Increase In Apparent Resistance Research Articles

Determination of hydraulic flushing impact range by DC resistivity test method

Chinese coal mines often apply hydraulic flushing technology for pressure relief and permeability increase of difficult-to-extract coal seams In order to study effective detection methods for the impact range of hydraulic flushing on coal seams, we first experimentally investigated the response characteristics of the apparent resistivity of small-scale coal mass to hydraulic flushing, then theoretically analyzed the characteristics of its temporal variation and spatial distribution, and lastly measured the changes in apparent resistivity in the Hydraulic flushing process at Nuodong Coal Mine, Guizhou Province, China, and the effective range of Hydraulic flushing in the coal bed. The experimental results showed that the spatial distribution and variation of coal and rock mass structure could be reflected by apparent resistivity. Under high-pressure water, coal structure was changed (loosen, broken, or out-washed), forming cavities and a network of cracks in the surroundings of the drilling borehole, which reduced the conductivity of coal and caused a significant increase in apparent resistivity. In both laboratory and field experiments, the coal/rock apparent resistivity was relatively consistent, and the apparent resistivity in the hydraulic flushing impact region significantly grew. The hydraulic flushing impact range was gradually increasing, and the response of apparent resistance was closely associated with the role of high-pressure water and the evolution of the fracture field.

Electrical properties of lightly Ga-doped ZnO nanowires

We investigated the growth, crystal structure, elemental composition and electrical transport characteristics of ZnO nanowires, a promising candidate for optoelectronic applications in the UV-range. Nominally-undoped and Ga-doped ZnO nanowires were grown by metal-organic chemical vapor deposition. Photoluminescence measurements confirmed the incorporation of Ga via donor-bound exciton emission. With atom-probe tomography we estimated an upper limit of the Ga impurity concentration (). We studied the electrical transport characteristics of these nanowires with a W-nanoprobe technique inside a scanning electron microscope and with lithographically-defined contacts allowing back-gated measurements. An increase in apparent resistivity by two orders of magnitude with decreasing radius was measured with both techniques with a much larger distribution width for the nanoprobe method. A drop in the effective carrier concentration and mobility was found with decreasing radius which can be attributed to carrier depletion and enhanced scattering due to surface states. Little evidence of a change in resistivity was observed with Ga doping, which indicates that the concentration of native or background dopants is higher than the Ga doping concentration.

Sap velocity as an indicator of diurnal and long-term hydraulic resistance changes in mature ‘Walker’ hybrid poplar treesThis article is one of a selection of papers published in the Special Issue on Poplar Research in Canada.

Hydraulic resistance directly limits sap flow in trees so that understanding water-use patterns in poplar trees requires knowledge about diurnal and long-term changes in resistances in the soil–plant–atmosphere continuum. Granier thermal dissipation probes were used for continuous measurements of sap velocity (V) changes over time and whole-tree water-use patterns in the main stems of ‘Walker’ hybrid poplar trees. When atmospheric demand was low, V depended only on solar radiation (Rg) and vapour pressure difference (VPD). However, on warm, sunny days, midday V decreased, despite a constant leaf water potential (Ψl) of –1.5 MPa. Because bulk soil water potential (Ψs) did not change significantly, the constant Ψl implied that the water potential difference (ΔΨ) remained diurnally constant. Although stomatal control would explain changes in V when VPD was low, an increase in hydraulic resistance was required to explain the decrease in V when VPD was high. Xylem cavitation was ruled out as a cause because the apparent increase in resistance reversed in late afternoon, while VPD was still high. Mid-August irrigation immediately increased daily V, which was attributed to a change in ΔΨ because of less negative Ψs. A gradual increase in sap flow over two weeks following irrigation implied a decrease in plant hydraulic resistance, most likely attributable to new root growth. It was concluded that hydraulic resistance changes, both diurnal and in the longer term, occurred primarily at the rhizosphere or root level.

Magnetotelluric static shift and direct current sensitivity

SUMMARY Magnetotelluric (MT) static shift is a non-inductive change of the MT apparent resistivity response that severely impairs the interpretation of data. Since MT static shift is a pure direct current (DC) phenomenon, we can apply the DC sensitivity distribution to predict its behaviour. We consider a four-point DC arrangement in Schlumberger configuration with current electrode spacing AB. Then, negative sensitivities form two semi-spherical or bowl-shaped structures between current and potential electrodes. With the limit of AB towards infinity, the semi-spheroids grow infinitely large, leaving positive sensitivities only within a vertical dyke structure between the potential electrodes. This arrangement represents the DC analogue to the MT static-shift case, yielding horizontal current systems at the position of the potential electrodes or, in MT terms, the telluric probes. Thus, the sign of static shift is due not only to the resistivity of the perturbing structure but also to its location with respect to the probes, which are separated by a non-infinitesimal distance. This is of crucial importance when investigating MT static-shift phenomena. For example, a conductive body between the telluric probes will decrease the MT apparent resistivity (positive sensitivity), whereas the same body located away from both the probes yields an increase in apparent resistivity (negative sensitivity). 3-D modelling clearly verifies this behaviour. A quantitative relationship between the static-shift factors for MT and DC is derived. In practice, however, a sufficiently large layout has to be chosen to approximate the behaviour at infinity or, if this is not possible, models derived from DC have to be incorporated into MT modelling. Thus, DC investigations are apt to quantify MT static shifts. A field survey comprising MT and large-scale DC resistivity soundings was carried out in the Grabfeld area in Unterfranken, Germany, and serves as a practical example. The MT response pattern in the target area is remarkably homogeneous. However, one site is severely disturbed by static shift. Although serious near-surface resistivity contrasts do exist at the other stations, it is shown that only those at the disturbed station can cause static shift.

Drug sensitivity and metastatic ability in B16 melanoma cells

We have previously reported that highly metastatic cell lines derived from KHT fibrosarcoma (KHT 35L1) and B16 melanoma (B16F10) are more resistant to N-phosphonacetyl-L-aspartate (PALA) and methotrexate (MTX) than the parental cell lines. This correlation between drug resistance and metastatic ability suggested the possibility that both phenotypes might have arisen in parallel as a result of a similar mechanism. In this study, we examined this possibility by reproducing the selection procedure for B16F10 cells (by serial passage of B16F1 cells as lung nodules) and testing the cells at each passage for changes in resistance to PALA and MTX. The results confirm that serial passage of B16F1 cells as lung nodules (LP) selects for cells with increasing metastatic ability (100-fold after seven passages), but these cells did not develop increased resistance to PALA and became more sensitive to MTX. For comparison B16F1 cells were also serially passaged (six passages) as leg tumors (LT). These cells became slightly more metastatic (3-fold) than B16F1 cells maintained in tissue culture, and demonstrated a small increase in sensitivity to MTX, as in the LP lines. There was also an apparent increase in resistance to PALA. In no instance was there a parallel increase in drug resistance and metastatic ability indicating that these two phenotypes do not necessarily arise in parallel in this cell line.

Geophysical anomalies over epithermal systems

The interpretation of most physical anomalies in epithermal systems needs to be integrated with available geological and geochemical data, and incorporated into a conceptual model of the original hydrology of the system. These anomalies result from the passage of hydrothermal fluids through rocks. Pervasive clay minerals associated with argillic-propylitic alteration zones cause low-resistivity anomalies comparable in size to the original geothermal system (typically 10–30 km 2). Resistivities are usually in the range 10–30 Ωm, and decrease to less than 10 Ωm in areas of advanced argillic alteration. These very low-resistivity zones may be caused by the reaction of the rock with acid, steam-heated waters. Most hydrothermal fluids, including relatively cool CO 2-bearing waters, destroy magnetite and cause demagnetisation anomalies. Airborne electrical and magnetic survey methods are therefore ideal for detecting epithermal systems in large exploration areas. Gamma-ray spectroscopy also has potential because of the concentration of potassium minerals (adularia and illite) in high-temperature alteration assemblages. In contrast to system-wide anomalies which have characteristic widths of several kilometres, epithermal ore bodies have characteristic widths of hundreds of metres. The precipitation of quartz and adularia which commonly accompanies gold-silver mineralisation causes an increase in resistivity, with values locally exceeding 1000 Ωm. Although this mineral assemblage causes an increase in apparent resistivity when near-surface, its subsurface extension may be difficult to delineate by electrical methods because of current channelling in adjacent low resistivity zones. The main I.P. anomalies are more likely to be associated with argillic alteration zones, unless quartz-adularia zones have significant sulphide concentrations. Gravity high anomalies of the order of mgals may occur as a result of mineral precipitation, especially in low-density, high-porosity host rocks. In relatively dense host rocks, local fracturing, alteration and late-stage leaching may actually cause gravity low anomalies of several mgal.

EPSPs in rat neocortical neurons in vitro. II. Involvement of N-methyl-D-aspartate receptors in the generation of EPSPs

1. Intracellular recordings were obtained from neurons in layer II/III of rat frontal cortex. Single-electrode current- and voltage-clamp techniques were employed to compare the sensitivity of excitatory postsynaptic potentials (EPSPs) and iontophoretically evoked responses to N-methyl-D-aspartate (NMDA) to the selective NMDA antagonist D-2-amino-5-phosphonovaleric acid (D-2-APV). The voltage dependence of the amplitudes of the EPSPs before and after pharmacologic changes in the neuron's current-voltage relationship was also examined. 2. NMDA depolarized the membrane potential, increased the neuron's apparent input resistance (RN), and evoked bursts of action potentials. The NMDA-induced membrane current (INMDA) gradually increased with depolarization from -80 to -40 mV. The relationship between INMDA and membrane potential displayed a region of negative slope conductance in the potential range between -70 and -40 mV which was sufficient to explain the apparent increase in RN and the burst discharges during the NMDA-induced depolarization. 3. Short-latency EPSPs (eEPSPs) were evoked by low-intensity electrical stimulation of cortical layer IV. Changes in the eEPSP waveform following membrane depolarization and hyperpolarization resembled those of NMDA-mediated responses. However, the eEPSP was insensitive to D-2-APV applied at concentrations (up to 20 microM) that blocked NMDA responses. 4. EPSPs with latencies between 10 and 40 ms [late EPSPs (lEPSPs)] were evoked by electrical stimulation using intensities just subthreshold to the activation of IPSPs. The amplitude of the lEPSP increased with hyperpolarization and decreased with depolarization. 5. The lidocaine derivative QX-314, injected intracellularly, suppressed sodium-dependent action potentials and depolarizing inward rectification. Simultaneously, the amplitude of the eEPSP significantly decreased with depolarization. Neither the amplitude of a long-latency EPSP nor the amplitude of inhibitory postsynaptic potentials (IPSPs) was significantly affected by QX-314. 6. Cesium ions (0.5-2.0 mM) added to the bathing solution reduced or blocked hyperpolarizing inward rectification. Under these conditions, the amplitude of the eEPSP increased with hyperpolarization. The amplitude of the lEPSP was unaltered or enhanced. 7. The lEPSP was reversibly blocked by D-2-APV (5-20 microM), although the voltage-dependence of its amplitude did not resemble the action of NMDA on neocortical neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Interpretation of dipole-dipole resistivity monitoring data at Cerro Prieto

Repetitive dipole-dipole resistivity data have been taken on a yearly basis by LBL at Cerro Prieto since 1978. Stations along a single profile line extending from the Cucapa Mountains to the center of the Mexicali Valley and passing over the present production zone have been remeasured with sufficient accuracy to detect subsurface changes in resistivity, some of which are probably related to fluid production. The precision of the most recent measurements (November, 1981) averages about 1%. Results from 2.5 years of monitoring indicate a 5% annual increase in apparent resistivity over the present production area and decreases in apparent resistivity of the same magnitude in the regions immediately eastward and westward from the production zone. The increase in resistivity in the production zone is most likely due to dilution of reservoir fluids with fresher water, as evidenced by a drop in chloride content of produced waters. An attempt was made to determine whether specific lithologic zones in wellbores show resistivity changes with time by comparing well logs from newly drilled wells with logs from older nearby wells. Results show that lateral resistivity variations within stratigraphic units between closely-spaced wells are sufficient to obscure possible temporal changes. The area of decreasing resistivity in the eastern part of the field is associated with a steeply dipping conductive body, a zone of higher thermal gradients and an increase in shale thickness in the section. Well log analysis shows that the low resistivity is mostly due to higher temperatures. Decreasing resistivity in this area may be caused by an influx of hotter and more saline brines from depth. Recent measurements also show dramatic increase in near-surface resistivity at the western end of the monitored line. This is most likely due to recent changes in local irrigation practices which resulted in a general improvement in groundwater quality. To investigate the phenomenon of resistivity changes caused by groundwater movement and chemical reactions, we propose the establishment of an additional resistivity line crossing both the new eastern production zone and the present survey line at an angle of 60°. This line would permit the acquisition of baseline data over the future production zone (Cerro Prieto 11 and 111 areas), in conjunction with the present line, and would establish a grid of stations which could be used to map subsurface groundwater fronts.

Slow potential changes in mammalian muscle fibers during prolonged hyperpolarization: transport number effects and chloride depletion.

Mammalian skeletal muscle fibers exhibit large slow changes in membrane potential when hyperpolarized in standard chloride solutions. These large slow potential changes are radically reduced in low chloride solutions, where the faster and smaller potential change ("creep"), usually observed in amphibian fibers, becomes apparent. The slow potential change during a hyperpolarizing current pulse leads to an increase in apparent resistance of up to nine times the instantaneous value and takes minutes to reach a steady value. It then takes a similar time to decay very slowly back to the resting membrane potential after the current pulse. The halftime for the slow potential change was found to be inversely proportional to the current magnitude. From measurements of immediate post-pulse membrane potentials, assuming constant ionic permeabilities, the internal chloride concentration was calculated to decrease exponentially towards a steady value (e.g., for one fiber from 12.3 to 6.6 mM after a 330-sec pulse). The time course and magnitude of the concentration change were predicted from chloride transport number differences, and the known and measured properties of the fibers, and were found to agree very well with the values obtained from experimental measurements. In addition, the shapes of the V2-V1 responses, measured in the three-electrode current clamp set-up with either potassium chloride or potassium citrate current electrodes, were as predicted by transport number chloride depletion effects and were at variance with the predictions of a permeability change mechanism.

Studies on some aspects of bioassay of stomach poisons

Using DDT dust as insecticide and third and fourth instar larvae ofEuproctis lunata Walk. as test insect the following fundamental aspects of the bioassay of stomach poisons have been investigated. Starvation and resistance.—It has been indicated that, contrary to previous notions, resistance of E. lunata larvae decreases with increase in period of starvation. The apparent increase in resistance due to starvation is obviously due to the fact that prolonged periods of starvation (24–48 hours) make theE. lunata larvse somewhat lethargic and these lethargic larvae actually eat lesser quantity of both poisoned and unpoisoned food and therefore their percentage mortality is also low. Body weight and resistance.—It has been shown that resistance to insecticide increases with increase in body weight. An increase in amount of poisoned sandwich eaten with increase in body weight has been observed, but the amount of poison ingested per gm. body weight of insect varied within a comparatively narrow range and showed no definite order of variation. It has been suggested that more or less constant amount of DDT per gm. body weight begins to have an inhibitory effect on further feeding. Several smaller doses versusone large dose.—It has been shown that a comparatively larger dose given to larvae in one instalment is more effective than the same dose given in several instalments of smaller doses. It is suggested that the detoxification of some of the poison may be the cause of this phenomenon.