Emergent Angle Research Articles

Extensional fault geometry and evolution within rifted margin hyper-extended continental crust leading to mantle exhumation and allochthon formation

Abstract. Seismic reflection interpretation at magma-poor rifted margins shows that crustal thinning within the hyper-extended domain occurs by in-sequence oceanward extensional faulting which terminates in a sub-horizontal reflector in the topmost mantle immediately beneath tilted crustal fault blocks. This sub-horizontal reflector is interpreted to be a detachment surface that develops sequentially with oceanward in-sequence crustal faulting. We investigate the geometry and evolution of active and inactive extensional faulting due to flexural isostatic rotation during magma-poor margin hyper-extension using a recursive adaptation of the rolling-hinge model of Buck (1988) and compare modelling results with published seismic interpretation. In the case of progressive in-sequence faulting, we show that sub-horizontal reflectors imaged on published seismic reflection profiles can be generated by the flexural isostatic rotation of faults with initially high-angle geometry. Our modelling supports the hypothesis of Lymer et al. (2019) that the S reflector on the Galician margin is a sub-horizontal detachment generated by the in-sequence incremental addition of the isostatically rotated soles of block-bounding extensional faults. Flexural isostatic rotation produces shallowing of emergent fault angles, fault locking, and the development of new high-angle shortcut fault segments within the hanging wall. This results in the transfer and isostatic rotation of triangular pieces of hanging wall onto exhumed fault footwall, forming extensional allochthons which our modelling predicts are typically limited to a few kilometres in lateral extent and thickness. The initial geometry of basement extensional faults is a long-standing question. Our modelling results show that a sequence of extensional listric or planar faults with otherwise identical tectonic parameters produce very similar seabed bathymetric relief but distinct Moho and allochthon shapes. Our preferred interpretation of our modelling results and seismic observations is that faults are initially planar in geometry but are isostatically rotated and coalesce at depth to form the seismically observed sub-horizontal detachment in the topmost mantle. In-sequence extensional faulting of hyper-extended continental crust results in a smooth bathymetric transition from thinned continental crust to exhumed mantle. In contrast, out-of-sequence faulting results in a transition to exhumed mantle with bathymetric relief.

Distribution of emergent aquatic vegetation leaf inclination angle and its simulation

Leaf normal inclination angle distribution (LAD) determines the interception of radiation by leaves and the transmission of radiation by the vegetation canopy, which affects the distribution of incident photosynthetically active radiation on plant leaves and ultimately directly affects the net primary productivity of vegetation. However, the contact measurement method for leaf inclination angle is difficult to perform due to the complex growing environment of wetland aquatic vegetation. In this study, the method of measuring normal leaf inclination angles was improved for wetland aquatic vegetation based on horizontal digital photography combined with the application of a protractor. Phragmites australis and Typha orientalis, two typical emergent aquatic vegetation (EAV) species growing in wetlands, were used to evaluate the robustness of leaf inclination measurement methods. In addition, LAD values were measured and analyzed for their variations across seasonal and vertical gradients within the canopy. Finally, the applicability of the two simulation algorithms for leaf normal inclination was evaluated and compared. Analysis revealed that Phragmites australis LAD exhibited a variation with seasonal changes and vertical canopy height, which is beneficial for light transmission within the canopy. The LAD of Typha orientalis was always an erectophile pattern and changed only slightly with the seasons. The new findings provided additional insights and evidence on aquatic plant resistance and environmental adaptation. Comparing simulation models for EAV, the trigonometric algorithm outperforms other algorithms in accuracy and deviation. The results of this research provide a valuable reference for exploring radiation transfer within aquatic vegetation canopies and understanding vegetation growth.

Directional modulated light-emitting technology based on airborne display

ABSTRACT High-quality airborne display benefits pilots of making correct interpretation and operation in time, and avoids from moving body due to the unclear transmission of flight data which may lead to flying risk. A directional modulated light-emitting technology based on airborne display is proposed in this paper which can be used on Diamond DA42 New Generation aircraft. Using the proposed technology, a composite optical film with micro-structure is designed which can realise the redistribution of light intensity according to the viewing demand. A collimated display is introduced to reduce the colour shift. When the optical film is attached on a collimated display, the incident light of small viewing angles can be directionally modulated and concentrated to the large viewing angles which are just effective pilot and co-pilot viewing angles. The calculation results show that the brightness of airborne display on two pilots’ viewing angles can be increased by 38.9% and 58.7% respectively after using the proposed technology. Meanwhile, the emergent angles from the collimated display are smaller than both two pilots’ viewing angles, so the proposed optical film is beneficial to reduce the colour shift and resulting in a better display quality.

Laboratory photometry of regolith analogues: Effect of porosity-II

Context:Numerous minor bodies of our solar system are covered by loosely bound dust particles; these layers are called regolith. Light scattered by regolith surfaces is a function of their bulk porosity, of the sizes, shapes, structures and compositions of the constituent particles. Aims:To increase our data base, the present work is an extension of our previous work by Kar et al. (2016), where we reported light scattering data for regolith surfaces with different porosities, sizes, and composition of the particles (from very low to moderate absorption). The new samples have larger particles with moderate to high absorption, three originate from industry, and three are natural (among them a mixture of two previously studied samples). Methods:The samples were prepared with different bulk porosities. Photometric phase curves were built for two different geometrical configurations. The light source was a He–Ne laser at 632.8 nm. The phase angle (g) covered for the first configuration is from 45 to 126° and for the second configuration it is from 45 to 108°, in steps of 9 and 4.5° respectively. We maintained incident angle (i)= emergent angle (e) for the first configuration and i=45°, while e varying for the second configuration. The experimental data were fitted successfully to the semi-empirical model proposed by Hapke (2008) and interpreted in terms of the porosities and sizes of the particles. The albedo values obtained from the model for two samples are compared to those calculated directly from Mie theory. Results:The successful fit by the model is confirmed just as the increase of reflectivity with the decrease of porosities for a given composition and particle size. Further, it was observed that reflectivity increases with the decrease in particle size for a given composition.Finally, we have tested that for corundum and silicon carbide samples with a 22μm and 15μm particle sizes ( respectively), the best fitted albedo (ω) values that can be obtained from Hapke model, match very well with those calculated directly by Mie Theory. This also re-validated our approach adopted in the present work.

Optical design of a new folding scanning system in MEMS-based lidar

In order to solve the two disadvantages, serious divergence and limited scanning angle, of the traditional optical angle expansion in MEMS-based scanning system, a new method is proposed, i.e. an f-theta lens group and a wide-angle lens group are added into the traditional one. Principle of the optical angle expansion is discussed, while the relationship between emergent angle and incident angles has been given. Employing the optical design software ZEMAX, a folding scanning optical system based on MEMS mirror is designed thereafter. The folding scanning optical system we designed can not only scan in a large angle field, which is greater than 60°, but also obtain small spot size at distance of about 160 m, where the spot size is less than 10 cm. This folding scanning optical system has a significant application value in 3D imaging lidar system.

P- and S-mode separation of three-component VSP data

ABSTRACTIn an offset vertical seismic profile (VSP), elastic wavefields are recorded in three orthogonal components. For a given wave mode P, SV or SH, the amplitude is distributed among the three components because observations close to the target layer enable numerous seismic waves with large emergent angles to be recorded. Therefore, in VSP P and S mode separation, it is necessary to extract the same wave mode from every component and restore its true amplitude, rather than simply removing the P-wave projection from the horizontal component or the S-wave projection from the vertical component. Thus, we investigate a two-step method of VSP wave mode separation in which it is assumed that the P and SV (or SH) modes are not polarised orthogonally. First, the Z- and R-components are rotated into the nonorthogonal polarisation directions of the PV- and SV-waves. Here, the PV-wave is the P-mode projection onto the vertical–radial plane. The PV- and T-components are rotated into the nonorthogonal polarisation directions of the P- and SH-waves. To derive true polarisation directions for each component rotation, we employ a least-squares approach utilising the wave particle motions in different quadrants, which can determine the polarisation direction even under noisy conditions. Synthetic data test and application to VSP data from a coalfield reveal the stability and effectiveness of our method.

Advanced three‐dimensional seismic imaging of deep supercritical geothermal rocks in Southern Tuscany

ABSTRACTThe seismic K‐Horizon is the key to gaining understanding on the deep supercritical geothermal rocks in Southern Tuscany. The K‐Horizon is hosted in metamorphic rocks, which cause strong seismic wavefield scattering resulting in a poor signal‐to‐noise ratio. Our study aims to reveal high‐resolution seismic images of the K‐Horizon below a geothermal field in Southern Tuscany, using an advanced three‐dimensional seismic depth imaging approach. The key seismic pre‐processing steps in the time domain include muting a large amount of persistent noise based on the statistical analysis of the seismic amplitudes, and tomostatics technique to correct for static effects. We carried out seismic depth imaging using Kirchhoff Pre‐Stack Depth Migration and Fresnel Volume Migration techniques. Each migration technique was tested with constant and heterogeneous three‐dimensional velocity models. Due to the difficulties in determining emergent angles for this low signal‐to‐noise ratio data set, the migration results with the heterogeneous three‐dimensional velocity model show less coherent reflections compared to the migration results using the constant velocity model. Both velocity models however lead to relatively the same structure and depth of the K‐Horizon, indicating the similarity of the average velocities along the wave propagation paths in both velocity models. With both velocity models Fresnel Volume Migration yields the K‐Horizon with better reflection coherency and higher signal‐to‐noise ratio than standard Kirchhoff Pre‐Stack Depth Migration. Nevertheless, both migration techniques have been able to reveal the K‐Horizon with relatively high resolution and provide a reliable basis for geothermal rock characterization as well as steering of the first geothermal well penetrating the K‐Horizon.

Apparent magnitude calculation method for complex shaped space objects

The apparent magnitude of a complex shaped space object is difficult to calculate since its visible triangular facets are hard to be determined. The most common-used visible facets determination method is to judge the incident and emergent angle, but this method does not apply to a space object which is a combination of convex bodies or has a concave surface. In this paper, a new quadtree-based hidden vertex removal method is presented, and it is composed of four steps: coordinate transformation, hidden vertex removal, quadtree decompositions and visible triangular facets determination. This method can determine visible triangular facets of a space object of any shapes. Once a space object triangular facet model, the observed direction of a telescope and the incident direction of the sunlight are given, the visible triangular facets of the space object can be obtained. Two experiments are carried out to verify the visible triangular facet determination performance of this method qualitatively and quantitatively. Experiment results show the visible triangular facet calculated by the proposed method is accurate and reasonable.

Ray tracing of turning wave in elliptically anisotropic media with an irregular surface

Seismic ray tracing in anisotropic media with irregular surface is crucial for the exploration of the fine crustal structure. Elliptically anisotropic medium is the type of anisotropic media with only four independent elastic parameters. Usually, this medium can be described by only the vertical phase velocity and the horizontal phase velocity for seismic wave propagation. Model parameterization in this study is described by flexible triangular grids, which is beneficial for the description of irregular surface with high degree of approximation. Both the vertical and horizontal phase velocities are defined in the triangular grids, respectively, which are used for the description of phase velocity distribution everywhere in the model by linear interpolation. We develop a shooting ray tracing method of turning wave in the elliptically anisotropic media with irregular surface. Runge-Kutta method is applied to solve the partial differential equation of seismic ray in elliptically anisotropic media. Linearly modified method is used for adjusting emergent phase angles in the shooting scheme. Numerical tests demonstrate that ray paths coincide well with analytical trajectories in transversely homogeneous elliptically anisotropic media. Seismic ray tracing results in transversely inhomogeneous elliptically anisotropic media demonstrate that our method is effective for further first-arrival tomography in elliptically anisotropic media with an irregular surface.

Improved structural characterization of the Earth's crust at the German Continental Deep Drilling Site using advanced seismic imaging techniques

AbstractThis paper describes the principles of three novel seismic imaging techniques and their application to two deep seismic reflection data sets from the vicinity of the German Continental Deep Drilling Site (KTB). These imaging techniques are based on Kirchhoff prestack depth migration and use an inherent restriction of the migration operator to focus the wavefield to its actual reflection point. For Fresnel volume migration, the emergent angle at the receivers is estimated and then used to propagate the wavefield back into the subsurface along which the Fresnel volume is determined. The migration operator is restricted to this volume, thereby focusing the image to the part of the isochrone which physically contributes to the reflection. For coherency migration, the coherency of the wavefield at neighboring traces is calculated and used as a weighting factor within the migration integral, leading to a comparable focusing to the reflection point. For coherency‐based Fresnel volume migration, both approaches are combined, resulting in an even more focused seismic image with significantly increased image quality. We applied these methods to two seismic data sets from the area around the KTB: a survey with standard split‐spread geometry (KTB8502) and a sparse data set with a small number of source points in combination with short receiver lines (INSTRUCT93). The focusing approaches yield major improvements in the final images for both data sets. Incoherent noise and migration artifacts are reduced and the visibility of crustal structures is strongly enhanced, allowing for an improved geologic and tectonic characterization.

A method for determining d–D neutron energies in a large sample

The energy of monoenergetic neutrons generated by the D(d,n)3He reaction was determined as a function of emergent angle and incidence energy of d+-beam, En(θ,Ed). Based on the geometric size of the experimental sample, position of the sample relative to the Ti–D solid or D2 gas targets, volume distribution of D2 gas targets, theoretical formulas were obtained for calculating the mean neutron energy required to irradiate a large sample. Using these formulas, the mean neutron energies of the Ti–D solid and D2 gas targets irradiating a large sample were calculated under various conditions. The results were compared to those reported in the literature. The formulas obtained in this study were found to be applicable for the determination of mean neutron energy irradiating a large sample for the Ti–D solid and D2 gas targets.

Optical simulation of phosphor layer of white LEDs

A Matlab (2009a) program was developed to simulate the photons emitted by the LED chips. The photons' transmission, reflection, and refraction in phosphor layers with different shapes were calculated in this program. The CCT was calculated at different emergent angles with different layer profiles, such as planar layer, hemispherical layer (with different diameters), half ellipsoid layer and other irregular shapes. As a consequence of optimization of angular color uniformity, the trend of configuration of the phosphor layer was discussed and analyzed.

Wavefield-separation methods for dual-sensor towed-streamer data

A dual-sensor towed streamer records the pressure and vertical component of particle motion associated with the incident wavefield that may be used to separate the wavefield into its up- and downgoing parts. This procedure requires information about the water properties (wave-propagation velocity and density) and is robust in the presence of errors in the estimation of these quantities of the magnitude likely to be encountered. In practice, the particle motion data recorded by current towed marine streamers encounter very strong mechanical noise such that, for the lowest frequencies, the wavefield separation must be approximated by deconvolving the ghost function from the pressure data. This procedure requires information about the streamer depth and is robust to small depth errors over the frequency range for which it is required for dual-sensor streamer processing, but it is much more sensitive if applied over the bandwidth necessary to deghost pressure data acquired at a conventional streamer depth. The signal-to-noise ratio can be further enhanced by recombining the up- and downgoing pressure fields at the sea surface, which has the effect of applying a ghostlike filter to noise that is recorded by only one of the two sensors. In practical marine acquisition scenarios, spatial sampling is often insufficient to yield an accurate result, especially in the crossline direction. If each streamer is processed independently assuming that the wavefield propagation is purely inline, significant errors can be introduced. For arrivals with high emergent angles, errors may also be introduced even if the wavefield propagation actually is purely inline due to incorrect treatment of spatially aliased energy. However, these effects are almost entirely confined to very shallow events. They can be mitigated by using independently derived information about the crossline propagation angle and, for data comprising predominantly forward scattered energy, appropriate application of linear moveout.

A method for determining fast neutron energies in a large sample

The energy of 14MeV monoenergetic neutron generated by the T(d,n)4He reaction as a function of the d beam emergent angle, En(θ), is considered. Based on the geometric size of the experimental sample and the position of the sample relative to the T–Ti target, a theoretical formula for calculating the mean neutron energy irradiating a large sample is obtained. This formula is related to the distance L between the sample and the T–Ti target, to the emergent angle β and the radius of the sample R. Using the Zr/Nb method, a fitting formula for the mean neutron energy is derived based on the evaluated values of the 90Zr(n,2n)89m+gZr and 93Nb(n,2n)92mNb reaction cross-sections. Combining this formula with the measured σ(Zr)/σ(Nb) ratio, the mean neutron energy irradiating a large sample was obtained. The theoretical calculations and the experimental measurements show that our formula can be used to determine the mean neutron energy irradiating on a large sample.

Fresnel volume migration of single-component seismic data

Standard implementations of Kirchhoff prestack depth migration (PSDM) distribute the recorded wavefield along two-way-traveltime isochrons and an image is generated by constructive interference of these isochrons along the actual reflector elements. Beside the recent developments of wave-equation-based approaches, Kirchhoff PSDM is still considered widely as a state-of-the-art technique in obtaining high-quality images of the subsurface, particularly for highly irregular survey layouts and target-oriented imaging tasks. However, for sparse sampling or limited aperture, the resulting image is affected by significant migration noise as a result of limited constructive interference of the back-propagated wavefield. Some modifications have been proposed to reduce these artifacts. These modifications include constructing a specular path of wave propagation, derived from estimates of the emergent angle of coherent phases in the seismogram section, and the mainly heuristic restriction of the imaging operator to the neighborhood of that wavepath. Our approach uses Fresnel volumes to restrict the migration operator in a physically frequency-dependent way. Using the emergent angle at the receiver, determined by a local slowness analysis, a ray is propagated into the subsurface; the back-propagation of the wavefield is restricted to the vicinity of this ray according to its approximated Fresnel volume. This so-called Fresnel volume migration approach enhances image quality significantly compared with standard Kirchhoff PSDM because of the inherent focusing and the restriction of the back-propagation to the region around the actual reflection point.

Diffraction imaging by multifocusing

Correct identification of geologic discontinuities, such as faults, pinch-outs, and small-size scattering objects, is a primary challenge of the seismic method. Seismic response from these objects is encoded in diffractions. Our method images local heterogeneities of the subsurface using diffracted seismic events. The method is based on coherent summation of diffracted waves arising in media that include interface discontinuities and local velocity heterogeneities. This is done using a correlation procedure that coherently focuses diffraction energy on a seismic section by flattening diffraction events using a new local-time-correction formula to parameterize diffraction traveltime curves. This time correction, which is based on the multifocusing method, depends on two parameters: the emergent angle and the radius of curvature of the diffracted wavefront. These parameters are estimated directly from prestack seismic traces. The diffraction multifocusing stack (DMFS) can separate diffracted and reflected energy on a stacked section by focusing diffractions to the diffraction location and defocusing the reflection energy over a large area.

Bidirectional reflectance of rough particulate media: Ray-tracing solution

We investigate the volume and surface shadowing in particulate porous random media with macroscale surface roughness over the full scattering hemisphere using Monte-Carlo ray tracing. We also study the volume and rough-surface effects using two diffuse bidirectional reflectance models, namely, the Lambertian and Lommel–Seeliger models. First, we examine the volume effects arising from the porosity of the loosely packed particulate medium. Next, we consider additional shadowing due to the macroscale surface roughness modelled as two-dimensional random fields following Gaussian correlation and fractional-Brownian-motion statistics. We pay special attention to the relative difference of the reflectance as a function of angles of incidence and emergence from a particulate medium with and without macroscale roughness, since it is the major factor when considering what information can safely be deduced from measured reflectance data. The main conclusions of our study concern the distribution of the reflectance over the scattering hemisphere. The rough-surface shadowing due to macroscale roughness has a relatively small effect on the total reflectance of porous media for small emergent azimuth angles, but the importance of rough-surface shadowing increases for large angles. Also, when compared to a solid rough surface which shows a single extended brightness maximum at the opposition observation geometry, a particulate surface can have two separated maxima along the zero azimuth angle. Thus, observations at the azimuth angle of 180 ∘ and large emergence angle, and observations sampling well the azimuth angle for large values of incidence angle would seem to give optimal data when considering the inversion of the properties affecting the shadowing of regolith-covered surfaces.

Dependence of secondary-electron emission on the emergent angle of frozen-chargedH0andH+projectiles penetrating a thin carbon foil

The statistical distributions of the number of secondary electrons (SEs) emitted per projectile from a thin carbon foil under irradiation with the frozen-charged ${\mathrm{H}}^{0}$ and ${\mathrm{H}}^{+}$ projectiles of $2.5--3.5\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ have been measured as a function of the emergent angle of projectiles in the range from $0.0\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.0\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}$ in every $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}$ step. The measurement of SEs was carried out in the forward and the backward directions of the incident beam independently. From $0.0\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}$ to $\ensuremath{\sim}1.0\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}$, the SE yields, that is, the mean number of emitted electrons per incident ion, increase with increasing emergent angle and tend to be saturated at larger angles. This is a common trend, regardless of incident energies and charge states of a projectile and also of emitted directions of SEs. Quantitatively, the saturated SE yields at larger angles for the ${\mathrm{H}}^{+}$ penetration are about 40--50 % larger than those at $0.0\phantom{\rule{0.3em}{0ex}}\mathrm{mrad}$. On the other hand, the corresponding relative increase for the ${\mathrm{H}}^{0}$ penetration reaches as high as $\ensuremath{\sim}100%$ or more. The observed proton-hydrogen difference is well reproduced in the calculated energy losses by a Monte Carlo simulation including the impact parameter dependent stopping cross sections in a single collision of a hydrogen or a proton with a carbon atom.

Topographic corrections to slowness measurements in parsimonious migration of land data

Parsimonious migration requires that incident and emergent angles be measured, via apparent slownesses, from the seismic data being migrated. When slownesses are measured from land data, it is the apparent slowness along the topography, not the horizontal slowness that is being measured. Thus, errors are introduced into the incident and emergent angle estimates, which are defined via horizontal slownesses. These errors can be corrected using the local topographic dips. A 2D field data example shows that, after correction, a migrated image has significantly improved coherency.

Dependence of secondary electron emission on the emergent angle of 1 MeV/u H+, He2+ and Li3+ penetrating a thin carbon foil

With 1MeV/u H+, He2+ and Li3+ beams incident on a carbon foil of 2.2μg/cm2 in thickness, the statistical distributions of the number of simultaneously emitted secondary electrons (SE’s) have been measured as a function of the emergent angle of foil-transmitted ions in the range from 0.0mrad to 4.0mrad for every 0.5mrad step. The measurement of SE’s was carried out in the forward and backward directions of the incident beam independently. The SE yields, that is, the mean number of emitted electrons per incident ion, increase with increasing the emergent angle and saturate at larger angles. This trend is common to the projectiles used and also to the forward and backward directions. As a result of a Monte Carlo simulation taking account of the impact parameter dependent energy loss in a single collision of a proton with a carbon atom, it is found that the calculated energy losses exhibit an emergent angular dependence quite similar to that of the measured SE yields. However the measured SE yields divided by the square of the projectile atomic number, ZP, becomes smaller with increasing ZP, especially for the backward emission. A similar observation has been reported by other authors and this trend seems to originate from the preferencial fast internal SE production by heavier projectiles followed by their escape from a thin target foil.