Frequency-domain Profile Research Articles

Seismic deconvolution based on SNR-weighted stacking

We propose a seismic deconvolution algorithm based on signal-to-noise ratio (SNR)-weighted stacking. First, we applied the time-frequency analysis to decompose the common midpoint (CMP) or common reflection point (CRP) gather. It is considered that there are differences in noise between different traces in the frequency domain profile. Then, we employ an SNR-weighted stacking strategy in each frequency-domain profile. Finally, we construct the resolution-enhancement amplitude spectrum, apply a linear inversion approach to calculate the overall weights of each frequency-domain profile, perform weighted stacking between the frequency-domain profiles, and obtain the resolution-enhancement post-stack data. The frequency decomposition of the pre-stack gather is efficiently utilized for the primary data of the proposed method. The SNR-weighted stacking adaptively selects the components with high SNR at each frequency. In addition, the resolution-oriented stacking weights are determined by the linear inversion approach. This algorithm can be adaptively weighted by considering the SNR of each frequency component, which can effectively overcome the problem of low SNR in spectral simulation and obtain high-resolution seismic data. The algorithm does not include wavelet extraction and regularization; thus, the resolution-enhancement data is more reliable. Applications to synthetic and field seismic datasets demonstrate that the proposed resolution-enhancement methodology is of great benefit to the succeeding structural interpretation and thin-bed identification.

Time dependent dihedral angle oscillations of the spike protein of SARS-CoV-2 reveal favored frequencies of dihedral angle rotations

The spike protein of SARS-CoV-2 is critical to viral infection of human host cells which ultimately results in COVID-19. In this study we analyzed the behavior of dihedral angles (phi and psi) of the wild-type spike protein over time from molecular dynamics and identified that their oscillations are dominated by a few discrete, relatively low frequencies in the 23–63 MHz range with 42.969 MHz being the most prevalent frequency sampled by the oscillations. We thus observed the spike protein to favor certain frequencies more than others. Gaps in the tally of all observed frequencies for low-abundance amino acids also suggests that the frequency components of dihedral angle oscillations may be a function of position in the primary structure since relatively more abundant amino acids lacked gaps. Lastly, certain residues identified in the literature as constituting the inside of a druggable pocket, as well as others identified as allosteric sites, are observed in our data to have distinctive time domain profiles. This motivated us to propose additional residues with similar time domain profiles, which may be of potential interest to the vaccine and drug design communities for further investigation. Thus these findings indicate that there is a particular frequency domain profile for the spike protein hidden within the time domain data and this information, perhaps with the suggested residues, might provide additional insight into therapeutic development strategies for COVID-19 and beyond.

HVDC grid fault location method using genetic algorithm on reconstructed frequency-domain voltage profiles

This paper presents a novel frequency domain based fault location method for High Voltage Direct Current (HVDC) grids. The method relies only on short data windows (below two milliseconds) of single-ended voltage measurements, thus rendering the technique practically applicable to HVDC grids employing high-speed circuit breakers. Based on the available post-fault voltage traces, a frequency domain profile of voltage transfer function is initially constructed, which is then iteratively evaluated against theoretical voltage profiles, synthesized with the use of travelling wave principles. The evaluation routine is based on a genetic algorithm (GA) regime, which once terminated indicates a close approximation to the feeder’s fault location. The result of this process is further refined using a weighted averaging function to obtain the final fault distance estimation. The performance of the proposed method is demonstrated using comprehensive electromagnetic transient (EMT) simulation studies conducted on a four-terminal meshed HVDC grid using PSCAD/EMTDC. The results revealed that the method can estimate the fault location with high accuracy for all transmission media types, including hybrid lines that comprise of cable and overhead line segments. Moreover, it has been found that the method maintains its accuracy for very long transmission lines, highly-resistive faults and different fault types.

Measurement of Non-Linear Impedances and Zero Free Parameter Modeling Approach for Predicting Battery Voltages

Choosing the appropriate energy storage system for a given application is one of the most important and deciding issues that need to be tackled in designing electric powered equipment. In particular, for electric vehicles, a plethora of power consumption regimes are described by regulatory bodies. These power consumption profiles can be easily transformed to current requirements of the energy storage system which results in countless such current profiles. The problem is further complicated with the vast variety of available energy storage systems, that all need to be tested for a given application specified by the required current profile. Therefore, rather than testing every single combination, modelling approaches are utilized to predict various combinations. Models in the literature usually employ impedance measurements as their basis, either to obtain relevant parameters for a first principles model or to fit an equivalent circuit which is subsequently taken to the time domain. In our modelling approach[1], the current profile to be modeled is converted to the frequency domain via a Fourier Transform. This frequency domain profile is multiplied by the impedance at each frequency, yielding a voltage response again in the frequency domain. This frequency domain voltage is then inverse Fourier transformed to obtain the time domain voltage response. After correcting for the state of charge with a simple DC discharge map, the voltage response of any electrochemical storage system can be obtained with errors less than 1%[1,2]. The developed modelling approach does not fit any models to the impedance data, thus has no free parameters. Thus, modeling battery systems require the accurate measurement of impedance in both our method [1,2]and others. In order to acquire an accurate impedance spectrum, the system needs to be stable throughout the measurement and the sine wave applied needs to lead to a linear response. The impedance spectrum measured can either have non-linearities due to the high amplitude used, or due to the chemistry of the system being irreversible. In either case, the measured impedance and/or the response of the battery would have contributions due to non-linearities. EIS assumes linearity which is usually valid for small amplitudes (of both voltage and current) for energy storage systems. In real applications, however, larger amplitudes are required from these systems which is not easily modeled using equivalent circuit models. In our approach, however, the non-linearities can be accounted for by simply adding the relevant harmonics to the otherwise linear impedance data. We will show the applicability of our model for nonlinear systems using a supercapacitor under high current loads. For the latter case, a primary chemistry, SOCl2 is investigated. In the literature the impedance of SOCl2 batteries is ill-characterized where the reports either have obvious non-linearities[3] or omit data [4]. If the following half reactions and the resulting total reaction is considered for the chemistry, a cell potential that is independent of any concentration is seen, which causes the cell voltage to stay constant at around 3.6V (minus a small decrease due to i.R) as shown in Figure a. Li(s) --> Li+(dis.) + e- (Anode) 4Li+(dis.)+ 4e- + 2SOCl2 (l)--> 4LiCl(s) + SO2 (g)+ S(s) (Cathode) Even a small perturbation such as the small amplitude voltage signal of a potentiostatic impedance measurement is not viable, where in the discharge region the cell potential drops below 3.6V only at full discharge and in the charge region, the reaction is not defined. Utilizing a galvanostatic measurement with a DC offset and dynamic adjustment of the measurement parameters, we will show Kramers-Kronig transformable impedance data for SOCl2 batteries as shown in Figure b.

ENDOR with band-selective shaped inversion pulses

Electron Nuclear DOuble Resonance (ENDOR) is based on the measurement of nuclear transition frequencies through detection of changes in the polarization of electron transitions. In Davies ENDOR, the initial polarization is generated by a selective microwave inversion pulse. The rectangular inversion pulses typically used are characterized by a relatively low selectivity, with full inversion achieved only for a limited number of spin packets with small resonance offsets. With the introduction of pulse shaping to EPR, the rectangular inversion pulses can be replaced with shaped pulses with increased selectivity. Band-selective inversion pulses are characterized by almost rectangular inversion profiles, leading to full inversion for spin packets with resonance offsets within the pulse excitation bandwidth and leaving spin packets outside the excitation bandwidth largely unaffected. Here, we explore the consequences of using different band-selective amplitude-modulated pulses designed for NMR as the inversion pulse in ENDOR. We find an increased sensitivity for small hyperfine couplings compared to rectangular pulses of the same bandwidth. In echo-detected Davies-type ENDOR, finite Fourier series inversion pulses combine the advantages of increased absolute ENDOR sensitivity of short rectangular inversion pulses and increased sensitivity for small hyperfine couplings of long rectangular inversion pulses. The use of pulses with an almost rectangular frequency-domain profile also allows for increased control of the hyperfine contrast selectivity. At X-band, acquisition of echo transients as a function of radiofrequency and appropriate selection of integration windows during data processing allows efficient separation of contributions from weakly and strongly coupled nuclei in overlapping ENDOR spectra within a single experiment.

Time-domain damping models in structural acoustics using digital filtering

This paper describes a new approach in order to formulate well-posed time-domain damping models able to represent various frequency domain profiles of damping properties. The novelty of this approach is to represent the behavior law of a given material directly in a discrete-time framework as a digital filter, which is synthesized for each material from a discrete set of frequency-domain data such as complex modulus through an optimization process. A key point is the addition of specific constraints to this process in order to guarantee stability, causality and verification of thermodynamics second law when transposing the resulting discrete-time behavior law into the time domain. Thus, this method offers a framework which is particularly suitable for time-domain simulations in structural dynamics and acoustics for a wide range of materials (polymers, wood, foam, etc.), allowing to control and even reduce the distortion effects induced by time-discretization schemes on the frequency response of continuous-time behavior laws.

Impact of Microwave Pulses on Thermoacoustic Imaging Applications

Thermoacoustic imaging is a promising modality for breast cancer detection. Acoustic signals generated by tumor targets with different sizes subject to microwave pulses with various widths and waveforms are analytically studied in this letter. Time- and frequency-domain profiles of the acoustic signals are achieved. Some key parameters of the acoustic signals, including peak value and peak-to-peak time interval of the time-domain acoustic pressure and frequency of the global peak and magnitude ratio of the second highest peak to the global peak of the frequency-domain pressure, are discussed. Impact of pulse width on acoustic signal peak-to-peak interval and image spatial resolution is developed. Square pulse is demonstrated to be superior in acquiring higher image resolution and signal-to-noise ratio. This study is helpful to optimizing the microwave pulse, selecting acoustic transducer, and evaluating image resolution.

Logarithmic cyclic frequency domain profile for automatic modulation recognition

Cyclostationary techniques have been applied widely to the problem of recognising communication modulation schemes. As these techniques are processing intensive, much effort has been invested in researching algorithms that can reduce the number of computational steps required, with fast Fourier transform approaches predominating. A novel approach to improve the extent of the cyclic frequency (α) is proposed. By using the constant Q transform (CQT), a logarithmic form of the spectral correlation function (SCF) can be produced. This allows the α-axis to be extended, which can be advantageous when the receiver bandwidth cannot be well matched to the signal frequency and bandwidth using a priori knowledge of spectrum allocation. It is found that a CQT-based SCF can form the basis of a logarithmic cyclic frequency domain profile algorithm without loss of sensitivity compared with the conventional, linear form.

Theoretical Analysis of Relaxation During Shaped Pulses. I. The Effects of Short T1 and T2

A detailed theoretical treatment of spin-lattice and spin-spin relaxation effects which occur during the application of long selective pulses is presented. The effects of T1 and T2 on the degradation of the frequency-domain profile are discussed for a wide variety of pulse shapes. The predicted effects are observed experimentally for residual HOD in a 2:1 (v/v) mixture of DMSO/D20. The relaxation effects for a given pulse are dependent on the ratio of pulse length to relaxation time, and differences between pulses are due to the trajectories of the magnetization produced by each pulse envelope. A comparison of the various pulse shapes is presented for aid in experimental design and data interpretation.

Photochemically accumulated photon echoes and Stark effect

We investigate photon echoes stimulated by diffraction from photochemically accumulated gratings in the presence of an externally applied electric field. In the experiment carried out with picosecond laser pulses using a chlorin-doped polyvinylbutyral film at 1.6 K we observe an oscillatory behavior of the diffracted signal intensity depending on the strength of the applied electric field. The unusual oscillating behavior is due to a linear Stark effect, which influences the frequency domain profile of the photochemically accumulated gratings.

Band-selective radiofrequency pulses

A theoretical treatment is given of the general problem of designing amplitude-modulated radiofrequency pulses that will excite a specified band of frequencies within a high-resolution NMR spectrum with uniform intensity and phase but with negligible excitation elsewhere. First a trial pulse envelope is defined in terms of a finite Fourier series and its frequency-domain profile calculated through the Bloch equations. The result is compared with the desired target profile to give a multidimensional error surface. The method of simulated annealing is then used to find the global minimum on this surface and the result refined by standard gradient-descent optimization. In this manner, a family of new shaped radio-frequency pulses, known as BURP ( band-selective, uniform response, pure-phase) pulses, has been created. These are of two classes—pulses that excite or invert z magnetization and those that act as general-rotation πr/2 or π pulses irrespective of the initial condition of the nuclear magnetization. It was found convenient to design the latter class as amplitude-modulated time-symmetric pulses. Tables of Fourier coefficients and pulse-shape ordinates are given for practical implementation of BURP pulses, together with the calculated frequency-domain responses and experimental verifications. Examples of the application of band-selective pulses in conventional and multidimensional spectroscopy are given. Pure-phase pulses of this type should also find applications in magnetic resonance imaging where refocusing schemes are undesirable.

Efficient DFT-based model reduction for continuous systems

A method for online identification of reduced-order models (ROM) of stable continuous systems is presented. The method is unique because it utilizes the moving discrete Fourier transform (MDFT) to continuously monitor the frequency-domain profile of the plant input and output signals. These signals need not be sinusoidal, although they must be accurately represented by their DFTs. Also, the input must contain at least n frequency components (for an nth-order ROM). A computer simulation demonstrates the method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>