Search

In this work, the propagation and attenuation of vertically polarized surface waves when interacting with terraced slopes is studied experimentally and numerically. To validate the devised simulation, a laboratory-scale physical model is tested in order to examine the attenuation properties of this well-known artificial landform. The experiment involves formation of a terraced slope, in a laboratory setup, via use of an unconsolidated granular medium made of silica microbeads. This granular medium exhibits a gravity-induced power-law stiffness profile, resulting in a depth-dependent velocity profile. A piezoelectric actuator was used to excite vertically polarized surface acoustic modes localized near the surface of the medium. The three components of the particle velocity field of these modes were measured by means of a 3-D laser Doppler vibrometer. In accordance with the terraced slope, a simple inclined plane was further tested to investigate and highlight the differences in terms of wave propagation along these two different ground formations. The results of this research provide significant experimental evidence that the terraced slopes form mechanisms which attenuate low-frequency surface waves, thus acting as metasurfaces. This work suggests the use of laboratory-scale physical models to investigate the wave propagation in different landforms, which extend beyond typical horizontal ground morphologies, and which could be linked to atypical wave propagation properties, possibly even influencing propagation of seismic waves. [ABSTRACT FROM AUTHOR]

Tsunami generation by offshore earthquakes is a problem of scientific interest and practical relevance, and one that requires numerical modelling for data interpretation and hazard assessment. Most numerical models utilize two-step methods with one-way coupling between separate earthquake and tsunami models, based on approximations that might limit the applicability and accuracy of the resulting solution. In particular, standard methods focus exclusively on tsunami wave modelling, neglecting larger amplitude ocean acoustic and seismic waves that are superimposed on tsunami waves in the source region. In this study, we compare four earthquake-tsunami modelling methods. We identify dimensionless parameters to quantitatively approximate dominant wave modes in the earthquake-tsunami source region, highlighting how the method assumptions affect the results and discuss which methods are appropriate for various applications such as interpretation of data from offshore instruments in the source region. Most methods couple a 3-D solid earth model, which provides the seismic wavefield or at least the static elastic displacements, with a 2-D depth-averaged shallow water tsunami model. Assuming the ocean is incompressible and tsunami propagation is negligible over the earthquake duration leads to the instantaneous source method, which equates the static earthquake seafloor uplift with the initial tsunami sea surface height. For longer duration earthquakes, it is appropriate to follow the time-dependent source method, which uses time-dependent earthquake seafloor velocity as a forcing term in the tsunami mass balance. Neither method captures ocean acoustic or seismic waves, motivating more advanced methods that capture the full wavefield. The superposition method of Saito et al. solves the 3-D elastic and acoustic equations to model the seismic wavefield and response of a compressible ocean without gravity. Then, changes in sea surface height from the zero-gravity solution are used as a forcing term in a separate tsunami simulation, typically run with a shallow water solver. A superposition of the earthquake and tsunami solutions provides an approximation to the complete wavefield. This method is algorithmically a two-step method. The complete wavefield is captured in the fully coupled method, which utilizes a coupled solid Earth and compressible ocean model with gravity. The fully coupled method, recently incorporated into the 3-D open-source code SeisSol, simultaneously solves earthquake rupture, seismic waves and ocean response (including gravity). We show that the superposition method emerges as an approximation to the fully coupled method subject to often well-justified assumptions. Furthermore, using the fully coupled method, we examine how the source spectrum and ocean depth influence the expression of oceanic Rayleigh waves. Understanding the range of validity of each method, as well as its computational expense, facilitates the selection of modelling methods for the accurate assessment of earthquake and tsunami hazards and the interpretation of data from offshore instruments. [ABSTRACT FROM AUTHOR]

Enigmatically strong tube waves continue to exist long after the direct wave during repeat crosswell-monitoring surveys at the Nagaoka CO2 injection site in Japan. The tube waves, which have linear moveouts with velocities of 1.29 and 1.41 km s−1 at plastic and steel casings, respectively, are generated by double scattering on the shallow side of the wells. We characterize wavefields to confirm that these late coda waves are tube waves that are excited at the source well, propagate upward, then travel to a receiver well as a body wave, and finally propagate downward along the receiver well. Even though these tube waves result from second-order scattering, they have large amplitudes because of the relatively short distance between source and receiver wells. Because these waves propagate along both wells many times, we can extract detailed information of wave propagation by averaging them to increase the signal-to-noise ratio. We first use the tube waves to relocate sources and receivers between multiple monitoring surveys, demonstrating our ability to correct the severe noise caused by location errors that frequently degrades the repeatability of time-lapse surveys. Then, we apply an inversion based on the physics of tube waves to estimate the location and strength of the scatterers and find that they are predominantly located in the shallow segment of the wells, above the sources, and infer that they are related to local fractures and/or wellbore conditions because their locations do not correspond to the well geometry. Lastly, we use the tube waves to accurately estimate subsurface velocities along the wells. The estimation is stable and robust, and the velocities follow the general trend of subsurface structure seen in the well log data. Due to the receiver spacing, tube-wave analysis cannot resolve a thin, high-velocity layer at the CO2 reservoir. By combining tube waves observed during different stages of the monitoring survey, we can estimate the time-lapse changes of the subsurface velocities. [ABSTRACT FROM AUTHOR]

This study is concerned with the mechanics of wave propagation in a type of architectured, granular, material system. Specifically, we investigate wave propagation in a topologically interlocked material (TIM) system. TIM systems are assemblies of polyhedrons in which individual polyhedrons cannot be removed from the assembly without complete disassembly due to the geometric interlocking of the polyhedrons. The study employs an explicit finite element code to compute phase velocities, amplitude distributions, and wave patterns in a linear assembly of topologically interlocking tetrahedra. Tetrahedra are considered fully 3D linear elastic bodies interacting with neighboring tetrahedra by contact and friction. We discuss the characteristics of waves traveling along the row of tetrahedra. The velocity of the wave is found to be independent of amplitude for an idealized contact, but dependent on the amplitude for a realistic asperity contact. In the absence of friction and for constant contact stiffness the effective wave velocity is determined to be approximately 50% the material wave speed, but increases in the presence of friction to about 80% of the material wave speed. We attribute the wave velocity to wave-guiding as set by the geometry of the tetrahedra assembly geometry and a rocking motion of tetrahedra on an axis perpendicular to the wave propagation direction. The degree to which such rocking motion occurs is affected by friction. [ABSTRACT FROM AUTHOR]

In this work we introduce an identification procedure for the numerical assessment of the constitutive parameters of strain gradient elasticity (SGE) for architectured materials. The key idea is to measure the dispersion diagram of guided waves propagating in an architectured plate and then, by solving an inverse problem, to retrieve the constitutive parameters of the homogeneous equivalent material by fitting the response of the SGE model. In the present contribution this procedure is tested in the case study of a hexagonal honeycomb lattice. The strength of such approach, which is investigated here from a numerical viewpoint only, is that it could also be readily applied in an experimental setting. • Numerical identification of the constitutive parameters of strain gradient elasticity. • Identification based on the dispersion diagram of guided waves. • Case study of hexagonal honeycomb. [ABSTRACT FROM AUTHOR]

Compared with surface waves, guided waves are rarely applied in near-surface investigation. The main reason may lie in the complexity of their dispersion curves. Besides, the study and understanding of guided wave dispersion characteristics are now also inadequate and not deep enough. In this paper, we derived the complete theoretical dispersion curves of P – SV -wave and pure P -wave systems in layered media based on the transmission matrix method and obtained the relative displacement amplitude coefficients at the free surface as a function of frequency and phase velocity for both surface and guided waves. By assigning the value of relative displacement amplitude coefficient to the corresponding point (f,v) on dispersion curve, we got a multi-information diagram called relative amplitude dispersion map (RADM). As a unified description of surface and guided waves, RADM not only shows the velocity–frequency relationship but also represents the polarized energy ratio at the free surface by display colours. The accuracy of RADM was proved by synthetic seismic records, in which RADMs fit well with the corresponding dispersion energy of surface and guided waves. In addition, we designed six models with different Poisson's ratio (PR) and different layer numbers for comparison. It shows that the dispersive vertical-to-horizontal amplitude ratio of guided waves is complex and discontinuous in RADM, which brings great difficulty for mode identification and even affects the subsequent inversion. Tests also show that for high PR layers, the trends of guided P – SV -wave dispersion curves are basically consistent with those of pure P wave. With the decrease of PR, dispersion curves of guided P – SV wave gradually deviate from those of pure P wave. However, RADMs can be greatly consistent with the dispersion energy in either case. This is of great significance for the inversion of near-surface P and S velocities by using dispersion relationships of multimode surface and guided waves. [ABSTRACT FROM AUTHOR]

We present observations and modeling of spatial eigen‐functions of resonating waves within fault zone waveguide, using data recorded on a dense seismic array across the San Jacinto Fault Zone (SJFZ) in southern California. The array consists of 5‐Hz geophones that cross the SJFZ with ~10–30 m spacing at the Blackburn Saddle near the Hemet Stepover. Wavefield snapshots after the S wave arrival are consistent for more than 50 near‐fault events, suggesting that this pattern is controlled by the fault zone structure rather than source properties. Data from example event with high signal to noise ratio show three main frequency peaks at ~1.3, ~2.0, and ~2.8 Hz in the amplitude spectra of resonance waves averaged over stations near the fault. The data are modeled with analytical expressions for eigen‐functions of resonance waves in a low‐velocity layer (fault zone) between two quarter‐spaces. Using a grid search‐based method, we investigate the possible width of the waveguide, location within the array, and shear wave velocities of the media that fit well the resonance signal at ~1.3 Hz. The results indicate a ~300 m wide damaged fault zone layer with ~65% S wave velocity reduction compared to the host rock. The SW edge of the low‐velocity zone is near the mapped fault surface trace, indicating that the damage zone is asymmetrically located at the regionally faster NE crustal block. The imaging resolution of the fault zone structure can be improved by modeling fault zone resonance modes and trapped waves together. Key Points: Source‐independent resonance fault zone wavefield is consistently observed after S wave arrival in data recorded by a dense arrayA fault zone model with a low velocity layer between two quarter spaces can fit well the resonance wavefield at ~1.3 HzModeling the resonance wavefield provides independent constraints on fault zone properties complementary to previous studies [ABSTRACT FROM AUTHOR]

We present a new approach to simulate high-frequency seismic wave propagation in and under the oceans. Based upon AxiSEM3D, this method supports a fluid ocean layer, with associated water-depth phases and seafloor topography (bathymetry). The computational efficiency and flexibility of this formulation means that high-frequency calculations may be carried out with relatively light computational loads. A validation of the fluid ocean implementation is shown, as is an evaluation of the oft-used ocean loading formulation, which we find breaks down at longer periods than was previously believed. An initial consideration of the effects of seafloor bathymetry on seismic wave propagation is also given, wherein we find that the surface waveforms are significantly modified in both amplitude and duration. When compared to observed data from isolated island stations in the Pacific, synthetics which include a global ocean and seafloor topography appear to more closely match the observed waveform features than synthetics generated from a model with topography on the solid surface alone. We envisage that such a method will be of use in understanding the new and exciting ocean-bottom and floating seismometer data sets now being regularly collected. [ABSTRACT FROM AUTHOR]

With the comparison to the resistivity ultra-deep measurement, the single-well reflection survey in acoustic logging-while-drilling (ALWD) measurement lags far behind, especially ALWD dipole measurement has long been thought to be little added value. In this paper, we extended the dipole shear-wave (S -wave) reflection survey technology in wireline logging into ALWD and demonstrated the theoretical feasibility of adopting a dipole source–receiver system to perform ALWD reflection survey. For this purpose, we investigated the radiation patterns of radiant SH, SV and P waves, the energy fluxes of guided and radiant waves and their acoustical radiation efficiencies from an LWD dipole acoustic source by comparisons with the wireline results. The analysis results reveal that a dominant excitation-frequency band does exist in ALWD dipole S -wave reflection. Consequently, the expected excitation frequency should be located in the band of the signal with high radiation efficiency, guaranteeing the best radiation performance. In fast formations, SH wave is the best candidate for ALWD reflection survey due to its highest radiation efficiency. In contrast, the dominant excitation-frequency band of SH wave gets wider in a slow formation. Besides, the SV - and P -wave radiation efficiencies are also remarkable, implying that both waves can also be used for ALWD reflection survey in slow formations. We expounded the SH-, SV- and P -reflection behaviours at three typical excitation frequencies by our 3-D finite difference. Simulations to single-well reflection validate the key role of dominant excitation-frequency band and demonstrate the theoretical feasibility of applying the technology to ALWD. Our results can guide the design and measurement methods of ALWD dipole S -wave reflection survey tool, which could have extensive application prospect for geo-steering. [ABSTRACT FROM AUTHOR]

As part of the TIPAGE (TIen shan – PAmir GEodynamic program) project, passive seismological observations were made along an approximately N-S profile crossing the Pamir seismic zone for about one year. From these observations guided waves were recognized. These guided waves occur as a single, continuous, secondary, compressional (P) wave phase behind the first P-wave arrivals. An equivalent phase in the shear (S) wavefield is hardly recognizable. Modelling of the phase shows that an approximately 10 km thick low velocity zone (LVZ) between the Moho and about 160 km depth reproduces the guided waves as a single, continuous phase much better than a 15–20 km thick LVZ. Modelling of the arrival times of the guided waves reveals that a model with a P-wave velocity of 6.3 km/s above about 100 km depth, and a velocity of 7.6 km/s between this depth and the deep cluster of earthquakes at about 150 km depth provides the best fit to the observed travel-time data. One plausible way to explain the low velocity of 6.3 km/s is to invoke the presence of melts in the LVZ. Then, taking a velocity of 6.9 km/s for the lower crust being subducted, about 10–13% melt is required to obtain a velocity of about 6.3 km/s in the LVZ between the Moho and about 100 km depth. This would be in keeping with the estimated burial depths from xenoliths of Gondwana terrane affinity brought to the surface in the southeastern Pamir around 11 million yr. ago. The present-day LVZ is interpreted to comprise continental lower crust. Although guided waves are known to exist associated with subducted oceanic crust or fault zones, this is the first time to the knowledge of the authors that guided waves have been observed resulting from a LVZ associated with subducted continental lower crust. Unlabelled Image • Guided waves from earthquakes within the Pamir seismic zone have been recognized. • The guided waves occur as a secondary phase behind the first arrivals. • The guided waves are generated by a low velocity layer about 10 km thick. • The low velocity layer is caused by subducting lower continental crust. • The velocities in the low velocity layer are partly due to the presence of melts. [ABSTRACT FROM AUTHOR]

In actual monopole acoustic logging-while-drilling (LWD), the acquisition of formation velocities is rather difficult because of a strong collar wave interference. In this study, a method is proposed to enhance the amplitudes of formation gliding waves in an LWD borehole, based on the directional radiation technology of linear phased array (LPA) acoustic transmitters. Waveforms in the LWD borehole generated by a monopole acoustic transmitter and LPA acoustic transmitters are numerically simulated using the finite-difference method. The influence of the LPA parameters on the various waves in the LWD borehole is also analysed. The simulation results show that the guided waves in the LWD borehole generated by the LPA acoustic transmitter have the same type but different relative amplitudes compared to those generated by the monopole transmitter. The angular width and deflection angle of the main radiated acoustic beam in the borehole can be controlled by adjusting the LPA parameters. As the delay time between the excitation signals applied to adjacent elements is increased, the deflection angle of the main radiated acoustic beam in the borehole satisfies the generation conditions for the gliding compressional (P) and shear (S) waves, successively. When the delay time is set such that the deflection angle is equal to the first (or second) critical angle of the formation, the amplitude of the gliding P (or S) wave is greatly enhanced. Moreover, the amplitude of the gliding P (or S) wave increases linearly as the element number increases under such conditions. Therefore, compared to the monopole acoustic transmitter, the LPA acoustic transmitter can be utilized to effectively improve the reliability of the acoustic LWD tool in measuring formation velocities. This study establishes a theoretical foundation for the next-generation acoustic LWD tool development. [ABSTRACT FROM AUTHOR]

Determining accurate microseismic event locations at the Groningen gas field in the Netherlands has important implications for understanding the ongoing induced seismicity and its associated seismic hazard. To improve the depth constraint of the microseismicity, downhole monitoring arrays have been deployed in the central region of the Groningen field. The observed seismicity at these receivers is characterized by significant complexity in the waveforms, due to the high velocity contrasts that exist and the acquisition geometry. Reliably identifying and picking phases for use in earthquake location algorithms is therefore particularly challenging. Using a well constrained and highly detailed 3-D velocity model, we show how full waveform modelling can be used to understand the causes of the observed phase complexity. By identifying the different modelled phase arrivals in detail, we look to identify any systematic changes in waveform complexity with source location, to aid phase identification of the recorded downhole data. Theoretical traveltimes are often the foundation of earthquake location algorithms. We highlight the associated problems of their computation for the downhole monitoring setup for the Groningen model by complementing the full waveform simulations with traveltime computations and their associated ray paths from both an eikonal solver and using the wavefront construction method. We highlight large inconsistencies in the traveltimes, demonstrate the limitations and sensitivity of ray tracing in a layered velocity model, and show how the theoretical traveltimes do not equate with the dominant phase arrivals observed within the modelled waveforms. Finally, we propose an approach based on computing P - and S -wave traveltimes directly from the full waveform modelling, such that phase picking is based on an amplitude threshold rather than individual phase identification, which can also be adjusted for any given moment tensor. [ABSTRACT FROM AUTHOR]

Infrasound signals from the announced underground nuclear explosions at the North Korean test site were first identified in 2009 at regional infrasound stations in South Korea. For the subsequent nuclear test in 2013 corresponding arrivals were again identified at these stations, but could also be detected at the two closest IMS stations in Russia (IS45) and Japan (IS30) and were associated to the event in the Reviewed Event Bulletin of the CTBTO. When the nuclear test in January 2016 occurred and infrasound signals for IS45 were reported, it spurred this investigation of potential observations from such events conducted over the past 10 or more years. By applying F–K analyses interactively on the data of IMS infrasound stations IS45 and IS30 we verified the detections for the 2013 event, but found additional signals related to other tests except for the 2009 and September 2016 events. Additionally, we found a signal in 2009 at IS45 that we interpret as being related to the source for which a precursory signal was identified at the regional infrasound station network in South Korea. We also investigated the potential for infrasound observations for a small seismic event in May 2010 on the test site and obtained some evidence for a very weak signal. By atmospheric propagation modelling using ray tracing and parabolic equation calculations we aim at interpreting the observations and attempt to explain the corresponding arrival times of signals. As most of the signals are hidden in the background noise, we also studied the detection pattern of IS45 and IS30 in order to assess the probability of the identified signals to be random observations coming from preferred arrival directions. [ABSTRACT FROM AUTHOR]

An efficient numerical method is developed for the simulation of dynamic response and the prediction of the wave propagation in composite plate structures. The method is termed finite wavelet domain method and takes advantage of the outstanding properties of compactly supported 2D Daubechies wavelet scaling functions for the spatial interpolation of displacements in a finite domain of a plate structure. The development of the 2D wavelet element, based on the first order shear deformation laminated plate theory is described and equivalent stiffness, mass matrices and force vectors are calculated and synthesized in the wavelet domain. The transient response is predicted using the explicit central difference time integration scheme. Numerical results for the simulation of wave propagation in isotropic, quasi-isotropic and cross-ply laminated plates are presented and demonstrate the high spatial convergence and problem size reduction obtained by the present method. [ABSTRACT FROM AUTHOR]

Several days of passive seismic broad-band recordings (vertical component) from a dense 3 × 6 km array installed near Chémery (France), with about 100 seismometers, are analysed for wavefield characterization between 0.1 and 3 Hz. Backazimuth is determined by using the Multiple Signal Characterization (MUSIC) algorithm at frequencies below 1 Hz, and non-coherent cross-correlation beamforming above 1 Hz, since the latter is less sensitive to aliasing issues. A novel method of determining the wavefield velocity is introduced, consisting of processing a cross-correlation common-offset gather by the MUSIC algorithm. The fundamental and three higher modes of Rayleigh waves (R0, R1, R2 and R3) are identified under 1 Hz. Above 1.5 Hz, the L g phase is detected, while R0 and R1 are also present. Roughly between 1 and 1.5 Hz, a quicker phase, probably P g, is detected. Both P g and L g are dominant during night time, suggesting they have a natural origin, which is also consistent with their backazimuth pointing towards the Atlantic. Large scale 2-D spectral-element simulations using deep- and shallow-water ocean sources confirm the possibility of the L g phase excitation. Thus, even above 1 Hz, natural sources can explain the major part of the ambient noise energy during quiet time periods. [ABSTRACT FROM AUTHOR]

14 microbarograph arrays recorded infrasonic signals at distances of between 53 and 930 km from six large chemical explosions detonated at White Sands Missile Range, New Mexico, US, over an 18 d period during August 2012: the Humming Roadrunner trials. Atmospheric sound speed profiles derived from state-of-the-art meteorological specifications predicted a weak stratospheric duct to stations due west of the six explosions, with effective sound speed ratios between the stratosphere and ground surface of between 0.99 and 1.04. Only thermospheric ducts were predicted along meridional paths. For all explosions, a wider range of signals than predicted by infrasonic propagation modelling is observed. The geographical extent of zonal path observations is significantly greater than predicted, with the first stratospheric return recorded within the modelled shadow zones and out to distances of 900 km. In addition, extended signal coda lasting up to 300 s was recorded, indicative of acoustic multipathing from fine-scale unmodelled atmospheric structure. Along predominantly meridional path arrivals consistent with acoustic refraction in the stratosphere, mesosphere and thermosphere were recorded with celerities between 235 and 295 m s−1. 24 U-shaped phases were recorded across the six explosions; those with the slowest celerities (220–240 m s−1) exhibited periods of up to 16 s, in comparison to phases with stratospheric celerities (>280 m s−1) that had periods <3 s. This is consistent with nonlinear waveform stretching along thermospheric paths. Within the geometric stratospheric shadow zone, at distances of between 53 and 95 km from the source, several tens of seconds of signal coda were recorded, with increasing apparent velocity with time. Preliminary modelling suggests these returns were partially reflected from lower stratospheric (<35 km) altitudes. The presence of geographically extensive stratospheric zonal returns, meridional stratospheric returns and extended signal coda adds to a growing body of evidence that operational meteorological specifications do not capture the detail required to synthesize regional infrasonic waveforms. [ABSTRACT FROM AUTHOR]

We introduce a time-domain, high-order in space, hybridizable discontinuous Galerkin (DG) spectral element method (HDG-SEM) for wave equations in coupled elastic-acoustic media. The method is based on a first-order hyperbolic velocity-strain formulation of the wave equations written in conservative form. This method follows theHDGapproach by introducing a hybrid unknown, which is the approximation of the velocity on the elements boundaries, as the only globally (i.e. interelement) coupled degrees of freedom. In this paper, we first present a hybridized formulation of the exact Riemann solver at the element boundaries, taking into account elastic-elastic, acoustic-acoustic and elastic-acoustic interfaces. We then use this Riemann solver to derive an explicit construction of the HDG stabilization function τ for all the above-mentioned interfaces. We thus obtain an HDG scheme for coupled elastic-acoustic problems. This scheme is then discretized in space on quadrangular/hexahedral meshes using arbitrary high-order polynomial basis for both volumetric and hybrid fields, using an approach similar to the spectral element methods. This leads to a semi-discrete system of algebraic differential equations (ADEs), which thanks to the structure of the global conservativity condition can be reformulated easily as a classical system of first-order ordinary differential equations in time, allowing the use of classical explicit or implicit time integration schemes. When an explicit time scheme is used, the HDG method can be seen as a reformulation of a DG with upwind fluxes. The introduction of the velocity hybrid unknown leads to relatively simple computations at the element boundaries which, in turn, makes the HDG approach competitive with the DG-upwind methods. Extensive numerical results are provided to illustrate and assess the accuracy and convergence properties of this HDG-SEM. The approximate velocity is shown to converge with the optimal order of k+1 in the L²-norm,when element polynomials of order k are used, and to exhibit the classical spectral convergence of SEM. Additional inexpensive local post-processing in both the elastic and the acoustic case allow to achieve higher convergence orders. The HDG scheme provides a natural framework for coupling classical, continuous Galerkin SEM with HDG-SEM in the same simulation, and it is shown numerically in this paper. As such, the proposed HDG-SEM can combine the efficiency of the continuous SEM with the flexibility of the HDG approaches. Finally, more complex numerical results, inspired from real geophysical applications, are presented to illustrate the capabilities of the method for wave propagation in heterogeneous elastic--acoustic media with complex geometries. [ABSTRACT FROM AUTHOR]

The acoustic method, being the most effective method for cement bond evaluation, has been used by industry for more than a half century. However, the methods currently used are almost always focused on the first arrival (especially for sonic logging), which has limitations. We use a 3-D finite-difference method to numerically simulate the wavefields from a dipole source in a single-cased hole with different cement conditions. By using wavefield snapshots and dispersion curves, we interpret the characteristics of the modes in the models. We investigate the effect of source frequency, the thickness and location of fluid columns on different modes. The dipole wavefield in a single-cased hole consists of a leaky P (for frequency >10 kHz) from formation, formation flexural, and also some casing modes. Depending on the mode, their behaviour is sometimes sensitive to the existence of fluid between the cement and formation and sometimes sensitive to the existence of fluid between the casing and cement. The formation S velocity can be obtained from the formation flexural mode at low frequency. However, interference from high-order casing modes makes the leaky P invisible and P velocity determination difficult when the casing is not well cemented. The dispersion curve of the formation flexural mode is sensitive to the fluid thickness when fluid exists only at the interface between casing and cement. The fundamental casing dipole mode is only sensitive to the total fluid thickness in the annulus between casing and formation. Either the arrival time or amplitude of the high-order casing dipole mode is sensitive to the fluid column when the fluid column is next to the casing. We provide a table that summarizes the ability of different modes to detect fluid columns between various layers of casing, cement and formation. Based on the results, we suggest a data processing flow for field application, which will highly improve cement evaluation. [ABSTRACT FROM AUTHOR]

Meteoroids entering the Earth atmosphere at high hypersonic velocities are a source of sonic boom that is recorded as infrasound signal at the ground level. The Carancas meteorite (Peru, 2007) is re-examined in this way as a reference case with ground crater and nearby seismic and infrasonic recordings. A new trajectory is proposed for this meteorite by minimizing the difference between computed and observed times of arrivals for geometrical arrivals. A scenario based on diffraction is proposed to explain non-geometrical arrivals. Model frequency spectra show a reasonable agreement with data, which allows estimation of the meteorite diameter in a narrow range compatible with crater observations. [ABSTRACT FROM AUTHOR]

H-beams are usually made of structural steel and are widely used in construction, civil, and mechanical engineering. Both the rolling process and complex working condition can yield some defects on the H-beam structures. Characteristics of guided wave propagation in H-beam are investigated in this study for damage identification by using guided wave-based structural health monitoring (SHM) technique. Wave structures are obtained for wave mode selection in H-beam SHM applications. Guided waves are excited by using piezoelectric actuators. Two excitation ways, that is, symmetric excitation and circle excitation, are studied to select appropriate wave mode for damage identification. Based on the wave propagation characteristics, probability-based diagnostic image method is proposed to estimate locations of simulated defects. In the image reconstruction process, arithmetic average-based and geometric average-based methods are investigated for damage localization with high precision. The results demonstrate that guided wave-based SHM technique is applicable to damage identification for H-beam structures. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The long-range atmospheric propagation of explosion-like waves of frequency in the infrasound range is investigated using non-linear ray theory. Simulations are performed for sources of increasing amplitude on rays up to the lower thermosphere and for distances of hundreds of kilometres. A study of the attenuation of the waveforms observed at ground level induced by both the classical mechanisms and the vibrational relaxation of the molecules comprising the atmospheric gas is carried out. The relative importance of classical absorption and vibrational relaxation along the typical atmospheric propagation trajectories is assessed. Non-linear effects are highlighted as well and particular emphasis is placed on their strong interaction with absorption phenomena. A detailed description of the propagation model and of the numerical algorithm used in this work is first reported. Results are then discussed and the importance of the different mechanisms is clarified. [ABSTRACT FROM AUTHOR]

In this paper we present a wave-based technique for modelling waveguides equipped with piezoelectric actuators in which there is no need for common simplifications regarding their dynamic behaviour, the interaction with the waveguide or the bonding conditions. The proposed approach is based on the semi-analytical finite element (SAFE) method. We developed a new piezoelectric element and employed the analytical wave approach to model the distributed electrical excitation and scattering of the waves at discontinuities. The model was successfully verified numerically and validated against an experiment on a beam-like waveguide with emulated anechoic terminations. [ABSTRACT FROM AUTHOR]

Near-surface geophysical imaging is often performed by generating surface waves, and estimating the subsurface properties through inversion, that is, iteratively matching experimentally observed dispersion curves with predicted curves from a layered half-space model of the subsurface. Key to the effectiveness of inversion is the efficiency and accuracy of computing the dispersion curves and their derivatives. This paper presents improved methodologies for both dispersion curve and derivative computation. First, it is shown that the dispersion curves can be computed more efficiently by combining an unconventional complex-length finite element method (CFEM) to model the finite depth layers, with perfectly matched discrete layers (PMDL) to model the unbounded half-space. Second, based on analytical derivatives for theoretical dispersion curves, an approximate derivative is derived for the so-called effective dispersion curve for realistic geophysical surface response data. The new derivative computation has a smoothing effect on the computation of derivatives, in comparison with traditional finite difference (FD) approach, and results in faster convergence. In addition, while the computational cost of FD differentiation is proportional to the number of model parameters, the new differentiation formula has a computational cost that is almost independent of the number of model parameters. At the end, as confirmed by synthetic and real-life imaging examples, the combination of CFEM + PMDL for dispersion calculation and the new differentiation formula results in more accurate estimates of the subsurface characteristics than the traditional methods, at a small fraction of computational effort. [ABSTRACT FROM AUTHOR]

Iranian Plateau reflects several different tectonic styles of collision, and large-scale strike-slip faults. We calculate a high-resolution 2-D ML shear velocity map for the Iranian Plateau to detect lateral crustal thickness changes associated with different tectonic boundaries. The ML velocity is very sensitive to strong lateral variations of crustal thickness and varies between the velocity of Lg and Sn phases. Our data set consists of 65 795 ML amplitude velocity measurements from 2531 precisely relocated events recorded by Iranian networks in the period 1996-2014. Using a constrained least-squares inversion scheme, we inverted theML velocities for a 2-D shear velocity map of Iran. Our results show that the Zagros and South Caspian Basin (SCB) have shear wave velocities close to the Sn phase, and are thus Lg-blocking regions. High velocities in the High Zagros and the Simply Folded Belt imply significant crustal undulations within these zones. We note that in the central and south Zagros, the velocity border between the Zagros and central Iran is not coincident with the Zagros suture line that marks underthrusting of the Arabian plate beneath central Iran. The low plains of Gilan and Gorgan to the south of the Caspian Sea show high shear velocities similar to the SCB, implying that they are either underlain by an oceanic type crust or a transitional crust with a strong lateral crustal thickness gradient. The Lut block is an Lg-passing block implying that it is not surrounded by any sudden crustal thickness changes along its borders with central Iran. In the Alborz, NW Iran, Kopeh-Dagh, Binalud and most of the central Iran, low shear velocity near the Lg velocity is attributed to smooth or minor Moho undulations within these regions. [ABSTRACT FROM AUTHOR]

Seismic wavefields interact with low-velocity fault damage zones. Waveforms of ballistic fault zone head waves, trapped waves, reflected waves and signatures of trapped noise can provide important information on structural and mechanical fault zone properties. Here we extend the class of observable fault zone waves and reconstruct in-fault reverberations or multiples in a strike-slip faulting environment. Manifestations of the reverberations are significant, consistent wave fronts in the coda of cross-correlation functions that are obtained from scattered earthquake waveforms and seismic noise recorded by a linear fault zone array. The physical reconstruction of Green's functions is evident from the high similarity between the signals obtained from the two different scattered wavefields. Modal partitioning of the reverberation wavefield can be tuned using different data normalization techniques. The results imply that fault zones create their own ambiance, and that the here reconstructed reverberations are a key seismic signature of wear zones. Using synthetic waveform modelling we show that reverberations can be used for the imaging of structural units by estimating the location, extend and magnitude of lateral velocity contrasts. The robust reconstruction of the reverberations from noise records suggests the possibility to resolve the response of the damage zone material to various external and internal loading mechanisms. [ABSTRACT FROM AUTHOR]

Seismoelectric logging has potential applications in exploring the porous formation characteristics. In this study, we theoretically address how the amplitudes of the Stoneley wave and its accompanying borehole seismoelectric field depend on the porous formation parameters such as porosity, permeability and tortuosity. We calculate the ratio of the electric field amplitude to the pressure amplitude (REP amplitude) for the low-frequency Stoneley wave of different formations and find that the REP amplitude is sensitive to porosity and tortuosity but insensitive to permeability. To confirm this conclusion, we derive the approximate expression of the REP amplitude in the wavenumber-frequency domain which shows that the REP amplitude is dependent on tortuosity but independent of permeability. This contradicts the result concluded by previous researchers from experiments that the REP amplitude is directly proportional to the permeability. The reason is probably attributed to the fact that the rock samples with different permeabilities typically have different tortuosities. Since the REP amplitude is sensitive to tortuosity, we propose a method of estimating formation tortuosity from seismoelectric logs. It is implemented by using the REP amplitude and the wavenumber of the Stoneley wave at a given frequency when the porosity and the pore fluid viscosity and salinity are known. We test the method by estimating the tortuosities from the synthetic seismoelectric waveforms in different formations. The result shows that the errors relative to the input tortuosities are lower than 5.0 per cent without considering the uncertainties of other input parameters. [ABSTRACT FROM AUTHOR]

We analyse the volumetric sensitivity of fault zone seismic head and trapped waves by constructing finite-frequency sensitivity (Fr'echet) kernels for these phases using a suite of idealized and tomographically derived velocity models of fault zones. We first validate numerical calculations by waveform comparisons with analytical results for two simple fault zone models: a vertical bimaterial interface separating two solids of differing elastic properties, and a 'vertical sandwich' with a vertical low velocity zone surrounded on both sides by higher velocity media. Establishing numerical accuracy up to 12 Hz, we compute sensitivity kernels for various phases that arise in these and more realistic models. In contrast to direct P body waves, which have little or no sensitivity to the internal fault zone structure, the sensitivity kernels for head waves have sharp peaks with high values near the fault in the faster medium. Surface wave kernels show the broadest spatial distribution of sensitivity, while trapped wave kernels are extremely narrow with sensitivity focused entirely inside the low-velocity fault zone layer. Trapped waves are shown to exhibit sensitivity patterns similar to Love waves, with decreasing width as a function of frequency and multiple Fresnel zones of alternating polarity. In models that include smoothing of the boundaries of the low velocity zone, there is little effect on the trapped wave kernels, which are focused in the central core of the low velocity zone. When the source is located outside a shallow fault zone layer, trapped waves propagate through the surrounding medium with body wave sensitivity before becoming confined. The results provide building blocks for full waveform tomography of fault zone regions combining high-frequency head, trapped, body, and surface waves. Such an imaging approach can constrain fault zone structure across a larger range of scales than has previously been possible. [ABSTRACT FROM AUTHOR]

Employing dynamic reciprocity can be an effective tool to simplify the calculation of elastic wavefields for borehole problems and to check the results. We analytically obtain the reciprocity relations for the elastodynamic fields generated by multipole sources in a fluid-solid configuration: if the multipole sources are located in the fluid, the particle displacement due to a dipole source is reciprocal to the particle acceleration due to a single force; the fluid pressure due to a dipole source is reciprocal to the particle acceleration due to a monopole source; the particle displacement due to a quadrupole source is reciprocal to the spatial partial derivative of the particle acceleration due to a single force; the fluid pressure due to a quadrupole source is reciprocal to the spatial partial derivative of the particle acceleration due to a monopole source. These relations are tested by numerical experiments for different borehole problems, including acoustic logging, single-well imaging and vertical seismic profiling. A reciprocity test can be used as a quick check of a finite-difference algorithm and the implementation of the sources, although it cannot detect errors due to improper discretization of the interfaces. [ABSTRACT FROM AUTHOR]

We develop an algorithm for automatic identification of fault zone trapped waves in data recorded by seismic fault zone arrays. Automatic S picks are used to identify time windows in the seismograms for subsequent search for trapped waves. The algorithm calculates five features in each seismogram recorded by each station: predominant period, 1 s duration energy (representative of trapped waves), relative peak strength, arrival delay and 6 s duration energy (representative of the entire seismogram). These features are used collectively to identify stations in the array with seismograms that are statistical outliers. Applying the algorithm to large data sets allows for distinguishing genuine trapped waves from occasional localized site amplification in seismograms of other stations. The method is verified on a test data set recorded across the rupture zone of the 1992 Landers earthquake, for which trapped waves were previously identified manually, and is then applied to a larger data set with several thousand events recorded across the San Jacinto fault zone. The developed technique provides an important tool for systematic objective processing of large seismic waveform data sets recorded near fault zones. [ABSTRACT FROM AUTHOR]

We discuss several outstanding aspects of seismograms recorded during >4 weeks by a spatially dense Nodal array, straddling the damage zone of the San Jacinto fault in southern California, and some example results. The waveforms contain numerous spikes and bursts of high-frequency waves (up to the recorded 200 Hz) produced in part by minute failure events in the shallow crust. The high spatial density of the array facilitates the detection of 120 small local earthquakes in a single day, most of which not detected by the surrounding ANZA and regional southern California networks. Beamforming results identify likely ongoing cultural noise sources dominant in the frequency range 1-10 Hz and likely ongoing earthquake sources dominant in the frequency range 20-40 Hz. Matched-field processing and back-projection of seismograms provide alternate event location. Themedian noise levels during the experiment at different stations, waves generated by Betsy gunshots, and wavefields from nearby earthquakes point consistently to several structural units across the fault. Seismic trapping structure and local sedimentary basin produce localized motion amplification and stronger attenuation than adjacent regions.Cross correlations of high-frequency noise recorded at closely spaced stations provide a structural image of the subsurface material across the fault zone. The high spatial density and broad frequency range of the data can be used for additional high resolution studies of structure and source properties in the shallow crust. [ABSTRACT FROM AUTHOR]

Multichannel analysis of surface waves (MASW) method analyses high-frequency surface waves to determine shear (S)-wave velocities of near-surface materials, which are usually unconsolidated and possess higher Poisson's ratios. One of key steps using the MASW method to obtain the near-surface S-wave velocities is to pick correct phase velocities in dispersive images. A high-frequency seismic survey conducted over near-surface materials with a higher Poisson's ratio will often result in data that contains non-geometric wave, which will raise an additional energy in the dispersion image. Failure to identify it may result in misidentification. In this paper, we have presented a description about leaky surface wave and the influence caused by the existence of leaky waves in a high-frequency seismic record. We first introduce leaky wave and non-geometric wave. Next, we use two synthetic tests to demonstrate that non-geometric wave is leaky wave and show the properties about leaky surface wave by eigenfunctions using Chen's algorithm. We show that misidentification may occur in picking the dispersion curves of normal Rayleigh wave modes because the leaky-wave energy normally connects energy of fundamental and/or higher modes. Meanwhile, we use a real-world example to demonstrate the influence of leaky wave. We also propose that muting and filtering should been applied to raw seismic records prior to generating dispersive images to prevent misidentifying leaky surface waves as modal surface waves by a real-world example. Finally, we use a three-layer model with a low-velocity half-space to illustrate that leaky surface waves appear on condition that the phase velocities are higher than maximum S-wave velocity of the earth model when solving the Rayleigh equation. [ABSTRACT FROM AUTHOR]

The aseismic Murray ridge (MR) is a continuation of the Owen fracture zone which marks the boundary between the Indian and Arabian plates. Due to large variation in morphology and structure within this NE-SW trending ridge in the Arabian Sea a large variation of the bathymetry from few hundred metres to about 4000 m is seen. Observed seismicity on the ridge system is predominantly strike-slip. Tsunamis generated in the Makran subduction zone (MSZ) will propagate through the MR system due to its proximity. As the tsunami speed depends on the depth of the ocean, bathymetry plays a vital role on tsunami propagation. In this paper, the effect of tsunami propagation through the MR system is carried out with the existing bathymetry and comparing the results by removing the bathymetry. To study this phenomenon the 1945 Makran tsunami and worst possible tsunamigenic earthquakes form eastern and western MSZ are considered. The directivity of tsunami propagation with the ridge system is seen to change after crossing the MR towards the southeast direction for tsunamis generated in the eastern MSZ. For tsunami generated in the western MSZ there is no change in its directivity and is almost same as without the ridge with propagation being towards the open sea. Hence the MR not only affects the amplitude of the tsunamis but also the directionality and the arrival times. [ABSTRACT FROM AUTHOR]

Multichannel analysis of surface waves (MASW) method analyses high-frequency surface waves to determine shear (S)-wave velocities of near-surface materials, which are usually unconsolidated and possess higher Poisson's ratios. One of key steps using the MASW method to obtain the near-surface S-wave velocities is to pick correct phase velocities in dispersive images. A high-frequency seismic survey conducted over near-surface materials with a higher Poisson's ratio will often result in data that contains non-geometric wave, which will raise an additional energy in the dispersion image. Failure to identify it may result in misidentification. In this paper, we have presented a description about leaky surface wave and the influence caused by the existence of leaky waves in a high-frequency seismic record. We first introduce leaky wave and non-geometric wave. Next, we use two synthetic tests to demonstrate that non-geometric wave is leaky wave and show the properties about leaky surface wave by eigenfunctions using Chen's algorithm. We show that misidentification may occur in picking the dispersion curves of normal Rayleigh wave modes because the leaky-wave energy normally connects energy of fundamental and/or higher modes. Meanwhile, we use a real-world example to demonstrate the influence of leaky wave. We also propose that muting and filtering should been applied to raw seismic records prior to generating dispersive images to prevent misidentifying leaky surface waves as modal surface waves by a real-world example. Finally, we use a three-layer model with a low-velocity half-space to illustrate that leaky surface waves appear on condition that the phase velocities are higher than maximum S-wave velocity of the earth model when solving the Rayleigh equation. [ABSTRACT FROM AUTHOR]

A geotechnical and geophysical campaign was performed at sites located in the alluvial plain of the river of Beirut (Lebanon), which is characterized by a significant lateral and vertical geological variability, along with anthropogenic disturbances in the first metres. The method combination has allowed detecting a shallow conductive low velocity layer of varying depth and thickness, corresponding to a soft clay layer embedded in coarser formations. This layer was found to exert strong control on the experimental dispersion curves (estimated from both active and passive experiments) characterized by a continuous mode superposition at high frequency, associated with an increase in phase velocity. Vs profiles in boreholes turned out to be of prime importance for adequately defining the parametrization before inversion and for ensuring the reliability of the inversion dispersive estimates at low frequency. A major output of this study is also to show that this low velocity layer, along with the strong shear velocity contrast at its bottom, significantly contributes to the site seismic response, and could make it difficult to use the measured H/V peak frequency as a proxy for the soil thickness over bedrock. [ABSTRACT FROM AUTHOR]

Laboratory experiments using laser-based ultrasonic techniques can be used to simulate seismic surveys on highly controlled small-scale physical models of the subsurface. Most of the time, such models consist in assemblies of homogeneous and consolidated materials. To enable the physical modelling of unconsolidated, heterogeneous and porous media, the use of granular materials is suggested here. We describe a simple technique to build a two-layer physical model characterized by lateral variations, strong property contrasts and velocity gradients. We use this model to address the efficiency of an innovative surface-wave processing technique developed to retrieve 2-D structures from a limited number of receivers. A step by step inversion procedure of the extracted dispersion curves yields accurate results so that the 2-D structure of the physical model is satisfactorily reconstructed. The velocity gradients within each layer are accurately retrieved as well, confirming current theoretical and experimental studies regarding guided surface acoustic modes in unconsolidated granular media. [ABSTRACT FROM AUTHOR]

In many parts of the ocean high-frequency seismic energy is carried to very great distances from the source. The onsets of the P and S energy travel with speeds characteristic of the mantle lithosphere. The complex and elongated waveforms of such Po and So waves and their efficient transport of high frequencies (>10 Hz) have proved difficult to explain in full. Much of the character can be captured with stratified models, provided a full allowance is made for reverberations in the ocean and the basal sediments. The nature of the observations implies a strong scattering environment. By analysing the nature of the long-distance propagation we are able to identify the critical role played by shallow reverberations in the water and sediments, and the way that these link with propagation in a heterogeneous mantle. 2-D finite difference modelling to 10 Hz for ranges over 1000 km demonstrates the way in which heterogeneity shapes the wavefield, and the way in which the properties of the lithosphere and asthenosphere control the nature of the propagation processes. The nature of the Po and So phases are consistent with pervasive heterogeneity in the oceanic lithosphere with a horizontal correlation length (∼10 km) much larger than the vertical correlation length (∼0.5 km). [ABSTRACT FROM AUTHOR]

Reliable high-resolution 3-D characterization of aquifers helps to improve our understanding of flow and transport processes when small-scale structures have a strong influence. Crosshole ground penetrating radar (GPR) is a powerful tool for characterizing aquifers due to the method's high-resolution and sensitivity to porosity and soil water content. Recently, a novel GPR full-waveform inversion algorithm was introduced, which is here applied and used for 3-D characterization by inverting six crosshole GPR cross-sections collected between four wells arranged in a square configuration close to the Thur River in Switzerland. The inversion results in the saturated part of this gravel aquifer reveals a significant improvement in resolution for the dielectric permittivity and electrical conductivity images compared to ray-based methods. Consistent structures where acquisition planes intersect indicate the robustness of the inversion process. A decimetre-scale layer with high dielectric permittivity was revealed at a depth of 5–6 m in all six cross-sections analysed here, and a less prominent zone with high dielectric permittivity was found at a depth of 7.5–9 m. These high-permittivity layers act as low-velocity waveguides and they are interpreted as high-porosity layers and possible zones of preferential flow. Porosity estimates from the permittivity models agree well with estimates from Neutron–Neutron logging data at the intersecting diagonal planes. Moreover, estimates of hydraulic permeability based on flowmeter logs confirm the presence of zones of preferential flow in these depth intervals. A detailed analysis of the measured data for transmitters located within the waveguides, revealed increased trace energy due to late-arrival elongated wave trains, which were observed for receiver positions straddling this zone. For the same receiver positions within the waveguide, a distinct minimum in the trace energy was visible when the transmitter was located outside the waveguide. A novel amplitude analysis was proposed to explore these maxima and minima of the trace energy. Laterally continuous low-velocity waveguides and their boundaries were identified in the measured data alone. In contrast to the full-waveform inversion, this method follows a simple workflow and needs no detailed and time consuming processing or inversion of the data. Comparison with the full-waveform inversion results confirmed the presence of the waveguides illustrating that full-waveform inversion return reliable results at the highest resolution currently possible at these scales. We envision that full-waveform inversion of GPR data will play an important role in a wide range of geological, hydrological, glacial and periglacial studies in the critical zone. [ABSTRACT FROM AUTHOR]

Acoustic logging-while-drilling (LWD) technology has been commercially used in the petroleum industry. However it remains a rather difficult task to invert formation compressional and shear velocities from acoustic LWD signals due to the unwanted strong collar wave, which covers or interferes with signals from the formation. In this paper, seismoelectric LWD is investigated for solving that problem. The seismoelectric field is calculated by solving a modified Poisson's equation, whose source term is the electric disturbance induced electrokinetically by the travelling seismic wave. The seismic wavefield itself is obtained by solving Biot's equations for poroelastic waves. From the simulated waveforms and the semblance plots for monopole, dipole and quadrupole sources, it is found that the electric field accompanies the collar wave as well as other wave groups of the acoustic pressure, despite the fact that seismoelectric conversion occurs only in porous formations. The collar wave in the electric field, however, is significantly weakened compared with that in the acoustic pressure, in terms of its amplitude relative to the other wave groups in the full waveforms. Thus less and shallower grooves are required to damp the collar wave if the seismoelectric LWD signals are recorded for extracting formation compressional and shear velocities. [ABSTRACT FROM AUTHOR]

Sedimentary rocks can exhibit strong permeability anisotropy due to layering, pre-stresses and the presence of aligned microcracks or fractures. In this paper, we develop a modified cylindrical finite-difference algorithm to simulate the borehole acoustic wavefield in a saturated poroelastic medium with transverse isotropy of permeability and tortuosity. A linear interpolation process is proposed to guarantee the leapfrog finite difference scheme for the generalized dynamic equations and Darcy's law for anisotropic porous media. First, the modified algorithm is validated by comparison against the analytical solution when the borehole axis is parallel to the symmetry axis of the formation. The same algorithm is then used to numerically model the dipole acoustic log in a borehole with its axis being arbitrarily deviated from the symmetry axis of transverse isotropy. The simulation results show that the amplitudes of flexural modes vary with the dipole orientation because the permeability tensor of the formation is dependent on the wellbore azimuth. It is revealed that the attenuation of the flexural wave increases approximately linearly with the radial permeability component in the direction of the transmitting dipole. Particularly, when the borehole axis is perpendicular to the symmetry axis of the formation, it is possible to estimate the anisotropy of permeability by evaluating attenuation of the flexural wave using a cross-dipole sonic logging tool according to the results of sensitivity analyses. Finally, the dipole sonic logs in a deviated borehole surrounded by a stratified porous formation are modelled using the proposed finite difference code. Numerical results show that the arrivals and amplitudes of transmitted flexural modes near the layer interface are sensitive to the wellbore inclination. [ABSTRACT FROM PUBLISHER]

SUMMARY The Rayleigh wave is dispersive in a stratified half-space. The modes of Rayleigh waves in a stratified half-space are usually defined by the order of the dispersion curves, such as fundamental mode, first higher mode, etc. However, the propagation characteristics of the same mode can be essentially different (e.g. surface wave and trapped wave) at different frequencies, and there is no rule to explain which parts of dispersion curves correspond to the surface wave or trapped wave. Actually, when the frequency is high, by decomposition of dispersion equation, three basic modes (R mode, R-period mode and S-period mode) of Rayleigh waves can be defined, and the mode can be further named by the phase velocities such as R1 mode, R1∼2 mode, etc. By the new definition, the dispersion curves can be divided into several velocity zones, and the modes can then be determined. In this paper, with generalized R/T coefficient method, the eigendisplacements of the three basic modes at high frequencies are studied. The study shows the physical advantage of the new definition of modes: Rayleigh waves of the same mode share the similar characteristics, and the main eigendisplacement characteristics of one mode remain the same when the medium is changed. With the definition of modes, the study makes it possible to know the basic eigendisplacement characteristics of high-frequency Rayleigh waves at different parts of the dispersion curves immediately. Especially, it presents a rule to explain which parts of dispersion curves correspond to the surface wave or trapped wave. Besides, based on the new definition, the modes in the vicinity of the osculation points are analysed. The 'coupled modes' that show the characteristics of two different modes simultaneously are found. It is found that the two neighbouring dispersion curves can exchange their corresponding modes of Rayleigh waves sometimes: one mode changes into another one gradually via coupled modes on each dispersion curve. The mode conversion can happen not only between surface wave mode and trapped wave mode, but also between two different trapped wave modes. However, when there is only one mode in the vicinity of the osculation points, there will be no mode conversion. [ABSTRACT FROM AUTHOR]

SUMMARY This paper addresses the issue of defect signature in stiffened panels at medium frequencies. With a tool for estimating the wave propagation characteristics of two-dimensional structures by k-space analysis, namely the Inhomogeneous Wave Correlation, the estimation of wavenumbers for waves propagating in stiffened panels can be achieved. Here, the method is employed with the sparse-simulated normal displacements of a set of stiffened panels. New insights into the use of their k-space behavior are hence highlighted. The guided wave behavior, which starts to appear at medium frequencies, can be extracted and treated in order to detect the presence of damage through a multi-modal wave propagation analysis in stiffened panels. The sensitivity of guided waves to defects in stiffened panels at medium frequency range is studied numerically, and the phenomena observed are explained with an energy diffusion analysis of waves propagating in stiffened panels. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY High-sensitivity, dense seismic observations and numerical simulations revealed a distinct guided SP converted wave of oceanic mantle earthquakes that occurred within the subducting Philippine Sea (PHS) plate and a waveguide effect of the PHS Plate beneath southwest Japan. Based on seismograms, we identify the oceanic mantle earthquakes of the double seismic zone and find a new type of later phase for these earthquakes. The primary features of these later phases are as follows: (1) these phases are observed at the northeastern inland stations of the convex slab axis, (2) the vertical and radial components are dominant and (3) the apparent velocity is approximately 8.0 km s-1. To examine the origin of these later phases, we simulate a 2-D P- SV seismic wavefield considering the PHS slab geometry. Based on the numerical models, we conclude that the later phases are guided SP converted waves trapped within the oceanic crust, which acts as low-velocity layer. A series of numerical simulations indicates that two factors are necessary to reproduce the observed SP guided waves: (i) a low-velocity channel must exist between stations and sources and (ii) the oceanic crust must only be in contact with the lower crust near the stations. [ABSTRACT FROM AUTHOR]

SUMMARY Simulations of earthquake rupture on the southern San Andreas Fault (SAF) reveal large amplifications in the San Gabriel and Los Angeles Basins (SGB and LAB) apparently associated with long-range path effects. Geometrically similar excitation patterns can be recognized repeatedly in different SAF simulations (e.g. Love wave-like energy with predominant period around 4 s, channelled southwestwardly from the SGB into LAB), yet the amplitudes with which these distinctive wavefield patterns are excited change, depending upon source details (slip distribution, direction and velocity of rupture). We describe a method for rapid calculation of the sensitivity of such predicted wavefield features to perturbations of the source kinematics, using a time-reversed (adjoint) wavefield simulation. The calculations are analogous to those done in adjoint tomography, and the same time-reversed calculation also yields path-sensitivity kernels that give further insight into the excitation mechanism. For rupture on the southernmost 300 km of SAF, LAB excitation is greatest for slip concentrated between the northern Coachella Valley and the transverse ranges, propagating to the NE and with rupture velocities between 3250 and 3500 m s-1 along that fault segment; that is, within or slightly above the velocity range (between Rayleigh and S velocities) that is energetically precluded in the limit of a sharp rupture front, highlighting the potential value of imposing physical constraints (such as from spontaneous rupture models) on source parametrizations. LAB excitation is weak for rupture to the SW and for ruptures in either direction located north of the transverse transverse ranges, whereas Ventura Basin (VTB) is preferentially excited by NE ruptures situated north of the transverse ranges. Path kernels show that LAB excitation is mediated by surface waves deflected by the velocity contrast along the southern margin of the transverse ranges, having most of their energy in basement rock until they impinge on the eastern edge of SGB, through which they are then funnelled into LAB. VTB amplification is enhanced by a similar waveguide effect. [ABSTRACT FROM AUTHOR]

In quantum theory, any Hamiltonian describing a physical system is mathematically represented by a self-adjoint linear operator to ensure the reality of the associated observables. In an attempt to extend quantum mechanics into the complex domain, it was realized few years ago that certain non-Hermitian parity-time ( $\mathcal{PT}$) symmetric Hamiltonians can exhibit an entirely real spectrum. Much of the reported progress has been remained theoretical, and therefore hasn't led to a viable experimental proposal for which non Hermitian quantum effects could be observed in laboratory experiments. Quite recently however, it was suggested that the concept of $\mathcal{PT}$-symmetry could be physically realized within the framework of classical optics. This proposal has, in turn, stimulated extensive investigations and research studies related to $\mathcal{PT}$-symmetric Optics and paved the way for the first experimental observation of $\mathcal{PT}$-symmetry breaking in any physical system. In this paper, we present recent results regarding $\mathcal{PT}$-symmetric Optics. [ABSTRACT FROM AUTHOR]