Travel-time Curves Research Articles

Wave transforms of transient electromagnetic field in conductive earth

The study aims at finding a stable method for transformation of time-domain electromagnetic diffusion field to an electromagnetic wavefield. Two ways of transformation are considered: singular-value decomposition (SVD) and Tikhonov’s regularizations. Transformation is applied to TEM responses of a conductive half-space, a conductive S film, and to a series of horizontally layered models. The wave transforms are used to plot travel-time curves and to estimate the velocity of EM field propagation.

Study on the characteristics of travel time based on the method of wavelet decomposition

Abstract The fixed-point detectors using the technology of Automatic Vehicle Identification (AVI) is able to gather samples of vehicles’ travel time on the corresponding section of urban main roads. Through the application of wavelet decomposition method, the irregular travel time data of the road section would be denoised into travel-time curves. These curves would be classified and analyzed to reveal the characteristics and the regular pattern of vehicles’ travel time on certain urban road section in order to identify its relationship with land use and urban space distribution.

Rupture imaging for the 2016 August 24, Mw=6.0 central Italy earthquake, from back-projection of strong-motion array data

<p class="western" align="justify"><span style="font-family: 'Bitstream Charter', serif;"><span>By extending the conventional Beam-Forming frequency-wavenumber power spectral estimate to the case of arbitrarily-shaped wavefronts, we obtained images of rupture propagation during the 2016 August 24, Mw=6.0 central Italy earthquake. Using a set of strong-motion accelerometers, we evaluate the beam power along the travel time curves associated with synthetic sources spanning a model fault surface. This allows deriving time-dependent images of the distribution of energy radiation throughout the fault plane. Results indicate bi-lateral rupture propagation toward SE and NW, in rough agreement with surface co-seismic displacement and surface damage pattern. To a first order, our results are also consistent with those obtained from full-waveform inversion of strong-motion data.</span></span></p>

Interpretation of hard rock seismic data using 3D prestack diffraction imaging

Mineral deposits are associated with geological settings characterized by discontinuities and complex structural heterogeneities such as fracture zones, small-scale objects, intrusive and steeply dipping structures. Therefore, detecting such heterogeneities is of primary interest in mineral exploration. Usually, the scale and shape of such heterogeneities cause the seismic energy to diffract rather than reflect. Despite the natural lack of reflectors and potentially abundant number of diffractors, there are only few case studies of diffraction imaging in hard rock environments with almost no examples of diffraction imaging in prestack domain. Herein, we fill this gap by implementing a 3D prestack diffraction imaging technique to detect point diffractors in hard rock environment. The technique includes computation of diffraction traveltime curves followed by semblance analysis along the curves, with high value of semblance corresponding to high diffractivity. The performance of the method is demonstrated on a 3D synthetic seismic data and applied to a 3D field seismic data set recorded over Kevitsa mineral deposit in the northern Finland. The results of the 3D prestack diffraction imaging method suggest that diffractivity is a powerful attribute that can be used with other seismic attributes for the interpretation of seismic data in hard rock environments.

Continuous estimation of 3-D reflector orientations along 2-D deep seismic reflection profiles

Interpretation of 2-D deep seismic reflection data can be limited by the recording of out-of-plane reflections that cannot be readily distinguished from those originating beneath a seismic line. Here I present a method analogous to semblance velocity analysis that utilizes varying source-receiver azimuths to derive continuous estimates of 3-D reflector orientations along onshore 2-D reflection profiles. For each zero-offset time within a common depth point supergather, the semblance is calculated along 3-D travel time curves, and the dip and strike of the most coherent reflection is determined. Relative errors in these angles are derived from the range of travel time curves that have semblance values greater than a specified fraction, for example 90%, of the maximum. The method is illustrated using a section from line 10GA-YU1 from the Youanmi terrane of the Yilgarn craton in Australia in which the original field data have been replaced with synthetic in-line and cross-line reflections. Reflector orientations are generally well recovered where the range of available source-receiver azimuths is greater than 20o, but the method fails at lower ranges where the seismic line is almost linear, and this behavior is also observed in analysis of the field data. Nevertheless when using a realistic 1-D velocity function the orientation of upper crustal shear zones can be readily determined, and on unmigrated sections subhorizontal sills can be distinguished on the basis of their geometry from the mid-crustal reflectivity. In future surveys, reflector orientations can be determined along the near-linear sections of a seismic profile by deploying additional receivers, perhaps as cross-line recording spreads, to supplement the limited range of azimuths available from the in-line acquisition. The method can in principle be extended to marine reflection surveys, and more complex sub-surface velocity models.

Diffractivity — Another attribute for the interpretation of seismic data in hard rock environment, a case study

The primary objective of seismic exploration in a hard rock environment is the detection of heterogeneities such as fracture zones, small-scale geobodies, intrusions, and steeply dipping structures that are often associated with mineral deposits. Prospecting in such environments using seismic-reflection methods is more challenging than in sedimentary settings due to lack of continuous reflector beds and predominance of steeply dipping hard rock formations. The heterogeneities and “fractal” aspect of hard rock geologic environment produce considerable scattering of the seismic energy in the form of diffracted waves. These scatterers can be traced back to irregular and often “sharp-shaped” mineral bodies, magmatic intrusions, faults, and complex and heterogeneous shear zones. Due to the natural lack of reflectors and abundant number of diffractors, there are only a few case studies of diffraction imaging in hard rock environments. There are almost no theoretical models or field examples of diffraction imaging in prestack domain. We have filled this gap by applying a 3D prestack diffraction imaging method to image point diffractors. We calculated the diffractivity by computing the semblance of seismic data along diffraction traveltime curves in the prestack domain. The performance of the method is evaluated on a synthetic case and a field seismic data set collected over the Kevitsa mineral deposit in northern Finland. The high-resolution results obtained by the application of prestack diffraction imaging suggest that diffractivity is a robust attribute that can be used in addition to other seismic attributes for the interpretation of seismic data in hard rock environment.

The earthquake of February 3, 2015, near Sumy, Ukraine: Source parameters and focal mechanism

The parameters of the earthquake that took place February 3, 2015, near the city of Sumy, Ukraine, were calculated from an analysis of records obtained by both Russian and Ukrainian seismic stations (Poltava, Skvira, Nikolaev, Dneropetrovsk, and Desna). The calculated hypocenter depth of 54 km was verified by several approaches: isolation of deep PP, SP phases from the records of remote stations and solution of the kinematic problem for the Poltava station. The focal mechanism as shear with a complex fault component was determined by the first arrivals of P-waves. The data on the azimuthal travel-time curve confirm the focal mechanism. We have calculated the earthquake parameters; they are as follows: length gap L1 = 8.08 km, L2 = 6.68 km, a destruction rate of C = 2 km/s. We have obtained the dynamic parameters of the event. The calculated fault length (L = 5.46 km) within the accuracy limits of the method coincides with the early result obtained by the azimuthal travel-time curve. On the basis of these results, we suggest that elastic energy release and formation of the dislocations in the earthquake source occurred on a smooth, prefractured fault (σr > 0). Association of the hypocenter with the tectonic node of the northern marginal fault of the Dnieper–Donets graben and northern branch of Kryvyi Rih–Kremenchuk suture confirm this. Here, we observe a considerable Moho depth, structural alteration, and high gradients of the temperature and magnetic and electric rock properties in the lower Earth’s crust and upper mantle. These circumstances are favorable for the earthquake occurring here.

Traffic Analysis and OD Travel Time Matrix Based on Two-Fluid Model

The traditional four-step traffic demand forecast model is limited by the long processing time and high cost of origin destination (OD) matrix investigation. A dynamic traffic assignment model is difficult to establish for a large-scale network. To obtain the traffic character and quickly search the shortest path of different zones for dynamic route guidance, the authors analyze travel distance and travel time using taxi global positioning system (GPS) data, fit the travel time and stop time relation curve, and propose a method of traffic analysis using the two-fluid curve. The authors find that the bandwidth of the two-fluid curves is valuable for traffic operation and guidance. Subsequently, the authors analyze the relationship between unit distance travel time and unit distance stop time of different zones. The results verify that the sensitivity of different traffic zones varies. Finally, matrix iteration is used to calculate the shortest travel time path under different unit distance stop times, and the OD travel time matrix is analyzed. The findings indicate that the two-fluid method can be used for dynamic route guidance.

Runoff Prediction for Dam Safety Evaluations Based on Variable Time of Concentration

AbstractThis paper describes a method for estimating runoff hydrographs for dam safety evaluations in which the time of concentration is considered dependent on the magnitude of the excess rainfall intensity. Current practice for hydrologic routing assumes the time of concentration to be invariant and a function of basin characteristics. The proposed method addresses the limitations arising from the assumed linearity in the unit hydrograph by showing that the time of concentration is dependent on the excess rainfall intensity through a power relationship. The proposed method uses a variable flow velocity approach to compute travel time and construct time-area curves for a range of excess rainfall intensities. From the derived relationships between time of concentration, flow, and excess rainfall intensity, an explicit nonlinear convolution is performed to compute runoff hydrographs. In its current implementation, the method assumes a uniform distribution of rainfall over the entire watershed. This method ...

Insights into the crustal structure of the transition between Nares Strait and Baffin Bay

The crustal structure and continental margin between southern Nares Strait and northern Baffin Bay were studied based on seismic refraction and gravity data acquired in 2010. We present the resulting P wave velocity, density and geological models of the crustal structure of a profile, which extends from the Greenlandic margin of the Nares Strait into the deep basin of central northern Baffin Bay. For the first time, the crustal structure of the continent-ocean transition of the very northern part of Baffin Bay could be imaged. We divide the profile into three parts: continental, thin oceanic, and transitional crust. On top of the three-layered continental crust, a low-velocity zone characterizes the lowermost layer of the three-layered Thule Supergroup underneath Steensby Basin. The 4.3–6.3km thick oceanic crust in the southern part of the profile can be divided into a northern and southern section, more or less separated by a fracture zone. The oceanic crust adjacent to the continent-ocean transition is composed of 3 layers and characterized by oceanic layer 3 velocities of 6.7–7.3km/s. Toward the south only two oceanic crustal layers are necessary to model the travel time curves. Here, the lower oceanic crust has lower seismic velocities (6.4–6.8km/s) than in the north. Rather low velocities of 7.7km/s characterize the upper mantle underneath the oceanic crust, which we interpret as an indication for the presence of upper mantle serpentinization. In the continent-ocean transition zone, the velocities are lower than in the adjacent continental and oceanic crustal units. There are no signs for massive magmatism or the existence of a transform margin in our study area.

Glaciological and geophysical investigations aimed at organization of a new airfield at the Station Mirny (East Antarctica)

Main results of glaciological and geophysical engineering surveys, conducted during three summer field seasons of 2013– 2016 (59–61st Russian Antarctic Expeditions – RAE) near the Russian Station Mirny (East Antarctica), are discussed in the paper. Objective of these works was to site and then to organize a new airfield for landing of medium-range aircrafts with ski landing gears. Investigations included aerial photography, GPR surveys (georadar profiling), ice core drilling, and installation of landmarks to measure velocity of the glacier motion. The GSSI ground-penetrating radars with the main frequencies of 270 MHz and 900 MHz were used. In addition, special explorations were conducted for detecting the englacial crevasses by means of remote-sensing methods. The GPR data allowed a revealing the boundary between snow-andfirn thickness and atmospheric ice. In the course of processing of 252 travel-time curves of the diffracted waves a kinematic model of the sub-surface part of the glacier has been constructed. It was found that the dielectric permittivity of the snowfirn thickness averages 2.43; similar value for the atmospheric ice amounts to 3.0. The GPR data made it possible to determine intraglacial (englacial) crevasses and to choose the most favorable field for the landing. On February 10, 2016, the first middle-range aircraft DC-3T (BT-67) had landed onto the new run-way near the station Mirny.

A Fault Diagnosis Method of High Voltage Circuit Breaker Based on Moving Contact Motion Trajectory and ELM

In order to avoid the deficiencies of conventional high voltage circuit breaker mechanical properties detection methods, a new algorithm based on image block matching with diamond search strategy is presented in this paper. The motion of auxiliary mark on the pull rod or shaft is firstly recorded by a high-speed and high-definition digital camera when the circuit breaker is open or close. Then the motion trajectory is acquired through diamond image block matching method. The mechanical parameters, such as travel and open and close velocity, are calculated according to the travel-time curve of the circuit breaker. Finally, evaluation model is constructed taking mechanical parameters characteristic values as inputs of ELM. Comparing to the existing techniques, our method is a noncontact measurement based on computer vision. It is easy and convenient for practical application since it need not any electrical and mechanical connection to the breaker. Another advantage of our method is that it can obtain the line and angle displacement simultaneously. The experiment results on the circuit breaker of 220 kv show that our method is effective for breaker mechanical properties detection.

Imaging in 2D media

Stacking by CDP technique is inapplicable for processing of data from bottom seismic stations or acoustic sonobuoys. In addition, big amount of unknown velocity and structural parameters of the real layered medium do not allow these parameters to be defined by standard processing methods. Local sloped stacking is proposed for simultaneous obtaining the stacked tracks, travel time curve of a chosen wave, and the first derivative of this travel time curve. The additionally defined parameters are second derivative of this travel time curve and integrated average of squared travel time curve. These data are sufficient to reduce the amount of unknown parameters (down to one–two for each boundary) when layer-by-layer top-to-bottom processing. As a result, the stable estimates of velocity parameters of the layered (isotropic or anisotropic) medium can be obtained and stacked tracks obtained by local sloped staking can be transformed into boundaries in the time and depth sections.

Upper mantle and transition zone structure beneath Leizhou–Hainan region: Seismic evidence for a lower-mantle origin of the Hainan plume

The origin of the widespread volcanism at the Leizhou–Hainan (Leiqiong) region in the Southern China remains obscure. We take advantage of the highly active seismicity and dense seismic networks surrounding this region to investigate its upper mantle and Mantle Transition Zone (MTZ) structure. Over 5000 P-wave waveforms whose raypaths bottom at depths around the MTZ are collected, and traveltimes of their first arrivals are hand-picked. By matching the traveltime curve variation over the epicentral distance range from 10° to 35°, we first construct a 1-D upper mantle and MTZ velocity structure for the region. This initial model is then refined by forward modeling, in which the observed triplicated waveforms from selected earthquakes are compared with the synthetic seismograms with varying velocity structure. In our preferred model for Leiqiong, the P-wave velocities deeper than 200km at the upper mantle are 0.8–1.2% lower than the IASP91, and 0.6% slower in the MTZ, while the top and bottom boundaries of the MTZ depresses 12km and slightly uplifted, respectively, compared to the global averages. This model provides independent constraints on the structure beneath Leiqiong, suggesting a thermal anomaly within the MTZ and a lower mantle origin for the volcanism seen in this region.

Seismic data two-step recovery approach combining sparsity-promoting and hyperbolic Radon transform methods

In current research of seismic data recovery problems, the sparsity-promoting method usually produces an insufficient recovery result at the locations of null traces. The HRT (hyperbolic Radon transform) method can be applied to problems of seismic data recovery with approximately hyperbolic events. Influenced by deviations of hyperbolic characteristics between real and ideal travel-time curves, some spurious events are usually introduced and the recovery effect of intermediate and far-offset traces is worse than that of near-offset traces. Sparsity-promoting recovery is primarily dependent on the sparsity of seismic data in the sparse transform domain (i.e. on the local waveform characteristics), whereas HRT recovery is severely affected by the global characteristics of the seismic events. Inspired by the above conclusion, a two-step recovery approach combining sparsity-promoting and time-invariant HRT methods is proposed, which is based on both local and global characteristics of the seismic data. Two implementation strategies are presented in detail, and the selection criteria of the relevant strategies is also discussed. Numerical examples of synthetic and real data verify that the new approach can achieve a better recovery effect by simultaneously overcoming the shortcomings of sparsity-promoting recovery and HRT recovery.

Automatic identification of multiply diffracted waves and their ordered scattering paths.

An automated algorithm uses recordings of acoustic energy across a spatially-distributed array to derive information about multiply scattered acoustic waves in heterogeneous media. The arrival time and scattering-order of each recorded diffracted acoustic wave, and the exact sequence of diffractors encountered by that wave, are estimated without requiring an explicit model of the medium through which the wave propagated. Individual diffractors are identified on the basis of their unique single-scattering relative travel-time curves (move-outs) across the array, and secondary (twice-scattered) waves are detected using semblance analysis along temporally offset primary move-outs. This information is sufficient to estimate travel times and scattering paths of all multiply diffracted waves of any order, and for these events to be identified in recorded data. The algorithm is applied to synthetic acoustic data sets from a variety of media, including different numbers of point-diffractors and a medium with strong heterogeneity and non-hyperbolic move-outs.

Gene expression models for prediction of longitudinal dispersion coefficient in streams

Summary Longitudinal dispersion is the key hydrologic process that governs transport of pollutants in natural streams. It is critical for spill action centers to be able to predict the pollutant travel time and break-through curves accurately following accidental spills in urban streams. This study presents a novel gene expression model for longitudinal dispersion developed using 150 published data sets of geometric and hydraulic parameters in natural streams in the United States, Canada, Europe, and New Zealand. The training and testing of the model were accomplished using randomly-selected 67% (100 data sets) and 33% (50 data sets) of the data sets, respectively. Gene expression programming (GEP) is used to develop empirical relations between the longitudinal dispersion coefficient and various control variables, including the Froude number which reflects the effect of reach slope, aspect ratio, and the bed material roughness on the dispersion coefficient. Two GEP models have been developed, and the prediction uncertainties of the developed GEP models are quantified and compared with those of existing models, showing improved prediction accuracy in favor of GEP models. Finally, a parametric analysis is performed for further verification of the developed GEP models. The main reason for the higher accuracy of the GEP models compared to the existing regression models is that exponents of the key variables (aspect ratio and bed material roughness) are not constants but a function of the Froude number. The proposed relations are both simple and accurate and can be effectively used to predict the longitudinal dispersion coefficients in natural streams.

Diagnostic possibilities of body waves for studying the interior structure of mars

The diagnostic possibilities of body waves for refining the models of the Martian interior structure are explored. The MATLAB-based software is developed for calculating the theoretical travel time curves and estimating the amplitudes of P- and S-waves for the predicted values of seismic moments of the marsquakes. The traveltimes of the P- and S-waves, as well as the reflected phases and the phases that passed through the core as the functions of epicentral distance, are compared for different test models of the interior of Mars.

A program for the inversion of travel-time curves of refracted and reflected seismic waves propagating in vertically inhomogeneous elastic media for MS-DOS and Windows

A program for the inversion of hodographs of seismic waves propagating in vertically inhomogeneous media is described. Two versions of the program are proposed: for MS-DOS and Windows, respectively. Detailed descriptions of input-data preparation in both operating systems are given.

Nonstretch normal moveout through iterative partial correction and deconvolution

Source to receiver distances used in seismic data acquisition have been steadily increasing and it is now common to work with data acquired with more than 10 km of offset. Subbasalt exploration and seismic undershooting are just two applications in which long-offset reflections are sought. However, such reflections are often subjected to muting to suppress normal moveout (NMO) stretch artifacts, thus causing a loss of valuable information. To retrieve these portions of the recorded wavefield, we developed a nonstretch NMO correction based on wavelet estimation and on an iterative procedure of partial NMO correction and deconvolution. We evaluated this methodology using fourth-order traveltime curve approximations to increase the offset of usable reflections, but it can be adapted to traveltime curves of any order. Time- and space-variant wavelets, estimated by means of singular value decomposition along the sought traveltimes, were used to build the desired output for the deconvolution that aims at retrieving the original shape of the partially stretched wavelets. We tested our method on a synthetic gather presenting time and offset varying wavelets, on a real-marine line simulating an undershooting pattern and on true undershooting land-marine data. These examples demonstrated that our new algorithm effectively limits the stretching associated with the NMO correction and enables the recovery of those portions of the stacked sections that are typically lost from muting in the standard NMO correction.