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vEGU21: Gather Online | 19–30 April 2021, Seismic noise recorded at the surface of Mars has been monitored since February 2019, using the InSight seismometers.This noise can reach -200 dB and is 500 times lower than on Earth at night and it increases of 30 dB during the day. We analyze its polarization as a function of time and frequency in the band 0.03-1Hz. We use the degree of polarization to extract signals with stable polarization independent of their amplitude and type of polarization. We detect polarized signals at all frequencies and all times. Glitches correspond to linear polarized signals which are more abundant during the night. For signals with elliptical polarization, the ellipse is in the horizontal plane below 0.3 Hz (LF). Above 0.3 Hz (HF) and except in the evening, the ellipse is in the vertical plane and the major axis is tilted. While polarization azimuths are different in the two frequency bands, they both vary as a function of local hour and season. They are also correlated with wind direction, particularly during the daytime. We investigate possible aseismic and seismic origins of the polarized signals. Lander or tether noise can be discarded. Pressure fluctuations transported by wind may explain part of the HF polarization but not the tilt of the ellipse. This tilt can be obtained if the source is an acoustic emission coming from high altitude at critical angle. Finally, in the evening when the wind is low, the polarized signals may correspond to the seismic wavefield of the Mars background noise.

AGU Fall Meeting 2019 in San Francisco , 9-13 December 2019, EIS/InSIght team, InSight is the first planetary mission with a seismometer package, SEIS, since the Apollo Lunar Surface Experiments Package. SEIS is complimented by APSS, which has as a goal to document the atmospheric source of seismic noise and signals. Since June 2019, SEIS has been delivering 6 axis 20 sps continuous seismic data, a rate one order of magnitude larger originally planned. More than 50 events have been detected by the end of July 2019 but only three have amplitudes significantly above the SEIS instrument requirement. Two have clear and coherent arrivals of P and S waves, enabling location, diffusion/attenuation characterization and receiver function analysis. The event¿s magnitudes are likely ¿ 3 and no clear surface waves nor deep interior phases have been identified. This suggests deep events with scattering along their final propagation paths and with large propagation differences as compared to Earth and Moon quakes. Most of the event¿s detections are made possible due to the very low noise achieved by the instrument installation strategy and the very low VBB self-noise. Most of the SEIS signals have amplitudes of spectral densities in the 0.03-5Hz frequency bandwidth ranging from 10-10 m/s2/Hz1/2 to 5 10-9 m/s2/Hz1/2. The smallest noise levels occurs during the early night, with angstrom displacements or nano-radian tilts. This monitors the elastic and seismic interaction of a planetary surface with its atmosphere, illustrated not only by a wide range of SEIS signals correlated with pressure vortexes, dust devils or wind activity but also by modulation of resonances above 1 Hz, amplified by ultra-low velocity surface layers. After about one half of a Martian year, clear seasonal changes appear also in the noise, which will be discussed. One year after landing, the seismic noise is therefore better and better understood, and noise correction techniques begun to be implemented, either thanks to the APSS wind and pressure sensors, or by SEIS only data processing techniques. These data processing techniques open not only the possibility of better signal to noise ratio of the events, but are also used for various noise auto-correlation techniques as well as searches of long period signals. Noise and seismic signals on Mars are therefore completely different from what seismology encountered previously on Earth and Moon.

AGU Fall Meeting 2019 in San Francisco, 9-13 December 2019, Since the two seismometers of the InSight mission have been dropped on the ground of the landing site at Elysium Planitia on December, 2018 more than 200 martian days (called SOL) of seismic record have been repatriated on Earth. With a long enough continuous seismic signal the application of seismic interferometry techniques can be considered to extract information on the shallows structures or the elastic properties of the medium from auto-correlation of seismic ambient noise. In single-station configuration the zero-offset global reflection response at the vertical of the seismometer can be approximated by the stacked Z-Z auto-correlation function for P-waves velocity and the stacked E-E and N-N auto-correlations functions for S-waves velocity, assuming a perfectly vertical layered medium and homogeneously distributed and mutually uncorrelated noise sources. As good signal-to-noise ratios are needed to apply this kind of analysis we experiment various pre-processing approaches before the computation of the correlations functions and try to take into account all the potentials disturbances coming from the lander¿s modes or the various artificial signals in the data. We focused our analysis on a continuous 10sps-very-broadband (VBB) record. In each SOL we computed the correlations functions in 24 windows of one martian hour duration in order to obtain a total correlation tensor for various Mars time. Finally we applied a time-frequency adaptive method to filter the incoherent phase arrivals. Potential interpretation of the retrieved auto-correlations to the crustal structure of Mars and its temporal variations will be discussed.agu

A narrow rectilinear valley in the French Pyrenees, affected in the past by damaging earthquakes, has been chosen as a test site for soil response characterization. The main purpose of this initiative was to compare experimental and numerical approaches. A temporary network of 10 stations has been deployed along and across the valley during two years; parallel various experiments have been conducted, in particular ambient noise recording, and seismic profiles with active sources for structure determination at the 10 sites. Classical observables have been measured for site amplification evaluation, such as spectral ratios of horizontal or vertical motions between site and reference stations using direct S waves and S coda, and spectral ratios between horizontal and vertical (H/V) motions at single stations using noise and S-coda records. Vertical shear-velocity profiles at the stations have first been obtained from a joint inversion of Rayleigh wave dispersion curves and ellipticity. They have subsequently been used to model the H/V spectral ratios of noise data from synthetic seismograms, the H/V ratio of S-coda waves based on equipartition theory, and the 3D seismic response of the basin using the spectral element method. General good agreement is found between simulations and observations. The 3D simulation reveals that topography has a much lower contribution to site effects than sedimentary filling, except at the narrow ridge crests. We find clear evidence of a basin edge effect, with an increase of the amplitude of ground motion at some distance from the edge inside the basin and a decrease immediately at the slope foot.

Program abstracts of the 162nd meeting of the Acoustical Society of America.; International audience; We describe a time-resolved monitoring technique for heterogeneous media, especially multiply scattering media. Our approach is based on the spatial variations of the cross-coherence of diffuse waves acquired at fixed positions but at different dates. The technique applies to all kind of waves, but a particular attention will be paid to ultrasound propagating in concrete. To locate and characterize a defect occurring between successive acquisitions, we use a maximum likelihood approach combined with a diffusive propagation model. We quantify the performance of this technique called LOCADIFF with numerical simulations. In several illustrative examples, we show that the change can be located with a precision of a few wavelengths and that its effective scattering cross-section can be retrieved. We investigate how the accuracy and precision of the method depends on the number of source-receiver pairs, on the time window used to compute the cross-correlation and on the errors in the propagation model. Applications can be found in nondestructive testing (civil engineering), seismology, radar, and sonar location.