Backscattering Of Waves Research Articles

Spaceborne radar identification of desert regions as suppliers of dust into the atmosphere

Atmosphere dust represents a mixture of minute particles of various salts and minerals. Also it contains remains of animal and vegetable organisms, spores of causative organisms and so forth. The dust is carried by winds over considerable distances and its presence in the atmosphere is among the factors which have essential influence on the global climate of the Earth. At present the Aeolian processes and their consequences are monitored from space using multispectral optical systems (TOMS, METEOSAT, MODIS etc.) only. These are not capable of reliable identification of the areas themselves that are sources of raising the dust in the atmosphere independently of the cloudiness, solar illumination and transparency of the atmosphere. This problem can be solved with the use of space radar systems of the Earth remote sensing. The paper presents the first results of the development of a radar technique of identification of desert regions in which dust from the surface is transported up into the atmosphere under the action of Aeolian processes. The work was performed using data of Earth remote sensing SAR Envisat-1 obtained over deserts of Mauritania. Specific features of display of the narrow-beam backscattering of radio waves in radar images in dependence on the surface wind speed and direction and direction of radar illumination of the surface. It is concluded that the radar means of remote sensing represent an efficient tool for detecting regions of dust raise into the atmosphere. Results of the study can be used for the development of new methods of remote monitoring of the processes in desert areas that affect the climate of vast regions of the Earth.

Opportunities and pitfalls in surface-wave interpretation

Many explorationists think of surface waves as the most damaging noise in land seismic data. Thus, much effort is spent in designing geophone arrays and filtering methods that attenuate these noisy events. It is now becoming apparent that surface waves can be a valuable ally in characterizing the near-surface geology. This review aims to find out how the interpreter can exploit some of the many opportunities available in surface waves recorded in land seismic data. For example, the dispersion curves associated with surface waves can be inverted to give the S-wave velocity tomogram, the common-offset gathers can reveal the presence of near-surface faults or velocity anomalies, and back-scattered surface waves can be migrated to detect the location of near-surface faults. However, the main limitation of surface waves is that they are typically sensitive to S-wave velocity variations no deeper than approximately half to one-third the dominant wavelength. For many exploration surveys, this limits the depth of investigation to be no deeper than approximately 0.5–1.0 km.

Stimulated Brillouin backscattering of hollow Gaussian laser beam in collisionless plasma under relativistic–ponderomotive regime

AbstractStimulated Brillouin backscattering of an intense hollow Gaussian laser beam (HGLB) from collisionless plasma has been investigated under relativistic–ponderomotive regime. The main feature of considered hollow Gaussian laser beam is having the same power at different beam orders with null intensity at the center. Backscattered radiation is generated due to nonlinear interaction between main beam (pump beam) with pre-excited ion acoustic wave (IAW). Modified coupled equations has been set up for the beam width parameters of the main beam, ion-acoustic wave, back-scattered wave, and back reflectivity of stimulated Brillouin scattering (SBS) with the help of the Wentzel–Kramers–Brillouin approximation, fluid equations and paraxial theory approach. These coupled equations are solved analytically and numerically to study the laser intensity in the plasma, the variation of amplitude of the excited IAW and back reflectivity of SBS. The back reflectivity of SBS is found to be highly sensitive to the order of the HGLB, intensity of main laser beam, and plasma density for typical laser and plasma parameters. The focusing of main laser beam (hollow Gaussian) and IAW significantly affected the back reflectivity of SBS. The results show that the self-focusing and back reflectivity is enhanced for higher order modes of HGLB.

Three‐component decomposition technique for hybrid‐pol SAR data

This study describes a three-component decomposition of Stokes vector representing the hybrid-polarimetry (hybrid-pol) data. The three components are single-bounce, double-bounce and volume scattering. At the outset, the received backscattered wave is decomposed into completely polarised (CP) and completely depolarised (CD) parts based on Born and Wolf's wave dichotomy theorem. Then, the CP part is further decomposed into three independent components which indicate the contribution of three basic scattering mechanisms. In the proposed method, the CD part is considered as random noise and no physical meaning is attached to it. Whereas in existing hybrid-pol decomposition techniques such as m − δ and m − α, this component has been considered as volume scattering. This results in volume scattering being overestimated by these techniques. The authors demonstrated that the performance of the proposed decomposition technique is better than these existing decomposition techniques.

Gravitational-wave tail effects to quartic non-linear order

Gravitational-wave tails are due to the backscattering of linear waves onto the space-time curvature generated by the total mass of the matter source. The dominant tails correspond to quadratic non-linear interactions and arise at the one-and-a-half post-Newtonian (1.5 PN) order in the gravitational waveform. The ‘tails-of-tails’, which are cubic non-linear effects appearing at the 3 PN order in the waveform, are also known. We derive here higher non-linear tail effects, namely those associated with quartic non-linear interactions or ‘tails-of-tails-of-tails’, which are shown to arise at the 4.5 PN order. As an application, we obtain at that order the complete coefficient in the total gravitational-wave energy flux of compact binary systems moving on circular orbits. Our result perfectly agrees with black-hole perturbation calculations in the limit of extreme mass ratio of the two compact objects.

A contactless ultrasonic surface wave approach to characterize distributed cracking damage in concrete

We describe an approach that utilizes ultrasonic surface wave backscatter measurements to characterize the volume content of relatively small distributed defects (microcrack networks) in concrete. A simplified weak scattering model is used to demonstrate that the scattered wave field projected in the direction of the surface wave propagation is relatively insensitive to scatterers that are smaller than the propagating wavelength, while the scattered field projected in the opposite direction is more sensitive to sub-wavelength scatterers. Distributed microcracks in the concrete serve as the small scatterers that interact with a propagating surface wave. Data from a finite element simulation were used to demonstrate the viability of the proposed approach, and also to optimize a testing configuration to collect data. Simulations were validated through experimental measurements of ultrasonic backscattered surface waves from test samples of concrete constructed with different concentrations of fiber filler (0.0, 0.3 and 0.6%) to mimic increasing microcrack volume density and then samples with actual cracking induced by controlled thermal cycles. A surface wave was induced in the concrete samples by a 50kHz ultrasonic source operating 10mm above the surface at an angle of incidence of 9°. Silicon-based miniature MEMS acoustic sensors located a few millimeters above the concrete surface both behind and in front of the sender were used to detect leaky ultrasonic surface waves emanating from concrete. A normalized backscattered energy parameter was calculated from the signals. Statistically significant differences in the normalized backscattered energy were observed between concrete samples with varying levels of simulated and actual cracking damage volume.

On the link between the speckle free nature of optoacoustics and visibility of structures in limited-view tomography

Similar to pulse-echo ultrasound, optoacoustic imaging encodes the location of optical absorbers by the time-of-flight of ultrasound waves. Yet, signal generation mechanisms are fundamentally different for the two modalities, leading to significant distinction between the optimum image formation strategies. While interference of back-scattered ultrasound waves with random phases causes speckle noise in ultrasound images, speckle formation is hindered by the strong correlation between the optoacoustic responses corresponding to individual sources. However, visibility of structures is severely hampered when attempting to acquire optoacoustic images under limited-view tomographic geometries. In this tutorial article, we systematically describe the basic principles of optoacoustic signal generation and image formation for objects ranging from individual sub-resolution absorbers to a continuous absorption distribution. The results are of relevance for the proper interpretation of optoacoustic images acquired under limited-view scenarios and may also serve as a basis for optimal design of tomographic acquisition geometries and image formation strategies.

Modulational instability and solitary waves in polariton topological insulators

Optical microcavities supporting exciton–polariton quasi-particles offer one of the most powerful platforms for the investigation of the rapidly developing area of topological photonics in general, and of photonic topological insulators in particular. Energy bands of the microcavity polariton graphene are readily controlled by a magnetic field and influenced by the spin-orbit (SO) coupling effects, a combination leading to the formation of linear unidirectional edge states in polariton topological insulators as very recently predicted. In this work we depart from the linear limit of non-interacting polaritons and predict instabilities of the nonlinear topological edge states resulting in the formation of the localized topological quasi-solitons, which are exceptionally robust and immune to backscattering wave packets propagating along the graphene lattice edge. Our results provide a background for experimental studies of nonlinear polariton topological insulators and can influence other subareas of photonics and condensed matter physics, where nonlinearities and SO effects are often important and utilized for applications.

Imaging of local porosity/voids using a fully non-contact air-coupled transducer and laser Doppler vibrometer system

This study exploits the feasibility of imaging zones of local porosity/voids simulated by introducing microspheres during layup of a unidirectional carbon fiber–reinforced polymer composite panel. A fully non-contact hybrid system primarily composed of an air-coupled transducer and a laser Doppler vibrometer was used for imaging the local porosity/void zones from the guided wave response. To improve image resolution, several preprocessing techniques are performed. The wavefield reconstructed from the laser Doppler vibrometer measurements was first “denoised” using a one-dimensional wavelet transform in the time domain followed by a two-dimensional wavelet transform in the spatial domain. From the total wavefield, the much weaker backscattered waves were separated from the stronger incident wave by frequency–wavenumber domain filtering. In order to further enhance the signal-to-noise ratio and sharpen the image, the attenuation of incident wave propagation to the damage site was compensated through two proposed weight functions. Finally, a zero-lag cross-correlation was performed for imaging the zone where the compensated incident and backscattered waves were in phase. This improved imaging condition, the “denoised” weighted zero-lag cross-correlation, was proposed and tested for defect imaging in the composite panel with eight intentionally introduced zones of high porosity/voids of varying diameters (1.59–6.35 mm) and depths (0.36–1.08 mm). As expected, the sensitivity of the non-contact air-coupled transducer/laser Doppler vibrometer hybrid system was limited by the wavelength of the excitation signal. The system incorporated with the denoised weighted zero-lag cross-correlation imaging condition for guided wave interrogation gave similar image quality in comparison with that by the immersion C-scan.

Attenuation of Bragg backscattering of electromagnetic waves from density fluctuations near the region of polarization degeneracy in magnetoactive plasma

Specific features of Bragg backscattering under conditions of strong polarization degeneracy near the cutoff surface in an anisotropic medium are studied analytically and numerically. It is shown that the linear interaction of normal waves can substantially affect wave scattering by suppressing the amplification of Bragg backscattering near the cutoff region in the case of weak coupling between normal waves.

Temporal Separation Detection for Chipless Depolarizing Frequency-Coded RFID

This paper studies the detection of chipless frequency-coded tags. The detection exploits a temporal separation that allows obtaining the identification (ID) with only one measurement. In this way, the flexibility of reading of this type of chipless tags is clearly improved, which is highly expected for future real implementation. This temporal separation is possible when the tag presents a long-time signature, longer than the backscattering wave corresponding to the surrounding objects. A technique based on the short-time Fourier transform (STFT) is used to differentiate the useful parts of the signal, which contain the tag ID. Up to now, this was done by using a calibration process based on two measurements at least to remove coupling and clutter contribution. With the proposed approach the acquisition of the tag ID is direct, and it is not necessary to have further information such as the measurement of the environment without the tag. A study on the time duration of several frequency-coded tags is performed based on simulations and measurements. The study shows that this approach can be used with classical depolarizing chipless tags already proposed in the literature. It is proven that the proposed approach is useful to detect the tag response with a single measurement.

Near-surface fault detection by migrating back-scattered surface waves with and without velocity profiles

We demonstrate that diffraction stack migration can be used to discover the distribution of near-surface faults. The methodology is based on the assumption that near-surface faults generate detectable back-scattered surface waves from impinging surface waves. We first isolate the back-scattered surface waves by muting or FK filtering, and then migrate them by diffraction migration using the surface wave velocity as the migration velocity. Instead of summing events along trial quasi-hyperbolas, surface wave migration sums events along trial quasi-linear trajectories that correspond to the moveout of back-scattered surface waves. We have also proposed a natural migration method that utilizes the intrinsic traveltime property of the direct and the back-scattered waves at faults. For the synthetic data sets and the land data collected in Aqaba, where surface wave velocity has unexpected perturbations, we migrate the back-scattered surface waves with both predicted velocity profiles and natural Green's function without velocity information. Because the latter approach avoids the need for an accurate velocity model in event summation, both the prestack and stacked migration images show competitive quality. Results with both synthetic data and field records validate the feasibility of this method. We believe applying this method to global or passive seismic data can open new opportunities in unveiling tectonic features.

Study of acoustic wave backscattering by discrete inhomogeneities of different sizes

The influence of the distribution statistics on the sizes of discrete random bottom inhomogeneities on the mean intensity of acoustic signal backscattering is studied. The simultaneous presence of two effects leading to an increase in the mean intensity of backscattering are considered: the correlation in the positional relationship of particles on the plane of the bottom and their size distribution.

Re-extraction of the Second Order Terms of High Frequency Radio Wave Backscattering in the Presence of Uniform Surface Current

HF radar systems are widely used for the measurement of ocean surface currents and wave parameters. Theoretical analysis of high frequency radio wave backscattering from the sea has been derived in the past. Also, the relationship between Bragg frequencies and surface currents is determined and has been proved by HF radars. It is known that when surface currents are present, the Bragg frequencies move to new frequencies. Their movement is proportional to the surface current in the direction toward radar. In addition, it is observed that sea surface current changes the second order part of the spectrum of radar reflection from the sea surface. In this paper, a theoretical analysis of high-frequency radio wave backscattering from the sea in the presence of constant surface current provides a formula for the second order part of the spectrum of sea echo. This resulted in similar formulas of others but the process of formula extraction is different. This method may be used for the determination of the second order spectra of radar cross section in the presence of other types of surface current.

Broadband non-reciprocal transmission of sound with invariant frequency.

We design and experimentally demonstrate a broadband yet compact acoustic diode (AD) by using an acoustic nonlinear material and a pair of gain and lossy materials. Due to the capabilities of maintaining the original frequency and high forward transmission while blocking backscattered wave, our design is closer to the desired features of a perfect AD and is promising to play the essential diode-like role in realistic acoustic systems, such as ultrasound imaging, noise control and nondestructive testing. Furthermore, our design enables improving the sensitivity and the robustness of device simultaneously by tailoring an individual structural parameter. We envision our design will take a significant step towards the realization of applicable acoustic one-way devices, and inspire the research of non-reciprocal wave manipulation in other fields.

Imaging near-surface heterogeneities by natural migration of backscattered surface waves

S U M M A R Y We present a migration method that does not require a velocity model to migrate backscattered surface waves to their projected locations on the surface. This migration method, denoted as natural migration, uses recorded Green’s functions along the surface instead of simulated Green’s functions. The key assumptions are that the scattering bodies are within the depth interrogated by the surface waves, and the Green’s functions are recorded with dense receiver sampling along the free surface. This natural migration takes into account all orders of multiples, mode conversions and non-linear effects of surface waves in the data. The natural imaging formulae are derived for both active source and ambient-noise data, and computer simulations show that natural migration can effectively image near-surface heterogeneities with typical ambient-noise sources and geophone distributions.

Measuring the characteristics of backscattering of sound on a rough surface in the near-field zone of a phased array

The paper presents the results of an experiment on measuring the characteristics of backscattering of sound waves by a rough surface. Measurements were conducted in the near zone of a vertical array, which made it possible with a high degree of accuracy to control the propagation conditions and the angles of incidence of an acoustic wave onto the surface. Tonal signals were emitted in a constant mode at frequencies of 0.5–3.5 kHz. The Doppler spectrum of the scattered signal was analyzed in detail, the spectral component corresponding to Bragg scattering was isolated, and the width of this component was determined. The possibilities of calculating the scattered field using two forms of the sea state spectrum are studied and compared: the two-dimensional spatiotemporal sea state spectrum, which was measured optically, and the conventional one-dimensional time spectrum, measured with a single surveying mark.

Effects of surface roughness on diffuse ultrasonic backscatter in the solids

In the diffuse ultrasonic backscatter describing the scattering of elastic waves from polycrystalline metal material, the spatial variance of the signal is used as a primary measure of microstructure.Previously,theoretical singly-scattered response models have been developed for the diffuse backscatters of elastic waves within polycrystalline materials,which take into consideration both transducer beams and microstructural scattering information.However,the surface roughness of the liquid-solid interface induces a noticeable change of spatial variance amplitude,and its effect on the diffuse ultrasonic backscatter that can severely degrade the accuracy and practicability of the microstructure parameter evaluation was neglected in previous models.Therefore,a new singly-scattered response model for the rough surface polycrystalline samples is developed by following the forms similar to previous models for longitudinal-to-longitudinal scattering at normal incidence.In particular, we assume that the surface is slightly rough,specifically,the surface roughness value should not be larger than the magnitude of the wavelength.Hence,the modified expressions of ultrasonic reflection and transmission coefficients for the randomly rough interface can be applied to the singly-scattered response model.Then,with the modified transmission coefficient,a Gaussian beam is adopted to model the transducer beam pattern at normal incidence for longitudinal wave propagation through a rough liquid-solid interface to the polycrystal.Next,the Wigner transform of the displacement field is derived with a parameter of the surface roughness root mean square value.After that,a new expression of the calibration parameter including the modified reflection coefficient is given to provide a conversion between the displacement field and the experimental transducer voltage.Finally,the rough surface singly-scattered response model is built and the surface roughness correction coefficient is presented here to quantify the effect of the surface roughness on diffuse ultrasonic backscatter.The numerical results show that the Wigner distribution amplitude decreases and the acoustic energy coverage shrinks with the increase of the surface roughness.The theoretical spatial variance amplitude decreases by about 79.2% when the root mean square roughness value is set to be 40 m.The surface roughness correction coefficient is usually smaller than 1 when the reference calibration sample is smooth,but it can be bigger than 1 when the reference sample is rough.The results from the developed theory are then compared with the experimental measurements associated with a pulse echo transducer configuration for 304 stainless steel by using the smooth and rough surface samples.From these measurements,the mean grain size of the stainless steel can be determined.The experimental results show that although the corrected and uncorrected models both fit the experimental spatial variance curve from the smooth surface sample well,the uncorrected model fails to extract the grain size of the rough surface sample.The relative error of the grain size between optical microscopy and the uncorrected model can reach -21.35%.In contrast,good agreement with optical microscopy is observed by using the surface roughness corrected model,and the relative error is only 1.35%.In conclusion,the ultrasonic waves transmit though the rough interface twice,and the diffuse scattering which happens in these processes reduces the number of backscatter waves that can return to the transducer,so the spatial variance amplitudes drop dramatically.The correction coefficient presented here can describe the effect of surface roughness on diffuse ultrasonic backscatter.Moreover,it can improve the accuracy of grain size evaluation effectively.Thus,the surface roughness corrected ultrasonic backscatter model may be applicable for quality control of roughwrought castings or forgings during the manufacturing.

Coherent backscattering of phase conjugate ultrasound waves in dispersive systems

Processes of the propagation of phase conjugate ultrasound waves in stationary and moving random media containing micro-inclusions in the form of gas bubbles and solid spheres are studied experimentally and theoretically. The wave phase conjugation of sound at a frequency of 10 MHz is conducted via supercritical parametric conversion. The qualitative difference between the statistical properties of scattered and phase conjugate acoustic fields is established. Unlike the detected stochastic signals from scattered waves, the signals of the phase conjugate waves are regular and more useful in determining the concentrations and parameters of the scatterers. A theoretical model describing the observed features of the coherent backscattering of phase conjugate waves is developed.

Phase conjugation of ultrasound waves in comparison with backscattering in disordered medium

Ultrasound wave phase conjugation in randomly scattering media is studied experimentally and treated theoretically in comparison with ordinary backscattering phenomenon. The experiments are carried out on a scattering sample made of a cylindrical agar gel matrix containing a set of glass micro-spheres of 300μm diameter. The sample was immersed in water and turned slowly for simulation of a statistical ensemble of scatterers. Phase conjugation of the scattered acoustic waves at frequency 10 MHz was implemented by the parametric technique. The signals of backscattered (BSW) and phase conjugate (PCW) waves were detected by the same ultrasonic transceiver. In contrast with the stochastic behaviour of the BSW signal, the PCW amplitude is found almost regular and insensitive to the statistical features of the scattering ensemble. The mean values of amplitudes and the signal to noise ratios for PCW are one order of value higher than for BSW. A variation of the number of scatterers in the aperture of the incident acoustic beam revealed the proportionality of the mean values of PCW amplitude and BSW signal intensity. The results are in quantitative agreement with the developed theory. The advantages of the phase conjugation technique in comparison with the backscattering method for testing of randomly scattering media are discussed.