Types Of Source Models Research Articles

FEMfuns: A Volume Conduction Modeling Pipeline that Includes Resistive, Capacitive or Dispersive Tissue and Electrodes

Applications such as brain computer interfaces require recordings of relevant neuronal population activity with high precision, for example, with electrocorticography (ECoG) grids. In order to achieve this, both the placement of the electrode grid on the cortex and the electrode properties, such as the electrode size and material, need to be optimized. For this purpose, it is essential to have a reliable tool that is able to simulate the extracellular potential, i.e., to solve the so-called ECoG forward problem, and to incorporate the properties of the electrodes explicitly in the model. In this study, this need is addressed by introducing the first open-source pipeline, FEMfuns (finite element method for useful neuroscience simulations), that allows neuroscientists to solve the forward problem in a variety of different geometrical domains, including different types of source models and electrode properties, such as resistive and capacitive materials. FEMfuns is based on the finite element method (FEM) implemented in FEniCS and includes the geometry tessellation, several electrode-electrolyte implementations and adaptive refinement options. The Python code of the pipeline is available under the GNU General Public License version 3 at https://github.com/meronvermaas/FEMfuns. We tested our pipeline with several geometries and source configurations such as a dipolar source in a multi-layer sphere model and a five-compartment realistically-shaped head model. Furthermore, we describe the main scripts in the pipeline, illustrating its flexible and versatile use. Provided with a sufficiently fine tessellation, the numerical solution of the forward problem approximates the analytical solution. Furthermore, we show dispersive material and interface effects in line with previous literature. Our results indicate substantial capacitive and dispersive effects due to the electrode-electrolyte interface when using stimulating electrodes. The results demonstrate that the pipeline presented in this paper is an accurate and flexible tool to simulate signals generated on electrode grids by the spatiotemporal electrical activity patterns produced by sources and thereby allows the user to optimize grids for brain computer interfaces including exploration of alternative electrode materials/properties.

Synthesising the intrinsic FRB population using frbpoppy

Context. Fast radio bursts (FRBs) are radio transients of an unknown origin whose nature we wish to determine. The number of detected FRBs is large enough for a statistical approach to parts of this challenge to be feasible. Aims. Our goal is to determine the current best-fit FRB population model. Our secondary aim is to provide an easy-to-use tool for simulating and understanding FRB detections. This tool can compare surveys, or provide information about the intrinsic FRB population. Methods. To understand the crucial link between detected FRBs and the underlying FRB source classes, we performed an FRB population synthesis to determine how the underlying population behaves. The Python package we developed for this synthesis, frbpoppy, is open source and freely available. frbpoppy simulates intrinsic FRB populations and the surveys that find them with the aim to produce virtual observed populations. These populations can then be compared with real data, which allows constraints to be placed on the underlying physics and selection effects. Results. We are able to replicate real Parkes and ASKAP FRB surveys in terms of detection rates and observed distributions. We also show the effect of beam patterns on the observed dispersion measure distributions. We compare four types of source models. The “complex” model, featuring a range of luminosities, pulse widths, and spectral indices, reproduces current detections best. Conclusions. Using frbpoppy, an open-source FRB population synthesis package, we explain current FRB detections and offer a first glimpse of what the true population must be.

A method to generate equivalent energy spectra and filtration models based on measurement for multidetector CT Monte Carlo dosimetry simulations.

The purpose of this study was to present a method for generating x-ray source models for performing Monte Carlo (MC) radiation dosimetry simulations of multidetector row CT (MDCT) scanners. These so-called "equivalent" source models consist of an energy spectrum and filtration description that are generated based wholly on the measured values and can be used in place of proprietary manufacturer's data for scanner-specific MDCT MC simulations. Required measurements include the half value layers (HVL1 and HVL2) and the bowtie profile (exposure values across the fan beam) for the MDCT scanner of interest. Using these measured values, a method was described (a) to numerically construct a spectrum with the calculated HVLs approximately equal to those measured (equivalent spectrum) and then (b) to determine a filtration scheme (equivalent filter) that attenuates the equivalent spectrum in a similar fashion as the actual filtration attenuates the actual x-ray beam, as measured by the bowtie profile measurements. Using this method, two types of equivalent source models were generated: One using a spectrum based on both HVL1 and HVL2 measurements and its corresponding filtration scheme and the second consisting of a spectrum based only on the measured HVL1 and its corresponding filtration scheme. Finally, a third type of source model was built based on the spectrum and filtration data provided by the scanner's manufacturer. MC simulations using each of these three source model types were evaluated by comparing the accuracy of multiple CT dose index (CTDI) simulations to measured CTDI values for 64-slice scanners from the four major MDCT manufacturers. Comprehensive evaluations were carried out for each scanner using each kVp and bowtie filter combination available. CTDI experiments were performed for both head (16 cm in diameter) and body (32 cm in diameter) CTDI phantoms using both central and peripheral measurement positions. Both equivalent source model types result in simulations with an average root mean square (RMS) error between the measured and simulated values of approximately 5% across all scanner and bowtie filter combinations, all kVps, both phantom sizes, and both measurement positions, while data provided from the manufacturers gave an average RMS error of approximately 12% pooled across all conditions. While there was no statistically significant difference between the two types of equivalent source models, both of these model types were shown to be statistically significantly different from the source model based on manufacturer's data. These results demonstrate that an equivalent source model based only on measured values can be used in place of manufacturer's data for Monte Carlo simulations for MDCT dosimetry.

Spectral characteristics of the excitation and propagation of Lg from underground nuclear explosions in central Asia

The nonlinear inverse method of Xie [1993] is applied to analyze Lg spectra from many recent underground nuclear explosions in central Asia. The analysis simultaneously determines Lg seismic moments (M0) and corner frequencies (fc) of events, as well as path‐variable Lg Q values and their frequency dependences at 1 Hz. Lg Q at 1 Hz estimated for many paths from the Lop Nor and Balapan test sites to various broadband Incorporated Institution for Seismology, Chinese Digital Seismic Network, and Kyrghizstan Network stations agree well with path‐averaged Lg coda Q values from previously obtained tomographic maps for Eurasia. This suggests that the Lg Q values obtained in this inversion have not been significantly affected by focusing/defocusing or other effects of three‐dimensional structural complexities. The logarithms of Lg M0 values obtained in this study correlate linearly, with a slope of about 1.2, with International Seismological Centre (ISC) body wave magnitudes (Mb). The linear relationship is consistent with a previous empirical relationship found between the logarithm of P wave seismic moments and ISC Mb values. The slope of the linear relationship also suggests that the logarithm of Lg M0 is a more direct measure of event size than is Mb, which, for larger events, is affected by high‐frequency amplitude falloff. Another important finding of this study is that the M0 values for the explosions scale with fc−4, rather than with fc−3, as would be expected if stress drops were constant. This is consistent with previous scaling relations derived using Lg from earthquake sources and suggests that the difference between Lg and local S excitation by the same source types (explosion or earthquake) may overwhelm that between the excitation of Lg by different source types. A comparison between Lg M0 and fc values obtained using an explosion source model with those obtained using an earthquake source model suggests that the M0 values estimated using the explosion source model are systematically lower (by a factor of 0.27) than those estimated using the earthquake source model. This occurs even though the data used are the same in the inversions with the two types of source model. The factor of 0.27 is therefore due to the source model dependence of the M0, fc estimates. The overshoot and lower spectral levels at lower frequencies in the explosion source model tend to reduce both the estimated M0 and fc values. This suggests that in any nuclear monitoring program, the same type of source model (either an earthquake or explosion) should always be used to discriminate explosions using Lg. The source model dependence also suggests that more effort should be made to estimate the amount of overshoot in the Lg excitation caused by various source types.

Estimation of seismic source functions of some explosions in the U.S.S.R. using AR method

Autoregressive (AR) modelling of seismograms is used to estimate the high frequency slope of the source spectral variations of synthetic seismograms and of seismograms recorded at Gauribidanur seismometer array (GBA), India, from explosions in the U.S.S.R. region. Analysis of synthetic seismograms shows that the AR method is able to reproduce the expected spectral variation for different T∗ values and yields. The analysis of explosions in central U.S.S.R. on 7 July 1980, 4 April 1980 and 1 November 1980 shows a high frequency spectral variation ω −2 similar to the predicted variation by von Seggern and Blandford while eastern Kazakh explosions on 29 June 1980 and 18 August 1980 show a ω −4 spectral falloff in the high frequency region as proposed in the source function model of Haskell. The recovered spectrum of western Kazakh event on 14 July 1980 shows a ω −3 variation in the high frequency which could have occurred due to Helmberger's type of source model or due to interference between P and pP.

Analysis of cylindrical antennas--A spectral iteration technique

The cylindrical antenna problem has been tackled using the spectral iteration technique. An iterative scheme is employed for improving on an initially assumed form of the current distribution. Use is made of the fast Fourier transform (FFT) algorithm, and the cumbersome process of matrix inversion is circumvented. Consequently, this method is capable of handling a larger number of unknown coefficients in the expansion of the current distribution. Furthermore, it provides a convenient means of testing for the satisfaction of the boundary conditions on the surface of the antenna. Convergence criteria for the iteration process have been established and the use of an acceleration procedure is illustrated. Different types of source models have been investigated, and the convergence of both local and nonlocal parameters is also discussed.