Observed Reflection Coefficients Research Articles

On acoustic reflection from sand-sized water-saturated granular media at MHz frequencies: Measurements, models and the role of speckle.

Acoustic reflection coefficients are reported for water-saturated granular media at frequencies from 1.2 to 2.0 MHz using a narrow-beam broadband transducer in a monostatic geometry at near-normal incidence. Natural sand and glass beads with median grain diameters ranging from 0.22 to 0.40 mm were used. For each granular medium, bed elevation and root-mean-square roughness were measured using side-on photographs of the sediment-water interface. The probability density distributions of the bed elevations are Gaussian. The roughness parameter is close to 1, indicating that the reflected pressure field is mainly due to coherent scattering. The probability distribution of the observed reflection coefficients is nearly Gaussian, consistent with the predictions from a coherent single-scattering model. The horizontal decorrelation length of the observed reflection coefficients is ∼4 mm, with no consistent dependence on either frequency or grain size, and approximately equal to 20% of the transducer diameter. This behaviour, which is reproduced by the single-scattering model, is due to speckle. The size/frequency-dependence of the reflection coefficients are well described by Eckart's [(1953). J. Acoust. Soc. Am. 25(3), 566-570] prediction for a rough surface with Gaussian-distributed surface elevations. Comparisons are made to previously reported reflection coefficient measurements.

Measurement of axial field distribution in a W-band extended interaction resonant cavity based on perturbation technique

The theoretical analysis and the experiment of perturbation techniques to the quantitative measurement of electric field characteristics in a 94 GHz extended interaction resonant cavity (EIRC) are described. Parameters such as the observed reflection coefficient (S11) and resonant frequency at the disturbance point can be adopted to determine the Q-factor, relative electric field intensity, and distribution of the EIRC. In the experiment stage, the frequency deviation characteristics of the tested EIRC under the influence of the metallic sphere are consistent with the simulation results, and the relative error of the frequency deviation is 4.40%. This provides an effective solution for studies of the performance monitoring of millimeter–submillimeter-wave electronic devices and the exploration of high-frequency material science.

Energy Transfers and Reflection of Infragravity Waves at a Dissipative Beach Under Storm Waves

AbstractThis study presents unpublished field observations of infragravity waves, collected at the dissipative beach of Saint‐Trojan (Oléron Island, France) during the storm Kurt (3 February 2017), characterized by incident short waves of significant heights reaching 9.5 m and peak periods reaching 22 s. Data analysis reveals the development of exceptionally large infragravity waves, with significant heights reaching 1.85 m close to shore. Field observations are complemented by numerical modeling with XBeach, which well reproduces the development of such infragravity waves. Model results reveal that infragravity waves were generated mainly through the bound wave mechanism, enhanced by the development of a phase lag with the shortwave energy envelope. Spectral analysis of the free surface elevation shows the generation of superharmonic and subharmonic infragravity waves, the latter dominating the free surface elevation variance close to shore. Modeling results suggest that subharmonic infragravity waves result, at least partly, from infragravity‐wave merging, promoted by the combination of free and bound infragravity waves propagating across a several kilometer‐wide surf zone. Due to the steeper slope of the upper part of the beach profile, observed and modeled reflection coefficients under moderate‐energy show a strong tidal modulation, with a weak reflection at low tide (R2<0.2) and a full reflection at high tide (R2∼1.0). Under storm waves, the observed reflection coefficients remain unusually high for a dissipative beach (R2∼0.5−1.0), which is explained by the development of subharmonic infragravity waves with frequencies around 0.005 Hz, too long to suffer a substantial dissipation.

Wavelet analysis ultra-thin metasurface for hypersonic flow control

Metasurfaces are one of the new frontier of material science and engineering, finding widespread applications in a number of fields, including acoustics and aeroacoustics. In the present paper we propose a novel metasurface specifically fabricated for the hypersonic flow transition control, together with a new strategy for metasurface characterization in the ultrasonic regime. Instead of a conventional porous layer, the metasurface here presented consists in a flat plate with a set of regularly distributed sharp slots. We experimentally observed that such a geometry significantly reduces the wall reflection coefficient, which is known to play a fundamental role in the boundary layer transition phenomenon. Numerical simulations led us to interpret the incident wave scattering as the underlying mechanism related to the observed reflection coefficient reduction. The metasurface characterization has been carried out by comparing the conventional reflection coefficient in the Fourier domain with an innovative wavelet transform-based strategy. As an overall result, the surface geometry here proposed has been shown to offer a twofold advantage: i) it is more effective in ultrasonic incident wave control with respect to conventional porous layers, ii) it needs less manufacturing time, a primary requirement of this type of technology. More interesting, we have put in evidence how a multiresolution approach, like that here proposed, can be highly promising as characterization tool for further metasurfaces with a more complex and multi-scale geometry, being wavelets able to capture the multiscale behaviour of the reflected wave, overcoming the well-known limits of Fourier-based strategies of data analysis.

Reflection from bound microbubbles at high ultrasound frequencies

Targeted contrast agents and ultrasound imaging are now used in combination for the assessment and tracking of biomarkers in animal models in vivo. These applications have triggered interest in the understanding and prediction of the ultrasound echoes from contrast agents attached to cells. This study describes the reflection enhancement due to microbubbles bound on a gelatin surface. The reflection enhancement was measured using ultrasound pulses at high frequency (40 MHz) and low pressure (38 kPa peak-negativepressure) allowing a linear approximation to be applied. The observed reflection coefficient increased with the number of microbubbles, until reaching saturation at 0.9 when the surface coverage fraction was 35%. A multiple scattering model assuming that the targeted microbubbles are confined within an infinitesimally thin layer appeared suitable in predicting the reflection coefficient even at very high surface densities. These results could permit the optimization of the sensitivity of highfrequency ultrasound to targeted contrast agents.

In situ reflection coefficient measurements from 5–50 kHz from the experiments for validation of acoustic modeling techniques (EVA) sea test

An accurate model of high-frequency reflection loss at the ocean bottom is critical to understanding shallow water sound propagation and the detection and classification of buried objects such as mines. The value of the observed reflection coefficient can be perturbed by such effects as scattering from a rough interface and dispersion in the ocean sediment. To obtain ground truth measurements for model development and verification, reflection coefficients from a rough water/sediment interface were measured as part of the experiments for validation of acoustic modeling techniques (EVA) sea test on the north shore of Isola dElba, Italy. Data at grazing angles from 10–80 deg and frequencies from 5–50 kHz were obtained. Experimental effects such as transducer response and spherical wave effects were quantified. The data will be analyzed to determine the influence of dispersion and scattering effects. [Work supported by ONR, Ocean Acoustics.]

Seismic detection of transient changes beneath Black Rapids Glacier, Alaska, U.S.A.: II. Basal morphology and processes

AbstractUsing changes observed in daily seismic reflections, we have investigated the basal morphology of Black Rapids Glacier, Alaska, U.S.A. The englacial drainage of ice-marginal lakes caused significant changes in the daily reflections, as well as dramatic increases in basal motion. Changes in reflection arrival times and amplitudes indicate that there is a basal till layer at least 5 m thick at some locations beneath this surge-type glacier. Rapid changes in the observed reflection coefficients during the drainage events indicate that changes in till properties must occur throughout the entire 5 m thick layer, they must last for several days following the lake drainages and they must be completely reversible over as little as 36 min. Our seismic analysis shows that changes in effective pressure of the till are unlikely to cause the required changes in the reflection coefficients, but that a decrease in till saturation is likely. We therefore interpret the cause of the seismic anomalies as being a temporary decrease in saturation as water is input to the subglacial hydraulic system, and propose that such a change may occur quickly and reversibly by a redistribution of overburden pressure. Higher water pressures within the hydraulic system cause that region to support more of the glacier’s weight, leaving the remaining areas to support less. Any till within these areas of decreased normal stress would experience a consequent decrease in pore-water pressure, causing gas to exolve, thus decreasing saturation. This decrease in saturation would cause a change in the strength of the basal layer and may affect basal dynamics.

Seismic detection of transient changes beneath Black Rapids Glacier, Alaska, U.S.A.: II. Basal morphology and processes

AbstractUsing changes observed in daily seismic reflections, we have investigated the basal morphology of Black Rapids Glacier, Alaska, U.S.A. The englacial drainage of ice-marginal lakes caused significant changes in the daily reflections, as well as dramatic increases in basal motion. Changes in reflection arrival times and amplitudes indicate that there is a basal till layer at least 5 m thick at some locations beneath this surge-type glacier. Rapid changes in the observed reflection coefficients during the drainage events indicate that changes in till properties must occur throughout the entire 5 m thick layer, they must last for several days following the lake drainages and they must be completely reversible over as little as 36 min. Our seismic analysis shows that changes in effective pressure of the till are unlikely to cause the required changes in the reflection coefficients, but that a decrease in till saturation is likely. We therefore interpret the cause of the seismic anomalies as being a temporary decrease in saturation as water is input to the subglacial hydraulic system, and propose that such a change may occur quickly and reversibly by a redistribution of overburden pressure. Higher water pressures within the hydraulic system cause that region to support more of the glacier’s weight, leaving the remaining areas to support less. Any till within these areas of decreased normal stress would experience a consequent decrease in pore-water pressure, causing gas to exolve, thus decreasing saturation. This decrease in saturation would cause a change in the strength of the basal layer and may affect basal dynamics.

Some attributes of wavefields scattered from Ivrea-type lower crust

The Ivrea Zone in northern Italy is a well studied sliver of extended lower continental crust which was brought to the surface in the course of Alpine lithospheric shortening. Based on two 1:25,000 geological maps from the central Ivrea Zone and petrophysical parameters of the corresponding lithologies we have constructed geologically constrained, stochastic seismic models of Ivrea-type lower crust. A stochastic rather than a deterministic approach is applied due to the limited availability of. detailed geological maps and the high degree of small-scale structural and petrophysical complexity of the Ivrea Zone. The primary characteristic of the resulting model is its “layered” self-affine or fractal structure and its bimodal velocity distribution. While we are not suggesting that Ivrea-type structure is universal for extended lower continental crust, synthetic seismograms may help to constrain some pertinent quantitative aspects observed in deep seismic reflection data: 1. (1) Ivrea-type lower crust can explain the igh observed reflection coefficients and bright reflectivity in the lower crust; 2. (2) observed lower-crustal Q-factors can be largely explained by scattering losses and intrinsic Q-factors of reflective lower crust are likely to be larger than 1000, which is incompatible with the presence of free volatiles; 3. (3) in agreement with recent broad band studies of the lower crust, Ivrea-type lower crust does not create any tuning effects; and 4. (4) for Ivrea-type heterogeneity the lateral correlation of the reflected signal is largely independent of the lateral characteristic scale of the scattering structures, which suggests that multiple scattering may be important in the reflective lower crust.

Reflective probing of laser generated multi-kbar compressional shocks in water

The structure of the shock front generated by propagating a multikilobar laser driven compressional ramp wave into water was probed using laser reflectivity. Good agreement between the observed reflection coefficient of the front and the Fresnel formula implies that the spatial extent of the leading edge of the shock was less than 0.06 000000mm, corresponding to a risetime of less than 20 ps. A graphical technique is also presented that describes both the shock formation and decay process for laser driven condensed phase compressional waves. Using this procedure, the shock was found to fully form after propagating approximately 100 μ m in water consistent with experimental observation.

Ice sheet internal radio‐echo reflections and associated physical property changes with depth

The use of radio‐echo sounding records to indicate the presence of internal layers within large ice sheets is of interest to glaciologists because it offers a possible means of tracking the internal properties of the ice sheets over large distances. In this paper we use the measured physical properties of core to bedrock taken at Cape Folger, East Antarctica (66°22′S, 111°E, 324‐m depth), to compute a depth‐reflection coefficient profile for comparison with the observed radio‐echo reflections. The measurements available on physical properties are density variations, bubble size and shape changes, and crystal fabric variations. In calculations to differentiate the effects of the physical properties, it appears that density variations account for the primary contributions to the calculated dielectric property changes corresponding to the highest observed reflection coefficients. However, bubble changes alone can also account for reasonable, though lower, reflection coefficients at the depths corresponding to observed reflections. Crystal fabric variations correspond poorly with the reflection locations. The close correspondence between the depths of the bubble shape changes (which are definitely deformational features) and the depths of the density variations, and between both of these and the radio‐echo layers, indicates that deformational events in the ice sheet's history are represented by the variations in physical properties and associated radio‐echo records.

Osmotic reflection coefficients of canine subcutaneous adipose tissue endothelium

Volume changes were produced in isolated perfused canine subcutaneous adipose tissue when hydrophilic solutes were added to the perfusate. The initial volume slopes per unit osmolality change yielded the following product: (observed reflection coefficient × filtration coefficient) for the endothelial barrier. The filtration coefficient (CFC), determined from the sustained constant volume change per increment of venous pressure, averaged 0.027 ml × min −1 × 100 g −1. The observed reflection coefficients were (mean ± SE): sodium chloride, 0.0227 ± 0.002; sucrose, 0.0333 ± 0.003; raffinose, 0.0340 ± 0.002; and Ficoll 70, 0.9110 ± 0.160. The relatively high capacity for filtration of adipose tissue capillaries (compared with those in skeletal muscle) was confirmed. The observed reflection coefficients for exchange vessels in fat were about one tenth of the values reported for lung, cardiac, and skeletal muscle endothelium.

Reflection coefficients of dog lung endothelium to small hydrophilic solutes.

Step additions of small hydrophilic solutes to the perfusate of isolated dog lungs produce weight changes (Vargas-Johnson). The product: (observed reflection coefficient) times (observed filtration coefficient) for the lung endothelial barrier is obtained from the initial weight slope per unit osmolality change. The observed filtration coefficient determined from the sustained constant weight slope per increment of pulmonary outflow pressure was 0.038 plus or minus 0.011 cm/s (assumed exchange area 500 cm2/g lung). Correction to average transcapillary pressure (32% upward) and for osmotic buffering by resident solute (25% upward) gave 0.063 plus or minus 0.018 cm/s. The observed reflection coefficients were (mean plus or minus 2 SE): sodium chloride, 0.30 plus or minus 0.11; urea, 0.30 plus or minus 0.12; glucose, 0.48 plus or minus 0.26; sucrose, 0.39 plus or minus 0.17; raffinose, 0.35 plus or minus 0.16; hypotonic response: interstitial solute, 0.26 plus or minus 0.10. The observed reflection coefficient of sodium chloride, corrected for the preceding filtration coefficient effects (65% downward) and for combined flow limitation and osmotic buffering effects (50% upward) was 0.27. Transcapillary gradients of small hydrophilic solutes produce substantial osmotic effects for short times (seconds). Passive exchanges and net fluxes across the endothelium of the lung capillaries have parameters similar to those of the endothelium of skeletal muscle.

BONDING OF LAYERS IN TWO‐DIMENSIONAL SEISMIC MODELING

In constructing two‐dimensional layered seismic models, the most commonly used technique is to butt‐join the layers with some bonding agent. Whether or not this type of bonding satisfies the “welded‐contact” hypothesis which is assumed in theoretical treatments is investigated experimentally. Two pieces of a steel plate are bonded together with several agents (water, castor oil, Vaseline, epoxy resin, Devcon plastic steel, and two different solders), and the reflections of the normally incident compressional and Rayleigh waves from the interface are measured. From these, it is found that the bonds do not satisfy the “welded‐contact” condition. Furthermore, comparison with the theoretically computed values demonstrates that the observed reflection coefficients are, in general, too large to be solely due to the contrasts between the elastic coefficients of the plate and the bonding materials. Imperfect adherence of the bonding agent to the edges is offered as an explanation. Theoretical curves are presented to demonstrate the significance even the least possible effect of the bonding layer on the phase velocities of dispersive Rayleigh waves.