Short-wavelength Structures Research Articles

Elastic time-reverse imaging of backscattered surface-wave data

Imaging shallow near-vertical or short-wavelength structure using seismic data is an important goal in near-surface geophysical applications. Numerous seismic analysis approaches use backscattered analysis of surface waves for these purposes; however, there is a need for new associated frameworks that can take advantage of these events to generate interpretable images in the depth-migrated domain. By drawing a connection with elastic time-reverse imaging (E-TRI) approaches developed for microseismic event analysis, an efficient elastic migration approach is developed that uses two adjoint wavefields defined by partitioned outward-propagating transmission and inward-propagating scattered surface-wave data. Subsurface images are generated by applying the energy-norm imaging condition that correlates temporally and spatially colocated energy throughout the two time-reversed adjoint wavefield domains. Synthetic tests highlight the ability of the E-TRI framework to image shallow (quasi)vertical subsurface discontinuities. Results from a field test using near-surface seismic data acquired in the Majes region of Arequipa, Peru, demonstrate the ability of the E-TRI framework to highlight short-wavelength structure identified in a parallel elastic full-waveform inversion analysis. The results suggest that the E-TRI technique should be applicable in a range of near-surface geotechnical and environmental applications.

Origin of oblique structures controlled by pre-tectonic thickness variations in frictional and salt-bearing fold-and-thrust belts: Insights from analogue modelling

This work includes, first, a synthesis of experimental analogue modelling programs assessing the influence of along-strike thickness variations in the development of oblique structures in fold-and-thrust belts. As a second part, we present a new experimental program testing, systematically, the influence of tapered décollement-cover thrust wedges.Previous experiments show that frictional, brittle models feature numerous, short-wavelength structures where the cover is thinner. Deformation fronts are totally or locally oblique to the backstop when the cover thinning is progressive or sharp, respectively. Low (<30°) or higher structural obliquities occur when thickness variations are progressive or sharp, respectively. The addition of a basal or intermediate décollement, commonly entails the transference of deformation towards its external pinch-out. Consequently salients occur and transfer zones and oblique structures form over décollement boundaries. Their location and orientation strongly depend on the pre-compressional shape of the décollements. Furthermore, as demonstrated by our modelling results, tapered brittle covers over viscous décollements result in asymmetric thrust wedges formed by structures that end or change their vergence laterally, resulting in obliquities generally <30°.Two natural case studies, showing strong (South Pyrenean Central Salient) or moderately oblique (Keping Shan fold-and-thrust belt) structures are revisited and compared to the described analogue models.

Seismic Characterization of a Landslide Complex: A Case History from Majes, Peru

Seismic characterization of landslides offers the potential for developing high-resolution models on subsurface shear-wave velocity profile. However, seismic methods based on reflection processing are challenging to apply in such scenarios as a consequence of the disturbance to the often well-defined structural and stratigraphic layering by the landslide process itself. We evaluate the use of alternative seismic characterization methods based on elastic full waveform inversion (E-FWI) to probe the subsurface of a landslide complex in Majes, southern Peru, where recent agricultural development and irrigation activities have altered the hydrology and groundwater table and are thought to have contributed to increased regional landslide activities that present continuing sustainability community development challenges. We apply E-FWI to a 2D near-surface seismic data set for the purpose of better understanding the subsurface in the vicinity of a recent landslide location. We use seismic first-arrival travel-time tomography to generate the inputs required for E-FWI to generate the final high-resolution 2D compressional- and shear-wave (P- and S-wave) velocity models. At distances greater than 140 m from the cliff, the inverted models show a predominantly vertically stratified velocity structure with a low-velocity near-surface layer between 5–15 m depth. At distances closer than 140 m from the cliff, though, the models exhibit significantly reduced shear-wave velocities, stronger heterogeneity, and localized shorter wavelength structure in the top 20 m. These observations are consistent with those expected for a recent landslide complex; however, follow-on geotechnical analysis is required to confirm these assertions. Overall, the E-FWI seismic approach may be helpful for future landslide characterization projects and, when augmented with additional geophysical and geotechnical analyses, may allow for improved understanding of the hydrogeophysical properties associated with suspected ground-water-driven landslide activity.

New free-Air and Bouguer Gravity Anomalies Maps of Brazil

Here we perform the integration of all available data from the BNDG (Banco Nacional de Dados Gravimétricos) and the BDEP (Banco de Dados de Exploração e Produção da Agência Brasileira de Petróleo) to provide new free-air and Bouguer gravity anomaly maps for the Brazilian territory with newly acquired data over the years, mainly in regions with no data coverage in the past. Quality controls and subsequent gridding processes, in the same system of the whole dataset, are developed on the Oasis Montaj software (OM). Subsets of data from various gravity surveys are gridded and upward continued up to 3000 m to avoid high-frequency noise, allowing them to be gathered. We fill areas with no data coverage in the North Region of Brazil with gravity values from the XGM2019e geopotential model. To join the subset grids of ground and airborne surveys and the geopotential model, we use a collection of grids knitting methods from the OM. To verify the consistency of our grids, we compare them with previously derived gravity anomalies maps and geopotential models. Our new free-air and Bouguer gravity anomalies maps showmore detailed short-wavelength geological structures than their predecessors. Therefore, these new gravity anomalies maps may be helpful for the development of recent tectonic, oil, and mining studies.

Experimental study of bubble competition and spike competition in Richtmyer–Meshkov flows

Shock-tube experiments on various two-bubble and two-spike interfaces are performed to examine the dependence of bubble competition and spike competition on the initial spectra and density ratio of the interface. The differences in the influences of bubble competition and spike competition on the Richtmyer–Meshkov instability are highlighted for the first time. The bubble-competition effect is mainly dependent on the initial spectra of the two-bubble configuration. In contrast, the spike-competition effect is determined by both the initial spectra and density ratio. The extended buoyancy–drag model is adopted to explain the variation of the drag force imposed on the long-wavelength and short-wavelength structures as the initial conditions change. Based on the spectrum analysis, it is found that the constituent modes of two-bubble and two-spike interfaces have different contributions to the long-wavelength and short-wavelength perturbation growths. A generalised, nonlinear, analytical model is then established to quantify the bubble-competition effect and spike-competition effect considering arbitrary initial spectra and density ratio. The bubble-competition effect is believed to be stronger than the spike-competition effect at a high density ratio because it suppresses the high-frequency perturbation growth more evidently.

Thermal Vibrational Convection in a Rotating Plane Layer

The influence of rotation on the thermal vibrational convection of liquid in a horizontal plane layer is studied experimentally. The convection in the layer is excited by the vibrations of circular polarization in the horizontal plane. The research is carried out in a liquid layer heated from above, under conditions of a strong stabilizing effect of gravity. In this case, the vibrational convection is excited at a relatively high intensity of vibrations and manifests itself in the development of short-wavelength convective structures in the form of beads. It is found that the rotation has a stabilizing effect and increase the vibrational convection excitation threshold. A map of convective stability is plotted on the plane of dimensionless parameters: vibrational parameter, and dimensionless velocity of rotation, for different values of the gravitational Rayleigh number. The results are compared with the classic case of gravitational convection in a rotating layer. It is shown that, similarly to the case of gravitational convection, rotation has a stabilizing effect on vibroconvective stability; the threshold value of the vibration parameter increases with the dimensionless velocity. It is found that at large negative values of the gravitational Rayleigh number, when cellular convective structures of vibroconvective nature are characterized by large wave numbers, in the studied area of the dimensionless frequency, rotation has little effect on the wave number of spatial convective structures, but changes their shape. (Clinical trials: no any clinical trials were performed).

Multiphase deformation of an inverted Permian deepwater rift basin: The Nong Pong Formation, Khao Khwang Fold and Thrust Belt, Thailand

• Khao Khwang Fold and Thrust Belt (KKFTB) is mixture of thrusted carbonate platforms over inverted deepwater rifts. • Nong Pong Formation affected by multiphase folding, whose structural style changes with sedimentary facies . • Estimated shortening of the KKFTB (300 km), supportive of E-W suture bisecting Indochina Terrane . The bathyal Permian Nong Pong Formation is crucial for understanding the deformation style, and amount of shortening in the E-W trending, Triassic (Indosinian Orogeny) Khao Khwang Fold and Thrust Belt. The formation was deposited in a series of marine rift basins, with carbonates forming on intervening highs. The platform carbonates are shortened by a series of dominantly north-directed thrusts. The Nong Pong Formation is extensively affected by tight to isoclinal, predominantly chevron style folding. Five major variations in stratigraphy (ranging between carbonate-dominated and shale-dominated) affect the deformation styles within the formation. Shortening locally is estimated to be up to 60% in sections up to 300 m long and is concentrated into short-wavelength (10 s m) structures. The absence of proximal slope depo-belts, and juxtaposition of carbonate platform facies with deepwater deposits suggest thrusts (some initiated as inverted normal faults) located on the basin margins have displacements of c. 10–25 km. Estimates of shortening from outcrops, and analogy with Zealandia, suggest the Nong Pong Formation was deposited in a basin up to 100 km wide. Such estimates point to a significant E-W trending seaway that bisected the Indochina Terrane. Multiple fold orientations and phases are present in the Nong Pong Formation, which are related to early mass transport events, multiple tectonic phases (Indosinian and Palaeogene), the influence of inherited trends (e.g. Permian rift basin morphology), and 3D deformation during a single tectonic phase. Deformation of the Nong Pong Formation demonstrates significant differences in structural style between ribbon continents and major continents, particularly, the Indochina ribbon continent shows the widespread inversion of rift basins across a submarine continent (similar to Zealandia) 100 s kms wide.

Data-Domain Traveltime Inversion of Reflected Waves Using Segment Dynamic Image Warping

Reflection traveltime information is generally used for producing a kinematically accurate velocity model for seismic imaging. Accurately estimating the traveltime difference between the demigrated and observed data is crucial for data-domain reflection traveltime tomography. To enhance the robustness of signal's alignment, we propose a new method of segment dynamic image warping (SDIW). It first exploits windowed polynomial fitting for signal processing and then aligns the signals based on point-wise segment-to-segment matching. Compared with the conventional point-to-point matching strategy, our method is more robust and insensitive toward strong random noise. By minimizing the traveltime difference computed by SDIW, data-domain reflection traveltime inversion (DRTI) is then used to find an accurate background velocity model. Synthetic tests show that SDIW provides reliable time shifts between the demigrated and observed data even if the signal-to-noise ratio of input data is rather low. This enables DRTI to automatically reconstruct the deep background velocity model. The subsequent full waveform inversion (FWI) gradually added short-wavelength velocity structures and finally recovers a high-resolution and high-fidelity model. The results demonstrate that the DRTI provides a kinematically accurate background velocity model sufficient for following FWI to fully retrieve the detail of the subsurface structures even when the low frequencies are missing.

Implications of tectonic anomalies from potential field data in some parts of Southeast Nigeria

Tectonic structures controlling mineralization in some parts of Southeast Nigeria were evaluated using airborne potential field data. High- and low-frequency filters and depth determination tools were adopted to evaluate short- and long-wavelength anomalies, resolve the spatial spreading of igneous intrusions, depths to geologic sources and basin topography. The high-frequency results exhibited high concentration of short-wavelength anomalies in the Obudu Plateau and Ikom–Mamfe Rift. The underlying main tectonics of the area elucidated by the low-frequency results caused the widespread occurrences of short-wavelength geologic structures that are revealed by the high-frequency maps. The study area is characterized by comparatively thin (~ 13.0 to < 3000 m) sedimentation. The observed thin thickness is as a result of the massive Precambrian basement outcrops in some locations in the Obudu Plateau and the proliferation of igneous intrusions within this part of the Lower Benue Trough. The 2-D models showed the undulating nature of the underlying basin topography, the location of intrusions, domal structures and related normal faults. The locations and neighbourhood of intrusions and/or short-wavelength structures are viable sites for lead–zinc–barite, brine and metallogenic minerals.

Restructuring of rotating spokes in response to changes in the radial electric field and the neutral pressure of a cylindrical magnetron plasma

Nonlinear plasma structures in the partially magnetized E×B plasma of a cylindrical magnetron are investigated using 2D3V particle-in-cell Monte Carlo collision simulations. In the early phase of the discharge, plasma gradients and radial electric fields excite a lower hybrid type instability that forms long wavelength rotating density spokes. As the discharge grows in density by ionization and the cathode gets shielded by the formation of an ion sheath, radial electric fields diminish in the quasineutral region of the discharge. This induces a transition of the spokes into short-scale spoke-on-spoke modes. The short wavelength structures can be reversed back into a long spiral spoke by lowering the neutral pressure, which revives the radial electric fields via a turbulent plasma expansion. Plasma phenomena connected to the rotating spoke include the anomalous radial transport and loss of electrons through the spoke, azimuthal dragging of ions by the spoke’s field, plasma temperature modulations by the spoke structure, and formation of electron vortices around equipotential islands, in some cases with opposing rotations to the underlying E×B drift. Electron scattering from non-ionizing collisions with neutrals also has a minor influence on the instability.

Acoustic full-waveform inversion to match far-offset reflections with pseudo-horizontal particle acceleration data

In the early stage of acoustic full-waveform inversion (AFWI), it is important to exploit the long-wavelength features of the gradient and suppress its short-wavelength features to update the background velocity. However, due to strong near-offset PP reflections and multiples within the pressure data, conventional AFWI sometimes primarily reconstructs the short-wavelength features of a given model and fails to converge on a reliable subsurface P-wave velocity model. Therefore, this study, we propose the usage of pseudo-horizontal particle acceleration (pseudo-ax) data for AFWI, rather than the original pressure data. Because the amplitudes of PP reflections are implicitly weighted according to the reflection angle in pseudo-ax data, the near-offset PP reflections and multiples of the pseudo-ax data are much weaker than those of the pressure data. As a result, AFWI using pseudo-ax data focuses on reconstructing the longer-wavelength features of a given model in the early iterations, and then gradually reconstructs short-wavelength features as the inversion process continues. Using synthetic and real data examples for the Volve oilfield in the North Sea, we examined the effects and feasibility of this strategy. In the synthetic data example, the proposed strategy rapidly reduced far-offset data misfits related to long-wavelength feature updates and achieved smaller model misfits than the conventional strategy. The real data example showed that the velocity model reconstructed using the proposed strategy flattened some of the reflectors at the angle-domain common-image gathers (ADCIGs) better than the velocity model obtained using the conventional strategy. These findings demonstrate that our strategy converges closer to the real P-wave velocity model than the conventional strategy by building a hierarchical velocity model from long- to short-wavelength structures.

Strain engineering of digitally alloyed AlN/GaN nanorods for far-UVC emission as short as 220 nm

Far-UVC light with emission wavelengths between 207 nm and 222 nm has shown significant potential for killing pathogens without damaging exposed human tissues and can be an alternative for safe sterilization. This work first reports on different compressively strained (AlN) 8 /(GaN) 2 nanorods constructing by strain engineering digitally alloyed GaN embedded in an AlN barrier. By controlling the atomically thin GaN well under increasing compressive stress, we use the top-down etching method to realize regular nanorod arrays based on (AlN) 8 /(GaN) 2 with different compressive strains in the GaN well. The emission wavelength is as short as 220 nm in the far-UVC, as expected by the theoretical calculations. We believe that this study will play an essential role in the design and fabrication of short-wavelength and high-efficiency LED structures with far-UVC emissions and potential use in effective, reliable, and safe UV disinfection systems.

Влияние внутренних оптических потерь на генерацию стимулированного излучения в среднем ИК диапазоне в волноводных гетероструктурах с квантовыми ямами HgCdTe/CdHgTe

We study two heterostructures with an array of 10 HgCdTe/CdHgTe quantum wells, grown within a single technological series and designed to generate stimulated emission (SE) in the 20 – 30 µm wavelength range. In the more short-wavelength structure we obtain SE at the wavelength λ ~ 23.9 µm at 10 K, while no SE was obtained in the more long-wavelength structure. We calculate optical absorption in passive layers for both structures and show it to be higher in the more long-wavelength structure. Finally, we consider several ways to minimize absorption influence on SE generation.

Orbital evolution of Saturn’s satellites due to the interaction between the moons and the massive rings

Context. Saturn’s mid-sized moons (satellites) have a puzzling orbital configuration with trapping in mean-motion resonances with every-other pairs (Mimas-Tethys 4:2 and Enceladus-Dione 2:1). To reproduce their current orbital configuration on the basis of a recent model of satellite formation from a hypothetical ancient massive ring, adjacent pairs must pass first-order mean-motion resonances without being trapped. Aims. The trapping could be avoided by fast orbital migration and/or excitation of the satellite’s eccentricity caused by gravitational interactions between the satellites and the rings (the disk), which are still unknown. In our research we investigate the satellite orbital evolution due to interactions with the disk through full N-body simulations. Methods. We performed global high-resolution N-body simulations of a self-gravitating particle disk interacting with a single satellite. We used N ∼ 105 particles for the disk. Gravitational forces of all the particles and their inelastic collisions are taken into account. Results. Dense short-wavelength wake structure is created by the disk self-gravity and a few global spiral arms are induced by the satellite. The self-gravity wakes regulate the orbital evolution of the satellite, which has been considered as a disk spreading mechanism, but not as a driver for the orbital evolution. Conclusions. The self-gravity wake torque to the satellite is so effective that the satellite migration is much faster than was predicted with the spiral arm torque. It provides a possible model to avoid the resonance capture of adjacent satellite pairs and establish the current orbital configuration of Saturn’s mid-sized satellites.

Testing the limits of virtual deep seismic sounding via new crustal thickness estimates of the Australian continent

We apply virtual deep seismic sounding (VDSS) to data collected from both permanent and temporary seismic stations in Australia with the goal of examining (i) the resilience of the method to the presence of complex lithospheric structure, and (ii) the effectiveness of different approaches for estimating bulk crustal properties (namely thickness and Vp). Data from the permanent WRAB in the Northern Territory station is ideal for benchmarking VDSS (large number and favourable distribution of recorded earthquakes), with the results from several approaches agreeing on a thickness of 40-42 km. Application of VDSS to data from the temporary BILBY array, a linear distribution of broadband stations that traverses central Australia, shows that strong Moho reflections can be retrieved with as few as two earthquakes even at the transition between crustal blocks of different character and in the presence of thick sedimentary basins. Crustal thickness varies between 36-54 km and compare well with the reflectivity character of nearby deep seismic reflection lines. Furthermore, we find that off-line estimates of crustal thickness, calculated by binning the source regions according to back-azimuth, produce estimates of crustal thickness that are consistent with the regional geology. Overall, we find that VDSS is a powerful technique for estimating crustal thickness and velocity due to its insensitivity to complex short-wavelength structure and requirement of a small number earthquakes to produce a stable result. However, not all schemes tested for extracting bulk crustal properties appear to be robust and stringent data quality checking is still required during implementation.

Multiscale structures and noise in magnetized plasmas line-tied at conducting surfaces

In magnetized plasma situations where magnetic fields intersect massive conducting boundaries, ‘line-tied’ boundary conditions are often used, analytically and in numerical simulations. For ideal magnetohydrodynamic (MHD) plasmas, these conditions are arrived at given the relatively long time scales for magnetic fields penetrating resistively into good conductors. Under line-tied boundary conditions, numerical simulations often exhibit what could be construed as numerical ‘noise’ emanating from the boundaries. We show here that this ‘noise’ is real. By combining numerical and analytical methods, we highlight the existence of sharp spatial structures near the conductors and confirm the appearance of short wavelength structures riding on long wavelength modes. We conclude that, for numerical fidelity, the short multiscale structures need to be resolved. Generally, the short structure widths scale as the square root of the plasma $\unicode[STIX]{x1D6FD}$.

Crustal Heating and Lithospheric Alteration and Erosion Associated With Asthenospheric Upwelling Beneath Southern New England (USA)

AbstractThe Northern Appalachian Anomaly (NAA), a region of exceptionally low seismic velocities in the asthenosphere beneath southern New England and easternmost New York State, has been interpreted as a site of mantle upwelling. We synthesize a combination of new and previously published data that indicates the following: (1) The upwelling has eroded or delaminated the lithosphere in a localized region centered in southern Vermont that we call the “Green Mountains Anomaly.” Forty‐second period Rayleigh wave phase velocities, which have peak sensitivity at lithospheric depths, are slow in this region, and S wave receiver functions are dominated by shallow (60‐km depth) mantle structures indicating reduced velocities. Thermal springs and sites with anomalously high concentrations of mantle‐derived helium‐3 are concentrated at the borders of the Green Mountains anomaly, perhaps due to stress concentrations produced by geologically recent, uncompensated delamination of the lower lithosphere. (2) S wave receiver functions indicate intense (&gt;10%), shallow (60‐km depth) short wavelength structures along the southern and western edges of the NAA, indicating that the lithosphere there is being intensely altered, possibly by a combination of shearing and introduction of volatiles. And (3) Notwithstanding the localized thinning and alteration, the NAA lithosphere as a whole does not appear to have experienced pervasive heating, for the compressional wave quality factor of QP = 870 inferred from the decay rate of Po waves is very significantly above the QP ≈ 60 value previously reported for the NAA asthenosphere.

Resolving the fine-scale velocity structure of continental hyperextension at the Deep Galicia Margin using full-waveform inversion

Continental hyperextension during magma-poor rifting at the Deep Galicia Margin is characterised by a complex pattern of faulting, thin continental fault blocks, and the serpentinisation, with local exhumation, of mantle peridotites along the S-reflector, interpreted as a detachment surface. In order to understand fully the evolution of these features, it is important to image seismically the structure and to model the velocity structure to the greatest resolution possible. Travel-time tomography models have revealed the long-wavelength velocity structure of this hyperextended domain, but are often insufficient to match accurately the short-wavelength structure observed in reflection seismic imaging. Here we demonstrate the application of two-dimensional (2D) time-domain acoustic full-waveform inversion to deep water seismic data collected at the Deep Galicia Margin, in order to attain a high resolution velocity model of continental hyperextension. We have used several quality assurance procedures to assess the velocity model, including comparison of the observed and modelled waveforms, checkerboard tests, testing of parameter and inversion strategy, and comparison with the migrated reflection image. Our final model exhibits an increase in the resolution of subsurface velocities, with particular improvement observed in the westernmost continental fault blocks, with a clear rotation of the velocity field to match steeply dipping reflectors. Across the S-reflector there is a sharpening in the velocity contrast, with lower velocities beneath S indicative of preferential mantle serpentinisation. This study supports the hypothesis that normal faulting acts to hydrate the upper mantle peridotite, observed as a systematic decrease in seismic velocities, consistent with increased serpentinisation. Our results confirm the feasibility of applying the full-waveform inversion method to sparse, deep water crustal datasets.

Design considerations for λ ∼ 3.0- to 3.5-μm-emitting quantum cascade lasers on metamorphic buffer layers

Quantum cascade lasers (QCLs) that employ metamorphic buffer layers as substrates of variable lattice constant have been designed for emission in the 3.0- to 3.5-μm wavelength range. Theoretical analysis of the active-region (AR) energy band structure, while using an 8-band k•p model, reveals that one can achieve both effective carrier-leakage suppression as well as fast carrier extraction in QCL structures of relatively low strain. Significantly lower indium-content quantum wells (QWs) can be employed for the AR compared to QWs employed for conventional short-wavelength QCL structures grown on InP, which, in turn, is expected to eliminate carrier leakage to indirect-gap valleys (X, L). An analysis of thermo-optical characteristics for the complete device design indicates that high-Al-content AlInAs cladding layers are more effective for both optical confinement and thermal dissipation than InGaP cladding layers. An electroluminescence-spectrum full-width half-maximum linewidth of 54.6 meV is estimated from interface roughness scattering and, by considering both inelastic and elastic scattering, the threshold-current density for 3.39-μm-emitting, 3-mm-long back-facet-coated QCLs is projected to be 1.40 kA/cm2.

Field-induced reentrant magnetoelectric phase in LiNiPO4

Using pulsed magnetic fields up to $30\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ we have measured the bulk magnetization and electrical polarization of ${\mathrm{LiNiPO}}_{4}$ and have studied its magnetic structure by time-of-flight neutron Laue diffraction. Our data establish the existence of a reentrant magnetoelectric phase between $19\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ and $21\phantom{\rule{0.28em}{0ex}}\mathrm{T}$. We show that a magnetized version of the zero field commensurate structure explains the magnetoelectric response quantitatively. The stability of this structure suggests a field-dependent spin anisotropy. Above $21\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, a magnetoelectrically inactive, short-wavelength incommensurate structure is identified. Our results demonstrate the combination of pulsed fields with epithermal neutron Laue diffraction as a powerful method to probe even complex phase diagrams in strong magnetic fields.