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Surface Data (surface + data)
Selected AbstractsTowards automatic computer-aided knee surgery by innovative methods for processing the femur surface modelTHE INTERNATIONAL JOURNAL OF MEDICAL ROBOTICS AND COMPUTER ASSISTED SURGERY, Issue 3 2010Pietro Cerveri Abstract Background The femoral shaft (FDA) and transepicondylar (TA), anterior,posterior (WL) and posterior condylar (PCL) axes are fundamental quantities in planning knee arthroplasty surgery. As an alternative to the TA, we introduce the anatomical flexion axis (AFA). Obtaining such axes from image data without any manual supervision remains a practical objective. We propose a novel method that automatically computes the axes of the distal femur by processing the femur mesh surface. Methods Surface data were processed by exploiting specific geometric, anatomical and functional properties. Robust ellipse fitting of the two-dimensional (2D) condylar profiles was utilized to determine the AFA alternative to the TA. The repeatability of the method was tested upon 20 femur surfaces reconstructed from CT scans taken on cadavers. Results At the highest surface resolutions, the relative median error in the direction of the FDA, AFA, PCL, WL and TA was < 0.50°, 1.20°, 1.0°, 1.30° and 1.50°, respectively. As expected, at the lowest surface resolution, the repeatability decreased to 1.20°, 2.70°, 3.30°, 3.0° and 4.70°, respectively. The computed directions of the FDA, PCL, WL and TA were in agreement (0.60°, 1.55°, 1.90°, 2.40°) with the corresponding reference parameters manually identified in the original CT images by medical experts and with the literature. Conclusions The proposed method proved that: (a) the AFA can be robustly computed by a geometrical analysis of the posterior profiles of the two condyles and can be considered a useful alternative to the TA; (b) higher surface resolutions leads to higher repeatability of all computed quantities; (c) the TA is less repeatable than the other axes. Copyright © 2010 John Wiley & Sons, Ltd. [source] The role of groundwater in cliff instability: an example at Cape Blanc-Nez (Pas-de-Calais, France)EARTH SURFACE PROCESSES AND LANDFORMS, Issue 1 2006Guillaume Pierre Abstract Cliff retreat in northern Boulonnais is described. The distribution of the amount of retreat is related to structural and topographic factors. Both control the runoff,infiltration balance and therefore the mechanical behaviour of the rocks, which determines the modes of failure. The spatial variability of the retreat rate is explained, but predicting the temporal variability of the retreat rate, which is central to risk management, is much more difficult. Rainfall and piezometric surface data enhanced a ,piston flow' mechanism during November 2000. The result of its occurrence on the stability of the cliff and conditions of its recurrence are examined with a view to better understanding the rate of recession of coastal cliffs and wiser management of risk. Copyright © 2005 John Wiley & Sons, Ltd. [source] Migration velocity analysis and waveform inversionGEOPHYSICAL PROSPECTING, Issue 6 2008William W. Symes ABSTRACT Least-squares inversion of seismic reflection waveform data can reconstruct remarkably detailed models of subsurface structure and take into account essentially any physics of seismic wave propagation that can be modelled. However, the waveform inversion objective has many spurious local minima, hence convergence of descent methods (mandatory because of problem size) to useful Earth models requires accurate initial estimates of long-scale velocity structure. Migration velocity analysis, on the other hand, is capable of correcting substantially erroneous initial estimates of velocity at long scales. Migration velocity analysis is based on prestack depth migration, which is in turn based on linearized acoustic modelling (Born or single-scattering approximation). Two major variants of prestack depth migration, using binning of surface data and Claerbout's survey-sinking concept respectively, are in widespread use. Each type of prestack migration produces an image volume depending on redundant parameters and supplies a condition on the image volume, which expresses consistency between data and velocity model and is hence a basis for velocity analysis. The survey-sinking (depth-oriented) approach to prestack migration is less subject to kinematic artefacts than is the binning-based (surface-oriented) approach. Because kinematic artefacts strongly violate the consistency or semblance conditions, this observation suggests that velocity analysis based on depth-oriented prestack migration may be more appropriate in kinematically complex areas. Appropriate choice of objective (differential semblance) turns either form of migration velocity analysis into an optimization problem, for which Newton-like methods exhibit little tendency to stagnate at nonglobal minima. The extended modelling concept links migration velocity analysis to the apparently unrelated waveform inversion approach to estimation of Earth structure: from this point of view, migration velocity analysis is a solution method for the linearized waveform inversion problem. Extended modelling also provides a basis for a nonlinear generalization of migration velocity analysis. Preliminary numerical evidence suggests a new approach to nonlinear waveform inversion, which may combine the global convergence of velocity analysis with the physical fidelity of model-based data fitting. [source] The effects of near-surface conditions on anisotropy parameter estimations from 4C seismic dataGEOPHYSICAL PROSPECTING, Issue 1 2006Bärbel Traub ABSTRACT We present a study of anisotropic parameter estimation in the near-surface layers for P-wave and converted-wave (C-wave) data. Near-surface data is affected by apparent anisotropy due to a vertical velocity compaction gradient. We have carried out a modelling study, which showed that a velocity gradient introduces apparent anisotropy into an isotropic medium. Thus, parameter estimation will give anomalous values that affect the imaging of the target area. The parameter estimation technique is also influenced by phase reversals with diminishing amplitude, leading to erroneous parameters. In a modelling study using a near-surface model, we have observed phase reversals in near-surface PP reflections. The values of the P-wave anisotropy parameter , estimated from these events are about an order of magnitude larger than the model values. Next, we use C-wave data to estimate the effect of anisotropy (,) and compute , from these values. These calculated ,-values are closer to the model values, and NMO correction with both ,-values shows a better correction for the calculated value. Hence, we believe that calculating , from , gives a better representation of the anisotropy than picked , from the P-wave. Finally, we extract the anisotropy parameters , and , from real data from the Alba Field in the North Sea. Comparing the results with reference values from a model built according to well-log, VSP and surface data, we find that the parameters show differences of up to an order of magnitude. The ,-values calculated from the C-wave anisotropy parameter , fit the reference values much better and show values of the same order of magnitude. [source] Towards ice-core-based synoptic reconstructions of west antarctic climate with artificial neural networksINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 5 2005David B. Reusch Abstract Ice cores have, in recent decades, produced a wealth of palaeoclimatic insights over widely ranging temporal and spatial scales. Nonetheless, interpretation of ice-core-based climate proxies is still problematic due to a variety of issues unrelated to the quality of the ice-core data. Instead, many of these problems are related to our poor understanding of key transfer functions that link the atmosphere to the ice. This study uses two tools from the field of artificial neural networks (ANNs) to investigate the relationship between the atmosphere and surface records of climate in West Antarctica. The first, self-organizing maps (SOMs), provides an unsupervised classification of variables from the mid-troposphere (700 hPa temperature, geopotential height and specific humidity) into groups of similar synoptic patterns. An SOM-based climatology at annual resolution (to match ice-core data) has been developed for the period 1979,93 based on the European Centre for Medium-Range Weather Forecasts (ECMWF) 15-year reanalysis (ERA-15) dataset. This analysis produced a robust mapping of years to annual-average synoptic conditions as generalized atmospheric patterns or states. Feed-forward ANNs, our second ANN-based tool, were then used to upscale from surface data to the SOM-based classifications, thereby relating the surface sampling of the atmosphere to the large-scale circulation of the mid-troposphere. Two recorders of surface climate were used in this step: automatic weather stations (AWSs) and ice cores. Six AWS sites provided 15 years of near-surface temperature and pressure data. Four ice-core sites provided 40 years of annual accumulation and major ion chemistry. Although the ANN training methodology was properly designed and followed standard principles, limited training data and noise in the ice-core data reduced the effectiveness of the upscaling predictions. Despite these shortcomings, which might be expected to preclude successful analyses, we find that the combined techniques do allow ice-core reconstruction of annual-average synoptic conditions with some skill. We thus consider the ANN-based approach to upscaling to be a useful tool, but one that would benefit from additional training data. Copyright © 2005 Royal Meteorological Society. [source] Construction of statistical shape atlases for bone structures based on a two-level framework,THE INTERNATIONAL JOURNAL OF MEDICAL ROBOTICS AND COMPUTER ASSISTED SURGERY, Issue 1 2010Chenyu Wu Abstract Background The statistical shape atlas is a 3D medical image analysis tool that encodes shape variations between populations. However, efficiency, accuracy and finding the correct correspondence are still unsolved issues during the construction of the atlas. Methods We developed a two-level-based framework that speeds up the registration process while maintaining accuracy of the atlas. We also proposed a semi-automatic strategy to achieve segmentation and registration simultaneously, without knowing any prior information about the shape. Results We have separately constructed the atlas for the femur and spine. The experimental results demonstrate the efficiency and accuracy of our methods. Conclusions Our two-level framework and semi-automatic strategy are able to efficiently construct the atlas for bone structures without losing accuracy. We can handle either 3D surface data or raw DICOM images. Copyright © 2009 John Wiley & Sons, Ltd. [source] Evaluation of Information Loss in Digital Elevation Models With Digital Photogrammetric SystemsTHE PHOTOGRAMMETRIC RECORD, Issue 95 2000Y. D. Huang Information loss is caused when a surface is sampled with a finite interval, such as in the production of a digital elevation model (DEM). This information loss can become the dominant part of the error in a DEM. The ability to quantify information loss enables guidance to be provided for an appropriate choice of grid interval and better accuracy assessment for the DEM. With the use of digital photogrammetric systems, evaluation of information loss has become much easier. This paper describes three methods of evaluating information loss. An example is given of the method which is most appropriate for use with a digital photogrammetric system, based on rock cliff surface data and the VirtuoZo system. [source] Atmospheric tides over the Pyrenees: observational study and numerical simulationTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 650 2010J. Díaz de Argandoña Abstract Barometric tides around the Pyrenees mountain range are analyzed by means of synoptic surface-station data recorded during one year, surface data from the Pyrenees Experiment (PYREX) and the CRA/LA Very High Frequency (VHF) wind profiler installed in the north of the range. Tides are decomposed into their diurnal and semi-diurnal components. Diurnal tides show a strong non-migrating component and are very dependent on local conditions. Semi-diurnal tides are more homogeneous and present a north,south asymmetry, also noted in the Alps. This cross-range asymmetry could be related to some interference effect caused by the mountain range in the migrating semi-diurnal tide wave. The asymmetry of the diurnal component presents a very strong seasonal variation, probably related to local diabatic effects. A three-month long simulation has been carried out with the National Center for Atmospheric Research's Weather Research and Forecasting (WRF) limited-area model to try to reproduce the tide structure. The validation of the results with wind-profiler data shows reasonable agreement with the observed diurnal tide and poorer results for the semi-diurnal component. At surface level, however, the model reproduces some of the features of the observed semi-diurnal tide, and especially the cross-range asymmetry. Copyright © 2010 Royal Meteorological Society [source] | ||||||||||