Frequency-dependent Reflection Research Articles

A method to determine the acoustic reflection and absorption coefficients of porous media by using modal dispersion in a waveguide.

The measurement of acoustic material characteristics using a standard impedance tube method is generally limited to the plane wave regime below the tube cut-on frequency. This implies that the size of the tube and, consequently, the size of the material specimen must remain smaller than a half of the wavelength. This paper presents a method that enables the extension of the frequency range beyond the plane wave regime by at least a factor of 3, so that the size of the material specimen can be much larger than the wavelength. The proposed method is based on measuring of the sound pressure at different axial locations and applying the spatial Fourier transform. A normal mode decomposition approach is used together with an optimization algorithm to minimize the discrepancy between the measured and predicted sound pressure spectra. This allows the frequency and angle dependent reflection and absorption coefficients of the material specimen to be calculated in an extended frequency range. The method has been tested successfully on samples of melamine foam and wood fiber. The measured data are in close agreement with the predictions by the equivalent fluid model for the acoustical properties of porous media.

Intrinsic Modeling of Near-Field Electromagnetic Induction Antennas for Layered Medium Characterization

We present a closed-form equation for intrinsic modeling of near-field electromagnetic induction (EMI) antennas for planar layered media characterization. Resorting to a decomposition of the backscattered EM field into elementary distributions over the antenna aperture, the EMI transmitting and receiving antennas are modeled using infinitesimal magnetic dipoles and field points, and characteristic frequency-dependent global reflection and transmission coefficients. Low-frequency propagation of the EM fields in the medium is described using 3-D planar layered media Green's functions. We performed measurements with a loop antenna situated at different heights, ranging from near-field to far-field conditions, above water of known electrical conductivity to determine its intrinsic properties, and a range of salinity conditions was applied to subsequently validate the proposed model. The EMI system was set up using a vector network analyzer equipped with a prototype EMI antenna specifically designed for this application. The model showed good accuracy for reproducing the observed data, and model inversion provided good estimates of the medium electrical conductivity. Yet, insensitivity of the EMI signal to water electrical conductivity was encountered for low salinity due to the presence of a copper sheet as the bottom boundary condition of the experimental setup. Moreover, the efficiency of the antenna decreased rapidly as antenna height above water surface increases, leading to increasing discrepancies between estimated and measured water electrical conductivity values as the antenna moves away from the water surface. Although some technical improvements are still needed, the proposed approach is promising for quantitative estimation of soil electrical conductivity from EMI data.

Phase transition, electronic and optical properties of III-Sb compounds under pressure

III-V semiconductors are the backbone of optoelectronic industry. Here, we have performed first principle calculations to investigate the structural, electronic and optical properties of III-Sb (III = B, Al, Ga, Sb) compounds under the effect of pressure. The structural phase transition from zincblende to rocksalt phases is determined by the common tangent of the two E–V curves. The obtained results are in good agreement with the available literature. Compounds make electronic transition from semiconductors to metals under pressure. The calculated band structure in zincblende structure was compared with experimental and theoretical findings. Optical properties including real and imaginary parts of the complex dielectric function, frequency-dependent reflectivity and optical conductivity are explained to characterize the optical nature of these compounds in both phases.

Frequency-dependent reflection coefficients in diffusive-viscous media

Amplitude variation with offset/angle of incidence inversion has been playing a key role in hydrocarbon detection and reservoir characterization. Traditionally, it has been limited to elastic even viscoelastic cases. We investigated the reflectivity in diffusive-viscous media. Our study demonstrated that the magnitude of the reflection coefficients in such a media is not only related to the incident angle and the parameters of the medium but also strongly depends on the frequency. We first demonstrated the validity of diffusive-viscous wave equation, proposed empirically, based on the continuum equations and basic laws of physics. Then, we investigated the frequency-dependent phase velocity and the quality factor [Formula: see text], whose impacts typically increase toward high frequencies, and we studied the magnitude and phase of the reflection coefficient at an interface between two diffusive-viscous media. A general equation of the reflection coefficients was also obtained for a stack of arbitrary number of plane layers in such media. Finally, we computed synthetic vertical seismic profile seismograms using the reflectivity method for a model consisting of five fluid-saturated layers to demonstrate that the diffusive and viscous terms in diffusive-viscous theory have a big impact on the synthetic seismograms, and stronger attenuation is observed in the layer having larger attenuation parameters.

Causality and scattering from cylindrical shells

Acoustic scattering from a cylindrical shell is required to be causal, so that the incident wave must precede the scattered wave that it creates. In the frequency domain, this statement may be explored by forming a frequency-dependent complex-valued reflection coefficient that relates the scattered wave to the incident wave. The real and imaginary parts of the reflection coefficient must therefore satisfy Hilbert Transform relations that involve integrals over frequency. As a result, one may find the real part of the reflection coefficient given only its imaginary part over a frequency range, and vice-versa. The reflection coefficient is not required to be minimum phase and rarely is minimum phase, so the causality condition cannot be used directly to estimate the phase of the reflection coefficient from its magnitude. However, the effective impedance associated with the reflection coefficient is required to be minimum phase. An approach is presented for using these relations to estimate the phase of a reflection coefficient given only its magnitude. Examples are presented that illustrate these relationships for cylindrical shells.

Phase transition, electronic and optical properties of NaCl under pressure

In this paper, we have carried out a theoretical investigation on the structural and optoelectronic properties of NaCl under pressure effect via first principle calculations within the density functional theory. The structural phase transition from NaCl(B1) to CsCl(B2) -type structures is determined. The compound has a very wide bandgap in both phases. Optical properties including the absorption coefficient, optical conductivity and frequency dependent reflectivity are explained to characterize the optical nature of NaCl up to pressure of 134 GPa.

Phase transition, electronic and optical properties of mercury chalcogenides under pressure

We have carried out a theoretical investigation of the structural, electronic and optical properties of mercury chalcogenides under pressure via first-principle calculations within the density functional theory. The structural phase transition from zincblende (ZB) to rock salt phases is determined. The calculated band structure in ZB phase was compared with experimental and theoretical findings. Optical properties including the real and imaginary parts of the complex dielectric function and the frequency-dependent reflectivity are explained to characterize the optical nature of mercury chalcogenides in both phases.

Radiative Topological States in Resonant Photonic Crystals

We present a theory of topological edge states in one-dimensional resonant photonic crystals with a compound unit cell. Contrary to the conventional electronic topological states, the modes under consideration are radiative; i.e., they decay in time due to the light escape through the structure boundaries. We demonstrate that the edge states survive despite their radiative decay and can be detected both in time- and frequency-dependent light reflection.

An AVO inversion method in the frequency domain based on a layered model—a Bakken Shale case study

Conventional AVO techniques are based on the linear approximation of Zoeppritz equations for the inversion of elastic properties across a subsurface interface. However, for a layered model consisting of internal multiple layers beyond seismic resolution, it can be difficult to use the traditional AVO inversion methods due to the practicalities of picking amplitudes, resolution problems, and thin-layer effects. We propose an improved AVO inversion method applied to a layered model for inversion of layer thickness and properties by incorporating the frequency content of the wavelet. The proposed method uses the propagator matrix method as a theoretical description of the frequency-dependent reflection coefficients for the layered model in an inversion scheme. We test the proposed method on a single layered model with synthetic data to produce an inversion for layer thickness and porosity, and then investigate the feasibility of the method for the characterization of the fracture zone in the Bakken formation. For the inversion of the reference crack density and layer thickness of the fracture zone in the Bakken formation, the minima of the objective functions generate inversion results that indicate a reasonable fit with the true model parameters. The inversion error may result from the intrinsic complexity of reflections from a layered model, in which several different combinations of layer thicknesses and associated properties may produce similar frequency-dependent coefficients. By contrast, in the multicomponent model data example, the inversion of converted PS-wave data seems to be less stable compared to PP-wave data. The potential of the proposed AVO inversion method may include applications to complex models, such as a sandstone/shale interbedded system, or a formation that presents internal heterogeneity.

Electronic and optical properties of the SiB2O4 (B=Mg, Zn, and Cd) spinel oxides: An ab initio study with the Tran–Blaha-modified Becke–Johnson density functional

We report ab initio density functional theory calculations of the structural, electronic and optical properties of the spinel oxides SiMg2O4, SiZng2O4, and SiCd2O4 using the full-potential linearized augmented plane-wave method. The structural parameters calculated using both the local density and generalized gradient approximations to the exchange-correlation potential are consistent with the literature data. To calculate the electronic properties, the exchange-correlation potential is treated with various functionals, and we find that the newly developed Tran–Blaha-modified Becke–Johnson functional significantly improves the band gap. We predict a direct band gap in all of the considered SiB2O4 compounds, and the band gaps continuously decrease as the atomic size of the B element increases. The decrease in the fundamental direct band gap (Γ–Γ) from SiMg2O4 to SiZn2O4 to SiCd2O4 can be attributed to p–d mixing in the upper valence bands of SiZn2O4 and SiCd2O4. The lowest conduction band is well dispersive, similar to that found for transparent conducting oxides such as ZnO. This band is mainly defined by the s and p electrons of the Si and B (B=Mg, Zn, Cd) atoms. The topmost valence band is considerably less dispersive and is defined by O-2p and B–d electrons. The charge-carrier effective masses are evaluated at the topmost valence band and at the bottommost conduction band that were calculated. The frequency-dependent complex dielectric function, absorption coefficient, refractive index, extinction coefficient, reflectivity and electron energy loss function were estimated. We find that the value of the zero-frequency limit of the dielectric function ε(0) increases as the band gap decreases. The origins of the peaks and structures in the optical spectra are determined in terms of the calculated energy band structures.

Modeling and analysis of seismic wave dispersion based on the rock physics model

Seismic wave attenuation and dispersion are largely related to the fluid content in reservoirs, which has been verified by rock physics models, numerical modeling and some field case studies. The main mechanism of seismic wave dispersion is wave-induced fluid flow due to the heterogeneity at different scales, including ‘macroscopic’, ‘microscopic’ and ‘mesoscopic’. Recently, studies on dispersion have focused on mesoscopic rock. Previous studies have shown that there are many factors influencing seismic wave dispersion and attenuation, and amongst all the parameters, gas saturation is a very important one. Based on the White model, in this paper we firstly model the seismic wave dispersion and attenuation characteristics with varying gas saturation and porosity, and then analyze the velocity and normal-incidence (NI) reflection coefficient using three sandstone models. The results show that the frequency-dependent reflection coefficient (FDRC) is influenced by gas saturation, which corresponds to the frequency-dependent velocity characteristic. Finally, we derive approximate FDRC expressions using the NI reflection coefficient method, which can be used to estimate the dispersion characteristic. Analysis of the cross-plots of NI and dispersion indicates that dispersion and attenuation are more sensitive to gas saturation and porosity. Therefore, dispersion and NI cross-plots provide a potential way to predict the gas saturation. The modeling results of different consolidated sandstone reservoirs also indicate that there is a critical gas saturation point at which the dispersion magnitude reaches maximum value. Based on the critical point, the cross-plot can be divided into two parts and we can predict the gas saturation in either of them.

Modeling the acoustic scattering from large fish schools using the Bloch-Floquet theorem

Scattering from large fish schools can significantly contribute to volume reverberation in the open ocean measured by mid-frequency long-range SONAR systems (1–10 kHz). This can potentially cause large false-alarms, especially if the resonance frequencies of the fish’ air-filled swim bladder is excited. Hence, to ultimately improve the detection performance of long-range SONAR systems, we seek an efficient modeling technique for the acoustic scattering created by large school of fish, which readily accounts for the fish bio-acoustic properties, school’s spatial configuration and multiple scattering effects. We exploit here a key observation to simplify our problem: fish in larger schools tend to swim in a periodic arrangement whereby we approximate the large school as an infinite system with a periodic collection of fish’ air filled swimbladders. Thus, the Bloch-Floquet theorem, governing waves in periodic media, allows predictions of the acoustic field in an infinite media by simply modeling the dynamic response of a single unit cell only (containing one fish). This approach allows one to rapidly predict the frequency dependent reflection and transmission coefficient on a semi-infinite fish school for various incident waves. Good agreement was found with the results obtained from finite element modeling of realistic, finite sized fish schools.

Materials identification synthetic aperture radar: progress toward a realized capability

Most non-metallic materials have frequency-dependent reflectivity functions, that is, they reflect electromagnetic energy in a manner that depends on frequency. Pulsed-chirped synthetic aperture radar and other multispectral radar systems do not generally take into account the frequency dependence of material reflections in forming scenes or making other inferences. In this report, we introduce a simple mathematical approach to using existing pulsed chirp synthetic aperture systems in a manner which results in a determination of a frequency-dependent reflectivity function for each pixel in a computed scene. Our analysis of collected data suggests that the method may be useful to distinguish disturbed from non-disturbed earth, and to detect chemicals on the surface of the earth. The method we have developed provides the analyst with a vector above each pixel with each vector component referencing a frequency band. This additional information may be useful for considering surface texture, subsurface layering and materials identification.

Imaging frequency-dependent reflectivity from synthetic-aperture radar

This paper develops a method for using a synthetic-aperture radar system to obtain not only a spatial image of a scene but also the localized frequency dependence of the scene reflectivity. In other words, for each image pixel, we also obtain a plot of the frequency dependence of the reflectivity in that pixel. We present a method for extracting this information from the data, and also a formula that characterizes the performance of this imaging system. We conclude with some simulations suggesting that the method may be promising.

Microwave sintering carbon nanotube/Ni0.5Zn0.5Fe2O4 composites and their electromagnetic performance

Carbon nanotube (CNT)–Ni0.5Zn0.5Fe2O4 powders were prepared by in situ chemical precipitation and hydrothermal processing, and further sintered by microwave sintering technology. The results show that CNTs acted as ‘heating source’ and promoted the consolidation of composites during the microwave sintering process. However, too much CNTs (such as 5 wt%) led to phase decomposition and reduction of ferrite materials because of the ultra-high localized temperature building up in the interface of CNTs and ferrite grains. The electrical conductivity of composites increased by more than seven orders of magnitude when compared to that of pure Ni0.5Zn0.5Fe2O4, and remained a high value at the temperature of 70 K (for example, 1 wt% CNT/Ni0.5Zn0.5Fe2O4 sample kept conductivity of 0.1 S/m). The saturation magnetization was strongly dependent on the mass percentage of CNTs. With the increase in CNT content, both the real and the imaginary permittivity were increased in the frequency region 0.6–5 GHz (L and S bands). According to the measured results of ϵr and μr, the frequency-dependent reflectance loss (RL) of CNT/Ni0.5Zn0.5Fe2O4 composite ceramics with different CNT content was evaluated. The CNT-doped ferrite ceramics discussed herein is very promising to be used in an on-beam-line high-order mode (HOM) load in particle accelerators based on superconducting RF due to their excellent low-temperature characteristics.

Reflection and transmission of Lamb waves at an imperfect joint of plates

The reflection and transmission of Lamb waves at an imperfect joint of plates are analyzed numerically by the modal decomposition method and the hybrid finite element method. The joint is modeled as a spring-type interface characterized by distributed normal and tangential stiffnesses. The analysis is focused on a low-frequency range where the lowest-order symmetric and antisymmetric Lamb waves are the only propagating modes. The frequency-dependent reflection and transmission characteristics of these Lamb modes are computed for different interfacial stiffnesses, together with the generation behavior of localized, non-propagating higher-order Lamb modes. As a result, S0-mode Lamb wave is shown to exhibit the reflection and transmission characteristics which are monotonically frequency-dependent. On the other hand, A0-mode Lamb wave shows complicated and non-monotonic frequency dependence in the reflection and transmission characteristics. The obtained Lamb wave characteristics are discussed in the light of approximate one-dimensional models constructed based on classical plate theories. As a result, the reflection and transmission coefficients of S0-mode Lamb wave are accurately reproduced by a simple model of longitudinal wave in thin plates, while those of A0-mode Lamb wave are well described by the Mindlin plate model of flexural wave. It is also shown that stiffness reduction at the corners of the contacting edges of plates has only minor influence on the reflection and transmission characteristics.

TIDAL WAVE REFLECTION FROM THE CLOSURE DAM IN THE GUADALQUIVIR ESTUARY (SW SPAIN)

Closure dams in estuaries and tidal areas have a strong impact on their hydrodynamics and morphology. Among other reasons, this impact can be due to tidal reflection. In the Guadalquivir estuary (SW Spain), data measured allowed to identify a partially standing motion along the upper third of the estuary. We estimate the frequency-dependent complex reflection coefficients at the head dam, and analyze the effect of the combined action of the incident and reflected tidal waves on the residual currents. Erosion-deposition patterns induced by the tidal reflection were identified. The most energetic tidal constituent, the semidiurnal M2, favors sedimentation near the entrance of the Port of Seville, where the most intense dredging works take place. Finally, we briefly discuss the potential consequences of reflection on the turbidity spatial distribution.

Seismic low-frequency-based calculation of reservoir fluid mobility and its applications

Low frequency content of seismic signals contains information related to the reservoir fluid mobility. Based on the asymptotic analysis theory of frequency-dependent reflectivity from a fluid-saturated poroelastic medium, we derive the computational implementation of reservoir fluid mobility and present the determination of optimal frequency in the implementation. We then calculate the reservoir fluid mobility using the optimal frequency instantaneous spectra at the low-frequency end of the seismic spectrum. The methodology is applied to synthetic seismic data from a permeable gas-bearing reservoir model and real land and marine seismic data. The results demonstrate that the fluid mobility shows excellent quality in imaging the gas reservoirs. It is feasible to detect the location and spatial distribution of gas reservoirs and reduce the non-uniqueness and uncertainty in fluid identification.

Color Matrix Refractive Index Sensors Using Coupled Vertical Silicon Nanowire Arrays

Vivid colors are demonstrated in silicon nanowires with diameters ranging from 105 to 346 nm. The nanowires are vertically arranged in a square lattice with a pitch of 400 nm and are electromagnetically coupled to each other, resulting in frequency-dependent reflection spectra. Since the coupling is dependent on the refractive index of the medium surrounding the nanowires, the arrays can be used for sensing. A simple sensor is demonstrated by observing the change in the reflected color with changing refractive index of the surrounding medium. A refractive index resolution of 5 × 10(-5) is achieved by analyzing bright-field images captured with an optical microscope equipped with a charge coupled device camera.

Current transport through an n‐doped, nearly lattice matched GaN/AlInN/GaN heterostructure

AbstractThe low refractive index of AlInN makes it a strong candidate as a waveguide cladding layer for lasers emitting from the blue to the green. A sequence of AlInN layers interfaced periodically with GaN is needed to provide a smooth surface for growth of quantum wells. The current transport across a highly doped single 54 nm thick layer of AlInN clad by GaN is analysed and is shown to be dominated by the heterointerfaces. Greater than 1 V is required to obtain current densities required in lasers of several kA/cm2. Frequency dependent electrical reflection measurements show that a 20 nm wide depletion zone is present at the interfaces even under bias and despite the high doping levels. A thermally activated resistive shunting of the interface is relevant at low voltages while the current is dominated by tunnelling at laser current densities. A low defect density AlInN/GaN multilayer structure requires > 10 V in order to drive current at levels needed in a laser diode. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)