Observed Travel Times Research Articles

Using a numerical model to understand the connection between the ocean and acoustic travel-time measurements

Measurements of acoustic ray travel-times in the ocean provide synoptic integrals of the ocean state between source and receiver. It is known that the ray travel-time is sensitive to variations in the ocean at the transmission time, but the sensitivity of the travel-time to spatial variations in the ocean prior to the acoustic transmission have not been quantified. This study examines the sensitivity of ray travel-time to the temporally and spatially evolving ocean state in the Philippine Sea using the adjoint of a numerical model. A one year series of five day backward integrations of the adjoint model quantify the sensitivity of travel-times to varying dynamics that can alter the travel-time of a 611 km ray by 200 ms. The early evolution of the sensitivities reveals high-mode internal waves that dissipate quickly, leaving the lowest three modes, providing a connection to variations in the internal tide generation prior to the sample time. They are also strongly sensitive to advective effects that alter density along the ray path. These sensitivities reveal how travel-time measurements are affected by both nearby and distant waters. Temporal nonlinearity of the sensitivities suggests that prior knowledge of the ocean state is necessary to exploit the travel-time observations.

A study of solar and interplanetary parameters of CMEs causing major geomagnetic storms during SC 23

Abstract. In this paper we analyse 25 Earth-directed and strongly geoeffective interplanetary coronal mass ejections (ICMEs) which occurred during solar cycle 23, using data provided by instruments on SOHO (Solar and Heliospheric Observatory), ACE (Advanced Composition Explorer) and geomagnetic stations. We also examine the in situ parameters, the energy transfer into magnetosphere, and the geomagnetic indexes. We compare observed travel times with those calculated by observed speeds projected into the plane of the sky and de-projected by a simple model. The best fit was found with the projected speeds. No correlation was found between the importance of a flare and the geomagnetic Dst (disturbance storm time) index. By comparing the in situ parameters with the Dst index we find a strong connection between some of these parameters (such as Bz, Bs · V and the energy transfer into the magnetosphere) with the strength of the geomagnetic storm. No correlation was found with proton density and plasma temperature. A superposed epoch analysis revealed a strong dependence of the Dst index on the southward component of interplanetary magnetic field, Bz, and to the Akasofu coupling function, which evaluates the energy transfer between the ICME and the magnetosphere. The analysis also showed that the geomagnetic field at higher latitudes is disturbed before the field around the Earth's equator.

Bayesian Dynamic Linear Model with Switching for Real-Time Short-Term Freeway Travel Time Prediction with License Plate Recognition Data

AbstractThis paper presents a Bayesian inference-based dynamic linear model (DLM) with switching based on three-phase traffic flow theory to predict online short-term travel time with plate recognition data. The proposed method combines the DLM model with a Hidden Markov Model (HMM) to capture the probability of flow breakdown and delays associated with congestion. By viewing travel time fluctuations as a time-varying stochastic process due to unforeseen events (e.g., incidents, accidents, or bad weather), the proposed dynamic linear model with Markov switching (SDLM) employs the HMM to determine the optimal traffic state sequence corresponding to a given travel time and flow rate observation sequence. The experimental results based on automatic license plate recognition data of a Jingtong Expressway stretch in Beijing City suggest that the proposed method can provide accurate and reliable travel time prediction under various traffic conditions.

Empirical Validation of Three Macroscopic Dynamic Network Loading Models

The macroscopic dynamic network loading (DNL) model is the essential component to perform dynamic traffic assignment (DTA). The popular DNL models widely used in literature of DTA are Point-Queue (PQ) and Linear Travel Time (LTT) models because they fulfil of all the required desirable properties and require lesser computational cost. The Divided Linear Travel Time (DLT) model is presented as a variant model to overcome theoretical deficiencies found in PQ and LTT models. Literature of DTA focused on theoretical properties of these models; however, empirical validation of these models with real traffic data is scarce. This study prime objective is to validate PQ, LTT and DLT models with collected data in terms of inflow and outflow profiles from simultaneous recorded videos at both ends of the link under various conditions of link volume. The obtained results indicated that models are not able to replicate observed travel time profiles as Root Mean Square Error (RMSE) values are quite higher in almost all analysed cases. Investigation of properties such as FIFO and heterogeneity in transport modes using a fastest lane experiment indicated that model predictions are better when extent of these measures were lower. It implies that incorporation of these notions in future models are means to further progress in this direction.

Dynamic Modeling of VISSIM's Critical Gap Parameter at Unsignalized Intersections

Critical gaps are essential parameters in the modeling of traffic operations at unsignalized intersections and roundabouts and are the main determinants in the calculation of intersection capacity. In microscopic simulation tools, critical gaps are basic driver-related variables; the gaps’ values and variations within the population are user defined. Errors or oversimplifications of these basic variables may yield unrealistic results in terms of total throughput, queue lengths, waiting times, and so forth. In this study, a method is proposed for the calibration of the critical gap parameter in microscopic simulations; the method is based on individual travel time observations. A simple formulation for the calculation of critical gap parameters takes into account the increasing risk taken by drivers on a minor street with increasing volumes on the major road. This new formulation provides a better match between the accepted gaps observed in reality and the actual traffic conditions. The new methodology has been used to calibrate and validate the minimum gap parameter of the software program VISSIM through the use of data taken from several unsignalized intersections in Leuven, Belgium. The results show that the volume-dependent model outperforms the conventional constant critical gap parameters.

Estimating Travel Times of Coronal Mass Ejections to 1 AU Using Multi-spacecraft Coronagraph Data

We study the relationship between the speeds of coronal mass ejections (CMEs) obtained close to the Sun and in the interplanetary medium during the low solar-activity period from 2008 to 2010. We use a multi-spacecraft forward-modeling technique to fit a flux-rope-like model to white-light coronagraph images from the STEREO and SOHO spacecraft to estimate the geometrical configuration, propagation in three-dimensions (3D), and the radial speeds of the observed CMEs. The 3D speeds obtained in this way are used in existing CME travel-time prediction models. The results are compared to the actual CME transit times from the Sun to STEREO, ACE, and Wind spacecraft as well as to the transit times calculated using projected CME speeds. CME 3D speeds give slightly better predictions than projected CME speeds, but a large scatter is observed between the predicted and observed travel times, even when 3D speeds are used. We estimate the possible sources of errors and find a weak tendency for large interplanetary CMEs (ICMEs) with high magnetic fields to arrive faster than predicted and small, low-magnetic-field ICMEs to arrive later than predicted. The observed CME transit times from the Sun to 1 AU show a particularly good correlation with the upstream solar-wind speed. Similar trends have not been observed in previous studies using data sets near solar maximum. We suggest that near solar minimum a relatively narrow range of CME initial speeds, sizes, and magnetic-field magnitudes led to a situation where aerodynamic drag between CMEs and ambient solar wind was the primary cause of variations in CME arrival times from the Sun to 1 AU.

Adjusting survival estimates for premature transmitter failure: a case study from the Sacramento-San Joaquin Delta

In telemetry studies, premature tag failure causes negative bias in fish survival estimates because tag failure is interpreted as fish mortality. We used mark-recapture modeling to adjust estimates of fish survival for a previous study where premature tag failure was documented. High rates of tag failure occurred during the Vernalis Adaptive Management Plan’s (VAMP) 2008 study to estimate survival of fall-run Chinook salmon (Oncorhynchus tshawytscha) during migration through the San Joaquin River and Sacramento-San Joaquin Delta, California. Due to a high rate of tag failure, the observed travel time distribution was likely negatively biased, resulting in an underestimate of tag survival probability in this study. Consequently, the bias-adjustment method resulted in only a small increase in estimated fish survival when the observed travel time distribution was used to estimate the probability of tag survival. Since the bias-adjustment failed to remove bias, we used historical travel time data and conducted a sensitivity analysis to examine how fish survival might have varied across a range of tag survival probabilities. Our analysis suggested that fish survival estimates were low (95% confidence bounds range from 0.052 to 0.227) over a wide range of plausible tag survival probabilities (0.48–1.00), and this finding is consistent with other studies in this system. When tags fail at a high rate, available methods to adjust for the bias may perform poorly. Our example highlights the importance of evaluating the tag life assumption during survival studies, and presents a simple framework for evaluating adjusted survival estimates when auxiliary travel time data are available.

Tectonic evolution of southern Baffin Bay and Davis Strait: Results from a seismic refraction transect between Canada and Greenland

Wide‐angle reflection/refraction seismic data were acquired on a 450‐km‐long transect in southern Baffin Bay extending from Baffin Island to Greenland. Dense air gun shots were recorded on 22 ocean bottom seismometers. APwave velocity model was developed from forward and inverse modeling of the observed travel times. Beneath the Baffin Island shelf, a three‐layered continental crust is observed with velocities of 5.5 to 6.9 km/s. Typical for transform margins, there is a sharp transition between continental and oceanic crust. Off Baffin Island, 7‐km‐thick oceanic crust is interpreted to lie in a major transform fault identified on the gravity map. Beneath the deep Baffin Bay basin, 9‐km‐thick oceanic crust is encountered but thins to 6 km within an assumed fracture zone. The thicker than normal oceanic crust indicates an ample magma supply, possibly related to melt extracted from a mantle plume. Seaward of the Greenland continental crust, 20‐km‐thick igneous crust (6.3 to 7.3 km/s) is encountered in a 25‐km‐wide zone interpreted as a leaky transform fault that can be correlated southward through Davis Strait. The igneous crust is bounded by a 20‐km wide basin to the west, underlain by 4‐km‐thick crust of unknown affinity. This structure is probably associated with transform movements. A high‐velocity lower crustal layer (7.1 km/s) of 8 km thickness is indicated beneath the Greenland crust and can be correlated into the adjacent thick igneous crust. Both the thick igneous and Greenland crust are covered by up to 4 km of Paleogene volcanics (5.2 to 5.7 km/s).

Body surface scanner for the abdominal sound speed tomographic imaging

The ultrasound tomography has been studied for the reconstruction of the abdominal sound speed image to measure the visceral fat area. The method is based on the travel time observations of the sound waves transmitted through the abdominal medium. In the present study, aiming to realize the method as a clinically available equipment, body surface scanner machinery was developed keeping good contact between a transducer and a human abdominal body surface. To-and-fro movement scanning equipment including the attachment of the elastic coupling gel hemisphere in front of the transducer surface was installed. The optimum pushing status was controlled by monitoring the variation of the received sound wave amplitude. They were in stable over the wide pushing distance region, regardless of the contact surface angle. It was shown that measured precision of the travel time were good enough to discriminate the difference of the sound speed between the fat and protein regions in the human abdominal region.

Synthetic seismograms for a synthetic Earth: long-period P- and S-wave traveltime variations can be explained by temperature alone

Current interpretations of seismic observations typically argue for significant chemical heterogeneity being present in the two large low shear velocity provinces under Africa and the Pacific. Recently, however, it has been suggested that large lateral temperature variations in the lowermost mantle resulting from a strong thermal gradient across D? may provide an alternative explanation. In case of a high heat flux from the core into the mantle, the magnitude of shear wave velocity variations in tomographic models can be reconciled with isochemical whole mantle flow and a pyrolite composition. So far, the hypothesis of strong core heating has been tested in a consistent manner only against tomographic S-wave velocity models, but not against P-wave velocity models. Here, we explore a new approach to assess geodynamic models and test the assumption of isochemical whole mantle flow with strong core heating directly against the statistics of observed traveltime variations of both P and S waves. Using a spectral element method, we simulate 3-D global wave propagation for periods down to 10 s in synthetic 3-D elastic structures derived from a geodynamic model. Seismic heterogeneity is predicted by converting the temperature field of a high-resolution mantle circulation model (MCM) into seismic velocities using thermodynamic models of mantle mineralogy. Being based on forward modelling only, this approach avoids the problems of limited resolution and non-uniqueness inherent in tomographic inversions while taking all possible finite-frequency effects into account. Capturing the correct physics of wave propagation allows for a consistent test of the assumption of high core heat flow against seismic data. The statistics of long-period body wave traveltime observations show a markedly different behaviour for P and S waves: the standard deviation of P-wave delay times stays almost constant with turning depth, whereas that of the S-wave delay times increases strongly throughout the mantle. Surprisingly, synthetic traveltime variations computed for the isochemical MCM reproduce these different trends. This is not expected from a ray-theoretical point of view and highlights the importance of finite-frequency effects. Most importantly, the large lateral temperature variations in the lower mantle related to strong core heating are able to explain most of the standard deviation of observed P- and S-wave delay times. This is a strong indication that seismic heterogeneity in the lower mantle is likely dominated by thermal variations on the length scales relevant for long-period body waves.

Finding the most reliable path with and without link travel time correlation: A Lagrangian substitution based approach

Path travel time reliability is an essential measure of the quality of service for transportation systems and an important attribute in travelers’ route and departure time scheduling. This paper investigates a fundamental problem of finding the most reliable path under different spatial correlation assumptions, where the path travel time variability is represented by its standard deviation. To handle the non-linear and non-additive cost functions introduced by the quadratic forms of the standard deviation term, a Lagrangian substitution approach is adopted to estimate the lower bound of the most reliable path solution through solving a sequence of standard shortest path problems. A subgradient algorithm is used to iteratively improve the solution quality by reducing the optimality gap. To characterize the link travel time correlation structure associated with the end-to-end trip time reliability measure, this research develops a sampling-based method to dynamically construct a proxy objective function in terms of travel time observations from multiple days. The proposed algorithms are evaluated under a large-scale Bay Area, California network with real-world measurements.

Reliable Lg Arrival-Time Picks and Potential for Enhanced Epicenter Location

Lg waves are generally the most prominent phase in seismic records at local and regional distances. They are dispersive and have complex waveforms. Their propagation characteristics, such as phase and group velocity, are not as easily related to formal crustal model features, as are P and S velocities. However, slowly propagating Lg waves may provide important travel-time observations in the context of epicenter location, but this potential has not been realized. The main reason is that Lg phases arrive in the coda of S waves and that the original waveforms are complex. The latter was resolved by replacing waveforms with their envelopes using the short-term average (STA) transform. We address the Lg arrival-time picking problem by associating time picks with the Lg amplitude maxima ( Z component) in the record envelope. Validations of Lg onsets are not easy due to the lack of accurate Lg travel-time curves. Instead Lg times are converted to group velocities and station-wise consistency of velocities for closely spaced earthquake sources is required. With rare exceptions these group velocities agree within ![Graphic][1] . We also tested the potential of our Lg arrival times for the epicenters location using the grid search scheme of Rodi (2006). On a stand alone basis the locations were almost identical to the International Seismological Centre (ISC) and the National Earthquake Information Center (NEIC) epicenter solutions. A practical problem with introducing joint P - Lg locations is that these phases do not have similar wave paths, and besides Lg travel times are not focal depth dependent. The latter result stems from our Lg synthetic record analysis. We conclude that the Lg arrival-time picking procedure developed is robust and convenient for near real time analysis. Because of its slow propagation velocities between 2.8 and 3.7 km/s, use of Lg arrival times can enhance focal parameter estimates reported by seismological centers around the world. [1]: /embed/inline-graphic-1.gif

Observed travel times for Multiply-reflected ScS waves from a deep-focus earthquake

Tlie deep-focus Argentinean earthquake of December 81962, generated multiply reflected ScS phases which were recorded veryclearly at stations of the IGY and the TJSC&GS standardized worldwidenetworks and at Canadian stations. The data gathered from this earthquakefor the multiply-reflected ScS and sScS were used to construct thetravel times and to extend them to shorter epicentral distances. Thesenew data brought to light an error in published travel times for the 2(ScS)phase.

Field and Model Studies of the Boston Outfall

Near- and far-field oceanographic measurements of the plume from the Boston outfall were made over two days in April 2001. The results were consistent with the original physical model studies on which the diffuser design was based. New laboratory experiments were run to simulate the actual conditions at the time of the field tests using three-dimensional laser-induced florescence (3DLIF) to measure dilution. The 3DLIF results were very close to the field observations and provided considerable additional insight into the near-field mixing processes: the plumes from each port of the multiport risers merge, first with other ports on the same riser, and then with similar plumes from adjacent risers; mixing transverse to the current direction then gradually erases any vestiges of lateral concentration variations. The field results were consistent with predictions of the mathematical model NRFIELD. Measurements were also made in the far field on the second sampling day. For the observed travel times of up to 24 h, the increase in dilution was small and was due to horizontal diffusion only; vertical diffusion was negligible due to the stable oceanic density stratification. The results, and those of tests conducted in July 2001, indicate that the main features of the near-field processes of ocean wastewater outfalls can be closely predicted by small-scale physical model experiments and the empirical mathematical models derived from them. The results confirm that diffusers with widely spaced risers can effectively dilute and disperse the effluent.

Assessment of Tomography Models of Taiwan Using First-Arrival Times from the TAIGER Active-Source Experiment

In 2008, the TAIGER project conducted an active-source experiment in which seismic records from 10 explosions were collected at nearly 1400 locations on the island of Taiwan. In this study, we manually pick the first-arrival times from the explosion records to ensure reliable and accurate travel time observations, and use them to examine the complex P-wave structural variations in the crust beneath Taiwan.

An Improved Technique for Static Correction in a High Resolution Shallow Seismic Reflection Data using Difference in Reflection Times.

Shallow Seismic reflection survey was carried out in Zaria, located in the basement complex of central northern Nigeria, with the aim of characterising the granitic batholith. However, the effect of near surface material, which could make reflection events appear disjointed or deeper than they really are, could be enormous if not corrected for. This work sets out to correct for this events by making use of difference in reflection times estimated from initially known models of the subsurface. During the data acquisition the receivers where placed at 1 m interval, with a constant offset of 1 m. the common midpoint (CMP) method with a 12 fold coverage was employed. Previous refraction tomography model carried out in the area was used as a guide during the 2D velocity model generation, making use of the observed travel times. The observe time was used to generate the initial model, which was later corrected using the previous known subsurface model, thereby noting the difference in travel time induced by the near surface materials. The results obtained showed that the reflection events were more coherent and in their actual reflecting point on the statically corrected seismic sections, than the section without static correction because of the excess time from the near surface material with variable p wave velocity. This experiment has proved that difference in reflection travel times obtained from previous known model or borehole information in conjunction with the observed travel times serve as a better tool in applying static correction to seismic reflection data, than any previous known conventional methods that relies on a single technique. Key Words: Static correction, High resolution, Seismic reflection, Reflection times

Effect of a vertical fracture on horizontal unsaturated seepage flow

The movement of water flow in unsaturated fractured rock was investigated with the sandstone sample through experimental research and numerical simulation. The experimental results show that the arriving time of wetting front is delayed by the fracture, resulting in the increase of water saturation in the domain on the upstream side of the fracture, which will locally enhance water flow through the matrix. The numerical simulation with the finite volume method captures effectively these characteristics. The comparison between simulated and observed travel time and arriving time of wetting front shows that their difference are very small and the simulated results are in agreement with the observed results, which implies calibrated parameters are reliable and effective. Then according to the calibrated parameters, fractured models were established to examine how the change of large fractured aperture would affect the arriving time of wetting fronts, pressure heads and water saturation on the upstream and downstream sides of fracture.

Estimating Route Choice and Travel Time Reliability with Field Observations of Bluetooth Probe Vehicles

Route choice is often assessed with either a modeling technique or field observations. Field observations have historically used a variation of license plate matching. The proposed technique assesses route choice and travel time that uses an anonymous Bluetooth media access control (MAC) address sampling technique as a surrogate for license plate matching to assess route choice. The Bluetooth sampling technique was used to evaluate the impact of an unexpected bridge closure in northwest Indiana, including an assessment of the proportion of vehicles using each of four alternate routes. The Bluetooth technology also provided a means to collect travel time data for each alternate route; these observed travel times were also compared with travel time estimates obtained by route classification and link distance. In general, the route choice behavior was consistent with observed travel time estimates. The Bluetooth sampling technique is cost-effective to deploy, and although results are approximate, direct measurement of travel times and route choice is useful for public agencies to assess mobility and travel time reliability along alternate routes.

Monitoring travel times in an urban network using video, GPS and Bluetooth

The travel time is an important measure for the quality of traffic. This paper discusses a few methods to measure or estimate the travel time in urban road networks. First of all, it is important to know that urban travel times display a large variation, so that the measurement of a single (average) travel time is not so meaningful. The travel time distribution is more relevant than the single value of the average. This distribution can be obtained from observations of travel times of individual vehicles, for instance by tracing probe vehicles with GPS. The distribution of travel time can also be measured by Bluetooth scanners that can recognize Bluetooth devices in a car. Travel time over a link can be estimated by comparing passing times at its beginning and end. Automated Number Plate Recognition cameras have a similar possibility to follow individual vehicles from point to point. In the study of travel time measured in urban areas, the quality of data from Bluetooth scanners appeared to be disappointing. The most important reason is that outliers cannot easily be eliminated. This is especially a problem in urban areas. Another reason is the uncertainty about the Bluetooth devices’ carriers. It is difficult to identify whether the Bluetooth device belongs to a car or bus passenger, a cyclist or a pedestrian. Probe vehicles with GPS are very well appropriate for the measurements of travel times in urban areas.

Location-Based Data for Estimated Traffic on Urban Arterial in Heterogeneous Traffic Conditions

Accurate estimation of traffic in intelligent transportation system applications, such as the advanced traveler information system and the advanced traffic management system, requires fixed location-based measurements, vehicle-based measurements, or both. Using both data sources is too expensive for most government agencies, especially in developing countries such as India, and also leads to issues related to installation and maintenance, especially on urban roads. The main drawback of vehicle-based measurements is the potential lack of participation because of privacy concerns; lack of participation would limit data collection to a sample of the population, primarily on public transport vehicles. The study aims to overcome such difficulties by using only location-based flow data for the estimation of spatial parameters, such as density and travel time. These parameters are difficult to measure or estimate on an urban arterial, especially under heterogeneous traffic conditions, because of lack of lane discipline and because of complex interactions among different vehicle types. The Lighthill–Whitham–Richards macroscopic traffic flow model discretized in both space and time was employed in the estimation scheme. The resulting partial differential equations were solved numerically with the finite difference formulation of forward–time backward–space. Both linear and exponential speed–density relationships were considered and incorporated into the macroscopic model. Linear and cubic spline interpolations of input flow values were compared. The estimated density was corroborated with the density obtained from input–output analysis. Estimated travel times were compared with manually observed travel times and travel times obtained from probe vehicles fitted with Global Positioning System devices.