Steep Surface Slopes Research Articles

Frequency selective illumination for high aperture coherence scanning interferometry

Abstract Coherence scanning interferometry is a widely used optical topography measurement technique, which can achieve axial resolutions in the sub-nanometer regime. Nevertheless, in the lateral dimension it is inherently diffraction limited and multiple problems arise when approaching this limit. Especially for challenging surface topographies like steep slopes or small grating periods measurement artifacts start to cause massive deviations in the 3D reconstruction of the surface due to the increased influence of noise on increasingly weak signals. In this study we present an illumination approach for Linnik-type CSI, which highlights oblique incident angles of the illuminating light cone in high numerical aperture (0.95) systems. It is demonstrated, that this approach can significantly improve the signal-to-noise ratio for measurements at steep surface slopes and therefore increase the subsequent surface reconstruction.

Interferometric metrology probe for highly sloped freeform optics.

A wide variety of systems are employed to measure the surface profile of aspheric and freeform optical surfaces. Freeform metrology systems must accurately characterize the full surface under test, which can be difficult with steep surface slopes. Here we present an interferometric surface metrology probe for highly sloped aspheric and freeform optical surfaces. The optical design of this probe allows the measurement of surface slopes up to 50deg without tilting the probe, which simplifies stage design and increases the accuracy of the system. A spectrally controlled light source is used to create a virtual ball in front of the probe tip to measure the surface distance and angle. This system produces a cloud of points, to which Zernike polynomials are fit and used to reconstruct the surface. We will show sensitivity tests and accuracy results.

Observational constraints on the sensitivity of two calving glaciers to external forcings

AbstractFuture mass loss projections of the Greenland ice sheet require understanding of the processes at a glacier terminus, especially of iceberg calving. We present detailed and high-rate terrestrial radar interferometer observations of Eqip Sermia and Bowdoin Glacier, two outlet glaciers in Greenland with comparable dimensions and investigate iceberg calving, surface elevation, velocity, strain rates and their links to air temperature, tides and topography. The results reveal that the two glaciers exhibit very different flow and calving behaviour on different timescales. Ice flow driven by a steep surface slope with several topographic steps leads to high velocities, areas of extension and intense crevassing, which triggers frequent but small calving events independent of local velocity gradients. In contrast, ice flow under smooth surface slopes leaves the ice relatively intact, such that sporadic large-scale calving events dominate, which initiate in areas with high shearing. Flow acceleration caused by enhanced meltwater input and tidal velocity variations were observed for terminus sections close to floatation. Firmly grounded terminus sections showed no tidal signal and a weak short-term reaction to air temperature. These results demonstrate reaction timescales to external forcings from hours to months, which are, however, strongly dependent on local terminus geometry.

Engineering application of submerged water jets for sediment removal in a tidal riverbed

Sediment deposition problems have attracted the interest of engineers and researchers. Several experimental studies have been conducted on scour depth using turbulent jets. However, field observation and monitoring have rarely been reported. This study aimed to eliminate sediments on a tidal riverbed using a prototype device, which consisted of a set of submerged vertical water nozzles and submerged horizontal air nozzles. The effectiveness of the water jet in sediment removal during spring and neap tides was evaluated. The quantitative relationships of dimensionless parameters, such as (1) the relative sediment scour volume versus the number of flows from the jet exit, (2) the relative sediment scour volume versus the relative scour depth, and (3) the relative scour size versus the relative jet intensity, were analyzed. The results showed that the freshwater flowing to the sea affected the sediment scour volume during the falling cycle of spring tides. In contrast, the rising cycle of spring tides retarded the freshwater flow, resulting in a decrease in the sediment scour volume. A steep water surface slope accelerated the river flow and further influenced the cross-flow current around the study area. As a result, a highly diffusive turbulent flow was produced, causing suspended sediments to be rapidly removed from the scour hole center. An increase in the number of flows from the jets led to intensified diffusion of turbulent energy into the flow. The rapidly varying water depth caused jet energy to be dissipated before approaching the riverbed, and it significantly affected the scour process during the spring-tide period. The proposed equations can be used to estimate the scour volume, scour size, and re-suspended sediments in tidal rivers within defined ranges of parameters.

Blue-ice moraines formation in the Heritage Range, West Antarctica: Implications for ice sheet history and climate reconstruction

Blue ice is found in areas of Antarctica where katabatic winds, focussed by steep surface slopes or by topography around nunataks, cause enhanced surface ablation. This process draws up deeper, older ice to the ice sheet surface, often bringing with it englacial sediment. Prevailing theories for dynamically stable moraine surfaces in East Antarctica suggest that: (i) it is this material, once concentrated, that forms blue-ice moraines (BIM), (ii) that the moraine formation can be dated using cosmogenic isotope approaches, and that, (iii) since we expect an increase in exposure age moving away from the ice margin towards bedrock, dating across the moraine can be used to constrain ice-sheet history. To test this lateral accretion model for BIM formation we visited Patriot, Marble and Independence Hills in the southern Heritage Range, West Antarctica. Detailed field surveys of surface form, sediment and moraine dynamics were combined with geophysical surveys of the englacial structure of the moraines and cosmogenic nuclide analysis of surface clasts. Results suggest sediment is supplied mainly by basal entrainment, supplemented by debris-covered valley glaciers transferring material onto the ice sheet surface, direct deposition from rock-fall and slope processes from nunataks. We find that once sediment coalesces in BIM, significant reworking occurs through differential ablation, slope and periglacial processes. We bring these processes together in a conceptual model, concluding that many BIM in West Antarctica are dynamic and, whilst they persist through glacial cycles, they do not always neatly record ice sheet retreat patterns since linear distance from the ice margin does not always relate to increased clast exposure age. Understanding the dynamic processes involved in moraine formation is critical to the effective interpretation of the typically large scatter of cosmogenic nuclide exposure ages, opening a deep window into the million-year history of the West Antarctic Ice Sheet.

The sublimative torques of Jupiter Family Comets and mass wasting events on their nuclei

Sublimative outgassing of comets produces torques that alter the rotation state of their nuclei. Recently, parameterized sublimative torque models have been developed to study rotation state changes of individual comet nuclei and populations of cometary bodies. However, these models simplify the interactions between the escaping gas and cometary surface into only a few parameters that hide the details of these complex interactions. Here we directly compare the X-parameter model (Samarasinha and Mueller, 2013) with the SYORP model (Steckloff and Jacobson, 2016) to tease out insights into the details of the gas-surface interactions driving sublimative torques. We find that, for both of these models to accurately model sublimative torques, the number of sublimating molecules that contribute to the net torque is largely independent of the detailed shape and activity of the nucleus, but rather depends primarily on the size of the nucleus and the effective heliocentric distance of the comet. We suggest that cometary activity must be largely restricted to regions of steep gravitational surface slopes (above the angle of repose), where mass wasting can refresh activity by shedding mantles of refractory materials and exposing fresh volatiles. We propose a new classification scheme for comets based on the frequency of this mass-wasting process (relative to the timescale of activity fading): quasi-equilibrium, episodic, quasi-dormant, and extinct.

Multimode Coherent Pattern in Bistatic Scattering From Randomly Corrugated Surfaces With Irregular Grooves at L-Band

This paper investigates the bistatic scattering from randomly corrugated surfaces with irregular grooves. For a given incident wave, the surface fields and the scattered field were computed by the method of moments in which the rooftop basis function was used to account for fast phase changes due to steep surface slopes. The total scattered field is decomposed into coherent and incoherent components to analyze their respective contributions. We found that, for randomly corrugated surfaces with irregular ridges or grooves, the coherent scattering is profound at several scattering angles with strong main lobes, whose beamwidths are strongly correlated with the ridge density. The numerical simulation has shown that the lobe angular shift away from the specular direction is quasi-linearly dependent on the ridge density, suggesting that the coherent scattering pattern substantially contains the surface geometric information. We expect this paper to offer deeper understanding of coherent imaging of rough surface and to help in designing a novel imaging system.

Sandbox modelling of sequential thrusting in a mechanically two-layered system and its implications in fold-and-thrust belts

Many fold-and-thrust belts display multi-storied thrust sequences, characterizing a composite architecture of the thrust wedges. Despite dramatic progress in sandbox modelling over the last three decades, our understanding of such composite thrust-wedge mechanics is limited and demands a re-visit to the problem of sequential thrusting in mechanically layered systems. This study offers a new approach to sandbox modelling, designed with a two-layered sandpack simulating a mechanically weak Coulomb layer, resting coherently upon a stronger Coulomb layer. Our experimental models reproduce strikingly similar styles of the multi-storied frontal thrust sequences observed in natural fold-and- thrust belts. The upper weak horizon undergoes sequential thrusting at a high spatial frequency, forming numerous, closely spaced frontal thrusts, whereas the lower strong horizon produces widely spaced thrusts with progressive horizontal shortening. This contrasting thrust progression behaviour gives rise to composite thrust architecture in the layered sandpack. We show the evolution of such composite thrust sequences as a function of frictional strength (μb) at the basal detachment and thickness ratio (Tr) between the weak and strong layers. For any given values of Tr and μb, the two thrust sequences progress at different rates; the closely-spaced, upper thrust sequence advances forelandward at a faster rate than the widely-spaced, lower thrust sequence. Basal friction (μb) has little effects on the vergence of thrusts in the upper weak layer; they verge always towards foreland, irrespective of Tr values. But, the lower strong layer develops back-vergent thrusts when μb is low (∼0.36). In our experiments, closely spaced thrusts in the upper sequence experience intense reactivation due to their interaction with widely spaced thrusts in the lower sequence. The interaction eventually affects the wedge topography, leading to two distinct parts: inner and outer wedges, characterised by steep and gentle surface slopes, respectively.

Deformation structures in the frontal prism near the Japan Trench: Insights from sandbox models

We have used sandbox experiments to explore the mechanics of the frontal prism structures documented by seismic reflection data and new borehole from IODP Expedition 343 (JFAST). This study investigated the effects of down-dip (normal to trench axis) variations in frictional resistance along a decollement on the structural development of the frontal wedges near subduction zones. Interpretation of seismic reflection images indicates that the wedge has been effected by trench-parallel horst-and-graben structures in the subducting plate. We performed sandbox experiments with down-dip patches of relatively high and low friction on the basal decollement to simulate the effect of variable coupling over subducting oceanic plate topography. Our experiments verify that high frictional resistance on the basal fault can produce the internal deformation and fault-and-fold structures observed in the frontal wedge by the JFAST expedition. Subduction of patches of varying friction caused a temporal change in the style of internal deformation within the wedge and gave rise to two distinctive structural domains, separated by a break in the surface slope of the wedge: (i) complexly deformed inner wedge with steep surface slope and (ii) shallow taper outer wedge, with a sequence of imbricate thrusts. Our experiments further demonstrate that the topographic slope-break in the wedge develops when the hinterland part of the wedge essentially stops deforming internally, leading to in-sequence thrusting with the formation of an outer wedge with low taper angle. For a series of alternate high and low frictional conditions on the basal fault the slope of the wedge varies temporally between a topographic slope-break and uniformly sloping wedge.

Phase structure and surface morphology evolution of Al–Ti–O films irradiated by electron beam

Al–Ti–O films were prepared on Si substrates by reactive magnetron sputtering technology. Then the as-deposited and annealed films were treated by electron beam irradiation. The phase structure and surface morphology of the films were investigated by scanning electron microscopy and atomic force microscopy. Especially, height–height correlation function measurement was introduced to quantitatively characterize the film surface evolution. The results show that both electron irradiation and annealing induce well-crystallization of as-deposited films, while the irradiation leads to the phase change of annealed films. In contrast to those of as-deposited films, the surface morphologies of annealed films exhibits roughening characteristic and steep local surface slope due to the formation of new phases and the preferred grain growth. The electron irradiation can result in a rougher surface due to the irradiation-induced structural damage.

Geomorphological analysis of the drainage system on the growing Makran accretionary wedge

The morphology of six adjacent major catchments draining the onshore Makran accretionary wedge in southeast Iran and southwest Pakistan was studied to examine how the channel pattern and the length profiles may reflect the recent and active growth of the wedge. Qualitative field surveys were combined with the quantitative analysis of channel steepness and concavity measured from digital elevation models. These profiles were compared with modelled profiles using a stream power approach assuming homogeneously uplifting, uniform rock substratum. Results show a distinct difference between the studied western and eastern catchments. The three western rivers are in morphological equilibrium; whereas the two eastern rivers exhibit profiles with prominent convexities and knickpoints, thus notably diverging from equilibrium concave-up shapes. All the studied catchments share the same base level, flow on similar lithologies, and developed under uniform climate conditions. Therefore, we interpret the morphometric differences in terms of differential rock uplift rate as a response to local tectonic activity. This interpretation is consistent with both uplift rates of marine terraces along the coast of Makran and the recorded seismicity. The geomorphological analysis presented here extends coastal information to wide inland areas and documents longer term tectonic behaviour than the seismotectonic record. Hence, the steeper surface slope and faster surface uplift rates recorded by the eastern catchments compared to equilibrium of the western catchments are regional, long-term signals relevant for a wide part of the Makran accretionary wedge. We attribute the regional geomorphic difference to Quaternary variations in tectonic regimes that forced differential uplift rates of the wedge surface. We hypothesize that the different tectonic regimes are related to different subduction rates.

Dynamics and retreat of the Late Weichselian Kongsfjorden ice stream, NW Svalbard

Terrestrial cosmogenic nuclide dating in combination with detailed landform mapping in the Kongsfjordhallet area, NW Svalbard, have provided new insight on configuration, dynamics, and deglaciation of the Late Weichselian Kongsfjorden ice stream. The minimum Late Weichselian ice surface elevation in Kongsfjorden was >449 m a.s.l. indicating considerably thicker ice and a steeper surface gradient than earlier suggested. For the adjacent inter-ice stream area an even steeper surface slope is reconstructed. The glacial landforms, as well as the surface exposure ages of erratic boulders at different elevations, suggest a gradual lowering of the ice surface. Deglaciation of the higher elevations was probably underway by 20.0 ka. At ca 16.6 ka a large moraine complex (‘the Kongsfjorden moraine’) was deposited close to the fjord mouth. The shape of the moraines, the steep ice surface gradient, as well as the correlation to fine laminated clay lacking ice rafted debris deposited in the trough beyond the moraine suggest that ice dynamics switched from ice-stream behaviour to a slower flowing outlet (tidewater) glacier. A Younger Dryas or Early Holocene advance of local valley glaciers is shown by moraine lobes cross-cutting the Late Weichselian lateral moraines.

Biogeochemical Processes in the Soil-Groundwater System of a Forest-Peatland Complex, North Coast British Columbia, Canada

Abstract There is a limited understanding of the distribution and transport of solutes in the coastal forest-peatland systems of northern British Columbia, Canada. The purpose of this research was to evaluate the sources, concentration and flux of major ions and dissolved organic carbon (DOC) in the soil-groundwater system of a forest-peatland complex in the coastal western hemlock (CWH) forest zone. DOC dominated the groundwater flux, with most of it exported during high flow conditions. Inorganic ion concentration increased and DOC decreased with ground surface slope and community type from open peatland to bog woodland, bog forest, swamp forest and upland forest. Base cations, HC03 - and DOC in the peatland forest communities differed significantly (P < 0.05) from concentrations in the upland forest. The greatest concentrations of ions derived from mineral weathering were found in the upland forest where the absence of peat, deep mineral soil (85-465 cm) and steep surface slope (26%) combined to increa...

Lee slope sediment processes leading to avalanche initiation on an aeolian dune

In order to detail the governing conditions through which a slipface matures to the point of failure, dry sand avalanches were observed in the Dune Simulation Wind Tunnel on a 1:1 replica transverse dune with a crest height of approximately 1.2 m. Areal distributions of grainfall and reptation were measured using traps. Changes in the slipface elevation were observed using 3‐D laser scanning with a vertical accuracy of 0.096 mm for approximately every 1 mm2 of surface area. Grainfall decayed exponentially from the brink with a constant rate across all wind velocities. Reptation removed sediment from areas close to the brink and deposited it downslope, creating low amplitude, cross‐slope ripples on the slipface. A critical length scale separating grainscale and bulk sediment behavior is identified, and it defines the lower limit to the validity of angle of repose measurements. Avalanche initiation occurred in an area of steep surface slope below a sediment bulge, with distance from the brink independent of wind velocity. The time between avalanches was found to be constant for constant wind velocity.

Influence of persistent wind scour on the surface mass balance of Antarctica

Accurate quantification of surface snow accumulation over Antarctica is a key constraint for estimates of the Antarctic mass balance, as well as climatic interpretations of ice-core records. Over Antarctica, near-surface winds accelerate down relatively steep surface slopes, eroding and sublimating the snow. This wind scour results in numerous localized regions (< or = 200 sq km) with reduced surface accumulation. Estimates of Antarctic surface mass balance rely on sparse point measurements or coarse atmospheric models that do not capture these local processes, and overestimate the net mass input in wind-scour zones. Here we combine airborne radar observations of unconformable stratigraphic layers with lidar-derived surface roughness measurements to identify extensive wind-scour zones over Dome A, in the interior of East Antarctica. The scour zones are persistent because they are controlled by bedrock topography. On the basis of our Dome A observations, we develop an empirical model to predict wind-scour zones across the Antarctic continent and find that these zones are predominantly located in East Antarctica. We estimate that approx. 2.7-6.6% of the surface area of Antarctica has persistent negative net accumulation due to wind scour, which suggests that, across the continent, the snow mass input is overestimated by 11-36.5 Gt /yr in present surface-mass-balance calculations.

Topographic optical profilometry by absorption in liquids

Optical absorbance within a liquid is used as a photometric probe to measure the topography of optical surfaces relative to a reference. The liquid fills the gap between the reference surface and the measuring surface. By comparing two transmission images at different wavelengths we can profile the height distribution in a simple and reliable way. The presented method handles steep surface slopes (<90°) without difficulty. It adapts well to any field of view and height range (peak to valley). A height resolution in the order of the nanometer may be achieved and the height range can be tailored by adapting the concentration of water soluble dyes. It is especially appropriate for 3D profiling of transparent complex optical surfaces, like those found in micro-optic arrays and for Fresnel, aspheric or free-form lenses, which are very difficult to measure by other optical methods. We show some experimental results to validate its capabilities as a metrological tool and handling of steep surface slopes.

Large‐scale deformation in the India‐Asia collision constrained by earthquakes and topography

AbstractAnalysis of earthquake focal mechanisms allows division of the India‐Asia collision into kinematic domains that strongly correlate with topography. These kinematic domains indicate strain partitioning dominated by oblique slip deformation. The Kunlun and south Tibetan fault systems mark discontinuities in the strain field and bound the high, flat topography of the plateau which deforms by transtension. The northern and southern margins of Tibet deform by transpression or contraction and are topographically steep. Correlations between seismicity and topography are due to Mohr–Coulomb wedge mechanics at the northern and southern plateau margins which produce naturally steep surface slopes, whereas the flat interior and eastern margin of the Tibetan Plateau is underlain by viscous crust which supports subdued topography further muted by Cenozoic basin fill. These data indicate that the long wavelength topography of the India‐Asia collision is controlled by seismically caused surface displacements which are linked to deep crustal deformation mechanics.

Transition of accretionary wedge structures around the up-dip limit of the seismogenic subduction zone

The Nankai accretionary prism is divided into three segments: outer and inner wedges and their transition zone. These wedges reflect different aspects of wedge taper, internal deformation, and basal plate boundary fault. The outer wedge is characterized by narrow critical taper, internal deformation by in-sequence-fold-and-thrust and aseismic décollement. The inner wedge represents a stable narrow taper, weakly deformed internal structure and seismogenic plate boundary fault along its base. The transition zone between the two wedges shows large critical taper with steep surface slope, internal structure of out-of-sequence thrust, and step-down of décollement onto the sediment–oceanic basement interface. The trench slope break and oceanward margin of forearc basin is located around the landward edge of this transition zone. These common aspects might be related to the lithification of both accreted and underthrust sediments and the resultant switch of the plate boundary fault. Deformation and lithification process recorded in exhumed on-land mélange of accretionary complexes suggest that the step-down of the plate boundary décollement occurs around the up-dip limit of seismogenic subduction zone.

Heparine perfusiondoes not improve mechanical recovery of intervertebral discs in vitro

Many fold-and-thrust belts display multi-storied thrust sequences, characterizing a composite architecture of the thrust wedges. Despite dramatic progress in sandbox modelling over the last three decades, our understanding of such composite thrust-wedge mechanics is limited and demands a re-visit to the problem of sequential thrusting in mechanically layered systems. This study offers a new approach to sandbox modelling, designed with a two-layered sandpack simulating a mechanically weak Coulomb layer, resting coherently upon a stronger Coulomb layer. Our experimental models reproduce strikingly similar styles of the multi-storied frontal thrust sequences observed in natural fold-and- thrust belts. The upper weak horizon undergoes sequential thrusting at a high spatial frequency, forming numerous, closely spaced frontal thrusts, whereas the lower strong horizon produces widely spaced thrusts with progressive horizontal shortening. This contrasting thrust progression behaviour gives rise to composite thrust architecture in the layered sandpack. We show the evolution of such composite thrust sequences as a function of frictional strength (μb) at the basal detachment and thickness ratio (Tr) between the weak and strong layers. For any given values of Tr and μb, the two thrust sequences progress at different rates; the closely-spaced, upper thrust sequence advances forelandward at a faster rate than the widely-spaced, lower thrust sequence. Basal friction (μb) has little effects on the vergence of thrusts in the upper weak layer; they verge always towards foreland, irrespective of Tr values. But, the lower strong layer develops back-vergent thrusts when μb is low (∼0.36). In our experiments, closely spaced thrusts in the upper sequence experience intense reactivation due to their interaction with widely spaced thrusts in the lower sequence. The interaction eventually affects the wedge topography, leading to two distinct parts: inner and outer wedges, characterised by steep and gentle surface slopes, respectively.

Of isbræ and ice streams

AbstractFast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.