Amplitude Restoration Research Articles

Amplitude death and restoration in networks of oscillators with random-walk diffusion

Systems composed of reactive particles diffusing in a network display emergent dynamics. While Fick’s diffusion can lead to Turing patterns, other diffusion schemes might display more complex phenomena. Here we study the death and restoration of collective oscillations in networks of oscillators coupled by random-walk diffusion, which modifies both the original unstable fixed point and the stable limit-cycle, making them topology-dependent. By means of numerical simulations we show that, in some cases, the diffusion-induced heterogeneity stabilizes the initially unstable fixed point via a Hopf bifurcation. Further increasing the coupling strength can moreover restore the oscillations. A numerical stability analysis indicates that this phenomenology corresponds to a case of amplitude death, where the inhomogeneous stabilized solution arises from the interplay of random walk diffusion and heterogeneous topology. Our results are relevant in the fields of epidemic spreading or ecological dispersion, where random walk diffusion is more prevalent.

Seismic imaging of subsurface geological structures by Kirchhoff’s migration based on extended Born approximation

Complex seismic signatures are generated because of the multifaceted nature of the subsurface. These features make the interpretation very complex. To understand the seismic behaviour, different numerical tools are available. In this present study, an attempt has been made to demonstrate both the modelling and imaging aspects of these complex subsurface features commonly encountered in petroleum exploration. The present work is an extended form of the Born approximation by using Green’s function based asymptotic ray theory. Subsequently, Kirchhoff’s depth migration has been applied to generate seismic shot gathers over structural as well as stratigraphic traps. From this analysis, it is observed that the technique is able to efficiently migrate both the structural and stratigraphic traps. The proposed technique also intends to handle strong velocity variation and amplitude restoration. However, some noise in terms of over-critical reflection has been observed in the depth migrated section corresponding to pinch-out and unconformity respectively.

Time-lapse surface seismic processing for Stage 2C of CO2CRC Otway Project

Stage 2C of the Otway project aims to detect a small injection of CO2-rich mixture into a saline aquifer, verify stabilisation of the injection and evaluate the detectability threshold of CO2 achievable by surface seismic. Over the past three years, we have produced five vintages of high-quality 4D seismic data showing the evolution of the 15,000 tonnes of injected scCO2/CH4 gas mixture into the saline aquifer at 1500 m depth. The time-lapse seismic processing workflow was built based on the findings from a synthetic feasibility study and processing of the baseline dataset. Through this workflow we processed the time-lapse data which allowed monitoring of small incremental injections (about 5 000 tonnes each). Here we elaborate on effect of various processing routines on the quality of 4D image, namely, amplitude restoration, prestack cross-equalization and imaging. We aim at preservation and restoration of time-lapse signal and suppression of time-lapse noise. We evaluate the results of the processing efforts in post-stack domain through estimates of 4D signal-to-noise ratio in vicinity of the injection interval. The usefulness of individual processing routines for improvement of time-lapse signal cannot be established independently from other routines in the same workflow. Surprisingly, images with pre-stack AGC applied have consistently higher time-lapse signal-to-noise ratio compared to images produced without it. Improvement of the resolution and appearance of 3D images does not guarantee increase of time-lapse signal-to-noise ratio.

Diffusion-weighted magnetic resonance spectroscopy boosted by simultaneously acquired water reference signals.

To combine the metabolite-cycling technique with diffusion-weighted 1 H-MR spectroscopy and to use the inherent water reference for compensation of motion-related signal loss for improved estimation of metabolite apparent diffusion coefficients (ADCs). Diffusion-weighted spectra of water and metabolites were acquired simultaneously using metabolite-cycling at 3 T. The water information was used for signal correction of phase, frequency, and eddy currents, as well as for compensation of motion-induced signal loss. ADCs were estimated by 2D simultaneous fitting. The quality of ADC restoration was investigated in vitro. Subsequently, the new approach was applied in 13 subjects for enhanced metabolite ADC estimation in gray matter. Metabolite-cycled diffusion 1 H-MRS is suitable to measure metabolite and water ADCs simultaneously. The water reference facilitates signal amplitude restoration, compensating for motion-related artefacts. 2D fitting stabilizes the fitting procedure and allows the estimation of ADCs even for low signal-to-noise metabolites. Use of the motion-compensation scheme leads to estimation of smaller ADCs for virtually all metabolites (44% smaller ADC on average), to a reduction of fitting uncertainties for metabolite ADCs in individual subjects and reduced variance over the cohort (45% smaller SD on average). Using the simultaneously acquired water signal as internal reference allows not only for compensation of phase and frequency fluctuations but also for signal amplitude restoration, and thus improved metabolite ADC estimation. Combination with 2D simultaneous fitting promises access to the diffusion properties even for low signal-to-noise metabolites. The combination of both techniques increases the specificity and sensitivity of estimated metabolite ADC values in the cohort.

Hydrology signal from GRACE gravity data in the Nelson River basin, Canada: a comparison of two approaches

The Gravity Recovery and Climate Experiment (GRACE) satellite mission measures the combined gravity signal of several overlapping processes. A common approach to separate the hydrological signal in previous ice-covered regions is to apply numerical models to simulate the glacial isostatic adjustment (GIA) signals related to the vanished ice load and then remove them from the observed GRACE data. However, the results of this method are strongly affected by the uncertainties of the ice and viscosity models of GIA. To avoid this, Wang et al. (Nat Geosci 6(1):38–42, 2013. https://doi.org/10.1038/NGEO1652; Geodesy Geodyn 6(4):267–273, 2015) followed the theory of Wahr et al. (Geophys Res Lett 22(8):977–980, 1995) and isolated water storage changes from GRACE in North America and Scandinavia with the help of Global Positioning System (GPS) data. Lambert et al. (Postglacial rebound and total water storage variations in the Nelson River drainage basin: a gravity GPS Study, Geological Survey of Canada Open File, 7317, 2013a, Geophys Res Lett 40(23):6118–6122, https://doi.org/10.1002/2013GL057973, 2013b) did a similar study for the Nelson River basin in North America but applying GPS and absolute gravity measurements. However, the results of the two studies in the Nelson River basin differ largely, especially for the magnitude of the hydrology signal which differs about 35%. Through detailed comparison and analysis of the input data, data post-processing techniques, methods and results of these two works, we find that the different GRACE data post-processing techniques may lead to this difference. Also the GRACE input has a larger effect on the hydrology signal amplitude than the GPS input in the Nelson River basin due to the relatively small uplift signal in this region. Meanwhile, the influence of the value of alpha , which represents the ratio between GIA-induced uplift rate and GIA-induced gravity-rate-of-change (before the correction for surface uplift), is more obvious in areas with high vertical uplift, but is smaller in the Nelson River basin. From Gaussian filtering of simulated data, we found that the magnitude of the peak gravity signal value can decrease significantly after Gaussian filtering with large average radius filter, but the effect in the Nelson River basin is rather small. More work is needed to understand the effect of amplitude restoration in the post-processing of GRACE g-dot signal. However, it is encouraging to find that both the methodologies of Wang et al. (2013, 2015) and Lambert et al. (2013a, b) can produce very similar results if their inputs are the same. This means that their methodologies can be applied to study the hydrology in other areas that are also affected by GIA provided that the effects of post-processing of their inputs are under control.

An Improved Droop Control Strategy for Low-Voltage Microgrids Based on Distributed Secondary Power Optimization Control

To achieve accurate reactive power sharing and voltage frequency and amplitude restoration in low-voltage microgrids, a control strategy combining an improved droop control with distributed secondary power optimization control is proposed. The active and reactive power that each distributed generator (DG) shares is calculated by extracting load information and utilizing a power sharing ratio, and is reset to be the nominal power to recalculate droop gains. The droop control curves are reconstructed according to the nominal active and reactive power and the recalculated droop gains. The reconstructed active power-frequency droop control can regulate active power adaptively and keep frequency at a nominal value. Meanwhile, the reconstructed reactive power voltage droop control can reduce voltage amplitude deviation to a certain extent. A distributed secondary power optimization control is added to the reconstructed reactive power voltage droop control by using average system voltage. The average system voltage is obtained by using a consensus algorithm in a distributed, sparse communication network which is constituted by all controllers of DGs. As a result, accurate reactive power sharing is realized, average system voltage is kept at a nominal value, and all voltage amplitude deviations are further reduced. Due to the absence of a microgrid central controller, the reliability of the strategy is enhanced. Finally, the simulation results validate the proposed method.

Signal Transmission With Long Cable for Design of PET Detector for Hybrid PET-MRI

Recently, we reported a hybrid positron emission tomography-magnetic resonance imaging (PET-MRI) approach involving the transmission of a photo-sensor charge signal to a preamplifier using a long cable. However, in this application, pulse distortions may degrade the performance of the PET detector. The purpose of this study is to further extend the charge signal transmission (CST) approach and to design a PET detector module capable of transmitting the photo-sensor charge signal to PET electronics remotely located outside the MRI room for the hybrid PET-MRI. To achieve this, we developed a pulse restoration circuit (PRC) compensating the performance degradations caused by using the long cable from Geiger-mode avalanche photodiode (GAPD) to PET electronics. The effect of the cable length on the PET detector performance was examined using different lengths of flexible flat cable ranging from 0 to 21 m. Rise time, fall time and amplitude of the GAPD output were measured as a function of the cable length. Photopeak position, energy resolution and time resolution were also measured using a 100 MS/s data acquisition unit. PRC based on the filtering and shaping networks consisted of amplitude, and rise and fall restorations. The amplitude restoration was designed using op-amp and resistors. The rise time-fall time restoration was designed using RC shaping filter. The measured pulse shapes passing through the long cable with PRC were similar to the output of GAPD with the cable length of 0 m (amplitude 169 mV, rise time 35 ns and fall time 210 ns), and the degrading PET detector performance caused by long cable were restored (energy resolution 16%, photopeak position 400 ch and time resolution 1.3 ns). There were no significant changes in the PET detector module performance using the CST approach with PRC as a function of the cable length, both outside and inside the MRI room. There were no obvious artifacts or changes in the MR phantom images. These results show that the proposed approach using extended charge signal transmission method with long cable and simple pulse restoration circuit is reliable and useful for the development of a hybrid PET-MRI system.

Control Architecture for Parallel Inverter in Uninterruptible Power Systems - Part I: Control Design and Experimental Results

In this paper, a control strategy for the parallel operation of three-phase inverters forming an online uninterruptible power system (UPS) is presented. The UPS system consists of a cluster of paralleled inverters with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filters directly connected to an ac critical bus and an ac/dc forming a dc bus. The proposed control scheme comprises two layers: 1) a local layer that contains a “reactive power-to-phase droop” in order to synchronize the phase angle of each inverter and a virtual resistance loop that guarantees equal power sharing among inverters; and 2) a central controller that guarantees synchronization with an external real/fictitious utility, and critical bus voltage amplitude restoration. Improved transient and steady-state frequency, active, reactive, and harmonic power sharing, and global phase-locked loop resynchronization capability are achieved. Detailed system topology and control architecture are presented in this paper. Furthermore, a mathematical model was derived in order to analyze critical parameters effects on system stability. The proposed control approach has been validated by means of experimental results obtained for several case-study scenarios.

Multilevel nonuniform grid algorithm for acceleration of integral equation-based solvers for acoustic scattering

A fast algorithm for the evaluation of acoustic fields produced by given source distributions is developed with the aim of accelerating iterative boundary element method (BEM) solvers. The algorithm is based on field smoothing by phase and amplitude compensation, which allows for sampling of the fields radiated by finite-size source distributions over coarse nonuniform (spherical) grids (NGs). Subsequently, the fields at the desired target points can be obtained by an interpolation and phase and amplitude restoration. Combining this approach with the divide-and-conquer strategy, the total field is computed via a hierarchical decomposition of the source domain. In this computational scheme, the phase and amplitude compensated fields produced by neighboring subdomains are gradually aggregated through a multilevel process involving interpolation between increasingly dense NGs and the scatterer surface. This multilevel NG algorithm is used to reduce the computational cost of applying the field evaluation operator and its adjoint, as required in each iteration of the conjugate gradient solver based on the BEM-discretized integral representation of scattering problems. Accuracy and computational efficiency of the NG algorithm are demonstrated on representative examples of elongated, quasi-planar, and full 3-D scatterers.

Beta-Adrenergic Stimulation Does Not Affect Calcium Sparks Refractoriness in Ventricular Myocytes

Cardiac Ca2+ sparks are intracellular Ca2+ release events from clusters of ryanodine receptors (RyR2) in the junctional sarcoplasmic reticulum (jSR). L-type Ca2+ channels (LCC) are located in the nearby apposing sarcolemma (SL) mainly at the transverse tubules. Cellular depolarization permits local Ca2+ influx through LCC that activates RyR2 clusters by Ca2+-induced Ca2+-release (CICR). Ca2+ sparks also occur during diastole due to the finite opening rate of the RyR2s that are sensitive to both cytosolic Ca2+ ([Ca2+]i) and to SR Ca2+ ([Ca2+]SR). There is a significant (at least 50%) depletion of jSR Ca2+ during each Ca2+ spark and this depletion (measured as Ca2+ blinks, Brochet et al. 2005) suggests that refractoriness of Ca2+ sparks is due to the reduction of [Ca2+]SR (Sobie et al. 2006). Additional factors (beyond [Ca2+]i and [Ca2+]SR) have been reported to affect the opening and closing rates of RyR2. Here we examine RyR2 modulation by protein kinase A (PKA) during Beta-adrenergic stimulation. Studying RyR2 refractoriness is complicated because it overlaps with ICa restitution and with the slower SR Ca2+ uptake by SERCA. We assessed RyR2 refractoriness in permeabilized ventricular myocytes from phospholamban-KO mice by studying repeated spontaneous Ca2+ sparks at the same Ca2+ release location in the absence and presence of cAMP (10 μM). We observed under control conditions that Ca2+ spark amplitude restoration (time constant ∼70 ms) was ∼2 fold slower than the reported jSR Ca2+ refilling. RyR2 phosphorylation did not affect Ca2+ sparks amplitude restoration, and the Ca2+ spark frequency distribution peak was slightly diminished, with small increases at longer delays. We conclude that under conditions when neither LCC nor [Ca2+]SR can change to influence Ca2+ sparks rate or Ca2+ sparks refractoriness, RyR2 phosphorylation by PKA activation does not alter RyR2 refractoriness.

Nonuniform grid time domain (NGTD) algorithm for fast evaluation of transient wave fields

A novel algorithm to efficiently compute transient wave fields produced by known three-dimensional source constellations is proposed. The algorithm uses domain decomposition concepts and comprises two steps to be repeated for each subdomain considered. In the first step, delay- and amplitude- compensated fields, produced by sources residing inside each subdomain are computed at a sparse set of points surrounding the observation domain. In the second step, total fields in the observer domain are evaluated by interpolation, delay and amplitude restoration, and aggregation of subdomain fields. The proposed scheme is well-suited to accelerate the solution of time domain integral equations by marching on in time, to carry out time domain physical optics calculations, and to realize near- to far-field transformations of transients. Moreover, the scheme automatically adapts to, and takes advantage of, special geometrical features of the source-observer constellation studied, a key benefit when analyzing quasi-planar configurations. In addition, it realizes a seamless transition from the dynamic to the quasi-static regime, thus facilitating a unified treatment of electrically large and small problems. Last but not least, the scheme is remarkably simple to implement.

Correcting spurious resolution

Spurious resolution is a byproduct of optical defocus whereby sinusoidal image components are reversed in sign: light stripes become dark and vice-versa. Such contrast-reversal effects are readily apparent in defocused projected images, and in subjective visual experience while viewing targets closer than the near point of the eye. Mathematically, spurious resolution is due to negative zones in the optical transfer function. Such zones appear in the transfer functions that correspond to blurring by convolution with a uniform disk (as in geometrical optics), and in those implied by physical optics for a defocused eye limited only by diffraction. What happens if these spurious negative zones are re-inverted, restoring the original contrast signs? The answer for both geometrical and physical optics is that the pointspread function is greatly re-sharpened. Since sign correction is purely a phase operation, this means that substantial deblurring can be produced by a simple phase-correcting filter that entails no amplitude restoration at all. In principle such a filter could be inserted at any point between a target (e.g., printed text) and its retinal image, by convolving the image at that stage (e.g., on the page) with the inverse Fourier transform of the sign of the blur transfer function.

Nonuniform polar grid algorithm for fast field evaluation

A novel algorithm to efficiently compute time-harmonic fields produced by known two-dimensional source constellations is proposed. The algorithm relies on domain decomposition and comprises two steps to be repeated for each subdomain. In the first step, phase, and amplitude compensated fields, produced by currents residing within each subdomain are computed over a sparse set of points surrounding the observation domain. During the second step, total fields in the observer domain are evaluated by interpolation, phase and amplitude restoration and aggregation of subdomain fields. The proposed approach is applied to the fast iterative analysis of scattering phenomena using the method of moments.

Temporal evolution of amplitude restoration and thresholding in an all-optical regenerative memory

Temporal evolution of amplitude restoration and thresholding in an all-optical regenerative memory

Temporal evolution of amplitude restoration and thresholding in an all-optical regenerative memory

In this paper, we experimentally measure the temporal evolution of amplitude restoration and thresholding in an all-optical regenerative memory. The memory architecture comprises two coupled all-optical switching gates and different wavelength optical pulses are used for clock and data sources. The all-optical gates are nonlinear interferometers incorporating semiconductor optical amplifiers as the nonlinear optical material. The concatenated response of these nonlinear interferometers provides the required square top transmission function that allows the stored optical pulses to be equalized in amplitude. The edge of the transmission function forms the threshold for pulse storage in the memory. We input a variable amplitude data packet into the memory and record the temporal evolution of the pulses with a fast real-time digitizing oscilloscope. Complete amplitude restoration of the pulses occurs after ∼5 circulations of the optical fibre memory loop. This measurement is in good agreement with the theoretical prediction and indicates that the square top transmission response could also be achieved for data ‚on the fly’ with a linear concatenation of only a few all-optical switching gates.

Storage threshold and amplitude restoration in an all-optical regenerative memory

We demonstrate an all-optical regenerative memory in which a variable threshold level for optical pulse storage can be predetermined. Only optical pulses with amplitudes above the threshold level are stored and their amplitudes are equalised by the sequential transfer function of the concatenated nonlinear optical switching gates.

T2 restoration and noise suppression of hybrid MR images using Wiener and linear prediction techniques

The authors address the problem of enhancing hybrid magnetic resonance (MR) images degraded by T2 effects and additive measurement noise. To reduce imaging time, MR signals are acquired using hybrid imaging (HI) sequences such as rapid acquisition relaxation-enhanced (RARE) and fast spin-echo (FSE). With these techniques, T2 effects act as a distortion filter. This T2 filter affects the signal and results in image spatial resolution and/or contrast loss. Furthermore, the amplitude and phase discontinuities in the T2 filter frequency response function may generate serious ringing artifacts. These distortions will damage image quality and affect object detectability. The authors use the Wiener filter and linear prediction (LP) technique to process HI MR signals in the spatial frequency domain (K-space) and the hybrid domain, respectively. Based on the average amplitude symmetry constraint of the spin echo signal, the amplitude frequency response function of the T2 distortion filter can be estimated and used in the Wiener filter for a global T2 amplitude restoration. Then, the linear prediction technique is utilized to obtain the local signal amplitude and phase estimates around the discontinuities of the frequency response function of the T2 filter. These estimates are used to make local amplitude and phase corrections. The effectiveness of this combined technique in correcting T2 distortion and reducing the measurement noise is analyzed and demonstrated using experiments on both phantoms and human studies.

Bit-serial architecture for optical computing

The design of a complete, stored-program digital optical computer is described. A fully functional, proof-of-principle prototype can be achieved by using LiNbO(3) directional couplers as logic elements and fiber-optic delay lines as memory elements. The key design issues are computation in a realm where propagation delays are much greater than logic delays and implementation of circuits without fip-flops. The techniques developed to address these issues yield architectures that do not change as their clocking speed is scaled upward and the size is scaled downward proportionally; these are called speed-scalable architectures. Signal amplitude restoration and resynchronization are accomplished by the novel technique of switching in a fresh copy of the system clock. Device characteristics that are important to the proof-of-principle demonstration are discussed, including the special properties and limitations that are important when designing with them. Design principles are exemplified by the design of an n-bit counter. Following this, the design for a stored-program bit-serial computer is described. We estimate that the described prototype architecture can be operated in the 100-MHz region with off-the-shelf components, and in the O. 1-1-THz region with foreseeable future components.

All-optical regenerator based on nonlinear fibre sagnac interferometer

For the first time, all-optical regeneration using a nonlinear fibre Sagnac interferometer switch (NSIS) and pump-probe walk-off is demonstrated. In the NSIS, all-optical regeneration including timing and amplitude restoration is performed by switching clock pulses with amplified input signals. Timing restoration is evaluated by performing error-rate measurements at 5 Gbit/s. >

Minimum-Variance and Maximum-Likelihood Deconvolution for Noncausal Channel Models

This paper extends the previous works of Mendel and his students on the subject of deconvolution from causal channel (wavelet) models to noncausal channel models. Noncausal wavelets occur, for example, in seismic data processing when a land vibrator is used to excite the Earth. Minimum-variance and maximum-likelihood deconvolution algorithms are developed herein for symmetrical and/or nonsymmetrical time-invariant wavelets that are excited by stationary and/or nonstationary white noise inputs. Minimum-variance deconvolution algorithms for a noncausal wavelet turn out to be quite different than those for a causal wavelet; however, maximum-likelihood deconvolution algorithms for a noncausal wavelet, which involve event detection and amplitude restoration, are essentially the same as those for a causal wavelet. Examples are provided that illustrate the performance of the different deconvolution algorithms.