The effect of pressure on the electrical conductivity of KTB rocks

Complex electrical resistivity and permeability were measured on two gneiss samples and nine amphibolites (originally located at a depth of 4150 m to 5012 m) from the main drilling of the German deep drilling project (KTB). Measurements were performed as a function of hydrostatic pressures up to 240 MPa on core samples (30 mm in diameter and 10–20 mm high). For each measurement, two samples were used, one being parallel, and one perpendicular to the borehole axis. At low pressures and again at maximum pressure the frequency dispersion (1 kHz up to 1 MHz) of the complex resistivity was measured using a two electrode device. An unusual pressure effect was detected on some of the samples and was established to be due to the oriented deposition of good conducting phases in the foliation. Rock fabric and the orientation of ore mineralization was measured on thin sections and polished sections prepared from the same samples.     

The electrical conductivity of the Moon

Investigations of the lunar electrical conductivity were described in excellent reviews by Sonett (1974) and Dyalet al. (1976). In this paper we will try to consider some new aspects of this problem in comparison with the Earth's data.     

The electrical conductivity of clouds

Summary A modified Gerdien cell was designed, evaluated, and built for measurement of the polar conductivities in clouds. This conductivity dropsonde was attached to a U.S. Weather Bureau, 1680 mHz, radiosonde for telemetry and to measure pressure, temperature, and relative humidity profiles. The combined instruments were ejected from aircraft, and others were released from balloons into the region of interest.Eight flights were made during the 1967 thunderstorm season. Three of these drops were successful in measuring conductivity inside of electrically active clouds. Two fair-weather profiles were measured for comparison purposes, and three of the drops were faulty.These very preliminary results tend to indicate considerable electrical conductivity in thunderclouds. The data are too few to support a strong statement in favor of increased conductivity, but the instruments were sufficiently reliable to prove that the conductivity was not reduced, as is normally assumed, in the clouds investigated.This research was supported by the Atmospheric Sciences Section, National Science Foundation under Grant GA-701.     

The bedrock electrical conductivity map of the UK

Airborne electromagnetic (AEM) surveys, when regionally extensive, may sample a wide-range of geological formations. The majority of AEM surveys can provide estimates of apparent (half-space) conductivity and such derived data provide a mapping capability. Depth discrimination of the geophysical mapping information is controlled by the bandwidth of each particular system. The objective of this study is to assess the geological information contained in accumulated frequency-domain AEM survey data from the UK where existing geological mapping can be considered well-established. The methodology adopted involves a simple GIS-based, spatial join of AEM and geological databases. A lithology-based classification of bedrock is used to provide an inherent association with the petrophysical rock parameters controlling bulk conductivity. At a scale of 1:625k, the UK digital bedrock geological lexicon comprises just 86 lithological classifications compared with 244 standard lithostratigraphic assignments. The lowest common AEM survey frequency of 3 kHz is found to provide an 87% coverage (by area) of the UK formations. The conductivities of the unsampled classes have been assigned on the basis of inherent lithological associations between formations. The statistical analysis conducted uses over 8 M conductivity estimates and provides a new UK national scale digital map of near-surface bedrock conductivity. The new baseline map, formed from central moments of the statistical distributions, allows assessments/interpretations of data exhibiting departures from the norm. The digital conductivity map developed here is believed to be the first such UK geophysical map compilation for over 75 years. The methodology described can also be applied to many existing AEM data sets.     

西藏南部蛇绿岩套电导率研究

大地电磁(MT)资料显示,青藏高原地壳及地幔中普遍存在着高导层.作为大陆造山带中古洋盆岩石圈残片,蛇绿岩套的电导率测量可为了解古洋盆地区地壳及地幔的电性结构提供极其有用的信息.本研究中,我们在压力为1 GPa或3 GPa下,用交流阻抗谱法测量了采自西藏南部地区的蚀变辉长岩、玄武岩、角闪橄榄岩及方辉橄榄岩四个样品的阻抗谱,并进一步得出样品的电导率,不同样品电导率与温度之间的关系满足Arrhenius关系式.在实验温度范围内,蛇绿岩套电导率的对数logσ位于-6.0~-0.5 S/m之间,且随着温度的增高,不同样品电导率增大约4~5.5个量级.样品在未脱水的情况下,低温段的活化焓变化范围在0.4~0.6 eV之间,高温段的活化焓变化范围为1.7~2.6 eV之间.同时,我们研究了样品中结构水含量及铁含量对实验电导率的影响,验证了样品电导率与铁含量之间呈正比关系.当对样品结构水含量进行归一化后,相同温度下各样品的电导率随铁含量的增加而增大,而对样品铁含量归一化后,相同温度下各样品的电导率随样品中水含量的增加而增大.将实验电导率与藏南地区大地电磁结果进行了对比,发现本研究中各样品高温段实验电导率结果均落在大地电磁结果范围内.     

The influence of oxidation state on the electrical conductivity of olivine

The electrical conductivity of a single crystal of San Carlos olivine (Fo₉₂, 0.16 wt.% Fe₂O₃) has been measured as a function of temperature and oxygen fugacity (fO₂). After heating to 1338°C at fO₂ = 10^?12 atm., the conductivity at 950°C was 10^?5 (ohm-m)^?1, almost 3 orders of magnitude less than that measured in air. This decrease is due to the reduction of Fe³⁺ to Fe²⁺. Further heating to 1500°C at fO₂ = 10^?14 atm., decreased the electrical conductivity at 950°C to 10^?6 (ohm-m)^?1. When recovered at room temperature, the speciment had Fo₉₆ composition and contained small, opaque blebs distributed throughout the crystal. Derivations of temperature profiles for the earth's mantle from conductivity-depth models must take account of the important role played by iron oxidation state in the electrical conductivity of olivine.     

Negative pressure effect on the electrical conductivity of San Carlos olivine and its implication to the electrical structure in the upper mantle

Pressure effect on the electrical conductivity of San Carlos olivine was investigated by the newly installed electrical conductivity measurement system at China University of Geosciences. Electrical conductivity of San Carlos olivine aggregates was measured up to 12 GPa and 1475 K using the Walker-type multi-anvil apparatus equipped with eight WC cubes as the second-stage anvils. The pressure generation against applied load for the experimental assemblage was examined by phase transition of Bi,quartz, forsterite under different P-T conditions. To check the data validity of this new system, electrical conductivities of the serpentinites and talc samples were measured. The results are consistent with the published data of the same samples. Electrical conductivity(σ) of the San Carlos olivine aggregates and temperature(T) satisfy the Arrhenian formula: σ=σ0exp[.(ΔE+PΔV)/kT].The pre-exponential factor(σ0), activation energy(ΔE) and activation volume(ΔV) yield value of 7.74 S/m, 0.85 eV and 0.94cm3/mol, respectively. Electrical conductivities of the San Carlos olivine aggregates decline with increasing pressure at same temperatures. The negative pressure effect can be interpreted by strain energy model of defect energy together with the lattice deformation. In addition, the electrical conductivity-depth 1-D profile of the upper mantle was constructed based on our results and some assumptions. The calculated profile is concordant with the geophysical observation at the depth of 180–350 km beneath Europe, which indicates that the upper mantle beneath Europe might be dry.     

碳酸盐化橄榄岩的电性研究

为进一步探讨上地幔的高导层成因,了解碳酸盐在上地幔电性方面的作用并估算上地幔高导层的碳酸盐含量,本文对不同碳酸盐含量的橄榄岩及玄武岩样品在2~3 GPa、300~1300℃的条件下进行了电性实验研究.研究初步发现:碳酸盐熔体显著增强橄榄岩、玄武岩样品的导电能力;单纯用含硅酸盐熔体的橄榄岩或单纯用含水橄榄岩可能难以解释上地幔某些区域的异常高导现象;同样,单纯用碳酸盐化的橄榄岩可能也难以解释上地幔某些区域的高导现象;上地幔的高导区很可能是碳酸盐熔体、硅酸盐熔体及水的共存区域.     

碳酸盐化橄榄岩的电性研究

为进一步探讨上地幔的高导层成因,了解碳酸盐在上地幔电性方面的作用并估算上地幔高导层的碳酸盐含量,本文对不同碳酸盐含量的橄榄岩及玄武岩样品在2~3 GPa、300~1300℃的条件下进行了电性实验研究.研究初步发现:碳酸盐熔体显著增强橄榄岩、玄武岩样品的导电能力;单纯用含硅酸盐熔体的橄榄岩或单纯用含水橄榄岩可能难以解释上地幔某些区域的异常高导现象;同样,单纯用碳酸盐化的橄榄岩可能也难以解释上地幔某些区域的高导现象;上地幔的高导区很可能是碳酸盐熔体、硅酸盐熔体及水的共存区域.     

Global electrical conductivity of the earth

The Banks (1969, 1972) and Parker (1970) models of the electrical conductivity distribution are critically reviewed along with classical models by Chapman (1919), Lahiri and Price (1939), Rikitake (1950a, b, c) and others. The modern models do not seem to account for the geomagnetic variations having a continuum spectrum and S_q at the same time. A large difference in response between the 1-day and 0.5-day period components of S_q is suspected to be caused by a resonance-like induction in the superficial layer of the earth. Dufficulties in determining the conductivity of the earth's top layer are also emphasized.An overall distribution of conductivity within the earth which seems to be the most reliable at present, is drawn mostly on the basis of Banks' model.     

The qualitative character of the global electrical conductivity of the earth

A spectral stacking and smoothing procedure has been applied to unbroken hourly values of H and Z, for 1964 and 1965, from 17 observatories, in order to estimate the magnitude and phase of the P₁⁰ response of the Earth to long-period geomagnetic fluctuations. Exploratory techniques have been used to gauge when sufficient smoothing has been applied, and to identify the qualitative character of the global electrical conductivity of the Earth.     

The effect of baric disturbances in the atmosphere on microseismic processes in the crust

The effect of baric variations of different origins on characteristics of seismic noise is analyzed in the frequency range 0.03–20 Hz. Long period variations in atmospheric pressure caused by cyclones, whose period T ranges from half a day to a few days, are shown to increase the microseismic background amplitude by two to four times in the frequency range 0.03–1 Hz (the coefficient of linear correlation between time variations in the amplitude and atmospheric pressure is K = 0.65 at a significance level of r = 0.95). Short-period baric variations with T ～ 5–30 min associated with the passage of cold fronts lead to a tenfold increase in the microseismic background amplitude in the frequency range 4–8 Hz (K = 0.67 at r = 0.95). In this case, disturbances of seismic background are recorded for 20–60 min after the passage of an atmospheric front and display an exponential drop in the amplitude. In distinction to cyclones, an atmospheric front increases the number of impulsive microseismic events of the resonance type.     

The electrical conductivity of eclogite in Tibet and its geophysical implications

The electrical conductivity of Tibetan eclogite was investigated at pressures of 1.5–3.5 GPa and temperatures of 500–803 K using impedance spectroscopy within a frequency range of 10-1–106 Hz. The electrical conductivity of eclogite increases with increasing temperature(which can be approximated by the Arrhenius equation), and is weakly affected by pressure. At each tested pressure, the electrical conductivity is weakly temperature dependent below ~650 K and more strongly temperature dependent above ~650 K. The calculated activation energies and volumes are 44±1 kJ/mol and-0.6±0.1 cm3/mol for low temperatures and 97±3 kJ/mol and-1.2±0.2 cm3/mol for high temperatures, respectively. When applied to the depth range of 45–100 km in Tibet, the laboratory data give conductivities on the order of 10-1.5–10-4.5 S/m, within the range of geophysical conductivity profiles.     

The effect of aerosols on the temperatures of a zonal average climate model

The effect of atmospheric aerosols on the temperatures of a zonal average climate model is investigated. This is done by introducing into the climate model the results of extensive calculations of the effect of aerosols on the partitioning of solar radiation. Calculations are performed for a non-absorbing and a sloghtly absorbing aerosol, for average and for heavy aerosol amounts. The results indicate that the presence of atmospheric aerosols causes a cooling of the earth's surface and atmosphere, at all latitudes, with aerosol amounts, especially for the absorbing aerosols. The results are compared with some of the previous estimates of aerosol effects on climate.     

Temperature-time dependence of the electrical conductivity of garnets

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