Sensitive Voltmeter Research Articles

Determination of the permeability of seepage flow paths in dams from self-potential measurements

The flow of the pore water in porous media generates an electrical current known as the streaming current. This current is due to the drag of the excess of charges contained in the electrical diffuse layer coating the surface of the grains. This current is associated with an electric field called the streaming potential field. The fluctuations of this field can be remotely measured using a set of non-polarizable electrodes located at the ground surface or in wells and a sensitive voltmeter. The self-potential method (SP) aims at passively measuring the streaming potential anomalies associated with ground water flow. We present a stochastic numerical framework for inverting self-potential data in order to localize seeps in dams and characterize their permeability and Darcy velocity. Our approach is based on the use of Markov chains Monte Carlo (McMC) method for solving the inverse problem. We performed first a validation of the method on a synthetic case study and then on large-scale field surveys on three different dams. Our approach is successful in localizing seeps and determining their permeability. A sensitivity study is performed on each of these three dams to better define the hydraulic and electrical parameters influencing the self-potential signal and the uncertainties associated with the estimation of those parameters. Our results show that the self-potential method can provide quantitative hydrogeological information for the characterization of seeps in dams and dikes.

Délais diagnostiques des sarcomes d’Ewing de l’enfant : distribution, déterminants et conséquences

The flow of the pore water in porous media generates an electrical current known as the streaming current. This current is due to the drag of the excess of charges contained in the electrical diffuse layer coating the surface of the grains. This current is associated with an electric field called the streaming potential field. The fluctuations of this field can be remotely measured using a set of non-polarizable electrodes located at the ground surface or in wells and a sensitive voltmeter. The self-potential method (SP) aims at passively measuring the streaming potential anomalies associated with ground water flow. We present a stochastic numerical framework for inverting self-potential data in order to localize seeps in dams and characterize their permeability and Darcy velocity. Our approach is based on the use of Markov chains Monte Carlo (McMC) method for solving the inverse problem. We performed first a validation of the method on a synthetic case study and then on large-scale field surveys on three different dams. Our approach is successful in localizing seeps and determining their permeability. A sensitivity study is performed on each of these three dams to better define the hydraulic and electrical parameters influencing the self-potential signal and the uncertainties associated with the estimation of those parameters. Our results show that the self-potential method can provide quantitative hydrogeological information for the characterization of seeps in dams and dikes.

Saline pulse test monitoring with the self‐potential method to nonintrusively determine the velocity of the pore water in leaking areas of earth dams and embankments

A method is proposed to localize preferential fluid flow pathways in porous media on the basis of time‐lapse self‐potential measurements associated with salt tracer injection upstream. This method is first tested using laboratory data. A network of nonpolarizing electrodes located is connected to a highly sensitive voltmeter used to record the resulting electrical field fluctuations occurring over time at the surface of the tank. The transport of the conductive salt plume through the permeable porous materials changes the localized streaming potential coupling coefficient associated with the advective drag of the excess charge of the pore water and is also responsible for a diffusion current associated with the salinity gradient. Monitoring of the electrical potential distribution at the ground surface can be used to localize the pulse of saline water over time and to determine its velocity. This method applies in real time and can be used to track highly localized flow pathways characterized by high permeability. Our sandbox experiment demonstrates the applicability of this new method under well‐controlled conditions with a coarse‐sand channel embedded between fine‐sand banks. A finite element model allows us to reproduce the time‐lapse electrical potential distribution over the channel, but some discrepancies were observed on the banks. Finally, we performed a numerical simulation for a synthetic case study inspired by a recently published field case study. A Markov chain Monte Carlo sampler is used to determine the permeability and the porosity of the preferential fluid flow pathway of this synthetic case study.

Broad-Band Microwave Voltmeter Using Thermoelectric Effects of Hot Carriers

The performance, especially of the minimum detectable voltage, the dynamic range, and the frequency characteristics on the microwave voltmeter based on the thermoelectric effects of hot carriers in a semiconductor placed in the strip line are described. Parasitic reactance of whisker and mount may be neglected in comparison with spreading resistance of not less than 500 Ω, so that broad-band microwave voltmeter can be obtained. The VSWR was less than 1.2 within the range of frequency from 1 to 12.4 GHz. Making use semiconductor of low resistivity and small diameter point contact diode, a highly sensitive voltmeter is obtained. It was found that the dynamic range depends on the resistivity and the diameter of the electrode.

Electron microscope measurements: a digital read-out across the intermediate lens.

SUMMARYIn order to ensure reproducibility of magnification and to determine the accuracy of figures taken from the magnification meter on the side of a Siemens electron microscope, a sensitive voltmeter with a digital read‐out was put in the intermediate as well as the objective and projector lens circuits. The microscope was calibrated on different days and the results of the magnification measurements taken from the scale were compared with the results obtained using the read‐out system. The conclusions underline the necessity for exercising caution in interpreting magnification figures and suggest a relatively inexpensive method of improving reproducibility.

Transistorized Control System Analyser

Quite often it becomes necessary to know the transfer function of a control system for its analysis and improvement. One of the many methods of determining the transfer function experimentally is to plot the Bode Diagram for gain and phase curves of the system. From these diagrams, the transfer function is determined by standard procedure. The present paper describes one simple transistorized instrument by which such determination of the transfer function is possible. Gain and phase are determined by determining the Fourier expression for inphase and quadrature components of the a.c. signal output with respect to the reference input. The instrument consists of an oscillator of variable frequency ranging from 0·01 to 10 c/s. and a phase sensitive voltmeter by which the inphase and quadrature components are determined for different value of oscillator frequency. The oscillator and the different units of the phase sensitive voltmeter are described in the later sections. Finally the accuracy of the instrument is discussed.

A paleomagnetic spinner magnetometer using a fluxgate gradiometer

A spinner magnetometer, for paleomagnetic research, consisting of a 5 cps spinner shaft, a fluxgate gradiometer pickup, and a phase sensitive voltmeter is described. A major consideration in the design of the system is that it may be duplicated with a minimum of available technical facilities. A discussion is given of possible improvements in sensitivity. The present sensitivity is 1 × 10 −7 emu/cm 3 for an 11.5 cm 3 specimen.

The use of a phase sensitive voltmeter in electromagnetic prospecting

The use of a phase sensitive voltmeter in electromagnetic prospecting

A rapid method for measuring coercive force and other ferromagnetic properties of very small samples

A description is given of a method for the quantitative investigation of the permanent magnetic properties, especially the coercive force, of samples of about one milligram. The method is based essentially on the measurement of the alternating e.m.f., generated by letting the sample vibrate in a searchcoil, with a sensitive voltmeter. For the measurement of magnetic moments the instrument has to be calibrated. It is pointed out how this can be done. The time needed for a measurement is about two minutes.