Reflectance Ratio Research Articles

A step towards pigmentation independent retinal vessel oximetry

AbstractPurposeRetinal vessel oximetry is a promising tool to gain information on the microcirculation of the retina. Due to the ratio of reflectance’s of the vessel plane and the neighbouring tissue, computation of oxygen saturation values (SO2) varies between individuals. The aim of this study was to develop a data transformation approach to overcome pigmentation‐ dependency, allowing for inter‐subject comparison of SO2 values.MethodsArterial and venous SO2 results of 67 healthy subjects (mixed ethnicities) were related to the pigmentation value (RP) provided by the Vesselmap software (RVA Imedos, Jena). After normalization of the axis values, a regression line was rotated around the intercept with the y‐axis applying a rotation matrix (clockwise rotation).ResultsRegression lines before rotation were aSO2=0.808*RP+84.61 (R2 = 0.74) and vSO2 = 0.358*RP+39.17 (R2 = 0.56) for arteries and veins, respectively. Transformed arterial and venous regression lines were aSO2 = 0.066*RP+84.57 (R2 = 0.05) and vSO2 = 0.028*RP+39.09 (R2 = 0.01), respectively.ConclusionClockwise rotation of the regression lines for arteries and veins was able to reduce R2 from 0.74 to 0.05 for arteries and from 0.56 to 0.01 for veins, confirming that the data transformation resulted in retinal pigmentation independent SO2 results. The proposed approach might improve comparability of inter‐subject results and allow for determination of normative data.

On the Adequacy of Representing Water Reflectance by Semi-Analytical Models in Ocean Color Remote Sensing

Deterministic or statistical inversion schemes to retrieve ocean color from space often use a simplified water reflectance model that may introduce unrealistic constraints on the solution, a disadvantage compared with standard, two-step algorithms that make minimal assumptions about the water signal. In view of this, the semi-analytical models of Morel and Maritorena (2001), MM01, and Park and Ruddick (2005), PR05, used in the spectral matching POLYMER algorithm (Steinmetz et al., 2011), are examined in terms of their ability to restitute properly, i.e., with sufficient accuracy, water reflectance. The approach is to infer water reflectance at MODIS wavelengths, as in POLYMER, from theoretical simulations (using Hydrolight with fluorescence and Raman scattering) and, separately, from measurements (AERONET-OC network). A wide range of Case 1 and Case 2 waters, except extremely turbid waters, are included in the simulations and sampled in the measurements. The reflectance model parameters that give the best fit with the simulated data or the measurements are determined. The accuracy of the reconstructed water reflectance and its effect on the retrieval of inherent optical properties (IOPs) is quantified. The impact of cloud and aerosol transmittance, fixed to unity in the POLYMER scheme, on model performance is also evaluated. Agreement is generally good between model results and Hydrolight simulations or AERONET-OC values, even in optically complex waters, with discrepancies much smaller than typical atmospheric correction errors. Significant differences exist in some cases, but having a more intricate model (i.e., using more parameters) makes convergence more difficult. The trade-off is between efficiency/robustness and accuracy. Notable errors are obtained when using the model estimates to retrieve IOPs. Importantly, the model parameters that best fit the input data, in particular chlorophyll-a concentration, do not represent adequately actual values. The reconstructed water reflectance should be used in bio-optical algorithms. While neglecting cloud and aerosol transmittances degrades the accuracy of the reconstructed water reflectance and the retrieved IOPs, it negligibly affects water reflectance ratios and, therefore, any variable derived from such ratios.

Lacustrine environmental evolution and implications on source rock deposition in the Upper Cretaceous-Paleocene of the South Yellow Sea Basin, offshore eastern China

Lacustrine source rocks are well developed within the Upper Cretaceous-Paleocene in the South Yellow Sea Basin (SYSB) in offshore eastern China. Understanding the deposition of these source rocks could provide critical information for hydrocarbon exploration in the SYSB, as well as reveal the paleoenvironment across the Cretaceous/Paleogene boundary in eastern China. This study investigates the lacustrine environmental evolution during the Upper Cretaceous Taizhou Formation (K2t) to Paleocene Funing Formation (E1f) within the SYSB and its implications on source rock deposition based on core observation combined with organic geochemical indicators. Multiple maturity parameters (vitrinite reflectance, Tmax and biomarker isomerization ratios) reveal that the organic matter (OM) has reached early oil window maturity. The OM of source rocks is dominated by kerogen Type II-III (liptinite and vitrinite). Anoxic, saline-brackish deep to semi-deep lacustrine environments prevailed during the deposition of K2t2 (second member of the K2t), E1f2 (second member of the E1f) and E1f4L (lower unit of fourth member of the E1f). The deposition of three intervals was influenced by marine incursion, as evidenced by geochemical and paleontological results. Low to moderate TOC contents (avg. 0.78 wt% to 1.91 wt%), which corresponds to low to moderate terrestrial OM input, are observed through three intervals. Abundant freshwater, together with high terrestrial OM flowed into the lacustrine environments during the deposition of E1f4U (upper unit of fourth member of the E1f), leading to dysoxic-oxic and freshwater conditions. High terrestrial OM input is probably the major controlling factor for relatively high TOC contents (avg. 2.49%) in the E1f4U. An evolution from balanced-fill to overfilled lake-type is observed in the Upper Cretaceous-Paleocene of SYSB, consistent with an evolution from semi-arid to humid paleoclimate conditions. The E1f4U and E1f2 source rocks exhibit fair to good potential, whereas poor to fair potential is present in the E1f4L and K2t2 source rocks. Compared with the Paleogene source rocks in onshore eastern China, the Upper Cretaceous-Paleocene source rocks in the SYSB probably hold good potential for generating commercial quantity of oil, although further study on hydrocarbon generation kinetics of source rocks and overall basin history are needed.

Thermo-molecular gas-film lubrication (t-MGL) considering temperature distributions and accommodation coefficients: analyses by quasi-free-molecular t-MGL equation

In the present paper, the air-bearing characteristics of a fixed plane inclined slider (Case 1) and a step slider flying in either air or He (Case 2) over a running boundary wall with local temperature distributions and imperfect surface accommodation coefficients, the molecular reflections at the surface of which are a mixture of diffuse reflection (α) and specular reflection (1−α) (Maxwell-type reflection), are analyzed using the thermo-molecular gas-film lubrication (t-MGL) equation. In particular, since the minimum spacing is ultra-small (~ 1 nm), the quasi-free-molecular t-MGL equation, in which the gas temperature, τG, is assumed to be that in the free molecular limit, τGfm, defined by temperatures and accommodation coefficients at the disk, τW0 and α0, respectively, and those at the slider, τW1 and α1, respectively, is used. For a plane inclined slider (Case 1), the fundamental static and dynamic characteristics are analyzed numerically and are compared with the approximation results for an infinite bearing number (t-MGLqfm, Λ→∞). When the spacing is ultra-small and only the running boundary (disk) has a local temperature distribution, the additional static pressure produced by the local boundary temperature distribution is approximately proportional to the equivalent accommodation coefficient $$ \gamma_{\alpha } \equiv \alpha_{0} (2 - \alpha_{1} )/[2\{ 1 - (1 - \alpha_{0} )(1 - \alpha_{1} )\} ] $$. For a step slider (Case 2), the following results are revealed: (1) the spacing decreases occur as the accommodation coefficient of the disk, α0, decrease i.e., the ratio of specular reflection at the disk increases, (2) the increases in the minimum spacing due to local temperature distribution are negligible in both air and He because the heat spot size is very small, (3) the decreases in the minimum spacing for a slider flying in He are significant, and (4) the spacing fluctuation caused by a running wavy disk varies according to both the surface accommodation coefficients and the ambient gas (air/He) and can be estimated precisely.

Microphysics of Snowfall Over Coastal East Antarctica Simulated by Polar WRF and Observed by Radar

AbstractThe current assessment of the Antarctic surface mass balance mostly relies on reanalysis products or climate model simulations. However, little is known about the ability of models to reliably represent the microphysical processes governing the precipitation. This study makes use of recent ground‐based precipitation measurements at Dumont d'Urville station in Adélie Land to evaluate the representation of the precipitation microphysics in the Polar version of the Weather Research Forecast (Polar WRF) atmospheric model. During two summertime snowfall events, high‐resolution simulations are compared to measurements from an X‐band polarimetric radar and from a Multi‐Angle Snowflake Camera (MASC). A radar simulator and a “MASC” simulator in Polar WRF make it possible to compare similar observed and simulated variables. Radiosoundings and surface‐meteorological observations were used to assess the representation of the regional dynamics in the model. Five different microphysical parameterizations are tested. The simulated temperature, wind, and humidity fields are in good agreement with the observations. However, the amount of simulated surface precipitation shows large discrepancies with respect to observations, and it strongly differs between the simulations themselves, evidencing the critical role of the microphysics. The inspection of vertical profiles of reflectivity and mixing ratios revealed that the representation of the sublimation process by the low‐level dry katabatic winds strongly influences the actual amount of precipitation at the ground surface. By comparing the simulated radar signal as well as MASC and model particle size distributions, it is also possible to identify the microphysical processes involved and to pinpoint shortcomings within the tested parameterizations.

Detection of point scatterers using diffraction imaging and deep learning

ABSTRACTDiffracted waves carry high‐resolution information that can help interpreting fine structural details at a scale smaller than the seismic wavelength. However, the diffraction energy tends to be weak compared to the reflected energy and is also sensitive to inaccuracies in the migration velocity, making the identification of its signal challenging. In this work, we present an innovative workflow to automatically detect scattering points in the migration dip angle domain using deep learning. By taking advantage of the different kinematic properties of reflected and diffracted waves, we separate the two types of signals by migrating the seismic amplitudes to dip angle gathers using prestack depth imaging in the local angle domain. Convolutional neural networks are a class of deep learning algorithms able to learn to extract spatial information about the data in order to identify its characteristics. They have now become the method of choice to solve supervised pattern recognition problems. In this work, we use wave equation modelling to create a large and diversified dataset of synthetic examples to train a network into identifying the probable position of scattering objects in the subsurface. After giving an intuitive introduction to diffraction imaging and deep learning and discussing some of the pitfalls of the methods, we evaluate the trained network on field data and demonstrate the validity and good generalization performance of our algorithm. We successfully identify with a high‐accuracy and high‐resolution diffraction points, including those which have a low signal to noise and reflection ratio. We also show how our method allows us to quickly scan through high dimensional data consisting of several versions of a dataset migrated with a range of velocities to overcome the strong effect of incorrect migration velocity on the diffraction signal.

Sources of Variation in Assessing Canopy Reflectance of Processing Tomato by Means of Multispectral Radiometry.

Canopy reflectance sensors are a viable technology to optimize the fertilization management of crops. In this research, canopy reflectance was measured through a passive sensor to evaluate the effects of either crop features (N fertilization, soil mulching, appearance of red fruits, and cultivars) or sampling methods (sampling size, sensor position, and hour of sampling) on the reliability of vegetation indices (VIs). Sixteen VIs were derived, including seven simple wavelength reflectance ratios (NIR/R460, NIR/R510, NIR/R560, NIR/R610, NIR/R660, NIR/R710, NIR/R760), seven normalized indices (NDVI, G-NDVI, MCARISAVI, OSAVI, TSAVI, TCARI), and two combined indices (TCARI/OSAVI; MCARI/OSAVI). NIR/560 and G-NDVI (Normalized Difference Vegetation Index on Greenness) were the most reliable in discriminating among fertilization rates, with results unaffected by the appearance of maturing fruits, and the most stable in response to different cultivars. Black mulching film did not affect NIR/560 and G-NDVI behavior at the beginning of the growing season, when the crop is more responsive to N management. Due to a moderate variability of NIR/560 and G-NDVI, a small sample size (5–10 observations) is sufficient to obtain reliable measurements. Performing the measurements at 11:00 and 14:00 and maintaining a greater distance (1.8 m) between plants and instrument enhanced measurement consistency. Accordingly, NIR/560 and G-NDVI resulted in the most reliable VIs.

The use of reflectance spectroscopy and the prehnite spectral index to target gold mineralization at the Bonfim polymetallic skarn deposit, Seridó Mobile Belt, Borborema Province, Brazil

Abstract The Serido Mobile Belt hosts numerous mineralized skarns in the Borborema Province, northeastern Brazil. The Bonfim W-Mo-Au-Bi-Te skarn occurs within a Neoproterozoic metasedimentary sequence (Serido Group), within marble and schist lenses, or at the contacts between both. An ASD-FieldSpec™ 3 High-Resolution spectrometer was used to characterize the mineralogy of a representative drill core interval (181 m) from the Bonfim deposit, in order to generate the spectral characterization to define the gold mineralization. Skarns are characterized by the presence of diopside, actinolite-tremolite, clinozoisite-zoisite, muscovite and prehnite. Prehnite and epidote constitute the main components of the calc-silicate assemblage associated with late-stage gold mineralization. The diagnostic absorption feature of prehnite at 1478 nm (OH) can be used to identify its presence, and the prehnite index proposed in this case study (a reflectance ratio of 1465 nm/1478 nm) allows highlighting the prehnite enriched sections along the core. Geochemical data show a positive correlation between gold and the presence of a prehnite absorption at 1478 nm. The results suggest that the prehnite spectral signature can be used as a reliable and efficient (rapid) indicator for gold skarns, perhaps under geologically similar circumstances to those at Bonfim.

Rapid, label-free detection of cerebral ischemia in rats using hyperspectral imaging

Stroke is the third most common cause of disability and the second most common cause of death worldwide. Ischemia, one of the two broad categories of stroke, is characterized by a lack of sufficient amounts of blood in order to supply an adequate amount of oxygen and nutrients. It is important to assess the part of the brain that becomes ischemic and necrotic during neurosurgery or experiments in real time. However, there is currently no effective means to achieve this goal. We proposed a method based on hyperspectral imaging (HSI) for the real-time detection of a varied range of ischemic brain tissues in vivo or ex vivo and assessed the practical utility of a model of ischemic stroke in rats. The results showed that hyperspectral images processed with a ratio of spectral reflectance at 545 and 560 nm (R545/R560) could identify early brain ischemia and accurately show regions of ischemia. We verified the area imaged by HSI using hematoxylin and eosin (HE) and 2, 3, 5-triphenyltetrazolium chloride (TTC) staining methods. This technique could precisely image the ischemic part of the brain in vivo and ex vivo. These results demonstrate the practical utility of HSI for the real-time detection of cerebral ischemia in rats. By providing rapid assessment of brain tissue perfusion, HSI may help doctors recognize ischemic regions quickly and precisely during surgery as well as have great utility in the experimental process.

Acceleration of Interstellar Pickup He+ at Earth's Perpendicular Bow Shock

AbstractOn 05 December 2015 the Magnetospheric MultiScale constellation observed interstellar pickup ion distributions during an inbound crossing of Earth's perpendicular bow shock near the subsolar point, which provides new insights into shock acceleration of pickup ions throughout the heliosphere. In this study we analyze the upstream and downstream velocity distributions of H+ and He+ using data from the Hot Plasma Composition Analyzer on Magnetospheric MultiScale. We derive average two‐dimensional pitch angle distributions in the upstream and downstream field‐aligned bulk plasma frame, as well as integrated one‐dimensional velocity distributions. By comparing the upstream and downstream distributions, we find evidence of accelerated H+ and He+ downstream of the shock. By comparing measured and theoretical reflection ratios for He+, we attribute a significant part of this acceleration to a single reflection at the shock.

The Impact of the Radar-Sampling Volume on Multiwavelength Spaceborne Radar Measurements Using Airborne Radar Observations

Multiwavelength radar observations have demonstrated great potential in improving microphysical retrievals of cloud properties especially in ice and snow precipitation systems. Advancements in spaceborne radar technology have already fostered the launch in 2014 of the first multiwavelength radar system in space, while several future spaceborne multiwavelength radar concepts are under consideration. However, due to antenna size limitations, the sampling volume of spaceborne radars is considerably larger than those achieved by surface- and airborne-based radars. Here, the impact of these large sampling volumes in the information content of the Dual-Wavelength Ratio estimates at Ka-W, Ku-Ka is investigated. High-resolution airborne multiwavelength radar observations during the Olympic Mountain Experiment (OLYMPEx) are used to perform retrievals of ice/snow characteristic particle size, such as mass-weighted particle diameter. To mimic the different satellite sampling volumes, a moving average is applied to the airborne measurements. The radar-observed variables (reflectivity and dual-wavelength ratios) and retrieved microphysical properties at the coarser resolution are compared against those at the original resolution. Our analysis indicates that future Ka-W spaceborne radar missions should take into account the impact of the radar resolution volume on the retrieval of microphysical properties and avoid footprints larger than 2–3 km.

The Influence of Temperature and Community Structure on Light Absorption by Phytoplankton in the North Atlantic.

We present a model that estimates the spectral phytoplankton absorption coefficient () of four phytoplankton groups (picophytoplankton, nanophytoplankton, dinoflagellates, and diatoms) as a function of the total chlorophyll-a concentration (C) and sea surface temperature (SST). Concurrent data on (at 12 visible wavelengths), C and SST, from the surface layer (<20 m depth) of the North Atlantic Ocean, were partitioned into training and independent validation data, the validation data being matched with satellite ocean-colour observations. Model parameters (the chlorophyll-specific phytoplankton absorption coefficients of the four groups) were tuned using the training data and found to compare favourably (in magnitude and shape) with results of earlier studies. Using the independent validation data, the new model was found to retrieve total with a similar performance to two earlier models, using either in situ or satellite data as input. Although more complex, the new model has the advantage of being able to determine for four phytoplankton groups and of incorporating the influence of SST on the composition of the four groups. We integrate the new four-population absorption model into a simple model of ocean colour, to illustrate the influence of changes in SST on phytoplankton community structure, and consequently, the blue-to-green ratio of remote-sensing reflectance. We also present a method of propagating error through the model and illustrate the technique by mapping errors in group-specific using a satellite image. We envisage the model will be useful for ecosystem model validation and assimilation exercises and for investigating the influence of temperature change on ocean colour.

Bias Correction to Antenna Frequency Response for Wideband Polarimetric Phased Array Radar

The frequency response of wideband polarimetric phased array radar antenna biases polarization measurement. A bias correction method named the iterative frequency division (IFD) algorithm is proposed in this paper to eliminate the effect of frequency response of the antenna system. The method is built on a new measurement bias model in which the effects of frequency response on the backscattering covariance matrix are analyzed. To demonstrate the effectiveness of the proposed method, a wideband microstrip patch array element is designed. Simulation results show that the biases of differential reflectivity ( Z D R ) and linear depolarization ratio ( L D R ) can be decreased to less than 0.1 dB and − 40 dB even for wide beam direction. The proposed method maintains a favorable polarization measurement accuracy for wideband polarimetric phased array radar.

Glucose Sensing inEx-VivoHuman Gingival Tissue With Enhanced Sensitivity in Combination Band

The proposed technique based on frequency domain dual wavelength low coherence interferometry (FD-DWLCI) is demonstrated for sensing glucose in ex-vivo human gingival tissue, with preliminary studies performed on solid bilayer gingival tissue phantom and saline. A broad band supercontinuum source filtered using bandpass filters with centre wavelengths at 1300, 1580 (corresponding to the first overtone region), and 2100 nm (corresponding to the combination band of glucose) was employed for the studies. Enhanced glucose resolutions of 11 and 15 mg/dl were estimated for the ratio of relative reflectivity of wavelengths 2100 /1300 nm, in the gingival tissue phantom and ex-vivo human gingival tissue, respectively.

Observational and Modeling Study of Ice Hydrometeor Radar Dual-Wavelength Ratios

AbstractThe influence of ice hydrometeor shape on the dual-wavelength ratio (DWR) of radar reflectivities at millimeter-wavelength frequencies is studied theoretically and on the basis of observations. Data from dual-frequency (Ka–W bands) radar show that, for vertically pointing measurements, DWR increasing trends with reflectivity Ze are very pronounced when Ka-band Ze is greater than about 0 dBZ and that DWR and Ze values are usually well correlated. This correlation is explained by strong relations between hydrometeor characteristic size and both of these radar variables. The observed DWR variability for a given level of reflectivity is as large as 8 dB, which is in part due to changes in mean hydrometeor shape as expressed in terms of the particle aspect ratio. Hydrometeors with a higher degree of nonsphericity exhibit lower DWR values when compared with quasi-spherical particles because of near-zenith reflectivity enhancements for particles outside the Rayleigh-scattering regime. When particle mass–size relations do not change significantly (e.g., for low-rime conditions), DWR can be used to differentiate between quasi-spherical and highly nonspherical hydrometeors because (for a given reflectivity value) DWR tends to increase as particles become more spherical. Another approach for differentiating among different degrees of nonsphericity for larger scatterers is based on analyzing DWR changes as a function of radar elevation angle. These changes are more pronounced for highly nonspherical particles and can exceed 10 dB. Measurements of snowfall spatiotemporally collocated with spaceborne CloudSat W-band radar and ground-based S-band operational weather radars also indicate that DWR values are generally smaller for ice hydrometeors with higher degrees of nonsphericity, which, for the same level of S-band reflectivity, exhibit greater differential reflectivity values.

Designing on scaled absorbing materials at THz band using Ni@mica composite

The scaled measurement on the target model coated with the frequency dispersion absorber with a large-scaled factor was still an unsolved problem. In this work, the scaled absorbing material with the scale factor 100 was designed at Terahertz (THz) band using the hybrid Ni@mica particle. The designing method on the scaled absorber was composed with the particle distribution designing, permittivity interpolation method, and multilayer structure optimization. The complex permittivity and permeability were measured first on the THz time-domain spectroscopy (TDS) measurement system in frequency 0.8–1.8 THz as the Ni@mica particle-filling ratio of the absorber was 30%, and then, the reflection loss could be calculated. As the different holes were set in the absorbing squared body element, the transmission ratio and the reflection ratio of the absorbing composite were simulated in the computer simulation technology (CST) software, and the effective permittivity and permeability could be calculated. Then, the permittivity data base with different holes was built by the Lagrange interpolation method. Finally, the scaled absorber materials could be optimized for the magnetic coating plate at 1 THz and 0.8 THz, and the designed scaled absorber included two layers: the bottom layer thickness was 0.12 mm with volume content 30% and the upper layer thickness was 0.17 mm with volume content 27%. The calculation result showed that the radar cross section (RCS) deviation between the two similar models was − 40.02dB@8GHz and − 40.21dB@10GHz, which were very close to the theoretical value − 40 dB. It indicated that the Ni@mica was one of the effective candidates to construct the scaled absorbing material.

A Multibeam Interference Model for Analyzing Complex Near‐Field Images of Polaritons in 2D van der Waals Microstructures

AbstractVan der Waals (vdW) materials are among the most promising candidates for photonic integrated circuits because they support a full set of polaritons that can manipulate light at deep subdiffraction nanoscale. It is possible to directly probe the propagating polaritons in vdW materials in real space via scattering‐type scanning near‐field optical microscopy, such that the wave vector and lifetime of the polaritons can be extracted from as‐measured interference fringes by Fourier analysis. However, this method is unsuitable for clutter interference patterns in samples exhibiting inadequate fringes due to small size (less than 10 µm) or complex edges that are often encountered in nanophotonic devices and new material characterization. Here, a multibeam interference model is developed to analyze complex images by disentangling them into periodic patterns and residue. By employing phase stationary approximation, polariton wave vector can be derived from offset ratio of the center point, and the ratio of polariton reflection and scattering rates at the edge is obtained from the ratio of the periodic and aperiodic patterns. This method can be widely used in the optical characterization of new vdW materials that are difficult to synthesize into large crystals, as well as nanophotonic integrated devices with unique boundaries.

Implementation of four-port MIMO diversity microstrip antenna with suppressed mutual coupling and cross-polarized radiations

A compact four-element MIMO microstrip antenna is designed, analyzed and proposed for high frequency modern wireless applications. Proposed antenna is compact in size and offers a frequency band of about 6 GHz ranging from 23 to 29 GHz with a peak gain of 7.1 dBi and suppressed mutual coupling and cross-polarization level. Presented antenna possess better diversity with minimum mutual coupling among its all four elements. Diversity is verified in terms of envelope correlation coefficient, directive gain, total active reflection coefficient and ratio of mean effective gain. All the diversity parameters are found within the appropriate limit standardized for a MIMO device. Experimental results of fabricated prototypes verified the proposed results.

Study on propagation properties of electromagnetic waves through a magnetized, collisional and inhomogeneous plasma under oblique incidence

The properties of electromagnetic waves propagation through a collisional, magnetized and inhomogeneous plasma with obliquely incident angles are investigated in this paper. The reflection, transmission, and attenuation ratios are calculated first with the scattering matrix method. Based on the derived formulas, we then analyze the effects of different parameters on the propagation properties under oblique incidence. Simulation results indicate that a larger incident angle causes a longer wave transmission distance in the plasma and is more likely to impede the waves propagation. Under an obliquely incident angle, both the transmission and attenuation ratios are greatly affected by other parameters, such as the effective plasma collisional frequency, background magnetic flux density, plasma thickness and electron density, additionally. These conclusions are of guiding significance to the applications of the interaction between electromagnetic waves and plasma.

Scattering of the Fundamental Shear Guided Wave From a Surface-Breaking Crack in Plate-Like Structures.

Cracks in critical sections of steel structures pose a major safety concern in many industries. Existing high-frequency ultrasonic techniques offer high detection sensitivity to cracks but have poor inspection volume coverage, limiting their practical use for monitoring large areas of structures. Low-frequency guided waves have relatively high inspection area coverage and are currently used in pipeline monitoring for corrosion defects but face challenges in detecting critical cracks that often cause over an order of magnitude lower cross-sectional area loss. A study of scattering from small cracks in a thin-walled (<12 mm) section with an incident plane SH0 guided wave at higher frequencies but remaining below the SH1 cutoff is presented here using quasi-static approximations, the aim being to explore the possibility of using this regime for crack growth monitoring applications. A 3-D solution was developed using dimensional analysis, which showed that the SH0 reflection ratio is proportional to frequency to the power 1.5, to the effective crack size cubed, and is inversely proportional to the plate thickness and to the square root of the distance from the crack to the receiving sensor. Finite element analysis was used to validate these power coefficients and to calculate the proportionality constant. The results show that a higher inspection frequency offers improved sensitivity, but the validity of the results here is limited to the SH1 cutoff frequency. The predicted 3-D solution was validated by measurements on a pipe with a progressively grown notch.