Travel Time Maps Research Articles

Novel automatic first-arrival picking method for ultrasound sound-speed tomography

Ultrasound sound-speed tomography (USST) is a promising technique for breast cancer diagnosis that is currently under investigation. Compared with two-dimensional X-ray mammography, it not only provides three-dimensional images but also avoids radiation exposure. However, the image quality of USST is highly dependent on the accuracy of travel time map (TTM). To improve the accuracy, a novel automatic first-arrival picking method is proposed in this study. With this method, Akaike information criterion is used to obtain travel time roughly, then cross-correlation of neighboring traces is employed to correct the obtained travel time. Simulation, phantom, and ex vivo experiments are implemented. The simulation experiments showed that the absolute errors of the proposed method were 52 and 98 ns for simple and complex structure data, respectively. The phantom and ex vivo experiments demonstrated the feasibility of the proposed method. In this study, a novel and robust first-arrival picking method was proposed for USST.

Quantifying the impact of accessibility on preventive healthcare in sub-Saharan Africa using mobile phone data.

Poor physical access to health facilities has been identified as an important contributor to reduced uptake of preventive health services and is likely to be most critical in low-income settings. However, the relation among physical access, travel behavior, and the uptake of healthcare is difficult to quantify. Using anonymized mobile phone data from 2008 to 2009, we analyze individual and spatially aggregated travel patterns of 14,816,521 subscribers across Kenya and compare these measures to (1) estimated travel times to health facilities and (2) data on the uptake of 2 preventive healthcare interventions in an area of western Kenya: childhood immunizations and antenatal care. We document that long travel times to health facilities are strongly correlated with increased mobility in geographically isolated areas. Furthermore, we found that in areas with equal physical access to healthcare, mobile phone-derived measures of mobility predict which regions are lacking preventive care. Routinely collected mobile phone data provide a simple and low-cost approach to mapping the uptake of preventive healthcare in low-income settings.

Helioseismology of sunspots: defocusing, folding, and healing of wavefronts

We observe and characterize the scattering of acoustic wave packets by a sunspot in a regime where the wavelength is comparable to the size of the sunspot. Spatial maps of wave travel times and amplitudes are measured from the cross-covariance function of the random wave field observed by SOHO/MDI around the sunspot in active region NOAO 9787. We consider separately incoming plane wave packets consisting of f modes and p modes with radial orders up to four. Observations show that the travel-time perturbations diminish with distance far away from the sunspot – a finite-wavelength phenomenon known as wavefront healing in scattering theory. Observations also show a reduction of the amplitude of the waves after their passage through the sunspot. We suggest that a significant fraction of this amplitude reduction is due to the defocusing of wave energy by the fast wave-speed perturbation introduced by the sunspot. This “geometrical attenuation” will contribute to the wave amplitude reduction in addition to the physical absorption of waves by sunspots. We also observe an enhancement of wave amplitude away from the central path: diffracted rays intersect with unperturbed rays (caustics) and wavefronts fold and triplicate. Wave amplitude measurements in time-distance helioseismology provide independent information that can be used in concert with travel-time measurements.

A Search for Helioseismic Signature of Emerging Active Regions

Using SOHO/MDI and GONG observations we present time–distance deep-focusing measurements to examine the deeper layers of the solar convective zone. The constructed travel-time maps show 10 – 15 second perturbations at depths of 40 – 75 Mm around active region locations before their emergence to the solar surface. The majority of the active regions used in this study were the same as those used in the recent work published by Ilonidis, Zhao, and Kosovichev (Science 333, 993, 2011). In order to confirm the capability of time–distance measurements to detect emerging active regions, we used a technique similar to their time–distance scheme. Our measurements only in some cases show a similar travel-time anomaly. Additionally, we have shown that the technique utilized in our study can provide more spatial details of the emerging flux configurations.

3-D seismic analysis of a carbonate platform in the Molasse Basin - reef distribution and internal separation with seismic attributes

Carbonate platforms differ from clastic sedimentary environments by a greater heterogeneity, so that key horizons for mapping and compartmentalisation of the reservoir are generally missing. We show that different seismic attributes help to compete with these difficulties and to identify different carbonate facies within the platform.The Upper Jurassic carbonate platform in Southern Germany in the Molasse Basin is a main exploration target for hydrogeothermal projects. Knowledge about the distribution of different carbonate facies within the platform, which is overprinted by faults, is important for a realistic reservoir simulation. The platform with an average thickness of 600 meters was artificially divided into four layers of equal thickness. Within each layer the characteristic seismic pattern was visualized by different attributes (travel time mapping, spectral decomposition), allowing additionally for further depositional classification.Within the uppermost layer the coral reef distribution could be mapped. The reefs form several complexes of up to 12 square kilometres in size. The surrounding slope and trough areas are identified as well. Within the platform , the distribution of sponge reefs could be visualized. They form either amalgamations in distinct areas, or are spread as small singular structures with diameters of approximately less than hundred meters.Comparing tectonic elements and reef distribution within the whole platform reveals that the early topography triggered the reef distribution, while these lithologic inhomogenities influenced later on the local shape of tectonic lineaments.The fault system which dominates the structural style in the area is visible in the different transformations but does not obscure the facies distribution, which hindered former interpretations of the data set. In this way a reservoir model can incorporate now the first time the reef distribution within an area.

Featured Graphic. Travel Time Maps of Urban Areas in the Netherlands

Featured Graphic. Travel Time Maps of Urban Areas in the Netherlands

Characterizing seismic bright spots in deeply buried, Ordovician Paleokarst strata, Central Tabei uplift, Tarim Basin, Western China

Anomalous seismic-amplitude bright spots are a common feature in deeply buried (5500 to 6500 m) Ordovician limestone strata in the Central Tabei Uplift area of the Tarim Basin in northwest China. Those anomalies have proven to be useful indicators of reservoir quality. The bright spots as seen on seismic data are tied to high-gamma ray, low-velocity zones in wireline logs, and correspond to clastic cave sediment-fills in the host limestone in core. Synthetic seismic models confirm this relationship between seismic bright spots and cave-sediment fills. A seismic traveltime map of the top Ordovician unconformity illustrates erosional topography and seismic geomorphologic patterns associated with the unconformity with numerous sinuous fluvial channels and canyons, fluvial valleys, sinkholes, and tower karsts and hills. A mature surface drainage system interacted with a near-surface karst system and allowed terrigenous sediments to enter an underground cave system. Karst-related bright spots probably correspond to paleocaves that initiated along an early regional fracture network and later were enhanced and altered by additional discharges in the surface drainage system. A few examples of bright spots are interpreted to be related to postkarstification faults that might have had a hydrothermal origin. Distribution of bright spots provides a useful reference in mapping regional collapsed-paleokarst systems. Bright spots typically are associated with circular and linear faults, and V-shaped depression patterns are related to a collapsed paleocave complex. Seismic-scale mapping and visualization of the paleokarst system can be achieved by interpreting circular and linear faults using continuity attributes and by analyzing the relationship among faults, local depressions, and bright spots.

Validated helioseismic inversions for 3D vector flows

According to time-distance helioseismology, information about internal fluid motions is encoded in the travel times of solar waves. The inverse problem consists of inferring 3-D vector flows from a set of travel-time measurements. Here we investigate the potential of time-distance helioseismology to infer 3-D convective velocities in the near-surface layers of the Sun. We developed a new Subtractive Optimally Localised Averaging (SOLA) code suitable for pipeline pseudo-automatic processing. Compared to its predecessor, the code was improved by accounting for additional constraints in order to get the right answer within a given noise level. The main aim of this study is to validate results obtained by our inversion code. We simulate travel-time maps using a snapshot from a numerical simulation of solar convective flows, realistic Born travel-time sensitivity kernels, and a realistic model of travel-time noise. These synthetic travel times are inverted for flows and the results compared with the known input flow field. Additional constraints are implemented in the inversion: cross-talk minimization between flow components and spatial localization of inversion coefficients. Using modes f, p1 through p4, we show that horizontal convective flow velocities can be inferred without bias, at a signal-to-noise ratio greater than one in the top 3.5 Mm, provided that observations span at least four days. The vertical component of velocity (v_z), if it were to be weak, is more difficult to infer and is seriously affected by cross-talk from horizontal velocity components. We emphasise that this cross-talk must be explicitly minimised in order to retrieve v_z in the top 1 Mm. We also show that statistical averaging over many different areas of the Sun allows for reliably measuring of average properties of all three flow components in the top 5.5 Mm of the convection zone.

Implementation and Comparison of Acoustic Travel-Time Measurement Procedures for the Solar Dynamics Observatory/Helioseismic and Magnetic Imager Time – Distance Helioseismology Pipeline

The Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO) satellite is designed to produce high-resolution Doppler-velocity maps of oscillations at the solar surface with high temporal cadence. To take advantage of these high-quality oscillation data, a time – distance helioseismology pipeline (Zhao et al., Solar Phys. submitted, 2010) has been implemented at the Joint Science Operations Center (JSOC) at Stanford University. The aim of this pipeline is to generate maps of acoustic travel times from oscillations on the solar surface, and to infer subsurface 3D flow velocities and sound-speed perturbations. The wave travel times are measured from cross-covariances of the observed solar oscillation signals. For implementation into the pipeline we have investigated three different travel-time definitions developed in time – distance helioseismology: a Gabor-wavelet fitting (Kosovichev and Duvall, SCORE’96: Solar Convection and Oscillations and Their Relationship, ASSL, Dordrecht, 241, 1997), a minimization relative to a reference cross-covariance function (Gizon and Birch, Astrophys. J. 571, 966, 2002), and a linearized version of the minimization method (Gizon and Birch, Astrophys. J. 614, 472, 2004). Using Doppler-velocity data from the Michelson Doppler Imager (MDI) instrument onboard SOHO, we tested and compared these definitions for the mean and difference travel-time perturbations measured from reciprocal signals. Although all three procedures return similar travel times in a quiet-Sun region, the method of Gizon and Birch (Astrophys. J. 614, 472, 2004) gives travel times that are significantly different from the others in a magnetic (active) region. Thus, for the pipeline implementation we chose the procedures of Kosovichev and Duvall (SCORE’96: Solar Convection and Oscillations and Their Relationship, ASSL, Dordrecht, 241, 1997) and Gizon and Birch (Astrophys. J. 571, 966, 2002). We investigated the relationships among these three travel-time definitions, their sensitivities to fitting parameters, and estimated the random errors that they produce.

Estimation of Steam Chamber Extent Using 4D Seismic

Summary The Steam-Assisted Gravity Drainage (SAGD) process has been successfully implemented to produce ultra-viscous bitumen from the Athabasca oil sands in the province of Alberta. In the Hangingstone area, 15 pairs of SAGD wells had been drilled in the reservoir by 2006, each a maximum of 30 m in thickness and approximately 300 m in depth. The production reached an average of 8,000 BOPD in recent years. The reservoir is geologically characterized as a stacked incised valley with fills in fluvial to upper-estuarine channels. Thin mudstone layers and abrupt changes in facies caused by sedimentary deposits present complexities and difficulties for SAGD implementation. A 3D seismic survey was conducted in 2002 to obtain a clear view of geology that was fully utilized for planning additional wells. In order to evaluate SAGD efficiency and performance, a time-lapse 3D seismic survey was carried out in 2006. In this paper, P-wave velocity (Vp) maps, transformed from the seismic travel-time maps, were interpreted with a new methodology for evaluating the areal extent of the steam chamber zone created by the SAGD process. In the previous experimental study of seismic velocity measurements with oil sands cores, Vp was found to steeply drop with an increase in temperature and to gently decrease with an increase in pore-pressure. Based on the experimental results, a petrophysical model was formulated to express Vp as a function of temperature, pressure and water saturation. The high-pressure and high-temperature zone of the SAGD process should generate differences between the first (2002) and second (2006) Vp maps from which we can estimate the area of the reduced bitumen viscosity with a temperature increase. As effects of pressure are probably more areally extensive than effects of temperature, these two effects on the Vp maps need to be segregated. As a new method, a scaling factor for the Vp reduction was first estimated to adjust the laboratory-scale and field-scale. We then calculated a distribution of Vp reduction corresponding to steam chamber conditions in order to decouple composite effects of temperature and pressure, based on the petrophysical model. Distinguishing the high-temperature and high pore-pressure zone from the low-temperature and high pore-pressure zone, we could determine a steam chamber distribution. The bitumen volume in the steam chamber zone was estimated and compared with the actual production. The methodology, interpretation procedures and results obtained are presented in detail.

Travel Time and Transfer Analysis Using Transit Smart Card Data

Automatic fare collection systems using smart card technology have become popular because they provide an efficient and cost-saving alternative to the manual fare collection method. In 2004, the City of Seoul, South Korea, introduced a smart card-based transit fare scheme, which was a distance-based, integrated fare collection and calculation system. Over the years, the system was extended twice and now can provide detailed information about public transit use in the region. This information includes each trip's boarding and alighting times and locations, as well as the connected trip chains with transfers. This paper examines possibilities for using such data for transportation planning application. First, a process to generate a travel time map is presented. For this, more than 100 million trip data are used to estimate travel times among stops. It is also demonstrated that transfer data can be readily obtainable because the on- and off-boarding information reside in the data set. Although transfers are considered to be important information for public transit planning, it has not been easy to collect such information. This study illustrates that transfer data can be used to locate the critical transfer points that need improvement. It is also demonstrated that a simple data query can quickly identify these locations. In addition, transfer trip patterns between two zones are analyzed, which provides meaningful information about passengers’ transfer location choice.

Truly remote regions of Earth revealed

In our hyperconnected world, getting away from it all is easier said than done, as a new set of travel-time maps reveal

Utilization of GPS Travel Time and Delay Data for Optimal Routing

Abstract Traffic congestion during peak hours has become one of the major problems affecting the road networks. GPS, in coordination with data retrieving software has been used for various purposes ranging from commercial utilization (route guidance systems) to monitoring traffic by different transportation organizations. In this research, collection of travel time and delay data on a congested network is used to obtain results that enable a commuter to make intelligent choices regarding his or her route. Travel time and delay data retrieved from a GPS instrument attached to a test vehicle traversing the study area during peak hours has been used to determine an optimal route between any given origin and destination in the area. The data collected on each road segment was analyzed and fitted into distributions. Using these distributions, the estimated travel time on each segment was inferred and the probability of being delayed was determined. The travel time and delay data in the form of Microsoft Access tables was imported into the GIS and programmed using a Visual Basic code, such that a user-friendly tool written in Visual Basic using ArcObjects was developed which provides the user with the optimal route between two selected points in the network, the travel time required to traverse the route, and the probability of being delayed on the route. In addition to this data, travel time maps are generated for the network, which provide a visual display illustrating the road network around a reference point for a given time.

Scenarios of Earthquake-Generated Tsunamis for the Italian Coast of the Adriatic Sea

We calculated the expected impact on the Italian coast of the Adriatic Sea of a large set of tsunamis resulting from potential earthquakes generated by major fault zones. Our approach merges updated knowledge on the regional tectonics and scenario-like calculations of expected tsunami impact. We selected six elongated potential source zones. For each of them we determined a Maximum Credible Earthquake and the associated Typical Fault, described by its size, geometry and kinematics. We then let the Typical Fault float along strike of its parent source zone and simulated all tsunamis it could generate. Simulations are based on the solution of the nonlinear shallow water equations through a finite-difference technique. For each run we calculated the wavefields at specified simulation times and the maximum water height field (above mean sea level), then generated travel-time maps and maximum water height profiles along the target coastline. Maxima were also classified in a three-level code of expected tsunami threat. We found that the southern portion of Apulia facing Albania and the Gargano promontory are especially prone to the tsunami threat. We also found that some bathymetric features are crucial in determining the focalization-defocalization of tsunami energy. We suggest that our results be taken into account in the design of early-warning strategies.

HELAS: local helioseismology data website

The Local Helioseismology Network Activity is part of the European Helio-and Asteroseismology Network (HELAS). One aspect of the network activity is to collate multipurpose data sets and make them available to the community for local helioseismic analysis. The first stage of the project is underway whereby high quality and useful data sets have been selected and acquired. The HELAS Local Helioseismology Network Activity website at http://www.mps.mpg.de/projects/seismo/NA4/ provides this data ready to download. Furthermore, the data is supplemented with relevant documentation necessary for further analysis, including details about the data reduction process that has already been applied. The data primarily consists of Doppler velocity observations but also includes observations of the line-of-sight magnetic field, vector magnetic field measurements, intensity and travel time maps. The website will be continuously updated with data thereby providing convenient access to comprehensive data sets appropriate for use in local helioseismology.

Earthquake‐generated tsunamis in the Mediterranean Sea: Scenarios of potential threats to Southern Italy

We calculated the impact on Southern Italy of a large set of tsunamis resulting from earthquakes generated by major fault zones of the Mediterranean Sea. Our approach merges updated knowledge on the regional tectonic setting and scenario‐like calculations of expected tsunami impact. We selected three potential source zones located at short, intermediate and large distance from our target coastlines: the Southern Tyrrhenian thrust belt; the Tell‐Atlas thrust belt; and the western Hellenic Arc. For each zone we determined a Maximum Credible Earthquake and described the geometry, kinematics and size of its associated Typical Fault. We then let the Typical Fault float along strike of its parent source zone and simulated all tsunamis it could trigger. Simulations are based on the solution of the nonlinear shallow water equations through a finite difference technique. For each run we calculated the wavefields at desired simulation times and the maximum water elevation field, then produced traveltime maps and maximum wave‐height profiles along the target coastlines. The results show a highly variable impact for tsunamis generated by the different source zones. For example, a large Hellenic Arc earthquake will produce a much higher tsunami wave (up to 5 m) than those of the other two source zones (up to 1.5 m). This implies that tsunami scenarios for Mediterranean Sea countries must necessarily be computed at the scale of the entire basin. Our work represents a pilot study for constructing a basin‐wide tsunami scenario database to be used for tsunami hazard assessment and early warning.

Local Helioseismology and Correlation Tracking Analysis of Surface Structures in Realistic Simulations of Solar Convection

We apply time-distance helioseismology, local correlation tracking and Fourier spatial-temporal filtering methods to realistic supergranule scale simulations of solar convection and compare the results with high-resolution observations from the SOHO Michelson Doppler Imager (MDI). Our objective is to investigate the surface and sub-surface convective structures and test helioseismic measurements. The size and grid of the computational domain are sufficient to resolve various convective scales from granulation to supergranulation. The spatial velocity spectrum is approximately a power law for scales larger than granules, with a continuous decrease in velocity amplitude with increasing size. Aside from granulation no special scales exist, although a small enhancement in power at supergranulation scales can be seen. We calculate the time-distance diagram for f- and p-modes and show that it is consistent with the SOHO/MDI observations. From the simulation data we calculate travel time maps for surface gravity waves (f-mode). We also apply correlation tracking to the simulated vertical velocity in the photosphere to calculate the corresponding horizontal flows. We compare both of these to the actual large-scale (filtered) simulation velocities. All three methods reveal similar large scale convective patterns and provide an initial test of time-distance methods.

Geologic Sensitivity and Groundwater Travel Time Map of the Marine Corps Air Station, Beaufort, South Carolina USA

Geologic Sensitivity and Groundwater Travel Time Map of the Marine Corps Air Station, Beaufort, South Carolina USA

Geologic Sensitivity and Groundwater Travel Time Map of the Marine Corps Air Station, Beaufort, South Carolina USA

Please click here to download the map associated with this article. Geographic information system (GIS) mapping of geologic sensitivity, defined in this study as the sensitivity of groundwater contamination based on geologic units, and modeling groundwater flow are important aids in establishing protocols for groundwater monitoring, determining guidelines for proper handling of hazardous substances, siting future facilities, emergency response, and designing remediation plans for the clean up of toxic spills. GIS technology is utilized to develop maps for the Marine Corps Air Station (MCAS) in Beaufort, South Carolina, USA of (1) the subsurface geology in three dimensions from land surface to the top of the Floridan aquifer, a regionally extensive and highly productive groundwater resource, (2) derivative geologic sensitivity/groundwater contamination potential, and (3) hydrogeology and groundwater flow. Soils data are also incorporated into the model to better understand how soil drainage enhances or inhibits recharge of groundwater and, together with organic matter affects the movement of contaminants through the soil column. A calibrated 3D groundwater flow model of MCAS-Beaufort and subsequent detailed flow path and travel time analyses are based on the 3D geologic characterization delineated on the geologic sensitivity map. The groundwater flow model can be displayed in two ways. One mode is as an ArcGIS project where groundwater pathlines with travel times of 5 to 500 years can be plotted from any 30 X 30 m cell within the boundaries of MCAS-Beaufort. Another mode is as a static map, which defines areas of the MCAS-Beaufort where groundwater travel time from the water table to the top of the Floridan aquifer is ≤ 10 years or ≤ 25 years. The groundwater travel-time analysis can be used to evaluate the impact of particular land uses on the spread of contaminants introduced at the water table and transported to the Upper Floridan aquifer. Methodologies developed for this study are applicable to other shallow aquifer settings.

Optimal Gaussian Phase-Speed Filters in Time-Distance Helioseismology

Gaussian phase-speed filters are widely used in time-distance helioseismology to select specific wave packets whose travel times are then measured at the solar surface. This filtering increases the signal-to-noise (S/N) ratio of the temporal cross-covariances that are fitted to derive the travel times. The central phase speeds of these Gaussian filters are prescribed by a solar model; their widths are typically chosen empirically. No systematic study has been published on the effect of this filter width on the S/N ratio of the travel times. Such an analysis requires the ability to generate both noise and signal travel-time perturbations, this is now possible due to the recent introduction of a noise model and Born-approximation sensitivity kernels. These kernels allow for a derivation of travel-time perturbations as functions of a given sound-speed perturbation and are dependent on the phase-speed filters applied to the data, unlike simpler kernels. In this paper, we show that there is indeed an optimum value of the filter width that results in a maximum S/N ratio for the travel-time maps. Narrower filters exclude too much signal to produce useful travel-time perturbation maps, while broader filters are not selective enough.