139 research outputs found
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\ud
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
Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing
Establishing models for the formation of well-mixed polyphase domains in ultramylonites is difficult because the effects of large strains and thermo-hydro-chemo-mechanical feedbacks can obscure the transient phenomena that may be responsible for domain production. We use scanning electron microscopy and nanotomography to offer critical insights into how the microstructure of a highly deformed quartzo-feldspathic ultramylonite evolved. The dispersal of monomineralic quartz domains in the ultramylonite is interpreted to be the result of the emergence of synkinematic pores, called creep cavities. The cavities can be considered the product of two distinct mechanisms that formed hierarchically: Zener-Stroh cracking and viscous grain-boundary sliding. In initially thick and coherent quartz ribbons deforming by grain-size-insensitive creep, cavities were generated by the Zener-Stroh mechanism on grain boundaries aligned with the YZ plane of finite strain. The opening of creep cavities promoted the ingress of fluids to sites of low stress. The local addition of a fluid lowered the adhesion and cohesion of grain boundaries and promoted viscous grain-boundary sliding. With the increased contribution of viscous grain-boundary sliding, a second population of cavities formed to accommodate strain incompatibilities. Ultimately, the emergence of creep cavities is interpreted to be responsible for the transition of quartz domains from a grain-size-insensitive to a grain-size-sensitive rheology
Thermal-elastic stresses and the criticality of the continental crust
Heating or cooling can lead to high stresses in rocks due to the different thermal-elastic properties of minerals. In the upper 4 km of the crust, such internal stresses might cause fracturing. Yet it is unclear if thermal elasticity contributes significantly to critical stresses and failure deeper in Earth's continental crust, where ductile creep causes stress relaxation. We combined a heating experiment conducted in a Synchrotron microtomograph (Advanced Photon Source, USA) with numerical simulations to calculate the grain-scale stress field in granite generated by slow burial. We find that deviatoric stresses >100 MPa can be stored during burial, with relaxation times from 100's to 1000's ka, even in the ductile crust. Hence, grain-scale thermal-elastic stresses may serve as nuclei for instabilities, thus rendering the continental crust close to criticality
Patient and provider perceptions of a peer-delivered intervention ('Khanya') to improve anti-retroviral adherence and substance use in South Africa: a mixed methods analysis
BACKGROUND: Despite a high prevalence of problematic substance use among people living with HIV in South Africa, there remains limited access to substance use services within the HIV care system. To address this gap, our team previously developed and adapted a six-session, peer-delivered problem-solving and behavioral activation-based intervention (Khanya) to improve HIV medication adherence and reduce substance use in Cape Town. This study evaluated patient and provider perspectives on the intervention to inform implementation and future adaptation. METHODS: Following intervention completion, we conducted semi-structured individual interviews with patients (n = 23) and providers (n = 9) to understand perspectives on the feasibility, acceptability, and appropriateness of Khanya and its implementation by a peer. Patients also quantitatively ranked the usefulness of individual intervention components (problem solving for medication adherence 'Life-Steps', behavioral activation, mindfulness training, and relapse prevention) at post-treatment and six months follow-up, which we triangulated with qualitative feedback to examine convergence and divergence across methods. RESULTS: Patients and providers reported high overall acceptability, feasibility, and appropriateness of Khanya, although there were several feasibility challenges. Mindfulness and Life-Steps were identified as particularly acceptable, feasible, and appropriate components by patients across methods, whereas relapse prevention strategies were less salient. Behavioral activation results were less consistent across methods. CONCLUSIONS: Findings underscore the importance of examining patients' perspectives on specific intervention components within intervention packages. While mindfulness training and peer delivery models were positively perceived by consumers, they are rarely used within task-shared behavioral interventions in low- and middle-income countries
Habitable Zones in the Universe
Habitability varies dramatically with location and time in the universe. This
was recognized centuries ago, but it was only in the last few decades that
astronomers began to systematize the study of habitability. The introduction of
the concept of the habitable zone was key to progress in this area. The
habitable zone concept was first applied to the space around a star, now called
the Circumstellar Habitable Zone. Recently, other, vastly broader, habitable
zones have been proposed. We review the historical development of the concept
of habitable zones and the present state of the research. We also suggest ways
to make progress on each of the habitable zones and to unify them into a single
concept encompassing the entire universe.Comment: 71 pages, 3 figures, 1 table; to be published in Origins of Life and
Evolution of Biospheres; table slightly revise
Acceleration of slip motion in deep extensions of seismogenic faults in and below the seismogenic region
- …