Electrical Conductivity Depth Profiles Research Articles

Effect of Different Mineralogical Proportions on the Electrical Conductivity of Dry Hot-Pressed Sintering Gabbro at High Temperatures and Pressures

Electrical conductivities of the dry hot-pressed sintering gabbro with various mineralogical proportions (CpxXPl100−X, X = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 vol% (the signals of Cpx and Pl denote clinopyroxene and plagioclase, respectively) were measured in the YJ-3000t multi-anvil pressure and Solartron-1260 impedance spectroscopy analyzer at temperatures of 773–1073 K and pressures of 1.0–3.0 GPa. At the given pressure conditions, the electrical conductivity and temperature conformed to an Arrhenius relation. For the fixed mineralogical composition of Cpx50Pl50, the electrical conductivities of the samples significantly increased with the rise of temperature, but slightly decreased with increasing pressure. Furthermore, the activation energy and activation volume were determined as 1.06 ± 0.12 eV and 6.00 ± 2.00 cm3/mole, respectively. As for the various mineralogical compositions of dry gabbro, the electrical conductivities of the samples increased with the rise of volume percentage of clinopyroxene (Cpx) at 1.0 GPa. It is proposed that the main conduction mechanism is the small polaron, owing to the positive relation between the electrical conductivity and the iron content in samples. On the basis of these obtained conductivity results, laboratory-based electrical conductivity–depth profiles for the hot-pressed sintering gabbro with various mineralogical proportions and temperature gradients were successfully established. In conclusion, although the present acquired electrical conductivity results on the dry hot-pressed sintering gabbro with various mineralogical proportions cannot explain the high conductivity anomaly in the oceanic crust and West African craton, it can provide one reasonable constraint on the mineralogical composition in these representative gabbro-rich regions.

English

Solar quiet (Sq) daily currents variation obtained in the Hermanus (34.34oS, 19.22oE), Tsumeb (19.24oS, 17.72oE), Hartebeesthoek (25.68oS, 28.09oE) and Maputo (25.97oS, 32.57oE), were employed in determining the mantle electrical conductivity depth profile of the Southern African region. The external and internal contributions in the solar quiet field were separated using the spherical harmonic analysis (SHA), after which, the transfer functions were used to compute the electrical conductivity depth profiles of the region. A downward increase was observed in electrical conductivities and deep depth of penetration within the Earth regions. In Hartebeesthoek, Hermanus, Maputo and Tsumeb, the evaluated average electrical conductivity values are 0.028 Sm-1, 0.039 Sm-1, 0.057 Sm-1 and 0.0.025 Sm-1 at depths of 76.4 km, 84.1 km,141.0 km and 111.5 km  at the respective maximum depths of penetration of 1052.8 km, 1467.0 km, 1160.8 km and 1289.5 km in Hartebeesthoek, Hermanus, Maputo and Tsumeb, the calculated electrical conductivity reached the maximum values of 0.498 Sm-1, 0.323 Sm-1, 0.387 Sm-1 and 0.187 Sm-1 respectively. Discontinuities were observed in all the profiles but are more prominent in Tsumeb region near 390.0 - 750.0 km, 820.0 - 980.0 km and 200.0 - 300.0 km. From these results, we are stating that the effects from the deeper 3-D structures (such as gold, copper etc), the hydrated transition zone and effect from the ocean contribute to the greater depth of Sq penetration. Key words: Solar quiet currents, Southern Africa, electrical conductivity, spherical harmonic analysis, Mantle.

A comparison of non-linear optimisation algorithms for recovering the conductivity depth profile of an electrically conductive block using eddy current inspection

This paper compares 15 different non-linear optimisation algorithms for reconstructing the depth profile of electrical conductivity for a conductive block of material using multi-frequency measurements from an eddy current probe. Synthetic data was generated using a finite element model, where the underlying conductivity profile was randomly generated and smoothed, noise was then superimposed on the synthetic data. The compared algorithms were designed such that optimal performance was achieved for each algorithm by using the knowledge of the true underlying conductivity profile used to generate the synthetic data. The most competitive algorithm after ten iterations was determined to be dependent on the a priori estimate and level of noise, where the Broyden–Fletcher–Goldfarb–Shanno, augmented Levenberg-Marquardt and Levenberg-Marquardt are each top of different case studies. The augmented Levenberg-Marquardt and Levenberg-Marquardt were then employed on measured data, where the algorithms used a novel update procedure (in the nuclear graphite application) for the regularisation parameter. The study considers the conductivity profile of graphite blocks of the type used in the nuclear industry.

Temperature and electrical conductivity of water in Lake Nyos transmitted by an automatic observation buoy

An automatic observation buoy (AOB) was installed at Lake Nyos in March 2014. The temperature and electrical conductivity (EC) of lake water were continuously observed from March 10, 2014 to April 15, 2016. The thermocline and chemocline located deeper than −90 m subsided 15 m in the observation period due to the bottom water removal by the CO2 degassing pipes. The velocity of subsidence was estimated quantitatively by correlating the time variation of EC obtained by AOB and the depth profile of EC. The velocity of subsidence was not constant, namely, it was fast in dry season and slow in rainy season. The time variation in the velocity of subsidence can be attributed to the seasonal variation of water flux in the degassing pipes. Considering the water balance in the deep part of lake, the flux of ground water supplied to lake was estimated to be 6.5*106 (m3/year), which is comparable to the recharge rate of ground water within the catchment area surrounding Lake Nyos. Abrupt drops in temperature and conductivity were observed at −120 m in January 2016, when the temperature at -4 m reached 21.8 °C as the lowest value. The abrupt drops are due to the sinking of surface cold water.

Bayesian Analysis of Spatially-Dependent Functional Responses with Spatially-Dependent Multi-Dimensional Functional Predictors

Modeling high-dimensional functional responses utilizing multi-dimensional functional covariates is complicated by spatial and/or temporal dependence in the observations in addition to high-dimensional predictors. To utilize such rich sources of information we develop multi-dimensional spatial functional models that employ low-rank basis function expansions to facilitate model implementation. These models are developed within a hierarchical Bayesian framework that accounts for several sources of uncertainty, including the error that arises from truncating the infinite dimensional basis function expansions, error in the observations, and uncertainty in the parameters. We illustrate the predictive ability of such a model through a simulation study and an application that considers spatial models of soil electrical conductivity depth profiles using spatially dependent near-infrared spectral images of electrical conductivity covariates.

Evidence for spatial variability in hydraulic redistribution within an oak–pine forest from resistivity imaging

Electrical resistivity imaging (ERI) was used to determine spatiotemporal resistivity changes in the vadose root zone of an oak–pine forest during an 8-day period of drying following a rainfall event. Electrical images from 0 to 2 m depth, incorporating the shallow root zone, were inferred to represent spatiotemporal variability of moisture between tree and non-tree areas resulting from hydraulic redistribution (HR). Electrical resistance measurements on a grid of 96 electrodes were collected, six times a day, from July 2nd to July 10th, 2009. Images were produced from a 3-D inversion of resistance measurements that were assigned weights based on error models that accounted for both measurement (reciprocal) and modeling errors. To account for temperature variations, a sinusoidal diurnal temperature correction based on the 1-D heat conduction equation and measurements at 5 cm depth was applied to the resistivity results. Laboratory measurements of resistivity as a function of moisture content for samples from the site obeyed Archie’s Law, suggesting that electrolytic conduction is the dominant current pathway in these soils. Spatiotemporal variability in moisture dynamics due to root zone processes was therefore inferred from spatial variation in changes of electrical conductivity over time. Electrical conductivity depth profiles were generated for tree versus non-tree locations using conductivities from corresponding finite element mesh voxel coordinates. These profiles reveal a stable temporal profile beneath trees relative to a pronounced decrease in electrical conductivity over time at non-tree locations. Relative changes in electrical conductivity between 6 am and 6 pm surveys revealed that both tree and non-tree locations are more conductive at 6 am, although changes are greater at tree locations. This finding is likely diagnostic of hydraulic lift (HL) overnight by tree roots. Regardless of location, the differences between 6 am and 6 pm surveys are consistent with HR, impacting moisture content across the entire survey area.

Effects of Microstructure on Eddy Current Residual Stress Characterization of Shot-Peened Inconel 718

Recent studies have shown the possibility of exploiting the eddy current (EC) method for nondestructive characterization of near-surface residual stresses in surface-treated nickel-base superalloy components. The method involves measuring swept frequency EC signals to obtain near-surface electrical conductivity depth profiles, which can be converted into residual stress profiles using the empirical piezoresistivity relationship. However, the relationship between EC responses and residual stress profiles appears to depend on the microstructure. This paper reports on a systematic study of microstructural effects on the EC responses and near-surface conductivity deviation profiles induced by shot peening, using a series of heat-treated Inconel 718 samples with different secondary phase contents and thus a variety of hardness levels. Calibrated EC signals from 100 kHz to 50 MHz were measured from aged and unaged samples, before and after shot peening at various Almen intensities. Strong dependence of the EC responses on the sample hardness and microstructure has been observed. A procedure is introduced to convert the EC signals into shot-peening-induced conductivity deviation profiles where the microstructure dependency is suppressed. The resulting estimates of the conductivity changes are larger than expected, indicating that characterization of shot-peening-induced residual stresses by EC requires further understanding of the material responses beyond the empirical piezoresistivity relationship. Other mechanisms that could contribute to the observed conductivity changes are discussed.

Measurements of wave velocity and electrical conductivity of an amphibolite from southwestern margin of the Tarim Basin at pressures to 1.0 GPa and temperatures to 700 °C: comparison with field observations

SUMMARY In situ measurements of elastic wave velocities and electrical conductivities in the three structural directions (normal to foliation Z, perpendicular to lineation in foliation Y and parallel to lineation X) for an amphibolite collected from southwestern margin of the Tarim Basin, northwest China, were carried out in the laboratory. The elastic wave velocity was measured with thecombinedtransmission–reflectionmethodatpressuresupto1.0GPa(atroomtemperature) and temperatures up to 700 ◦ C (at 1.0 GPa) and the electrical conductivity was measured with the impedance spectroscopy from 250 to 700 ◦ C at 1.0 GPa. The experimentally determined data included compressional (Vp) and shear wave velocities (Vs), velocity anisotropy (Av), intrinsic pressure and temperature derivatives of Vp and Vs, electrical conductivity (σ), electrical conductivity anisotropy (Aσ) and the parameters of the Arrhenius relationship. Elastic wave velocities increase in the structural directions Z, Y, X, with Vp of 6.63, 6.78 and 6.95 km s −1 and Vs of 3.75, 3.82 and 3.96 km s −1 for Z, Y and X, respectively, at pressure of 1.0 GPa. Elastic wave velocities increase linearly with pressure at room temperature and pressures between 0.25 and 1.0 GPa and decrease linearly with increasing temperature at 1.0 GPa. The pressure coefficients of the sample are in the range of 0.1883–0.2308 km s −1 GPa −1 for Vp and 0.1149–0.1678 km s −1 GPa −1 for Vs. The temperature coefficients are in the range of 2.09–2.35 × 10 −4 km s −1 GPa −1 for Vp and 1.28–1.68 × 10 −4 km s −1 GPa −1 for Vs .T he electrical conductivity increases with increasing temperature, consistent with the Arrhenius relationship. Activation energies for the three structural directions of the amphibolite are in the range of 0.71–0.75 eV. The amphibolite shows velocity anisotropy (4.15–4.86 per cent for Vp and 5.29–5.84 per cent for Vs at 0.25–1.0 GPa) and electrical conductivity anisotropy (11.1–25.2 per cent). Based on the regional crust model and geothermal gradient, velocity and electrical conductivity-depth profiles were calculated for the sample. These profiles were then compared with those derived from seismic reflection/refraction data and from electromagnetic data. Our results showed that the amphibolite sample has Vp and Vs in agreement with those of the middle and lower crust obtained from seismic reflection/refraction data, and σ in accord with that of the lower crust deduced from electromagnetic data. The lower crust of the electromagnetic crust model is roughly equivalent to the middle and lower crust layers of the seismic crust model. Therefore, it is suggest that the amphibolite may be one of the constituents of the present middle and lower crust in the Tarim Basin.

Water distribution across the mantle transition zone and its implications for global material circulation

Various methods for inferring the water distribution in Earth's mantle are reviewed including geochemical and geophysical methods. The geochemical approach using the water contents of basalts shows that the water content in the source regions of ocean island basalt is generally larger than that of the source region of mid-ocean ridge basalt, but the location of the source regions of ocean island basalts is poorly constrained. Geophysical approaches have potential of providing constraints on the spatial distribution of water but their usefulness depends critically on the sensitivity of geophysical observations to water content relative to other factors, in addition to the resolution of geophysical observations. Existing experimental data on the influence of water on seismologically observable properties and on electrical conductivity are reviewed. Frequently used seismological observations such as the anomalies in seismic wave velocities and of the topography on the mantle discontinuities are only weakly sensitive to water content but more sensitive to other factors such as the major element chemistry and temperature for a typical range of water contents. In contrast, electrical conductivity is highly sensitive to water content and only modestly sensitive to other factors such as temperature, oxygen fugacity and major element chemistry. Models of electrical conductivity–depth profiles are constructed where the influence of hydrogen and iron partitioning among coexisting minerals and of the depth variation in oxygen fugacity are incorporated. It is shown (i) that the electrical conductivity varies more than two orders of magnitude for a plausible range of water content in the mantle (~ 10 ppm wt to ~ 1 wt.%) and (ii) that if water content is constant with depth, there will be a drop in electrical conductivity at ~ 410-km. Although the resolution is not as high as seismological observations, geophysically inferred electrical conductivity distributions generally show higher conductivity in the mantle transition zone than the upper mantle, suggesting that the water content in the transition zone is higher than that in the upper mantle with some lateral variations. Implications of inferred water distribution are discussed including the possible partial melting near 410-km and its role in global water circulation.

Evaluation of a Geonics EM31-3RT probe to delineate hydrologic regimes in a tile-drained field

A world-wide need to use water resources efficiently necessitates more effective approaches to study water and contaminant transport in soil. This study examined the effectiveness of a multi-receiver electromagnetic induction probe (Geonics EM31-3RT) and modeling software (EMIGMA) to delineate hydrological regimes at field scale. The site consisted of 20 (15 m × 15 m) tile-drained plots in Southern Ontario, Canada. Measurements of apparent soil electrical conductivity (ECa) and magnetic susceptibility were obtained using the EM31-3RT in each plot at four distances (0, 2.25, 4.5 and 7.5 m) from the tile drain, and on three occasions (August 22, 26 and 29) in 2003. The EMIGMA was used to simulate a depth profile of electrical conductivity (ECs) from EM31-3RT readings. The near-surface soil showed significantly (p < 0.01) smaller ECa values than at greater depth. The ECa measurements made directly over the tile drains were smaller than those observed further away due to the presence of the drains. Cluster analysis indicated that the largest ECa values were at the lower elevations of the site related to the redistribution of moisture from higher elevations. The effect of tile drains and rainfall events on ECa was simulated well by EMIGMA, with smaller ECs values above the drains compared to further away, and showing an increase in ECs in the near-surface soil after rain. This study suggests that EM31-3RT measurements combined with EMIGMA simulation of electrical conductivity can provide valuable information on depth profiles of ECa and water dynamics in soil.

Electromagnetic detection of a 410-km-deep melt layer in the southwestern United States

A deep-seated melt or fluid layer on top of the 410-km-deep seismic discontinuity in Earth's upper mantle, as proposed in the transition-zone 'water filter' hypothesis, may have significant bearing on mantle dynamics and chemical differentiation. The geophysical detection of such a layer has, however, proved difficult. Magnetotelluric and geomagnetic depth sounding are geophysical methods sensitive to mantle melt. Here we use these methods to search for a distinct structure near 410-km depth. We calculate one-dimensional forward models of the response of electrical conductivity depth profiles, based on mineral physics studies of the effect of incorporating hydrogen in upper-mantle and transition-zone minerals. These models indicate that a melt layer at 410-km depth is consistent with regional magnetotelluric and geomagnetic depth sounding data from the southwestern United States (Tucson). The 410-km-deep melt layer in this model has a conductance of 3.0 x 10(4) S and an estimated thickness of 5-30 km. This is the only regional data set that we have examined for which such a melt layer structure was found, consistent with regional seismic studies. We infer that the hypothesized transition-zone water filter occurs regionally, but that such a layer is unlikely to be a global feature.

Simple analytical approximations for eddy current profiling of the near-surface residual stress in shot-peened metals

Because of their frequency-dependent penetration depth, eddy current measurements are capable of mapping the near-surface depth profile of the electrical conductivity. This technique can be used to nondestructively characterize the subsurface residual stress distribution in certain types of shot-peened metals, e.g., in nickel-base superalloys. For quantitative evaluation of the experimental results, analytical and computational techniques are needed to solve the direct and inverse problems, i.e., to predict the frequency-dependent apparent eddy current conductivity from the depth profile of the frequency-independent intrinsic electrical conductivity of the specimen and vice versa. Simple analytical approximations are presented for both the direct and inverse eddy current problems by exploiting two specific features of the subsurface electrical conductivity variation caused by near-surface residual stresses in shot-peened metals. First, compressive residual stresses are limited to a shallow surface region of depth, much less than typical probe coil diameters. Second, the change in electrical conductivity due to residual stresses is always very small, typically less than 1%. The proposed approximations are verified by numerical comparison to much more complicated numerical solutions.

Tikhonov Regularization of Electrical Conductivity Depth Profiles in Field Soils

AbstractElectromagneetic induction is an established, noninvasive technique for measuring the apparent electrical conductivity of soils and contaminants in soils and aquifers. Nevertheless, the noninvasive determination of electrical conductivity depth profiles using only aboveground electromagnetic induction measurements remains difficult since it involves ill‐posed inverse problems. Tikhonov regularization is an established technique for the solution of ill‐posed linear inverse problems. We used Tikhonov regularization for estimating the electrical conductivity depth profile of the soil from aboveground electromagnetic induction measurements. Results from the inversion of sets of Geonics EM‐38 measurements are reported. It was found that this technique is superior to other inverse techniques previously reported in the literature.

International conference on Lake Nyos disaster, Yaoundé, Cameroon 16–20 March, 1987: Conclusions and recommendations

An automatic observation buoy (AOB) was installed at Lake Nyos in March 2014. The temperature and electrical conductivity (EC) of lake water were continuously observed from March 10, 2014 to April 15, 2016. The thermocline and chemocline located deeper than −90 m subsided 15 m in the observation period due to the bottom water removal by the CO2 degassing pipes. The velocity of subsidence was estimated quantitatively by correlating the time variation of EC obtained by AOB and the depth profile of EC. The velocity of subsidence was not constant, namely, it was fast in dry season and slow in rainy season. The time variation in the velocity of subsidence can be attributed to the seasonal variation of water flux in the degassing pipes. Considering the water balance in the deep part of lake, the flux of ground water supplied to lake was estimated to be 6.5*106 (m3/year), which is comparable to the recharge rate of ground water within the catchment area surrounding Lake Nyos. Abrupt drops in temperature and conductivity were observed at −120 m in January 2016, when the temperature at -4 m reached 21.8 °C as the lowest value. The abrupt drops are due to the sinking of surface cold water.