Mode Conversion and Period Doubling at Plasma-β\beta Unity in an Alfv\'en-Wave Experiment with Liquid Rubidium

We report Alfv\'en-wave experiments with liquid rubidium at the Dresden High Magnetic Field Laboratory (HLD). Reaching up to 63 T, the pulsed magnetic field exceeds the critical value of 54 T at which the Alfv\'en speed becomes equal to the sound speed (plasma-$\beta$ unity). At this threshold we observe a period doubling of an applied 8 kHz CW excitation, a clear footprint for a parametric resonance between magnetosonic waves and Alfv\'en waves.Comment: 5 pages, 4 figure

2D characterization of near-surface V P/V S: surface-wave dispersion inversion versus refraction tomography

International audienceThe joint study of pressure (P-) and shear (S-) wave velocities (Vp and Vs ), as well as their ratio (Vp /Vs), has been used for many years at large scales but remains marginal in near-surface applications. For these applications, and are generally retrieved with seismic refraction tomography combining P and SH (shear-horizontal) waves, thus requiring two separate acquisitions. Surface-wave prospecting methods are proposed here as an alternative to SH-wave tomography in order to retrieve pseudo-2D Vs sections from typical P-wave shot gathers and assess the applicability of combined P-wave refraction tomography and surface-wave dispersion analysis to estimate Vp/Vs ratio. We carried out a simultaneous P- and surface-wave survey on a well-characterized granite-micaschists contact at Ploemeur hydrological observatory (France), supplemented with an SH-wave acquisition along the same line in order to compare Vs results obtained from SH-wave refraction tomography and surface-wave profiling. Travel-time tomography was performed with P- and SH- wave first arrivals observed along the line to retrieve Vtomo p and Vtomo s models. Windowing and stacking techniques were then used to extract evenly spaced dispersion data from P-wave shot gathers along the line. Successive 1D Monte Carlo inversions of these dispersion data were performed using fixed Vp values extracted from Vtomo p the model and no lateral constraints between two adjacent 1D inversions. The resulting 1D Vsw s models were then assembled to create a pseudo-2D Vsw s section, which appears to be correctly matching the general features observed on the section. If the pseudo-section is characterized by strong velocity incertainties in the deepest layers, it provides a more detailed description of the lateral variations in the shallow layers. Theoretical dispersion curves were also computed along the line with both and models. While the dispersion curves computed from models provide results consistent with the coherent maxima observed on dispersion images, dispersion curves computed from models are generally not fitting the observed propagation modes at low frequency. Surface-wave analysis could therefore improve models both in terms of reliability and ability to describe lateral variations. Finally, we were able to compute / sections from both and models. The two sections present similar features, but the section obtained from shows a higher lateral resolution and is consistent with the features observed on electrical resistivity tomography, thus validating our approach for retrieving Vp/Vs ratio from combined P-wave tomography and surface-wave profiling

a review and some new issues on the theory of the h v technique for ambient vibrations

In spite of the Horizontal-to-Vertical Spectral Ratio (HVSR or H/V) technique obtained by the ambient vibrations is a very popular tool, a full theoretical explanation of it has been not reached yet. A short excursus is here presented on the theoretical models explaining the H/V spectral ratio that have been development in last decades. It leads to the present two main research lines: one aims at describing the H/V curve by taking in account the whole ambient-vibration wavefield, and another just studies the Rayleigh ellipticity. For the first theoretical branch, a comparison between the most recent two models of the ambient-vibration wavefield is presented, which are the Distributed Surface Sources (DSS) one and the Diffuse Field Approach (DFA). A mention is done of the current developments of these models and of the use of the DSS for comparing the H/V spectral ratio definitions present in literature. For the second research branch, some insights about the connection between the so-called osculation points of the Rayleigh dispersion curves and the behaviour of the H/V curve are discussed

Near-field seismic displacement and tilt associated with the explosive activity of Stromboli

Broadband seismic recordings in the near-field of Strombolian explosions, at 500 m distance, show pronounced effects of tilt. The tilt signal is predominant in the horizontal components beyond about 50 s period while it is negligible in the vertical component. The waveform of the tilt signal at the seismometer output is a double time integral of the waveform due to ground displacement. Since the waveform of the displacement is known from the vertical component, the waveform of the tilt signal in the horizontal seismogram can be reconstructed and both contributions can be separated from each other with a linear regression. We have analyzed data recorded in the summit region of Stromboli in 1995 and 1996. The regional tilt can be determined from the differential vertical displacement between instruments a few tens of meters apart. Local tilts determined with individual instruments scatter around the regional value, most probably due to local strain-tilt-coupling. Mogi's (1958) formulae for a pressure source in a homogeneous halfspace are used to interpret the results. The source displaces a volume of several tens of cubic meters of the surrounding rock before the explosive discharge; typical volumes were 25 m3 in July 1995 and 60 m3 in September 1996

Near-field seismic displacement and tilt associated with the explosive activity of Stromboli

Broadband seismic recordings in the near-field of Strombolian explosions, at 500 m distance, show pronounced effects of tilt. The tilt signal is predominant in the horizontal components beyond about 50 s period while it is negligible in the vertical component. The waveform of the tilt signal at the seismometer output is a double time integral of the waveform due to ground displacement. Since the waveform of the displacement is known from the vertical component, the waveform of the tilt signal in the horizontal seismogram can be reconstructed and both contributions can be separated from each other with a linear regression. We have analyzed data recorded in the summit region of Stromboli in 1995 and 1996. The regional tilt can be determined from the differential vertical displacement between instruments a few tens of meters apart. Local tilts determined with individual instruments scatter around the regional value, most probably due to local strain-tilt-coupling. Mogi's (1958) formulae for a pressure source in a homogeneous halfspace are used to interpret the results. The source displaces a volume of several tens of cubic meters of the surrounding rock before the explosive discharge; typical volumes were 25 m3 in July 1995 and 60 m3 in September 1996