Normalized Partial Scattering Cross Section for Performance Evaluation of Low-Observability Scattering Structures

The development of diffusion metasurfaces created new opportunities to elevate the stealthiness of combat aircraft. Despite the potential significance of metasurfaces, their rigorous design methodologies are still lacking, especially in the context of meticulous control over the scattering of electromagnetic (EM) waves through geometry parameter tuning. Another practical issue is insufficiency of the existing performance metrics, specifically, monostatic and bistatic evaluation of the reflectivity, especially at the design stage of metasurfaces. Both provide limited insight into the RCS reduction properties, with the latter being dependent on the selection of the planes over which the evaluation takes place. This paper introduces a novel performance metric for evaluating scattering characteristics of a metasurface, referred to as Normalized Partial Scattering Cross Section (NPSCS). The metric involves integration of the scattered energy over a specific solid angle, which allows for a comprehensive assessment of the structure performance in a format largely independent of the particular arrangement of the scattering lobes. We demonstrate the utility of the introduced metric using two specific metasurface architectures. In particular, we show that the integral-based metric can be used to discriminate between the various surface configurations (e.g., checkerboard versus random), which cannot be conclusively compared using traditional methods. Consequently, the proposed approach can be a useful tool in benchmarking radar cross section reduction performance of metamaterial-based, and other types of scattering structures.

Delineation of Fractures Using a 2D GPR Processing Strategy for 3D Imaging: Weak Zones within Carbonates at the Archaeological Site of Xochicalco in Mexico

The use of GPR data multipath summation on data acquired over parallel study lines is presented here within the framework of a study on the effects of natural hazards on cultural heritage areas in order to image weak zones within carbonates, such as fractures and caverns. This study was realized at the archeological site of Xochicalco in Mexico, where fractures and caverns are potential sources of the degradation of the archeological remains. Dense parallel GPR study lines spaced every 0.25 m were surveyed using a 400 MHz monostatic antenna with the aim to image possible weak zones in three dimensions. We used a 2D imaging approach, namely, the method of multipath summation, which efficiently focused the scattered energy within the GPR sections. The study revealed, at depths of 1.6m and 1.8m, several linear events attributed to fractures, leading to the preliminary conclusion of this on-going project that cracks on the walls of the Quetzalcoatl Temple after a large earthquake in 2017 are prone to instability of carbonates rocks.

Handling gaps in acquisition geometries - improving Marchenko-based imaging using sparsity-promoting inversion and joint inversion of time-lapse data

Marchenko focusing and imaging are novel methods for correctly handling multiple scattered energy while processing seismic data. However, strict requirements in the acquisition geometry, specifically co-location of sources and receivers as well as dense and regular sampling, currently constrain their practical applicability. We reformulate the Marchenko equations to handle the case where there are gaps in the source geometry while receiver sampling remains regular (or the opposite, by means of reciprocity). Using synthetic data based on a velocity model that produces strong interbed multiples, we test different solvers for the newly formulated inversion problem and we compare these results to results obtained by applying standard Marchenko inversion to a previously reconstructed dataset. When using the unreconstructed dataset, the ability of the Marchenko equations to retrace multiple reflected energy deteriorates. Sparsity-promoting Marchenko inversion, while improving the appearance of focusing functions, barely decreases multiple leakage in gathers and does not visibly improve the final image when compared to standard least-squares inversion. On the other hand, reconstructing the wavefield in advance restores the proper functioning of the Marchenko methods. Further, we test a joint inversion technique designed for time-lapse data with non-repeated geometries and originally intended to be solved using sparsity-promoting inversion. Motivated by our previous results, we compare images produced by this method to images produced by solving the same joint inversion problem without sparsity constraint. We find that the joint inversion alone hardly improves the resulting images but, when combined with the sparsity constraint, it leads to better noise and multiple suppression and produces a clean time-lapse image. Overall, none of the results from sparsity-promoting inversion techniques match the results obtained when reconstructing the wavefield in advance. We show that this can be explained by the comparatively slow convergence rate of sparsity-promoting Marchenko inversion.

Frequency-domain least-squares generalized internal multiple imaging with the energy norm

Recorded seismic data contain various types of scattered energy, including those corresponding to multiples. Traditional imaging techniques are focused on single-scattering events and, thus, may fail to image crucial structures, such as salt flanks and faults that sometimes are illuminated only by the multiple scattered energy. The recently introduced generalized internal multiple imaging (GIMI) method offers an opportunity to image multiples by projecting the recorded data back into the subsurface, followed by an interferometric crosscorrelation of the subsurface wavefield with the recorded data. During this process, the interferometric step converts the first-order scattering to a tomographic component and the double-scattering forms the primary reflectivity. Dealing with a large volume of data consisting of full wavefields over the image space renders the interferometric step computationally expensive in the time domain. To make the implementation of GIMI tractable, we formulate its frequency-domain version. Moreover, we use the energy norm imaging condition to separate the reflectivity portion from the tomographic component. We demonstrate these features with numerical experiments.

ЕКСПЕРИМЕНТАЛЬНЕ ВИЗНАЧЕННЯ ЕНЕРГЕТИЧНИХ УТОМНИХ ХАРАКТЕРИСТИК СПЛАВУ Д16Т (2024) МЕТОДОМ ВІЛЬНИХ КОЛИВАНЬ

In the process of studying fatigue characteristics of materials, components, assemblies and entire structures, as well as the prediction of durability, it is necessary to know the energy fatigue characteristics of various materials. These parameters include α – a fatigue failure parameter and W-1– irreversibly dissipated energy per loading cycle at stresses equal to the fatigue limit based on N = 107 cycles. The logarithmic damping decrement shows the fraction of irreversibly scattered energy when the oscillations decay from the total elastic energy of the oscillations. It has been proven that for metals fatigue failure is a constant "dangerous" energy. This paper describes the experimental determination of these energy characteristics for one of the most common materials in aviation, D16T (2024). By measuring the logarithmic damping factor in the fatigue test process, the dissipated energy is found over the loading cycle. Tests are conducted at two different loading levels. This allows to obtain a system of two equations to determine the two energy characteristics α and W-1.

Coupling Coefficient of Flux Density and Density Gradient of Reflected Sound Energy in Quasi-Diffuse Sound Fields

Sound pressure levels in rooms are determined by the energy of direct sound and the energy generated by reflections of sound from enclosures. In some areas of a room, the reflected sound energy exceeds the energy of the direct sound. For this reason, calculations of the energy characteristics of reflected sound energy are of great importance. In most rooms, the reflection of sound from enclosures has a specular-diffuse character. In this case, one part of the reflected energy transferred by specular reflected waves is distributed according to the laws of geometric acoustics, and the other part, scattered diffusely, according to Lambert’s law. The calculation of the specular component of the reflected energy is made by the methods of ray tracing, and the scattered energy is calculated by the statistical energy methods developed by the authors for the quasi-diffuse sound fields of the rooms. The basis of the latter methods is the relationship between the flux density of reflected sound energy and the density gradient of this energy. The accuracy of these methods depends on the accuracy of the adopted value of the coupling coefficient between the flux density and the density gradient of the reflected sound energy. Currently, there are different ideas about its magnitude. The paper presents the results of studies of coupling coefficients with regard to space-planning and acoustic characteristics of premises. The investigations were carried out using Kuttruff’s integral equation. Based on the results obtained, recommendations are given on the choice of the coupling coefficient.

Внутреннее трение на границах зерен, содержащих протяженные поры

A model of internal friction at a grain boundary containing equidistant parallel cylindrical pores is presented. Variable shear stress induces a mutual displacement of the interfacial regions matched at the segments between pores depending on their position. The values of scattered energy at each segment and total internal friction are determined. The temperature dependence of the internal friction has a form of a wide peak.