ADDENDUM TO “STUDIES OF ELASTIC WAVE ATTENUATION IN POROUS MEDIA” BY M. R. J. WYLLIE, G. H. F. GARDNER, AND A. R. GREGORY (GEOPHYSICS, OCTOBER, 1962, PP. 569–589)

Geophysics ◽  
1963 ◽  
Vol 28 (6) ◽  
pp. 1074-1074 ◽  
Author(s):  
M. R. J. Wyllie ◽  
G. H. F. Gardner ◽  
A. R. Gregory
Keyword(s):  
High Frequency ◽  
Critical Angle ◽  
Wave Attenuation ◽  
Low Frequency ◽  
Shear Velocity ◽  
Confining Pressure ◽  
Biot’S Theory ◽  
Biot's Theory ◽  
Angle Method ◽  
Frequency Pulse

In the paper published last year we noted that the shear velocity through liquid‐saturated rocks often appeared to exceed the shear velocity through the same rocks when dry. Our results were based on measurements made by the critical‐angle method on rocks subjected to heavy confining pressure. We felt constrained to observe that, if shear velocities through liquid‐saturated rocks were higher than through their dry counterparts, the applicability of Biot’s theory to any but low‐frequency resonating systems was open to question. Because Biot’s theory will be of maximum use only if it can be applied to systems in which velocities are measured by high‐frequency pulse techniques, our warning diminished the practical value of our other results.

A theory of compressional wave attenuation in noncohesive sediments

Geophysics ◽  
1985 ◽  
Vol 50 (8) ◽  
pp. 1311-1317 ◽  
Author(s):  
C. McCann ◽  
D. M. McCann
Keyword(s):  
Wave Attenuation ◽  
Compressional Wave ◽  
Solid Particles ◽  
Biot’S Theory ◽  
Attenuation Coefficients ◽  
Low Frequencies ◽  
Biot's Theory ◽  
Compressional Waves ◽  
High Frequencies ◽  
Water Saturated

Published reviews indicate that attenuation coefficients of compressional waves in noncohesive, water‐saturated sediments vary linearly with frequency. Biot’s theory, which accounts for attenuation in terms of the viscous interaction between the solid particles and pore fluid, predicts in its presently published form variation proportional to [Formula: see text] at low frequencies and [Formula: see text] at high frequencies. A modification of Biot’s theory which incorporates a distribution of pore sizes is presented and shown to give excellent agreement with new and published attenuation data in the frequency range 10 kHz to 2.25 MHz. In particular, a linear variation of attenuation with frequency is predicted in that range.

scholarly journals From unspecific to adjusted, how the BOLD response in the rat hippocampus develops during consecutive stimulations

2016 ◽  
Vol 37 (2) ◽  
pp. 590-604 ◽  
Author(s):  
Stephanie Riemann ◽  
Cornelia Helbing ◽  
Frank Angenstein
Keyword(s):  
High Frequency ◽  
Pulse Sequences ◽  
Low Frequency ◽  
Bold Response ◽  
Deconvolution Analysis ◽  
Bold Responses ◽  
Electrical Pulses ◽  
Stimulation Period ◽  
Frequency Pulse ◽  
Spiking Activity

To determine the possibility to deconvolve measured BOLD responses to neuronal signals, the rat perforant pathway was electrically stimulated with 10 related stimulation protocols. All stimulation protocols were composed of low-frequency pulse sequences with superimposed high-frequency pulse bursts. Because high-frequency pulse bursts trigger only one synchronized spiking of granular cells, variations of the stimulation protocol were used: (a) to keep the spiking activity similar during the presentation of different numbers of pulses, (b) to apply identical numbers of pulses to induce different amounts of spiking activity, and (c) to concurrently vary the number of applied electrical pulses and resultant spiking activity. When complex pulse sequences enter the hippocampus, an unspecific default-like BOLD response is first generated, which relates neither to the number of incoming pulses nor to the induced spiking activity. Only during subsequent stimulations does the initial unspecific response adjust to a more adequate response, which in turn either strongly related to spiking activity when low-frequency pulses were applied or depended on the incoming activity when high-frequency pulse bursts were presented. Thus, only the development of BOLD responses during repetitive stimulations can predict the underlying neuronal activity and deconvolution analysis should not be performed during an initial stimulation period.

Fatigue of Paralyzed and Control Thenar Muscles Induced by Variable or Constant Frequency Stimulation

2003 ◽  
Vol 89 (4) ◽  
pp. 2055-2064 ◽  
Author(s):  
Christine K. Thomas ◽  
Lisa Griffin ◽  
Sharlene Godfrey ◽  
Edith Ribot-Ciscar ◽  
Jane E. Butler
Keyword(s):  
Spinal Cord ◽  
High Frequency ◽  
Low Frequency ◽  
Constant Frequency ◽  
Time Integral ◽  
Frequency Stimulation ◽  
Force Time ◽  
And Control ◽  
Frequency Pulse ◽  
Force Time Integral

Muscles paralyzed by chronic (>1 yr) spinal cord injury fatigue readily. Our aim was to evaluate whether the fatigability of paralyzed thenar muscles ( n = 10) could be reduced by the repeated delivery of variable versus constant frequency pulse trains. Fatigue was induced in four ways. Intermittent supramaximal median nerve stimulation (300-ms-duration trains) was delivered at 1) constant high frequency (13 pulses at 40 Hz each second for 2 min); 2) variable high frequency (each second for 2 min). The first two intervals of each variable frequency train were 5 and 20 ms. The remaining pulses were evenly distributed in time across 275 ms. The number of pulses varied for each subject such that the force time integral in the unfatigued state matched that evoked by a constant 40-Hz train; 3) constant low frequency (7 pulses at 20 Hz each second for 4 min); and 4) variable low frequency (each second for 4 min). The pulse pattern was the same as that for variable high frequency except that the force-time integral was matched to that produced by the constant low-frequency stimulation. These same experiments were performed on the thenar muscles of five able-bodied control subjects. The variable high-frequency trains used to fatigue paralyzed and control muscles had an average (± SE) of 12 ± 2 and 10 ± 1 pulses, respectively. Variable low-frequency trains had 7 ± 1 and 6 ± 1 pulses, respectively. Significant mean force declines of comparable magnitude (to 20–25% initial fatigue force or to 13–21% initial 50 Hz force) were seen in paralyzed muscles with all four stimulation protocols. The force reductions in paralyzed muscles were always accompanied by significant increases in half-relaxation time and decreases in force-time integral, irrespective of the stimulation protocol. Significant force decreases also occurred in control muscles during each fatigue test. Again, these force declines were similar whether constant or variable pulse patterns were used at high or low frequencies (to 40–60% initial fatigue force or to 29–36% initial 50 Hz force). The force reductions in control muscles were significantly less than those seen in paralyzed muscles, except when constant high-frequency stimulation was used. The variations in stimulation frequency, pulse pattern, and pulse number used in this study therefore had little influence on thenar muscle fatigue in control subjects or in spinal cord–injured subjects with chronic paralysis.

Design and Implementation of a Dimmable LED Driver with Low-Frequency PWM Control

2013 ◽  
Vol 284-287 ◽  
pp. 2538-2542
Author(s):  
Hung Liang Cheng ◽  
Chun An Cheng ◽  
Chao Shun Chen ◽  
Kuan Lung Huang
Keyword(s):  
High Frequency ◽  
High Efficiency ◽  
Low Frequency ◽  
Buck Converter ◽  
Duty Ratio ◽  
Buck Converters ◽  
Resonant Converter ◽  
Switching Loss ◽  
Led Driver ◽  
Frequency Pulse

This paper proposes a high-efficiency dimmable LED driver for light emitting diodes (LED). The developed LED driver consists of a full-bridge resonant converter and six buck converters. The function of the full-bridge resonant converter is to obtain a smooth dc-link voltage for the buck converters by phase-shift modulation (PSM) while that of the six buck converters is to drive six LED modules, respectively. The gate voltage of the active switch of each buck converter is a combination of high-frequency and low-frequency pulses. The duty ratio of the high-frequency pulse controls the LED voltage and thereby, controls the amplitude of LED current. LEDs are dimmed by low-frequency pulse-width modulation (PWM) to vary the average current flowing through LED. Circuit equations are derived and circuit parameters are designed. High circuit efficiency is ensured by operating the active switches at zero-voltage switching-on to reduce the switching loss. Finally, a prototype circuit was built to verify the accuracy and feasibility of the proposed LED driver.

STUDIES OF ELASTIC WAVE ATTENUATION IN POROUS MEDIA

Geophysics ◽  
1962 ◽  
Vol 27 (5) ◽  
pp. 569-589 ◽  
Author(s):  
M. R. J. Wyllie ◽  
G. H. F. Gardner ◽  
A. R. Gregory
Keyword(s):  
Porous Media ◽  
Elastic Wave ◽  
Wave Attenuation ◽  
Fluid Motion ◽  
Fluid Viscosity ◽  
Biot’S Theory ◽  
Biot's Theory ◽  
High Frequencies ◽  
Very High Frequencies

Elastic wave attenuation in porous media is due in part to the relative motion of the liquid and the solid. Biot’s theory expresses this component in terms of permeability, fluid viscosity, frequency, and the elastic constants of the material. Experiments were performed to measure attenuation in the frequency range f <20,000 cps by a resonant bar method; attempts to measure attenuation at very high frequencies gave more equivocal results. Alundum bars were used to test the validity of the theory, for with these the loss not due to fluid motion is relatively small. Experiments were also made with natural specimens of rock. These showed that when not subjected to compacting pressure both the velocities and decrements of specimens were affected chemically and physically by the presence of liquid pore saturants. It is concluded that Biot’s theory seems generally applicable to the determination of the fluid‐solid or “sloshing” losses in resonated porous media. There is still some doubt about the applicability of the theory in the case of measurements made by pulse techniques. The use of attenuation measurements as a logging technique, possibly to estimate permeability, is also discussed.

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