Travel-time Curves Research Articles

Crustal structure of the Caucasus from the Stepnoe-Bakuriani and Volgograd-Nakhichevan DSS profiles (reinterpretation of the primary data)

This work presents the results of reinterpretation of the deep seismic sounding (DSS) data for the Stepnoe-Bakuriani profile and the southern part of the Volgograd-Nakhichevan profile, carried out using new processing methods. Both the profiles cut the trend of the Greater Caucasus. They were acquired in the 1960s by multichannel continuous profiling, which provided high-quality records; however, only the travel-time curves for the main wave types have survived till now. The waves recorded on these profiles have a rather complex origin and their processing by the methods existing at that time was a challenge. At present, the modern computer technologies allowed us to invert the preserved travel-time curves for the velocity models of the Earth’s crust and the very tops of the mantle down to 80 km. It is shown that the crustal thickness increases under the Greater Caucasus up to 50–60 km and this increase is not gradual, as implied in the previous reconstructions, but occurs through a system of deep dislocations. Traced by the oblique reflections and sharply contrasting seismic velocities, these dislocations extend into the crust. An extended, north-dipping boundary is revealed at a depth of 50–80 km. The velocity model of the Greater Caucasian crust exhibits slightly decreased velocities compared to the surrounding platform regions. At the same time, the velocities sharply increase in the middle and even upper parts of the crust in the Kura depression.

GPR prospecting of cylindrical structures in cultural heritage applications: a review of geometric issues

ABSTRACTCultural heritage diagnostics often involves the application of geophysical methods like ground‐penetrating radar (GPR) along non‐planar surfaces. Standard processing algorithms are based on the hypothesis of half‐space geometry. In the investigation of cylindrical structures like columns with a small radius, this basic assumption is strongly violated and the application of conventional algorithms may be inappropriate. Although frequently overlooked, this fact implies that appropriate algorithms should be developed to take into account the additional data complexity arising from the unusual acquisition setting and exceptional care should be used in interpreting the radargrams. The first part of this paper offers a systematic review of problems related to geometric issues (air waves, multiple reflections, shape of the various traveltime curves) performing a theoretical analysis of major differences between cylindrical and half‐space models. The second part shows, through numerical simulations and real data examples, the inappropriateness of the half‐space model in the processing and interpretation of GPR surveys along cylindrical structures. The numerical simulations help visualizing the differences in reflection/diffraction features from buried and above‐surface objects and errors arising from the use of inappropriate models. The real data examples offer a tangible demonstration of the benefits obtained from exploiting the correct cylindrical model and the additional information gained from later arrivals, such as multiple reflections, in constraining the estimation of electrical parameters (propagation velocity) related to constitutive and health states of the structure.

The Beam-Forming Technique for Enhancement of Noisy Seismic Refraction Data

We have described a method of obtaining useful information from noisy seismic refraction data. The simple method, tagged beam-forming technique, is based on the basic time-distance equations of refraction seismology. It involves basically of introducing relative time delays to individual seismic traces of seismic refraction spread to correct for the non-coincidence of the incidence seismic energy at different geophones, and averaging the traces to obtain the beam. The assumption here is that the signal is coherent between the geophones while the noise is random, and for groups of geophones corresponding to the same refraction segments of the travel time curve, this basic assumption is valid. The process of beam forming therefore leads to improvement in signal-to-noise ratio (SNR) and correct determination of the intercept times which are subsequently used to compute other geologic layer parameters. The ability of the applied technique to filter out or minimize random noise has been tested using a modified random number routine. The performance test on computation of geologic layer parameters using very noisy synthetic data reveals that the method is still very reliable even with very poor quality data having SNR as small as 0.05.

Offset continuation (OCO) ray tracing using OCO trajectories

Offset continuation (OCO) is a seismic configuration transform designed to simulate a seismic section as if obtained with a certain source-receiver offset using the data measured with another offset. Since OCO is dependent on the velocity model used in the process, comparison of the simulated section to an acquired section allows for the extraction of velocity information. An algorithm for such a horizon-oriented velocity analysis is based on so-called OCO rays. These OCO rays describe the output point of an OCO as a function of the Root Mean Square (RMS) velocity. The intersection point of an OCO ray with the picked traveltime curve in the acquired data corresponding to the output half-offset defines the RMS velocity at that position. We theoretically relate the OCO rays to the kinematic properties of OCO image waves that describe the continuous transformation of the common-offset reflection event from one offset to another. By applying the method of characteristics to the OCO image-wave equation, we obtain a raytracing-like procedure that allows to construct OCO trajectories describing the position of the OCO output point under varying offset. The endpoints of these OCO trajectories for a single input point and different values of the RMS velocity form then the OCO rays. A numerical example demonstrates that the developed ray-tracing procedure leads to reliable OCO rays, which in turn provide high-quality RMS velocities. The proposed procedure can be carried out fully automatically, while conventional velocity analysis needs human intervention. Moreover, since velocities are extracted using offset sections, more redundancy is available or, alternatively, OCO velocities can be studied as a function of offset.

Dromocrone delle onde P dedotte dallo studio del terremoto profondo del basso Tirreno del 3 Gennaio 1960

By means of a statistica! metliod of Caloi-Peronaci, theauthor determines the space-time coordinates of the deep earthquake whichtook place in the lower Tirrenian sea on Jan. 3, 1960.Through a series of attemps performed by making the recording timesof the P's intervene in divers station groupings (intermediate, intermediateand far, intermediate and near), emphasins is given to the utility of thedata of near stations with regard to a better determination of depth andorigin time.The most probable equations of tlie travel-time curves of the P wavesare worked out, valid up to distances of 30°. The equations thus obtainedmake evident anticipations of the order of 2S in the stations of Messina andReggio C. with regard to the times foreseen according to tlie Jeffreys and Bullentables. The travel-time curves obtained for the group of scandinavianstations ( 18°,6 ^ A < 30°) shows anticipations from 6 about (Goteborg) to zero.

Seismic ray tomography using L1 integral norm

Seismic ray tomography methods are usually associated with substantial computer processing time. The reason for this is that at each step of the iterative inversion process defined by the tomographic method the two-point ray tracing problem must be solved for each source-receiver pair. In order to resolve this, an Euclidean norm (L2 vector norm), commonly used in error functions which are to be minimized in inversion procedures, is substituted by an L1 integral norm, which enables the estimation of model parameters by minimizing the area between observed and calculated traveltime curves that are interpolated (or adjusted) to the data points. Relatively simple mathematical developments and numerical experiments with two-dimensional compressional seismic wave velocity field models showthat L1 integral norm saves an enormous amount of processing time with no significant loss of accuracy. Occasionally, parameters of the model can be better estimated using L1 integral norm than the L2 vector norm that is traditionally utilized in seismic inversion tomography.

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

La « discontinuità 20° »

In reference to some circumstances ascertained in arecent study of the seismic novement that occurred in Sicily on the 23rdDecember 1959, two a t t e m p t s have been carried out in order to determinethe depth of the discontinuity surface t h a t we suppose to be the cause ofanomalous behaviour observed in t h e travel-time curve of P-waves near 20°I n the first part some possible q u a n t i t a t i v e consequences deriving fromt h e observed disagreement with Jeffreys-Bullen travel-time curves have beencharacterized, on the assumption that the velocity of P-waves resumesordinary behaviour just below the Moliorovicic discontinuity. On thisassumption, a probable velocity law has been determined t h a t presents aremarkable discontinuity at a depth of 586 km.I n the second part it has been demonstrated t h a t , assuming the numericalvalues of velocity proposed by Gutenberg in the range 40 <= h <= 80 kin, the deduced equations of the two branches of P-waves travel-timecurve give a distribution of velocity values as function of depth t h a t presentsa marked discontinuity at the depth of 536 km, immediately followedby a marked increase in the rate of increase of velocity with increase ofdepth.

Crustal and uppermost mantle structure beneath the External Dinarides, Croatia, determined from teleseismic receiver functions

SUMMARY Broad-band seismograms of teleseismic events recorded at the Croatian Seismological Network were used to compute radial receiver functions (RFs) for eight locations in the External Dinarides. Waveform modelling was performed by a multistep matching of the theoretical RFs computed for horizontally layered 1-D isotropic models with the averaged observed RFs. Constraints from existing deep seismic sounding profiles, traveltime curves of regional crustal seismic phases and intuitive inferences gained from interactive forward modelling were used to construct initial 1-D models of the Earth. A non-linear inversion was performed in two steps—a grid search followed by the Monte Carlo search for the model parameters. Concurrently, RFs from different azimuths were stacked to obtain trade-off estimates of crustal thickness versus Vp/Vs ratios. The Moho depths were found in the range from around 40 km for Northern Adriatic stations to over 55 km for stations in the central part of the External Dinarides. Comparing our results with recent maps of the Moho topography inferred from seismic and gravimetric data, we find that for some stations the agreement between our results and the existing Moho maps is very good. For the others, we find the Mohorovicic discontinuity to be considerably deeper, indicating some of the thickest crust in Europe. Although it is plausible that such a deep Moho could be a consequence of a complex tectonic setting of the region (e.g. overlapping of two large tectonic units—the Adriatic microplate and the Dinarides), this result will have to be verified in the future studies using various other geophysical techniques.

Analysis of remote detection travel time curves measured from microfluidic channels

Remote detection technique can increase sensitivity of an NMR experiment by several orders of magnitude in microfluidic applications. Travel time experiment is a basic remote detection NMR experiment, which reveals the travel time distribution of the molecules flowing from the encoding coil region to the detector. In this article, we focus on analyzing how flow type (Poiseuille or plug flow), diffusion, dispersion and geometry of the flow channels are manifested in the travel time curves measured from microfluidic channels. We demonstrate that remote detection travel time experiment could be used even as an alternative NMR method for measuring self-diffusion coefficient of a fluid without magnetic field gradients. In addition, we introduce a modified travel time pulse sequence, which removes the signal of unencoded fluid spins as well as the background signal arising from the material inside or close to the detector.

Ｐ波反射法測線に加えた３成分地震計１個でのＳ波速度の簡易推定法

Seismic reflection profiling using P-to-SV converted waves with 3-component seismometers has an advantage for examining deeper S-wave velocity profile compared with other methods such as SH-wave reflection profiling. In spite of this advantage, the reflection profiling with 3-component seismometers has not been popular because it requires three-times more channels than reflection profiling with vertical-component seismometers, which is popular for examining P-wave velocity profile. In our previous paper, we showed a possibility of examining S-wave velocity profile by analyzing P-to-SV reflected waves observed at one 3-component seismometer together with P-wave reflection profiling. Although the method in the previous paper has ambiguity to obtain S-wave velocity, the approach is attractive because one additional 3-component seismometer requires little additional cost. In this paper, we show the validity of the approach using another method. This new method is easy to apply, if P-wave velocity profile was already determined using vertical component reflection profiling. We picked P-to-SV reflected waves on observed radial-component records, and adjusted theoretical travel-time curves to the observed waves at one 3-component seismometer. P-wave velocity profile and depths of discontinuities were fixed to the result of P-wave reflection profiling. When the travel time was calculated, a ratio of P-wave velocity to S-wave velocity was assumed to be the same for all layers. The ratio for well-matched theoretical time was considered as the ratio of vertical travel times of S-wave to P-wave from the surface to the reflected layer. Shear-wave velocity for each layer was calculated from the ratio. Slowness of the P-to-SV reflected wave at large offsets depends on P-wave velocity and a depth of the reflector, but shows little dependence on S-wave velocity at large offsets. This characteristic makes it possible to separate the influence of the reflector depth and of S-wave velocity on travel-time curves of the reflected waves. Tests for simple horizontal layer models and dipping layer models showed that S-wave velocities were obtained within about 10% in error, and that obtained S-wave velocity was stable in errors of P-wave velocity and of an inclination of the layer. We applied this method to a previous seismic reflection survey with 3-component seismometers which was carried out at Fuchu-city, Tokyo. Previous study analyzed P-and S-wave velocity profiles to 2-km depth using P-and converted-wave reflection profiling. Another survey also examined P-and S-wave velocity profiles near the survey line using VSP method. We selected one station near the VSP well among the survey stations, and analyzed the data using the method. S-wave velocity profile that we obtained was consistent with the profiles of the previous studies to 2-km depth. Only one 3-component seismometer was necessary for this method with P-wave reflection profiling. Our results show that this method provides adequate shear-wave velocity profile with little additional cost.

The velocity of P-waves below the " 20" discontinuity "

Velocity values for P-waves below the " 2 0 ° discontinuit y " are calculated by applying the Wadati-Masuda method, and by using separately calculated travel-time curves for the P-waves and the PdPwaves of each of the five earthquakes examined. The velocities calculated have a mean value of 10.21 ± 0.24 km/sec, and are without exception greater than those obtained in a previous piece of research (9.57 km/sec). As regards the value of the velocity of the P-waves above the "20° discontinuity" (8.96 kin/sec), the percentage difference between the velocity above and the velocity below the "20° discontinuity" is 14%. Application of the Herglotz-Wiechert procedure in the zone of the mantle below the "20° discontinuity" does not give significant variations in the correlation between velocity and depth, in comparison with the results obtained by other authors.

Some seismologieal results and geostructural suggestions from a study of the Reggio Calabria earthquake of 16 January, 1975

Starting from the analytic determination of the most important parameters of the Reggio Calabria earthquake, several geostructural hypotheses are analyzed and discussed. The statistical hvpocentre, calculated in two separate phases for (lie epicentral coordinates (q> = 38°04'28"; A = 15°38'55") and for the depth (h = 10.04 km), is consistent with an earlier macroseismic estimation. The travel times for the first arrivals, limited to the distance interval 0 ^ ¿1° ^ 12 can be approximated by means of the three linear equations: which can be interpreted as the travel-time curves of the direct (Pg) and refracted (Pn i , P„2) longitudinal waves. The existence of a focal mechanism, compatible with a couple + oppositely directed force model is deduced from a study of the signs of the first impulses. This model includes faulting and displacement with dislocation plane N 22"K and inclination of ~ 75° towards XI 12°E. Considering the focal location and our geostructural knowledge of the area involved, the Reggio Calabria earthquake can be explained by a process of tectonic distension due to the drift of the Calabrian Arc and to other reliable geostructural elements. The explanation is critically evaluated in relation to the tectonic forces involving the Strait of Messina and the volcanic zone of Etna.

The Correction of Shot and OBS Position in the 3D Seismic Experiment of the SW Indian Ocean Ridge

AbstractIn a three‐dimensional (3D) seismic experiment carried out at sea, due to the distance between GPS antenna on ship and the center of air gun sources in sea water, and some environmental factors such as wind and sea current, the survey ship may adjust its heading direction, which makes the real shot position deviate from designed shot track; meanwhile, the real locations of Ocean Bottom Seismometers (OBS)s on the seafloor may drift from designed points (deployed locations) since OBSs are of free‐fall type and usually affected by sea currents during its descending. So, the calibration of air‐gun shots and OBSs' positions are basic steps for later studies on fine seismic velocity tomography. Our research is mainly focused on the correction of shot and OBS positions with up‐to‐date data from a new 3D seismic experiment in the SW Indian Ocean Ridge. The shot positions were corrected with geometric relationship among ship's heading, GPS antenna and air‐gun array. The correction of OBS positions were based on travel time of direct water wave, Monte Carlo method and least square method. In addition, we discuss the relationship between reduced travel time curves of direct water wave and OBS's deviation. Our results show that OBS's locations generally drift from the designed points about 1 km with an error range of 0~20 m, and shots are adjusted to its actual positions. The correction results not only supply a fundamental database for the 3D seismic tomography in our study area, but also provide valuable experiences for 3D seismic survey in the South China Sea in future.

Fifty years of seismic anisotropy studies in Russia

This is a historic overview of seismic anisotropy studies in Russia run as part of seismic exploration work in the 1940s through the 1980s, with a focus on main research lines. At the early stage in the 1940s through 1950s, most important contributions belonged to A.G. Tarkhov, Yu.V. Risnichenko, and S.M. Rytov (averaging the parameters of stratified media), I.I. Gurvich (processing reflection and refraction traveltime curves in media with elliptical anisotropy), and N.I. Berdennikova (shear-wave velocity anisotropy). In the 1960s–1980s, there were two basic schools of thought: one of G.I. Petrashen’, with a more theoretical approach, and the other of N.N. Puzyrev dealing more with experimental work. Most of experiments addressed a newly discovered phenomenon of azimuthal anisotropy. This anisotropy appearing as “anomalous” polarization of shear and converted waves was found out to result from vertical fractures in rocks. The unusual polarization became understood owing to Klem-Musatov’s model of a subsurface with a system of aligned cracks. The problem was fully resolved after field data had been processed with an algorithm by I.R. Obolentseva and S.B. Gorshkalev, for separating the total field of interfering shear waves of two types into fast and slow phases polarized in crack-parallel and crack-orthogonal directions, respectively.

Time-lapse refraction seismic tomography for the detection of ground ice degradation

Abstract. The ice content of the subsurface is a major factor controlling the natural hazard potential of permafrost degradation in alpine terrain. Monitoring of changes in ice content is therefore similarly important as temperature monitoring in mountain permafrost. Although electrical resistivity tomography monitoring (ERTM) proved to be a valuable tool for the observation of ice degradation, results are often ambiguous or contaminated by inversion artefacts. In theory, the sensitivity of P-wave velocity of seismic waves to phase changes between unfrozen water and ice is similar to the sensitivity of electric resistivity. Provided that the general conditions (lithology, stratigraphy, state of weathering, pore space) remain unchanged over the observation period, temporal changes in the observed travel times of repeated seismic measurements should indicate changes in the ice and water content within the pores and fractures of the subsurface material. In this paper, a time-lapse refraction seismic tomography (TLST) approach is applied as an independent method to ERTM at two test sites in the Swiss Alps. The approach was tested and validated based on a) the comparison of time-lapse seismograms and analysis of reproducibility of the seismic signal, b) the analysis of time-lapse travel time curves with respect to shifts in travel times and changes in P-wave velocities, and c) the comparison of inverted tomograms including the quantification of velocity changes. Results show a high potential of the TLST approach concerning the detection of altered subsurface conditions caused by freezing and thawing processes. For velocity changes on the order of 3000 m/s even an unambiguous identification of significant ice loss is possible.

Characterization and Development of the Ideal Pedal Force, Pedal Travel, and Response Time in the Brake System for the Translation of the Voice of the Customer to Engineering Specifications

It is a well-known fact that braking is one of the most important factors in customer satisfaction and a key factor in the voice of the customer (VOC). The VOC has to be translated into engineering specifications in due course and various methods have been developed. Current engineering specifications utilize only a few discrete points on the pedal force (PF), pedal travel (PT), and response time (RT) curves owing to the multi-dimensional nature of these curves, and thus may not represent the entire curves well. Principal component analysis (PCA) is applied in order to analyse the entire shapes of the three curves and to identify the underlying variation modes. Then, buyer satisfaction scores from a mail survey are regressed on the variation modes. Finally, the ideal shapes of the PF, PT, and RT curves are developed for the sport segment, the coupé and sedan segment, and the truck segment to aid new brake system and vehicle design. Translating the VOC from marketing research into engineering specifications is a critical step and PCA is a valuable tool for this translation.

On time-distance curves for Pn waves in the Friuli earthquake of 6 May 1976

Results obtained in the study of the arrival times of the Pn phase for the Friuli earthquake of 6 May 1976 (H = 20h00™15s,5; Normal 0 14 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:Tabella normale; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:; mso-padding-alt:0pt 5.4pt 0pt 5.4pt; mso-para-margin:0pt; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:Times New Roman; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} φ = 46° 16'30 N; Normal 0 14 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:Tabella normale; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:; mso-padding-alt:0pt 5.4pt 0pt 5.4pt; mso-para-margin:0pt; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:Times New Roman; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} λ = 13° 06' 00 E; M = 6.3) with respect to the epicentral distance are reported. Particular emphasis is given to the fact that the travel-time curves relative to the Pn waves and the corresponding equations are only valid for selected alignment. By plotting the time-distance curves it was possible to derive the depth of the lower surface of the upper mantle for different zones; the results thus obtained exibit significant differences.

On shear‐wave triplications in a multilayered transeversely isotropic medium with vertical symmetry axis

ABSTRACTThe presence of triplications (caustics) can be a serious problem in seismic data processing and analysis. The traveltime curve becomes multi‐valued and the geometrical spreading correction factor tends to zero due to energy focusing.We analyse the conditions for the qSV‐wave triplications in a homogeneous transversely isotropic medium with vertical symmetry axis. The proposed technique can easily be extended to the case of horizontally layered vertical symmetry axis medium. We show that the triplications of the qSV‐wave in a multilayered medium imply certain algebra. We illustrate this algebra on a two‐layer vertical symmetry axis model.

Generation of Shear Waves from Explosions in Water-Filled Cavities

Abstract We analyze shear waves from a 27 kiloton nuclear explosion at 597 m depth in salt at Azgir, Kazakhstan, and four subsequent small nuclear explosions in the water-filled cavity of the original explosion. We use local recordings on a common set of radial and vertical component instruments to identify the source of these shear waves. These small explosions caused minimal damage to the cavity walls and so are similar in some important ways to cavity decoupled explosions; however, the signal size is comparable to a tamped explosion. The shear wave amplitudes relative to P generated by the water-filled cavity explosions are comparable to each other and to the tamped explosion. Comparison of the water-filled cavity explosions’ P spectra with a distinct bubble pulse peak and two distinct S phases, which lack that spectral peak, indicates that the S phases are not scattered from P . The initial S -waves’ travel-time curve, impulsiveness, and compactness are consistent with generation at or very near the source and are inconsistent with significant scattering much outside the source volume. We estimate that the S waves are consistent with an S -wave source at the same depth and with the same moment as the spherical component of the explosion source. Nonlinear axisymmetric numerical modeling of the nonlinear deformation of the salt medium indicates that neither fracturing nor asphericity of the nonlinear region outside of the cavity, for a variety of source asymmetries, would produce the observed shear waves for any of the water-filled cavity explosions. Oscillations of the cavity itself due to variations in the amplitude and timing of the pressure around the cavity wall can, however, produce shear waves at the frequencies observed. Both offset of the source and asphericity of the cavity lead to such variations in pressure.