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The methods of direction finding for an immobile point source have been analyzed to obtain initial data for constructing acoustic distance measuring and tomography algorithms as applied to the deep sea. It is found that, to obtain reliable bearing estimates in the near- and far-field acoustic illumination zones (NFAIZ and FFAIZ), both in summer and in winter, it is sufficient to use the values of effective phase velocity (EPV) or effective group velocity (EGV) of sound, which are close to the measured speed of sound in water. However, in the shadow zone (SZ) under summer conditions, the effective velocities differ significantly from the speed of sound in water, and their values depend on distance, complicating additionally the solution of this problem. Therefore, to estimate the EPV and EGV, one must have information about the distance to the source. It is shown that application of vertically oriented antennas makes it possible to estimate the distance in the SZ and calculate independently the EPV and EGV values for each distance, which is necessary for direction finding. Thus, under summer conditions, conventional signal direction finding is performed in acoustic illumination zones, whereas in the SZ, in the case of simultaneous application of horizontal and vertical antennas, one must previously determine the distance to the source for calculating the bearing. The shadow zone is abscent in winter; thus, to phase a horizontal antenna on almost all distances, one can use the average speed of sound in water, but the antenna range must be determined. [ABSTRACT FROM AUTHOR]

Some results on modeling and observation of infrasound propagation in the atmosphere in a presence of mesoscale and anisotropic wind velocity and temperature fluctuations are presented. The theoretical model of infrasound scattering from anisotropic wind velocity and temperature inhomogeneities of the atmosphere is developed. With this model, the appearance of the stratospheric, mesospheric, and thermospheric arrivals of the infrasound signals in the acoustic shadow zones is explained. The analytic relation between the wave field of the scattered infrasound signal and the vertical profile of the effective sound speed fluctuations is obtained. Using this relation, the vertical profiles of the fluctuations within the upper stratosphere (25–55 km) and the lower thermosphere (105–140 km) were retrieved from the waveforms and travel times of the signals recorded in the acoustic shadow zones. The theoretical frequency spectrum of the infrasound wavefield reflected from the fine-scale layered structure of the atmosphere is obtained and compared with the spectra of the observed stratospheric arrivals. [ABSTRACT FROM AUTHOR]

The south polar layered deposits (SPLD) of Mars record its Amazonian climate history. Detailed stratigraphic analyses of the SPLD will improve our understanding of the Martian depositional system. Radar reflection patterns correlate with the varying composition of the internal layers, thus providing a key to unlocking the depositional history of the SPLD and Martian paleoclimate. Using data from SHAllow RADar (SHARAD) onboard the Mars Reconnaissance Orbiter, we studied a new type of weak reflection pattern (shadow zone) across the SPLD. These shadow zones lack identifiable bases and are generally deeper than previously reported low‐reflection zones interpreted as sealed CO2 ice. The discrepancy between the waveforms of shadow zones in focused and unfocused radargrams was used to identify whether the shadow zones were associated with fog‐related data‐processing artifacts. Numerical simulations utilizing various dielectric models demonstrate that lossy materials are most likely the cause of the shadow zone. These materials are expected to exhibit a gradual increase in concentration with depth, ranging from pure water ice to lossy materials at a scale of >50 m. The presence of such lossy materials is consistent with the compositional heterogeneity of SPLD, but their origins remain obscure. Plain Language Summary: Orbital radar data reveal diverse reflection patterns within the south polar layered deposits (SPLD) of Mars, providing valuable insights into their internal structure and composition. The reflection powers are low or even non‐existent when there is weak compositional variation or the material is lossy. This study reports and investigates a low‐reflection zone, referred to as the shadow zone, in the SHAllow RADar (SHARAD) data across the SPLD. The shadow zones exist in the background of a diffuse echo signature known as radar fog. Compared to previously reported low‐reflection zones, shadow zones typically lack a detectable base and exhibit significantly greater depth. We analyzed the waveform of the SHARAD radargrams and the results indicate the presence of two distinct types of shadow zones: one attributed to fog‐related artifacts, and the other classified as real shadow zones. Numerical simulations indicate the real shadow zones are likely linked to lossy materials such as salts, clays, and/or mafic minerals. These materials are expected to exhibit a distribution of gradual increasing concentration with depth in the SPLD. This research provides new insights into the internal composition of the SPLD and its potential relationship with the climate of Mars. Key Points: We explored shadow zones in SHAllow RADar data poleward at 80°S using radargrams before and after synthetic aperture processingThe shadow zones exhibit distinct morphologies from the previously identified reflectivity‐free zones, which were inferred to be dry iceForward modeling reveals that shadow zones likely associate with lossy materials conforming to a gradual increasing concentration with depth [ABSTRACT FROM AUTHOR]

Coverage in mobile communications systems requires an acceptable link gain over a maximum service area, with areas of low link gain being problematic. A standard approach to maximize link gain is to deploy multiple-input–multiple-output (MIMO) antennas that adapt to the changing multipath. There is also recent interest in using active surfaces for adaptive reflections within the multipath environments, and this calls for new ideas at the system and component levels. A critical mechanism in urban propagation is corner diffraction, which illuminates shadow areas but with low link gain. We present a simple add-on system for corners, comprising a simple scattering dipole or scattering array, which can significantly improve link gain in the shadow region over a wide bandwidth. The concept is demonstrated through analysis, simulation, and physical measurement. Only shadowed areas are boosted, and the line-of-sight coverage is not significantly affected. Our demonstration is for a passive, fixed configuration, which is suitable for retrofitting to existing systems. [ABSTRACT FROM AUTHOR]

It is established that the interference invariant can be calculated not only by analyzing the frequency and spatial distribution of the intensity field but also by studying the phase-gradient field. It is found that the interference structure of the intensity field and the field of value inverse to the phase gradient (effective phase velocity) can be described by identical dependences, based on the use of invariant. It is shown that, in contrast to shallow water, which is characterized by relative stability of invariant values, the interference invariant in deep water changes significantly with variation in distance. In the shadow zone, with detectors and sources located at real depths (less than 400 m), the interference invariant can be described by an analytical dependence, which is in essence affected by only the water depth and distance; it is affected by neither sound frequency, nor source depth, nor detector depth and is only slightly affected by the characteristics of speed-of-sound vertical profile. Hence, the thus obtained analytical dependence can be referred to as invariant. [ABSTRACT FROM AUTHOR]

Array gain (AG) is significant in evaluating the detection performance of the vertical line array, which is directly determined by the correlation of signal and noise, respectively. In this paper, we analyze the vertical correlation for a 16-element vertical line array experimented in the deep ocean in 2016. The ray interference theory is utilized to interpret the mechanism of the vertical correlation of the sound field in different zones. In the direct-arrival zone, the direct rays and once-surface-reflected rays are two dominated components, whose arrival time difference for each element are nearly the same, and the vertical correlation is high. In the shadow zone, the sound field is mainly dominated by bottom-reflected rays and the vertical correlation decreases due to different grazing angles and arrival times of each ray. Different from the previous assumption of noise independence, the effect of noise correlation on the AG is analyzed through the measured marine environmental noise. Results indicate that the noise correlation coefficients in two zones are low but not 0. In the direct-arrival zone, AG is about 10 dB, very close to the ideal value of 10 log M . AG even exceeds it when NG is negative. Moreover, AG in the direct-arrival zone is higher than the one in the shadow zone. [ABSTRACT FROM AUTHOR]

Since Gangneung Harbor breakwaters were built in the east coast of Korea from 1992 to 2002, the shoreline inside the harbor had severely changed as it was accreted in the area inside the harbor but eroded further south during the observation period (1979–2005). We investigated this process using Telemac-2DH model to calculate flows, waves, sediments and morphological changes in unstructured grids. The performance of the model was validated using the experiment data by Graven and Wang (2007). The model was run in two cases by turning on/off the diffraction mode using the formula by Holthujisen et al. (2003). For this, we tuned the model by setting a shadow zone behind the breakwaters and by applying the diffraction formula only inside the shadow zone. The model results showed that wave heights significantly increased inside the shadow zone when the diffraction mode was on, compared to the case when the diffraction mode was off. This effect of diffraction was confirmed by the observations as the wave heights measured inside the shadow zone became 10–20% lower than those measured outside, which were nicely simulated by the model. In addition to the wave height, the wave-induced currents became stronger in the innermost area of the shadow zone with the diffraction mode. The model also successfully predicted the observed morphological change pattern because it simulated the shoreline erosion at the southern end of the shadow zone where the currents were bifurcated. Inside the shadow zone, sediment deposition occurred, corresponding to the observation, when the diffraction mode was on, whereas this deposition process was not simulated when the mode was off. The results support that the observed shoreline accretion in the harbor was mainly due to the reduced wave and current energy, which emphasizes the importance of accurate modelling of diffraction effect in the prediction of shoreline evolution. [ABSTRACT FROM AUTHOR]

Array gain (AG) is significant in evaluating the detection performance of the vertical line array, which is directly determined by the correlation of signal and noise, respectively. In this paper, we analyze the vertical correlation for a 16-element vertical line array experimented in the deep ocean in 2016. The ray interference theory is utilized to interpret the mechanism of the vertical correlation of the sound field in different zones. In the direct-arrival zone, the direct rays and once-surface-reflected rays are two dominated components, whose arrival time difference for each element are nearly the same, and the vertical correlation is high. In the shadow zone, the sound field is mainly dominated by bottom-reflected rays and the vertical correlation decreases due to different grazing angles and arrival times of each ray. Different from the previous assumption of noise independence, the effect of noise correlation on the AG is analyzed through the measured marine environmental noise. Results indicate that the noise correlation coefficients in two zones are low but not 0. In the direct-arrival zone, AG is about 10 dB, very close to the ideal value of 10 log M . AG even exceeds it when NG is negative. Moreover, AG in the direct-arrival zone is higher than the one in the shadow zone.

• Reachability issues of the MQL oil droplets during micro-milling is investigated. • Detrimental roles of top-burr on MQL oil deposition are conceptualized and assessed. • Lubricating capability of a fixed MQL supply reduces gradually with machining time. • Loss of wetted area increases exponentially from 79 to 407 μm2 within 46 s. • MQL effectiveness reduces from 40% to 8% owing to droplet restriction by burrs. During Minimum Quantity Lubrication (MQL), ensuring the reachability of the oil droplets to the cutting region is important to achieve better lubrication effect and enhanced machinability. During end-milling, the side walls and top-burrs can obstruct a fraction of oil droplets from reaching the cutting region resulting in the formation of "Shadow Zone". For a fixed nozzle orientation, shadow zone size is governed by the axial depth and top-burr height. For a constant slot depth, shadow zone expands as the burr height increases. Since a worn-out tool produces taller top-burrs, increasingly larger fractions of the oil droplets fail to reach the cutting zone as the tool undergoes progressive wear. This article, for the first time, investigates the reachability issue of the MQL oil droplets and corresponding loss in wetting potential of a constant MQL oil flow rate owing to the growth of top-burrs with tool wear during Ti-6Al-4V micro-milling at varying aspect ratios. At 50 μm axial depth, burr height increases from 14 to 120 μm within 46.8 s of micro-milling. Corresponding expansion of shadow zone results in an exponential loss in lubricated area from 38 to 407 μm2. At a higher axial depth of 200 μm, the burr height increases from 21 to 190 μm within 125 mm cutting length, and consequently, 1036 to 3776 μm2 wetted area is lost. Inappropriate wetting compels the cutting edge to rotate in partially dry condition, and thus, MQL gradually becomes ineffective in controlling machinability (surface roughness) with machining duration. [Display omitted] [ABSTRACT FROM AUTHOR]

The goal of the present paper is to numerically investigate the effects produced by a vertical barrier located inside the ground, when the soil is subjected to seismic waves coming from a seismic source with a planar wave front. Firstly, the acceleration field on the ground surface is computed by varying both geometrical and mechanical properties of the barrier in order to determine, through a parametric study, the optimal barrier configuration to minimize the maximum magnitude of surface acceleration inside the shielded zone. Then, the above configuration is employed in the framework of a seismic analysis: a portion of a colonnade in Pompeii (Naples, Italy) is examined, with such a colonnade assumed to be located inside the shielded zone. A comparison of the seismic behaviour of such a heritage building is provided for the two cases of ground with or without the above vertical barrier. [ABSTRACT FROM AUTHOR]

We analyze the results of an experiment using an explosive sound source in the tropical part of the Indian Ocean. We consider the time structure of sound signals in geometric shadow zones to a distance of 270 km and the scheme of how the sound field in the shadow zone is formed by rays reflected from horizontally extended fine-structured sound velocity layers. From the results of calculation using a wave program that realizes the method of psuedodifferential parabolic equations, we analyze the influence of signal scattering by fine-structure sound velocity inhomogeneities on the sound field distribution in a waveguide. We show that the field formed by spots of light in each of the shadow zones is generated by a regular field and propagates in parallel to it, taking energy from the regular zone in the near field and in each subsequent convergence zone. This mechanism causes an additional decrease in the field in illuminated zones, which can be interpreted as additional attenuation of the regular sound field. [ABSTRACT FROM AUTHOR]