Quasi Two-dimensional Transfer of Elastic Waves

A theory for multiple scattering of elastic waves is presented in a random medium bounded by two ideal free surfaces, whose horizontal size is infinite and whose transverse size is smaller than the mean free path of the waves. This geometry is relevant for seismic wave propagation in the Earth crust. We derive a time-dependent, quasi-2D radiative transfer equation, that describes the coupling of the eigenmodes of the layer (surface Rayleigh waves, SH waves, and Lamb waves). Expressions are found that relate the small-scale fluctuations to the life time of the modes and to their coupling rates. We discuss a diffusion approximation that simplifies the mathematics of this model significantly, and which should apply at large lapse times. Finally, coherent backscattering is studied within the quasi-2D radiative transfer equation for different source and detection configurations.Comment: REVTeX, 36 pages with 10 figures. Submitted to Phys. Rev.

Time-resolved dynamics of electron wave packets in chaotic and regular quantum billiards with leads

We perform numerical studies of the wave packet propagation through open quantum billiards whose classical counterparts exhibit regular and chaotic dynamics. We show that for t less or similar to tau (tau being the Heisenberg time), the features in the transmitted and reflected currents are directly related to specific classical trajectories connecting the billiard leads. In contrast, the long-time asymptotics of the wave packet dynamics is qualitatively different for classical and quantum billiards. In particularly, the decay of the quantum system obeys a power law that depends on the number of decay channels, and is not sensitive to the nature of classical dynamics (chaotic or regular).Comment: 5 pages, 4 figure

Crystal Structure and Size-Dependent Neutralization Properties of HK20, a Human Monoclonal Antibody Binding to the Highly Conserved Heptad Repeat 1 of gp41

The human monoclonal antibody (mAb) HK20 neutralizes a broad spectrum of primary HIV-1 isolates by targeting the highly conserved heptad repeat 1 (HR1) of gp41, which is transiently exposed during HIV-1 entry. Here we present the crystal structure of the HK20 Fab in complex with a gp41 mimetic 5-Helix at 2.3 Å resolution. HK20 employs its heavy chain CDR H2 and H3 loops to bind into a conserved hydrophobic HR1 pocket that is occupied by HR2 residues in the gp41 post fusion conformation. Compared to the previously described HR1-specific mAb D5, HK20 approaches its epitope with a different angle which might favor epitope access and thus contribute to its higher neutralization breadth and potency. Comparison of the neutralization activities of HK20 IgG, Fab and scFv employing both single cycle and multiple cycle neutralization assays revealed much higher potencies for the smaller Fab and scFv over IgG, implying that the target site is difficult to access for complete antibodies. Nevertheless, two thirds of sera from HIV-1 infected individuals contain significant titers of HK20-inhibiting antibodies. The breadth of neutralization of primary isolates across all clades, the higher potencies for C-clade viruses and the targeting of a distinct site as compared to the fusion inhibitor T-20 demonstrate the potential of HK20 scFv as a therapeutic tool

Detecting seismic activity with a covariance matrix analysis of data recorded on seismic arrays

International audienceModern seismic networks are recording the ground motion continuously at the Earth's surface, providing dense spatial samples of the seismic wavefield. The aim of our study is to analyse these records with statistical array-based approaches to identify coherent time-series as a function of time and frequency. Using ideas mainly brought from the random matrix theory, we analyse the spatial coherence of the seismic wavefield from the width of the covariance matrix eigenvalue distribution. We propose a robust detection method that could be used for the analysis of weak and emergent signals embedded in background noise, such as the volcanic or tectonic tremors and local microseismicity, without any prior knowledge about the studied wavefields. We apply our algorithm to the records of the seismic monitoring network of the Piton de la Fournaise volcano located at La Réunion Island and composed of 21 receivers with an aperture of ∼15 km. This array recorded many teleseismic earthquakes as well as seismovolcanic events during the year 2010. We show that the analysis of the wavefield at frequencies smaller than ∼0.1 Hz results in detection of the majority of teleseismic events from the Global Centroid Moment Tensor database. The seismic activity related to the Piton de la Fournaise volcano is well detected at frequencies above 1 Hz

Experimental Study of Passive Defect Detection and Localization in Thin Plates from Noise Correlation

AbstractThis paper reports experimental results on a passive imaging technique for structural health monitoring to detect the occurrence of defects in plate-like structures. This technique is based on the fact that the active transient response between two sensors can be passively retrieved by cross-correlating the ambient noise-field recorded on these two sensors. A correlation matrix is estimated from friction noise recorded on a transducer array. It is observed that the accuracy of the estimated transient responses strongly depends on the spatial distribution of noise sources. The best convergence is obtained when the noise is uniformly distributed over the whole plate area. Defects are localized by applying a dispersive beamforming algorithm to the difference between the correlation matrices obtained with and without (w/o) defect. It is shown that the quality of the active transient response reconstruction is not a strong requirement for the defect localization. Indeed, the defect is successfully localized even if the noise source distribution is not uniform, provided that it remains spatially stationary between the states w/o defect. A simple theoretical framework is proposed to interpret these results

Depth Migration of Seismovolcanic Tremor Sources Below the Klyuchevskoy Volcanic Group (Kamchatka) Determined From aNetwork-Based Analysis

International audienceWe present a method for automatic location of dominant sources of seismovolcanic tremor in 3-D, based on the spatial coherence of the continuously recorded wavefield at a seismic network. We analyze 4.5 years of records from the seismic network at the Klyuchevskoy volcanic group in Kamchatka, Russia, when four volcanoes experienced tremor episodes. After enhancing the tremor signal with spectral whitening, we compute the daily cross-correlation functions related to the dominant tremor sources from the first eigenvector of the spectral covariance matrix and infer their daily positions in 3-D. We apply our technique to the tremors beneath Shiveluch, Klyuchevskoy, Tolbachik, and Kizimen volcanoes and observe the yearlong preeruptive volcanic tremor beneath Klyuchevskoy from deep to shallow partsof the plumbing system. This observation of deep volcanic tremor sources demonstrates that the cross-correlation-based method is a very powerful tool for volcano monitoring