Hugo G. Espinosa

Surface Impedance Mapping Using Sferics

Naturally occurring radio emissions from discrete sferics in the frequency range 500 Hz to 30 kHz have the potential for shallow conductivity profiling beneath the surface of the earth. A dual-channel time-domain receiver was constructed and used over selected geophysical targets. The instrumentation was validated through a comparison with very low-frequency surface impedance measurements from a distant navigation transmitter and 2-D modeling using the impedance method. The multifrequency results were then verified using this 2-D modeling. The skin effect shows the frequency dependence of electromagnetic field strength dissipation as a signal enters a multilayered earth. Single-frequency methods offer fixed depth conductivity profiles whereas multiple frequencies reveal multiple depth conductivity profiles that can help with 3-D modeling of subsurface features and anomalies. Several electromagnetic techniques offer multiple-frequency operation capitalizing upon this effect however these techniques generally use a local artificial signal source, which leads to near-field distortion effects in the measured data. This method shows significant promise for cost-effective, high-speed, ground-level surface impedance measurements targeting subsurface features down to 100 m.

Evaluating the Use of Inertial-Magnetic Sensors to Assess Fatigue in Boxing During Intensive Training

Automating measures of performance can allow for heightened understanding of how an athlete is performing, not only during a session but over time. Physical and mental performance degrades in high-intensity skilled sports like boxing as the athlete fatigues. Although the use of low cost, ubiquitous inertial sensors have been reported effective for performance classification in boxing, no assessment of a boxers efficacy has been reported under fatigue conditions. This letter evaluates the use of inertial sensors for automatic classification of fatigue by assessing the punch consistency in terms of pitch angle, punch force through acceleration, and hand speed, using the inverse time between punches. To achieve this, bespoke software was created in MATLAB, aided by the use of an attitude and heading reference system orientation filter. Six right-handed male elite boxers from the Pacific Nations, in preparation for the 2018 Gold Coast Commonwealth Games, Australia, consented to participate in the study. A noticeable decrease in performance for both hand speed (inverse time between punches) and force production (acceleration) was observed over time during intensive training, resulting in a Pearsons correlation coefficient of r 0.97 for the acceleration component and r 0.89 for the timing component. Inertial-magnetic sensors, with bespoke software, were experimentally found to be an effective tool for the automatic classification of boxing fatigue performance metrics. This study was conducted under ethical approval (ENG1413HREC).

Efficient forward modelling using the self-consistent impedance method for electromagnetic surface impedance*

The two-dimensional self-consistent impedance method is used to calculate the electromagnetic surface impedance above subsurface structures at very low frequencies. The method was derived from Faraday’s and Ampere’s Laws and results in a linear matrix equation where the right hand side of the equation corresponds to the source field introduced into the model as a fixed magnetic value. An air layer above the earth’s surface is included to allow the scattered magnetic field to be calculated at the surface. The source field is applied above the earth’s surface as a Dirichlet boundary condition, and a Neumann boundary condition is applied to all other boundaries in the solution space. The left hand side of the linear equation corresponds to the impedance matrix determined by discretising the solution space into two-dimensional rectangular pixels or cells bounded by lumped impedance elements, with values determined by the electromagnetic properties of the local media and the size of the pixel in the model. The resulting sparse matrix offers the flexibility of cells of any shape or size. Due to the large matrix dimensions, an iterative solver with a preconditioning technique was used to improve the speed, size and convergence of the solution. The efficient forward modelling has been applied to the analysis of a coal seam with various structural anomalies and line of oxidation along a line defined by 500 m with 0.5 m resolution. This improved technique allows in-field inverse modelling of surface impedance data. This paper reports several likely coal-seam scenarios relevant to surface mining operations. The two-dimensional self-consistent impedance method was used to calculate the electromagnetic surface impedance above subsurface structures at very low frequencies. The technique has been applied to the analysis of a coal seam with various structural anomalies and line of oxidation. This improved technique allows in-field inverse modelling of surface impedance data.

Wearable slot antenna at 2.45 GHz for off-body radiation: Analysis of efficiency, frequency shift, and body absorption

The interaction of body-worn antennas with the human body causes a significant decrease in antenna efficiency and a shift in resonant frequency. A resonant slot in a small conductive box placed on the body has been shown to reduce these effects. The specific absorption rate is less than international health standards for most wearable antennas due to small transmitter power. This paper reports the linear relationship between power absorbed by biological tissues at different locations on the body and radiation efficiency based on numerical modeling (r = 0.99). While the -10 dB bandwidth of the antenna remained constant and equal to 12.5%, the maximum frequency shift occurred when the antenna was close to the elbow (6.61%) and on the thigh (5.86%). The smallest change was found on the torso (4.21%). Participants with body-mass index (BMI) between 17 and 29 kg/m2 took part in experimental measurements, where the maximum frequency shift was 2.51%. Measurements showed better agreement with simulations on the upper arm. These experimental results demonstrate that the BMI for each individual had little effect on the performance of the antenna. Bioelectromagnetics. 39:25-34, 2018.

Surface Field Measurements From a Buried UHF Transmitter: Theory, Modeling and Experimental Results

Electromagnetic interference between a direct ray and a lateral wave was postulated theoretically but has been only observed near to the ground surface in a forest canopy. This communication reports surface measurements of the electric field profile from a 433 MHz transmitter buried up to 600 mm in sand (relative permittivity 2.13, conductivity 10−4 S/m). A strong interference pattern with nulls greater than −20 dB were observed extending laterally to more than 5 m. This interference was evident when both the transmitter and receiver lay on the surface and when separated by an above ground conducting shield. Forward modeling using the impedance method and the finite integral technique adequately predict the interference pattern. These observations impact open-pit mining, surface and buried wireless sensor networks, and under-road communications systems.

A numerical comparison between EFIE/MoM and CCD methods for EM scattering in two dimensions

Two methods for calculating the electromagnetic radiation scattered by a perfectly conducting object: the Electric Field Integral Equation formulated within the Method of Moments, and the Clifford-Cauchy-Dirac technique; are compared numerically for a particular test case. Both methods involve the calculation of fields over the surface of the scatterer from integral equations; one integral with a kernel of Greens functions and the other with a Cauchy kernel. Although both start with Maxwells equations the two methods differ fundamentally in several ways.

Accelerometer Artefacts from Body-Worn Sensors

Inertial sensors are commonly found in commercial products such as smartphones and sports bands. When these sensors are stationary, the variation in the recorded signal (noise level) is very small (approaching 0.1% of the gravitational acceleration). In many sports applications, the recorded acceleration has noise spikes related to the sensor itself and impulsive body movements. The wrist acceleration (100 samples/s) from a boxing jab (posterior–anterior) to a stationary bag using five subjects demonstrated that following the impact, the sensor oscillated in a manner dependent on the sensor weight. Mathematically, the integrated acceleration at the end point of the jab should be zero. This was used to remove the vibration artefacts and verified using a high-speed camera (2014 fps). A male participant with a rotator cuff shoulder reconstruction was asked to raise his arms (front lateral raise) individually from rest (vertical) to above the horizontal position carrying several different weights. The shoulder instability of the participant was evident in the accelerometer record as significantly larger anterior–posterior vibration at 9 Hz. This was not evident in the opposite shoulder which had no reported injury. This technique might prove a useful tool in quantifying shoulder instability over long periods of time.

A Literature Review Informing an Operational Guideline for Inertial Sensor Propulsion Measurement in Wheelchair Court Sports

With the increasing rise of professionalism in sport, teams and coaches are looking to technology to monitor performance in both games and training to find a competitive advantage. Wheelchair court sports (wheelchair rugby, wheelchair tennis, and wheelchair basketball) are no exception, and the use of microelectromechanical systems (MEMS)-based inertial measurement unit (IMU) within this domain is one innovation researchers have employed to monitor aspects of performance. A systematic literature review was conducted which, after the exclusion criteria was applied, comprised of 16 records. These records highlighted the efficacy of IMUs in terms of device validity and accuracy. IMUs are ubiquitous, low-cost, and non-invasive. The implementation in terms of algorithms and hardware choices was evidenced as a barrier to widespread adoption. This paper, through the information collected from the systematic review, proposes a set of implementation guidelines for using IMUs for wheelchair data capture. These guidelines, through the use of flow-charts and data tables, will aid researchers in reducing the barriers to IMU implementation for propulsion assessment.

MATLAB-Based Interactive Tool for Teaching Electromagnetics

Presents information on the MATLAB-Based Interactive tool for teaching electromagnetics. This was created for a second-year university-level electromagnetics course using a MATLAB-based graphical user interface (GUI). It aims to improve learning outcomes, reduce staff workload, and engage online students through blended learning. The tool allows for the random generation of inputs for a large number of problems, provides automatic feedback when the answers are incorrect, and generates a report when the answers are correct. Examples of electrostatics, magnetostatics, dynamic fields, and transmission lines problem sets are presented.

An assessment of simulation methodologies for the analysis of near-field radiation zones related to human exposure

The reliable evaluation of electric and magnetic field exposures with relevant radiofrequency exposure limit standards is important in order to provide protection against known adverse health effects. The evaluation of human exposure levels to radiofrequency, electric and magnetic fields and power density is required for assessment of compliance with technical standards, and it can be conducted by either measurement or numerical calculation. This paper presents a comparison between different computational tools (ProX5 EME, Cylindrical wave model, Flat-zone method and NEC as a reference) for the analysis of zone boundaries for human exposure to radiation from a Yagi-Uda antenna, operating at 450MHz with a maximum power of 50W. The results are based on the guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields, created by the International Commission on Non-Ionizing Radiation Protection (ICNIRP).