Function Of Arrival Time Research Articles

An advective‐dispersive stream tube approach for the transfer of conservative‐tracer data to reactive transport

Conservative‐tracer data are used for the parameterization of mixing‐controlled reactive transport. Temporal moments of the tracer breakthrough curve integrated over the outflow boundary of the domain yield the average velocity and the path‐averaged macrodispersion coefficient. On the basis of this information alone, no distinction is possible between spreading and mixing of the tracer. Analyzing the temporal moments of breakthrough curves locally obtained at single points in the domain gives additional information about the dilution of the tracer. In an accompanying paper [Cirpka and Kitanidis, this issue] we derive an apparent Péclet number of mixing Pea from local temporal moments. Assuming that Pea is constant over the cross section of the outflow boundary, a corrected probability density function of arrival times is determined. By interpreting the spatially integrated breakthrough curve as the result of advective‐ dispersive transport in independent stream tubes with identical Péclet number but differing seepage velocity, it is possible to transfer results of conservative transport to the transport of interacting compounds for cases in which mixing of the compounds is influenced significantly by local‐scale dispersion. This is an improvement to the stochastic‐convective model of Simmons et al. [1995] for the transfer of integrated tracer data to reactive transport. The approach is applied to the hypothetical case of a bimolecular reaction in a heterogeneous two‐dimensional aquifer.

Focal regions in spectrograms of long range propagation

A source near the deep sound channel axis excites mode groups (or paths) that involve both deep sound channel and boundary interacting propagation. Modal group speeds have a functional transition when passing through purely refractive to boundary reflecting phase speed regions. The result is that arrivals in this transition region line up in time across frequency. The combination of this alignment together with a similar, though broader coincidence of the last deep sound channel arrivals provide two time markers on a single phone spectrogram (intensity as a function of arrival time and frequency). Indeed, data from the Acoustic Thermometry of the Ocean (ATOC) program [ATOC Consortium, Science 281, 1327 (1998)] show this effect. These time markers, depending on auxiliary information provide source localization or inversion capability. [Work supported by ONR and DTRA.]

Short-term synaptic plasticity and network behavior.

We develop a minimal time-continuous model for use-dependent synaptic short-term plasticity that can account for both short-term depression and short-term facilitation. It is analyzed in the context of the spike response neuron model. Explicit expressions are derived for the synaptic strength as a function of previous spike arrival times. These results are then used to investigate the behavior of large networks of highly interconnected neurons in the presence of short-term synaptic plasticity. We extend previous results so as to elucidate the existence and stability of limit cycles with coherently firing neurons. After the onset of an external stimulus, we have found complex transient network behavior that manifests itself as a sequence of different modes of coherent firing until a stable limit cycle is reached.

Variables associated with hospital arrival time after stroke: effect of delay on the clinical efficiency of early treatment.

A limiting criterion for the eligibility of patients in clinical trials investigating acute stroke therapies is that time between onset of symptoms and arrival in the hospital should fall within the "therapeutic window." The aims of this study were to estimate hospital arrival time in an unselected sample of stroke patients, to assess the association with some clinical and demographic variables, and to evaluate the effects of the delay on the clinical efficiency of an effective treatment. We evaluated the delay in hospital arrival time in 189 patients (84 men, 105 women; mean age, 76.5 years) prospectively collected in the S Orsola-Malpighi Community Teaching Hospital in Bologna, Italy. Cutoffs of 2 and 5 hours were chosen to allow for hypothetical treatment within 3 and 6 hours, respectively. Exact multiple logistic regression was used to predict the delay as a function of dichotomized age, sex, symptoms on awakening, day of the week, hour of the day, area of residence, level of consciousness, and level of motor power defect. We then projected the effectiveness of tissue plasminogen activator (TPA) on disability as estimated with the aid of the odds ratio from the National Institute of Neurological Disorders and Stroke (NINDS) rt-PA Stroke Trial onto our unselected sample to evaluate clinical efficiency of treatment as a function of arrival time and of hypothetical effects of educational efforts to reduce it. The mean interval between onset of symptoms and hospital arrival was 680 minutes; 59 patients (31%) arrived within 2 hours and 100 (53%) within 5 hours. Onset of symptoms when awake, drowsiness or coma, and paralysis of at least one limb were the only independent predictors of hospital arrival within 2 and 5 hours in both the total sample and the subgroup of patients who were awake at stroke onset. The effectiveness of 17%, extrapolated with the aid of the odds ratio of 1.6 of having a favorable outcome (Barthel Index > or = 95 at 3 months) in treated versus untreated patients in the NINDS rt-PA Stroke Trial, corresponded to a projected clinical efficiency of 5%. This could be doubled by hypothesizing a 100% effect of educational efforts in reducing the delay in hospital arrival time. Patients with milder symptoms, for whom treatment might be more effective, were less likely to arrive in time for therapy. The proposed model of the relationship between the delay in hospital presentation after a stroke and the clinical efficiency of a given treatment might be useful for planning future clinical trials on early stroke treatment and predicting the impact of an educational program aimed at shortening arrival time.

Complex rays and waveforms near caustics

Ray theory can be carried into a shadow zone using rays of complex launch angle but the search for eigenrays is usually impractical. A new approach first finds the ‘‘measurement front,’’ i.e., the continuous curve of ray depth, as a function of arrival time at a fixed range. The measurement front folds back on itself when the rays touch a caustic. The measurement front can be continued beyond the caustic into the shadow zone using rays of complex launch angle. Points at a fixed depth on the measurement front represent ray arrivals at a hydrophone. Waveforms can be calculated directly from the ray arrivals. Airy functions are used for ray amplitudes near caustics.

Wavelet analysis of velocity dispersion of elastic interface waves propagating along a fracture

A wavelet analysis is performed on seismic waveforms of elastic interface waves that propagate along a fracture. The wavelet analysis provides a direct quantitative measure of spectral content as a function of arrival time. We find that the spectral content of the interface wave signals is not stationary, but exhibits increasing frequency content for later arrival times, representing negative velocity dispersion. The dispersion increases from −11 m/sec/MHz to −116 m/sec/MHz as the stress on the fracture is increased from 3.5 kPa to 33 MPa. The negative velocity dispersion agrees with predictions from the displacement‐discontinuity theory of the seismic response of fractures, and can be used to fit fracture stiffness.

Theoretical research on intensity envelope function of strong ground motion

After the time history of seismic motion is represented by superposition of a series of narrow frequency band wave groups, we obtain a general relation between wave group arrival time and derivative of phase spectra in the paper. On the basis of the relation, frequency number distribution function of wave group arrival time is completely equivalent to that of phase difference spectra. Under the assumption that phase angles of seismic motion obey uniform distribution ranged from 0 to −2π, a quantitative relation between intensity envelope function of seismic motion and energy distribution function with wave group arrival time has been derived in this paper. The relation illuminates inner links among Fourier amplitude spectra and derivative of phase spectra and intensity envelope function. Some examples given by the paper support the conclusions mentioned above.

The transverse coherence function at NORSAR over a wide range of separations

We have used data from the 80‐km‐diameter NORSAR array and the 3‐km‐diameter NORESS array to determine the Transverse Coherence Function (TCF) of arrival time for spatial separations ranging from a few hundred meters to 80 km. To accomplish our objective, we have devised a method to combine data from arrays of different aperture. The procedure is based on information about rms fluctuations and rms tilts on each array. Synthetic fluctuation fields with different statistical properties were generated to test the combination method. Our results for the TCF from NORSAR and NORESS are in reasonable agreement with the predictions of the Flatté‐Wu model of heterogeneities under NORSAR.

A molecular beam study of the O 2[sbnd]Pt(111) interaction

We have performed scattering experiments for O 2 on Pt(111) for translational energies up to 1.7 eV (1.09 eV normal energy). The scattered O 2 flux from the surface is measured as function of arrival time and outgoing angle. The experimental results reveal pronounced lobular scattering. The final energy shows for most experimental conditions a maximum near the specular direction. At higher incident energy, however, no such maximum is observed and the final energy is increasing monotonically with final angle. These results suggest a transition from scattering at a “flat” vibrating surface for low energies to scattering from a corrugated surface for the higher energies. The angular width at low incoming energy is comparable to that for Ar scattering, and is quite low compared with O 2 scattering from Ag(111) where a broadening of the angular spectrum towards the surface normal is observed. For Pt(111) we see a much reduced broadening despite the fact that the binding energies of the physisorption, chemisorption and dissociation states are similar for both systems. The broadening in the case of both Ag(111) and Pt(111) at higher normal energies can be ascribed to the influence of the chemisorption precursor in the interaction potential. In the case of Pt(111), however, the direct inelastic distributions are additionally modified by the significant probability of dissociation.

Post-sector beam deflection in time-resolved ion momentum spectrometry (trims)

Time-resolved ion momentum spectrometry is a technique that combines magnetic sector and time-of-flight analyzers to give energy-independent mass determination, enabling seperation of parent ions and products of metastable or collisionally activated dissociations. In the time-resolving stage of this mass spectrometer, ion packet formation by beam deflection is shown to be superior to that formed by ion source pulsing with simultaneous time and momentum resolutions of 650 and 500, respectively. Loss in detectability over that obtained using the magnetic sector without time resolution is only a factor of 65, despite using only 0.01% of the continuous beam. Precise beam intensity profiles as a function of magnetic field strength and arrival time are obtained.

Timing analysis using functional analysis

The usual block-oriented timing analysis for logic circuits does not take into account functional relations between signals. If functional relations are taken into consideration, it could be found that a long path is never activated. This results in more accurate delays. A comparison is made of three arrival time functions, A, B, and R. A is the arrival time as given by exhaustive simulation; B is the arrival time as calculated by a usual block-oriented algorithm; and R is the arrival time, that does functional analysis. It is shown that B contained in R contained in A. The first relation means that R is never more conservative than B and whenever the containment is proper, R is an improvement over B. The second relation means that R is correct in the sense that it will never assert a signal to be valid when it is not valid according to the ideal A. Experimental results showing how often R is an improvement over B are presented. >

“Spatial nodes” in discrete location problems

Discrete location models often assume an underlying network where demands originate at point nodes. To apply these models to planar regions with continuously distributed demand, the region is usually partitioned into “zones” and the demand from each zone is assumed to originate at a point, usually the zone centroid. Thus, the point node in the underlying network represents a spatial zone with a finite area. This paper examines the effect of approximating these “spatial nodes” by point nodes. In some problem scenarios, the approximation does not affect the solution. However, especially when the locational criterion includes the consideration of intra-zonal travel cost variances (e.g. travel time variance) and demands may originate anywhere within zones of nonzero area, point nodes do not give an accurate evaluation of the performance of a locational design. To illustrate the application of the concept of spatial nodes, a model is formulated for locatingp (fire-fighting) units in a region having continuously distributed demand with the objective of minimizing a nonlinear function of arrival times of the first and second closest units to any (fire) incident. A heuristic “site-substitution” procedure is presented that solves the formulated model.

Use of ray theory to calculate high-frequency radiation from earthquake sources having spatially variable rupture velocity and stress drop

AbstractWe analyze the problem of calculating high-frequency ground motions (>1 Hz) caused by earthquakes having arbitrary spatial variations of rupture velocity and slip velocity (or stress drop) over the fault. We approximate the elastic wave Green's functions by far-field body waves, which we calculate using geometric ray theory. However, we do not make the traditional Fraunhofer approximation, so our method may be used close to large faults. The method is confined to high frequencies (greater than about 1 Hz) due to the omisson of near-field terms, and must be used at source-observer distances less than a few source depths, due to the omission of surface waves. It is easily used in laterally varying velocity structures. Assuming a simple parameterization of the slip function, the computational problem collapses to the evaluation of a series of line integrals over the fault, with one line integral per each time ti in the observer seismogram. The path of integration corresponding to observation time ti consists of only those points on the fault which radiate body waves arriving at the observer at exactly time ti. This path is an isochron of the arrival time function. An isochron velocity may be defined that depends on rupture velocity and resembles the usual directivity function. Observed ground motions are directly dependent upon this isochron velocity. Ground velocity is proportional to isochron velocity and ground acceleration is proportional to isochron acceleration in dislocation models of rupture. Ground acceleration may also be related to spatial variations of slip velocity on the fault, using the square of isochron velocity as a constant of proportionality. We show two rupture models, one with variable slip velocity and the other with variable rupture velocity, that cause the same ground acceleration at a single observer. The computational method is shown to produce reasonably accurate synthetic seismograms, compared to a method using complete Green's functions, and requires about 0.5 per cent of the computer time. It may be very effective in calculating ground motions in the frequency band 1 to 10 Hz at observers within a few source depths of large earthquakes, where most of the high-frequency motions may be caused by direct P and S waves. We suggest a possible method for inverting ground motions for both slip velocity and rupture velocity over the fault.

Transient Response of an Elastic Half Space to Moving Loads

In this paper the transient response of an elastic half space to a non-uniformly moving point load is considered and a solution procedure is developed for the problem of moving loads. By using the well-known Cagniard's technique, a general solution of displacement is obtained in the form of integral for the inner and the surface responses. Then, the title problem is reduced to a simple consideration on Arrival Time Function, the behaviour of which governs the range of integration. The solution obtained here should be applied to miscellaneous problems of a moving load, the speed of which is non-uniform and largely arbitrary.

Transient Response of an Infinite Elastic Solid to a Moving Point Load

Transient response of an infinite elastic solid to a moving point load is considered. An analytical solution procedure is presented for solving a three dimensional problem of the moving load. The load motion is largely arbitrary. The subject problem is reduced to a simple consideration on "Arrival Time Function, " defined in this paper. Three typical ceses of the load motion, stationary, uniformly moving and accelerating loads, are discussed in detail. It is shown that analyses of waves and responses are carried out simultaneously by considering the behaviour of the Arrival Time Function and that extremum of the Arrival Time Function gives characteristic wave fronts such as shock waves ( leading waves ).

SEISMIC VELOCITY ESTIMATION*

AbstractThe accuracy of the two most common arrival time functions used in seismic velocity estimation is investigated. It is shown that the hyperbolic arrival time function is more accurate than the parabolic arrival time function for a horizontally layered elastic medium. An upper bound on the difference between the two arrival time functions is given.A maximum‐likehood detector for estimating the arrival time of the signals is given. For the signal‐in‐noise model that is used the maximum‐likelihood detector is equivalent to a least‐squares detector which corresponds to using the signal energy as coherency measure. The semblance coefficient corresponds to a normalized least‐squares detector. The semblance coefficient is very similar to a filter performance measure that is used in least‐squares filter design.

Airport Access Volumes from Airline Schedules

A methodology is described for estimating air passenger and visitor automobile traffic volumes at an airport as a function of an airline flight time. The methodology allows for the estimation of inbound and outbound traffic volumes for any desired time interval as a function of airline flight arrival and departure times and probability distributions for the times that passenger automobiles enter and leave the airport relative to flight times. The total expected number of air passenger vehicles using the access facilities in a given time period is obtained by superimposing these probability distributions over all airline flights. Previous methods of measuring airport trip generation are reviewed. This is followed by a description of a 2-day travel survey conducted at the Dallas/Fort Worth Regional Airport (DFW) to obtain data for the model. Model estimates are compared with measured traffic counts in preliminary model validation.

Asymmetrical sound channel axis propagation

An acoustic experiment was conducted utilizing the asymmetrical sound channel axis propagation paths of the Mediterranean within the Algero-Provencal Basin. Acoustic transmission loss in 94-Hz bands from 94 to 2250 Hz was obtained as a function of range using shipboard deployed explosive caps, half and one pound charges, and a fixed receiver in the channel. The experimental results are compared with the output of an acoustic model at 94 and 562 Hz. A measure of the attenuation coefficients is obtained over the frequency range which is independent of the acoustic model and is in agreement with data from other investigators. This axis data was transformed to develop the acoustic field for the case of off-axis source locations by a procedure devised to decrement the signal intensity as a function of arrival time. Estimates of the arrival time are based upon an inverse sound speed gradient representation of the medium. The resulting range-depth contours for 281 Hz are compared with outputs from NRL acoustic model TRIMAIN.[Work supported by NAVSEA, Code 06H1.]

A STUDY OF μ-MESONS INCIDENT AT LARGE ZENITH ANGLES

A Geiger counter telescope has been employed to study μ-mesons incident at large zenith angles. The counters were hodoscoped in pairs enabling the direction and angle of incidence of the particles to be determined. The intensity has been analyzed as a function of arrival time but no significant variation has been found either in solar time or sidereal time. However, a variation in solar time has been noted on analysis of extensive air showers measured concurrently. The angular variation of intensity has been studied and absolute rates obtained; these rates have been compared with those of an earlier determination showing excellent agreement. Some evidence has been obtained of a directional anisotropy in the μ-meson flux as measured at the apparatus.