Multichannel Spectral Analysis Research Articles

Role of the EEG theta network during software production: a connectivity study.

Software programming is an acquired evolutionary skill originating from consolidated cognitive functions (i.e., attentive, logical, coordination, mathematic calculation, and language comprehension), but the underlying neurophysiological processes are still not completely known. In the present study, we investigated and compared the brain activities supporting realistic programming, text and code reading tasks, analyzing Electroencephalographic (EEG) signals acquired from 11 experienced programmers. Multichannel spectral analysis and a phase-based effective connectivity study were carried out. Our results highlighted that both realistic programming and reading tasks are supported by modulations of the Theta fronto-parietal network, in which parietal areas behave as sources of information, while frontal areas behave as receivers. Nevertheless, during realistic programming, both an increase in Theta power and changes in network topology emerged, suggesting a task-related adaptation of the supporting network system. This reorganization mainly regarded the parietal area, which assumes a prominent role, increasing its hub functioning and its connectivity in the network in terms of centrality and degree.

Integrated 2D beam emission spectroscopy diagnostic at the Huan-Liuqi-2A (HL-2A) tokamak.

Two newly developed, eight-channel, integrated Beam Emission Spectroscopy (BES) detectors have been installed at Huan-Liuqi-2A tokamak, which extends the existing 16 single-channel modular BES system with additional 16 spatial channels. The BES collects the Doppler-shifted Balmer Dα emission with a spatial resolution of 1cm (radial) × 1.5cm (poloidal) and a temporal resolution of 0.5 µs to measure long-wavelength (k⊥ρi < 1) density fluctuations. Compared to the modular BES, the dark noise of the integrated BES is reduced by 50%-60% on average. The signal-to-noise ratio of the integrated BES system is optimized by the high light throughput front-end optics, high quantum efficiency photodiodes, high-gain, low-noise preamplifiers, and sufficient cooling capacity provided by the thermoelectric cooling (TEC) units that maintain the detectors at -20 °C. Crosstalk between channels that share the same optical system is found to be negligible. High-quality density fluctuation data enables 2D (radial-poloidal) imaging of turbulence, which allows for multi-channel spectral analysis, multi-channel cross-correlation analysis and velocimetry analysis. Preliminary results show that BES successfully captures the spatiotemporal features of the local turbulence and obtains statistically consistent turbulence characterization results.

Determination of the damping ratio by multi-channel spectral analysis of seismic downhole data

Soil dynamic parameters such as shear wave velocity and damping ratio are of major interest in earthquake engineering. While the shear wave velocity, directly linked to the shear modulus, can be determined by a number of laboratory and in-situ tests with satisfying accuracy the damping ratio is much more difficult to obtain. Especially the results of in-situ experiments show often large variations. This is in general due to the troublesome determination of precise signal amplitudes whether in time or frequency domain related with these techniques.The paper presented comes back to a relationship between attenuation and velocity dispersion of body waves which replaces the measurement of the amplitude characteristics of seismic signals by a frequency dependent velocity function. The implementation of this method has previously shown to be difficult because of the very small levels of dispersion observed in seismic data. Our approach aims to overcome the problem by applying a multi-channel spectral analysis which is widely used in surface wave testing to calculate a velocity dispersion. Multi-channel measurements have shown to be more tolerant to erroneous phase characteristics of single seismic traces than the more common two station measurements.The velocity dispersion curve is extracted from a phase velocity–frequency spectrum and the damping ratio is calculated by fitting a theoretical dispersion curve to the extracted curve. The method is demonstrated on correlated data of a seismic downhole test performed using a S-wave vibrator source. The obtained results show a reasonable agreement with damping ratios found in the literature for similar soils.

Seismic inversion of soil damping and stiffness using multichannel analysis of surface wave measurements in the marine environment

SUMMARY Determination of soil material damping is known to be difficult and uncertain, especially in the offshore environment. Using an advanced inversion methodology based on multichannel spectral analysis, Scholte and Love wave measurements are used to characterize subsea soil from a North Sea site. After normalization, a determinant-based objective function is used in a genetic algorithm optimization to estimate the soil shear modulus. The inverted shear-modulus profile is comparable to previously published results for the same data, although a higher degree of certainty is achieved in the near-surface layers. The half-power bandwidth method is used for extracting the attenuation curve from the measurements and efficient reference data points are chosen based on wavelet compression. The material-damping ratio inversion is performed using a modified stochastic optimization algorithm. Accounting for measurement errors, the material-damping ratio profile is retrieved from the fundamental-mode Scholte wave with a high degree of certainty. Furthermore, a method is proposed for identifying the frequency dependence of the material-damping ratio from in situ measurements. No evidence for frequency dependence is found and the small-strain soil material-damping ratio at this site can be said to be frequency independent for the measured conditions.

EEG Parameters of Treatment Response Prediction in Manic–Delusional and Manic–Paraphrenic States

The study of clinical, psychopathological and neurobiological characteristics of affective-delusional states is important for clarifying the diagnosis and individual prognosis of disease development. Quantitative EEG allows us to assess objectively the brain functional state of such patients and to clarify the neurophysiological mechanisms underlying the characteristics of syndrome structure of mental disorders. The aim of the study was to search for baseline amplitude–frequency and spatial characteristics of EEG as possible indicators for prediction of therapeutic response in patients with manic–paraphrenic and manic–delusional states in the framework of endogenous mental disorders with different syndrome structure. The study was conducted in 73 women aged 18–55 years with affective–delusional disorders meeting the criteria of diagnosis codes F25.0 and F25.2 according to ICD-10 and differing in the syndrome structure: with a predominance of sensual-delusional (28 patients) or ideational–delusional (31 patients) disorders, as well as a group with manic–delusional states (14 patients). The quantitative assessment of patient’s mental state in the dynamics of therapy was carried out by the YMRS and PANSS clinical scales. The brain functional state was assessed using multichannel recording and spectral analysis of resting EEG. The correlation analysis revealed the relationships between individual quantitative clinical estimates after a course of therapy and EEG parameters recorded prior to therapy (potential predictors of therapeutic response). The intergroup peculiarities of the structure of correlations between the baseline (recorded before the therapy) individual EEG spectral power values and the quantitative YMRS and PANSS clinical estimates after the therapy were revealed in three syndromally different groups of patients. These EEG parameters can be considered as candidates for the role of predictors of individual therapeutic response in patients with manic–delusional and manic–paraphrenic states. They reflect either the initially reduced brain functional state, in particular, the frontotemporal cortical areas, or its initial hyperactivation, i.e., disturbance of the normal ratio of excitation and inhibition processes. Thus, variations in the ratio of excitation and inhibition processes, reflected in the baseline resting EEG parameters, apparently determine both the clinical manifestations of manic–paraphrenic and manic–delusional conditions and the brain adaptive resources in terms of the capacity and magnitude of patient’s individual therapeutic response.

Особенности ЭЭГ при маниакально-парафренных и маниакально-бредовых состояниях у больных приступообразными эндогенными психозами

Background: the study of psychopathological and neurobiological features of psychotic states, in particular, manic-delusional and manic-paraphrenic ones, in the framework of paroxysmal endogenous psychoses, is relevant to clarify the diagnosis and individual prognosis of the disease. Quantitative EEG parameters allow objective assessment of the brain functional state in these patients, and clarify the neurophysiological mechanisms underlying the features of the syndromal structure of psychotic states. Objective: to evaluate the amplitude-frequency and spatial characteristics of the EEG in patients with paroxysmal endogenous psychosis with manic-paraphrenic and manic-delusional conditions. Patients and methods: 73 inpatient women aged from 18 to 55 years with manifest manic-paraphrenic and manic-delusional conditions in the structure of paroxysmal endogenous psychoses (F20.01, F20.02, F25.0, F30.2, F31.2 according to ICD-10) participated in a multidisciplinary study, Three groups were formed taking into account the peculiarities of the affective component of the syndrome and the mechanism of delusions: the 1st group (28 patients) with affective dominance of acute sensory delusion, the 2nd group (31 patients) with non-systematiting interpretative delusion, the 3rd group (14 patients) with manic delusion states with a dominance of visual-figurative delusions of imagination. Clinical psychopathological, psychometric, and neurophysiological methods were used. Evaluation of the mental state of patients was carried out using the YMRS and PANSS scales, and the brain functional state — using multi-channel recording and spectral analysis of the resting EEG. The correlation analysis method determined the relationships between clinical evaluations and EEG parameters. Results: the features of the EEG spectral power parameters and topography were revealed. Their correlations with clinical evaluations by the YMRS and PANSS scales significantly differed in the three groups of patients. It has been stated that they are caused by the different degree of hypofrontality and of the level of the brain stem structures activation. Perhaps these factors determine the characteristics of the psychopathic structure of manic-paraphrenic and manic-delusional conditions. Conclusions: the EEG quantitative characteristics and the structure of their relationshipswith clinical parameters differ in three clinically separated groups of patients with manic-paraphrenic and manic-delusional conditions indicate differences in the functional organization of the brain, mediating the clinical and psychopathological structure of syndromes, and confirm thevalidity of their typological division.

Assessing the Potential Benefits of the Geostationary Vantage Point for Generating Daily Chlorophyll-a Maps in the Baltic Sea

Currently, observations from low-Earth orbit (LEO) ocean color sensors represent one of the most used tools to study surface optical and biogeochemical properties of the ocean. LEO observations are available at daily temporal resolution, and are often combined into weekly, monthly, seasonal, and annual averages in order to obtain sufficient spatial coverage. Indeed, daily satellite maps of the main oceanic variables (e.g., surface phytoplankton chlorophyll-a) generally have many data gaps, mainly due to clouds, which can be filled using either Optimal Interpolation or the Empirical Orthogonal Functions approach. Such interpolations, however, may introduce large uncertainties in the final product. Here, our goal is to quantify the potential benefits of having high-temporal resolution observations from a geostationary (GEO) ocean color sensor to reduce interpolation errors in the reconstructed hourly and daily chlorophyll-a products. To this aim, we used modeled chlorophyll-a fields from the Copernicus Marine Environment Monitoring Service’s (CMEMS) Baltic Monitoring and Forecasting Centre (BAL MFC) and satellite cloud observations from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) sensor (on board the geostationary satellite METEOSAT). The sampling of a GEO was thus simulated by combining the hourly chlorophyll fields and clouds masks, then hourly and daily chlorophyll-a products were generated after interpolation from neighboring valid data using the Multi-Channel Singular Spectral Analysis (M-SSA). Two cases are discussed: (i) A reconstruction based on the typical sampling of a LEO and, (ii) a simulation of a GEO sampling with hourly observations. The results show that the root mean square and interpolation bias errors are significantly reduced using hourly observations.

THE IMPORTANCE OF AMOUNT OF SETTLEMENT IN DETERMINING THE BEARING CAPACITY OF SOILS

In this study, the aim was to determine safe bearing capacity of soils, which outcrop around Tamzi and Akcakale located in Gumushane, providing allowable settlement conditions for optimum foundation design. To define the geotechnical properties of soils, three trenches were dug and two seismic refractions with Multichannel Spectral Analysis of Surface Waves (MASW) were carried out in each research area. Sieve analyses, shear box tests, and triaxial compression tests were carried out on disturbed and undisturbed samples taken from the trenches. Seismic velocities of the soils were determined by seismic refraction and MASW methods. While determining the safe bearing capacity; the equations proposed by Terzaghi, Meyerhof, Kurtulus, Tezcan and Ozdemir, Turker, and Keceli were used and the obtained safe bearing capacity values were compared with each other. Then, the soils were modeled numerically by using finite element methods and safe bearing capacities providing allowable settlement conditions were determined. According to the results, safe bearing capacity values obtained from empirical equations are not satisfactory for optimum foundation design. For the optimum foundation design, safe bearing capacity should be accepted as 190 kN/m 2 for clayey soil (CL) and 485 kN/m 2 for the clayey sand (SC).

A global error estimator for the uncertainty of a multi-channel spectral analysis

A global estimator of the uncertainty of the average frequency response function in multi-channel spectral analysis measurements is proposed. The proposed global estimator is a generalization of the random error estimator of the frequency response function magnitude of a single-input–single-output system. In principle, the signal-to-noise ratio (and thus the quality of the frequency response function estimation) is increasing with increasing number of averages M, according to M. However, in the situation that, for practical reasons, there is a maximum imposed upon the total measurement time Tmax, it is clear that there is a trade-off between the number of averages M and the record length T (s) that is used to obtain an estimate of a single-average-frequency-response-function. There is a choice between a few long records or many short records, with the requirement that, assuming zero overlap, the number of averages M times the record length T may not exceed the total available measurement time, i.e. M×T⩽Tmax. In addition to the existence of such an optimum, a minimum record length is required as well which is related to the reverberation time of the system. The newly proposed global estimator is used to determine the optimal record length of a multi-channel system, such that a minimum error of the average frequency response function is obtained. It is also shown by experimental results that indeed the minimum allowable record length is related to the reverberation time of the system being measured.

Experimental estimation of Raman lidar sensitivity in the middle UV

The experimental results of remote detection of vapors of some atmospheric chemical compounds using a Raman lidar system with a narrow-line excimer KrF laser and a multichannel spectral analyzer, based on a diffraction spectrograph and an intensified CCD camera, are presented. The system sensitivity is estimated for a sounding range of 6–10 m. Using additional means for suppressing the intense Raman bands of N2 and O2 molecules, a detection threshold of 1 ppm is reached. Base rovibrational Raman bands of oxygen and nitrogen and their first overtones are observed during experiments on noise level estimation and Raman spectra recording in the atmosphere. The absence of overlapping of fluorescence and Raman signals is confirmed experimentally.

Fluorescence recognition of proteinaceous binders in works of art by a novel integrated system of investigation

AbstractFluorescence microscopy and microspectrofluorometry are important tools in the characterization and identification of proteins, offering a great range of applications in conservation science. Because of their high selectivity and sensitivity, the combination of these techniques can be exploited for improved recognition and quantification of proteinaceous binders in paintings and polychromed works of art. The present article explores an analytical protocol integrating fluorescence microscopy and fluorometry for both identification and mapping of proteinaceous binders (in particular egg and glues) in paint samples. The study has been carried out on historically accurate reconstructions simulating the structure and composition of tempera and oil paints containing these binders. To assess the spatial distribution of specific proteins within the paint layers, cross‐sections from the reconstructions were analyzed by fluorescence imaging after staining with an exogenous fluorophore. Reference fluorescence spectra for each layer were acquired by a multichannel spectral analyzer and compared after Gaussian deconvolution. The results obtained demonstrated the effectiveness of the integrated protocol, highlighting the potential for the use of fluorescent staining coupled with microspectrofluorometry as a routine diagnostic tool in conservation science. The current work creates a set of fully characterized reference samples for further comparison with those from actual works of art. Microsc. Res. Tech., 2011. © 2011 Wiley Periodicals, Inc.

Shallow-structure characterization by 2D elastic full-waveform inversion

Assessing the effectiveness of elastic full-waveform-inversion (FWI) algorithms when applied to shallow 2D structures in the presence of a complex topography is critically important. By using FWI, we overcome inherent limitations of conventional seismic methods used for near-surface prospecting (acoustic tomography and multichannel spectral analysis of surface waves). The elastic forward problem, formulated in the frequency domain, is based on a mixed finite-element P0-P1 discontinuous Galerkin method to ensure accurate modeling of complex topography effects at a reasonable computing cost. The inversion problem uses an FWI algorithm to minimize the misfit between observed and calculated data. Based on results from a numerical experiment performed on a realistic landslide model inspired from the morphostructure of the Super-Sauze earthflow, we analyzed the effect of using a hierarchical preconditioning strategy, based on a simultaneous multifrequency inversion of damped data, to mitigate the strong nonlinearities coming from the surface waves. This strategy is a key point in alleviating the strong near-surface effects and avoiding convergence toward a local minimum. Using a limited-memory quasi-Newton method improved the convergence level. These findings are analogous to recent applications on large-scale domains, although limited source-receiver offset ranges, low-frequency content of the source, and domination of surface waves on the signal led to some difficulties. Regarding the impact of data decimation on the inversion results, we have learned that an inversion restricted to the vertical data component can be successful without significant loss in terms of parameter imagery resolution. In our investigations of the effect of increased source spacing, we found that a sampling of 4 m (less than three times the theoretical maximum of one half-wavelength) led to severe aliasing.

KONUS-WIND OBSERVATIONS OF THE NEW SOFT GAMMA-RAY REPEATER SGR 0501+4516

In 2008 August, the new soft gamma-ray repeater SGR 0501+4516 was discovered by Swift. The source was soon confirmed by several groups in space- and ground-based multi-wavelength observations. In this letter we report the analysis of five short bursts from the recently discovered SGR, detected with Konus-Wind gamma-ray burst spectrometer. Properties of the time histories of the observed events, as well as results of multi-channel spectral analysis, both in the 20--300 keV energy range, show, that the source exhibits itself as a typical SGR. The bursts durations are <0.75 s and their spectra above 20 keV can be fitted by optically-thin thermal bremsstrahlung (OTTB) model with kT of 20--40 keV. The spectral evolution is observed, which resembles the SGR 1627-41 bursts, where a strong hardness-intensity correlation was noticed in the earlier Konus-Wind observations. The peak energy fluxes of all five events are comparable to highest those for known SGRs, so a less distant source is implied, consistent with the determined Galactic anti-center direction. Supposing the young supernova remnant HB9 (at the distance of 1.5 kpc) as a natal environment of the source, the peak luminosities of the bursts are estimated to be (2--5)x10^{40} erg s-1. The values of the total energy release, given the same assumptions, amount to (0.6--6)x10^{39} erg. These estimations of both parameters are typical for short SGR bursts.

GPS observations of TID events generated by the moving solar terminator

GPS-TEC data from the International GNSS Services (IGS) were used in this paper to analyze the medium scale traveling ionospheric disturbances (MSTIDs) which are generated by the moving solar terminator (ST) during the quiet days in 2005. We collected the most comprehensive and dense GPS network's data from North-America region and introduce here a strategy that combines polynomial fitting and multi-channel maximum entropy spectral analysis to obtain TID parameters. The results of our study are summarized as follows: (1) MSTIDs stimulated by the moving ST (ST-MSTIDs) are detected at mid-latitudes after the passage of the solar terminator with the life time of 2 similar to 3 h and the variation amplitude of 0.2 similar to 0.8 TECU. Spectral analysis indicated that the horizontal wavelength, average period, horizontal phase velocity of the MSTIDs are around 300 +/- 150 km,150 +/- 80 m/s, and 25 +/- 15 min, respectively. In addition, ST-MSTIDs have wavefronts elongated in the moving ST direction and almost parallel to ST. (2) The statistical results demonstrate that the dusk MSTIDs stimulated by ST is more obvious than the dawn MSTIDs in summer days. On the contrary, the more-pronounced dawn MSTIDs occur in winter days. Further analysis indicates that the seasonal variations of ST-MSTIDs occurrence frequency are most probably related to the seasonal differences of the variations of EUV flux in the ionosphere region and recombination process during sunrise and sunset period at mid-latitudes.

Multichannel Spectral and Coherence Analysis of Servoflap Main Rotor Blade

Flight performance of servoflap rotor helicopters is linked closely with the relationship between the servoflap and the rotor blade; rotor performance can be improved and fuselage vibration can be reduced when the feedback control for a servoflap is properly configured at multiple harmonics of the rotating (1/rev) frequency. In this paper, we have examined helicopter servoflap and main rotor blade parameters via multichannel spectral and coherence analysis. In particular, we have examined servoflap bending, rotor blade flatwise bending at 17 and 24% of the blade radius, and blade torsion at 38% of the blade radius using helicopter vibratory data collected during four representative flight conditions: hover, transition, minimum-power and cruising-speed forward flights. The analysis includes two steps: 1) identifying vibratory energy concentrations of servoflap bending and rotor blade flatwise bending and torsion at multiple harmonics of 1/rev frequency, and 2) quantifying the linear relationships of these spectral energy concentrations between servoflap bending and rotor blade flatwise bending and torsion with measures of magnitude squared coherence function and phase (time) delay. Using these quantitative descriptors, we analyzed helicopter vibratory data during four different flight conditions, summarized spectral energies of the first eight harmonics of 1/rev frequency, and delineated the linear relationships between the servoflap and rotating system parameters. The multichannel spectral and coherence analysis results have generated useful baseline data to assist continual improvement in servoflap rotor design.

Multi-channel spectral analysis of multi-pass acquisition measurements

Conditioned spectral analysis (CSA) and virtual source analysis (VSA) are two techniques currently used nowadays to post-treat measurements on multiple input multiple output (MIMO) systems. Those techniques are based on the estimation of cross-spectral quantities between outputs and the so-called “references”. In some measurement situations, all outputs cannot be acquired simultaneously, several measurement passes are necessary (using a scanning system, a robot or a laser vibrometer). An approach is proposed in this paper to adapt CSA and VSA to MIMO systems requiring a multi-pass acquisition. The approach is based on the average over all passes of a set of reference channels, and on the correction of cross-spectral quantities by using transfer functions and averaged reference spectra. An academic experiment is finally realized to illustrate the proposed approach.

Evaluation of MASW techniques to image steeply dipping cavities in laterally inhomogeneous terrain

This study evaluated the potential of using Multi-channel Spectral Analysis of Surface Wave (MASW) seismic techniques to image steeply dipping cavities in laterally inhomogeneous geological terrain. The primary test site was the Montague District, a historical gold mining area in Nova Scotia, Canada with numerous narrow steeply dipping mine workings, actively subsiding or collapsed crown pillars, and laterally inhomogeneous bedrock. Seismic surveys were conducted at two test sites using portable components and an effective geophone spacing of 1 m. A complimentary gravity survey was also conducted at one of the sites. Pre-processing focused on techniques which highlighted and then isolated the direct Rayleigh Waves for each waveform. Two MASW techniques were applied to the resulting pre-processed seismic sections; time delay mapping and shear velocity imaging. Both techniques showed the presence of time-delay or velocity anomalies where mine workings were mapped or inferred from surface subsidence patterns. Time-delay mapping was effective at determining the lateral position of the cavities, however, velocity imaging was able to generate images in both lateral distance and depth and thus has the greater potential to image both the shape and position of the cavities. Reduced velocity regions on the shear velocity images were consistent with the relative thickness of overburden at both test sites and the weakening of crown pillars at one site. Gravity surveying agreed with both the time-delay and velocity images and highlighted the use complimentary geophysical techniques to image near-surface cavities. Recommended future work includes additional field trials with rigorous ground truthing, improvements in the waveform processing methodology to reduce the processing time and the accuracy of depth resolution, and continued work in a related research program to quantify the relationships between seismic attributes and rock mass quality, subsidence potential, etc…

Detecting spatio-temporal precipitation variability in Portugal using multichannel singular spectral analysis

A spatio-temporal variability study of annual precipitation in Portugal has been performed using the multichannel singular spectral analysis (MSSA). The spatio-temporal variability patterns, which can represent progressive signals, reveal significant periodic characteristics that have not been unambiguously detected in previous studies using other methods. In this study, MSSA detects periodic behaviour in annual precipitation of about 3 and 9 years. Similar periodic signals of about 4 and 8 years are detected when singular spectral analysis (SSA) is applied to a series of a North Atlantic oscillation (NAO) index. The strong correlation between the NAO and precipitation variability in Portugal reinforces the importance of detection of these signals with similar periodic characteristics. Copyright © 2006 Royal Meteorological Society

Optical measurement and fabrication from a Morpho-butterfly-scale quasistructure by focused ion beam chemical vapor deposition

The Morpho-butterfly wing reflects interfered brilliant blue, which originates from nanostructures on its scales, for any incidence angle of white light. We have fabricated a Morpho-butterfly-scale quasistructure using focused ion beam chemical vapor deposition and observed brilliant blue reflection from this quasistructure with an optical microscope. We measured the reflection from real Morpho-butterfly scales and from the quasistructure with a photonic multichannel spectral analyzer system. The reflection spectra of the quasistructure were very similar to those of Morpho-butterfly scales.

Application of multi-channel spectral analysis to identify the source of a noise amplitude modulation in a diesel engine operating at idle

We are interested in the low frequency amplitude modulation of the noise generated by an engine operating at idle. This phenomenon, perceived inside the car, is particularly annoying. Modulated vibrations are transmitted to the frame mainly by one of the three engine mounts. The combustion is the first potential source to be inspected, but pragmatic observations on consecutive measurements show that it is not the cause of the amplitude modulation. Spectral analysis tools are applied on multi-channel measurements to identify the source of the phenomenon. A sensor is placed on each potential noise and vibration source. A virtual source analysis show that several uncorrelated sources are contributing to the operating response, particularly on frequencies for which a high amplitude modulation is observed. The computation of residual spectra obtained by means of conditioned spectral analysis proves that the diesel pump is involved in the amplitude modulation. Experiments are carried out to validate this diagnosis. Added masses appropriately placed on the injection circuit strongly attenuate the phenomenon.