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1
Allsop, N. W. H., et S. S. L. Hettiarachchi. « REFLECTIONS FROM COASTAL STRUCTURES ». Coastal Engineering Proceedings 1, no 21 (29 janvier 1988) : 58. http://dx.doi.org/10.9753/icce.v21.58.
Texte intégralRésumé :
Wave reflections at and within a coastal harbour may make a significant contribution to wave disturbance in the harbour. Reflected waves may lead to danger to vessels navigating close to structures, and may reduce the availability of berths within the harbour. Wave reflections may also increase local scour or general reduction in sea bed levels. In the design of breakwaters, sea walls, and coastal revetments, it is therefore important to estimate and compare the reflection performance of alternative structure types. In the use of numerical models of wave motion within harbours, it is essential to define realistically the reflection properties of each boundary. This paper presents results from a study of the reflection performance of a wide range of structures used in coastal and harbour engineering.
2
Isaacson, Michael, et Shiqin Qu. « Predicted wave field in a laboratory wave basin ». Canadian Journal of Civil Engineering 17, no 6 (1 décembre 1990) : 1005–14. http://dx.doi.org/10.1139/l90-111.
Texte intégralRésumé :
The present paper describes a numerical method for predicting the wave field produced by a segmented wave generator undergoing specified motions in a wave basin which may contain partially reflecting sides. The approach used is based on linear diffraction theory and utilizes a point source representation of the generator segments and any reflecting walls that are present. The method involves the application of a partial reflection boundary condition, which is discussed. Numerical results are presented for the propagating wave field due to specified wave generator motions in a rectangular basin. Cases that are considered include both perfectly absorbing and partially reflecting beaches along the basin sides, as well as the presence of perfectly reflecting short sidewalls near the generator. The method appears able to account adequately for the effects of wave diffraction and partial reflections, and to predict the generated wave field realistically. Key words: coastal engineering, hydrodynamics, laboratory facilities, ocean engineering, wave diffraction, wave generation, wave reflection.
3
Berger, D. S., J. K. Li, W. K. Laskey et A. Noordergraaf. « Repeated reflection of waves in the systemic arterial system ». American Journal of Physiology-Heart and Circulatory Physiology 264, no 1 (1 janvier 1993) : H269—H281. http://dx.doi.org/10.1152/ajpheart.1993.264.1.h269.
Texte intégralRésumé :
Traditional analysis of pulse-wave propagation and reflection in the arterial system treats measured pressure and flow waves as the sum of a single forward wave (traveling away from the heart) and a single backward wave (traveling toward the heart). The purpose of this study was to develop a more general wave reflection theory that allows repeated reflection of these waves. The arterial system was modeled as a uniform viscoelastic tube terminating in a complex load with reflections occurring at the tube load interface and the heart tube interface. The resulting framework considers the forward wave to be the sum of an initial wave plus a series of antegrade waves. Similarly, the backward wave is the sum of a series of retrograde waves. This repeated reflection theory contains within it the traditional forward/backward wave reflection analysis as a special case. In addition, the individual antegrade and retrograde waves, at the tube entrance, are shown to be independent of the tube length. Aortic pressure and flow data, from dog experiments, were used to illustrate the phenomenon of repeated reflections. Alteration of the arterial system loading conditions, brought about through pharmacological intervention, affected the number and morphology of repeated waves. These results are compared with those found in traditional forward/backward reflection analysis.
4
Hametner, Bernhard, Hannah Kastinger et Siegfried Wassertheurer. « Simulating re-reflections of arterial pressure waves at the aortic valve using difference equations ». Proceedings of the Institution of Mechanical Engineers, Part H : Journal of Engineering in Medicine 234, no 11 (20 juillet 2020) : 1243–52. http://dx.doi.org/10.1177/0954411920942704.
Texte intégralRésumé :
Re-reflections of arterial pressure waves at the aortic valve and their influence on aortic wave shape are only poorly understood so far. Therefore, the aim of this work is to establish a model enabling the simulation of re-reflection and to test its properties. A mathematical difference equation model is used for the simulations. In this model, the aortic blood pressure is split into its forward and backward components which are calculated separately. The respective equations include reflection percentages representing reflections throughout the arterial system and a reflection coefficient at the aortic valve. While the distal reflections are fixed, different scenarios for the reflection coefficient at the valve are simulated. The results show that the model is capable to provide physiological pressure curves only if re-reflections are assumed to be present during the whole cardiac cycle. The sensitivity analysis on the reflection coefficient at the aortic valve shows various effects of re-reflections on the modelled blood pressure curve. Higher levels of the reflection coefficient lead to higher systolic and diastolic pressure values. The augmentation index is notably influenced by the systolic level of the reflection coefficient. This difference equation model gives an adequate possibility to simulate aortic pressure incorporating re-reflections at the site of the aortic valve. Since a strong dependence of the aortic pressure wave on the choice of the reflection coefficient have been found, this indicates that re-reflections should be incorporated into models of wave transmission. Furthermore, re-reflections may also be considered in methods of arterial pulse wave analysis.
5
Vuillon, J., D. Zeitoun et G. Ben-Dor. « Reconsideration of oblique shock wave reflections in steady flows. Part 2. Numerical investigation ». Journal of Fluid Mechanics 301 (25 octobre 1995) : 37–50. http://dx.doi.org/10.1017/s0022112095003788.
Texte intégralRésumé :
The reflection of shock waves over straight reflecting surfaces in steady flows was investigated numerically with the aid of the LCPFCT algorithm. The findings completely supported the experimental results which were reported in Part 1 of this paper (Chpoun et al. 1995). In addition, the dependence of the resulting shock wave configuration on the distance between the trailing edge of the reflecting wedge and the bottom surface, inside the dual-solution domain, was studied. As a result of this study, as well as the one reported in Part 1, the state of the art of shock wave reflections in steady flows was reconsidered.
6
Isaacson, Michael, Enda O'Sullivan et John Baldwin. « Reflection effects on wave field within a harbour ». Canadian Journal of Civil Engineering 20, no 3 (1 juin 1993) : 386–97. http://dx.doi.org/10.1139/l93-054.
Texte intégralRésumé :
The present paper outlines a numerical model for predicting the wave field in a harbour with partially reflecting boundaries, and describes laboratory tests undertaken to assess the model. The numerical model is based on linear diffraction theory and involves the application of a partial reflection boundary condition. By utilizing a wave doublet representation of the fluid boundaries instead of the usual wave source representation, the extension is made to general harbour configurations that include breakwaters. Numerical results are compared with known solutions for specific reference configurations. Laboratory measurements have been made of the wave field within a particular harbour model having portions of the boundary corresponding to different degrees of wave reflection. A comparison with the numerical predictions is summarized and highlights the importance of adequately modelling the partial reflections within the harbour. Key words: breakwaters, coastal engineering, harbours, waves, wave diffraction, wave reflection.
7
Modra, Ben, Dan Howe, Anthony Folan et Kate McLean. « CHANNEL CONCENTRATION AND REFLECTIONS FROM DREDGE CHANNELS ». Coastal Engineering Proceedings, no 36v (28 décembre 2020) : 25. http://dx.doi.org/10.9753/icce.v36v.management.25.
Texte intégralRésumé :
Wave reflections from dredge channels are an important consideration for coastal infrastructure. A physical model study of a proposed development for the Port of Townsville demonstrated that channel reflection, and the relatively poorly understood channel concentration are significant processes that need to be considered in coastal developments. The study showed that channel reflection and channel concentration can significantly transform the local waves, resulting in complex multidirectional wave fields and higher design wave conditions.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/XsKsofNZzvQ
8
Vanden Eynden, Frederic, Bachar El-Oumeiri, Thierry Bové, Guido Van Nooten et Patrick Segers. « Proximal pressure reducing effect of wave reflection in the pulmonary circulation disappear in obstructive disease : insight from a rabbit model ». American Journal of Physiology-Heart and Circulatory Physiology 316, no 5 (1 mai 2019) : H992—H1004. http://dx.doi.org/10.1152/ajpheart.00635.2018.
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Locating the site of increased resistance within the vascular tree in pulmonary arterial hypertension could assist in both patient diagnosis and tailoring treatment. Wave intensity analysis (WIA) is a wave analysis method that may be capable of localizing the major site of reflection within a vascular system. We investigated the contribution of WIA to the analysis of the pulmonary circulation in a rabbit model with animals subjected to variable occlusive pulmonary disease. Animals were embolized with different sized microspheres for 6 wk ( n = 10) or underwent pulmonary artery (PA) ligation for 6 wk ( n = 3). These animals were compared with a control group ( n = 6) and acutely embolized animals ( n = 4). WIA was performed and compared with impedance-based methods to analyze wave reflections. The control group showed a relatively high extent of reflected waves (15.7 ± 10.6%); reflections had a net effect of pressure reduction during systole, suggesting an open-end reflector. The pattern of wave reflection was not different in the group with partial PA ligation (12.4 ± 4.1%). In the chronically embolized group, wave reflection was not observed (3.6 ± 1.5%). In the acute embolization group, wave reflection was more prominent (37.3 ± 12.6%), with the appearance of a novel wave increasing pressure, suggesting the appearance of a closed-end reflector. Wave reflections of an open-end type are present in the normal rabbit pulmonary circulation. However, the pattern and nature of reflections vary according to the extent of pulmonary vascular occlusion. NEW & NOTEWORTHY The study proposes an original framework of a complementary analysis of wave reflections in the time domain and in the frequency domain. The methodology was used in the pulmonary circulation with different forms of chronic obstructions. The results suggest that the pulmonary vascular tree generates a reflection pattern that could actually assist the heart during ejection, and chronic obstruction significantly modifies the pattern.
9
Monaldi, Lucas, Luis Gutiérrez Marcantoni et Sergio Elaskar. « OpenFOAMTM Simulation of the Shock Wave Reflection in Unsteady Flow ». Symmetry 14, no 10 (1 octobre 2022) : 2048. http://dx.doi.org/10.3390/sym14102048.
Texte intégralRésumé :
This work studies the impact of a shock wave traveling with non-constant velocity over straight surfaces, generating an unsteady and complex reflection process. Two types of shock waves generated by sudden energy released are studied: cylindrical and spherical. Several numerical tests were developed considering different distances between the shock wave origin and the reflecting surface. The Kurganov, Noelle, and Petrova (KNP) scheme implemented in the rhoCentralFoam solver of the OpenFOAMTM software is used to reproduce the different shock wave reflections and their transitions. The numerical simulations of the reflected angle, Mach number of the shock wave, and position of the triple point are compared with pseudo-steady theory numerical and experimental studies. The numerical results show good accuracy for the reflected angle and minor differences for the Mach number. However, the triple point position is more difficult to predict. The KNP scheme in the form used in this work demonstrates the ability to capture the phenomena involved in the unsteady reflections.
10
van der Baan, Mirko, et Dirk Smit. « Amplitude analysis of isotropic P-wave reflections ». GEOPHYSICS 71, no 6 (novembre 2006) : C93—C103. http://dx.doi.org/10.1190/1.2335877.
Texte intégralRésumé :
The analysis of amplitude variation with offset (AVO) of seismic reflections is a very popular tool for detecting gas sands. It is assumed in AVO, however, that plane-wave reflection coefficients can be used directly to analyze amplitudes measured in the time-offset domain. This is not true for near-critical angles of reflection. Plane-wave reflection coefficients incorporate the contribution of the head wave. A plane-wave decomposition such as a proper [Formula: see text] transform must be applied to the seismic data for accurate analysis of reflection coefficients near critical angles. Amplitudes after plane-wave decomposition are related directly to the plane-wave reflection coefficients; geometric-spreading corrections are no longer required, and polarization effects of P-P reflections recorded on the [Formula: see text]-component are also removed. Conventional, linearized expressions for the isotropic P-P-wave reflection coefficient depend on contrasts in three parameters, and they require background information about average P-wave/S-wave velocity ratios. We derive a new reduced-parameter expression that depends only on two free parameters without loss of accuracy. No extra prior parameter information is needed either. The reduction in free parameters is achieved by explicitly incorporating P-wave moveout information. A new AVO strategy is developed that requires moveout analysis of three reflections: the target horizon, the reflections directly above and below the target horizon, and the amplitudes of the target horizon. The new AVO expression can be used in the time-offset domain for precritical arrivals and in the [Formula: see text] domain for precritical and critical reflections.
11
Pan, Yue, Xiao He, Hao Chen et Xiuming Wang. « Reflection signals and wellbore scattering waves in acoustic logging while drilling ». Journal of Geophysics and Engineering 17, no 3 (18 mars 2020) : 552–61. http://dx.doi.org/10.1093/jge/gxaa014.
Texte intégralRésumé :
Abstract In sonic logging while drilling (LWD), it is difficult to extract reflection signals for the goal of geo-steering as the wave fields are so complicated. It is important to analyse the reflection and scattering effects based on the synthetic acoustic signals of the real LWD models, while considering the medium discontinuity at the end of the borehole. We numerically investigate the acoustic LWD responses to reflective boundaries out of the borehole. To simulate the received signals, the 3D finite difference in time domain method is implemented. Mode conversions between the collar and the Stoneley waves are revealed. Strong reflections are generated at the bottom of the well, which can be equivalent to an additional scattering source (i.e. an apparent point source). The scattering waves by the wellbore bottom are generally much stronger than the reflections from the layer interfaces of formations. By comparing the models with stratified interfaces of opposite inclination directions, the propagation mechanisms of two newly recognised reflection waves are revealed in addition to the traditional body wave reflections (P and S waves) in LWD models. The energy of the collar wave radiates outside the borehole and then reflects at the bedding boundaries; meanwhile, the scattering waves from the well bottom can generate reflections too. These reflection arrivals match well with the time predicted by ray theories, respectively. Finally, we propose a possible means to estimate the dipping directions of geological interfaces by reflection waves emitted from both LWD transmitters and the apparent source at the well bottom.
12
Skopintseva, Lyubov, Milana Ayzenberg, Martin Landrø, Tatyana Nefedkina et Arkady M. Aizenberg. « Long-offset AVO inversion of PP reflections from plane interfaces using effective reflection coefficients ». GEOPHYSICS 76, no 6 (novembre 2011) : C65—C79. http://dx.doi.org/10.1190/geo2010-0079.1.
Texte intégralRésumé :
A conventional amplitude variation with offset (AVO) inversion is based on geometrical seismics which exploit plane-wave reflection coefficients to describe the reflection phenomenon. Widely exploited linearizations of plane-wave coefficients are mostly valid at pre-critical offsets for media with almost flat and weak-contrast interfaces. Existing linearizations do not account for the seismic frequency range by ignoring the frequency content of the wavelet, which is a strong assumption. Plane-wave reflection coefficients do not fully describe the reflection of seismic waves at near-critical and post-critical offsets, because reflected seismic waves are typically generated by point sources. We propose an improved approach to AVO inversion, which is based on effective reflection coefficients (ERCs). ERCs generalize plane-wave coefficients for seismic waves generated by point sources and therefore more accurately describe near-critical and post-critical reflections where head waves are generated. Moreover, they are frequency-dependent and incorporate the local curvatures of the wavefront and the reflecting interface. In our study, we neglect the effect of interface curvature and demonstrate the advantages of our approach on synthetic data for a simple model with a plane interface separating two isotropic half-spaces. A comparison of the inversion results obtained with our approach and the results from an AVO inversion method based on the exact plane-wave reflection coefficient suggests that our method is superior, in particular for long-offset ranges which extend to and beyond the critical angle. We thus propose that long offsets can be successfully exploited in an AVO inversion under the correct assumption about the reflection coefficient. Such long-offset AVO inversion shows the potential of outperforming a conventional moderate-offset AVO inversion in the accuracy of estimated model parameters.
13
Kobler, H. « Arterial wave reflections. » Hypertension 22, no 2 (août 1993) : 268. http://dx.doi.org/10.1161/01.hyp.22.2.268.
Texte intégral
14
Zhu, Jianlin. « A transparent boundary technique for numerical modeling of elastic waves ». GEOPHYSICS 64, no 3 (mai 1999) : 963–66. http://dx.doi.org/10.1190/1.1444604.
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In numerical modeling of wave motions, strong reflections from artificial model boundaries may contaminate or mask true reflections from the interior model interfaces. Hence, developing a kind of exterior model boundary transparent to the outgoing waves is of critical importance. Among proposed solutions, e.g., Smith (1974), Kausel and Tassoulas (1981), and Higdon (1991), the most widely used may be the Clayton and Engquist (1977) method of absorbing boundary conditions, based on paraxial approximations for acoustic and elastic‐wave equations. However, absorbing boundary conditions make the reflection coefficients zero only for normal incidence, and suppression of reflected S-waves (Clayton and Engquist, 1977) becomes poorer as the ratio of P- to S-wave velocity ([Formula: see text]) becomes larger.
15
Vasilev, Eugene I., Tov Elperin et Gabi Ben-Dor. « Reconsideration of the So-Called von Neumann Paradox in the Reflection of a Shock Wave over a Wedge ». Materials Science Forum 566 (novembre 2007) : 1–8. http://dx.doi.org/10.4028/www.scientific.net/msf.566.1.
Texte intégralRésumé :
Numerous experimental investigations on the reflection of plane shock waves over straight wedges indicated that there is a domain, frequently referred to as the weak shock wave domain, inside which the resulted wave configurations resemble the wave configuration of a Mach reflection although the classical three-shock theory does not provide an analytical solution. This paradox is known in the literature as the von Neumann paradox. While numerically investigating this paradox Colella & Henderson [1] suggested that the observed reflections were not Mach reflections but another reflection, in which the reflected wave at the triple point was not a shock wave but a compression wave. They termed them it von Neumann reflection. Consequently, based on their study there was no paradox since the three-shock theory never aimed at predicting this wave configuration. Vasilev & Kraiko [2] who numerically investigated the same phenomenon a decade later concluded that the wave configuration, inside the questionable domain, includes in addition to the three shock waves a very tiny Prandtl-Meyer expansion fan centered at the triple point. This wave configuration, which was first predicted by Guderley [3], was recently observed experimentally by Skews & Ashworth [4] who named it Guderley reflection. The entire phenomenon was re-investigated by us analytically. It has been found that there are in fact three different reflection configurations inside the weak reflection domain: • A von Neumann reflection – vNR, • A yet not named reflection – ?R, • A Guderley reflection – GR. The transition boundaries between MR, vNR, ?R and GR and their domains have been determined analytically. The reported study presents for the first time a full solution of the weak shock wave domain, which has been puzzling the scientific community for a few decades. Although the present study has been conducted in a perfect gas, it is believed that the reported various wave configurations, namely, vNR, ?R and GR, exist also in the reflection of shock waves in condensed matter.
16
Cahill, Lindsay S., Yu-Qing Zhou, Johnathan Hoggarth, Lisa X. Yu, Anum Rahman, Greg Stortz, Clare L. Whitehead et al. « Placental vascular abnormalities in the mouse alter umbilical artery wave reflections ». American Journal of Physiology-Heart and Circulatory Physiology 316, no 3 (1 mars 2019) : H664—H672. http://dx.doi.org/10.1152/ajpheart.00733.2018.
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Current methods to detect placental vascular pathologies that monitor Doppler ultrasound changes in umbilical artery (UA) pulsatility have only moderate diagnostic utility, particularly in late gestation. In fetal mice, we recently demonstrated that reflected pressure waves propagate counter to the direction of flow in the UA and proposed the measurement of these reflections as a means to detect abnormalities in the placental circulation. In the present study, we used this approach in combination with microcomputed tomography to investigate the relationship between altered placental vascular architecture and changes in UA wave reflection metrics. Fetuses were assessed at embryonic day (E) 15.5 and E17.5 in control C57BL6/J mice and dams treated with combination antiretroviral therapy (cART), a known model of fetal growth restriction. Whereas the reflection coefficient was not different between groups at E15.5, it was 27% higher at E17.5 in cART-treated mice compared with control mice. This increase in reflection coefficient corresponded to a 36% increase in the total number of vessel segments, a measure of overall architectural complexity. Interestingly, there was no difference in UA pulsatility index between groups, suggesting that the wave reflections convey information about vascular architecture that is not captured by conventional ultrasound metrics. The wave reflection parameters were found to be associated with the morphology of the fetoplacental arterial tree, with the area ratio between the UA and first branch points correlating with the reflection coefficient. This study highlights the potential for wave reflection to aid in the noninvasive clinical assessment of placental vascular pathology. NEW & NOTEWORTHY We used a novel ultrasound methodology based on detecting pulse pressure waves that propagate along the umbilical artery to investigate the relationship between changes in wave reflection metrics and altered placental vascular architecture visualized by microcomputed tomography. Using pregnant mice treated with combination antiretroviral therapy, a model of fetal growth restriction, we demonstrated that reflections in the umbilical artery are sensitive to placental vascular abnormalities and associated with the geometry of the fetoplacental tree.
17
Rendleman, C. A., et F. K. Levin. « Reflection maxima for reflections from single interfaces ». GEOPHYSICS 53, no 2 (février 1988) : 271–75. http://dx.doi.org/10.1190/1.1442462.
Texte intégralRésumé :
At a workshop on refraction and wide‐angle reflections, Hilterman (1985) pointed out that, in contrast to the plane‐wave case, when there is a point source, a P-wave reflected from a plane interface attains its maximum amplitude at an offset greater than that corresponding to the critical angle (Figure 1). The same conclusion had been drawn earlier by Červený (1967). However, neither Červený’s results, which were based on very complicated mathematical expressions derived by Brekhovskikh (1960), nor Hilterman’s computer‐generated data shed light on the physics implied by the shifted maximum.
18
Ben-Dor, G., O. Igra et L. Wang. « Shock Wave Reflections in Dust-Gas Suspensions ». Journal of Fluids Engineering 123, no 1 (6 septembre 2000) : 145–53. http://dx.doi.org/10.1115/1.1331558.
Texte intégralRésumé :
The reflection of planar shock waves from straight wedges in dust-gas suspensions is investigated numerically. The GRP shock capturing scheme and the MacCormac scheme are applied to solve the governing equations of the gaseous and solid phases, respectively. These two schemes have a second-order accuracy both in time and space. It is shown that the presence of the dust significantly affects the shock-wave-reflection-induced flow field. The incident shock wave attenuates and hence unlike the shock wave reflection phenomenon in a pure gas, the flow field in the present case is not pseudo steady. The presence of the dust results in lower gas velocities and gas temperatures and higher gas densities and gas pressures than in dust-free shock wave reflections with identical initial conditions. It is also shown that the smaller is the diameter of the dust particle the larger are the above-mentioned differences. In addition, the smaller is the diameter of the dust particle the narrower is the width of the dust cloud behind the incident shock wave. Larger dust velocities, dust temperatures and dust spatial densities are obtained inside this dust cloud for smaller dust particles. The results provide a clear picture of whether and how the presence of dust particles affects the shock-wave-reflection-induced flow field.
19
Winterstein, D. F., et J. B. Hanten. « Supercritical reflections observed in P- and S- wave data ». GEOPHYSICS 50, no 2 (février 1985) : 185–95. http://dx.doi.org/10.1190/1.1441908.
Texte intégralRésumé :
We have observed a conspicuous example of supercritical reflection in both P- and SH- wave seismic data. Data were recorded in the Midland Basin (Texas) Project of the Conoco Shear Wave Group Shoot in 1977–1978. P- and S- wave critical angle phenomena, as observed in the data, are remarkably similar. Event amplitudes are small or undetectable at offsets out to about 2 000 ft, but at offsets from 2 500 to 3 600 ft amplitudes are higher than those of any other event. Head waves originating at the critical distance are weak but detectable. Long path multiplies of the supercritical parts of P and SH events appear at expected times and offsets. Constant velocity moveout corrections helped identify them. Sonic logs in combination with a knowledge of the lithology made it possible to model P- wave critical angle phenomena. Agreement of model results with the data was good when we assumed cylindrical wavefronts. As expected, modeling based on plane waves was unable to match observed phase and amplitude behavior. A number of potential uses for supercritical reflections in exploration and data processing readily come to mind, many of them related to the recording of relatively high amplitudes at distances where source noise is low. Observed rise in amplitude near the critical offset was very abrupt, particularly for SH-waves. This suggests that variations in the onset of high amplitudes may be useful for monitoring changes in velocity contrast at the reflecting interface.
20
Tiedeman, Simon Alexander, William Allsop, Viviana Russo et Andy Brown. « A DEMOUNTABLE WAVE ABSORBER FOR WAVE FLUMES AND BASINS ». Coastal Engineering Proceedings 1, no 33 (14 décembre 2012) : 37. http://dx.doi.org/10.9753/icce.v33.waves.37.
Texte intégralRésumé :
Passive wave absorption is an integral component of the physical modeling environment, used to minimise unwanted reflections of wave energy that compromise test results. This paper reviews data for methods of passive absorbers and then extends this knowledge through the design and implementation of a device that can be removed from the working water surface. The modeling tests that were carried out in this paper demonstrate that a parabolic spending beach can perform by absorbing waves with coefficients of reflection Cr(energy) significantly
21
Paul, Anne, et Michel Campillo. « Diffraction and conversion of elastic waves at a corrugated interface ». GEOPHYSICS 53, no 11 (novembre 1988) : 1415–24. http://dx.doi.org/10.1190/1.1442421.
Texte intégralRésumé :
Numerical modeling is used to investigate the effect of small‐scale irregularities of a reflecting boundary on elastic wave reflections. The scattered wave field is computed by using a discretized form of boundary integral equations and a plane‐wave decomposition of seismic wave fields. For various values of incidence angle of the P wave, we compute the distribution of diffracted energy for both P waves and S waves as a function of reflection angle. We show that corrugations with mean wavelength of the order of, or smaller than, the seismic wavelength have little effect on the reflected P wave. However, the pattern of P‐to‐S conversion is very different from that with a plane boundary. Scattered S waves appear at postcritical angles for any angle of incidence of the P wave. The amplitude of these nongeometrical shear waves decreases rapidly with decreasing amplitude of the corrugations, or when the mean wavelength of the corrugations becomes larger than the dominant seismic wavelength. The local geometry of the irregularities has a negligible effect on the scattered S waves. By analogy with perturbation theory, we propose interpreting the postcritically scattered S waves as the contribution to the shear wave field of converted inhomogeneous P waves diffracted along the boundary.
22
Cao, S., B. L. N. Kennett et B. R. Goleby. « A 3D isochronal modelling technique and its applications ». Exploration Geophysics 20, no 2 (1989) : 205. http://dx.doi.org/10.1071/eg989205.
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Reflection seismic datasets are obtained in both the exploration of oil and mineral resources and the probing of the deep crust and the upper mantle. To interpret the datasets, considerable effort has been spent on the understanding of seismic wave propagation phenomena by simulating seismic wave propagations in some a priori physical models. A rather simple and efficient modelling technique has been developed to study elastic wave reflections with full inclusion of diffractions.This modelling technique employs an integral representation of reflections from a surface or a scatterer. High frequency asymptotic approximations are used for the propagation between the seismic source or receiver and a surface or a scatterer. At a scatterer, first order scattering is assumed. At a surface, reflection and transmission effects are estimated using the assumption of a locally plane interface and plane incident wave. With these approximations, the reflected seismograms are calculated by convolving the time derivative of a source function with a model weight function for a particular source-receiver pair. The weight function at a particular time is evaluated by a line integral along a contour of equal total travel time from source to receiver via the scattering surface (an isochron). The kernel of this integral at a reflecting point is the local reflection coefficient which which represents the effects of the amplitude of material parameter contrasts at the reflecting point, the angles between the incoming and outgoing waves and the local surface normal and the local speed of advance of the isochron on the surface, and the geometrical spreading factors from the source and receiver to the reflecting point.This modelling technique is used to investigate the validity of some of the interpretations of a deep crustal reflection profile collected in central Australia. The modelling results confirm that even with a relatively short (4 km) field spread it would be possible to pick up the reflected energy from faults with dips of about 40�. The largest fault, the Redbank Zone, has significant displacement of the crust-mantle boundary and within the fault zone, it is conceivable to have considerable variability in physical properties.The deep seismic section shows this boundary as a thick (0.5s) band of complex reflections and diffractions at the reflection time appropriate to the crust-mantle transition. Two possible structures for the crust-mantle boundary were investigated, one where the crustal faults have displaced this interface and created a 'block-faulted' geometry and the other where the crustal faults are listric near the boundary and appear to sole out on the crust-mantle interface, giving rise to an undulation of the Moho. The modelling results (Figure 1) for an undulating boundary show a band of reflections which strongly resemble the observed seismic reflection data.
23
Zhu, Xinfa, et George A. McMechan. « Amplitude and phase versus angle for elastic wide-angle reflections in the τ‐p domain ». GEOPHYSICS 80, no 1 (1 janvier 2015) : N1—N9. http://dx.doi.org/10.1190/geo2013-0191.1.
Texte intégralRésumé :
Near- and postcritical spherical-wave reflections contain amplitude and phase variations with incident angle that are not predicted by plane-wave solutions. However, if a spherical wavefield is decomposed into plane waves by a time-intercept-slowness ([Formula: see text]) transform, then plane-wave reflection coefficients (the Zoeppritz) can be used as the basis of amplitude/phase versus angle analysis. The spherical-wave effects on reflection coefficients near the critical angle (in the time-offset domain) were decomposed by [Formula: see text] transformation into plane waves. Kinematic ray tracing linked the reflection angle at the target reflector and the apparent slowness at the surface receiver, which enabled extracting the amplitude/phase versus angle data at the reflector from the surface [Formula: see text] data. The most reliable inversion results were obtained by combining the extracted amplitudes and phases in a composite inversion for the elastic parameters below the target reflector.
24
Chabyshova, Elmira, et Gennady Goloshubin. « Seismic modeling of low-frequency “shadows” beneath gas reservoirs ». GEOPHYSICS 79, no 6 (1 novembre 2014) : D417—D423. http://dx.doi.org/10.1190/geo2013-0379.1.
Texte intégralRésumé :
P-wave amplitude anomalies below reservoir zones can be used as hydrocarbon markers. Some of those anomalies are considerably delayed relatively to the reflections from the reservoir zone. High P-wave attenuation and velocity dispersion of the observed P-waves cannot justify such delays. The hypothesis that these amplitude anomalies are caused by wave propagation through a layered permeable gaseous reservoir is evaluated. The wave propagation through highly interbedded reservoirs suggest an anomalous amount of mode conversions between fast and slow P-waves. The converted P-waves, which propagated even a short distance as slow P-waves, should be significantly delayed and attenuated comparatively, with the fast P-wave reflections. The amplitudes and arrival time variations of conventional and converted P-wave reflections at low seismic frequencies were evaluated by means of an asymptotic analysis. The calculations confirmed that the amplitude anomalies due to converted P-waves are noticeably delayed in time relatively to fast P-wave reflections. However, the amplitudes of the modeled converted P-waves were much lower than the amplitude anomalies observed from exploration cases.
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Chertova, N. B., et Yu V. Grinyaev. « Reflection of elastic waves at the boundary with the specified stresses ». Izvestiya vysshikh uchebnykh zavedenii. Fizika, no 5 (2021) : 52–59. http://dx.doi.org/10.17223/00213411/64/5/52.
Texte intégralRésumé :
The problem of elastic waves reflection at the boundary at given constant stresses is considered. Provided that the reflection laws are fulfilled, analytical expressions are obtained for the reflection coefficients of longitudinal and transverse waves, which allow us to determine the deformations amplitudes at the boundary. The dependences of the reflections coefficients and strain components at the boundary on the angle of incidence of the wave are calculated at nonzero values of normal and tangential stresses at the boundary and the specified wave parameters. The obtained results are analyzed and compared with the known results for the free surface.
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Feng, Zongcai, et Lianjie Huang. « Quasielastic least-squares reverse time migration of PS reflections ». GEOPHYSICS 87, no 3 (10 mars 2022) : S105—S116. http://dx.doi.org/10.1190/geo2021-0109.1.
Texte intégralRésumé :
Conventional analysis of amplitude variation with offset for elastic PS reflections is based on analytical reflection coefficients in a layered medium, and wave-equation-based PS migration is mainly used to produce a structural image. To overcome this problem, we have developed a least-squares reverse time migration (LSRTM) method for elastic PS reflections based on a quasielastic wave equation. The quasielastic wave equation can accurately model PS reflections with elastic amplitudes under the first-order Born approximation. Our LSRTM method inverts for perturbations of the S-wave velocity and density by minimizing the [Formula: see text] norm of the difference between recorded and predicted PS reflections modeled using a quasielastic wave equation. We refer to our new method as quasielastic LSRTM of PS reflections. Numerical tests on synthetic and field data indicate that our method can properly handle the amplitudes of elastic PS reflections and provides an accurate estimate of the perturbations of S-wave velocity and density. Extending the method to the 3D case is not straightforward and might require incorporating certain PS data processing techniques into the inversion itself.
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Jeng, Yih. « Shallow seismic investigation of a site with poor reflection quality ». GEOPHYSICS 60, no 6 (novembre 1995) : 1715–26. http://dx.doi.org/10.1190/1.1443904.
Texte intégralRésumé :
A shallow seismic reflection experiment was performed on a construction site to determine the feasibility of using reflection seismology to investigate the shallow structure in a weathered sand‐gravel interlayered zone that was known to be a poor transmission of high‐frequency seismic energy. Field‐recording parameters were designed to fit the limited space of the urban construction survey area. A 7 kg sledgehammer was used to generate P‐waves and SH‐waves. Single 100 Hz geophones were deployed at 1.0 m/0.5 m group intervals, and 200/100-Hz low‐cut filters were applied prior to A to D conversion to attenuate ground roll. For SH‐wave reflections, single 14 Hz geophones and a 70-Hz low‐cut filter on the seismograph were used. The dominant frequency bands ranged from 33 to 275 Hz and were centered around 110 Hz for P‐waves. Lower dominant frequency bands 20 to 160 Hz with a dominant frequency of around 85 Hz were observed on SH‐wave records. Four seismic lines, three P‐wave recordings and one SH‐wave recording, using different sets of recording parameters and an appropriate seismic‐wave generation method produced reflections from varying depth ranges and at different resolutions. The results show that the techniques employed in this experiment may resolve the structure of a site with poor reflection quality. An f-k dip filtering and deconvolution were necessary in processing the reflection data to eliminate various types of unwanted energy. The seismic interpretations in this study were verified by drilling and by a nearby excavation.
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Ren, Zhiming, Qianzong Bao et Bingluo Gu. « Joint wave-equation traveltime inversion of diving/direct and reflected waves for P- and S-wave velocity macromodel building ». GEOPHYSICS 86, no 4 (1 juillet 2021) : R603—R621. http://dx.doi.org/10.1190/geo2020-0762.1.
Texte intégralRésumé :
Full-waveform inversion (FWI) suffers from the local minima problem and requires a sufficiently accurate starting model to converge to the correct solution. Wave-equation traveltime inversion (WETI) is an effective tool to retrieve the long-wavelength components of the velocity model. We have developed a joint diving/direct and reflected wave WETI (JDRWETI) method to build P- and S-wave velocity macromodels. We estimate the traveltime shifts of seismic events (diving/direct waves and PP- and PS-reflections) through the dynamic warping scheme and construct a misfit function using the time shifts of diving/direct and reflected waves. We derive the adjoint wave equations and the gradients with respect to the background models based on the joint misfit function. We apply the kernel decomposition scheme to extract the kernel of the diving/direct wave and the tomography kernels of PP- and PS-reflections. For an explosive source, the kernels of the diving/direct wave and PP-reflections and the kernel of the PS-reflections are used to compute the P- and S-wave gradients of the background models, respectively. We implement JDRWETI by a two-stage inversion workflow: First, we invert the P- and S-wave velocity models using the P-wave gradients, and then we improve the S-wave velocity model using the S-wave gradients. Numerical tests on synthetic and field data sets reveal that the JDRWETI method successfully recovers the long-wavelength components of P- and S-wave velocity models, which can be used for an initial model for the subsequent elastic FWI. Moreover, the JDRWETI method prevails over the existing reflection WETI method and the cascaded diving/direct and reflected wave WETI method, especially when large velocity errors are present in the shallow part of the starting models. The JDRWETI method with the two-stage inversion workflow can give rise to reasonable inversion results even for the model with different P- and S-wave velocity structures.
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Tatavarti, V. S. N., David A. Huntley et Anthony J. Bowen. « INCOMING AND OUTGOING WAVE INTERACTIONS ON BEACHES ». Coastal Engineering Proceedings 1, no 21 (29 janvier 1988) : 9. http://dx.doi.org/10.9753/icce.v21.9.
Texte intégralRésumé :
A technique to decompose colocated random field measurements of wave elevation and current velocity into incoming (shoreward propagating) and outgoing (seaward propagating) components is presented. This decomposition technique, which is less sensitive to noise, enables us to determine the frequency dependent reflection coefficients and also the relative phase between the incoming and outgoing waves. The method is applied to C2S2 and NSTS data sets, from beaches with wide ranging characteristics and wave regimes. The results demonstrate the selective nature of beach absorption/reflection characteristics but are inconclusive in terms of a proper parameterization of reflections on natural beaches
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Qiu, Xinming, Chao Wang, Jun Lu et Yun Wang. « Surface-Wave Extraction Based on Morphological Diversity of Seismic Events ». Applied Sciences 9, no 1 (21 décembre 2018) : 17. http://dx.doi.org/10.3390/app9010017.
Texte intégralRésumé :
It is essential to extract high-fidelity surface waves in surface-wave surveys. Because reflections usually interfere with surface waves on X components in multicomponent seismic exploration, it is difficult to extract dispersion curves of surface waves. To make matters worse, the frequencies and velocities of higher-mode surface waves are close to those of PS-waves. A method for surface-wave extraction is proposed based on the morphological differences between surface waves and reflections. Frequency-domain high-resolution linear Radon transform (LRT) and time-domain high-resolution hyperbolic Radon transform (HRT) are used to represent surface waves and reflections, respectively. Then, a sparse representation problem based on morphological component analysis (MCA) is built and optimally solved to obtain high-fidelity surface waves. An advantage of our method is its ability to extract surface waves when their frequencies and velocities are close to those of reflections. Furthermore, the results of synthetic and field examples confirm that the proposed method can attenuate the distortion of surface-wave dispersive energy caused by reflections, which contributes to extraction of accurate dispersion curves.
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Thomas, Frédérique, Bruno Pannier, Nicolas Danchin et Michel E. Safar. « Wave reflections in hypertension ». Journal of Hypertension 37, no 3 (mars 2019) : 555–62. http://dx.doi.org/10.1097/hjh.0000000000001928.
Texte intégral
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Syme, Douglas A., A. Kurt Gamperl, Marvin H. Braun et David R. Jones. « Wave reflection effects in the central circulation of American alligators (Alligator mississippiensis) : what the heart sees ». American Journal of Physiology-Heart and Circulatory Physiology 291, no 4 (octobre 2006) : H1670—H1678. http://dx.doi.org/10.1152/ajpheart.00097.2006.
Texte intégralRésumé :
A large central compliance is thought to dominate the hemodynamics of all vertebrates except birds and mammals. Yet large crocodilians may adumbrate the avian and mammalian condition and set the stage for significant wave transmission (reflection) effects, with potentially detrimental impacts on cardiac performance. To investigate whether crocodilians exhibit wave reflection effects, pressures and flows were recorded from the right aorta, carotid artery, and femoral artery of six adult, anesthetized American alligators ( Alligator mississippiensis) during control conditions and after experimentally induced vasodilation and constriction. Hallmarks of wave reflection phenomena were observed, including marked differences between the measured profiles for flow and pressure, peaking of the femoral pressure pulse, and a diastolic wave in the right aortic pressure profile. Pulse wave velocity and peripheral input impedance increased with progressive constriction, and thus changes in both the timing and magnitude of reflections accounted for the altered reflection effects. Resolution of pressure and flow waves into incident and reflected components showed substantial reflection effects within the right aorta, with reflection coefficients at the first harmonic approaching 0.3 when constricted. Material properties measured from isolated segments of blood vessels revealed a major reflection site at the periphery and, surprisingly, at the junction of the truncus and right aorta. Thus, while our results clearly show that significant wave reflection phenomena are not restricted to birds and mammals, they also suggest that rather than cope with potential negative impacts of reflections, the crocodilian heart simply avoids them because of a large impedance mismatch at the truncus.
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Liu, J. J. « Sound wave structures downstream of pseudo-steady weak and strong Mach reflections ». Journal of Fluid Mechanics 324 (10 octobre 1996) : 309–32. http://dx.doi.org/10.1017/s0022112096007938.
Texte intégralRésumé :
Sound wave structures, downstream of moving incident shocks reflecting from straight compressive wedges, are analysed for both weak and strong Mach reflections (MR) using existing experiments. It is shown that the reflected waves can be well described by using the acoustic criterion or the weak oblique shock approximation, when the classical three-shock theory gives forward-facing reflected shock solutions. The predicted triple-point trajectory angles are found to be in close agreement with the experiments. The distinction between the applicabilities of the two methods is given by an analytically defined ‘smallness’ for the angle of reflecting wedges. The physics of the success of the two methods is discussed. It is concluded that forward-facing reflected shock solutions of pseudo-steady MR should be ruled out physically because sound waves cannot coalesce into Mach waves that propagate upstream of the triple point. In their place, MR-like phenomena occur with the reflected waves being normal Mach waves or finite compression waves for ‘small’ or ‘not-small’ reflecting wedge angles, respectively, and they are classified as the first- or second-king von Neumann reflections, respectively. Boundaries separating regimes between the first and second kinds of von Neumann reflections, and backward-facing MR are determined.
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Iverson, William P., Bill A. Fahmy et Scott B. Smithson. « VpVs from mode‐converted P-SV reflections ». GEOPHYSICS 54, no 7 (juillet 1989) : 843–52. http://dx.doi.org/10.1190/1.1442713.
Texte intégralRésumé :
P-SV reflections are generated by a compressional‐wave source and result from P waves that are converted to shear (SV) waves upon reflection. Recording both the P and SV components yields compressional and shear data simultaneously. Verifying that the easily detected events really are P-SV reflections is accomplished by noting the good correlation of surface CDP data with vertical seismic profile (VSP) reflections. Stacking velocities from P-SV CDP gathers determine the [Formula: see text] product when source‐to‐receiver offset is less than the depth of the reflector but data from synthetic models show that P-SV reflections are nonhyperbolic for shallow reflections or when source‐to‐receiver offset is too large. Shear velocity [Formula: see text] can be calculated from P-SV reflections by one of two techniques: comparison of stacked section P-P and P-SV reflection times or by using the P-P and P-SV stacking velocities. Unfortunately, most P-SV reflections on a P-SV seismic section do not necessarily originate from exactly the same depth as P-P reflections. When this depth discrepancy occurs, the reflection‐time comparison technique fails. In addition, [Formula: see text] cannot be calculated from P-SV reflections, and we must settle for the [Formula: see text] product from P-SV reflection stacking velocities. When P-SV stacking velocities are input to the familiar Dix equation, the resulting interval velocities yield the [Formula: see text] product.
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Liang, Chao, Ossian O’Reilly, Eric M. Dunham et Dan Moos. « Hydraulic fracture diagnostics from Krauklis-wave resonance and tube-wave reflections ». GEOPHYSICS 82, no 3 (1 mai 2017) : D171—D186. http://dx.doi.org/10.1190/geo2016-0480.1.
Texte intégralRésumé :
Fluid-filled fractures support guided waves known as Krauklis waves. The resonance of Krauklis waves within fractures occurs at specific frequencies; these frequencies, and the associated attenuation of the resonant modes, can be used to constrain the fracture geometry. We use numerical simulations of wave propagation along fluid-filled fractures to quantify fracture resonance. The simulations involve solution of an approximation to the compressible Navier-Stokes equation for the viscous fluid in the fracture coupled to the elastic-wave equation in the surrounding solid. Variable fracture aperture, narrow viscous boundary layers near the fracture walls, and additional attenuation from seismic radiation are accounted for in the simulations. We then determine how tube waves within a wellbore can be used to excite Krauklis waves within fractures that are hydraulically connected to the wellbore. The simulations provide the frequency-dependent hydraulic impedance of the fracture, which can then be used in a frequency-domain tube-wave code to model tube-wave reflection/transmission from fractures from a source in the wellbore or at the wellhead (e.g., water hammer from an abrupt shut-in). Tube waves at the resonance frequencies of the fracture can be selectively amplified by proper tuning of the length of a sealed section of the wellbore containing the fracture. The overall methodology presented here provides a framework for determining hydraulic fracture properties via interpretation of tube-wave data.
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Yen, David H. Y. « Green's Function for the Damped Wave Equation on a Finite Interval Subject to Two Robin Boundary Conditions ». Journal of Mechanics 19, no 1 (mars 2003) : 257–62. http://dx.doi.org/10.1017/s1727719100004299.
Texte intégralRésumé :
ABSTRACTThe Green's function for the damped wave equation on a finite interval subject to two Robin boundary conditions is studied. The problem for a semi-infinite interval with one Robin boundary is considered first by the Laplace transform method to establish the reflection principle at a Robin boundary. It is seen that the reflected wave generated by an exiting wave at a Robin boundary is a convolution involving the exiting wave and some kernel function. This generalizes the well known classical results for reflections at Dirichlet and Neumann boundaries. The reflection principle at a Robin boundary is then used for the finite interval case by considering multiple reflections and this leads to an infinite sequence of waves. In order to justify the results obtained here we also study the finite interval case by the Laplace transform method. The Laplace transform of the Green's function is expanded, for large values of the transform parameter, into an infinite series of negative exponentials and then inverted term by term. Agreement is reached between these two approaches.
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Mallick, Subhashis. « Amplitude-variation-with-offset, elastic-impedence, and wave-equation synthetics — A modeling study ». GEOPHYSICS 72, no 1 (janvier 2007) : C1—C7. http://dx.doi.org/10.1190/1.2387108.
Texte intégralRésumé :
Amplitude-variation-with-offset (AVO) and elastic-impedance (EI) analysis use an approximate plane P-wave reflection coefficient as a function of angle of incidence. AVO and EI both can be used in a three-term or a two-term formulation. This study uses synthetic data to demonstrate that the P-wave primary reflections at large offsets can be contaminated by reflections from other wave modes that can affect the quality of three-term AVO or EI results. The coupling of P-waves and S-waves in seismic-wave propagation through finely layered media generates the interfering wave modes. A methodology such as prestack-wave-equation modeling can properly account for these coupling effects. Both AVO and EI also assume a convolutional model whose accuracy decreases as incidence angles increase. On the other hand, wave-equation modeling is based on the rigorous solution to the wave equation and is valid for any incidence angle. Because wave interference is minimal at small angles, a two-term AVO/EI analysis that restricts input from small angles is likely to give more reliable parameter estimates than a three-term analysis. A three-term AVO/EI analysis should be used with caution and should be calibrated against well data and other data before being used for quantitative analysis.
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Vasarmidis, Panagiotis, Vasiliki Stratigaki, Tomohiro Suzuki, Marcel Zijlema et Peter Troch. « Internal Wave Generation in a Non-Hydrostatic Wave Model ». Water 11, no 5 (10 mai 2019) : 986. http://dx.doi.org/10.3390/w11050986.
Texte intégralRésumé :
In this work, internal wave generation techniques are developed in an open source non-hydrostatic wave model (Simulating WAves till SHore, SWASH) for accurate generation of regular and irregular long-crested waves. Two different internal wave generation techniques are examined: a source term addition method where additional surface elevation is added to the calculated surface elevation in a specific location in the domain and a spatially distributed source function where a spatially distributed mass is added in the continuity equation. These internal wave generation techniques in combination with numerical wave absorbing sponge layers are proposed as an alternative to the weakly reflective wave generation boundary to avoid re-reflections in case of dispersive and directional waves. The implemented techniques are validated against analytical solutions and experimental data including water surface elevations, orbital velocities, frequency spectra and wave heights. The numerical results show a very good agreement with the analytical solution and the experimental data indicating that SWASH with the addition of the proposed internal wave generation technique can be used to study coastal areas and wave energy converter (WEC) farms even under highly dispersive and directional waves without any spurious reflection from the wave generator.
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Duchesne, Mathieu J., André J. M. Pugin, Gabriel Fabien-Ouellet et Mathieu Sauvageau. « Detection of near-surface hydrocarbon seeps using P- and S-wave reflections ». Interpretation 4, no 3 (1 août 2016) : SH21—SH37. http://dx.doi.org/10.1190/int-2015-0175.1.
Texte intégralRésumé :
The combined use of P- and S-wave seismic reflection data is appealing for providing insights into active petroleum systems because P-waves are sensitive to fluids and S-waves are not. The method presented herein relies on the simultaneous acquisition of P- and S-wave data using a vibratory source operated in the inline horizontal mode. The combined analysis of P- and S-wave reflections is tested on two potential hydrocarbon seeps located in a prospective area of the St. Lawrence Lowlands in Eastern Canada. For both sites, P-wave data indicate local changes in the reflection amplitude and slow velocities, whereas S-wave data present an anomalous amplitude at one site. Differences between P- and S-wave reflection morphology and amplitude and the abrupt decrease in P-velocity are indirect lines of evidence for hydrocarbon migration toward the surface through unconsolidated sediments. Surface-gas analysis made on samples taken at one potential seeping site reveals the occurrence of thermogenic gas that presumably vents from the underlying fractured Utica Shale forming the top of the bedrock. The 3C shear data suggest that fluid migration locally disturbs the elastic properties of the matrix. The comparative analysis of P- and S-wave data along with 3C recordings makes this method not only attractive for the remote detection of shallow hydrocarbons but also for the exploration of how fluid migration impacts unconsolidated geologic media.
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Ursin, Bjørn, Martin Tygel et Einar Iversen. « SS-traveltime parameters from PP and PS reflections ». GEOPHYSICS 74, no 4 (juillet 2009) : R35—R47. http://dx.doi.org/10.1190/1.3147133.
Texte intégralRésumé :
The SS-wave traveltimes can be derived from PP- and PS-wave data with the previously derived [Formula: see text] method. We have extended this method as follows. (1) The previous requirement that sources and receivers be located on a common acquisition surface is removed, which makes the method directly applicable to PS-waves recorded on the ocean bottom and PP-waves recorded at the ocean surface. (2) By using the concept and properties of surface-to-surface propagator matrices, the second-order traveltime derivatives of the SS-waves are obtained. In the same way as for the original [Formula: see text] method, the proposed extension is valid for arbitrary anisotropic media. The propagator matrix and geometric spreading of an SS-wave reflected at a given point on a target reflector are obtained explicitly from the propagators of the PP- and PS-waves reflected at the same point. These additional parameters provided by the extended [Formula: see text] method can be used for a partial reconstruction of the SS-wave amplitude as well as for tomographic estimation of the elastic velocity model. A full simulation of the SS-wave, which includes reflection and transmission coefficients, cannot be obtained directly from recorded PP- and PS-wave amplitudes.
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Wu, Yulang, et George A. McMechan. « True amplitude recovery in reverse time extrapolation of plane and spherical waves ». GEOPHYSICS 83, no 3 (1 mai 2018) : T103—T122. http://dx.doi.org/10.1190/geo2017-0425.1.
Texte intégralRésumé :
A challenging outstanding problem in reverse time extrapolation is recovering accurate amplitudes at reflectors from the receiver wavefield. Various migrations have been developed to produce accurate image locations rather than correct amplitude information because of inadequate compensation of attenuation, dispersion, and transmission losses. We have evaluated the requirements, and determined the theoretical feasibility, of true amplitude recovery of 2D acoustic and elastic seismic data by using the analytic Zoeppritz equations for plane-wave reflection and transmission coefficients. Then, we used synthetic acoustic and elastic wavefield data generated by elastodynamic finite differences to verify the recovery, in the reverse time propagation, of spherical waves and illustrated the salient differences between the incident wavefields reconstructed from reflection data only and from the combination of reflection and transmission data. These examples quantitatively verify that recovering an incident plane or a spherical wave requires the reverse time propagation of all reflections and transmissions in a model with the correct velocity and density. Accurate reconstruction of an incident wave is not possible by backward propagation of only reflections. As an application, we removed downgoing internal multiple reflections generated by upgoing waves incident at reflectors shallower than a horizontal well, in which geophones are deployed. The subtraction of the downgoing reflection involves wavefield reconstruction at depths shallower than the horizontal well and separation of upgoing and downgoing wavefields. This approach assumes that the correct acoustic (or elastic) velocity and density models are available in, and shallower than, the layer where the horizontal well is located. Incident-wave reconstruction works equally well for smooth models, as for models with sharp boundaries. Uncertainties in the model used for reconstruction, and incompleteness of the data aperture are propagated into the equivalent uncertainties, and incompleteness of the reconstruction.
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Venugopal, Prem, et Luca Marinelli. « Localization of Arterial Bleeds Using Pulse Wave Reflections ». Military Medicine 186, Supplement_1 (1 janvier 2021) : 346–50. http://dx.doi.org/10.1093/milmed/usaa371.
Texte intégralRésumé :
ABSTRACT Introduction Localization of internal arterial bleeds is necessary for treatment in the battlefield. In this article, we describe a novel approach that utilizes pulse wave reflections generated by a bleed to locate it. Materials and Methods To demonstrate our approach, velocity and diameter waveforms in the presence of bleeds were simulated using the 1D wave propagation equations in a straight-vessel model of the human thoracic aorta. The simulated waveforms were then decomposed into forward and backward components using wave intensity analysis. Reflections arising from the bleed were identified from the decomposed waveforms. Results Reflection generated by the bleed introduced a new feature in the backward component, compared to the normal, no-bleed condition. The bleed location could be determined from the time delay between this reflection feature and the forward wave creating it, and the pulse wave velocity in the vessel. Conclusions The findings of this study could be utilized by ultrasound for hemorrhage localization.
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Igra, O., G. Hu, J. Falcovitz et W. Heilig. « Blast Wave Reflection From Wedges ». Journal of Fluids Engineering 125, no 3 (1 mai 2003) : 510–19. http://dx.doi.org/10.1115/1.1567310.
Texte intégralRésumé :
While a lot of attention was given to shock wave reflections from wedges during the past four decades, only little work was published regarding the similar case of blast wave reflection from wedges. In the present paper this subject is studied experimentally and theoretically/numerically. The obtained results show that the geometry of the reflected wave pattern is similar in the two cases when both incident waves have the same initial pressure jump across their fronts. However, different reflected pressure signatures (history) are observed in these two cases. The pressures obtained behind a reflected shock wave are always higher than those obtained behind the corresponding similar blast wave. In the present case differences as high as 17% were observed.
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Assis, Carlos A. M., Sérgio A. M. Oliveira, Roseane M. Misságia et Marco A. R. de Ceia. « Source wavelet and local wave propagation effects on the amplitude-variation-with-offset response of thin-layer models : A physical modeling study ». GEOPHYSICS 82, no 4 (1 juillet 2017) : N27—N41. http://dx.doi.org/10.1190/geo2016-0262.1.
Texte intégralRésumé :
In target layers with thicknesses below the vertical seismic resolution as thin layers, the tuning effect/interference between the wave propagation modes may increase the challenge of doing amplitude-variation-with-offset (AVO) analysis because it is difficult to recover the primary PP amplitudes embedded in the data by further seismic data processing. Thus, we have investigated the importance of the primary PP reflections, locally P-SV converted waves, and internal multiple reflections in the amplitude response of two thin-layer seismic physical models. One model consists of a thin water layer embedded between two nylon plates, and another model with a thin acrylic layer surrounded by water. Numerical modeling using the reflectivity method was applied to analyze each wave propagation mode and the source waveform role in the experimental data. Before the experimental reflection data acquisition, we characterized two source and receiver piezoelectric transducer (PET) pairs: one with a circular plane face and the other with a semispherical face. We measured the source wavelet, its dominant frequency, and the PETs’ directivity pattern. Semispherical PETs were chosen to acquire common midpoint reflection data. Thereafter, a processing workflow was applied to remove linear events interfering with the target reflections and to correct amplitudes due to transmission losses, source/receiver directivity, and geometric spreading effects. Finally, we investigated the thin-layer targets near incidence angle amplitude and the AVO response. The results showed that the interference between the primary PP reflections and the locally converted shear waves may considerably affect the observed amplitude response. The source wavelet bandwidth appeared as a second-order effect, and the internal multiple reflections were practically negligible. These results suggested that in real data sets, it is important to investigate the wave propagation modes and source wavelet role in the amplitudes observed, before deciding the AVO analysis/inversion workflow that should be adopted.
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Wang, Zijian, Binkai Shi et Chen Fang. « Characterisation of guided wave dispersion in isotropic tubes based on damping finite element boundaries ». Insight - Non-Destructive Testing and Condition Monitoring 65, no 1 (1 janvier 2023) : 28–35. http://dx.doi.org/10.1784/insi.2023.65.1.28.
Texte intégralRésumé :
Guided waves are suitable for non-destructive testing and structural health monitoring of tube-like structures. However, the dispersion phenomenon impedes the application of guided waves. Although the finite element (FE) method can simulate the guided wave propagation and help to study the dispersion phenomenon, boundary reflections can contaminate the wave field of interest and impede the FE simulation. In this paper, damping boundaries are developed as a set of FE frames with gradually increasing damping coefficients to alleviate boundary reflections. The wave signals simulated through the FE model with the damping boundaries only contain the waves from the transmitter to the receiver, without the interferences of the boundary reflections. The energy distribution on the frequency-velocity spectrum of the simulated signals agrees well with the analytical dispersion curves, indicating that the boundary reflections are effectively alleviated. The analytical solution of the guided wave equation and the FE modelling method presented in this paper can facilitate both research and applications of guided waves for tube-like structures.
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Nefedkina, T. V., P. A. Lykhin et G. A. Dugarov. « DETERMINATION OF AZIMUTHAL ANISOTROPIC MEDIA ELASTIC PARAMETERS FROM MULTIWAVE AVOA DATA BY NONLINEAR OPTIMIZATION METHOD ». Russian Journal of geophysical technologies, no 2 (29 janvier 2019) : 14–26. http://dx.doi.org/10.18303/2619-1563-2018-2-2.
Texte intégralRésumé :
In this paper, we investigate optimization algorithm of joint nonlinear AVOA inversion of PP+PS reflections in anisotropic media. Algorithm is based on the exact solution for PP and PS waves reflection coefficients in anisotropic HTI medium. The PP and PS wave’s reflections from the top of the anisotropic layer are examined. We use synthetic seismograms generated by ray method for the algorithm testing. We show that joint compressional and converted wave’s inversion allows increasing the robustness of the method and the accuracy of medium-parameter estimates. Coefficients of anisotropy are determined with better accuracy if signal-to-noise ratio is bigger than 5 for PP wave and bigger than 2 for PS wave.
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Minato, Shohei, Takeshi Tsuji, Toshifumi Matsuoka et Koichiro Obana. « Crosscorrelation of Earthquake Data Using Stationary Phase Evaluation : Insight into Reflection Structures of Oceanic Crust Surface in the Nankai Trough ». International Journal of Geophysics 2012 (2012) : 1–8. http://dx.doi.org/10.1155/2012/101545.
Texte intégralRésumé :
Seismic interferometry (SI) has been recently employed to retrieve the reflection response from natural earthquakes. We perform experimental study to apply SI to Ocean Bottom Seismogram (OBS) records in the Nankai Trough, southwest Japan in order to reveal the relatively shallow geological boundaries including surface of oceanic crust. Although the local earthquakes with short raypath we use to retrieve reflection response are expected to contain the higher-frequency components to detect fine-scale structures by SI, they cannot be assumed as plane waves and are inhomogeneously distributed. Since the condition of inhomogeneous source distribution violates the assumption of SI, the conventional processing yields to the deteriorated subsurface images. Here we adopt the raypath calculation for stationary phase evaluation of SI in order to overcome this problem. To find stationary phase, we estimate the raypaths of two reflections: (1) sea-surfaceP-wave reflection and (2) sea-surface multipleP-wave reflection. From the estimated raypath, we choose the crosscorrelation traces which are expected to produce objective reflections considering the stationary phase points. We use the numerical-modeling data and field data with 6 localized earthquakes and show that choosing the crosscorrelation traces by stationary phase evaluation improves the quality of the reflections of the oceanic crust surface.
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Holzschuh, Josef. « Low‐cost geophysical investigations of a paleochannel aquifer in the Eastern Goldfields, Western Australia ». GEOPHYSICS 67, no 3 (mai 2002) : 690–700. http://dx.doi.org/10.1190/1.1484512.
Texte intégralRésumé :
Compressional (P) wave and shear (S) wave seismic reflection techniques were used to delineate the sand and gravel aquifer within a highly saline clay‐filled paleochannel in the Eastern Goldfields of Western Australia. The seismic refraction and gravity methods were also used to investigate the paleochannel. The unsaturated loose fine‐grained sand up to 10 m in depth at the surface is a major factor in degrading subsurface imaging. The seismic processing needed to be precise, with accurate static corrections and normal moveout corrections. Deconvolution enhanced the aquifer and other paleochannel reflectors. P‐wave reflection and refraction layer depths had good correlation and showed a total of six boundaries: (1) water table, (2) change in velocity (compaction) in the paleochannel sediments, (3) sand and gravel aquifer, (4) red‐brown saprolite and green saprolite boundary, (5) weathered bedrock, and (6) unweathered bedrock. P‐wave explosive and hammer sources were found to have similar signal characteristics, and the aquifer and bedrock were both imaged using the hammer source. The deep shots below the water table have the most broadband frequency response for reflections, but stacking clear reflections was difficult. The S‐wave reflection results showed high lateral and vertical resolution of the basal saprolite clay, the sand and gravel aquifer, and very shallow clays above the aquifer. The S‐wave reflection stacking velocities were 10–20% of the P‐waves, increasing the resolution of the S‐wave section. The gravity data were modelled to fit the known drilling and P‐wave seismic reflection depths. The refraction results did not identify the top of bedrock, so refraction depths were not used for the gravity modeling in this highly weathered environment. The final gravity model mapped the bedrock topography beyond the lateral extent of the seismic and drilling data.
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Lynn, Heloise B., Wallace E. Beckham, K. Michele Simon, C. Richard Bates, M. Layman et Michael Jones. « P-wave and S-wave azimuthal anisotropy at a naturally fractured gas reservoir, Bluebell‐Altamont Field, Utah ». GEOPHYSICS 64, no 4 (juillet 1999) : 1312–28. http://dx.doi.org/10.1190/1.1444636.
Texte intégralRésumé :
Reflection P- and S-wave data were used in an investigation to determine the relative merits and strengths of these two data sets to characterize a naturally fractured gas reservoir in the Tertiary Upper Green River formation. The objective is to evaluate the viability of P-wave seismic to detect the presence of gas‐filled fractures, estimate fracture density and orientation, and compare the results with estimates obtained from the S-wave data. The P-wave response to vertical fractures must be evaluated at different source‐receiver azimuths (travelpaths) relative to fracture strike. Two perpendicular lines of multicomponent reflection data were acquired approximately parallel and normal to the dominant strike of Upper Green River fractures as obtained from outcrop, core analysis, and borehole image logs. The P-wave amplitude response is extracted from prestack amplitude variation with offset (AVO) analysis, which is compared to isotropic‐model AVO responses of gas sand versus brine sand in the Upper Green River. A nine‐component vertical seismic profile (VSP) was also obtained for calibration of S-wave reflections with P-wave reflections, and support of reflection S-wave results. The direction of the fast (S1) shear‐wave component from the reflection data and the VSP coincides with the northwest orientation of Upper Green River fractures, and the direction of maximum horizontal in‐situ stress as determined from borehole ellipticity logs. Significant differences were observed in the P-wave AVO gradient measured parallel and perpendicular to the orientation of Upper Green River fractures. Positive AVO gradients were associated with gas‐producing fractured intervals for propagation normal to fractures. AVO gradients measured normal to fractures at known waterwet zones were near zero or negative. A proportional relationship was observed between the azimuthal variation of the P-wave AVO gradient as measured at the tops of fractured intervals, and the fractional difference between the vertical traveltimes of split S-waves (the “S-wave anisotropy”) of the intervals.
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Bourgault, Daniel, David C. Janes et Peter S. Galbraith. « Observations of a Large-Amplitude Internal Wave Train and Its Reflection off a Steep Slope ». Journal of Physical Oceanography 41, no 3 (1 mars 2011) : 586–600. http://dx.doi.org/10.1175/2010jpo4464.1.
Texte intégralRésumé :
Abstract Remote and in situ field observations documenting the reflection of a normally incident, short, and large-amplitude internal wave train off a steep slope are presented and interpreted with the help of the Dubreil–Jacotin–Long theory. Of the seven remotely observed waves that composed the incoming wave train, five were observed to reflect. It is estimated that the incoming wave train carried Ei = (24 ± 4) × 104 J m−1 to the boundary. The reflection coefficient, defined as the ratio of reflected to incoming wave train energies, is estimated to be R = 0.5 ± 0.2. This is about 0.4 lower than parameterizations in the literature, which are based on reflections of single solitary waves, would suggest. It is also shown that the characteristics of the wave-boundary situation observed in the field are outside the parameter space examined in previous laboratory and numerical experiments on internal solitary wave reflectance. This casts doubts on extrapolating current laboratory-based knowledge to fjord-like systems and calls for more research on internal solitary wave reflectance.