Conductance Signatures Research Articles

Characterisation of sulphide-bearing waste-rock dumps using electrical resistivity imaging: the case study of the Rio Marina mining district (Elba Island, Italy)

Sulphide-bearing mine dumps are potential sources of pollution when acid mine drainage (AMD) occurs. Because the generation of AMD depends on the volume and composition of waste materials, their characterisation is crucial for the evaluation of geochemical hazards and for the design of remediation strategies to minimise their environmental impact. In this paper, a cost-effective strategy for the characterisation of an inactive mine dump in the Rio Marina mining district (Elba Island, Italy) using earth resistivity imaging (ERI) is presented. As no information regarding the nature of waste rocks is found in reports for the mine, five ERI profiles were acquired at the top of the waste pile. The results show that waste rocks are heterogeneous with a maximum thickness of 30m. Due to the large amounts of dispersed sulphide minerals, the waste rocks are characterised by an electrically conductive geophysical signature in comparison to the surrounding resistive metamorphic bedrock. A geostatistical approach was adopted to estimate the elevation of the edges of the mine dump, and the net volume of the waste rocks was computed through a raster analysis of the elevations of the upper and lower boundaries of the mine dump. High-conductivity anomalies were detected within the core of the mine dump. The integration of the hydrogeological, geochemical and geological framework of the Rio Marina mining district suggests that these anomalies could be a geophysical signature of subsurface regions where AMD is currently generated or stored, thus representing sources of environmental pollution.

Geophysical Monitoring of Foam Used to Deliver Remediation Treatments within the Vadose Zone

Foam is a promising vehicle for delivering amendments into the vadose zone for in situ remediation; it is an approach being considered for in situ treatment and stabilization of metals and radionuclides located within the deep vadose zone of the Department of Energy's Hanford Site in the state of Washington. A central aspect of evaluating the effectiveness of this approach is the ability to monitor foam distribution, its transformation, and the reactions that it induces in the subsurface, ideally in a noninvasive manner. In this study, we performed laboratory experiments to evaluate the potential of geophysical methods (complex resistivity and time‐domain reflectometry [TDR]) as tools for monitoring foam‐assisted amendment delivery in the deep vadose zone. Our results indicated great sensitivity of electrical methods to foam transportation and evolution in unsaturated porous media that were related to foam bubble coalescence and drainage processes. Specifically, we observed (i) a decrease in electrical resistivity (increase in electrical conductivity) by more than an order of magnitude in both silica sand and natural sediment matrices during foam transportation; (ii) an increase in resistivity (decrease in conductivity) of more than twofold during foam coalescence and drainage; and (iii) a distinct phase and imaginary conductivity signature related to the evolution of water films on sediment grains during foam injection and evolution processes. To assist with the interpretation of these data, TDR measurements were used to monitor moisture content, which provided complementary information about foam distribution and drainage. Our results clearly demonstrated the sensitivity of electrical and TDR signals to foam transportation and evolution in unsaturated porous media and suggest the potential of these methods for monitoring the response of a system to foam‐based remediation treatments at field scales.

Linker Dependent Bond Rupture Force Measurements in Single-Molecule Junctions

We use a modified conducting atomic force microscope to simultaneously probe the conductance of a single-molecule junction and the force required to rupture the junction formed by alkanes terminated with four different chemical link groups which vary in binding strength and mechanism to the gold electrodes. Molecular junctions with amine, methylsulfide, and diphenylphosphine terminated molecules show clear conductance signatures and rupture at a force that is significantly smaller than the measured 1.4 nN force required to rupture the single-atomic gold contact. In contrast, measurements with a thiol terminated alkane which can bind covalently to the gold electrode show conductance and force features unlike those of the other molecules studied. Specifically, the strong Au-S bond can cause structural rearrangements in the electrodes, which are accompanied by substantial conductance changes. Despite the strong Au-S bond and the evidence for disruption of the Au structure, the experiments show that on average these junctions also rupture at a smaller force than that measured for pristine single-atom gold contacts.

Two-Channel Kondo Physics in Two-Impurity Kondo Models

We consider the non-Fermi-liquid quantum critical state of the spin-S two-impurity Kondo model and its potential realization in a quantum dot device. Using conformal field theory and the numerical renormalization group, we show the critical point to be identical to that of the two-channel Kondo model with additional potential scattering, for any spin S. Distinct conductance signatures are shown to arise as a function of device asymmetry, with the square-root behavior commonly believed to arise at low-energies dominant only in certain regimes.

Geophysical Investigation of the Fresh-Saline Water Interface in the Coastal Area of Abergwyngregyn

The importance of the study of saline/fresh water incursion cannot be over-emphasized. Borehole sampling has been extensively used, but it is intrusive, quite expensive and time consuming. Electrical resistivity and electromagnetic techniques have proved successful in groundwater studies since geologic formation properties like porosity and permeability can be correlated with electrical conductivity signatures. Non-intrusive surface geophysical mapping comprising electrical resistivity and electromagnetic methods has been employed to investigate freshwater intrusion and delineate the fresh-saline water interface at the inter-tidal area of Abergwygregyn, North Wales, United Kingdom. Frequency Domain Electromagnetic Profiling and Constant Separation Traversing were used to produce 2-D images and contour plots enabling the identification of freshwater plumes onshore and in the central parts of the study area. Ground truth methods comprised chemical analyses and detailed, point specific information on the stratigraphy. The freshwater intruding from the coastal area appears to be pushing the saline-water further offshore due to the high piezometric head caused by the mountains and hills of Snowdonia adjacent to the study area. The fresh/saline water interface correlates quite well with previous studies carried out in the area. On the basis of the results of the resistivity and conductivity geophysical investigations the freshwater plumes and fresh/saline water interface in the study area were effectively identified and delineated.

Complex conductivity response to microbial growth and biofilm formation on phenanthrene spiked medium

Abstract Several laboratory studies have recently demonstrated the utility of geophysical methods for the investigation of microbial-induced changes over contaminated sites. However, it remains difficult to distinguish the effects due to the new physical properties imparted by microbial processes, to bacterial growth, or to the development of bacterial biofilm. We chose to study the influence of biofilm formation on geophysical response using complex conductivity measurements (0.1–1000 Hz) in phenanthrene-contaminated media. Biotic assays were conducted with two phenanthrene (PHE) degrading bacterial strains: Burkholderia sp (NAH1), which produced biofilm and Stenophomonas maltophilia (MATE10), which did not, and an abiotic control. Results showed that bacterial densities for NAH1 and MATE10 strains continuously increased at the same rate during the experiment. However, the complex conductivity signature showed noticeable differences between the two bacteria, with a phase shift of 50 mrad at 4 Hz for NAH1, which produced biofilm. Biofilm volume was quantified by Scanning Confocal Laser Microscopy (SCLM). Significant correlations were established between phase shift decrease and biofilm volume for NAH1 assays. Results suggest that complex conductivity measurements, specifically phase shift, can be a useful indicator of biofilm formation inside the overall signal of microbial activity on contaminated sites.

Conductance and noise signatures of Majorana backscattering

We propose a conductance measurement to detect the backscattering of chiral Majorana edge states. Because normal and Andreev processes have equal probability for backscattering of a single chiral Majorana edge state, there is qualitative difference from backscattering of a chiral Dirac edge state, giving rise to half-integer Hall conductivity and decoupling of fluctuation in incoming and outgoing modes. The latter can be detected through thermal noise measurement. These experimental signatures of Majorana fermions are robust at finite temperature and do not require the size of the backscattering region to be mesoscopic.

Signatures of continental collisions and magmatic activity in central Brazil as indicated by a magnetotelluric profile across distinct tectonic provinces

Broadband and long-period magnetotelluric soundings were collected along a 560km E–W profile in a region of central Brazil subjected to Neoproterozoic collision tectonics and Archean to Cretaceous magmatic events. The profile crosses the northeastern portion of Phanerozoic sediments and volcanics of the Paraná basin, the southern extension of the Neoproterozoic metasedimentary rocks of the Brasília belt, locally pervaded by Cretaceous alkaline magmas of the Alto Paranaíba igneous province (APIP), and Neoproterozoic sedimentary cover and exposed Archean basement of the southern São Francisco craton. 2D conductivity structures derived by joint inversions of the TE and TM polarization modes and a separate inversion of the tipper components show signatures of the past tectonomagmatic events that affected the area. A gravity-defined suture zone beneath the Paraná basin is detected in the models as a subvertical conductor extending from crustal to upper mantle depths, which strengthen the interpretation of a Neoproterozoic collision of the São Francisco craton and a continental block beneath the sedimentary basin. Deep underthrusting of organic graphite-bearing metasedimentary rocks composing a fossil suture zone is proposed to explain such increase in electrical conductivity. A similar conductivity signature beneath the sedimentary covered region of the São Francisco craton can be tentatively interpreted as another similar cryptic suture zone. Other isolated high-conductivity anomalies at midcrustal depths below the Paraná and APIP provinces are interpreted as fossil residues representing precipitated graphite and sulfide derived from percolating volatiles during the emplacement of Cretaceous mafic-ultramafic volcanics. A very high conductivity wedge into the lower lithosphere is highlighted in the deep mantle beneath the APIP volcanic complex, coincident with a zone of low velocity defined by seismic tomography. Geochemical evidence indicates that the alkaline magmatism stemmed from a metasomatized upper mantle at only slightly raised temperatures. Thus, interconnected carbonatite melts of low melting point and graphite in the lithospheric mantle are the most likely candidates to explain this high conductivity. The seismic and conductivity anomalies are probably triggered by the same source mechanisms (incipient melting) because they are plausibly related to the same magmatic process. Conductivity anomalies at crustal and mantle depths in the southern segment of the São Francisco craton suggest that its lithosphere was significantly affected by the several tectonomagmatic episodes it has experienced throughout its geological history. Consequently, the enhanced conductivity in the lower crust can be genetically related to upwelling volatile-rich intrusions whereas upper mantle high conductivity can be related to refertilization by infiltrations of low-degree carbonatitic melts from deeper-sourced metasomatic processes.

Effect of bioclogging in porous media on complex conductivity signatures

Flow through sand columns inoculated with Pseudomonas aeruginosa were used to investigate the effect of bioclogging on the complex conductivity and flow and transport properties. Complex conductivity (0.1–1000 Hz), the bulk hydraulic conductivity (K), volumetric flow rate (Q), dispersivity (D), and microbial cell concentrations were monitored over time. Environmental scanning electron microscope images of the sands were obtained at the end of the experiment. Bioclogging resulting from increases in microbial cell concentration and the production of exopolymeric substances (EPS) had a large impact on the imaginary conductivity (σ″), K, Q, D, and porosity (Φ). Changes in electrical and transport properties developed in three stages: an initial stage 1 with no significant changes in all measured parameters (Days 1–8), which we attribute to the reversible and irreversible attachment of microbial cells to the sand. In stage 2a (Days 9–16), bioclogging caused by the growth of biomass either as microcolonies filling pore throats and/or as uniform covering on the sand surfaces resulted in maximum decrease in Q and K, but with only moderate increases in σ″. In stage 2b (Days 17–24), significant production of EPS substances and increase in biofilm thickness resulted in a higher rate of increase in σ″ compared to stage 2a. In stage 3 (Days 25–32), biofilm growth reached a quasi steady state and insignificant changes are observed in all measured parameters. The results of this study suggest that σ″ can provide complimentary information in the assessment of bioclogging in porous media.

Conductance and Geometry of Pyridine-Linked Single-Molecule Junctions

We have measured the conductance and characterized molecule-electrode binding geometries of four pyridine-terminated molecules by elongating and then compressing gold point contacts in a solution of molecules. We have found that all pyridine-terminated molecules exhibit bistable conductance signatures, signifying that the nature of the pyridine-gold bond allows two distinct conductance states that are accessed as the gold-molecule-gold junction is elongated. We have identified the low-conductance state as corresponding to a molecule fully stretched out between the gold electrodes, where the distance between contacts correlates with the length of the molecule; the high-conductance state is due to a molecule bound at an angle. For all molecules, we have found that the distribution of junction elongations in the low-conductance state is the same, while in the high-conductance state, the most likely elongation length increases linearly with molecule length. The results of first-principles conductance calculations for the four molecules in the low-conductance geometry agree well with the experimental results and show that the dominant conducting channel in the conjugated pyridine-linked molecules is through the pi* orbital.

Simplified Impedance Model for Adhesively Bonded Piezo-Impedance Transducers

The electromechanical impedance technique employs surface-bonded lead zirconate titanate piezoelectric ceramic patches as impedance transducers for structural health monitoring and nondestructive evaluation. The patches are bonded to the monitored structures using finitely thick adhesive bond layer, which introduces shear lag effect, thus invariably influencing the electromechanical admittance signatures. This paper presents a new simplified impedance model to incorporate shear lag effect into electromechanical admittance formulations, both one-dimensional and two-dimensional. This provides a closed-form analytical solution of the inverse problem, i.e. to derive the true structural impedance from the measured conductance and susceptance signatures, thus an improvement over the existing models. The influence of various parameters (associated with the bond layer) on admittance signatures is investigated using the proposed model and the results compared with existing models. The results show that the new mod...

Real-time detection of deoxyribonucleic acid bases via their negative differential conductance signature

In this Brief Report, we present a method for the real-time detection of the bases of the deoxyribonucleic acid using their signatures in negative differential conductance measurements. The present methods of electronic detection of deoxyribonucleic acid bases are based on a statistical analysis because the electrical currents of the four bases are weak and do not differ significantly from one base to another. In contrast, we analyze a device that combines the accumulated knowledge in nanopore and scanning tunneling detection and which is able to provide very distinctive electronic signatures for the four bases.

Spin Inelastic Electron Tunneling Spectroscopy on Local Spin Adsorbed on Surface

The recent experimental conductance measurements taken on magnetic impurities on metallic surfaces, using scanning tunneling microscopy technique and suggesting occurrence of inelastic scattering processes, are theoretically addressed. We argue that the observed conductance signatures are caused by transitions between the spin states that have opened due to, for example, exchange coupling between the local spins and the tunneling electrons, and are directly interpretable in terms of inelastic transitions energies. Feasible measurements using spin-polarized scanning tunneling microscopy that would enable new information about the excitation spectrum of the local spins are discussed.

The geophysical response of the Tusker gold deposit, Lake Victoria gold fields, Tanzania

The Tusker gold deposit is located in the Lake Victoria Goldfields of Tanzania. The deposit contains total estimated resources of 123.27 Mt at 1.15g/t Au, as at 5th September, 2006. Geophysical techniques trialled over the deposit include down hole measurements, airborne time domain electromagnetics, dipole-dipole induced polarisation and resistivity and airborne magnetics. Petrophysical measurements suggest the deposit is associated with a conductive and chargeable signature. This is confirmed by airborne EM and dipole-dipole IP and resistivity data. Magnetic data map the unmineralised magnetic mudstone package overlying the deposit, and show that stratigraphy is deformed.

Contact Chemistry and Single-Molecule Conductance: A Comparison of Phosphines, Methyl Sulfides, and Amines

We compare the low bias conductance of a series of alkanes terminated on their ends with dimethyl phosphines, methyl sulfides, and amines and find that junctions formed with dimethyl phosphine terminated alkanes have the highest conductance. We see unambiguous conductance signatures with these link groups, indicating that the binding is well-defined and electronically selective. This allows a detailed analysis of the single-molecule junction elongation properties which correlate well with calculations based on density functional theory.

Case studies of magnetotelluric method applied to mapping active faults

Magnetotelluric method (MT) offers opportunity to detect crustal fluids along faults due to their high conductivity anomaly. Supposing that fluids deposited minerals in the conductive fractures (faults, dykes) decreasing the resistivity, the high seismicity in the area can be explained by the presence of these fluids. MT measurements were carried out in the period range 0.001–420 s crossing the Kalabsha fault (Aswan, Egypt) and Remiremont fault (Southern Vosges, France). In these work we detect geoelectrical resistivity anomalies of the Earth’s crust and link them to local seismic activity. Seismic events having magnitude (M<5) are found along fault zones in Kalabsha and Remiremont. The goals of our measurements are various. We would like to determine the precise location of the active faults, to study the connection of the Remiremont and Kalabsha seismicity to the MT resistivity structures, and to support the idea of the influence of the fluid-bearing conducting faults in the Remiremont and Kalabsha areas to the earthquake. These applications afford the unusual opportunity to study the percolation of water into the faults system and its effect on the seismicity, to reveal geological structures and the stress field covered by thin Quaternary formations. Data are analysed by 2D simultaneous inversion of both polarizations. The resulting models are compared with the local seismicity map. Our MT model reveals the conductive signature of the fault, as well as geological and tectonic stresses prevailing in active regions.

Benzo-homologated nucleobases in a nanotube-electrode set-up for DNA sequencing

Motivated by the possibility that the conductivity signatures of benzo-homologated DNAbases could be used to sequence DNA, we have investigated the conductivityproperties of these bases when they are non-covalently sandwiched between two(5,5) nanotube electrodes. It is found that these bases conduct poorly, making it verydifficult to distinguish them. An analysis of the changes in the conductivity ofbenzo-adenine as a function of the distance between the tips of the nanotubes revealedthat, even though the conductance increases by four orders of magnitude whenthe electrodes are brought closer together, the net conductance remains rathersmall. These results suggest that benzo-homologated bases, despite having smallerHOMO–LUMO gaps than their natural counterparts, when non-covalently boundto the electrodes cannot be used to sequence DNA by means of conductivitymeasurements.

Phonon runaway in carbon nanotube quantum dots

We explore electronic transport in a nanotube quantum dot strongly coupled with vibrations and weakly with leads and the thermal environment. We show that the recent observation of anomalous conductance signatures in single-walled carbon nanotube (SWCNT) quantum dots can be understood quantitatively in terms of current driven `hot phonons' that are strongly correlated with electrons. Using rate equations in the many-body configuration space for the joint electron-phonon distribution, we argue that the variations are indicative of strong electron-phonon coupling requiring an analysis beyond the traditional uncorrelated phonon-assisted transport (Tien-Gordon) approach.

Detection of preferential infiltration pathways in sinkholes using joint inversion of self‐potential and EM‐34 conductivity data

ABSTRACTThe percolation of water in the ground is responsible for measurable electric potentials called self‐potentials. These potentials are influenced by the distribution of the electrical conductivity of the ground. Because sinkholes are associated both with self‐potential and electrical conductivity anomalies, a joint inversion of EM‐34 conductivity and self‐potential data is proposed as a way of delineating the location of these features. Self‐potential and EM conductivity data were obtained at a test site in Normandy (France) where sinkholes and crypto‐sinkholes are present over a karstic area in a chalk substratum overlain by clay‐with‐flint and loess covers. The presence of sinkholes and crypto‐sinkholes is associated with negative self‐potential anomalies with respect to a reference electrode located outside the area where the sinkholes are clustered. The sinkholes also have a conductivity signature identified by the EM‐34 conductivity data. We used the simulated‐annealing method, which is a global optimization technique, to invert jointly EM‐34 conductivity and self‐potential data. Self‐potential and electrical conductivity provide clear complementary information to determine the interface between the loess and clay‐with‐flint formations. The sinkholes and crypto‐sinkholes are marked by depressions in this interface, focusing the groundwater flow towards the aquifer contained in the chalk substratum.

Electrical structure of the tectonically active Kalabsha Fault, Aswan, Egypt

In this work, we use the magnetotelluric (MT) method to detect geoelectrical conductivity anomalies in the Earth’s crust and link them to local seismic activity. This application affords the unusual opportunity to study the percolation of water from a lake into a fault system and its effect on the induced seismicity. MT measurements were carried out in the period range 0.0046–420 s at nine sites along a 15 km-long North–South profile crossing the Kalabsha Fault, on the western bank of Lake Aswan. Data were analysed by 2D simultaneous inversion of both polarisations. The resulting model is compared with the local seismicity map and reveals the conductive signature of the fault, as well as geological and tectonic stresses prevailing in the Aswan area. Our MT investigations show the following features: The measured MT strike aligns with the seismic epicentre axis corresponding to the Kalabsha Fault. While crossing the Fault, enhanced conductivity is found down to depths of 5 km on a 1–2 km profile segment. At mid-crustal depths (20 km), a very high conductive body is found to coincide with the main seismic cluster in the Aswan area. These observations indicate that seismic activity and high electrical conductivity are related. The link between them is the presence of crustal fluids which are presumably the cause of the high conductivity observed. Their presence is also required to trigger the observed seismicity. In addition, we explain the lower conductivity of the local upper crust in terms of stress-modulated rock porosity. We believe that these results are of general significance, as they could explain the mid-crustal seismicity of tectonically active zones. D 2005 Elsevier B.V. All rights reserved.