29 research outputs found
Quantum cavitation in liquid He: dissipation effects
We have investigated the effect that dissipation may have on the cavitation
process in normal liquid He. Our results indicate that a rather small
dissipation decreases sizeably the quantum-to-thermal crossover temperature
for cavitation in normal liquid He. This is a possible explanation
why recent experiments have not yet found clear evidence of quantum cavitation
at temperatures below the predicted by calculations which neglect
dissipation.Comment: To be published in Physical Review B6
Cavitation pressure in liquid helium
Recent experiments have suggested that, at low enough temperature, the
homogeneous nucleation of bubbles occurs in liquid helium near the calculated
spinodal limit. This was done in pure superfluid helium 4 and in pure normal
liquid helium 3. However, in such experiments, where the negative pressure is
produced by focusing an acoustic wave in the bulk liquid, the local amplitude
of the instantaneous pressure or density is not directly measurable. In this
article, we present a series of measurements as a function of the static
pressure in the experimental cell. They allowed us to obtain an upper bound for
the cavitation pressure P_cav (at low temperature, P_cav < -2.4 bar in helium
3, P_cav < -8.0 bar in helium 4). From a more precise study of the acoustic
transducer characteristics, we also obtained a lower bound (at low temperature,
P_cav > -3.0 bar in helium 3, P_cav > - 10.4 bar in helium 4). In this article
we thus present quantitative evidence that cavitation occurs at low temperature
near the calculated spinodal limit (-3.1 bar in helium 3 and -9.5 bar in helium
4). Further information is also obtained on the comparison between the two
helium isotopes. We finally discuss the magnitude of nonlinear effects in the
focusing of a sound wave in liquid helium, where the pressure dependence of the
compressibility is large.Comment: 11 pages, 9 figure
3D ray+born migration/inversion. Part I: Theory
Geophysics, v. 68, n. 4, p. 1348-1356, 2003. http://dx.doi.org/10.1190/1.1598128International audienc
3D ray+born migration/inversion - Part 2 : application to the SEG/EAGE OVERTHRUST experiment/Geophysics
Geophysics, v. 68, n. 4, p. 1357-1370, 2003. http://dx.doi.org/10.1190/1.1598129International audienc
3D Prestack preserved amplitude migration - Application to a 3D real marine dataset
International audienc
Building starting models for full waveform inversion from wide-aperture data by stereotomography,
International audienceBuilding an accurate initial velocity model for full waveform inversion (FWI) is a key issue to guarantee convergence of full waveform inversion towards the global minimum of a misfit function. In this study, we assess joint refraction and reflection stereotomography as a tool to build a reliable starting model for frequency-domain full waveform inversion from long-offset (i.e., wide-aperture) data. Stereotomography is a slope tomographic method that is based on the inversion of traveltimes and slopes of locally-coherent events in a data cube. One advantage of stereotomography compared to conventional traveltime reflection tomography is the semi-automatic picking procedure of locally-coherent events, which is easier than the picking of continuous events, and can lead to a higher density of picks. While conventional applications of stereotomography only consider short-offset reflected waves, we assess the benefits provided by the joint inversion of reflected and refracted arrivals. Introduction of the refracted waves allows the construction of a starting model that kinematically fits the first arrivals, a necessary requirement for full waveform inversion. In a similar way to frequency-domain full waveform inversion, we design a multiscale approach of stereotomography, which proceeds hierarchically from the wide-aperture to the short-aperture components of the data, to reduce the non-linearity of the stereotomographic inversion of long-offset data. This workflow which combines stereotomography and full waveform inversion, is applied to synthetic and real data case studies for the Valhall oil-field target. The synthetic results show that the joint refraction and reflection stereotomography for a 24-km maximum offset data set provides a more reliable initial model for full waveform inversion than reflection stereotomography performed for a 4-km maximum offset data set, in particular in low-velocity gas layers and in the deep part of a structure below a reservoir. Application of joint stereotomography, full waveform inversion and reverse-time migration to real data reveals that the FWI models and the reverse-time migration images computed from the stereotomography model shares several features with FWI velocity models and migrated images computed from an anisotropic reflection-traveltime tomography model, although stereotomography was performed in the isotropic approximation. Implementation of anisotropy in joint refraction and reflection stereotomography of long-offset data is a key issue to further improve the accuracy of the method