Differential Travel Times Research Articles

Heterogeneity-Driven Hydrodynamics Conditions the Hydrochemistry of Spring Water in Volcanic Islands.

Perched aquifers represent significant unexploited groundwater reserves in volcanic islands and contain valuable freshwater resources. These water reserves provide critical resources to indigenous populations suffering water scarcity. Groundwater discharged from a perched aquifer into two adjacent (14 m) springs in the volcanic summits constituted by basaltic and pyroclastic deposits of Gran Canaria Island (Spain) was examined. Based on springs discharge data, a three-dimensional groundwater flow and solute transport model of the investigated perched aquifer was calibrated to reproduce its hydraulic regime, as well as to explain the hydrochemical and isotopic composition of its main discharge systems, the studied springs. Groundwater flow simulations effectively replicated flow paths of the two springs affected by the existing geological heterogeneities, with differential travel times of 246 and 130 years, respectively, and with a convergent flow toward them partially explaining the averaged differences in electrical conductivity, δ18 O, and tritium observed between the springs. It can be concluded that, although water quality in both springs is similar and homogenous, as they come from the same aquifer system, geological heterogeneities in the upper elevation volcanic areas is likely the cause for the differences in the residence times of the two springs, which suggests that the flow regimes for the two springs are independent. The chemistry of the two springs, however, is essentially the same, with the exception of tritium, which is used to ascertain residence time.

Investigation of Effects of Near‐Surface Complexities on Measurement of Mantle Discontinuity Using SS and Its Precursors

AbstractSeismic observations of the mantle discontinuities at depths of 410 and 660 km provide crucial information to constrain the thermal and compositional states and further shape our understanding of mantle dynamics. The topography and impedance contrast of the mantle discontinuities, at both global and regional scales, have been imaged through differential travel times and amplitude ratios between SS and its precursors. However, a relatively simple reference phase (SS) is often assumed, which may overlook potential bias caused by near‐surface complexities. To assess these effects, we developed an algorithm to simulate the waveforms of SS and its precursors efficiently based on the propagation matrix method. We found that the crustal thickening increases the differential travel times but does not change the amplitude ratios much, while the sedimentary layer significantly affects both the differential travel times and amplitude ratios. The global anomalies of differential travel times and amplitude ratios are mapped incorporating earth models from CRUST1.0 and PREM. The time shift can reach ∼9 s, which will bring a depth bias of mantle discontinuity of more than 20 km, while the amplitude ratio anomaly is up to ∼40%, which will make the impedance contrast of the mantle discontinuity overestimated. The near‐surface complexities cannot be ignored in the investigation of mantle discontinuities with SS precursors, especially when the bounce points located in the regions with sediment.

Passenger car equivalent travel time of a truck

• Differences in the average travel time of trucks compared to passenger cars by area type. • Average travel times differ by day-of-the-week and time-of-the-day. • Difference in the average travel time tends to decrease with an increase in the reference speed. • Average travel time of trucks could be up to 1.18 times that of passenger cars. The Highway Capacity Manual (HCM) recommends the use of passenger car equivalent (PCE) to account for the influence of trucks on the operational performance of roads. Researchers have derived PCE based on capacity, headway, queue discharge, and delay in the past. However, it is not clear if the influence of trucks on the operational performance of roads would be the same in terms of travel time. The focus of this research is to assess the influence of trucks on the transportation system performance from a travel time perspective. The travel time data for Mecklenburg County (urban area) and Iredell County (rural area) were gathered for the year 2017 and processed to generate sixty-nine datasets accounting for the area type, temporal variation, reference speed, and traffic condition. Ordinary Least Square (OLS) regression models were then developed to examine the relationship between the travel time of trucks (dependent variable) and the travel time of passenger cars (explanatory variable). The coefficients indicate that the average travel time (ATT) of trucks is greater than the ATT of passenger cars irrespective of the dataset used for analysis, except in two cases of Iredell County. The ATT of trucks could be up to 1.18 times the ATT of passenger cars in some cases. The area type, day-of-the-week (DOW), time-of-the-day (TOD), reference speed of the link, and traffic condition at the time of data collection all seem to influence the ATT of trucks at different scales compared to the ATT of passenger cars.

Long‐Wavelength Topography and Multiscale Velocity Heterogeneity at the Core‐Mantle Boundary

AbstractThe structure of the lowermost mantle and the core‐mantle boundary (CMB) has profound implications for Earth's evolution and current‐day dynamics. Whilst tomographic studies of VS show good agreement in the lowermost mantle, consensus as to VP and especially CMB radius has not yet been reached. We perform a hierarchical Bayesian inversion for VP in the lowermost 300 km of the mantle and the radius of the CMB using differential travel time data. Concurrent with finding VP perturbations of 0.56% RMS amplitude that spatially agree with previous studies in areas of low posterior variance, we find 4.5 km RMS amplitude CMB radius perturbations with a broadly north‐south hemispherical character, with spherical harmonic power evenly distributed between degrees 1–3. These results suggest that CMB radial processes are set by a longer scale process than the VP perturbations.

Influence of shear wave velocity heterogeneity on SH-wave reverberation imaging of the mantle transition zone

SUMMARY Long-period (T > 10 s) shear wave reflections between the surface and reflecting boundaries below seismic stations are useful for studying phase transitions in the mantle transition zone (MTZ) but shear-velocity heterogeneity and finite-frequency effects complicate the interpretation of waveform stacks. We follow up on a recent study by Shearer & Buehler (hereafter SB19) of the top-side shear wave reflection Ssds as a probe for mapping the depths of the 410-km and 660-km discontinuities beneath the USArray. Like SB19, we observe that the recorded Ss410s-S and Ss660s-S traveltime differences are longer at stations in the western United States than in the central-eastern United States. The 410-km and 660-km discontinuities are about 40–50 km deeper beneath the western United States than the central-eastern United States if Ss410s-S and Ss660s-S traveltime differences are transformed to depth using a common-reflection point (CRP) mapping approach based on a 1-D seismic model (PREM in our case). However, the east-to-west deepening of the MTZ disappears in the CRP image if we account for 3-D shear wave velocity variations in the mantle according to global tomography. In addition, from spectral-element method synthetics, we find that ray theory overpredicts the traveltime delays of the reverberations. Undulations of the 410-km and 660-km discontinuities are underestimated when their wavelengths are smaller than the Fresnel zones of the wave reverberations in the MTZ. Therefore, modelling of layering in the upper mantle must be based on 3-D reference structures and accurate calculations of reverberation traveltimes.

Routing of autonomous vehicles for system optimal flows and average travel time equilibrium over time

We envisage a transportation management center (TMC) for future autonomous vehicles that distributes the traffic over a cycle comprising many days in such a manner that the average travel time (AVT) of vehicles of the same origin–destination (OD) over the cycle is identical at equilibrium while the entire network achieves system optimal (SO) flows every day. Vehicles are assigned to a set of paths, with different proportions of time within the cycle on different paths, but with the same AVT over the cycle at equilibrium. Since AVT is the same for each user, it is inherently fair to every-one. Thus, not only do users gain by subscribing to this system but the overall system also, as SO flows are followed each day. We then model the case of mixed equilibrium (ME), wherein drivers are given the option to subscribe to the TMC path combinations and hence assist in achieving SO, while non-subscribers only minimize their own private costs and follow their user equilibrium (UE) paths, which typically have shorter times than the corresponding AVT. To counter this, a tolling scheme is imposed on non-subscribers, i.e., UE users, based on their value-of-time (VOT) distribution and the travel time difference between subscribers' AVT and non-subscribers' UE time at ME. In the end, the total cost of travel of non-subscribers, toll plus UE time, is equal to the corresponding AVT of subscribers. The efficiency of this system is shown through numerical studies. Finally, the impact of market penetration and different VOT distributions on AVT and toll are investigated through sensitivity analysis.

Mode Choice Probability Between Bus and Railway Cianjur-Padalarang Route

Travel between cities in the Bogor, Sukabumi, and Cianjur areas is only accommodated by road-based public transportation that is interstate bus. The government issued a policy to reactivate railroads from Bogor to Padalarang, so that the southern region of West Java are connected to the rail network. This research intention to analyze characteristics of socio-economic users, characteristics of trip users, and analyze how much passengers move from the current bus to the Cianjur-Padalarang train. Data were collected by distributing questionnaires using stated preference techniques and analyzed using binary logit difference model with linear regression. The attributes to be analyzed are the difference between travel costs, travel time, and frequency of departure. The characteristic of users were analyzed with descriptive statictic method. The socio-economic characteristics of the passengers obtained were majority is male, aged 21-30 years old, education level senior high school, worked as entrepreneurship, income Rp2.000.001-Rp3.000.000, transportation costs for a month Rp100.001-Rp300.000. The characteristics of trip users are dominated by origin from Cianjur destination to Padalarang to work (economy), the frequency of trips is 1-2 times per week, the reason choosing the bus because of the bus have more easier mobility, the main travel time is 121-150 minutes. The results of the calculation of model sensitivity, the potential for passenger transfer from bus to train will increase if the difference in cost and travel time between trains and buses is getting smaller, and the difference in frequency is getting bigger.

Rural-Urban Disparities in Realized Spatial Access to General Practitioners, Orthopedic Surgeons, and Physiotherapists among People with Osteoarthritis in Alberta, Canada.

Rural Canadians have high health care needs due to high prevalence of osteoarthritis (OA) but lack access to care. Examining realized access to three types of providers (general practitioners (GPs), orthopedic surgeons (Ortho), and physiotherapists (PTs)) simultaneously helps identify gaps in access to needed OA care, inform accessibility assessment, and support health care resource allocation. Travel time from a patient’s postal code to the physician’s postal code was calculated using origin–destination network analysis. We applied descriptive statistics to summarize differences in travel time, hotspot analysis to explore geospatial patterns, and distance decay function to examine the travel pattern of health care utilization by urbanicity. The median travel time in Alberta was 11.6 min (IQR = 4.3–25.7) to GPs, 28.9 (IQR = 14.8–65.0) to Ortho, and 33.7 (IQR = 23.1–47.3) to PTs. We observed significant rural–urban disparities in realized access to GPs (2.9 and IQR = 0.0–92.1 in rural remote areas vs. 12.6 and IQR = 6.4–21.0 in metropolitan areas), Ortho (233.3 and IQR = 171.3–363.7 in rural remote areas vs. 21.3 and IQR = 14.0–29.3 in metropolitan areas), and PTs (62.4 and IQR = 0.0–232.1 in rural remote areas vs. 32.1 and IQR = 25.2–39.9 in metropolitan areas). We identified hotspots of realized access to all three types of providers in rural remote areas, where patients with OA tend to travel longer for health care. This study may provide insight on the choice of catchment size and the distance decay pattern of health care utilization for further studies on spatial accessibility.

Assessment of neighborhood sustainability in terms of urban mobility: A case study in Dhaka City, Bangladesh

Abstract For achieving sustainable cities, sustainable mobility is one of the key elements. Policymakers around the world are taking different strategies to ensure sustainable urban mobility at the local level. However, sustainable urban mobility assessment at neighborhoods of a developing country received a significant research gap. This study attempts to close this research gap by deriving and comparing neighborhood sustainability based on three indicators: modal share, travel time, and travel cost, in Dhaka city, Bangladesh. Using a two-step cluster model, neighborhoods were grouped into three clusters. Study results classified 41 (44.6%) neighborhoods as sustainable, 30 (32.6%) as potentially sustainable, and 21 (22.8%) as unsustainable neighborhoods due to less positive outcomes of sustainable mobility policies. Later, median income and jobs-housing ratio value for different neighborhood types validated the classification result. Findings from this study reveal insights for transport planners, development agencies, policymakers to identify areas where mobility of the residents needs to be improved on a priority basis; advocate further research on comprehensive sustainability assessment at the neighborhood level. Highlights for public administration, management and planning: • There is a significant difference in modal share, travel time, and travel cost in neighborhoods. • A traditional neighborhood of the city is comparatively more sustainable than newly developed ones. • Neighborhoods with similar sustainability status tend to be clustered on the city scale. • Median income and jobs-housing ratio can effectively capture the difference in sustainability level in terms of urban mobility. • Sustainable urban mobility assessment can contribute city planning and development process.

A before-after impact assessment of COVID-19 outbreak on bike-sharing ridership in Washington, DC

ABSTRACT The focus of this research is to assess the impact of COVID-19 outbreak in 2020 on the Capital Bikeshare (CaBi) ridership in Washington, D.C. area compared to 2019. Correlation analysis was conducted and mixed-effects negative binomial (MENB) models were developed to assess CaBi ridership by the type of CaBi user, as the available bike-sharing trip data followed a time-series cross-section panel data structure and the variables (season, stay-at-home order and lift, and weekday) have time-specific effects. The correlation coefficients between CaBi variables, COVID-19 variables, and characteristics of D.C.’s population profile and their statistical significance are sensitive to the year (2019 or 2020), the season of the year (Winter, Spring, Summer, or Fall), and the type of CaBi user (member or casual user). The number of stations between pickups and drop-offs, and the dummy variables representing the before and after stay-at-home order and lift were found significant in the MENB models. There was no significant difference in travel time of CaBi casual users and members during the outbreak. Riding for leisure or commuting has been potentially limited with the stay-at-home order. The findings have practical implications and help bike-sharing operators to increase their resilience during unexpected situations such as the pandemic.

Cycle-skipping mitigation using misfit measurements based on differentiable dynamic time warping

The dynamic time warping (DTW) misfit function has been used for full-waveform inversion to mitigate the local minima issue. However, it is not a smooth measurement and can yield strong discontinuities in the adjoint source. Such a weakness hampers the convergence of waveform inversion to informative minima. We have introduced a smooth DTW misfit function to remedy this shortcoming. The fundamental idea of the smooth DTW is to replace the min operator with its smooth relaxation. This replacement results in differentiable misfit measurements. Moreover, considering that the optimal warping plan indicates the traveltime difference between the observed and synthetic trace, we can construct a penalization term such that the penalized differentiable DTW distance further highlights the traveltime difference. Numerical examples demonstrate the advantage of the penalized differentiable DTW misfit function over the conventional nondifferentiable one.

The NICER “Reverberation Machine”: A Systematic Study of Time Lags in Black Hole X-Ray Binaries

We perform the first systematic search of all NICER archival observations of black hole (and candidate) low-mass X-ray binaries for signatures of reverberation. Reverberation lags result from the light travel time difference between the direct coronal emission and the reflected disk component, and therefore their properties are a useful probe of the disk-corona geometry. We detect new signatures of reverberation lags in eight sources, increasing the total sample from three to 11, and study the evolution of reverberation lag properties as the sources evolve in outbursts. We find that in all of the nine sources with more than one reverberation lag detection, the reverberation lags become longer and dominate at lower Fourier frequencies during the hard-to-soft state transition. This result shows that the evolution in reverberation lags is a global property of the state transitions of black hole low-mass X-ray binaries, which is valuable in constraining models of such state transitions. The reverberation lag evolution suggests that the corona is the base of a jet that vertically expands and/or gets ejected during state transition. We also discover that in the hard state, the reverberation lags get shorter, just as the quasiperiodic oscillations (QPOs) move to higher frequencies, but then in the state transition, while the QPOs continue to higher frequencies, the lags get longer. We discuss the implications of the coronal geometry and physical models of QPOs in light of this new finding.

Alternating Emission Features in Io's Footprint Tail: Magnetohydrodynamical Simulations of Possible Causes

AbstractIo's movement relative to the plasma in Jupiter's magnetosphere creates Alfvén waves propagating along the magnetic field lines which are partially reflected along their path. These waves are the root cause for auroral emission, which is subdivided into the Io Footprint (IFP), its tail and leading spot. New observations of the Juno spacecraft by Mura et al. (2018, https://doi.org/10.1126/science.aat1450) have shown puzzling substructure of the footprint and its tail. In these observations, the symmetry between the poleward and equatorward part of the footprint tail is broken and the tail spots are alternatingly displaced. We show that the location of these bright spots in the tail are consistent with Alfvén waves reflected at the boundary of the Io torus and Jupiter's ionosphere. Then, we investigate three different mechanisms to explain this phenomenon: (a) The Hall effect in Io's ionosphere, (b) travel time differences of Alfvén waves between Io's Jupiter facing and its opposing side and (c) asymmetries in Io's atmosphere. For that, we use magnetohydrodynamic simulations within an idealized geometry of the system. We use the Poynting flux near the Jovian ionosphere as a proxy for the morphology of the generated footprint and its tail. We find that the Hall effect is the most important mechanism under consideration to break the symmetry causing the “Alternating Alfvén spot street.” The travel time differences contributes to enhance this effect. We find no evidence that the inhomogeneities in Io's atmosphere contribute significantly to the location or shape of the tail spots.

The effect of frequency bandwidth on DSI anisotropy evaluation

Anisotropy evaluation in carbonate rocks plays a pivotal role in their reservoir characterization and modeling. This study demonstrates the benefits of advanced interpretation using the Dipole Shear Sonic Imager (DSI) log data for anisotropy evaluations of porous media. Splitting the shear wave into fast and slow components, with different polarizations and velocities, provides useful indications of formation anisotropy. Fast shear waves travel faster in directions parallel to layering and fracturing while slow shear waves travel more slowly perpendicular to layering and fracture networks. Quantifying the difference between fast and slow shear velocities provides a measure of acoustic anisotropy in the medium penetrated.The purpose of the research is to evaluate the effects of frequency bandwidth on the depth of the wave propagation rates and slowness values for anisotropy analysis, thus four bandwidth frequencies: 1–2 kHz, 2–4 kHz, 4–6 kHz, and 1–5 kHz are selected and examined in a vertical drilled well in the gas reservoirs of Upper Dalan Member, and Kangan Formations in the South Pars Gas-Condensate Field.The frequency filtering, semblance analysis, and slowness picking are done on waveforms for frequency-slowness dispersion analysis and slowness correction. The dispersion correction for bandwidth 4–6 kHz isn't performed completely in good hole condition intervals, because high- frequencies around 6 kHz are absorption due to the intrinsic attenuation of the rocks. Therefore, the considered bandwidth is decreased to 4–5.5 kHz. In the following, Alford rotation, align waveforms, and calculation and evaluation of derived fast and slow components are done for deriving the azimuth of the fast shear wave for each bandwidth of waveforms. The dominant trend of the azimuth of anisotropy is that aligned with the azimuth of the fast shear wave.The azimuth of the fast shear wave as azimuth of anisotropy and its corresponding intervals are compared with the results of image log data for the same well interval. This made it possible to reject or confirm the findings of the DSI acoustic anisotropy analysis for each bandwidth considered.In the low bandwidth frequency (1–2 kHz), the differences in slowness, energy, travel time, and penetration depth are more extreme than those recorded by the other bandwidths considered. The azimuth of anisotropy is NW-SE with a slight E-W trend. However, the azimuth of anisotropy was found to be contrary to the image log interpretation where the dominant trend is NE-SW.The dominant trend of the azimuth of anisotropy for bandwidth 2–4 kHz is NE-SW with a slight E-W trend. The anisotropy directions and zones are consistent with those identified from the wellbore image log results. Moreover, the penetration depth is meaningful and far away from the borehole drilling and mud effects. These results suggest that the bandwidth (2–4 kHz) is best suited for acoustic anisotropy analysis of wellbore sections using Dipole Shear Sonic Imager (DSI) recorded data in this well.The trends of the azimuth of anisotropy for bandwidth 4–5.5 kHz are NE-SW, and NW-SE, with expanded tolerance azimuth of anisotropy. The results that have been achieved in a small penetration depth and close to the borehole wall are affected by signal absorption and attenuation in flushed and transition areas. This leads to some errors and inconsistencies with the image log interpretation.The trends of the azimuth of anisotropy for bandwidth 1–5 kHz are NE-SW, NW-SE, and E-W. The outcomes are the consequence of the variety of directions and zones associated with this wider bandwidth, together with the penetration depths of the waveforms in the diverse zones considered. However, the azimuth of anisotropy is inconsistent with the image log interpretation.

Drone-Borne Ground-Penetrating Radar for Snow Cover Mapping

Ground-penetrating radar (GPR) is one of the most commonly used instruments to map the Snow Water Equivalent (SWE) in mountainous regions. However, some areas may be difficult or dangerous to access; besides, some surveys can be quite time-consuming. We test a new system to fulfill the need to speed up the acquisition process for the analysis of the SWE and to access remote or dangerous areas. A GPR antenna (900 MHz) is mounted on a drone prototype designed to carry heavy instruments, fly safely at high altitudes, and avoid interference of the GPR signal. A survey of two test sites of the Alpine region during winter 2020–2021 is presented, to check the prototype performance for mapping the snow thickness at the catchment scale. We process the data according to a standard flow-chart of radar processing and we pick both the travel times of the air–snow interface and the snow–ground interface to compute the travel time difference and to estimate the snow depth. The calibration of the radar snow depth is performed by comparing the radar travel times with snow depth measurements at preselected stations. The main results show fairly good reliability and performance in terms of data quality, accuracy, and spatial resolution in snow depth monitoring. We tested the device in the condition of low snow density (<200 kg/m3) and this limits the detectability of the air–snow interface. This is mainly caused by low values of the electrical permittivity of the dry soft snow, providing a weak reflectivity of the snow surface. To overcome this critical aspect, we use the data of the rangefinder to properly detect the travel time of the snow–air interface. This sensor is already installed in our prototype and in most commercial drones for flight purposes. Based on our experience with the prototype, various improvement strategies and limitations of drone-borne GPR acquisition are discussed. In conclusion, the drone technology is found to be ready to support GPR-based snow depth mapping applications at high altitudes, provided that the operators acquire adequate knowledge of the devices, in order to effectively build, tune, use and maintain a reliable acquisition system.

Model-data comparison of sound propagation in a glacierized fjord with a simulated brash ice surface.

Glacier ice loss impacts sound propagation within Arctic fjords. Regular calving events contribute to a collection of floating ice fragments, known as brash ice, at the ocean surface that obstruct the natural and anthropogenic acoustic signals, yet are difficult to characterize. Transmission loss measurements using a maximum length sequence (m-sequence) signal were conducted in September 2017 near Hansbreen glacier in Hornsund Fjord, Svalbard with dense brash ice present at the water surface. An acoustic model of the brash ice surface was inferred through consideration of the experimental geometry, arrival amplitude, and travel time difference between the direct and surface reflected arrivals from the source to two receivers. The inferred surface was then incorporated into a forward simulation of the environment using sound speed profiles measured during the experiment. BELLHOP ([Porter and Bucker (1987). J. Acoust. Soc. Am. 82(4), 1349-1359],), a ray tracing code available in the Acoustics Toolbox (HLS Inc., San Diego, CA), was used to track the time difference of arrivals and amplitudes of the modeled direct and surface reflected rays. Comparisons between the measured and simulated results provide insight into the geometric shape and reflection characteristics of the brash ice surface within this and similar environments.

The extent to which geography explains one of trauma's troubling trends: Insurance-based differences in appropriate interfacility transfer.

A growing body of literature suggests the persistence of a counterproductive triage pattern wherein uninsured adults with major injuries presenting to nontrauma centers (NTCs) are more likely than insured adults to be transferred. Geographic differences are frequently blamed. The objective of this study was to explore geography's influence on variations in insurance transfer patterns, asking whether differences in distance and travel time by road from NTCs to the nearest level 1 or 2 trauma center alter the effect. As a secondary objective, differences in neighborhood socioeconomic disadvantage were also assessed. Adults (16-64 years) with major injuries (Injury Severity Score, >15) presenting to NTC emergency departments (EDs) were abstracted from 2007 to 2014 state inpatient/ED claims. Differences in the risk-adjusted odds of admission versus transfer were compared using mixed-effect hierarchical logistic regression and spatial analysis. A total of 48,283 adults presenting to 492 NTC EDs were included. Among them, risk-adjusted admission differences based on insurance status exist (e.g., private vs. uninsured odds ratio [95% confidence interval], 1.60 [1.45-1.76]). Spatial analysis revealed significant geographic variation ( p < 0.001). However, in contrast to expectations, the largest insurance-based discrepancies were seen in less disadvantaged NTCs located closer to larger trauma centers. Stratified analyses comparing the closest versus furthest distance, shortest versus longest travel time, and least versus most deprived populations agreed, as did sensitivity analyses restricting uninsured transfer patients to those who remained uninsured versus subsequently became insured. Adults with major injuries presenting to NTCs were less likely to be transferred if insured. The trend persisted after accounting for differences in access to care, revealing that, while significant geographic variation in the phenomenon exists, geography alone does not explain the issue. Taken together, the findings suggest that additional and potentially subjective elements to insurance-based triage disparities at NTCs are likely to exist. Prognostic/Epidemiological, Level III.

Lowermost mantle (D″ layer) structure beneath the Indian Ocean: Insights from modeling of ScS-S and PcP-P residuals

• High Compressional (P) and shear (S) wave velocities in the lowermost mantle ( D ″ layer) beneath the Indian Ocean Geoid Low. • Evidence for slabs at the CMB beneath Indian Ocean Geoid Low. • Low R values in the lowermost mantle ( D ″ layer) beneath the western Indian Ocean Geoid Low. In this study, we investigate the seismic structure of the D ″ layer beneath the Indian Ocean by modeling the ScS-S and PcP-P differential travel time residuals corrected for the velocity structure above it. These times, representative of the anomalies in the D ″ layer, vary from −9.58 to 5.06 s for the shear wave (ScS-S) and −4.54 to 3.14 s for the compressional wave (PcP-P). Modeling of the residuals using a grid search approach reveals velocity perturbations in the range of −3.06% to 5.72% for the shear and −4.81% to 5.47% for the compressional waves, in the D ″ layer. Interestingly, the perturbations are positive below the Indian Ocean Geoid Low (IOGL) and negative below the adjoining region. The results clearly reveal presence of high velocity material atop the Core Mantle Boundary (CMB) beneath the IOGL, representing dehydrated Tethyan subducted slabs. Further, the low R SP values in the lowermost mantle beneath the western part of IOGL, calculated using the P and S velocity estimates from this study, mostly lie between 0.01 and 2.7. This implies that the anomalies may be thermal in origin, owing to the heterogeneities resulting from cold lithospheric slabs at the CMB.

Determination of coordinates of the earthquake hypocenter with a known structure of the earth's crust

Objective. The aim of the study is to develop methods for determining the coordinates of the hypocenter of an earthquake with a known structure of the earth's crust. Method. To solve the problem, the brute force method, the confidence interval, the comparative method and the enumeration of possible options on the grid are used. Result. A method has been developed for determining the coordinates of an earthquake source with a known structure of the earth's crust using points of various figures of the n-th order as geometric locations in calculations. Conclusion. The proposed method, with the known structure of the earth's crust, makes it possible to use all known methods that require information on the travel time differences of seismic waves to spaced seismic sensors and the difference in travel times of longitudinal and transverse seismic waves to the seismic sensor for calculating the coordinates of the earthquake hypocenter. The accuracy of determining the coordinates of the earthquake hypocenter depends on the quality of determining the structure of the earth's crust on which the seismic event was observed.

Estimating Truck Travel Time to Passenger Car or Traffic Stream Travel Time Ratio in North Carolina, USA

ABSTRACT The differences in travel times of passenger cars, traffic stream, and trucks depend on the area type, temporal factors, reference speed, and traffic condition. These explanatory variables account for the effect of geometric conditions and variations in the traffic flow. The focus of this research is to examine the correlations and estimate truck travel time to passenger car or traffic stream travel time ratio of a road link (dependent variable) as a function of these explanatory variables. Travel time data for Mecklenburg County and Iredell County in North Carolina, USA were gathered for the year 2017 to examine correlations, develop generalized estimating equations (GEE) models, and identify explanatory variables influencing the ratios. Gamma log-link distribution-based models are the best-fitted models to estimate the average travel time (ATT) of trucks to the ATT of passenger cars or traffic stream ratios. Notable differences in the coefficients were observed when the ATT of trucks was compared with the ATT of passenger cars or traffic stream. The area type (urban or rural) was observed to influence the ratios differently. The influence of traffic condition, reference speed (or free-flow speed), day-of-the-week (DOW) and time-of-the-day (TOD) on the ratios also varied with the area type.