Variation Of Propagation Velocity Research Articles

Migration based SAR imaging for ground penetrating radar systems

The authors consider migration based synthetic aperture radar (SAR) imaging of surface or shallowly buried objects using both down-looking and forward-looking ground penetrating radar (GPR). The well known migration approaches devised to image the interior of the Earth are based on wave equations and have been widely and successfully used in seismic signal processing for oil exploration for decades. They have the potential to image underground objects buried in complicated propagation media. Compared to ray-tracing based SAR imaging methods, migration based SAR imaging approaches are more suited to imaging underground objects owing to their simple and direct treatment of oblique incidence at the air-ground interface and propagation velocity variation in the soil. The authors apply the phase-shift migration approach to both constant-offset and common-shot experimental data collected by PSI GPR systems. They address the spatial aliasing problems related to the application of migration to the GPR data and the spatial zero-padding approach to circumvent the problem successfully.

Investigation of propagation modes and temperature/velocity variation on unstable combustion synthesis

Combustion synthesis/micropyretic synthesis is a technique in which material synthesis is accomplished by the propagation of a combustion front across the sample. In some cases, the combustion front may propagate in an unstable mode where the propagation velocity and combustion temperature of the combustion front are altered periodically. In this study, the processing conditions leading to unstable combustion reaction were first studied theoretically. The boundary temperatures separating stable and unstable reactions were then determined. The numerical analysis showed that the combustion temperature and the propagation velocity changed periodically during unstable combustion. As the combustion reaction became unstable, the average propagation velocity and the oscillatory frequency of front propagation decreased. The products of unstable combustion synthesis possessed the banded structures, implying the occurrence of the unstable oscillatory propagation, as demonstrated experimentally. In this study, high activation energy combustion (Ti + 2B reaction) and low activation energy combustion (Ni + Al reaction) were both chosen to illustrate the effect of unstable combustion. It is the first time the experimental and numerical results were combined to investigate the temperature and propagation velocity variations during unstable combustion.

A method to obtain reference images for evaluation of ultrasonic tissue characterization techniques

A general problem when evaluating ultrasonic methods for tissue characterization is that “a golden standard” is seldom known. This paper describes a manual method to obtain a reference image, with the same geometry as the ultrasound image, indicating spatial location of the different tissue types present in the biological tissue scanned in vitro. A 30×10×2 mm 3 piece of formalin fixed porcine tissue was molded into an agar block, which on the top surface, contained a set of fiducial markers, spaced 2.5 mm. The block was submerged into 20 °C water and a set of parallel 7.5 MHz spatial compound ultrasound images of tissue and fiducial markers were recorded each 0.5 mm. Guided by the fiducial markers, the agar block was subsequently cut into slices 2.5 mm thick, photographed and finally analyzed histologically identifying these tissues: collagen rich, collagen poor, micro vessels and muscle fibres. Due to: (1) the cutting procedure, (2) the finite size of the ultrasound beam and (3) the spatial variation in propagation velocity, the macroscopic photographs did not align completely with the ultrasound images. Likewise, the histological image is a geometrically distorted version of the macroscopic photograph, due to the histological preparation process. The histological information was “mapped back” into the format of the ultrasound images the following way: On the macroscopic images, outlines were drawn manually which defined the border of the tissue. These outlines were superimposed on the corresponding ultrasound images (identified via the fiducial markers) and modified to encompass what appeared to be tissue regions on the ultrasound images and subsequently re-applied to the macroscopic image. This modified macroscopic outline was used as guideline when drawing outlines identifying regions of the various tissue types. Specifically, the macroscopic image revealed the borders between the different tissues, while the histological image identified the four tissue types. A set of 12 reference images based on modified macroscopic outlines was created. The overlap between the ultrasound images and the macroscopic images—which are the geometrical basis for the final reference images—was between 77% and 93%. A set of 12 reference images spaced 2.5 mm, identifying spatial location of four different tissue types in porcine muscle has been created. With the reference images, it is possible to quantitatively compare different ultrasound based tissue classification techniques.

Acoustical characteristics of leak signals in plastic water distribution pipes

Acoustical characteristics of leak signals in plastic pipes were investigated in this study for several types of leaks simulated under controlled conditions at an experimental site. The investigation included the characterization of frequency content of sound or vibration signals as a function of leak type, flow rate, pipe pressure and season, the determination of the attenuation rate, and the variation of propagation velocity with frequency. The information presented in this paper for the acoustic characteristics of leak signals will help leak detection professionals in the selection of appropriate instrumentation, the design of appropriate measurement procedures, and in the case of propagation velocity, in accurately locating leaks using the cross-correlation method.

Migration method for subsurface radar considering variation of propagation velocity in depth direction

AbstractThere are two methods for improving the resolution in the scan axis direction of the subsurface radar, i.e., the F‐K migration method and the phase‐shift method. The resolution can be improved significantly by the F‐K migration method if the propagation velocity of the radio wave in the ground is uniform and by the phase‐shift method if the propagation speed varies in depth and is known. In general, the propagation speed of the radio wave often changes in depth and its distribution is unknown.In this paper, a method is proposed in which the radar image is divided in the depth direction and the propagation time difference up to the divided block is compensated for so that the F‐K migration method is applied to each block. The propagation speed of each block is estimated as the case in which the maximum reflection value of the reconstructed image is the largest after the F‐K migration process is carried out for several candidate values of the propagation speed. By means of this method, the distribution of the propagation speed can be estimated and a reconstructed image with high resolution can be obtained. This reconstructed image is of sufficiently good quality in comparison to the one by the phase‐shift method using the empirical propagation speed distribution. The processing time excluding the propagation speed estimation is reduced to one‐seventh that needed for the phase‐shift method.

Effect of recording electrode position along a muscle fibre on surface potential power spectrum

Extracellular action potentials produced by a muscle fibre of finite length were calculated for recordings at the skin surface. The sensitivity of power spectra to variations in propagation velocity (ν) and intracellular action potential (IAP) duration ( T in) was studied theoretically. The magnitude and distribution of the spectral power of muscle fibre potentials depend on the electrode longitudinal position. The relative shifts of the spectra in dB induced by variation in ν or T in hardly depend on the longitudinal position of the electrode. A variation in ν affects only the power spectrum positive slope and the initial part of the high-frequency roll-off and a variation in T in affects only the remaining part of the high-frequency roll-off. The total spectral amplitude is practically non-sensitive to variations in the wavelength, b = ν. T in. The total power is sensitive to variations in ν, T in as well as in b, and its relative changes depend on the electrode longitudinal position. The whole power spectrum is shifted along the frequency axis and mode ( F max), median ( F med) and mean ( F mean) frequencies have practically equal percentage changes only when ν and T in vary jointly in such a way that the product ν. T in keeps unchanged.

Saw sensors

This paper considers recent developments in surface acoustic wave sensors (SAWS) and illustrates the operating principle of both physical and chemical SAWS based on surface acoustic wave (SAW) propagation velocity variations. The SAWS that have appeared in the literature so far are qualitatively reviewed and compared with other existing sensors based on different working principles. It is concluded that for producing low-cost integrated SAWS for future applications, combined piezoelectric-silicon technology must be further developed.

Circadian variation in the propagation velocity of the migrating motor complex

Using twin intraluminal pressure-sensitive radiotelemetric capsules for prolonged monitoring of proximal small bowel motility in healthy volunteers, we observed a significant (p < 0.001) variation between daytime (6.44 ± 0.74 cm/min, mean ± SD) and nocturnal (2.90 ± 0.35 cm/min) propagation velocities of the migrating motor complex. An identical variation in propagation velocity was observed in patients suffering from the irritable bowel syndrome. These data suggested a circadian variation, but did not exclude alternative hypotheses. Data were therefore compiled from healthy sheep and pigs supplied with ad libitum rations each morning and adapted to a 12 h light, 12 h dark schedule. Recording from implanted electrodes showed a significant (p < 0.01) reduction in migrating motor complex velocity from 27.8 ± 5.1 cm/min by day to 16.7 ± 3.1 cm/min at night in sheep, and from 20.6 ± 2.4 to 13.4 ± 1.9 cm/min in pigs. The nocturnal reduction remained when the animals were fed in the evening. These observations suggest a circadian variation in mammalian migrating motor complex propagation velocity that is not merely a function of variation in intraluminal content.

A model for studying velocity variations in unmyelinated axons.

A simple electrical model of the nerve axon is presented. The only non-linear element in the model is the sodium conductance, which is assumed to vary in a step-like way. This allows the propagation velocity for action potentials to be expressed analytically. The aftereffects from an action potential are generally slow. By considering them quasi-stationary, the velocity of a subsequent action potential can be calculated and compared with that of the first one. Considering the effects of afterpotential and afterconductance, it is shown that the theory adequately describes the variations in propagation velocity in the squid giant axon for action potentials separated by 3 ms to 14 ms.

Surface acoustic waves in strained media

Variations of elastic constants and density with strain and the introduction of stresses can significantly affect the propagation velocity of surface acoustic waves. Digital computation using iterative methods is employed to obtain the variations of propagation velocity with strain. It is found that while the individual contributions of the initial stresses, changes in elastic constants, and changes in density with strain can be considerable, their combined effect in some materials may be reduced by mutual cancellation. Computed results for YX quartz YZ LiNbO3 are compared with experimentally measured values.

Synchronizing and multiplexing in a digital communications network

A concept of a totally digital communications network is described in which all signals are converted into digital form and remain so as they are multiplexed, switched, and transmitted. To permit time-division techniques for such network processing, and thus realize significant economic advantages, the digital signals must be either generated in synchronism or brought into synchronism. While the switching portion of this network is not yet in service, some of the transmission network already exists and the evolution toward a digital transmission hierarchy is well underway. The hierarchy consists of 1) terminals which convert analog Signals into digital form suitable for transmission, 2) transmission facilities which are available at various capacities, and 3) multiplexers which can derive several lower capacity digital facilities from a single high-capacity system. Such a network, when complete, can also disseminate time and frequency for other uses with the same accuracy as required by the network. The network requires that the relative phase difference between any two signals must be bounded, which means exact matching of the long-term average frequency throughout the network. Signal-processing time and variations in propagation velocity in various media-cable, radio, waveguide-control the short-term accuracy. This network can thus provide time-and-frequency information proportional to the observation time. Available techniques that can achieve such accuracy for the network are discussed.

Ultrasonic Doppler for Distance Measurement

An acoustic Doppler system can be used to provide true velocity information for distance traveled measurement. An ultrasonic system can be small, light in weight, and accurate. The Doppler equation is developed for a moving source in a moving medium. The results are applied to a Janus system and the error terms are separated. For an acoustic system propagation velocity variations must be considered, in addition to wind. A temperature compensated Janus arrangement will reduce errors in the acoustic system. The output of the velocity sensor is equivalent to noise passed through a narrow bandpass filter. Photographs of spectra obtained over various road surfaces are presented. The center frequency of the noise spectrum is proportional to the velocity of the sound source and provides input to a frequency tracker. Narrow band widths obtained with the acoustic system result in the synthesis of a sinusoidal current at the receiver output terminals such that a clear Doppler tone is audible if a loudspeaker is connected. Requirements on the frequency tracker are reduced in severity due to the narrow band width. A light weight sonic Doppler system is described which is comparable in size and weight to a modern autoradio. An experimental Janus system is discussed. Spectrum photographs for various road surfaces are presented.

Propagation velocity in electrically activated muscle fibres in man.

Summary.The arrival time of muscle action potentials evoked by direct local stimulation was determined in 3 to 5 points of the brachial biceps.The stimulus artifact was reduced by stimulating with a bipolar electrode via a doubly screened transformer with the screen around the secondary coil connected to the muscle.It was ascertained that potentials from the same few fibres were picked up at the different electrodes as indicated by an identical threshold for the steep potentials of short duration.The propagation velocity was constant over 10–11 cm muscle length investigated and, therefore, the muscle fibres must pass uninterrupted over this distance.In ten subjects the propagation velocity varied between 3.3 and 5.2 m/sec. and averaged 4.02 ± 0.13 m/sec. (36.5°C.). In different regions of the same muscle it varied up to 15 per cent. There was no significant difference in the velocity over muscles from subjects of different age (3–74) years). The small variation in propagation velocity in spite of large differences in fibre circumference (2.5–3) times, 19 biopsies) suggest the presence of a mechanism which synchronizes the propagation of the impulses from a small group of simultaneously activated fibres.Differences in propagation velocity of the order of magnitude found with electrical stimulation can only account for a minor part of the variation in muscle action potential duration obtained with random leads at voluntary effort.

Schlieren Photography of Sound Waves

This paper summarizes the historical background of the schlieren detection technique and gives a qualitative and quantitative description of its operation. The optical and electronic features of a working schlieren apparatus with high speed illumination are described. The equipment provides visual or photographic observation of an acoustic wave, conveniently generated by a spark gap, at a precisely adjustable time after its formation. Typical measurements of photographic density across wave fronts are shown and discussed. Applications of the schlieren technique in certain acoustical problems are mentioned, including the reflection, diffraction and refraction of sound and the variation of propagation velocity and wave form in sound waves of finite amplitude.