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Arrival Time (arrival + time)
Selected AbstractsClinical and Economic Factors Associated with Ambulance Use to the Emergency DepartmentACADEMIC EMERGENCY MEDICINE, Issue 8 2006Jennifer Prah Ruger PhD Background: Concern about ambulance diversion and emergency department (ED) overcrowding has increased scrutiny of ambulance use. Knowledge is limited, however, about clinical and economic factors associated with ambulance use compared to other arrival methods. Objectives: To compare clinical and economic factors associated with different arrival methods at a large, urban, academic hospital ED. Methods: This was a retrospective, cross-sectional study of all patients seen during 2001 (N= 80,209) at an urban academic hospital ED. Data were obtained from hospital clinical and financial records. Outcomes included acuity and severity level, primary complaint, medical diagnosis, disposition, payment, length of stay, costs, and mode of arrival (bus, car, air-medical transport, walk-in, or ambulance). Multivariate logistic regression identified independent factors associated with ambulance use. Results: In multivariate analysis, factors associated with ambulance use included: triage acuity A (resuscitation) (adjusted odds ratio [OR], 51.3; 95% confidence interval [CI] = 33.1 to 79.6) or B (emergent) (OR, 9.2; 95% CI = 6.1 to 13.7), Diagnosis Related Group severity level 4 (most severe) (OR, 1.4; 95% CI = 1.2 to 1.8), died (OR, 3.8; 95% CI = 1.5 to 9.0), hospital intensive care unit/operating room admission (OR, 1.9; 95% CI = 1.6 to 2.1), motor vehicle crash (OR, 7.1; 95% CI = 6.4 to 7.9), gunshot/stab wound (OR, 2.1; 95% CI = 1.5 to 2.8), fell 0,10 ft (OR, 2.0; 95% CI = 1.8 to 2.3). Medicaid Traditional (OR, 2.0; 95% CI = 1.4 to 2.4), Medicare Traditional (OR, 1.8; 95% CI = 1.7 to 2.1), arrived weekday midnight,8 AM (OR, 2.0; 95% CI = 1.8 to 2.1), and age ,65 years (OR, 1.3; 95% CI = 1.2 to 1.5). Conclusions: Ambulance use was related to severity of injury or illness, age, arrival time, and payer status. Patients arriving by ambulance were more likely to be acutely sick and severely injured and had longer ED length of stay and higher average costs, but they were less likely to have private managed care or to leave the ED against medical advice, compared to patients arriving by independent means. [source] Analysis of a Vertical Dipole Tracer Test in Highly Fractured RockGROUND WATER, Issue 5 2002William E. Sanford The results of a vertical dipole tracer experiment performed in highly fractured rocks of the Clare Valley, South Australia, are presented. The injection and withdrawal piezometers were both screened over 3 m and were separated by 6 m (midpoint to midpoint). Due to the long screen length, several fracture sets were intersected, some of which do not connect the two piezometers. Dissolved helium and bromide were injected into the dipole flow field for 75 minutes, followed by an additional 510 minutes of flushing. The breakthrough of helium was retarded relative to bromide, as was expected due to the greater aqueous diffusion coefficient of helium. Also, only 25% of the total mass injected of both tracers was recovered. Modeling of the tracer transport was accomplished using an analytical one-dimensional flow and transport model for flow through a fracture with diffusion into the matrix. The assumptions made include: streamlines connecting the injection and withdrawal point can be modeled as a dipole of equal strength, flow along each streamline is one dimensional, and there is a constant Peclet number for each streamline. In contrast to many other field tracer studies performed in fractured rock, the actual travel length between piezometers was not known. Modeling was accomplished by fitting the characteristics of the tracer breakthrough curves (BTCs), such as arrival times of the peak concentration and the center of mass. The important steps were to determine the fracture aperture (240 ,m) based on the parameters that influence the rate of matrix diffusion (this controls the arrival time of the peak concentration); estimating the travel distance (11 m) by fitting the time of arrival of the centers of mass of the tracers; and estimating fracture dispersivity (0.5 m) by fitting the times that the inflection points occurred on the front and back limbs of the BTCs. This method works even though there was dilution in the withdrawal well, the amount of which can be estimated by determining the value that the modeled concentrations need to be reduced to fit the data (,50%). The use of two tracers with different diffusion coefficients was not necessary, but it provides important checks in the modeling process because the apparent retardation between the two tracers is evidence of matrix diffusion and the BTCs of both tracers need to be accurately modeled by the best fit parameters. [source] Impact of Biomass-Decay Terms on the Simulation of Pulsed BioremediationGROUND WATER, Issue 2 2000Olaf A. Cirpka A numerical model is used for simulating the stimulation of biomass growth by injection of alternating pulses of a primary substrate and oxygen. We consider that the substrate sorbs, whereas oxygen does not undergo mass transfer, and mixing of the reacting compounds is dominated by the chromatographic effect. Different mathematical formulations for biomass growth and decay are compared. In models considering biomass decay, a minimal time of joint exposure to both reactants can be determined. This leads to a multimodal distribution of the biomass after multiple injection cycles. In multidimensional heterogeneous domains, the location of the biomass peaks is determined by the advective arrival time. The biomass is much more homogeneously distributed when biomass decay is neglected, because under this condition there is no constraint by a minimal joint exposure time. For the case of oxygen-dependent biomass decay, an injection scheme using shorter pulses of higher oxygen concentrations is shown to be superior to a scheme with equally long pulses of oxygen and the substrate. [source] Mate choice in Pied Flycatchers Ficedula hypoleuca: can females use song to find high-quality males and territories?IBIS, Issue 1 2003Helene M. Lampe High-quality male Pied Flycatchers Ficedula hypoleuca (defined by brighter plumage, better condition and more experience) have previously been shown to have larger syllable repertoires and greater song versatility than males of inferior quality. Thus, by preferring more complex songs, females could choose a high-quality male. Females may also use song as a cue to find a high-quality territory since early arriving males may obtain the best territories and these males have more complex songs than late-arriving males. We found that males with more complex songs had a greater chance of becoming paired and stayed unpaired for a shorter period than males with less elaborate songs. When controlling for arrival order, however, only strophe versatility was still correlated with pairing order. Males defending popular territories had more complex and longer songs and were also in better body condition than males in less popular territories. A multiple logistic regression showed that song length was important in explaining whether a male defended a popular nestbox or not. Thus, male arrival time seems to be important in deciding the quality of a male's territory, which in turn explains female choice. However, song quality seems to add important information. Thus, females could find both high-quality males and high-quality territories by using song cues during mate choice. [source] Shoreline tracking and implicit source terms for a well balanced inundation modelINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 10 2010Giovanni FranchelloArticle first published online: 31 JUL 200 Abstract The HyFlux2 model has been developed to simulate severe inundation scenario due to dam break, flash flood and tsunami-wave run-up. The model solves the conservative form of the two-dimensional shallow water equations using the finite volume method. The interface flux is computed by a Flux Vector Splitting method for shallow water equations based on a Godunov-type approach. A second-order scheme is applied to the water surface level and velocity, providing results with high accuracy and assuring the balance between fluxes and sources also for complex bathymetry and topography. Physical models are included to deal with bottom steps and shorelines. The second-order scheme together with the shoreline-tracking method and the implicit source term treatment makes the model well balanced in respect to mass and momentum conservation laws, providing reliable and robust results. The developed model is validated in this paper with a 2D numerical test case and with the Okushiri tsunami run up problem. It is shown that the HyFlux2 model is able to model inundation problems, with a satisfactory prediction of the major flow characteristics such as water depth, water velocity, flood extent, and flood-wave arrival time. The results provided by the model are of great importance for the risk assessment and management. Copyright © 2009 John Wiley & Sons, Ltd. [source] Locomotive assignment and freight train scheduling using genetic algorithmsINTERNATIONAL TRANSACTIONS IN OPERATIONAL RESEARCH, Issue 4 2006T. Godwin Abstract This study considers the movement of freight trains through a passenger rail network, a common occurrence in many developing countries. Passenger trains run according to a fixed schedule while freight trains need to be accommodated and run on the same track, ensuring that they do not interfere with passenger train movements. Operationally, this requires the assignment of a locomotive to a freight rake and then creating a workable schedule. Accordingly, we propose to solve the problem in two phases. In the first phase, we assign locomotives with partial scheduling with the objectives of minimizing total deadheading time and total coupling delay. We use a genetic algorithm to find non-dominant locomotive assignment solutions and propose a method for evaluating its performance. The solutions are then ranked using two approaches, based on the decision maker's preferences. In the second phase, we select a locomotive assignment solution based on the ranking and find the lower bound on the arrival time of freight trains at their destinations. We use a genetic algorithm again to schedule the freight trains in the passenger rail network, with prescribed locomotive assignment precedence constraints with the objective of minimizing total tardiness. Computational results confirm the efficacy of the proposed method. [source] Breeding in high-elevation habitat results in shift to slower life-history strategy within a single speciesJOURNAL OF ANIMAL ECOLOGY, Issue 2 2009H. Bears Summary 1Elevational gradients create environmental variation that is hypothesized to promote variation in life-history strategies. We tested whether differences in life-history strategies were associated with elevation in a songbird, the dark-eyed junco (Junco hyemalis; Aves; A.O.U. 1998). 2We monitored birds in four replicated sites per elevation, at 2000 m a.s.l. (high elevation) and 1000 m a.s.l. (low elevation), in the Rocky Mountains of Canada. 3Over 5 years, we measured the following traits and vital rates: egg-laying schedules, morphological indicators of reproductive stage, seasonal reproductive success, indicators of competitive class (age, size, arrival time), and survival rates. 4We found two main patterns: with an increase in breeding elevation, dark-eyed juncos delayed the development of structures necessary for reproduction (e.g. cloacal protuberance in males) and reduced the duration of their reproductive period to less than half of the time used by low-elevation birds; and 5Juncos at high-elevation sites had 55,61% lower annual reproductive success and 15 to 20% higher survival rates. While adult juncos at high elevations produced fewer offspring, those offspring were in better condition. Proportions of age and size classes in high- compared to low-elevation populations were similar, suggesting that a life-history trade-off is present, rather than competition forcing inferior competitors to breed in a peripheral habitat. The apparent trade-off between reproduction and survival corresponded to a shorter period of favourable weather and available food in high- compared to low-elevation habitats. 6Thus, elevation had a strong influence on life-history characteristics of a single species over a short spatial distance, suggesting a shift in life history from a high reproductive strategy at lower elevations to a high survivor strategy at high elevations. 7This is the first paper to show a shift in avian life-history strategies along an elevational gradient (in both genders, of multiple age classes) when region (latitude) and phylogenetic histories are controlled for. [source] Patterns of change in timing of spring migration in North European songbird populationsJOURNAL OF AVIAN BIOLOGY, Issue 1 2006Anders P. Tøttrup From 1976 to 1997 passerines were mist-netted and ringed on the island of Christiansø, in the Baltic Sea. Here we present analyses of phenological changes (i.e. time of arrival) for 25 species based on the entire populations of mist-netted songbirds during spring migration. We used two approaches (least square and quantile regression) to test for changes in arrival time of first individuals and three different parts of the songbird populations (i.e. first 5%, 50% and 95% of the total number of trapped individuals corrected for trapping effort). Our results generally confirm earlier spring arrival of migratory passerines with an overall earlier arrival of 0.26 days per year. Changes in the arrival time of first individuals are often the only data available. They are typically analysed on the assumption that they are representative of their respective population. We found a unidirectional, significant change towards earlier arrival for all four measures of arrival timing which seem to support this. However, the four measures of arrival are changing at different rates. First individuals changed arrival time more rapidly than the first 5%, 50% and 95% of the spring total. Such differences are likely to be important for our understanding of population-dynamic changes in relation to climate change. These differences may also have long-term evolutionary consequences. Migration distance seems to affect the degree of change in arrival time, but we found no difference between species wintering in different regions of Africa. [source] Climatic effects on timing of spring migration and breeding in a long-distance migrant, the pied flycatcher Ficedula hypoleucaJOURNAL OF AVIAN BIOLOGY, Issue 5 2005Christiaan Both Climate change has advanced the breeding dates of many bird species, but for few species we know whether this advancement is sufficient to track the advancement of the underlying levels of the food chain. For the long-distance migratory pied flycatcher Ficedula hypoleuca the advancement in breeding time has been insufficient to maintain the synchrony with their main food sources. The timing of arrival in the breeding areas from their African wintering grounds is likely to constrain the advancement of breeding date. We hypothesise that this is because in Africa they cannot predict the advancement of spring in their breeding habitat. However, long-distance migrants may advance their arrival time by migrating faster when circumstances en route are favourable. In this study we show that both arrival and breeding date depend on temperatures at their main North African staging grounds, as well as on temperature at the breeding grounds. Male arrival and average laying date were not correlated, but the positive effect of temperature in North Africa on breeding dates suggests that breeding date is indeed constrained by arrival of females. Long-distance migrants thus are able to adjust arrival and hence breeding by faster spring migration, but the degree of adjustment is probably limited as timing schedules in spring are tight. Furthermore, as climate change is affecting temperatures differently along the migratory flyway and the breeding areas, it is unlikely that arrival dates are advanced at the same rate as the timing of breeding should advance, given the advancement of the underlying levels of the food chain. [source] Improved bolus arrival time and arterial input function estimation for tracer kinetic analysis in DCE-MRIJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2009Anup Singh PhD Abstract Purpose To develop a methodology for improved estimation of bolus arrival time (BAT) and arterial input function (AIF) which are prerequisites for tracer kinetic analysis of dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) data and to verify the applicability of the same in the case of intracranial lesions (brain tumor and tuberculoma). Materials and Methods A continuous piecewise linear (PL) model (with BAT as one of the free parameters) is proposed for concentration time curve C(t) in T1 -weighted DCE-MRI. The resulting improved procedure suggested for automatic extraction of AIF is compared with earlier methods. The accuracy of BAT and other estimated parameters is tested over simulated as well as experimental data. Results The proposed PL model provides a good approximation of C(t) trends of interest and fit parameters show their significance in a better understanding and classification of different tissues. BAT was correctly estimated. The automatic and robust estimation of AIF obtained using the proposed methodology also corrects for partial volume effects. The accuracy of tracer kinetic analysis is improved and the proposed methodology also reduces the time complexity of the computations. Conclusion The PL model parameters along with AIF measured by the proposed procedure can be used for an improved tracer kinetic analysis of DCE-MRI data. J. Magn. Reson. Imaging 2009;29:166,176. © 2008 Wiley-Liss, Inc. [source] Colon Delivery Efficiencies of Intestinal Pressure-controlled Colon Delivery Capsules Prepared by a Coating Machine in Human SubjectsJOURNAL OF PHARMACY AND PHARMACOLOGY: AN INTERNATI ONAL JOURNAL OF PHARMACEUTICAL SCIENCE, Issue 10 2000ZHAOPENG HU Large quantities of pressure-controlled colon delivery capsules (PCDCs) were prepared by a Hicoater-mini pharmaceutical coating machine and colon delivery efficiencies were evaluated in man. Caffeine powder as a model drug was suspended with a polyethylene glycol (PEG) 1000 suppository base at 50°C, and was hardened in no. 0- and no. 2-sized capsular shapes. The capsule-shaped suppositories were coated with 5% w/v ethanolic ethylcellulose (7G grade) solution using the coating machine. By increasing the coating weight of ethylcellulose from 28.6 ± 1.1 mg to 45.3 ± 0.2 mg, the mean coating thickness of no. 0 PCDCs increased from 56 ± 1 ,m to 64 ± 1 ,m. With no. 2 PCDCs, the mean coating thickness increased from 50 ± 1 ,m to 57 ± 1 ,m by increasing the coating weight of ethylcellulose from 8.1 ± 0.5 mg to 11.2 ± 0.3 mg. The no. 0 PCDCs, having a mean ethylcellulose coating membrane thicknesses of 56± 1 ,m (type 1) and 64 ± 1 ,m (type 2), as well as no. 2 PCDCs, having thicknesses of 50 ± 1 ,m (type 3) and 57 ± 1 ,m (type 4), were used for in-vivo evaluation in man. After oral administration of test preparations containing 75 mg of caffeine, saliva samples were obtained and salivary caffeine levels were measured by an HPLC method. The first appearance time, Ti, of caffeine in the saliva was used as a parameter for the estimation of the release time of caffeine from PCDCs in the gastrointestinal tract. The mean Ti values of no. 0 PCDCs were 3.3 ± 0.3 h for type-1 and 5.3 ± 0.3 h for type-2 preparations while the mean Ti values of no. 2 PCDCs were 4.3 ± 0.5 h for type 3 and 5.3 ± 0.3 h for type 4. There were good correlations between ethylcellulose coating membrane thicknesses and in-vivo Ti values. A colon arrival time of 5 h was reported in our subjects by gastrointestinal magnetomarkergraphy. PCDCs having a mean coating thickness of 64± 1 ,m for no. 0 capsules and of 57 ± 1 ,m for no. 2 capsules were thought to deliver caffeine to the human colon efficiently. [source] Measurement of the two-photon correlation of synchrotron radiation in the VUV region by a delay-time modulation techniqueJOURNAL OF SYNCHROTRON RADIATION, Issue 4 2003Yasuhiro Takayama The two-photon correlation (second-order coherence) of synchrotron radiation in the VUV region ( = 55,eV) has been measured using a novel photon-counting method. A new technique has been developed to measure a small bunching effect by using a coincidence unit composed of a constant fraction discriminator, a time-to-amplitude converter (TAC), a single-channel analyzer (SCA) and two solid-state switches. The path of the circuit through which the stop signal for the TAC passes can be changed by a control voltage generated by a function generator, and the relative arrival time of two photons on condition that the output signal from the SCA appears is consequently changed. By modulating the arrival time and measuring the output rate from the SCA with a digital lock-in amplifier, an apparent bunching effect has been observed which is characteristic of the chaotic light. The electron-beam emittance in the horizontal direction was estimated as ,nm,rad by this experiment, and the value was consistent with the designed value of 36,nm,rad. [source] Robust ship scheduling with multiple time windowsNAVAL RESEARCH LOGISTICS: AN INTERNATIONAL JOURNAL, Issue 6 2002Marielle Christiansen We present a ship scheduling problem concerned with the pickup and delivery of bulk cargoes within given time windows. As the ports are closed for service at night and during weekends, the wide time windows can be regarded as multiple time windows. Another issue is that the loading/discharging times of cargoes may take several days. This means that a ship will stay idle much of the time in port, and the total time at port will depend on the ship's arrival time. Ship scheduling is associated with uncertainty due to bad weather at sea and unpredictable service times in ports. Our objective is to make robust schedules that are less likely to result in ships staying idle in ports during the weekend, and impose penalty costs for arrivals at risky times (i.e., close to weekends). A set partitioning approach is proposed to solve the problem. The columns correspond to feasible ship schedules that are found a priori. They are generated taking the uncertainty and multiple time windows into account. The computational results show that we can increase the robustness of the schedules at the sacrifice of increased transportation costs. © 2002 Wiley Periodicals, Inc. Naval Research Logistics 49: 611,625, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/nav.10033 [source] On-line algorithms for minimizing makespan on batch processing machinesNAVAL RESEARCH LOGISTICS: AN INTERNATIONAL JOURNAL, Issue 3 2001Gouchuan Zhang Abstract We consider problem of scheduling jobs on-line on batch processing machines with dynamic job arrivals to minimize makespan. A batch machine can handle up to B jobs simultaneously. The jobs that are processed together from a batch, and all jobs in a batch start and complete at the same time. The processing time of a batch is given by the longest processing time of any job in the batch. Each job becomes available at its arrival time, which is unknown in advance, and its processing time becomes known upon its arrival. In the first part of this paper, we address the single batch processing machine scheduling problem. First we deal with two variants: the unbounded model where B is sufficiently large and the bounded model where jobs have two distinct arrival times. For both variants, we provide on-line algorithms with worst-case ratio (the inverse of the Golden ratio) and prove that these results are the best possible. Furthermore, we generalize our algorithms to the general case and show a worst-case ratio of 2. We then consider the unbounded case for parallel batch processing machine scheduling. Lower bound are given, and two on-line algorithms are presented. © 2001 John Wiley & Sons, Inc. Naval Research Logistics 48: 241,258, 2001 [source] Toward faster algorithms for dynamic traffic assignment.NETWORKS: AN INTERNATIONAL JOURNAL, Issue 1 2003Abstract Being first in a three-part series promising a practical solution to the user-equilibrium dynamic traffic assignment problem, this paper devises a parametric quickest-path tree algorithm, whose model makes three practical assumptions: (i) the traversal time of an arc i , j is a piecewise linear function of the arrival time at its i -node; (ii) the traversal time of a path is the sum of its arcs' traversal times; and (iii) the FIFO constraint holds, that is, later departure implies later arrival. The algorithm finds a quickest path, and its associated earliest arrival time, to every node for every desired departure time from the origin. Its parametric approach transforms a min-path tree for one departure-time interval into another for the next adjacent interval, whose shared boundary the algorithm determines on the fly. By building relatively few trees, it provides the topology explicitly and the arrival times implicitly of all min-path trees. Tests show the algorithm running upward of 10 times faster than the conventional brute-force approach, which explicitly builds a min-path tree for every departure time. Besides dynamic traffic assignment, other applications for which these findings have utility include traffic control planning, vehicle routing and scheduling, real-time highway route guidance, etc. © 2002 Wiley Periodicals, Inc. [source] Retrospective cine 3He ventilation imaging under spontaneous breathing conditions: a non-invasive protocol for small-animal lung function imagingNMR IN BIOMEDICINE, Issue 2 2007Vasile Stupar Abstract A non-invasive and free-breathing hyperpolarized (HP) 3He imaging protocol for small animals was implemented and validated on rats for lung function imaging. Animals were allowed to breathe a mixture of air and 3He from a mask and a gas reservoir fitted to their heads. Radial imaging sequences were used, and MRI signal intensity changes were monitored for retrospective cine image reconstruction. The ventilation cycle of the animals was imaged with a 100,ms temporal resolution. The sliding window imaging technique was applied to reconstruct 5,ms time-shifted image series covering the complete breathing cycle. Image series were processed to extract quantitative ventilation parameters such as the gas arrival time. The reproducibility and the non-invasiveness of this ventilation imaging protocol were evaluated by multiple acquisitions on the same animals. Copyright © 2006 John Wiley & Sons, Ltd. [source] Polarization control of ultrashort mid-IR laser pulses for transient vibrational circular dichroism measurements,CHIRALITY, Issue 1E 2009Mathias Bonmarin Abstract Linear dichroism and birefringence artifacts are a major source of concern in transient circular dichroism measurements. They mainly arise from interaction of an imperfectly circular polarized probe beam with a nonisotropic sample. We present in this article a procedure to generate mid-IR pulses of highly symmetric left and right handed circular or elliptical polarization states for transient VCD measurements. An infrared femtosecond laser source is synchronized to the natural frequency of a photo elastic modulator. Residual static birefringence of the modulator and the sample cell can be largely compensated by carefully controlling the arrival time of the ultrashort probe pulses at the modulator. Chirality 21:E298,E306, 2009. © 2009 Wiley-Liss, Inc. [source] Emergency Department Overcrowding: Analysis of the Factors of Renege RateACADEMIC EMERGENCY MEDICINE, Issue 2 2007Phillip V. Asaro MD Background Reneging (i.e., leaving without being seen) is an important outcome of emergency department (ED) overcrowding. The input-throughput-output conceptualization of ED patient flow is helpful in understanding and measuring the impact of various factors on this outcome. Objectives To quantify the impact of input and output factors on ED renege rate. Methods The authors used patient-level and system-level data from multiple sources in their institution to build logistic regression models, with reneging as the dependent variable. This approach provides the impact of each input and output factor on renege rate expressed as an odds ratio (OR). Results The OR for reneging attributable to the difference between the 80th and 20th percentile values for inpatient bed utilization is 1.05. Comparing 80th and 20th percentile values for boarded ED admits as of 7 am, the OR is 1.73; for daily ED arrivals, the OR is 2.00; and for admission percentage, the OR is 1.12. The OR for evening versus morning patient arrival time is 3.9 and for patient arrival on a Monday versus a Sunday is 2.7. The OR for reneging for a patient presenting on Monday evening versus Sunday morning is 10.5. Conclusions The effects of ED input and output factors on renege rate are significant and quantifiable. At least some of the variation in these factors and subsequently their effects are predictable, suggesting that further refinement in the management of ED and inpatient resources could affect improvement in ED renege rate. Continued efforts at quantifying the effects are warranted. [source] The Econometrics of Ultra-high-frequency DataECONOMETRICA, Issue 1 2000Robert F. Engle Ultra-high-frequency data is defined to be a full record of transactions and their associated characteristics. The transaction arrival times and accompanying measures can be analyzed as marked point processes. The ACD point process developed by Engle and Russell (1998) is applied to IBM transactions arrival times to develop semiparametric hazard estimates and conditional intensities. Combining these intensities with a GARCH model of prices produces ultra-high-frequency measures of volatility. Both returns and variances are found to be negatively influenced by long durations as suggested by asymmetric information models of market micro-structure. [source] Delineating the rupture planes of an earthquake doublet using Source-Scanning Algorithm: application to the 2005 March 3 Ilan Doublet, northeast TaiwanGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2010Chih-Wen Kan SUMMARY Correct identification of the fault plane(s) associated with an earthquake doublet is a very challenging problem because the pair of events often occurs in close space and time with almost the same magnitude. Most long-period waveforms of an earthquake doublet are severely tangled and thus unsuitable for conventional waveform inversion methods. In this study, we try to resolve this issue by utilizing the recently developed Source-Scanning Algorithm (SSA). The SSA systematically searches the model space for seismic sources whose times and locations are most compatible with the observed arrivals of large amplitudes on seismograms. The identification of a seismic source is based on the brightness function, which is defined as the summation of the normalized waveform amplitudes at the predicted arrival times at all stations. By illuminating the spatiotemporal distribution of asperities during an earthquake's source process, we are able to constrain the orientation of the rupture propagation that, in turn, leads to the identification of the fault plane. A series of synthetic experiments are performed to test SSA's resolution under various scenarios including different directions of rupture propagation, imperfect station coverage and short origin time difference between the two events of a doublet. Because only short-period records are needed in the analysis, the proposed method is best suited for an earthquake doublet with a short time gap between the two events. Using the 2005 Ilan doublet (the origin time difference is only 70 s) that occurred in northeast Taiwan as an example, we show that the trace of the brightest spots moves towards the west and infer the E,W-striking plane to be the actual fault plane. [source] Optimal measurement of relative and absolute delay times by simulated annealingGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2002S. Chevrot Summary Conventional approaches to determine relative arrival times of body waves recorded by a local or a regional array rely on cross-correlations between pairs of traces. This problem is better posed as a non-linear inverse problem, which involves the minimization of a cost function that measures the difference between a reference unknown waveform that can be time delayed, and the observed data. The unknown parameters that are solved for, are the amplitude values at each time sample of the optimal waveform and its time delay defined for each trace. The problem, which has a large number of unknown parameters, is solved with simulated annealing. The algorithm is very efficient and can be used for a routine analysis of seismic data. The application of this method to several earthquakes recorded during different PASSCAL experiments demonstrates that it provides accurate and robust differential traveltime measurements even with very noisy data. [source] Improved microseismic event location by inclusion of a priori dip particle motion: a case study from EkofiskGEOPHYSICAL PROSPECTING, Issue 5 2010G.A. Jones ABSTRACT Microseismic monitoring in petroleum settings provides insights into induced and naturally occurring stress changes. Such data are commonly acquired using an array of sensors in a borehole, providing measures of arrival times and polarizations. Events are located using 1D velocity models, P- and S-wave arrival times and the azimuths of P-wave particle motions. However in the case of all the sensors being deployed in a vertical or near-vertical borehole, such analysis leads to an inherent 180° ambiguity in the source location. Here we present a location procedure that removes this ambiguity by using the dip of the particle motion as an a priori information to constrain the initial source location. The new procedure is demonstrated with a dataset acquired during hydraulic fracture stimulation, where we know which side of the monitoring well the events are located. Using a 5 -step location procedure, we then reinvestigate a microseismic data set acquired in April 1997 at the Ekofisk oilfield in the North Sea. Traveltimes for 2683 candidate events are manually picked. A noise-weighted analytic-signal polarization analysis is used to estimate the dip and azimuth of P-wave particle motions. A modified t-test is used to statistically assess the reliability of event location. As a result, 1462 events are located but 627 are deemed to be statistically reliable. The application of a hierarchal cluster analysis highlights coherent structures that cluster around wells and inferred faults. Most events cluster at a depth of roughly 3km in the Ekofisk chalk formation but very little seismicity is observed from the underlying Tor chalk formation, which is separated from the Ekofisk formation by an impermeable layer. We see no evidence for seismicity in the overburden but such events may be too distant to detect. The resulting picture of microseismicity at Ekofisk is very different from those presented in previous studies. [source] Constrained tomography of realistic velocity models in microseismic monitoring using calibration shotsGEOPHYSICAL PROSPECTING, Issue 5 2010T. Bardainne ABSTRACT The knowledge of the velocity model in microseismic jobs is critical to achieving statistically reliable microseismic event locations. The design of microseismic networks and the limited sources for calibration do not allow for a full tomographic inversion. We propose optimizing a priori velocity models using a few active shots and a non-linear inversion, suitable to poorly constrained systems. The considered models can be described by several layers with different P- and S-wave velocities. The velocities may be constant or have 3D gradients; the layer interfaces may be simple dipping planes or more complex 3D surfaces. In this process the P- and S- wave arrival times and polarizations measured on the seismograms constitute the observed data set. They are used to estimate two misfit functions: i) one based on the measurement residuals and ii) one based on the inaccuracy of the source relocation. These two functions are minimized thanks to a simulated annealing scheme, which decreases the risk of converging to a local solution within the velocity model. The case study used to illustrate this methodology highlights the ability of this technique to constrain a velocity model with dipping layers. This was performed by jointly using sixteen perforation shots recorded during a multi-stage fracturing operation from a single string of 3C-receivers. This decreased the location inaccuracies and the residuals by a factor of six. In addition, the retrieved layer dip was consistent with the pseudo-horizontal trajectories of the wells and the background information provided by the customer. Finally, the theoretical position of each calibration shot was contained in the uncertainty domain of the relocation of each shot. In contrast, single-stage inversions provided different velocity models that were neither consistent between each other nor with the well trajectories. This example showed that it is essential to perform a multi-stage inversion to derive a better updated velocity model. [source] Do changes in climate patterns in wintering areas affect the timing of the spring arrival of trans-Saharan migrant birds?GLOBAL CHANGE BIOLOGY, Issue 1 2005Oscar Gordo Abstract The life cycles of plants and animals are changing around the world in line with the predictions originated from hypotheses concerning the impact of global warming and climate change on biological systems. Commonly, the search for ecological mechanisms behind the observed changes in bird phenology has focused on the analysis of climatic patterns from the species breeding grounds. However, the ecology of bird migration suggests that the spring arrival of long-distance migrants (such as trans-Saharan birds) is more likely to be influenced by climate conditions in wintering areas given their direct impact on the onset of migration and its progression. We tested this hypothesis by analysing the first arrival dates (FADs) of six trans-Saharan migrants (cuckoo Cuculus canorus, swift Apus apus, hoopoe Upupa epops, swallow Hirundo rustica, house martin Delichon urbica and nightingale Luscinia megarhynchos), in a western Mediterranean area since from 1952 to 2003. By means of multiple regression analyses, FADs were analysed in relation to the monthly temperature and precipitation patterns of five African climatic regions south of the Sahara where species are thought to overwinter and from the European site from where FADs were collected. We obtained significant models for five species explaining 9,41% of the variation in FADs. The interpretation of the models suggests that: (1) The climate in wintering quarters, especially the precipitation, has a stronger influence on FADs than that in the species' potential European breeding grounds. (2) The accumulative effects of climate patterns prior to migration onset may be of considerable importance since those climate variables that served to summarize climate patterns 12 months prior to the onset of migration were selected by final models. (3) Temperature and precipitation in African regions are likely to affect departure decision in the species studied through their indirect effects on food availability and the build-up of reserves for migration. Our results concerning the factors that affect the arrival times of trans-Saharan migrants indicate that the effects of climate change are more complex than previously suggested, and that these effects might have an interacting impact on species ecology, for example by reversing ecological pressures during species' life cycles. [source] Calculation of Elapsed Decimal Time for Tracing StudiesGROUND WATER, Issue 1 2008Malcolm S. Field Calculation of time of travel from tracing studies in hydrologic systems is critical to establishing pollutant arrival times from points of inflow to points outflow, calculating subsurface flow velocities, and determining other important transport parameters such as longitudinal dispersion. In addition, breakthrough curve modeling demands accurate time of travel calculations if model results are to have any realistic meaning. However, accurate time of travel calculations are very difficult for long tracer tests in which sampling schedules are not consistent, or when there are major disruptions such as may occur when adverse weather conditions cause automatic sampling equipment to fail. Long and inconsistent sampling times may be accurately converted to decimal times of travel by converting the conventionally recorded Coordinated Universal Time for sampling date and time event to a baseline time standard. By converting to a baseline time standard, all recorded dates and times are linked to the established baseline standard so that each succeeding sampling date and time are correctly determined relative to the previous sampling date and time and to the injection date and time. [source] Analysis of a Vertical Dipole Tracer Test in Highly Fractured RockGROUND WATER, Issue 5 2002William E. Sanford The results of a vertical dipole tracer experiment performed in highly fractured rocks of the Clare Valley, South Australia, are presented. The injection and withdrawal piezometers were both screened over 3 m and were separated by 6 m (midpoint to midpoint). Due to the long screen length, several fracture sets were intersected, some of which do not connect the two piezometers. Dissolved helium and bromide were injected into the dipole flow field for 75 minutes, followed by an additional 510 minutes of flushing. The breakthrough of helium was retarded relative to bromide, as was expected due to the greater aqueous diffusion coefficient of helium. Also, only 25% of the total mass injected of both tracers was recovered. Modeling of the tracer transport was accomplished using an analytical one-dimensional flow and transport model for flow through a fracture with diffusion into the matrix. The assumptions made include: streamlines connecting the injection and withdrawal point can be modeled as a dipole of equal strength, flow along each streamline is one dimensional, and there is a constant Peclet number for each streamline. In contrast to many other field tracer studies performed in fractured rock, the actual travel length between piezometers was not known. Modeling was accomplished by fitting the characteristics of the tracer breakthrough curves (BTCs), such as arrival times of the peak concentration and the center of mass. The important steps were to determine the fracture aperture (240 ,m) based on the parameters that influence the rate of matrix diffusion (this controls the arrival time of the peak concentration); estimating the travel distance (11 m) by fitting the time of arrival of the centers of mass of the tracers; and estimating fracture dispersivity (0.5 m) by fitting the times that the inflection points occurred on the front and back limbs of the BTCs. This method works even though there was dilution in the withdrawal well, the amount of which can be estimated by determining the value that the modeled concentrations need to be reduced to fit the data (,50%). The use of two tracers with different diffusion coefficients was not necessary, but it provides important checks in the modeling process because the apparent retardation between the two tracers is evidence of matrix diffusion and the BTCs of both tracers need to be accurately modeled by the best fit parameters. [source] Effect of Heterogeneity on Radionuclide Retardation in the Alluvial Aquifer Near Yucca Mountain, NevadaGROUND WATER, Issue 3 2001S. Painter The U.S. Department of Energy is currently studying Yucca Mountain, Nevada, as a potential site for a geological high-level waste repository. In the current conceptual models of radionuclide transport at Yucca Mountain, part of the transport path to pumping locations would be through an alluvial aquifer. Interactions with minerals in the alluvium are expected to retard the downstream migration of radionuclides, thereby delaying arrival times and reducing ground water concentrations. We evaluate the effectiveness of the alluvial aquifer as a transport barrier using the stochastic Lagrangian framework. A transport model is developed to account for physical and chemical heterogeneities and rate-limited mass transfer between mobile and immobile zones. The latter process is caused by small-scale heterogeneity and is thought to control the macroscopic-scale retardation in some field experiments. A geostatistical model for the spatially varying sorption parameters is developed from a site-specific database created from hydrochemical measurements and a calibrated modeling approach (Turner and Pabalan 1999). Transport of neptunium is considered as an example. The results are sensitive to the rate of transfer between mobile and immobile zones, and to spatial variability in the hydraulic conductivity. Chemical heterogeneity has only a small effect, as does correlation between hydraulic conductivity and the neptunium distribution coefficient. These results illustrate how general sensitivities can be explored with modest effort within the Lagrangian framework. Such studies complement and guide the application of more detailed numerical simulations. [source] Using automatic passenger counter data in bus arrival time predictionJOURNAL OF ADVANCED TRANSPORTATION, Issue 3 2007Mei Chen Artificial neural networks have been used in a variety of prediction models because of their flexibility in modeling complicated systems. Using the automatic passenger counter data collected by New Jersey Transit, a model based on a neural network was developed to predict bus arrival times. Test runs showed that the predicted travel times generated by the models are reasonably close to the actual arrival times. [source] AUTOREGRESSIVE CONDITIONAL DURATION MODELS IN FINANCE: A SURVEY OF THE THEORETICAL AND EMPIRICAL LITERATUREJOURNAL OF ECONOMIC SURVEYS, Issue 4 2008Maria Pacurar Abstract This paper provides an up-to-date survey of the main theoretical developments in autoregressive conditional duration (ACD) modeling and empirical studies using financial data. First, we discuss the properties of the standard ACD specification and its extensions, existing diagnostic tests, and joint models for the arrival times of events and some market characteristics. Then, we present the empirical applications of ACD models to different types of events, and identify possible directions for future research. [source] Assessing normal pulse wave velocity in the proximal pulmonary arteries using transit time: A feasibility, repeatability, and observer reproducibility study by cardiovascular magnetic resonanceJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 5 2007MRCP, William M. Bradlow BM Abstract Purpose To calculate pulse wave velocity (PWV) in the proximal pulmonary arteries (PAs) by cardiovascular magnetic resonance (CMR) using the transit-time method, and address respiratory variation, repeatability, and observer reproducibility. Materials and Methods A 1.9-msec interleaved phase velocity sequence was repeated three times consecutively in 10 normal subjects. Pulse wave (PW) arrival times (ATs) were determined for the main and branch PAs. The PWV was calculated by dividing the path length traveled by the difference in ATs. Respiratory variation was considered by comparing acquisitions with and without respiratory gating. Results For navigated data the mean PWVs for the left PA (LPA) and right PA (RPA) were 2.09 ± 0.64 m/second and 2.33 ± 0.44 m/second, respectively. For non-navigated data the mean PWVs for the LPA and RPA were 2.14 ± 0.41 m/second and 2.31 ± 0.49 m/second, respectively. No statistically significant difference was found between respiratory non-navigated data and navigated data. Repeated on-table measurements were consistent (LPA non-navigated P = 0.95, RPA non-navigated P = 0.91, LPA navigated P = 0.96, RPA navigated P = 0.51). The coefficients of variation (CVs) were 12.2% and 12.5% for intra- and interobserver assessments, respectively. Conclusion One can measure PWV in the proximal PAs using transit-time in a reproducible manner without respiratory gating. J. Magn. Reson. Imaging 2007;25:974,981. © 2007 Wiley-Liss, Inc. [source] | ||||||||||||||||||||||||||||