2,095 research outputs found
Velocity Anisotropy Of Two Deep Crystalline Samples
Using ultrasonic velocity measurements taken over a multiplicity of directions we show
that samples exhibit weak to moderate anisotropy of seismic velocities. We further
define the anisotropic geometry with high resolution scanning electron microscopy.
Our data indicate that one sample, a granite, is transversely anisotropic, and that the
presence of fine to moderately fine microcracks is the most important factor effecting
the velocities. We model the angular velocity dependence using 5 elastic constants
and show that all 9 observed velocities fit these predictions to within 0.1 km/s. We
are unable to obtain similar fits to a second sample, a mica-schist, in the same fashion.
SEM observations indicate this rock displays orthorhombic symmetry. We made
additional velocity measurements in order to calculate 9 elastic constants, and found
that the predicted angular velocity dependence agreed much better with our velocity
observations.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumEnte nazionale per l'energia elettric
Well-to-Well Log Correlation Using Knowledge-Based Systems and Dynamic Depth Warping
We present a novel system for well-to-well log correlation using knowledge-based systems
and dynamic depth warping techniques. This approach overcomes a major drawback inherent
in previous methods, namely the difficulty in correlating missing or discontinuous
rock units.
The system has three components: (1) A Dynamic Programming algorithm to correlate
the logs and to find the minimum-cost or "best" match; (2) A set of "rules" to
guide the correlation; (3) A data base that contains the logs and other relevant geologic
and seismic information. The Dynamic Programming algorithm calculates the cost of
correlating each point in the first well with each of the points in the second well. The
resulting matrix of dissimilarity contains cost information about every possible operation which matches the well logs. The cost of matching the two wells is measured by
the difference in the log values. The dynamic programming approach allows correlation
across geologic structures, thinning beds, and missing or discontinuous units. A path
finding algorithm then traces through the matrix to define a function which maps the
first well onto the second. The minimum cost path is the optimal correlation between
the wells.
The system's database contains the well logs themselves and other relevant data
including information about the geologic setting, seismic ties, interpreted lithologies, and dipmeter information. Rules operating on the data affect the dynamic programming and
path finding algorithms in several ways: (1) Seismic ties or marker beds define a point in the warping path, thereby removing calculations over large portions of the search space; (2) Dipmeter results and knowledge of geologic structure further constrain the path to certain global areas and save calculation time; (3) The system assigns weights to different logs based on log quality and sensitivity; (4) Knowledge of the paleoenvironment allows the program to choose a set of rules (model) which accounts for changes in sediment type or thickness within a field. For example, when the program is operating in a deltaic environment, it will correlate the shales before attempting to correlate the sands. We demonstrate the method with synthetic examples in which the program successfully correlates across geologic structures and pinch-outs. We also applied the program to field examples from two widely separated oil provinces. In both cases, the automated correlation agreed very well with correlations provided by geologic experts
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Opportunities for system level improvement in antibiotic use across the surgical pathway
Optimizing antibiotic prescribing across the surgical pathway (before, during, and after surgery) is a key aspect of tackling important drivers of antimicrobial resistance and simultaneously decreasing the burden of infection at the global level. In the UK alone, 10 million patients undergo surgery every year, which is equivalent to 60% of the annual hospital admissions having a surgical intervention. The overwhelming majority of surgical procedures require effectively limited delivery of antibiotic prophylaxis to prevent infections. Evidence from around the world indicates that antibiotics for surgical prophylaxis are administered ineffectively, or are extended for an inappropriate duration of time postoperatively. Ineffective antibiotic prophylaxis can contribute to the development of surgical site infections (SSIs), which represent a significant global burden of disease. The World Health Organization estimates SSI rates of up to 50% in postoperative surgical patients (depending on the type of surgery), with a particular problem in low- and middle-income countries, where SSIs are the most frequently reported healthcare-associated infections. Across European hospitals, SSIs alone comprise 19.6% of all healthcare-acquired infections. Much of the scientific research in infection management in surgery is related to infection prevention and control in the operating room, surgical prophylaxis, and the management of SSIs, with many studies focusing on infection within the 30-day postoperative period. However it is important to note that SSIs represent only one of the many types of infection that can occur postoperatively. This article provides an overview of the surgical pathway and considers infection management and antibiotic prescribing at each step of the pathway. The aim was to identify the implications for research and opportunities for system improvement
Very Large and Reversible Stark-Shift Tuning of Single Emitters in Layered Hexagonal Boron Nitride
© 2019 American Physical Society. Combining solid-state single-photon emitters (SPEs) with nanophotonic platforms is a key goal in integrated quantum photonics. In order to realize functionality in potentially scalable elements, suitable SPEs have to be bright, stable, and widely tunable at room temperature. In this work, we show that selected SPEs embedded in a few-layer hexagonal boron nitride (h-BN) meet these demands. In order to show the wide tunability of these SPEs we employ an atomic force microscope (AFM) with a conductive tip to apply an electrostatic field to individual h-BN emitters sandwiched between the tip and an indium-tin-oxide-coated glass slide. A very large and reversible Stark shift of (5.5±0.3)nm at a zero-field wavelength of 670 nm is induced by applying just 20 V, which exceeds the typical resonance linewidths of nanodielectric and even nanoplasmonic resonators. Our results help to further understand the physical origin of SPEs in h-BN as well as for practical quantum photonic applications where wide spectral tuning and on/off resonance switching are required
Spontaneous periodic travelling waves in oscillatory systems with cross-diffusion
We identify a new type of pattern formation in spatially distributed active
systems. We simulate one-dimensional two-component systems with predator-prey
local interaction and pursuit-evasion taxis between the components. In a
sufficiently large domain, spatially uniform oscillations in such systems are
unstable with respect to small perturbations. This instability, through a
transient regime appearing as spontanous focal sources, leads to establishment
of periodic traveling waves. The traveling waves regime is established even if
boundary conditions do not favor such solutions. The stable wavelength are
within a range bounded both from above and from below, and this range does not
coincide with instability bands of the spatially uniform oscillations.Comment: 7 pages, 4 figures, as accepted to Phys Rev E 2009/10/2
The Viscous Nonlinear Dynamics of Twist and Writhe
Exploiting the "natural" frame of space curves, we formulate an intrinsic
dynamics of twisted elastic filaments in viscous fluids. A pair of coupled
nonlinear equations describing the temporal evolution of the filament's complex
curvature and twist density embodies the dynamic interplay of twist and writhe.
These are used to illustrate a novel nonlinear phenomenon: ``geometric
untwisting" of open filaments, whereby twisting strains relax through a
transient writhing instability without performing axial rotation. This may
explain certain experimentally observed motions of fibers of the bacterium B.
subtilis [N.H. Mendelson, et al., J. Bacteriol. 177, 7060 (1995)].Comment: 9 pages, 4 figure
The Clostridium difficile problem: A South African tertiary institution's prospective perspective
BACKGROUND AND OBJECTIVES: The aim of this study is to report the incidence of Clostridium difficile-associated disease (CDAD) in a tertiary-care hospital in South Africa and to identify risk factors, assess patient outcomes and determine the impact of the hypervirulent strain of the organism referred to as North American pulsed-field type 1 (NAP1). METHODS: Adults who presented with diarrhoea over a period of 15 months were prospectively evaluated for CDAD using stool toxin enzyme immunoassay (EIA). Positive specimens were evaluated by PCR. Patient demographics, laboratory parameters and outcomes were analysed. RESULTS: CDAD was diagnosed in 59 (9.2%) of 643 patients (median age 39 years, IQR 30 - 55). Thirty-four (58%) were female. Recent antibiotic exposure was reported in 39 (66%), 27 (46%) had been hospitalised within 3 months, and 14 (24%) had concomitant inflammatory bowel disease (IBD). Nineteen (32%) had community-acquired CDAD (CA-CDAD). The annual incidence of hospital-acquired CDAD (HA-CDAD) was 8.7 cases/10 000 hospitalisations. Two cases of the hypervirulent strain NAP1 were identified. Seven (12%) patients underwent colectomy (OR 6.83; 95% CI 2.41 - 19.3). On logistic regression, only antibiotic exposure independently predicted for CDAD (OR 2.9; 95% CI 1.6 - 5.1). Three (16%) cases of CA-CDAD reported antibiotic exposure (v. 90% of HA-CDAD, p<0.0001). Twelve (86%) patients had concomitant IBD (p<0.0001 v. HA-CDAD). CA-CDAD was significantly associated with antibiotic exposure (OR 0.04, 95% CI 0.01 - 0.24) and IBD (OR 9.6, 95% CI 1.15 - 79.8). CONCLUSION: The incidence of HA-CDAD in the South African setting is far lower than that reported in the West. While antibiotic use was a major risk factor for HA-CDAD, CA-CDAD was not associated with antibiotic therapy. Concurrent IBD was a predictor of CA-CDAD
Effective viscosity of microswimmer suspensions
The measurement of a quantitative and macroscopic parameter to estimate the
global motility of a large population of swimming biological cells is a
challenge Experiments on the rheology of active suspensions have been
performed. Effective viscosity of sheared suspensions of live unicellular
motile micro-algae (\textit{Chlamydomonas Reinhardtii}) is far greater than for
suspensions containing the same volume fraction of dead cells and suspensions
show shear thinning behaviour. We relate these macroscopic measurements to the
orientation of individual swimming cells under flow and discuss our results in
the light of several existing models
Twirling Elastica: Kinks, Viscous Drag, and Torsional Stress
Biological filaments such as DNA or bacterial flagella are typically curved
in their natural states. To elucidate the interplay of viscous drag, twisting,
and bending in the overdamped dynamics of such filaments, we compute the
steady-state torsional stress and shape of a rotating rod with a kink. Drag
deforms the rod, ultimately extending or folding it depending on the kink
angle. For certain kink angles and kink locations, both states are possible at
high rotation rates. The agreement between our macroscopic experiments and the
theory is good, with no adjustable parameters.Comment: 4 pages, 4 figure
Labels for non-individuals
Quasi-set theory is a first order theory without identity, which allows us to
cope with non-individuals in a sense. A weaker equivalence relation called
``indistinguishability'' is an extension of identity in the sense that if
is identical to then and are indistinguishable, although the
reciprocal is not always valid. The interesting point is that quasi-set theory
provides us a useful mathematical background for dealing with collections of
indistinguishable elementary quantum particles. In the present paper, however,
we show that even in quasi-set theory it is possible to label objects that are
considered as non-individuals. We intend to prove that individuality has
nothing to do with any labelling process at all, as suggested by some authors.
We discuss the physical interpretation of our results.Comment: 11 pages, no figure
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