Relative Spectral Amplitudes Research Articles

Higher-order thin layer method (HTLM) based wavefield modeling approach

Traditional free vibration-based forward modeling approaches provide only the theoretical modal solution that lacks information regarding modal amplitudes. Moreover, they ignore the influence of source-receiver layout on the wavefield. Numerical wavefield modeling approaches can generate a dispersion spectrum mimicking the actual field test; however, numerical methods are not efficient in terms of computational time and produce numerical artifacts. Furthermore, they can be contaminated with leaky waves for soil strata with a high Poisson's ratio. This study presents a new wavefield modeling approach for developing full dispersion spectra of stratified media incorporating source-receiver configuration. The proposed method calculates the theoretical modal amplitude using the higher-order thin layer method and determines the vertical displacement responses at specified virtual receiver positions in the frequency-space domain. Afterward, it searches for spectral values at each frequency over a broad range of trial phase velocities to create a full dispersion spectrum. The accuracy and computational efficacy of the present method are demonstrated on different soil profiles adopted from literature, including normally dispersive media, irregularly dispersive media with high-velocity contrast, and a crustal level model. Results indicate the proposed approach can clearly distinguish the relative spectral amplitude of each mode and dominant dispersion trend. Calculated seismograms and dispersion images are compared with the existing 2D staggered grid finite difference method. Although both methods yield similar results, the current method is at least two orders (x102) faster than the staggered grid finite difference method. Unlike numerical approaches, the proposed method does not require additional computational resources to model materials with a high Poisson's ratio. Furthermore, it minimizes the contamination of leaky waves in the dispersion spectrum when modeling a high Poisson's ratio. The proposed method does not produce any numerical artifacts and can be used to generate accurate synthetic seismograms as well as dispersion spectra. In addition, the proposed wavefield modeling approach can be effectively used to develop an accurate and faster inversion scheme by including the entire dispersion spectrum.

Dual-Wavelength Spectrum-Shaped Mid-Infrared Pulses and Steering High-Harmonic Generation in Solids

Mid-infrared (MIR) ultra-short pulses with multiple spectral-band coverage and good freedom in spectral and temporal shaping are desired by broad applications such as steering strong-field ionization, investigating bound-electron dynamics, and minimally invasive tissue ablation. However, the existing methods of light transient generation lack freedom in spectral tuning and require sophisticated apparatus for complicated phase and noise control. Here, with both numerical analysis and experimental demonstration, we report the first attempt, to the best our knowledge, at generating MIR pulses with dual-wavelength spectral shaping and exceptional freedom of tunability in both the lasing wavelength and relative spectral amplitudes, based on a relatively simple and compact apparatus compared to traditional pulse synthesizers. The proof-of-concept demonstration in steering the high-harmonic generation in a polycrystalline ZnSe plate is facilitated by dual-wavelength MIR pulses shaped in both spectral and temporal domains, spanning from 5.6 to 11.4 μm, with multi-microjoule pulse energy and hundred- milliwatt average power. Multisets of harmonics corresponding to different fundamental wavelengths are simultaneously generated in the deep ultraviolet region, and both the relative strength of individual harmonics sets and the spectral shapes of harmonics are harnessed with remarkable freedom and flexibility. This work would open new possibilities in exploring femtosecond control of electron dynamics and light–matter interaction in composite molecular systems.

Seismoelectric surface-wave analysis for characterization of formation properties, using dispersive relative spectral amplitudes

We have studied seismoelectric (SE) surface-wave signals and found that they can be used to infer changes in the SE coupling properties at depth. SE surface-wave signals have much higher amplitudes than SE body-wave signals. We measured the seismic and the electrical potential or electromagnetic (EM) field along the surface of the earth. We used dispersive relative spectral amplitudes (DRSAs) that measure the frequency-dependent relative strength of electrical signals versus seismic signals associated with SE surface-wave signals. We find that DRSAs have sensitivity to contrasts in the electrokinetic coupling coefficient and other relevant petrophysical properties at depth. Our discovery can mitigate the major limitation that plagues body-wave-based SE methods: the relative weakness of the converted EM signals from seismic body waves. We envision applications to characterize subsurface rock, fluid, and fluid-flow properties (e.g., porosity, permeability, and dynamic fluid viscosity, salinity) in the near surface, for aquifers, and shallow geothermal reservoirs.

A Method for Calibration of the Local Magnitude Scale Based on Relative Spectral Amplitudes, and Application to the San Juan Bautista, California, Area

We develop and use a spectral empirical Green’s function approach to estimate the relative source amplitudes of earthquakes near San Juan Bautista, California. We isolate the source amplitudes from path effects by comparing the recorded spectra of pairs of events with similar location and focal mechanism, without computing the path effect. With this method, we estimate the relative moments of 1600 M 1.5–4 local earthquakes, and we use these moments to recalibrate the duration magnitude scale in this region. The estimated moments of these small earthquakes increase with catalog magnitude M D roughly proportionally to 101.1 M D, slightly more slowly than a moment‐magnitude scaling of 101.5 M w. This more accurate magnitude scaling can be used in analyses of the local earthquakes, such as comparisons between the seismic moments and geodetic observations. [Electronic Supplement:][1] Additional description and figures showing estimated source amplitudes obtained with different parameters, and observed amplitudes of some earthquakes with large misfits. [1]: http://www.bssaonline.org/lookup/suppl/doi:10.1785/0120160141/-/DC1

Quantifying Vocal Mimicry in the Greater Racket-Tailed Drongo: A Comparison of Automated Methods and Human Assessment

Objective identification and description of mimicked calls is a primary component of any study on avian vocal mimicry but few studies have adopted a quantitative approach. We used spectral feature representations commonly used in human speech analysis in combination with various distance metrics to distinguish between mimicked and non-mimicked calls of the greater racket-tailed drongo, Dicrurus paradiseus and cross-validated the results with human assessment of spectral similarity. We found that the automated method and human subjects performed similarly in terms of the overall number of correct matches of mimicked calls to putative model calls. However, the two methods also misclassified different subsets of calls and we achieved a maximum accuracy of ninety five per cent only when we combined the results of both the methods. This study is the first to use Mel-frequency Cepstral Coefficients and Relative Spectral Amplitude - filtered Linear Predictive Coding coefficients to quantify vocal mimicry. Our findings also suggest that in spite of several advances in automated methods of song analysis, corresponding cross-validation by humans remains essential.

Simple model of the rf noise generated by multipacting electrons

A simplified analytical model is developed to predict the spectrum of electric current induced by the multipacting electrons between two parallel electrodes exposed to an rf voltage of fixed amplitude. The model is based on the resonant multipactor theory and makes it possible to calculate the relative spectral amplitudes of electric current at different harmonics and sub-harmonics of the applied rf frequency. The theoretical predictions are confirmed by numerical simulations of multipactor inside a rectangular waveguide. Specifically it is seen that the relative height of the spectral peaks decreases with increasing gap height.

Molecular dynamics study of the effect of pressure on the terahertz-region infrared spectrum of crystalline pentaerythritol tetranitrate

Terahertz infrared absorption spectra of crystalline pentaerythritol tetranitrate were obtained using classical molecular dynamics simulations for temperature T = 298 K and hydrostatic pressures P = 0, 1, 2, and 3 GPa. Two approaches were used to calculate the spectra; one based on combined normal-mode analysis and mode-relaxation calculations and the other on Fourier analysis of the dipole–dipole time autocorrelation function. The two methods yield similar, though not identical, positions and relative spectral amplitudes for all pressures studied. The predicted spectra exhibit an overall blue shift with increasing pressure, accompanied by a decrease in the integral absorption.

Perception of place of articulation for plosives and fricatives in noise

This study aims at uncovering perceptually-relevant acoustic cues for the labial versus alveolar place of articulation distinction in syllable-initial plosives {/b/, /d/, /p/, /t/} and fricatives {/f/, /s/, /v/, /z/} in noise. Speech materials consisted of naturally-spoken consonant–vowel (CV) syllables from four talkers where the vowel was one of {/a/, /i/, /u/}. Acoustic analyses using logistic regression show that formant frequency measurements, relative spectral amplitude measurements, and burst/noise durations are generally reliable cues for labial/alveolar classification. In a subsequent perceptual experiment, each pair of syllables with the labial/alveolar distinction (e.g., /ba, da/) was presented to listeners in various levels of signal-to-noise-ratio (SNR) in a 2-AFC task. A threshold SNR was obtained for each syllable pair using sigmoid fitting of the percent correct scores. Results show that the perception of the labial/alveolar distinction in noise depends on the manner of articulation, the vowel context, and the interaction between voicing and manner of articulation. Correlation analyses of the acoustic measurements and the threshold SNRs show that formant frequency measurements (such as F1 and F2 onset frequencies and F2 and F3 frequency changes) become increasingly important for the perception of labial/alveolar distinctions as the SNR degrades.

REM sleep EEG spectral analysis in patients with first-episode schizophrenia

The pathophysiology of schizophrenia includes abnormalities in subcortical–cortical transfer of information that can be studied using REM sleep EEG spectral analysis, a measure that reflects spontaneous and endogenous thalamocortical activity. We recorded 10 patients with first-episode schizophrenia and 30 healthy controls for two consecutive nights in a sleep laboratory, using a 10-electrode EEG montage. Sixty seconds of REM sleep EEG without artifact were analyzed using FFT spectral analysis. Absolute and relative spectral amplitudes of five frequency bands (delta, theta, alpha, beta1 and beta2) were extracted and compared between the two groups. Frequency bands with significant differences were correlated with BPRS positive and negative symptoms scores. Patients with schizophrenia showed lower relative alpha and higher relative beta2 spectral amplitudes compared to healthy controls over the averaged total scalp. Analysis using cortical regions showed lower relative alpha over frontal, central and temporal regions and higher relative beta2 over the occipital region. Absolute spectral amplitude was not different between groups for any given EEG band. However, absolute alpha activity correlated negatively with BPRS positive symptoms scores and correlated positively with negative symptoms scores. Since similar results have been reported following EEG spectral analysis during the waking state, we conclude that abnormalities of subcortical–cortical transfer of information in schizophrenia could be generated by mechanisms common to REM sleep and waking.

Age effect on fatigue-induced limb acceleration as a consequence of high-level sustained submaximal contraction

In reference to electromyographic measurement, the study was conducted to reassess differences in the behavior of fatigue-related neuromuscular function between young and elderly humans with limb acceleration (LA). Fourteen young and fourteen elderly subjects performed sustained index abduction at 75% of their maximal voluntary contractions (MVC) until task failure. Measures of neuromuscular function, including temporal/spectral features of muscle activity of the first dorsal interosseous (FDI) and LA of the index and hand, were monitored. The results showed a manifest fatigue-induced increase in LA of the index in the elderly group, but not in the young group. In contrast, only the young group developed a significant increase in amplitude of the electromyography (EMG) until task failure. Spectral analyses of LA in the index reflected marked age-dependent reorganization following muscle fatigue, with a greater reduction of relative spectral amplitude of LA in the range of 20-40 Hz, but a lesser reduction in coherence between EMG and LA in the elderly group. In line with fatigue-associated restructuring of LA, the mechanical coupling of the metacarpophalangeal joint was more severely undermined in the elderly group than in the young group. The present study manifested an age-related difference in the relative contributions of neural versus mechanical factors to muscle fatigue. Subsequent to a high-level sustained submaximal isometric contraction, a predominant mechanical failure of the musculotendon complex in the elderly was featured with LA, whereas EMG measurement characterized prevailing impairment of neuromuscular propagation in the young.

Hyperfine splittings in spin-frustrated trinuclear Cu(3) clusters.

The hyperfine structures of the EPR spectra of the spin-frustrated and distorted Cu(II) trimers were calculated in the spin-coupling model. The correlations between the hyperfine structures of the EPR spectra and geometry of the Cu(3) clusters (equilateral, isosceles, and scalene triangles) were found. For the EPR spectrum of the spin-frustrated ground state 2(S = 1/2) of an equilateral triangle Cu(3) cluster (J(12) = J(13) = J(23) = J), the calculated hyperfine structure represents the complicated spectrum of the 24 hyperfine lines, of total length 5a, where a is the hyperfine constant of the mononuclear Cu center. For an isosceles Cu(3) cluster (J(12) not equal J(13) = J(23)), the hyperfine splittings of the EPR spectra of the two split S = 1/2 levels with intermediate spins S(12) = 0 and S(12) = 1 are essentially different. The EPR signal of the |(S(12) = 0)S = 1/2> level is characterized by the four equally spaced hyperfine lines (interval A = a) with the same relative spectral amplitudes 16:16:16:16 and total length 3a. For the |(S(12) = 1)S = 1/2> level, the calculated hyperfine structure represents the spectrum of the 16 hyperfine lines with equal spacing (interval A' = a/3), the spectral intensity distribution 1:1:3:3:5:5:7:7:7:7:5:5:3:3:1:1 and total length 5a. These hyperfine spectra differ from the hyperfine structure (10 lines with interval a/3) of the EPR signals of the excited S = 3/2 level of the Cu(3) cluster. The quartet hyperfine structure, characteristic of a single Cu(2+) nucleus, which was observed experimentally for the doublet ground state of the spin-frustrated Cu(3)(II) clusters, corresponds to the hyperfine structure of the EPR signal of the |(S(12) = 0)S = 1/2> level. This hyperfine structure is evidence of the lowering of the Cu(3) cluster symmetry from trigonal to orthorhombic and the small splitting of the spin-frustrated 2(S = 1/2) ground state.

Influence of an alternating 3 Hz magnetic field with an induction of 0.1 millitesla on chosen parameters of the human occipital EEG

In 62 volunteers it was studied, whether an alternating 3 Hz magnetic field (induction 0.1 mT) vertically applied to the head over a period of 20 min causes changes in EEG parameters. The study's design was a random crossover controlled, blind one. The field was generated by a Helmholtz coils arrangement. The occipital surface EEGs (O1 and O2) were derived against the left earlobe. Significant differences (two-tailed P<0.05) between sham and real exposure were found for the relative spectral amplitudes of the theta (3.5–7.5 Hz) and beta band (12.5–25.0 Hz) and the theta/beta ratio. These observations can be interpreted as a more pronounced reduction of alertness under the real field condition compared with the control.

Accurate spectral scans of single chromatographic peaks

A detector equipped with two channels can fetch accurate luminescence spectra either from migrating chromatographic peaks or from other inputs of sharply varying concentration. To achieve this, one (the scanning) channel is continually referenced to another (the constant-wavelength) channel: the ratio of the two hence portrays correctly the intensity distribution of the spectrum. As a model system for testing this ratio approach on single gas chromatographic peaks, a flame photometric detector was connected to a 1 8 - m scanning spectrophotometer. No significant differences were found in relative spectral amplitudes between scanning the ascent versus the descent of a peak. Similar spectral constancy was obtained when scanning non-chromatographic variable inputs. With a view to practical use, three experiments were carried out that demonstrated the sensitivity and/or the spectral resolution obtainable from reference scans of single peaks. To wit, 5 pg of phosphorus produced a clearly recognizable HPO spectrum: the vibrational levels of S 2 resolved well; and the presence/absence of atomic iron lines confirmed an earlier postulated energy limit for the chemiluminescent excitation reaction.

Seismological aspects of the 1989–1990 eruptions at redoubt volcano, Alaska: the SSAM perspective

SSAM is a simple and inexpensive tool for continuous monitoring of average seismic amplitudes within selected frequency bands in near real-time on a PC-based data acquisition system. During the 1989–1990 eruption sequence at Redoubt Volcano, the potential of SSAM to aid in rapid identification of precursory Long-Period (LP) event swarms was realized, and since this time SSAM has been incorporated in routine monitoring efforts of the Alaska Volcano Observatory. In particular, an eruption that occurred on April 6 was successfully forecast primarily on the basis of recognizing the precursory LP activity on SSAM. Of twenty-two significant eruptions that occurred between December 14 and April 21, eleven had precursory swarms longer than one hour in duration that could be detected on SSAM. For individual swarms, the patterns of relative spectral amplitudes are distinct at each station and remain largely stationary through time, thus indicating that one source may have been preferentially and repeatedly activated throughout the swarm. Typically, a single spectral band dominates the signal at each seismic station: for the vigorous one-day swarm that preceded the first eruption on December 14, signals were sharply peaked in the 1.9–2.7 Hz band at the closest station, located 4 km from the vent, but were dominated by 1.3–1.9 Hz energy at three more distant stations located 7.5–22 km from the vent. The tendency for the signals from different swarms recorded at the same station to be peaked in the same frequency band suggests that all of the sources are characterized by a predominant length scale. Signals from the precursory LP swarms became weaker as the eruption sequence progressed, and swarms that occurred in March and April could only be detected at seismographs on the volcanic edifice. Onset times of precursory LP swarms prior to eruptions ranged from a few hours to about one week, but after the initial vent-clearing phase that ended December 19 these intervals tended to become progressively shorter for successive swarms. These trends in the relative onset times and intensities of successive precursory LP swarms are consistent with an overall depressurization of the magmatic system through time. In general, each of the swarms had an emergent onset, but the intensities did not always increase steadily until the eruptions. Instead, as the time of an eruption approached the intensity usually increased more rapidly before peaking and then declining prior to the eruption; for three of the swarms, two distinct peaks in intensity were apparent. The time intervals between final peaks in swarm intensity and ensuing eruptions ranged from about 2 hours to almost 2 days, but the peaks always occurred closer to the eruptions than to the swarm onsets. Both the onset of LP swarm activity and a decline in intensity prior to an eruption may represent critical points in the process of pressurization that drives the flow of fluids and gas in a sealed magmatic system. A notable exception to this pattern is the eruption of March 9 which lacked a detectable precursory LP swarm, but was followed by an unusually long period of strong LP seismicity that may have been stimulated by a depressurization of the magmatic system resulting from dome failure. On both December 14 and January 2, the spectra of early syn-eruptive signals have peaked signatures much like those of the spectra of precursory LP activity from shortly before the eruptions; these similarities may indicate that the source of precursory seismicity continued to be active during at least the early part of each eruption. In syn-eruptive signals from March and April recorded at stations on the volcanic edifice, the dominant spectral energy progressively shifts with time during the eruption to lower frequencies; at least part of the energy in these signals may have been generated by the debris flows associated with dome failures.

An explanation for experimental observations of harmonic cyclotron emission induced by fast ions

An explanation, supported by numerical simulations and analytical theory, is given for the harmonic cyclotron emission induced by fast ions in tokamak plasmas—in particular, for the emission observed at low harmonics in deuterium–deuterium and deuterium–tritium experiments in the Joint European Torus [e.g., Phys. Rev. Lett. 60, 33 (1988)]. It is shown that the first proton harmonic, whose field energy amplitude scales as the 0.84 power of the proton density, is one of the highest spectral peaks, whereas the first alpha harmonic is weak. The relative spectral amplitudes of different harmonics are compared. The results are consistent with the experimental observations. The simulations verify that the instabilities are caused by a weak relativistic mass effect. Simulation also shows that a nonuniform magnetic field leads to no appreciable change in the growth rate and saturation amplitude of the waves.

LOVE‐TYPE SEAM‐WAVES IN WASHOUT MODELS OF COAL SEAMS*

ABSTRACTThe propagation of Love seam‐waves across washouts of coal seams was studied by calculating synthetic seismograms with a finite‐difference method. Seam interruption, seam end and seam thinning models were investigated. The horizontal offset, the dip of the discontinuities and the degree of erosion served as variable parameters. Maximum displacement amplitudes, relative spectral amplitudes and phase and group slowness curves were extracted from the synthetic seismograms.Both seam interruption and seam thinning reduce the maximum displacement amplitudes of the transmitted Love seam‐waves. The degree of amplitude reduction depends on the horizontal offset and the degree of erosion. It is four times greater for a total seam interruption than for an equivalent seam thinning with a horizontal offset of four times the seam thickness. In a seam cut vertically, the impedance contrast between the coal and the washout filling determines the maximum displacement amplitudes of the reflected Love seam‐waves. They diminish by a maximum factor of four in oblique interruption zone discontinuities with a dip of maximum 27°, and by a maximum factor of ten in a seam thinning with a degree of erosion of at least 22%.The analysis of the relative spectral amplitudes indicates a preferential transmission of those phases with frequencies below, and a preferential reflection of those phases with frequencies above the first mode Airy‐phase. The relative spectral amplitudes of the reflected Love seam‐waves show a distinct interference pattern of the waves reflected at both interruption zone discontinuities.The dispersion analysis reveals a flattening of the phase and group slowness curves with increasing frequencies, horizontal offset and degrees of erosion.These results imply that a detection of washouts in‐mine will be possible in a frequency range including at least the first mode Airy‐phase. An interference pattern and a flattening of the dispersion curve indicate a washout rather than other seam obstructions and leads to an estimate of the washout dimension.

Split free oscillation amplitudes for the 1960 Chilean and 1964 Alaskan earthquakes

abstract Splitting of the Earth's normal modes was observed for both the 1960 Chilean and 1964 Alaskan earthquakes. The strong peaks in the observed spectrum of each split multiplet correspond to singlets with much higher amplitudes than the others. Using theoretical results we have derived elsewhere (Stein and Geller, 1977a), we are able to predict this pattern. We show that the source mechanisms inferred for these earthquakes from surface waves are consistent with the observed pattern of relative spectral amplitudes of the split modes. However other mechanisms, such as a slow isotropic volume change, are also consistent with the split-mode amplitudes and are excluded only by additional data.

Evoked Vocal Response of the Bullfrog

A quantitative study of a highly selective behavioral response to sound in the bullfrog is described. The males of a laboratory colony respond vocally to a restricted class of natural and synthetic sounds. A necessary condition to elicit calling is the simultaneous presence of sufficient sound energy in the frequency regions of 100–400 and 1000–2000 cps. The relative amplitudes of the spectral envelopes of these two regions can independently vary over a wide range (at least 50 dB) without noticeably affecting calling performance. Calling can be suppressed by the introduction of energy in the neighborhood of 500 cps. The degree of suppression depends on the relative spectral amplitudes of the introduced energy and the energy in the lower frequency range of 100–400 cps. A comparison of temporal fine structure indicates optimum calling to waveform periodicities of approximately 100 cps. In presentations of the mating calls of 34 different species, only the call of the bullfrog evokes the vocal response. [Work supported in part by the U. S. Army, U. S. Navy, and U. S. Air Force under contract DA-36-039-AMC-03200(E); National Institutes of Health, U. S. Department of Health, Education, and Welfare (grant MH-04737-04); National Science Foundation (grant GP-2495); and National Aeronautics and Space Administration (grant NsG-496).]