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Kim H. Parker - One of the best experts on this subject based on the ideXlab platform.

  • experimental evaluation of local wave speed in the presence of Reflected Waves
    Journal of Biomechanics, 2014
    Co-Authors: Alessandra Borlotti, Kim H. Parker, Ashraf W Khir
    Abstract:

    Wave speed (also called pulse wave velocity) is the speed by which disturbance travels along the medium and it depends on the mechanical and geometrical properties of the vessel and on the density of the blood. Wave speed is a parameter of clinical relevance because it is an indicator of arterial stiffness and cardiovascular diseases. The aim of this work is to compare different methods for the determination of local wave speed in bench experiments and investigate their relative accuracy when reflections are present. Pressure (P), flow (Q) and diameter (D) were measured along a flexible tube far and close to three positive and three negative reflection sites. Wave speed was calculated using PU-loop, (lnD)U-loop, QAloop, D 2 P-loop, sum of squares and characteristic impedance methods. Results were compared to the foot-to-foot method. We found that far from the reflections almost all methods give uniform results. Close to positive reflections the methods that rely on P and Q (or U) overestimate the wave speed value, while techniques based on D (or A) and Q (or U) underestimate it. On the contrary, close to negative reflections the methods that rely on P and Q (or U) underestimate the wave speed value, while techniques based on D (or A) and Q (or U) overestimate it. The D 2 P-loop does not seem to be affected by positive or negative

  • Augmentation of Coronary Blood Flow in Systole by Reflected Waves in the Proximal Aorta
    IFMBE Proceedings, 2009
    Co-Authors: Justin E. Davies, Kim H. Parker, Davies P. Francis, Nearchos Hadjiloizou, Zachary I. Whinnett, Charlotte Manisty, J Aguado-sierra, Iqbal S. Malik, Alun D. Hughes, Jamil Mayet
    Abstract:

    It has been proposed that blood flow in coronary arteries is augmented by backward travelling (Reflected) Waves in the proximal aorta that arise from distal reflection sites and propagate as forward travelling Waves in the coronary arteries. However, these Waves have never been identified and measured. We used wave intensity analysis to quantify forward and backward travelling Waves in the proximal aorta and coronary arteries at various times in the cardiac cycle, and assess their relationship to changes in coronary blood flow. In 19 subjects (age 35-73 years), sensor-tipped intra-arterial wires were used to measure pressure and flow velocity in the coronary arteries and in the proximal aorta. A backward travelling (Reflected) wave was seen in the proximal aorta in systole in all subjects (159±12ms after the ECGR-wave). A forward travelling wave that corresponded to this Reflected wave could be seen in the coronary arteries approximately 30ms later: left main stem (187±11 ms), left anterior descending (194 ±7 ms) and circumflex (189 ±7 ms) . In the coronary arteries the wave that corresponded to the Reflected wave in the aorta represented 20.3±2.1% of the peak intensity of the wave generated by ventricular systole (incident wave), and was associated with a 38.9±8.4% instantaneous increase in systolic coronary blood flow velocity (0.2 to 0.28m/s, p

  • simultaneous determination of wave speed and arrival time of Reflected Waves using the pressure velocity loop
    Medical & Biological Engineering & Computing, 2007
    Co-Authors: Ashraf W Khir, M J P Swalen, Jiling Feng, Kim H. Parker
    Abstract:

    In a previous paper we demonstrated that the linear portion of the pressure–velocity loop (PU-loop) corresponding to early systole could be used to calculate the local wave speed. In this paper we extend this work to show that determination of the time at which the PU-loop first deviates from linearity provides a convenient way to determine the arrival time of Reflected Waves (Tr). We also present a new technique using the PU-loop that allows for the determination of wave speed and Tr simultaneously. We measured pressure and flow in elastic tubes of different diameters, where a strong reflection site existed at known distances away form the measurement site. We also measured pressure and flow in the ascending aorta of 11 anaesthetised dogs where a strong reflection site was produced through total arterial occlusion at four different sites. Wave speed was determined from the initial slope of the PU-loop and Tr was determined using a new algorithm that detects the sampling point at which the initial linear part of the PU-loop deviates from linearity. The results of the new technique for detecting Tr were comparable to those determined using the foot-to-foot and wave intensity analysis methods. In elastic tubes Tr detected using the new algorithm was almost identical to that detected using wave intensity analysis and foot-to-foot methods with a maximum difference of 2%. Tr detected using the PU-loop in vivo highly correlated with that detected using wave intensity analysis (r 2 = 0.83, P < 0.001). We conclude that the new technique described in this paper offers a convenient and objective method for detecting Tr, and allows for the dynamic determination of wave speed and Tr, simultaneously.

  • Differentiation of stenosed and aneurysmal arteries by pulse wave propagation analysis based on a fluid-solid interaction computational method
    Technology and Health Care, 2007
    Co-Authors: Tomohiro Fukui, Ken-ichi Tsubota, Kim H. Parker, Shigeo Wada, Takami Yamaguchi
    Abstract:

    Pulse Wave Velocity (PWV) is recognized by clinicians as an index of the mechanical properties of human blood vessels. However, the measured PWV of real human blood vessels will not always obey the Moens-Korteweg equation, which describes the PWV in ideal elastic tubes. Waveform analysis has been studied as an alternative diagnosis for cardiovascular disease, and Reflected Waves that occur in the diseased region may be a key for the estimation of the severity of disease. In this study, we modeled stenosed and aneurysmal arteries in a three-dimensional coupled fluid-solid interaction scheme, and analyzed the pulse wave propagation in order to assess the Reflected Waves that occurred in the diseased region. A commercial code (Radioss, MECALOG, France) was used to solve the fluid-solid interactions. A steady flow with Reynolds number 1000 was imposed at the inlet of the artery as the basic flow, then a single rectangular pulse with Reynolds number 4000 was imposed upon the basic flow to produce a propagating wave. We showed that the Reflected Waves from the stenosis and the aneurysm are different in their phase, and the wavelength of the Reflected Waves from the aneurysm is affected by the aneurysm length.

  • time domain representation of ventricular arterial coupling as a windkessel and wave system
    American Journal of Physiology-heart and Circulatory Physiology, 2003
    Co-Authors: Jiunjr Wang, Kim H. Parker, Aoife B Obrien, Nigel G Shrive, John V Tyberg
    Abstract:

    The differences in shape between central aortic pressure (PAo) and flow waveforms have never been explained satisfactorily in that the assumed explanation (substantial Reflected Waves during diasto...

Jean M. Virieux - One of the best experts on this subject based on the ideXlab platform.

  • full waveform inversion of diving Reflected Waves for velocity model building with impedance inversion based on scale separation
    Geophysical Journal International, 2015
    Co-Authors: Wei Zhou, Romain Brossier, Stéphane Operto, Jean M. Virieux
    Abstract:

    Full waveform inversion (FWI) aims to reconstruct high-resolution subsurface models from the full wavefield, which includes diving Waves, post-critical reflections and short-spread reflections. Most successful applications of FWI are driven by the information carried by diving Waves and post-critical reflections to build the long-to-intermediate wavelengths of the velocity structure. Alternative approaches, referred to as reflection waveform inversion (RWI), have been recently revisited to retrieve these long-to-intermediate wavelengths from short-spread reflections by using some prior knowledge of the reflectivity and a scale separation between the velocity macromodel and the reflectivity. This study presents a unified formalism of FWI, named as Joint FWI, whose aim is to efficiently combine the diving and Reflected Waves for velocity model building. The two key ingredients of Joint FWI are, on the data side, the explicit separation between the short-spread reflections and the wide-angle arrivals and, on the model side, the scale separation between the velocity macromodel and the short-scale impedance model. The velocity model and the impedance model are updated in an alternate way by Joint FWI and waveform inversion of the reflection data (least-squares migration), respectively. Starting from a crude velocity model, Joint FWI is applied to the streamer seismic data computed in the synthetic Valhall model. While the conventional FWI is stuck into a local minimum due to cycle skipping, Joint FWI succeeds in building a reliable velocity macromodel. Compared with RWI, the use of diving Waves in Joint FWI improves the reconstruction of shallow velocities, which translates into an improved imaging at deeper depths. The smooth velocity model built by Joint FWI can be subsequently used as a reliable initial model for conventional FWI to increase the high-wavenumber content of the velocity model.

  • Full waveform inversion of diving & Reflected Waves for velocity model building with impedance inversion based on scale separation
    Geophysical Journal International, 2015
    Co-Authors: Wei Zhou, Romain Brossier, Stéphane Operto, Jean M. Virieux
    Abstract:

    Full waveform inversion (FWI) aims to reconstruct high-resolution subsurface models from the full wavefield, which includes diving Waves, post-critical reflections and short-spread reflections. Most successful applications of FWI are driven by the information carried by diving Waves and post-critical reflections to build the long-to-intermediate wavelengths of the velocity structure. Alternative approaches, referred to as reflection waveform inversion (RWI), have been recently revisited to retrieve these long-to-intermediate wavelengths from short-spread reflections by using some prior knowledge of the reflectivity and a scale separation between the velocity macromodel and the reflectivity. This study presents a unified formalism of FWI, named as Joint FWI, whose aim is to efficiently combine the diving and Reflected Waves for velocity model building. The two key ingredients of Joint FWI are, on the data side, the explicit separation between the short-spread reflections and the wide-angle arrivals and, on the model side, the scale separation between the velocity macromodel and the short-scale impedance model. The velocity model and the impedance model are updated in an alternate way by Joint FWI and waveform inversion of the reflection data (least-squares migration), respectively. Starting from a crude velocity model, Joint FWI is applied to the streamer seismic data computed in the synthetic Valhall model. While the conventional FWI is stuck into a local minimum due to cycle skipping, Joint FWI succeeds in building a reliable velocity macromodel. Compared with RWI, the use of diving Waves in Joint FWI improves the reconstruction of shallow velocities, which translates into an improved imaging at deeper depths. The smooth velocity model built by Joint FWI can be subsequently used as a reliable initial model for conventional FWI to increase the high-wavenumber content of the velocity model.

  • Near-surface Full Waveform Inversion Using Surface Waves and Reflected Waves
    2014
    Co-Authors: Isabella Masoni, Wei Zhou, Romain Brossier, Ludovic Métivier, Stéphane Operto, Jean M. Virieux
    Abstract:

    We investigate the capacity of extracting near-surface shear-wave velocity by considering dispersive surface Waves and non-dispersive Reflected Waves. We show that indeed the full waveform fitting of these Waves requires a dedicated approach by using lateral spatial and frequential coherence for surface Waves and by explicitely introduces the fitting of Reflected Waves in the inversion formulation. On a simple example as a two-layers model, lateral variations of the velocity are reconstructed while the low-wavenumber content of the velocity could be improved through reflection Waves. Combining these two sources of information on the shear-wave velocity could improve our shear-wave velocity imaging in the near-surface context.

  • toward data domain waveform inversion of Reflected Waves
    Seg Technical Program Expanded Abstracts, 2013
    Co-Authors: Romain Brossier, Jean M. Virieux, Stéphane Operto
    Abstract:

    Full Waveform Inversion (FWI) is becoming a powerful tool for quantitative seismic imaging from wide-azimuth seismic data. The method is today mainly used as a high-resolution tomography of the Earth zones sampled by both Reflected and diving events. However, the classical formulation of FWI prevents the reconstruction of the low part of the wavenumber spectrum of the velocity model from reflection-only data. The use of reflections in waveform inversion is a crucial challenge for deep imaging. This study, through a simple canonical example, first analyses the issue of classical waveform inversion when applied on reflection data. Then we show that, if a prior knowledge of the reflectivity is available, Reflected Waves can be exploited to retrieve the long wavelengths of the velocity, in particular when we use appropriate misfit measurement as the cross-correlation that overcome cycle-skipping issues, and inversion domain as the pseudo-depth domain that preserves the invariant property of zero-offset time.

Guohai Dong - One of the best experts on this subject based on the ideXlab platform.

  • separation of obliquely incident and Reflected irregular Waves by the morlet wavelet transform
    Coastal Engineering, 2011
    Co-Authors: Guohai Dong
    Abstract:

    Abstract An existing 2D time-domain method for separating irregular incident and Reflected Waves by wavelet transform [Ma et al., 2010. A new method for separation of 2D incident and Reflected Waves by the Morlet wavelet transform. Coastal Eng., 57(6):597–603] is extended to account for obliquely incident irregular Waves propagating over sloping bottoms. The linear shoaling and refraction coefficients are adopted to determine the amplitude and phase changes of Waves. The optimal central frequency of the Morlet wavelet is determined by the minimum Shannon wavelet entropy. Numerical tests show that the present method can accurately separate Waves over horizontal depths. For Waves at sloping bottoms, however, the separation errors increase as bottom slope increases and are significant for Waves with incident angle larger than π /3.

  • a new method for separation of 2d incident and Reflected Waves by the morlet wavelet transform
    Coastal Engineering, 2010
    Co-Authors: Guohai Dong, Gang Wang
    Abstract:

    A new method based on the Morlet wavelet transform for separating a 2D wave field into incident and Reflected Waves is proposed in this paper. The principle of this method, first, is derived for constant depths. Then, using the linear shoaling theory, the method is extended to an arbitrary sloping bathymetry. Owing to the time-frequency characteristic of wavelet transform, the present method can separate Waves in the real time domain and is not confined by the stationary assumption of Waves. The efficiency and accuracy of this method are demonstrated using numerical simulated data.

Wei Zhou - One of the best experts on this subject based on the ideXlab platform.

  • full waveform inversion of diving Reflected Waves for velocity model building with impedance inversion based on scale separation
    Geophysical Journal International, 2015
    Co-Authors: Wei Zhou, Romain Brossier, Stéphane Operto, Jean M. Virieux
    Abstract:

    Full waveform inversion (FWI) aims to reconstruct high-resolution subsurface models from the full wavefield, which includes diving Waves, post-critical reflections and short-spread reflections. Most successful applications of FWI are driven by the information carried by diving Waves and post-critical reflections to build the long-to-intermediate wavelengths of the velocity structure. Alternative approaches, referred to as reflection waveform inversion (RWI), have been recently revisited to retrieve these long-to-intermediate wavelengths from short-spread reflections by using some prior knowledge of the reflectivity and a scale separation between the velocity macromodel and the reflectivity. This study presents a unified formalism of FWI, named as Joint FWI, whose aim is to efficiently combine the diving and Reflected Waves for velocity model building. The two key ingredients of Joint FWI are, on the data side, the explicit separation between the short-spread reflections and the wide-angle arrivals and, on the model side, the scale separation between the velocity macromodel and the short-scale impedance model. The velocity model and the impedance model are updated in an alternate way by Joint FWI and waveform inversion of the reflection data (least-squares migration), respectively. Starting from a crude velocity model, Joint FWI is applied to the streamer seismic data computed in the synthetic Valhall model. While the conventional FWI is stuck into a local minimum due to cycle skipping, Joint FWI succeeds in building a reliable velocity macromodel. Compared with RWI, the use of diving Waves in Joint FWI improves the reconstruction of shallow velocities, which translates into an improved imaging at deeper depths. The smooth velocity model built by Joint FWI can be subsequently used as a reliable initial model for conventional FWI to increase the high-wavenumber content of the velocity model.

  • Full waveform inversion of diving & Reflected Waves for velocity model building with impedance inversion based on scale separation
    Geophysical Journal International, 2015
    Co-Authors: Wei Zhou, Romain Brossier, Stéphane Operto, Jean M. Virieux
    Abstract:

    Full waveform inversion (FWI) aims to reconstruct high-resolution subsurface models from the full wavefield, which includes diving Waves, post-critical reflections and short-spread reflections. Most successful applications of FWI are driven by the information carried by diving Waves and post-critical reflections to build the long-to-intermediate wavelengths of the velocity structure. Alternative approaches, referred to as reflection waveform inversion (RWI), have been recently revisited to retrieve these long-to-intermediate wavelengths from short-spread reflections by using some prior knowledge of the reflectivity and a scale separation between the velocity macromodel and the reflectivity. This study presents a unified formalism of FWI, named as Joint FWI, whose aim is to efficiently combine the diving and Reflected Waves for velocity model building. The two key ingredients of Joint FWI are, on the data side, the explicit separation between the short-spread reflections and the wide-angle arrivals and, on the model side, the scale separation between the velocity macromodel and the short-scale impedance model. The velocity model and the impedance model are updated in an alternate way by Joint FWI and waveform inversion of the reflection data (least-squares migration), respectively. Starting from a crude velocity model, Joint FWI is applied to the streamer seismic data computed in the synthetic Valhall model. While the conventional FWI is stuck into a local minimum due to cycle skipping, Joint FWI succeeds in building a reliable velocity macromodel. Compared with RWI, the use of diving Waves in Joint FWI improves the reconstruction of shallow velocities, which translates into an improved imaging at deeper depths. The smooth velocity model built by Joint FWI can be subsequently used as a reliable initial model for conventional FWI to increase the high-wavenumber content of the velocity model.

  • Near-surface Full Waveform Inversion Using Surface Waves and Reflected Waves
    2014
    Co-Authors: Isabella Masoni, Wei Zhou, Romain Brossier, Ludovic Métivier, Stéphane Operto, Jean M. Virieux
    Abstract:

    We investigate the capacity of extracting near-surface shear-wave velocity by considering dispersive surface Waves and non-dispersive Reflected Waves. We show that indeed the full waveform fitting of these Waves requires a dedicated approach by using lateral spatial and frequential coherence for surface Waves and by explicitely introduces the fitting of Reflected Waves in the inversion formulation. On a simple example as a two-layers model, lateral variations of the velocity are reconstructed while the low-wavenumber content of the velocity could be improved through reflection Waves. Combining these two sources of information on the shear-wave velocity could improve our shear-wave velocity imaging in the near-surface context.

Ashraf W Khir - One of the best experts on this subject based on the ideXlab platform.

  • experimental evaluation of local wave speed in the presence of Reflected Waves
    Journal of Biomechanics, 2014
    Co-Authors: Alessandra Borlotti, Kim H. Parker, Ashraf W Khir
    Abstract:

    Wave speed (also called pulse wave velocity) is the speed by which disturbance travels along the medium and it depends on the mechanical and geometrical properties of the vessel and on the density of the blood. Wave speed is a parameter of clinical relevance because it is an indicator of arterial stiffness and cardiovascular diseases. The aim of this work is to compare different methods for the determination of local wave speed in bench experiments and investigate their relative accuracy when reflections are present. Pressure (P), flow (Q) and diameter (D) were measured along a flexible tube far and close to three positive and three negative reflection sites. Wave speed was calculated using PU-loop, (lnD)U-loop, QAloop, D 2 P-loop, sum of squares and characteristic impedance methods. Results were compared to the foot-to-foot method. We found that far from the reflections almost all methods give uniform results. Close to positive reflections the methods that rely on P and Q (or U) overestimate the wave speed value, while techniques based on D (or A) and Q (or U) underestimate it. On the contrary, close to negative reflections the methods that rely on P and Q (or U) underestimate the wave speed value, while techniques based on D (or A) and Q (or U) overestimate it. The D 2 P-loop does not seem to be affected by positive or negative

  • simultaneous determination of wave speed and arrival time of Reflected Waves using the pressure velocity loop
    Medical & Biological Engineering & Computing, 2007
    Co-Authors: Ashraf W Khir, M J P Swalen, Jiling Feng, Kim H. Parker
    Abstract:

    In a previous paper we demonstrated that the linear portion of the pressure–velocity loop (PU-loop) corresponding to early systole could be used to calculate the local wave speed. In this paper we extend this work to show that determination of the time at which the PU-loop first deviates from linearity provides a convenient way to determine the arrival time of Reflected Waves (Tr). We also present a new technique using the PU-loop that allows for the determination of wave speed and Tr simultaneously. We measured pressure and flow in elastic tubes of different diameters, where a strong reflection site existed at known distances away form the measurement site. We also measured pressure and flow in the ascending aorta of 11 anaesthetised dogs where a strong reflection site was produced through total arterial occlusion at four different sites. Wave speed was determined from the initial slope of the PU-loop and Tr was determined using a new algorithm that detects the sampling point at which the initial linear part of the PU-loop deviates from linearity. The results of the new technique for detecting Tr were comparable to those determined using the foot-to-foot and wave intensity analysis methods. In elastic tubes Tr detected using the new algorithm was almost identical to that detected using wave intensity analysis and foot-to-foot methods with a maximum difference of 2%. Tr detected using the PU-loop in vivo highly correlated with that detected using wave intensity analysis (r 2 = 0.83, P < 0.001). We conclude that the new technique described in this paper offers a convenient and objective method for detecting Tr, and allows for the dynamic determination of wave speed and Tr, simultaneously.

  • measurements of wave speed and Reflected Waves in elastic tubes and bifurcations
    Journal of Biomechanics, 2002
    Co-Authors: Ashraf W Khir, Kim H. Parker
    Abstract:

    Abstract Wave intensity analysis is a time domain method for studying Waves in elastic tubes. Testing the ability of the method to extract information from complex pressure and velocity waveforms such as those generated by a wave passing through a mismatched elastic bifurcation is the primary aim of this research. The analysis provides a means for separating forward and backward Waves, but the separation requires knowledge of the wave speed. The PU-loop method is a technique for determining the wave speed from measurements of pressure and velocity, and investigating the relative accuracy of this method is another aim of this research. We generated a single semi-sinusoidal wave in long elastic tubes and measured pressure and velocity at the inlet, and pressure at the exit of the tubes. In our experiments, the results of the PU-loop and the traditional foot-to-foot methods for determining the wave speed are comparable and the difference is on the order of 2.9±0.8%. A single semi-sinusoidal wave running through a mismatched elastic bifurcation generated complicated pressure and velocity waveforms. By using wave intensity analysis we have decomposed the complex waveforms into simple information of the times and magnitudes of Waves passing by the observation site. We conclude that wave intensity analysis and the PU-loop method combined, provide a convenient, time-based technique for analysing Waves in elastic tubes.