Dipole Sensors Research Articles

Impedance of electric field sensors in magnetized plasmas: particle-in-cell simulations

To obtain both the intensities and phases of electric fields, it is necessary to calibrate the data observed by the sensor by consulting the sensor characteristics. The frequency profile of the sensor impedance obtained from the simulation is roughly consistent with that obtained from the theory. However, some differences are identified in the frequency profile of the sensor impedance obtained from the simulation compared to the theory. We conducted simulations using the particle-in-cell (PIC) simulation tool to analyze the impedance of electric field sensors immersed in magnetized space plasmas. The simulation model comprised a dipole sensor placed in a three-dimensional simulation box filled with electrons and ions. The dipole sensor was represented by perfect conductor rods. We carefully selected simulation parameters and employed an appropriate feeding technique to accurately evaluate the sensor impedance. We observed significant changes in the frequency dependence of the sensor impedance as cyclotron frequency varied. To understand the simulation results, we introduced the linear dispersion relation. By comparing the real part of the sensor impedance with the ω-k\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\omega-k$$\\end{document} diagram, we found that the impedance peaks corresponded to frequencies at which branches of plasma waves were present, with k=khalf\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$k=k_{\ ext{half}}$$\\end{document}, representing half the wavelength equal to the tip-to-tip sensor length, i.e., the sensor can be assumed as the half-wavelength dipole antenna.Graphical

High-sensitivity polarization-independent terahertz Taichi-like micro-ring sensors based on toroidal dipole resonance for concentration detection of Aβ protein

Abstract Terahertz (THz) metamaterial sensor is a newly-developing interdisciplinary technology, which combines the essential characteristics of THz spectroscopy and metamaterials, to obtain better sensitivity for trace detection of the different target analytes. Toroidal dipole resonances show great sensing potential due to their suppression of the radiative loss channel. Here, we found a high-quality planar toroidal dipole resonance in the breaking Chinese Taichi-like ring and then designed a novel polarization-independent terahertz toroidal sensor by combining four Taichi-like rings into a cycle unit. The sensor shows high-sensitivity sensing characteristics for the ultrathin analyte and refractive index. The optimized sensitivity of pure analytes under 4 μm coating thickness can numerically reach 258 GHz/RIU in the corresponding ∼1.345 THz frequency domain, which is much higher than that of previously reported sensors. We further fabricated experimentally the sensor and demonstrated its fascinating polarization-independent characteristics. Finally, it was successfully applied to the low-concentration detection (ranging from 0.0001 mg/mL to 10 mg/mL) of Aβ protein associated with Alzheimer’s disease. Our high-sensitivity polarization-independent THz toroidal dipole sensor would give access to rich applications in label-free biosensing.

Dispersion of flexural waves in a borehole with a tensile fracture in an anisotropic stress environment

ABSTRACTThe effect of tensile fracture in a vertical borehole under anisotropic horizontal stress conditions is numerically investigated in terms of the dispersion of flexural wave generated in dipole sonic logging. Our three‐dimensional model comprises a borehole filled with water and a tensile fracture intersecting the borehole in the borehole axial direction. Two shear waves are excited individually to produce particle displacements polarized in two orthogonal radial directions using two dipole sources aligned in the two polarized directions. A vertical array of equispaced dipole sensors is placed at the centre of the borehole along the borehole axis. We assumed that the surrounding formation possesses transversally isotropic anisotropy with the isotropy plane parallel to the borehole axis due to horizontal stress anisotropy. We examined the dispersion of flexural waves travelling along a borehole in our numerical models that include either fast or slow formation with various depths of tensile fractures. Our numerical results show that the deeper the penetration depth of a tensile fracture, the higher the slowness of shear waves polarized perpendicular to the tensile fracture for both slow and fast formation models. Our results indicate that the flexural dispersion behaviour could be used to investigate the depth of penetration of a tensile fracture that can be produced by either drilling or hydraulic fracturing.

Polarization-Independent Reconfigurable Graphene Gas Sensor Using Crescent Plasmonic Antenna

A polarization-independent gas sensor based on crescent plasmonic dipole antenna loaded with graphene monolayer is introduced in this paper for environment monitoring applications. Single, dual, and four arms crescent dipoles are designed, analyzed and investigated. The proposed gas sensor has reconfigurable absorption characteristics in the wavelength range from 900 to 1600 nm which covers the different gases responses. A parametric study on the effect of the sensor dimensions and polarization of the illuminated waves on the total absorption of different gases is investigated. A graphene monolayer is used to enhance the gas molecule absorption in its unbiased state. An enhancement of the total absorption cross section (ACS) values for n = 1.4 is 1.028 × 105 nm2 for the unloaded sensor case and is 1.77 × 105 for the graphene loaded sensor case. The unloaded graphene sensor sensitivity is 630 nm/RIU, and for the graphene loaded case is 677.5 nm/RIU. Dual-polarized crescent dipole consists of two orthogonal arms is designed to maintain the same sensitivity for x- and y-polarized E-field illumination. A polarization independent gas sensor consists of four crescents dipole arms rotated with 45° is explained. An enhancement of gas absorption is achieved by using a sensor array of four-arm dipole sensor. Different array sizes loaded with graphene monolayer are investigated. The total ACS peak value is 7.84 × 106 nm2 for 6 × 6 dipole array is obtained.

Moment method approach to analyze waveguide array radiator with crossed dipole as a near field sensor

Moment method approach is used to estimate the error encountered by the sensor in the near field and probe measures co-pole and cross-pole voltages at the same time. Conversely, due to several reflections between the waveguide and dipole sensor, near field values are changed. Calculations are done to analyze the absolute sampled co-pole voltage pattern and relative sampled electric field (without probe) pattern in the scan plane, and inaccuracy induced by the sensor is determined. The waveguide reflection coefficient, radiation admittance and susceptance with measuring sensor are reformed with respect to the equivalent values in the absence of the sensor in the near field.

Vertical–vertical controlled‐source electromagnetic instrumentation and acquisition

ABSTRACTVertical–vertical controlled‐source electromagnetic is an alternative to other techniques for providing three‐dimensional resistivity images of the subsurface. It utilizes a large and powerful vertical dipole transmitter and arrays of E‐field receivers with vertical and horizontal dipole sensors. The necessary instrumentation and acquisition procedures which differ strongly from other controlled‐source electromagnetic methods are described in the paper.

Study of UHF Frequency Response Characteristics of the Semicircle Dipole Inner Sensor for Partial Discharge Detection in GIS

The paper designed a semicircle dipole inner sensor for UHF partial discharge detection in GIS. By comparing the symmetric semicircle dipole sensor and asymmetric semicircle dipole sensor characteristics, the symmetric semicircle dipole sensor’s coupling property is better than that of asymmetric semicircle dipole sensor. At the same time, by changing the symmetric semicircle dipole transducer radius, the position of the feeder pole , the height of the feeder pole, analyze its influence on sensor features.

Real-time acoustic monitoring and source level estimates of walruses in the northeastern Chukchi Sea using particle velocity sensors

Particle motion sensors measure the vector component of the sound field. In underwater acoustics, they are used for studying the physics of the sound field, evaluating the potential effects of sound on fish, and defining the direction of arrival (DOA) of sound sources. Measuring the DOA in the vertical and horizontal plane allows two separate receivers to localize an acoustic source in three dimensions. In July 2013, we used two custom-built, real-time particle velocity acoustic recording systems to record and localize vocally active walruses in the water near groups hauled out on ice in the northeastern Chukchi Sea. The system was equipped with a three-axis dipole sensors and a calibrated omni-directional hydrophone. It was deployed at the water surface and transmitted data in real-time to a support skiff. The range between the recorders, support skiff, and calling animals was usually less than 200 m and typically within a few tens of meters, allowing for simultaneous visual observations. Calling walruses were localized using cross-fixes of acoustic bearings. Source levels were estimated by adding modeled frequency-dependent transmission losses to the received levels in each 1/3-octave-band obtained from the calibrated omni-directional hydrophone. Only calls with high signal-to-noise ratio were used in this analysis. The use of the particle velocity sensor allowed for the first source level measurements of walrus grunts and bell calls in the wild.

Effect of Aluminum Neutron Probe Access Tubes on the Apparent Electrical Conductivity Recorded by an Electromagnetic Soil Survey Sensor

Correlating soil moisture content to apparent electrical conductivity (σa), derived from above-ground, electromagnetic induction (EMI) dipole sensors, requires capacitance or neutron probe moisture meters. To this end, plastic or metallic access tubes (ca. 0.5-2 m long, 40-50 mm internal diameter, and 1-2 mm thickness walls) are inserted vertically into the soil to allow the probe to be lowered for moisture readings at a series of soil depths. The impact of these tubes on measurements derived from above-ground EMI sensors, when the sensor is in proximity or adjacent to these buried tubes, in unknown. We report on the impact of widely used aluminum (Al), as well as popular plastic alternatives of polyethylene (PE), polyvinylchloride (PVC) access tubes on the lateral σa profiles of an EM38 EMI meter as it is moved along survey transects that pass beside the access tubes. There was no significant difference observed between the EMI meter readings of the bare soil and the vertical holes created to house the access tubes nor when the plastic access tubes were in place. However, the Al tubes showed a considerable variation in readings once the EM38 meter was within 50 cm of the tube location. A theoretical model, based on a single dipole transmitter and receiver coil, and a thin, cylindrical conducting shell located beneath the earth' s surface confirmed the horizontal eddy currents, traveling around the tube shell to be responsible for the observed deviation in the sensor response when in proximity to the metallic tube.

Accuracy and resolution of bearing measurements for crossed-dipole receiver

A well-known system for estimating the bearing to an underwater acoustic source consists of two crossed dipole sensors and an omni-directional sensor. At DRDC Atlantic, several bearing estimation methods for this three-channel system in the presence of ocean noise have been examined with the goal of minimizing the bearing error. Example algorithms are arctangent and maximum likelihood beamformer. These algorithms have trade-offs between estimation accuracy and computational efficiency, because any practical bearing-estimation algorithm must remain efficient enough for implementation in a real-time sonar processor. The accuracy of these algorithms can be demonstrated through comparison with the Cramér-Rao lower bound. Previously the calculation of bearing accuracy was based on the assumption of isotropic noise and a single signal source. However, the case of anisotropic noise and multiple-sources is more realistic, especially for bistatic or multistatic active sonar. Adaptive methods such as the Minimum Variance Distortionless Response (MVDR) beamformer will be explored to address this problem. The paper will discuss the potential improvement of bearing estimate accuracy and bearing resolution through numerical models.

Maximum likelihood estimation of direction of arrival using an acoustic vector-sensor

A vector-sensor consisting of a monopole sensor collocated with orthogonally oriented dipole sensors is used for direction of arrival (DOA) estimation in the presence of an isotropic noise-field or internal device noise. A maximum likelihood (ML) DOA estimator is derived and subsequently shown to be a special case of DOA estimation by means of a search for the direction of maximum steered response power (SRP). The problem of SRP maximization with respect to a vector-sensor can be solved with a computationally inexpensive algorithm. The ML estimator achieves asymptotic efficiency and thus outperforms existing estimators with respect to the mean square angular error (MSAE) measure. The beampattern associated with the ML estimator is shown to be identical to that used by the minimum power distortionless response beamformer for the purpose of signal enhancement.

Robust Adaptive Vector Sensor Processing

A sizable literature exists on the theory of processing signals for many vector sensor array applications. In practice, however, mismatch and several noise processes can pose significant problems. If adaptive beamforming is also used, the "snapshot" issue potentially increases by a factor of four since each element in a vector sensor array consists of a scalar hydrophone and up to three spatially orthogonal particle motion sensors. Both of these sensor types have very different response and noise characteristics. Particle motion sensors are more sensitive to non-acoustic, motion-induced noise than hydrophones. In towed line array configuration, those sensors orthogonal to the direction of motion are exposed to higher intensities of flow noise at low frequencies than those coincident to the array axis. Similarly, different dipole sensors may be exposed to varying degrees of rotational mismatch. Sensors may also rest on the seafloor, creating asymmetries. Physical constraints also relate pressure and particle velocity measurements via conservation of momentum and mass equations. We examine adaptive processing methods customized to the unique characteristics of vector sensors. Array gain and other performance metrics in the presence of system mismatch and uncertainties is presented using common ocean noise models, including directional jamming, isotropic and surface noise.

Beamforming for a circular microphone array mounted on spherically shaped objects

This paper will discuss the concept of phase modes to generate a desired beam pattern for a circular microphone array mounted around a rigid sphere. The method will be described for arrays consisting of omnidirectional and dipole sensors. The sound diffraction caused by the sphere is taken into account. It will be seen that the method allows, with some restrictions, the design of a wide variety of broadband beam patterns for a given elevation which usually will be the plane of the array. The directivity index is used to characterize the three-dimensional behavior of the array. Simulations show the realization of different beam patterns, based on a 16-element circular array located at the equator of a sphere with radius 0.085 m. The frequency range of this array is from 300 Hz to 5 kHz. Especially at low frequencies, a very good combination of the directivity index and the white noise gain is achieved which cannot be realized with “conventional” beamforming for an array of similar dimensions. The simulations are verified by means of a measurement.

Adaptive beamforming against reverberation for a three-sensor array

Two algorithms for beamforming and bearing estimation of echoes from an active sonar transmission in a strongly reverberant bistatic environment are described. The receiving array consists of a single omnidirectional sensor and two collocated orthogonal dipole sensors, and is deployed in a bistatic configuration. Both beamforming algorithms are based on minimum-variance techniques. The first algorithm matches the pattern of the minimum-variance beamformed data with that expected from an impulse at a known bearing. The second algorithm reforms the sensor data from the minimum-variance beam response. Fixed-coefficient limaçon beamforming is then applied to estimate the beam power map with reduced reverberation. The detection performance of both techniques is evaluated by injection of a synthetic target echo into experimental reverberation data. The results suggest an enhanced array gain against reverberation of the order of 3 dB for reasonable values of signal strength and probability of false alarm, compared to a direct application of fixed-coefficient limaçon beamforming. The root-mean-squared bearing error for both techniques is reduced significantly, when compared to the limaçon beamformer, by factors varying from 2 to 5.

Measurement of the seismoelectric response from a shallow boundary

Field experiments carried out at a site near Vancouver, Canada have shown that a shallow lithologic boundary can be mapped on the basis of its seismoelectric response.As seismic waves cross the boundary between organic‐rich fill and impermeable glacial till, they induceelectric fields that can be measured at the surfacewith grounded dipole receivers. Sledgehammer and blasting cap seismic sources, positioned up to 7 m away from the interface, have produced clear seismoelectric conversions. Two types of seismoelectric signals are observed. The primary response is distinguished by near simultaneous arrivals at widely separated receivers. Its arrival time is equal to the time required for a seismic P‐wave to travel from the shotpoint to the fill/till boundary. On the surface, its maximum amplitude (about 1 mV/m) ismeasured by dipoles located within a few meters of the shotpoint. At greater distances, the amplitude of the primaryarrival decays rapidly with offset, and secondary seismoelectric arrivals become dominant. They differ from the primary response in that their arrival times increasewith dipole offset, and they appear to be generatedin the immediate vicinity of each dipole sensor. Our studies show that the responses cannot be attributedto piezoelectricity or to resistivity modulation in the presence of a uniform telluric current. We infer that seismically induced electrokinetic effects or streamingpotentials are responsible for the seismoelectric conversion,and a simple electrostatic model is proposed to account for the two types of arrivals. Although our experimentswere small in scale, the results are significant in that they suggest that the seismoelectric method may be used to map the boundaries of permeable formations.

Image derived second-order differential microphones

Second-order differential (SOD) toroidal and uniaxial microphones derived using a first-order differential (FOD) sensor and a reflecting plane are described. For a uniaxial implementation a dipole FOD sensor is positioned with its axis perpendicular to and suspended a few centimeters from a large acoustically reflecting surface. The resulting sensor image is phase reversed resulting in a transducer that is a linear quadrupole. The linear quadrupole can be described by two dimensions, the distance corresponding to the FOD sensor dipole distance and the distance from the reflecting plane. If the reflecting surface is large enough, or if the wall of an enclosure is used, the resulting microphone becomes a SOD axial microphone. The close match between the sensor and its image from a good acoustic reflector results in a uniaxial SOD microphone with a 3-dB beamwidth of 66°. The sensor will operate as a differential microphone at frequencies below 3 kHz for a 2.5-cm spacing between the sensor and reflecting surface. A wall-mounted toroid can be formed by using two FOD sensors at right angles to each other and with the axis of each sensor at 45° to the reflecting surface. Also, the spacing between sensors is twice the height of the sensors from the reflecting plane. Measurements show that these transducers can be realized with existing commercially available electret condenser dipole sensors.

Eccentric dipole sources in fluid-filled boreholes: Numerical and experimental results

A frequency-wavenumber (ω−k) representation is given for the response of eccentric multipole sensors in a fluid-filled borehole. Specializing to eccentric dipole sensors, synthetic time-domain waveforms from an array of receivers deployed along the axis of the borehole are displayed for a range of eccentricity magnitude and orientation, formation slowness, and borehole radius. Flexural wave slowness estimates within several frequency bands are extracted from the synthetic waveforms using a semblance technique. For eccentricity small relative to the borehole radius, the character of dipole waveforms and the estimated flexural slowness are little changed. When the eccentricity is a substantial fraction of the borehole radius, flexural wave amplitude is greatly increased, and strong nonmonotonic trends are introduced due to interference with other modes. If eccentricity is not parallel to the transmitting dipole axis, substantial signals are detected on cross-dipole receivers, perpendicular to that of the transmitting dipole. Eccentricity effects decrease with frequency. Even for large eccentricity, flexural slowness estimates are affected only slightly. Scaled laboratory experiments confirm the numerical results. Comparison of the experimental and numerical waveforms is made for both centered and eccentric dipoles in several model formations. The predicted increase and nonmonotonic variation of flexural wave amplitude versus receiver offset for eccentric sensors is observed. Experimental and numerical waveforms overlay with excellent agreement.

Low-frequency magnetic field technology: quantifying spinal range of motion.

Reliability of a sensitive, noninvasive technique for quantifying spinal range of motion was assessed by investigators who measured lumbar mobility of 19 subjects with no history of low-back pain or spinal abnormalities. The measurement method used low-frequency, quasistatic magnetic dipole field and sensors which disturb this field in a precisely quantifiable manner. Sensors, affixed to the skin over T12 to L1 interspace and over the sacrum at the level of S1, were directly interfaced with an IBM PC-AT microcomputer, which was used for error-free collection and storage of data. Measurement results compared favorably to those obtained from the biplanar radiographic technique. Statistical analysis showed an extremely high degree of intraobserver and interobserver reliability. Additional advantages included simplicity and noninvasiveness. Overall, the magnetic field technique proved a significant advancement in clinically quantifying spinal mobility, allowing precise determination of impairment under American Medical Association guidelines.