Electrical Conductivity Profiles Research Articles

Tidal Load and Salt Water Influences on Submarine Ground Water Discharge

AbstractField measurements were conducted to characterize the influences of salt water and tidal loading on submarine ground water discharge (SGD) into West Neck Bay on Shelter Island, New York. SGD was continuously recorded at three sites using an ultrasonic ground water seepage meter, and the spatial extents of the terrestrially derived SGD and the salt water/fresh water mixing zone were inferred from electrical conductivity profiling. While SGD increases with increasing hydraulic gradient in the far field of the coastal aquifer, shorter temporal variations of SGD were inversely correlated with tidal stage. SGD and hydraulic heads in the onshore unconfined aquifer near the coast responded asymmetrically to tidal loading, with discharge and head increasing faster than decreasing. A time lag of ∼1 h was observed between the tidal load and responses in onshore head and SGD. This lag represents a manifestation of the travel time required for tidal influences to propagate landward before the hydraulic gradient adjusts and influences seepage. The spatial extent of the fresh water component of SGD determined by electrical conductivity profiles, seepage velocity, and the time lag between tide and SGD amplitudes are in reasonable agreement with analytic estimates.

High-resolution conductivity profile measurements in detonating pressed explosive

An improved method for measuring the electric conductivity profile behind the detonation front in dense high explosives has been developed, which provides for a spatial resolution better than 0.1 mm. In comparison to the existing techniques, the proposed method ensures a more than tenfold increase in the range of conductivity measurements. The conductivity profile of detonating RDX exhibits a peak with an amplitude of ≈2 Ω−1 cm−1 and a width of about 1 mm, in agreement with known data on the chemical reaction zone thickness. The peak is followed by a “tail,” in which the conductivity is several times lower. In samples with a density of 1.75–1.8 g/cm3 (approaching the theoretical compaction limit), the conductivity peak exhibits a sharp increase in amplitude (up to 5–6 Ω−1 cm−1), while the peak width decreases to 0.5 mm.

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.

Water in the mantle: Results from electrical conductivity beneath the French Alps

A deep magnetotelluric sounding in the French Alps provided a vertical electrical conductivity profile between ∼200–1000 km. Two prominent features are observed. First, the conductivity in the depth range 400–800 km is smaller than the conductivity of a pyrolite mantle obtained from laboratory results for a normal geotherm. Second, the data do not require the conductivity to change throughout the transition zone (410–660 km). In this part of the mantle, a temperature of 350–450 C less than normal explains the magnetotelluric conductivity profile. At 200–400 km, our model favors a cold mantle with 1000–1500 ppm of water dissolved in olivine. If correct, this model suggests that the subducted slab is dehydrated before reaching the transition zone.

ESTIMATING SOIL PROFILE DEPTH WITH APPARENT ELECTRICAL CONDUCTIVITY FOR A TEXAS VERTISOL

This study aimed to develop a relationship between apparent electrical conductivity (ECa) and soil profile depthin a shrinking-swelling central Texas Vertisol. A strong relationship (R2 = 0.73) between ECa and soil profile depth to thecarbonate layer showed that soil profile depth could be estimated using ECa measurements in these soils. This relationshipbest predicted profile depth when soils were near field capacity and moisture-related electrical conductivity was high. A highcorrelation (R2 = 0.69) between two mapped ECa datasets gathered under comparatively wet and dry conditions, showedtemporal and spatial stability of ECa variation patterns. Using the relationship between ECa and soil profile depth along withmapped ECa data, ECa variation patterns could be related to spatial variation of soil profile depth within the field. Informationabout soil profile depth could be useful in crop management under rainfed conditions, especially as profile depth influencesthe water supplying capacity (WSC) of a soil. With a relationship between WSC and soil profile depth within a field, spatialvariation patterns of mapped ECa data could be used to divide a field into zones and management units of contrasting WSC.

Application of waves for remediation of contaminated aquifers.

A theory developed suggested that significant displacement of solute in saturated porous media results from the propagation of compression waves. Four independent one-dimensional experimental setups and a variety of laboratory methods were used to confirm the predictions of the theory, specifically aimed at developing a novel method of inducing compression waves for use in remediation of contaminated aquifers. Compaction and shock waves were emitted through granular porous media saturated with saline water. The changes in solute concentration at observation points along the propagating wave were used to verify the validity of theory. The first setup was designed mainly to provide a qualitative assessment (i.e., changes in pressure due to the propagating wave were not recorded). In situ quantitative measurements of the pressure and electrical conductivity profiles along a sand column were done with the second and third experimental setups, respectively, to short and long shock waves. In the fourth setup, solute displacement was visualized by X-ray absorption. The findings were consistent with the theory in all experimental setups.

Hydrogeological and Hydrochemical Investigation for Below-Sea-Level Quarrying at Cement Raw Material Site (Kocaeli-Darica, Turkey)

A research has been carried out to investigate the effects of below sea level mining on the cement raw material quality of a limestone quarry located adjacent to the shoreline near Darica-Kocaeli-Turkey. Field studies involved rock mass characterization through discontinuity surveys performed at the working benches of the quarry as well as on the core samples, monitoring of groundwater levels, performance of water pressure tests, and in-situ hydrochemical measurements. Hydrogeological data suggest that the carbonate sequence forms a poor unconfined aquifer having hydraulic conductivity values ranging between 10−6 m/s and 10−8 m/s. In the quarry, water seepages can only be observed at the shear zones. Electrical conductivity profiles taken from the boreholes located at various distances from the shore line indicated the present position of the salt water wedge.

A Bayesian approach to electromagnetic sounding in a marine environment

The problem of seafloor electromagnetic sounding is defined in terms of recovering the electrical conductivity profile beneath the seafloor from measurements of the electromagnetic field. From this viewpoint, electromagnetic sounding can be regarded as an inverse problem that infers parameters characterizing the environment. The propagation of the electromagnetic field in a multilayered marine environment is modeled by solving the Maxwell's equations and the boundary conditions for a horizontal electric dipole placed in seawater. Electromagnetic inversion represents a strongly nonlinear problem for which a direct solution is not available. A matched-field approach to this problem can be formulated based on Bayesian inversion theory, which provides environmental parameter estimates and their uncertainties. The goal of this paper is to investigate the matched-field inversion capability to recover the conductivity profile from measurements of the electromagnetic field.

Electrical conductivity and streaming potential coefficient in a moderately alkaline lava series

Coupled hydraulic and electrical problems are being investigated with renewed interest in geophysical applications. In this study, electrical conductivity and streaming potential coefficient (SPC) have been measured in the laboratory for crushed oceanite, hawaite, and trachyte samples from an almost complete moderately alkaline lava series from the Mururoa atoll (French Polynesia). For pH varying from 7 to 9.5 and ionic strength varying from 0.1 to 100 mmol/l, despite the closely related mineralogy of the rocks, contrasted results are obtained for the surface conductivity as defined by Revil and Glover: 0.10±0.07 mS/m, 8.3±0.2 mS/m and 1.05±0.16 mS/m for oceanite, hawaite, and trachyte, respectively, and for the inferred ζ potential: −6.6±0.3 mV, −26.3±0.5 mV, and −14.6±0.3 mV, respectively. The higher values obtained with hawaite suggest that electrical properties are controlled by secondary and accessory minerals such as clay minerals and zeolites resulting from low-temperature seawater alteration, rather than by the magmatic differentiation. Furthermore, a synthesis of results obtained with 20 rock samples indicates that signatures of the alteration process may emerge from the study of the ζ potential as a function of surface conductivity. The results with the lava series can be used to derive estimates of the scaling of basalt resistivity and associated SPC as a function of permeability. For permeability values smaller than 10−15 m2, the value of the SPC can be dramatically affected by the value of the surface conductivity. At least in the brittle crust, a better knowledge of the contributions of clay minerals and zeolites to surface conduction is therefore needed, both for the interpretation of electrical conductivity profiles and for the estimation of electrical variations associated with groundwater flow, as could be produced for example during volcanic or tectonic cycles.

Urethral and bladder neck functional length related to urethral electrical conductivity (UEC) profile in female stress urinary incontinence

Urethral and bladder neck functional length related to urethral electrical conductivity (UEC) profile in female stress urinary incontinence

Geoelectrical structure across the Bohemian Massif and the transition zone to the West Carpathians

Geoelectrical methods of magnetotelluric (MT) and geomagnetic deep soundings (GDS) were employed to infer the electrical conductivity structure at depths of specific regions of the Bohemian Massif (BM) and in the transition zone between the BM and the West Carpathians (WCP). Transfer functions between components of temporal variations of the natural electromagnetic field induced in the Earth by external sources were estimated and converted to electrical conductivity (resistivity) profiles. While the results from other regions of the BM are noted in references, the emphasis here is placed on the new analysis of MT/GDS data at field stations along the regional almost 500-km long Deep Seismic Sounding profile No. VI traversing the country from NW to SE. Inversion of its results suggests a complex geoelectrical structure, characteristic for each geological unit, with layers of increased electrical conductivity at crustal and uppermost mantle depths. The results also indicate strong fields of anomalous induction marking geoelectrical inhomogeneities at the eastern margin of the BM and in the foreland of the WCP.

Comparison of continental and oceanic mantle electrical conductivity: Is the Archean lithosphere dry?

Electrical conductivity profiles derived from magnetotelluric and geomagnetic sounding methods provide a means of constraining upper mantle properties that is complementary to seismic studies. We analyze conductivity profiles for an Archean craton and an oceanic setting, in conjunction with independent constraints on mantle geotherms, to estimate the water content of the upper mantle in these very different geologic environments. Results from this analysis indicate that the Archean lithosphere contains less water than the oceanic mantle in the depth range between ∼150 and ∼250 km. Below ∼250 km these oceanic and continental environments show similar conductivities, suggesting that the water content of the mantle does not vary significantly between ∼250 km and the 410‐km discontinuity. These observations indicate that the Archean lithosphere may be stabilized against convective instabilities partly because it has a high viscosity associated with a dry composition.

EFFECT of SALT and CITRIC ACID ON ELECTRICAL CONDUCTIVITIES and OHMIC HEATING of VISCOUS LIQUIDS

Four hydrocolloid solutions (starch, 4.3%; carrageenan, 1.7%; xanthan, 2%; pectin, 2.5%) were prepared in water with varying amounts of NaCl (0.25, 0.5, 0.75 and 1%). Hydrocolloid concentrations were adjusted to result in an apparent viscosity of 0.2 Pa.s at 20C and 300 s -1 . A voltage of 150 V was applied in a static ohmic heating cell to study the effect of hydrocolloid and salt on electrical conductivity (EC) and temperature profile at their pH. The pH levels were modified by citric acid addition and the combined effect of hydrocolloid, salt and citric acid was investigated. At low salt concentration, carrageenan and xanthan had shortest heating times and highest ECs. This was followed by pectin. Starch was the least effective, having slowest heating rates and lowest ECs. At 1% concentration, the salt effect overcame that of hydrocolloid giving similar temperature profiles and ECs. The effect of citric acid addition was present but negligible.

The electrical conductivity and thermal profile of the Earth's mid‐mantle

Electrical conductivity in the Earth's mantle is sensitive to temperature and chemical environment. Recent laboratory measurements of electrical conductivity are combined with candidate mantle geotherms to produce synthetic electrical conductivity profiles. These profiles are used to forward model the Earth's geomagnetic response function C, results of which are compared with the observed globally averaged response function at periods of 3.5 days to 4 months. Candidate lower mantle geotherms, representing whole‐mantle and layered convection end‐members, are compared using published electrical conductivity measurements on alumina‐bearing and alumina‐free perovskite in the conductivity models. Comparison of the predicted response functions with the observed geomagnetic response of the Earth shows that a) if lower mantle alumina is incorporated into perovskite, then the lower mantle must be cool, and b) if the alumina is not incorporated in perovskite then the results are only consistent with a hot lower mantle. In addition, the maximum alumina content of lower mantle MgSiO3 perovskite is constrained at 4%.

Using the Stabilization Plateau to Estimate Optimum Well Purge Volume

AbstractThis study concerns the problem of pump position in landfill monitoring wells and the correct time to stop purging and start sampling. Literature purge volume determination methods fail to use sufficient analyte values for comprehensive study of the stabilization plateau. From field studies it is recommended that prior to commencement of a sampling program a purge study be undertaken to determine pump position and optimum purge volume by conducting (1) a preliminary vertical electrical conductivity (EC) profile: (2) vertical profiling of formation water EC values by low‐flow pumping at 0.5 m vertical intervals‐while also noting EC vertical profile values to assure minimal effect on the water column water quality; and (3) a low‐flow, purge EC stabilization test with the pump positioned at highest formation water EC values. Sufficient EC values are needed to fulfill Conover's (1980) nonparametric tolerance interval sample size criteria, and to graphically delineate the stabilization plateau. This information will allow you to choose an optimum purge volume that allows a margin for error.

Fresh‐Saline Ground Water Interface in the Western Dead Sea Area

AbstractThe interface between fresh and saline ground water was studied in the Dead Sea area, using in situ profiles of electrical conductivity (EC) of water. The slope of the interface was found to be 10 times shallower than normally expected near the ocean because of the greater density contrast (the density of the Dead Sea is 1.23 g/cc). Although the Dead Sea system is extremely dynamic, the Ghyben‐Herzberg approximation was still found to be relevant. The shallow interface is expected to cause Dead Sea water to intrude much farther inland than does sea water adjacent to oceans. This extremely shallow interface needs to be taken into account when exploiting ground water in this area in order to avoid upconing of brines and salinization.In recent times, the hydrologic system of the Dead Sea area has changed in water levels and salinity. Simulations using SUTRA code were run for a confined subaquifer to examine the effects of such change on the location of the fresh‐saline water interface. Following the changes, the new interface is not parallel to the previous one because the increase in density causes a decrease in the interface's slope.

Calibrating an Electromagnetic Induction Instrument to Measure Salinity in Soil under Irrigated Cotton

Various calibration approaches have been proposed for determining profiles of apparent soil electrical conductivity (ECa) or soil electrical conductivity of a saturated soil paste extract (ECe) using an EM38 instrument. One of the most promising of these, the established‐coefficients approach, was selected for calibration of the EM38 to some irrigated cotton (Gossypium hirsutum L.)‐growing soil located in the Edgeroi district of the lower Namoi valley, northern NSW, Australia. However, the fitted salinity profiles were locally erratic, although global trends could be recognized. This method was compared with simple linear regression applied to the data from each depth increment separately. This was also unsatisfactory, and in fact, was not obviously different from the established‐coefficients method on the data. An alternative and statistically more rigorous approach was used in which a logistic curve is fitted to each calibration hole (the logistic profile model). This mixed random and fixed effects nonlinear model predicts ECe from measurements generated by an electromagnetic (EM) instrument (EM38) held in the horizontal mode of operation EM0,H The logistic profile model fitted the data well, with no obvious patterns in the residuals, and depended on far fewer parameters than the two alternative methods. In addition, unlike the established‐coefficients approach, the logistic model provided meaningful prediction errors.

Ionospheric quiet-day currents and their effects on the interpretation of upper mantle conductivity A review

A review of ionospheric currents,the external part of geomagnetic solar quiet (Sq)variation and the internal part of the induced currentwithin the Earth, has been carried out.The theoretical background has been reviewed,and the method of analysis, specificallythe spherical harmonic analysis (SHA),is presented. Various works are reviewed andfully discussed. It has been noted that anew model of the Sq current system could beused to obtain a mantle electrical conductivityprofile for hemispheres. It is suggested thatfurther research work be carriedout in other hemispheres using similarmodels for more robust interpretations.

Long-term performance monitoring for a permeable reactive barrier at the U.S. Coast Guard Support Center, Elizabeth City, North Carolina

A continuous hanging iron wall was installed in June, 1996, at the U.S. Coast Guard (USCG) Support Center near Elizabeth City, NC, United States, to treat overlapping plumes of chromate and chlorinated solvent compounds. The wall was emplaced using a continuous trenching machine whereby native soil and aquifer sediment was removed and the iron simultaneously emplaced in one continuous excavation and fill operation. To date, there have been seven rounds (November 1996, March 1997, June 1997, September 1997, December 1997, March 1998, and June 1998) of performance monitoring of the wall. At this time, this is the only full-scale continuous `hanging' wall installed as a permeable reactive barrier to remediate both chlorinated solvent compounds and chromate in groundwater. Performance monitoring entails the following: sampling of 10–5 cm PVC compliance wells and 15 multi-level samplers for the following constituents: TCE, cis-dichloroethylene ( c-DCE), vinyl chloride, ethane, ethene, acetylene, methane, major anions, metals, Cr(VI), Fe(II), total sulfides, dissolved H 2, Eh, pH, dissolved oxygen, specific conductance, alkalinity, and turbidity. Electrical conductivity profiles have been conducted using a Geoprobe™ to verify emplacement of the continuous wall as designed and to locate upgradient and downgradient wall interfaces for coring purposes. Coring has been conducted in November, 1996, in June and September, 1997, and March, 1998, to evaluate the rate of corrosion on the iron surfaces, precipitate buildup (particularly at the upgradient interface), and permeability changes due to wall emplacement. In addition to several continuous vertical cores, angled cores through the 0.6-m thick wall have been collected to capture upgradient and downgradient wall interfaces along approximate horizontal flow paths for mineralogic analyses.

The determination of electrical conductivity profiles using neural network inversion of multi-frequency eddy-current data

A neural network method for quantitative depth profiling of inhomogeneous materials with quasi-continuously varying electrical conductivity profiles is presented. The possibilities and limitations of eddy-current depth profiling are examined on the basis of insight into the physics of eddy currents and of the statistics of simulation results obtained for large sets of inversion examples. Neural network reconstructions of electrical conductivity profiles retrieved from experimental eddy-current data on artificial samples are discussed. The approach presented allows one to study the eddy-current inverse problem in a flexible way thanks to the combination of a versatile profile parametrization and the feasibility of fast neural network performance tests.