Quantum cavitation in liquid 3^3He: dissipation effects

We have investigated the effect that dissipation may have on the cavitation process in normal liquid $^3$He. Our results indicate that a rather small dissipation decreases sizeably the quantum-to-thermal crossover temperature $T^*$ for cavitation in normal liquid $^3$He. This is a possible explanation why recent experiments have not yet found clear evidence of quantum cavitation at temperatures below the $T^*$ 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

Seismic velocity analysis: in time or depth domain?

International audienc

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 Prestack preserved amplitude migration - Application to a 3D real marine dataset

International 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

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