Andrew Squelch

Improved estimates of percolation and anisotropic permeability from 3-D X-ray microtomography using stochastic analyses and visualization.

X-ray microtomography (micro-CT) with micron resolution enables new ways of characterizing microstructures and opens pathways for forward calculations of multiscale rock properties. A quantitative characterization of the microstructure is the first step in this challenge. We developed a new approach to extract scale-dependent characteristics of porosity, percolation, and anisotropic permeability from 3-D microstructural models of rocks. The Hoshen-Kopelman algorithm of percolation theory is employed for a standard percolation analysis. The anisotropy of permeability is calculated by means of the star volume distribution approach. The local porosity distribution and local percolation probability are obtained by using the local porosity theory. Additionally, the local anisotropy distribution is defined and analyzed through two empirical probability density functions, the isotropy index and the elongation index. For such a high-resolution data set, the typical data sizes of the CT images are on the order of gigabytes to tens of gigabytes; thus an extremely large number of calculations are required. To resolve this large memory problem parallelization in OpenMP was used to optimally harness the shared memory infrastructure on cache coherent Non-Uniform Memory Access architecture machines such as the iVEC SGI Altix 3700Bx2 Supercomputer. We see adequate visualization of the results as an important element in this first pioneering study.

Three-dimensional morphology of magmatic sulfides sheds light on ore formation and sulfide melt migration

The morphology of magmatic sulfides in igneous cumulates is controlled by the wetting properties of sulfide liquids against silicates. The formation of nickel sulfide ores, the behavior of sulfide liquids during mantle melting, and potentially the segregation of the Earth9s core, are all controlled by the ability of sulfide liquids to migrate through the pore space of partially molten silicates. Three-dimensional X-ray tomographic images of sulfide aggregates in komatiitic olivine cumulates indicate that sulfide liquids have a limited tendency to wet olivine crystals, forming interconnected networks only in the absence of silicate melt. Consequently, the ability of sulfide liquids to migrate through the pore space of olivine cumulates is limited. We conclude that disseminated sulfide ores in komatiites formed by accumulation of transported sulfide blebs a few millimeters in size, and not by settling of sulfide-olivine aggregates, and that sulfides accumulated in the proportions in which they are now found, rather than by percolation through cumulate pore space. It is unlikely that sulfide droplets can be entrained and carried from the mantle at low degrees of partial melting. Our results also support the hypothesis that segregation of the Earth9s core took place from a magma ocean, rather than by percolation of sulfidic melt through partially molten mantle.

Automatic detection of anisotropic features on rock surfaces

Surface roughness is an important rock property that is measured for structural geology and engineering purposes. We have developed an automatic technique to detect anisotropic features on rock faces based on fractal analysis. The analysis method has been applied to synthetic surfaces, and to digitally mapped point clouds of natural rock surfaces shaped by weathering, fault wear, and mining. We illustrate the technique using field examples from Permian sandstones containing brittle shear zones in northeast England, the surface of a neotectonic fault in Turkey, Proterozoic quartzite from central Australia, and Devonian Quartzite in an aggregate quarry in Germany. Roughness analysis of these natural examples suggests that a significant change of roughness value, anisotropy, and anisotropy direction can exist across scale. Our analysis method represents a step toward developing a toolkit to automatically detect and interpret surface characteristics from digitally acquired data sets. It has widespread potential for applications in rock engineering and the geosciences.

3D Stereoscopic Visualization of Fenestrated Stent Grafts

The purpose of this study was to present a technique of stereoscopic visualization in the evaluation of patients with abdominal aortic aneurysm treated with fenestrated stent grafts compared with conventional 2D visualizations. Two patients with abdominal aortic aneurysm undergoing fenestrated stent grafting were selected for inclusion in the study. Conventional 2D views including axial, multiplanar reformation, maximum-intensity projection, and volume rendering and 3D stereoscopic visualizations were assessed by two experienced reviewers independently with regard to the treatment outcomes of fenestrated repair. Interobserver agreement was assessed with Kendall’s W statistic. Multiplanar reformation and maximum-intensity projection visualizations were scored the highest in the evaluation of parameters related to the fenestrated stent grafting, while 3D stereoscopic visualization was scored as valuable in the evaluation of appearance (any distortions) of the fenestrated stent. Volume rendering was found to play a limited role in the follow-up of fenestrated stent grafting. 3D stereoscopic visualization adds additional information that assists endovascular specialists to identify any distortions of the fenestrated stents when compared with 2D visualizations.

High-resolution study of the 3D collagen fibrillary matrix of Achilles tendons without tissue labelling and dehydrating

Knowledge of the collagen structure of an Achilles tendon is critical to comprehend the physiology, biomechanics, homeostasis and remodelling of the tissue. Despite intensive studies, there are still uncertainties regarding the microstructure. The majority of studies have examined the longitudinally arranged collagen fibrils as they are primarily attributed to the principal tensile strength of the tendon. Few studies have considered the structural integrity of the entire three‐dimensional (3D) collagen meshwork, and how the longitudinal collagen fibrils are integrated as a strong unit in a 3D domain to provide the tendons with the essential tensile properties. Using second harmonic generation imaging, a 3D imaging technique was developed and used to study the 3D collagen matrix in the midportion of Achilles tendons without tissue labelling and dehydration. Therefore, the 3D collagen structure is presented in a condition closely representative of the in vivo status. Atomic force microscopy studies have confirmed that second harmonic generation reveals the internal collagen matrix of tendons in 3D at a fibril level. Achilles tendons primarily contain longitudinal collagen fibrils that braid spatially into a dense rope‐like collagen meshwork and are encapsulated or wound tightly by the oblique collagen fibrils emanating from the epitenon region. The arrangement of the collagen fibrils provides the longitudinal fibrils with essential structural integrity and endows the tendon with the unique mechanical function for withstanding tensile stresses. A novel 3D microscopic method has been developed to examine the 3D collagen microstructure of tendons without tissue dehydrating and labelling. The study also provides new knowledge about the collagen microstructure in an Achilles tendon, which enables understanding of the function of the tissue. The knowledge may be important for applying surgical and tissue engineering techniques to tendon reconstruction.

A reproducible framework for 3D acoustic forward modelling of hard rock geological models with Madagascar

A special challenge of hard rock exploration is to identify targets of interest within complex geological settings. Interpretation of the geology can be made from direct geological observations and knowledge of the area, and from 2D or 3D seismic surveys. These interpretations can be developed into 3D geological models that provide the basis for predictions as to likely targets for drilling and/or mining. To verify these predictions we need to simulate 3D seismic wave propagation in the proposed geological models and compare the simulation results to seismic survey data. To achieve this we convert geological surfaces created in an interpretation software package into discretised block models representing the different lithostratigraphic units, and segment these into discrete volumes to which appropriate density and seismic velocity values are assigned. This approach allows us to scale models appropriately for desired wave propagation parameters and to go from local to global geological models and vice versa. Then we use these digital models with forward modelling codes to undertake numerous 3D acoustic wave simulations. Simulations are performed with single shot and with exploding reflector (located on extracted geological surface) configurations.

Estimating porosity from CT scans of high permeability core plugs

Summary We have analysed CT scans of core plugs obtained from high permeability sandstones in the Wanneroo Sandstone member of the Leederville formation in the Perth Basin. Plugs taken from drill core at representative sections of aquifer horizons have been scanned in a SkyScan CT scanner and the resulting greyscale image stacks analysed to estimate hydraulic transport parameters of the aquifer horizon. These parameters are compared with laboratory measured porosity and permeability values obtained from standard physical tests. The analysis of the CT data provides support for understanding parameters derived from standard core plug analysis and wire line logging. However it also allows for a localised study of different zones within the core plug volume that is not possible with more ‘holistic’ laboratory measurement. Also, the mechanical framework of the grain and pore structure can be extracted as 3D geometric models for additional types of analysis and numerical modelling. We estimate values for porosity and permeability for distinct zones within the core plugs and for the full width of the core plug. The full width values are compared with the equivalent laboratory values and for calibration. In addition, the possible impact of millimetre to centimetre zonation for grain size and shape distribution is considered with reference to anisotropy in larger scale physical measurements from wire-line logging.

Visualizing and Interpreting Volumetric Datasets Using Volume Haptics

Volume Haptics is used for enhanced visualization and interaction of volumetric data. In this paper, evaluation of VHTK is presented which implements direct volume haptics with proxy based approach. The techniques are tested on medical and geophysical data. Different haptic modes are used that are controlled with transfer functions as parameters representing material properties.