Showing papers on "Phase velocity published in 2014"

Slow-Wave Substrate Integrated Waveguide

Abstract: This paper describes a new concept of substrate integrated waveguide (SIW): a slow-wave substrate integrated waveguide (SW-SIW). Compared to a conventional SIW, the proposed topology requires a double-layer substrate with a bottom layer including internal metallized via-holes connected to the bottom conductive plane. The slow-wave effect is obtained by the physical separation of electric and magnetic fields in the structure. Electromagnetic simulations show that this topology of SIW allows decreasing the longitudinal dimension by more than 40% since the phase velocity is significantly smaller than that of a classical SIW. Simultaneously, the lateral dimension of the waveguide is also reduced. By considering a double-layer technology, SW-SIWs exhibiting a cutoff frequency of 9.3 GHz were designed, fabricated, and measured. The transversal dimension and the phase velocity of the proposed SW-SIW are both reduced by 40% as compared to a classical SIW designed for the same cutoff frequency, leading to a significant surface reduction. Moreover, an original kind of taper is proposed to achieve a good return loss when the SW-SIW is fed by a microstrip transmission line.

Opposition control within the resolvent analysis framework

Abstract: This paper extends the resolvent analysis of McKeon & Sharma (J. Fluid Mech., vol. 658, 2010, pp. 336–382) to consider flow control techniques that employ linear control laws, focusing on opposition control (Choi, Moin & Kim, J. Fluid Mech., vol. 262, 1994, pp. 75–110) as an example. Under this formulation, the velocity field for turbulent pipe flow is decomposed into a series of highly amplified (rank-1) response modes, identified from a gain analysis of the Fourier-transformed Navier– Stokes equations. These rank-1 velocity responses represent propagating structures of given streamwise/spanwise wavelength and temporal frequency, whose wall-normal footprint depends on the phase speed of the mode. Opposition control, introduced via the boundary condition on wall-normal velocity, affects the amplification characteristics (and wall-normal structure) of these response modes; a decrease in gain indicates mode suppression, which leads to a decrease in the drag contribution from that mode. With basic assumptions, this rank-1 model reproduces trends observed in previous direct numerical simulation and large eddy simulation, without requiring high-performance computing facilities. Further, a wavenumber–frequency breakdown of control explains the deterioration of opposition control performance with increasing sensor elevation and Reynolds number. It is shown that slower-moving modes localized near the wall (i.e. attached modes) are suppressed by opposition control. Faster-moving detached modes, which are more energetic at higher Reynolds number and more likely to be detected by sensors far from the wall, are further amplified. These faster-moving modes require a phase lag between sensor and actuator velocity for suppression. Thus, the effectiveness of opposition control is determined by a trade-off between the modes detected by the sensor. However, it may be possible to develop control strategies optimized for individual modes. A brief exploration of such mode-optimized control suggests the potential for significant performance improvement.

3D Structures of equatorial waves and the resulting superrotation in the atmosphere of a tidally locked hot Jupiter

Abstract: Three-dimensional equatorial trapped waves excited by stellar isolation and the resulting equatorial superrotating jet in a vertical stratified atmosphere of a tidally-locked hot Jupiter are investigated. Taking the hot Jupiter HD 189733b as a fiducial example, we analytically solve a set of linear equations subject to stationary stellar heating with a uniform zonal-mean flow included. We also extract wave information in the final equilibrium state of the atmosphere from the radiative hydrodynamical simulation for HD 189733b by Dobbs-Dixon & Agol (2013). We find that the analytic wave solutions are able to qualitatively explain the three-dimensional simulation results. Studying the vertical structure of waves allows us to explore new wave features such as the westward tilt of wavefronts related to the Rossby-wave resonance as well as double gyres of dispersive Rossby waves. We also make an attempt to apply our linear wave analysis to explain some numerical features associated with the equatorial jet development seen in the GCM by Showman & Polvani (2011). During the spin-up phase of the equatorial jet, the acceleration of the jet as a result of the divergence of the wave momentum flux can be in principle boosted by the Rossby-wave resonance as the superrotating jet speed is close to the phase speed of the Rossby waves. However, we also find that as the jet speed increases, the Rossby-wave structure shifts eastward, while the Kelvin-wave structure remains approximately stationary, leading to the decline of the acceleration rate. Our analytic model of jet evolution implies that there exists only one stable equilibrium state of the atmosphere, possibly implying that the final state of the atmosphere is independent of initial conditions in the linear regime. Limitations and future improvements are also discussed.

Time-reverse modelling of acoustic wave propagation in attenuating media

Abstract: SUMMARY Time-reverse modelling (TRM) of acoustic wave propagation has been widely implemented in seismic migration and time-reversal source imaging. The basic assumption of this modelling is that the wave equation is time-invariant in non-attenuating media. In the Earth, attenuation often invalidates this assumption of time-invariance. To overcome this problem, I propose a TRM approach that compensates for attenuation and dispersion effects during the wave propagation in attenuating media. This approach is based on a viscoacoustic wave equation which explicitly separates attenuation and dispersion following a constant-Q model. Compensating for attenuation and dispersion during TRM is achieved by reversing the sign of the attenuation operator coefficient while leaving the counterpart dispersion parameter unchanged in this viscoacoustic wave equation. A low-pass filter is included to avoid amplifying high-frequency noise during TRM. I demonstrate the effects of the filter on the attenuation and the phase velocity by comparing with theoretical solutions in a 1-D Pierre shale homogeneous medium. Three synthetic examples are used to demonstrate the feasibility of attenuation compensation during TRM. The first example uses a 1-D homogeneous model to demonstrate the accuracy of the numerical implementation of the methodology. The second example shows the applicability of source location using a 2-D layering model. The last example uses a 2-D cross-well synthetic experiment to show that the methodology can also be implemented in conjunction with reverse-time migration to image subsurface reflectors. When attenuation compensation is included, I find improved estimation of the source location, the excitation timing of the point source, the magnitude of the focused source wavelet and the reflectivity image of reflectors, particularly for deep structures underneath strongly attenuating zones.

Multilayer magnetostrictive structure based surface acoustic wave devices

Abstract: This study addresses the experimental and theoretical investigations of guided elastic waves propagation in piezo-magnetic multi-layered structure. The structure is composed of a 20×TbCo2(5nm)/FeCo(5nm) nanostructured multi-layer deposited between two Aluminum (Al) Inter-Digitals Transducers forming a surface acoustic wave delay line, on a Y-cut LiNbO3 substrate. We compare the calculated and measured phase velocity variation under the action of the external magnetic field orientation and magnitude. We find quantitative agreement between the measured and modeled phase velocity shift for all external magnetic field configurations (hard axis and easy axis) and for different shape modes of elastic waves at their first and third harmonic operation frequencies. The shear horizontal mode exhibits a maximum phase velocity shift close to 20% for a ratio close to 1 between magneto-elastic film thickness and wavelength.

High-Speed Silicon Modulator With Slow-Wave Electrodes and Fully Independent Differential Drive

Abstract: We demonstrate a fully independent differential-drive capable of traveling-wave modulator in silicon using slow-wave transmission line electrode. The reported 3.5-mm device achieves a bandwidth of 27 GHz at $-$ 1 V bias with 7.8-V small signal $V_\pi$ and 50- $\Omega$ impedance. Raising the impedance to this extent requires effectively expanding the RF mode size and radically changes the RF phase velocity, but we show that this can be done with minimal crosstalk effects between the two arms and overall velocity mismatch, and thus, with a high EO bandwidth achieved. 40-Gb/s operation is demonstrated with 1.6-Vpp differential-drive, and performance comparisons to Lithium Niobate modulators are made.

Modal Stability TheoryLecture notes from the FLOW-NORDITA Summer School on Advanced Instability Methods for Complex Flows, Stockholm, Sweden, 2013

Abstract: This article contains a review of modal stability theory. It covers local stability analysis of parallel flows including temporal stability, spatial stability, phase velocity, group velocity, spati ...

Frequency and damping rate of fast sausage waves

Abstract: We investigate the frequency and damping rate of fast axisymmetric waves that are subject to wave leakage for a one-dimensional magnetic cylindrical structure in the solar corona. We consider the ideal magnetohydrodynamic (MHD) dispersion relation for axisymmetric MHD waves superimposed on a straight magnetic cylinder in the zero β limit, similar to a jet or loop in the solar corona. An analytic study accompanied by numerical calculations has been carried out to model the frequency, damping rate, and phase speed of the sausage wave around the cut-off frequency and in the long wavelength limit. Analytic expressions have been obtained based on equations around the points of interest. They are linear approximations of the dependence of the sausage frequency on the wave number around the cut-off wavelength for both leaky and non-leaky regimes and in the long wavelength limit. Moreover, an expression for the damping rate of the leaky sausage wave has been obtained both around the cut-off frequency and in the long wavelength limit. These analytic results are compared with numerical computations. The expressions show that the complex frequencies are mainly dominated by the density ratio. In addition, it is shown that the damping eventually becomes independent of the wave number in the long wavelength limit. We conclude that the sausage mode damping directly depends on the density ratios of the internal and external media where the damping declines in higher density contrasts. Even in the long wavelength limit, the sausage mode is weakly damped for high-density contrasts. As such, sausage modes could be observed for a significant number of periods in high-density contrast loops or jets.

Understanding Midlatitude Jet Variability and Change Using Rossby Wave Chromatography: Poleward-Shifted Jets in Response to External Forcing

Abstract: Rossby wave chromatography (RWC) is implemented in a linearized barotropic model as a tool to understand the response of the midlatitude jet to external forcing. Given the background zonal-mean flow and the space‐time structure of the baroclinic wave activity source, RWC calculates the space‐time structure of the upper-tropospheric eddy momentum fluxes. RWC is used to diagnose and understand the poleward shift of the jet in an idealized GCM using the convergence of the vertical EP flux in the upper troposphere as the wave activity source. The poleward-shifted jet is maintained via a selective ‘‘reflecting level’’ on the poleward flank of jet: for a given wavenumber, low phase speed waves are reflected but high phase speed waves are absorbed at the critical level on the polewardflank of jet. When the zonal-mean zonal wind increases on the polewardflank of the jet, a wider range of poleward-propagating waves encounter a reflecting level instead of a critical level on the poleward flank. The increased wave reflection leads to increased equatorward-propagating waves (and, therefore, poleward momentum flux) across the jet. Increases in wave phase speeds directly oppose the poleward shift because, in addition to the well-recognized effect of phase speed on wave dissipation in the subtropics, increased phase speeds imply more wave dissipation rather than reflection on the poleward flank via the selective reflecting level.

Seismic velocity structure and anisotropy of the Alaska subduction zone based on surface wave tomography

Abstract: Southcentral Alaska is a complex tectonic region that transitions from subduction of Pacific crust to flat slab subduction—and collision—of overthickened Yakutat crust. Because much of the Yakutat crust has been subducted, seismic imaging is needed in order to understand the crustal and upper mantle structural framework for this active tectonic setting. Here we use teleseismic Rayleigh waves to image large-scale variations in shear wave structure. Our imaging technique employs a two-plane wave representation with finite frequency sensitivity kernels. Our 3-D isotropic model reveals several features: the subducting Pacific/Yakutat slab, slow wave speeds characterizing the onshore Yakutat collision zone, slow wave speeds of the Wrangell subduction zone, and a deep tomographic contrast at the eastern edge of the Pacific/Yakutat slab. We produce anisotropic phase velocity maps that exhibit variations in the fast direction of azimuthal anisotropy. These maps show the dominance of the Yakutat slab on the observed pattern of anisotropy. West of the Yakutat slab the fast directions are approximately aligned with the plate convergence direction. In the region of the Yakutat slab the pattern is more complicated. Along the margins of the slab the fast directions are roughly parallel to the margins. We identify notable differences and similarities with published SKS splitting measurements. Integrative modeling using 3-D anisotropy models and different seismic measurements will be needed in order to establish a detailed 3-D anisotropic velocity model for Alaska. This study provides a large-scale starting point for such an effort.

Quasi-coherent modes and electron-driven turbulence

Abstract: This letter reports on quasi-coherent (QC) modes observed in fluctuation spectra from Tore Supra and TEXTOR reflectometers. QC modes have characteristics in between coherent and broad-band fluctuations as they oscillate around a given frequency but have a wide spectrum. They are ballooned at the LFS midplane and appear usually on a frequency ranging from 30 to 120 kHz. In ohmic plasmas from both tokamaks, QC modes are detected only in linear ohmic confinement (LOC) regime and disappear in saturated ohmic confinement (SOC) regime. Linear simulations from Tore Supra predict that the LOC and SOC regimes are dominated by electron and ion modes respectively. Measurements of the perpendicular velocity of density fluctuations have been made from the top of TEXTOR by poloidal correlation reflectometry. They suggest that QC modes have a phase velocity similar to 400 ms(-1) higher in the electron diamagnetic direction than lower frequency fluctuations. Additionally, the onset of QC modes during electron cyclotron resonance heating has been observed in a Tore Supra region where turbulence is suspected to be driven by electron modes. These experimental results and instability calculations show a correlation between onsets of QC modes and predictions of trapped electron modes.

Longitudinal wave propagation in a piezoelectric nanoplate considering surface effects and nonlocal elasticity theory

Abstract: The propagation characteristics of the longitudinal wave in a piezoelectric nanoplate were investigated in this study. The nonlocal elasticity theory was used and the surface effects were taken into account. In addition, the group velocity and phase velocity were derived and investigated, respectively. The dispersion relation was analyzed with different scale coefficients, wavenumbers, and voltages. The results showed that the dispersion degree can be strengthened by increasing the wavenumber and scale coefficient.

Nonlocal piezoelasticity based wave propagation of bonded double-piezoelectric nanobeam-systems

Abstract: Piezoelectric nanobeam (PNB) offer the possibility of being used in micro-electromechanical systems and nano-electromechanical systems and the dynamic testing of such structures often produces stress wave propagation in them. This work concerns with the size-dependent wave propagation of double-piezoelectric nanobeam-systems (DPNBSs) based on Euler–Bernoulli beam model. The two piezoelectric nanobeams are coupled by an enclosing elastic medium which is simulated by Pasternak foundation. Nonlocal piezoelasticity theory is used to derive the general differential equation based on Hamilton’s principal to include those scale effects. Particular attention is paid to the wave propagation piezoelectric control of the coupled system in three cases namely in-phase wave propagation, out-of-phase wave propagation and wave propagation when one PNB is stationary. In three mentioned cases, an analytical method is proposed to obtain phase velocity; cut-off and escape frequencies of the DPNBSs. Results indicate that the imposed external voltage is an effective controlling parameter for wave propagation of the coupled system. Furthermore, the phase velocity of in-phase wave propagation is independent of elastic medium stiffness.

Properties of Kelvin‐Helmholtz waves at the magnetopause under northward interplanetary magnetic field: Statistical study

Abstract: We search the plasma and magnetic field data of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes B and C during 2008 and 2009 for observation evidences of the Kelvin-Helmholtz instability (KHI). Fourteen KHI events with rolled-up vortices are identified under the northward interplanetary magnetic field (IMF) at the low-latitude boundary layer (LLBL). We collect another 42 events reported from the observations of the Geotail, Double Star TC-1, and Cluster for a statistical study of the KH wave properties. All the 56 rolled-up KH wave events are quantitatively characterized by the dominant period, phase velocity, and the wavelength. We further explore the relationship between the KH wave period and the solar wind velocity (VSW) and the IMF clock angle. It is found that the KH period tends to be shorter under a higher VSW, and longer with a larger IMF clock angle. The spatial distribution of the KH wavelength shows a longitudinal growth with increasing distance from the subsolar point along the flank magnetopause. The statistical results provide new insights for the development of KH waves and their connection with the interplanetary conditions and deepen our understanding of the KHI at the magnetopause.

A Transverse Oscillation Approach for Estimation of Three-Dimensional Velocity Vectors, Part II: Experimental Validation

Abstract: The 3-D transverse oscillation method is investi- gated by estimating 3-D velocities in an experimental flow-rig system. Measurements of the synthesized transverse oscillating fields are presented as well. The method employs a 2-D trans- ducer; decouples the velocity estimation; and estimates the axi- al, transverse, and elevation velocity components simultaneous- ly. Data are acquired using a research ultrasound scanner. The velocity measurements are conducted with steady flow in six- teen different directions. For a specific flow direction with (α, β) = (45, 15)°, the mean estimated velocity vector at the center of the vessel is (vx, vy, vz) = (33.8, 34.5, 15.2) ± (4.6, 5.0, 0.6) cm/s where the expected velocity is (34.2, 34.2, 13.0) cm/s. The ve- locity magnitude is 50.6 ± 5.2 cm/s with a bias of 0.7 cm/s. The flow angles α and β are estimated as 45.6 ± 4.9° and 17.6 ± 1.0°. Subsequently, the precision and accuracy are calculated over the entire velocity profiles. On average for all direction, the relative mean bias of the velocity magnitude is −0.08%. For α and β, the mean bias is −0.2° and −1.5°. The relative standard deviations of the velocity magnitude ranges from 8 to 16%. For the flow angles, the ranges of the mean angular deviations are 5° to 16° and 0.7° and 8°.

An overmoded relativistic backward wave oscillator with efficient dual-mode operation

Abstract: A dual-mode operation mechanism in an overmoded relativistic backward wave oscillator is presented. The electron beam interacts with the −1st space harmonic of TM01 mode synchronously in the slow wave structure. Then the backward propagating TM01 mode is converted to the forward propagating TM02 mode. As the phase velocity of the volume harmonic of TM02 mode is about twice that of the surface harmonic of TM01 mode, the TM02 mode also plays an important role in the high-power microwave generation. Particle-in-cell simulation shows that an efficiency of 48% and a significant improvement of the power capacity have been obtained.

Generation of surface waves by shear-flow instability

Abstract: We consider the linear stability of an inviscid parallel shear flow of air over water with gravity and capillarity. The velocity profile in the air is monotonically increasing upwards from the sea surface and is convex, while the velocity in the water is monotonically decreasing from the surface and is concave. An archetypical example, the ‘double-exponential’ profile, is solved analytically and studied in detail. We show that there are two types of unstable mode which can, in some cases, co-exist. The first type is the ‘Miles mode’ resulting from a resonant interaction between a surface gravity wave and a critical level in the air. The second unstable mode is an interaction between surface gravity waves and a critical level in the water, resulting in the growth of ripples. The gravity–capillary waves participating in this second resonance have negative intrinsic phase speed, but are Doppler shifted so that their actual phase speed is positive, and matches the speed of the base-state current at the critical level. In both cases, the Reynolds stresses of an exponentially growing wave transfer momentum from the vicinity of the critical level to the zone between the crests and troughs of a surface wave.

Electron acceleration in sub-relativistic wakefields driven by few-cycle laser pulses

Abstract: Using Particle-in-Cell simulations, we study the interaction of few mJ-few cycle laser pulses with an underdense plasma at resonant density. In this previously unexplored regime, it is found that group velocity dispersion is a key ingredient of the interaction. The concomitant effects of dispersion and plasma nonlinearities causes a deceleration of the wakefield phase velocity, which becomes sub-relativistic. Electron injection in this sub-relativistic wakefield is enhanced and leads to the production of a femtosecond electron bunch with picocoulomb of charge in the 5-10 MeV energy range. Such an electron bunch is of great interest for application to ultrafast electron diffraction. In addition, in this dispersion dominated regime, it is shown that positively chirped laser pulses can be used as a tuning knob for compensating plasma dispersion, increasing the laser amplitude during self-focusing and optimizing the trapped charge.

Nonlinear pulse propagation and phase velocity of laser-driven plasma waves

Abstract: Laser evolution and plasma wave excitation by a relativistically intense short-pulse laser in underdense plasma are investigated in the broad pulse limit, including the effects of pulse steepening, frequency redshifting, and energy depletion. The nonlinear plasma wave phase velocity is shown to be significantly lower than the laser group velocity and further decreases as the pulse propagates owing to laser evolution. This lowers the thresholds for trapping and wave breaking and reduces the energy gain and efficiency of laser-plasma accelerators that use a uniform plasma profile.

Propagation of torsional waves in a viscoelastic layer over an inhomogeneous half space

Abstract: This paper presents a theoretical study on propagation of torsional surface waves in a homogeneous viscoelastic isotropic layer with Voigt type viscosity over an inhomogeneous isotropic infinite half space. The non-homogeneity in half space is assumed to arise due to exponential variation in shear modulus and density. A closed-form solution has been obtained for the displacement in the layer as well as for a infinite half space. The dispersion and absorption relations for an torsional wave under the assumed geometry have been found. Numerical results are presented for propagation characteristics in terms of a number of non-dimensionalized parameters and have been produced graphically. This study investigates the effect of various parameters, namely non-homogeneity parameter, internal friction, the layer width and complex wave number on dissipation function and phase velocity of the torsional wave. Results in some special cases are also compared with existing solutions available from analytical methods, which show a close agreement.

Nonlinear evolution of ion acoustic solitary waves in space plasmas: Fluid and particle‐in‐cell simulations

Abstract: Spacecraft observations revealed the presence of electrostatic solitary waves (ESWs) in various regions of the Earth's magnetosphere. Over the years, many researchers have attempted to model these observations in terms of electron/ion acoustic solitary waves by using nonlinear fluid theory/simulations. The ESW structures predicted by fluid models can be inadequate due to its inability in handling kinetic effects. To provide clear view on the application of the fluid and kinetic treatments in modeling the ESWs, we perform both fluid and particle-in-cell (PIC) simulations of ion acoustic solitary waves (IASWs) and estimate the quantitative differences in their characteristics like speed, amplitude, and width. We find that the number of trapped electrons in the wave potential is higher for the IASW, which are generated by large-amplitude initial density perturbation (IDP). The present fluid and PIC simulation results are in close agreement for small amplitude IDPs, whereas for large IDPs they show discrepancy in the amplitude, width, and speed of the IASW, which is attributed to negligence of kinetic effects in the former approach. The speed of IASW in the fluid simulations increases with the increase of IASW amplitude, while the reverse tendency is seen in the PIC simulation. The present study suggests that the fluid treatment is appropriate when the magnitude of phase velocity of IASW is less than the ion acoustic (IA) speed obtained from their linear dispersion relation, whereas when it exceeds IA speed, it is necessary to include the kinetic effects in the model.

Multi-dimensional fractional wave equation and some properties of its fundamental solution

Abstract: In this paper, a multi-dimensional fractional wave equation that describes propaga- tion of damped waves is introduced and analyzed. In contrast to the fractional diffusion- wave equation, the fractional wave equation contains fractional derivatives of the same order α, 1 ≤ α ≤ 2 both in space and in time. This feature is a decisive factor for inher- iting some crucial characteristics of the wave equation, such as, a constant phase velocity of the damped waves which is now described by the fractional wave equation. Some new integral representations of the fundamental solution of the multi-dimensional wave equa- tion are presented. In the one- and three-dimensional cases, the fundamental solution is obtained in explicit form in terms of elementary functions. In the one-dimensional case, the fundamental solution is shown to be a spatial probability density function evolving in time. However, for the dimensions greater than one, the fundamental solution can be negative, and therefore, does not allow a probabilistic interpretation. To illustrate the analytical findings, results of numerical calculations and numerous plots are presented.

Mean flow stability analysis of oscillating jet experiments

Abstract: Linear stability analysis (LSA) is applied to the mean flow of an oscillating round jet with the aim of investigating the robustness and accuracy of mean flow stability wave models. The jet’s axisymmetric mode is excited at the nozzle lip through a sinusoidal modulation of the flow rate at amplitudes ranging from 0.1 % to 100 %. The instantaneous flow field is measured via particle image velocimetry (PIV) and decomposed into a mean and periodic part utilizing proper orthogonal decomposition (POD). Local LSA is applied to the measured mean flow adopting a weakly non-parallel flow approach. The resulting global perturbation field is carefully compared with the measurements in terms of spatial growth rate, phase velocity, and phase and amplitude distribution. It is shown that the stability wave model accurately predicts the excited flow oscillations during their entire growth phase and during a large part of their decay phase. The stability wave model applies over a wide range of forcing amplitudes, showing no pronounced sensitivity to the strength of nonlinear saturation. The upstream displacement of the neutral point and the successive reduction of gain with increasing forcing amplitude is very well captured by the stability wave model. At very strong forcing ( ), the flow becomes essentially stable to the axisymmetric mode. For these extreme cases, the prediction deteriorates from the measurements due to an interaction of the forced wave with the geometric confinement of the nozzle. Moreover, the model fails far downstream in a region where energy is transferred from the oscillation back to the mean flow. This study supports previously conducted mean flow stability analysis of self-excited flow oscillations in the cylinder wake and in the vortex breakdown bubble and extends the methodology to externally forced convectively unstable flows. The high accuracy of mean flow stability wave models as demonstrated here is of great importance for the analysis of coherent structures in turbulent shear flows.

Time-lapse seismic noise correlation tomography at Valhall

Abstract: We show that a reliable and statistically significant group velocity time-lapse difference between 2004 and 2010 can be retrieved from ambient seismic noise in an offshore hydrocarbon exploitation setting. We performed a direct comparison of Scholte wave group velocity images obtained using regularized tomography. We characterize the expected variation in group velocity images from the 2004 or 2010 recordings that result from fluctuations in the cross correlations by looking at cross correlations of portions of the recordings. We prove that the time-lapse difference is statistically significant. The time-lapse group velocity image from ambient noise data shows strong similarities with a time-lapse phase velocity map obtained from controlled source data. The most striking features are a northern and a southern group velocity increase due to compaction and subsidence as a result of reservoir production.