. The conductivity of dry crustal rocks is estimated to be less than 10 −4 S/m (e.g., Kariya and Shankland 1983) and that of crustal fluids 10-100 S/m (Nesbitt 1993). The normalized conductivity is thus expected to be less than 10 −5 at the crack density parameter of zero.…”

A Deep Transient EM Experiment in the Northern Part of Miyagi Prefecture, Northeastern Japan.

“…It may be considered that the thermal effects related to the volcanic zone reduce the resistivity in the crust. However, the effect of temperature or partial melt (e.g., Kariya and Shankland, 1983;Waff, 1974) is not likely to be a primary cause for the present conductive zone when we take the depth of such phenomena into account. The heat flow data observed around the present study area also show rather low values (Honda, 1985).…”

“…Figure 6 shows an example of the modeling process. When we initially calculate the stationary field of the layered earth model, which should be independent of the layered structure (Kaufman and Keller, 1983), it does not sufficiently agree with the amplitude of the observed data. Possible reasons of this difference can be ascribed to the anomalous field generated by the current channeling due to three dimensional structure, and ascribed to the inaccurate estimation of the source moment.…”

“…Theoretical curves for electrical responses of the stratified earth model are calculated using the mathematical expression by Kaufman and Keller (1983) and the subsurface resistivity structure is estimated by comparing these theoretical curves with the observed transient curve. We briefly summarize the fitting technique.…”

“…8 and 9. According to Kaufman and Keller (1983), amplitude of the magnetic field generated by a HED source depends on the angle 0 included between the source dipole direction and the source-receiver direction. In a cylindrical coordinate system, this dependency is simple trigonometric function; amplitudes of the vertical and the radial components is proportional to sin 0, while the azimuthal component is proportional to cos 0.…”