scholarly journals Bounding Surfaces in a Barchan Dune: Annual Cycles of Deposition? Seasonality or Erosion by Superimposed Bedforms?

2019 ◽  
Vol 11 (8) ◽  
pp. 965 ◽  
Author(s):  
Charles Bristow
Keyword(s):  
Migration Rate ◽  
Turnover Time ◽  
Bounding Surface ◽  
Annual Cycles ◽  
Potential Control ◽  
Barchan Dune ◽  
Ground Penetrating ◽  
Dipping Strata ◽  
Bounding Surfaces ◽  
Dune Crest

A barchan dune near Tarfaya in Morocco has been surveyed using ground-penetrating radar (GPR) revealing packages of dipping strata within the dune that are truncated by bounding surfaces. The bounding surfaces dip in the downwind direction, truncate sets of cross-stratification, and are themselves downlapped by dipping strata. Models of aeolian strata suggest that the bounding surfaces could be reactivation surfaces, an erosion surface formed when a dune is reshaped by a change in wind. Alternatively, they could be superposition surfaces formed by smaller bedforms migration over the dune surface. These two hypotheses are tested using a combination of field and satellite observations. The average annual migration rate for the barchan dune derived from satellite images, gives an annual migration rate of 21.4 m·yr−1. The number of reactivation surfaces imaged within the dune by GPR appears to scale with the annual migrating rate and dune turnover time suggesting that at this location, annual cycles in the wind regime are a potential control on dune stratigraphy with reactivation surfaces generated by changes in the wind direction, including wind reversals in the winter months. Alternatively, it is hypothesized that erosion in the lee of small superimposed bedforms as they pass the dune crest and approach the brink at the top of the slipface will create superposition surfaces. The migration rate of superimposed bedforms with a wavelength of 20 m has been measured at 2 m·day−1. This suggests that small superimposed bedforms will arrive at the dune crest approximately every 10 days. Thus, bounding surface created by erosion in the lee of superimposed dunes will be very common. Given that the turnover time of the barchan dune is estimated at 4.3 years, the number of superposition surfaces produced by the faster bedforms could be more than 100. The number of bounding surface imaged by a GPR profile along the length of the dune appears to support the wind-driven reactivation hypothesis. However, a GPR profile across the dune images many small trough sets, instead of a single slipface, suggesting that superimposed dunes play an important role in the stratigraphy of a relatively simple barchan dune.

Intestinal Structure and Function after Small Bowel by-Pass in the Rat

1972 ◽  
Vol 43 (6) ◽  
pp. 731-742 ◽  
Author(s):  
M. H. Gleeson ◽  
J. Cullen ◽  
R. H. Dowling
Keyword(s):  
Small Bowel ◽  
Migration Rate ◽  
Morphological Changes ◽  
Intestinal Resection ◽  
Glucose Absorption ◽  
Turnover Time ◽  
Structure And Function ◽  
Major Influence ◽  
Intestinal Continuity ◽  
And Function

1. To study further the influence of luminal nutrition on small bowel structure and function, segments of rat jejunum and ileum were completely excluded from intestinal continuity by Thiry-Vella by-pass operations. The effect of partial deprivation of luminal nutrition was also studied in jejunal segments that had been surgically transposed to a distal position in the intestinal tract. 2. Macroscopically, by-passed jejunum and ileum both became narrowed and atrophic, whereas the intestine in continuity showed hypertrophic changes similar to those seen after intestinal resection. 3. In by-passed intestine the pattern of villi changed from mucosal ridges to ‘fingers’ and ‘leaves’. Although villous height and total mucosal thickness were both reduced, light microscopy showed that even 6 months after exclusion from intestinal continuity villous morphology was still retained. 4. Epithelial-cell migration rate and turnover time were diminished in both jejunum and ileum after by-pass. In intestine in continuity the migration rate was increased, but the turnover time remained the same as in controls. 5. In by-passed jejunum, in vivo glucose absorption diminished progressively with time, but did not change in excluded ileal segments. 6. After ileo-jejunal transposition, the jejunum showed no major morphological changes, but glucose absorption became significantly depressed. 7. These results further support the concept that intraluminal nutrition exerts a major influence on cell turnover, villous morphology and absorptive function in the small intestine.

Imaging the structure and reconstructing the development of a barchan dune using ground-penetrating radar

Geomorphology ◽  
2019 ◽  
Vol 341 ◽  
pp. 192-202 ◽  
Author(s):  
Tianyang Fu ◽  
Yongqiu Wu ◽  
Lihua Tan ◽  
Dawei Li ◽  
Yanglei Wen
Keyword(s):  
Ground Penetrating Radar ◽  
Barchan Dune ◽  
Ground Penetrating

Interaction bounding surfaces exposed in migrating transverse aeolian ridges on Mars

Geology ◽  
2021 ◽  
Author(s):  
Mackenzie Day
Keyword(s):  
Satellite Images ◽  
Recent Literature ◽  
Sediment Supply ◽  
Formation Mechanisms ◽  
Planetary Surfaces ◽  
Bounding Surface ◽  
Surface Atmosphere ◽  
Aeolian Dunes ◽  
Bounding Surfaces

Wind-blown sand self-organizes into bedforms that have now been identified on six different planetary bodies. These bedforms, including ripples and dunes, exhibit patterns that are diagnostic of surface-atmosphere interactions and can be used to interpret winds and sediment supply from satellite images of planetary surfaces. Patterns in dune and ripple fields change when one or more bedforms interact, for example, by linking, colliding, or merging with one another. When two bedforms interact, the cross-strata developed by the bedforms include a bounding surface where the two bedforms combined. These “interaction bounding surfaces” have been interpreted from ancient and modern strata in recent literature, but they have not yet been identified beyond Earth. On Mars, aeolian dunes and ripples form much as they do on Earth, but additional enigmatic bedform types are also present. Transverse aeolian ridges are straight-crested bedforms found abundantly on Mars, but with few analogs on Earth. Formation mechanisms for these enigmatic bedforms range from dune-like migration and construction to growth in place via wedge stacking or kinetic sieving. In this work, I studied exposed stoss-slope stratification on these enigmatic Martian bedforms to (1) identify the first in situ examples of interaction bounding surfaces captured visually, and (2) demonstrate that the transverse aeolian ridges must have been forward migrating.

scholarly journals Contribution to the theory of attraction when the force varies as any power of the distance

1919 ◽  
Vol 95 (671) ◽  
pp. 390-399
Keyword(s):  
Uniform Density ◽  
Surfaces Of Revolution ◽  
Bounding Surface ◽  
Exterior Surface ◽  
Hollow Shell ◽  
Law Of Nature ◽  
Inner Surface ◽  
The Law ◽  
Bounding Surfaces ◽  
Similar Theory

The object of this paper is to make a contribution to the theory of attraction when the force is proportional to any given power of the distance. In the case of the law of nature, it is well known that when the attracting mass is a hollow shell of uniform density, whose exterior and interior bounding surfaces are both surfaces of revolution about the axis of z , the potential at any point exterior to the exterior surface can be simply obtained from the potential at any point on the axis exterior to the exterior bounding surface, and a similar theory connects the potential at any point within the inner bounding surface with that at any point on the axis which is also within the inner surface. In the expression for the point z on the z axis we have merely to write r for z , and intercalate the zonal harmonics P 0 , P 1 , P 2 ,... in the well-known manner. We consider the analogous theory when the law is that of the n th power of the distance instead of that of the inverse square.

scholarly journals Internal structure and development of an aeolian river dune in The Netherlands, using 3-D interpretation of ground-penetrating radar data

2002 ◽  
Vol 81 (1) ◽  
pp. 27-37 ◽  
Author(s):  
R.L. Van Dam
Keyword(s):  
Internal Structure ◽  
Ground Penetrating Radar ◽  
High Amplitude ◽  
Radar Data ◽  
Organic Horizon ◽  
Westerly Winds ◽  
Small Change ◽  
Ground Penetrating ◽  
Radar Facies ◽  
Bounding Surfaces

AbstractGround-penetrating radar data from a regular grid are used to study the internal structure and development of a 9-m high aeolian river dune in the Dutch Rhine-Meuse delta. The purpose of this investigation was to image the internal sedimentary structures to better understand the development of these aeolian river dunes. Three radar facies can be recognised in the GPR sections. Radar facies 1 has a maximum thickness of 5 to 6 m and is characterised by dipping, parallel reflections with a maximum length of at least 20 m. The reflections from perpendicular sections, analysed using closed-loop correlation in 3-D-interpretation software, form eastward dipping (14° maximum) surfaces. Radar facies 2 is one continuous, sub-horizontal reflection. This high amplitude reflection is most probably caused by a thin organic horizon. Radar facies 3 has a thickness of 3 to 4 m and is made up of sets of short, predominantly eastward to north-eastward dipping reflections separated by rather continuous, sub-horizontal reflections. The eastward dipping surfaces in radar facies 1 are foresets of a dune that was deposited by prevailing westerly winds in the Younger Dryas, the last cold period in the Pleistocene. During the Early Holocene, an increasing vegetation cover stabilised the dune and formed a thin organic horizon. Subsequent resumption of dune forming processes led to the formation of radar facies 3 on top of the vegetated Pleistocene dune. Sedimentation by small dunes, partly eroding each other, led to sets of cross-stratification separated by bounding surfaces. The results suggest a small change in palaeo wind direction.

Utility of Ground-Penetrating Radar in Near-Surface, High-Resolution Imaging of Lansing-Kansas City (Pennsylvanian) Limestone Reservoir Analogs

1998 ◽  
pp. 43-59
Author(s):  
Alex Martinez ◽  
Joseph M. Kruger ◽  
Evan K. Franseen
Keyword(s):  
High Resolution ◽  
Ground Penetrating Radar ◽  
Kansas City ◽  
Reservoir Modeling ◽  
Near Surface ◽  
Well Control ◽  
Imaging Tool ◽  
Velocity Information ◽  
Ground Penetrating ◽  
Bounding Surfaces

High-resolution ground-penetrating radar (GPR) is a subsurface imaging tool that can extend results gained from studies of reservoir-analog outcrops and add detailed information about reservoir analogs that is unavailable from either seismic data or well control alone. Integration of GPR-reflection information and outcrop photomosaics allowed detailed study of subtle changes in lithology and bedding surfaces by comparing lateral and vertical changes in GPR-reflection character with outcrop features. Outcrops are valuable for confirming interpretations of reflections and providing velocity information for the GPR data. Outcrops of two Lansing-Kansas City Group limestone units, the Captain Creek Limestone and the Plattsburg Limestone, were used as test sites to determine the vertical imaging resolution, penetration depth, and reflection character of high-frequency (500 MHz) GPR in interbedded carbonate and shale units, where the carbonate units contained thin interbeds of shale. Features as small as 0.1-0.2 m (0.3-0.7 ft)--including major architectural elements (such as major and minor bounding surfaces) and internal features (such as fractures, internal bedding, and crossbedding)--were successfully imaged and confirmed by outcrop data. Variations in GPR-reflection character between geologic units allowed recognition of argillaceous limestone units in the subsurface. Although shale and soil at the surface generally impeded GPR signal penetration, thin shale layers and shale at bounding surfaces actually enhanced reflectivity and aided in interpretation. Our results indicate that GPR can be successfully used as an aid in outcrop studies to provide quantitative data for use in reservoir modeling.

A bounding surface plasticity model for sands exhibiting particle crushing

2004 ◽  
Vol 41 (6) ◽  
pp. 1179-1192 ◽  
Author(s):  
Adrian R Russell ◽  
Nasser Khalili
Keyword(s):  
Critical State ◽  
Triaxial Tests ◽  
Compression Tests ◽  
Particle Crushing ◽  
Bounding Surface ◽  
Bounding Surface Plasticity ◽  
Isotropic Compression ◽  
Surface Plasticity ◽  
Wide Range ◽  
Bounding Surfaces

A new bounding surface constitutive model for sands is presented and is suited to a wide range of stresses, including those sufficient to cause particle crushing. The basic concepts of critical state soil mechanics are shown to be valid, and a uniquely shaped critical state line is defined to capture the three modes of plastic deformation observed across a wide range of stresses, including particle rearrangement, particle crushing, and pseudoelastic deformation. A limiting isotropic compression line is separated from the critical state line in the υ – In p′ plane by a constant shift along an elastic unload–reload line. In the deviator stress – mean effective stress (q–p′) plane, the loading and bounding surfaces are homologous about the origin and defined by a simple and versatile function. Isotropic hardening and softening of the loading and bounding surfaces are controlled by plastic volumetric strains. A commonly used non associative flow rule is adopted. Experimental results of monotonically loaded drained and undrained triaxial tests, isotropic compression tests, and oedometric compression tests are presented for a quartz sand and used to calibrate the model. Membrane penetration is accounted for in the model simulations of the test results. A single set of material parameters is introduced enabling rigorous and accurate predictions of stress–strain behaviour in sands.Key words: sand, bounding surface, plasticity, particle crushing.

Imaging molecules adsorbed onto electrodes under potential control by scanning probe microscopy

1991 ◽  
Vol 49 ◽  
pp. 376-377 ◽  
Author(s):  
N.J. Tao ◽  
J.A. DeRose ◽  
P.I. Oden ◽  
S.M. Lindsay
Keyword(s):  
Surface Charge ◽  
Environmental Effects ◽  
Scanning Probe Microscopy ◽  
Statistical Significance ◽  
Electrochemical Methods ◽  
Surface Structures ◽  
Scanning Probe ◽  
Potential Control ◽  
Afm Imaging ◽  
Special Circumstances

Clemmer and Beebe have pointed out that surface structures on graphite substrates can be misinterpreted as biopolymer images in STM experiments. We have been using electrochemical methods to react DNA fragments onto gold electrodes for STM and AFM imaging. The adsorbates produced in this way are only homogeneous in special circumstances. Searching an inhomogeneous substrate for ‘desired’ images limits the value of the data. Here, we report on a reversible method for imaging adsorbates. The molecules can be lifted onto and off the substrate during imaging. This leaves no doubt about the validity or statistical significance of the images. Furthermore, environmental effects (such as changes in electrolyte or surface charge) can be investigated easily.

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