Papers by Ludovic Margerin
Le Centre pour la Communication Scientifique Directe - HAL - INSU, Nov 15, 2021
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
<jats:p>&amp;lt;p&amp;gt;The scattering of seismic waves is the signature of random... more <jats:p>&amp;lt;p&amp;gt;The scattering of seismic waves is the signature of random heterogeneities, present in the lithospheric structure of a terrestrial planet. It is the result of refraction and reflection of the seismic waves generated by a quake, when they cross materials with different shear rigidity, bulk modulus, and density and therefore different seismic wave velocities, compared to the ambient space. &amp;amp;#160;On Earth, the seismic waves show relatively weak scattering, identified in later arriving coda waves that follow the main arrivals of body waves and decay with time. In contrast, seismic wave scattering is much more significant on the Moon, where the high heterogeneous structure of the lunar megaregolith, produced through millions of years of impact bombardment, is a structure that creates an extreme scattering environment.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;The landing of the NASA InSight mission on Mars in 2018, which carried and deployed a seismometer for the first time on the Martian ground, offered a pristine dataset for the investigation and analysis of the characteristics of the scattering attenuation of the Martian crust and uppermost mantle which is important for understanding the structure of the Martian interior. Lognonn&amp;amp;#233; et al. (2020) used a methodology based on the radiative transfer model (Margerin et al., 1998) to offer the first constraints for the scattering and attenuation in the Martian crust. In this study, we performed a further examination based on more and newer events of the Martian Seismic Catalog (InSight Marsquake Service, 2021).&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;The Marsquake Catalog contains events that are categorized according to the frequency content of the seismic signal (Clinton et al., 2021). In this study, we used 19 events of 4 different families, namely the Low Frequency, Broadband, High Frequency, and Very High Frequency events, for our investigation. We focused our investigation on the characteristics of the S-coda waveforms and for this reason, we worked on the respective energy envelopes. We manually picked the envelopes, defining the time window of the S-coda waves, as well the frequency range for each event, directly from the spectrograms of the events' signals, using an appropriately developed visual tool.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;We used a modeling approach (Dainty et al., 1974) that was developed for the computation of the energy envelopes of shallow events (Lunar impacts) and a diffusive, highly scattering layer, sitting over an elastic half-space. The energy envelope depends on the thickness of the diffusive layer, the range of the seismic ray, the diffusivity and the attenuation in the top layer, and the seismic wave velocity in the underneath elastic half-space. We analyzed all the tradeoffs between the terms of the modeling equation, namely the geometrical relationship of the velocity contrast between the diffusive layer and the elastic half-space with the seismic ray range and the diffusive layer thickness, the diffusivity with the diffusive layer thickness, and between the diffusivity and the velocity contrast of the two examined layers.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;The presence of the aforementioned tradeoffs made the definition of a unique model a very hard task, as the information for the azimuthal characteristics of the signal is not available for the examined events. This is a limitation that exists in seismology only while working with one station, with the InSight seismometer being the only station on a planet, and the amplitude of the seismic signal is not big enough to perform a specific polarization analysis and derive the azimuthal origin of the recorded signal. For this reason, we reviewed the fit between the modeling and the data, depending on the frequency content of the events.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;The Low Frequency and the Broadband events, which have a frequency content mainly below the tick noise detected at 1 Hz, could not satisfy the modeling approach of a simple diffusive layer. The spectral envelopes of the S-coda waves of these events are decaying very rapidly, which suggests an origin in a more elastic environment. This is in agreement with previous studies (Giardini et al., 2020) that suggest that these events are generated deeper in the Martian mantle. For this reason, we applied another approach to these signals, with an energy envelope equation designed for deep moonquakes (Dainty et al.,1974), but it was not either capable to fit the examined data envelopes, suggesting the absence of a very thick megaregolith structure on Mars.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;Based on the results of the High Frequency (HF) and Very High Frequency (VF) events we observed a range of possible paths and diffusivities that can satisfy the data and we investigated the tradeoffs between the parameters of a modeling equation that control the shape of the energy envelope for the…
The amplitudes of seismic waves decrease over distance due frequency dependent energy loss via sc... more The amplitudes of seismic waves decrease over distance due frequency dependent energy loss via scattering and attenuation. Scattering, or extrinsic attenuation, is dependent upon the size, strength, and abundance of elastic heterogeneities within the medium. Intrinsic attenuation results from dissipation due to anelastic processes and internal friction between molecules and defects within crystal lattices. Both forms of attenuation are described using the seismic quality factor, Q. Scattering can arise from compositional heterogeneities, mechanical boundaries (such as fractures, cracks, veins, cavities), and other major non-radial changes in material properties of an object. Intrinsic attenuation is tied to grain boundary processes, crystal defects, fluid-filled cracks, and other dissipative viscoelastic processes. Attenuation is vital for understanding the detectability, travel times, and propagation properties of seismic waves, as well as the properties of the interior of a world....
AGU Fall Meeting 2019 in San Francisco, 9-13 December 2019Since the two seismometers of the InSig... more AGU Fall Meeting 2019 in San Francisco, 9-13 December 2019Since 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.ag
Bulletin of the Seismological Society of America, 2021
ABSTRACTSince its deployment at the surface of Mars, the Seismic Experiment for Interior Structur... more ABSTRACTSince its deployment at the surface of Mars, the Seismic Experiment for Interior Structure (SEIS) instrument of the InSight mission has detected hundreds of small-magnitude seismic events. In this work, we highlight some features of two specific families: high-frequency (HF) and very-high-frequency (VF) events. We characterize the shape of the energy envelopes of HF and VF events with two parameters: (1) the delay time td between the onset and the peak of the dominant arrival; and (2) the quality factor Qc, which quantifies the energy decay rate in the coda. We observe that the envelope of HF and VF events is frequency independent. As a consequence, a single delay time suffices to characterize envelope broadening in the 2.5–7.5 Hz band. The typical coda decay time is also frequency independent, as attested by the close to linear increase of Qc with frequency. Finally, we use elastic radiative transfer theory to perform a series of inversion of seismogram envelopes for the at...
Journal of Geophysical Research: Planets, 2021
Since early February 2019, the SEIS (Seismic Experiment for Interior Structure) seismometer deplo... more Since early February 2019, the SEIS (Seismic Experiment for Interior Structure) seismometer deployed at the surface of Mars in the framework of the InSight mission has been continuously recording the ground motion at Elysium Planitia. In this study, we take advantage of this exceptional data set to put constraints on the crustal properties of Mars using seismic interferometry (SI). To carry out this task, we first examine the continuous records from the very broadband seismometer. Several deterministic sources of environmental noise are identified and specific preprocessing strategies are presented to mitigate their influence. Applying the principles of SI to the single-station configuration of InSight, we compute, for each Sol and each hour of the martian day, the diagonal elements of the time-domain correlation tensor of random ambient vibrations recorded by SEIS. A similar computation is performed on the diffuse waveforms generated by more than a hundred Marsquakes. A careful signalto-noise ratio analysis and an inter-comparison between the two datasets suggest that the results from SI are most reliable in a narrow frequency band around 2.4 Hz, where an amplification of both ambient vibrations and seismic events is observed. The average autocorrelation functions (ACFs) contain well identifiable seismic arrivals, that are very consistent between the two datasets. Interpreting the vertical and horizontal ACFs as, respectively, the P-and S-seismic reflectivity below InSight, we propose a simple stratified velocity model of the crust, which is mostly compatible with previous results from receiver function analysis. Our results are discussed and compared to recent works from the literature. Plain Language Summary The correlation of seismic records is the basis of seismic interferometry methods. These methods use seismic waves, either from ambient vibrations of the planet or from quakes, that are scattered in the medium in order to recover information about the structure between two seismic sensors. The method is implemented to compute the auto-correlation functions of the three components of the ground motion recorded by the SEIS seismometer. The comparison of the results obtained from earthquake data to the ones obtained from ambient vibrations demonstrates that the ambient seismic vibration is clearly above the self-noise of SEIS during early night hours around a specific frequency (2.4 Hz). The seismic vibrations appear to be amplified at this frequency by an unknown mechanism. Some seismic energy arrivals appear consistently in the auto-correlation functions, at specific propagation times, independent of the data sets and processing parameters tested. These arrivals are interpreted as vertically propagating seismic waves which are reflected on top of crustal layers. Their propagation times can be used to constrain a model of Mars crustal structure. COMPAIRE ET AL.
Earth and Planetary Science Letters, 2018
Physics of the Earth and Planetary Interiors, 2015
An analysis of coda waves excited in the 0.2-20 Hz frequency band and recorded by the underground... more An analysis of coda waves excited in the 0.2-20 Hz frequency band and recorded by the underground array Underseis (central Italy) has been performed to constrain both seismic attenuation at regional scale and velocity structure in the Mount Gran Sasso area. Attenuation was estimated with the MLTWA method, and shows a predominance of scattering phenomena over intrinsic absorption. The values of Q i and Q s are compatible with other estimates obtained in similar tectonic environments. Array methods allowed for a detailed study of the propagation characteristics, demonstrating that earthquake coda at frequencies greater than about 6 Hz is composed of only body waves. Coherence and spectral characteristics of seismic waves measured along the coda of local and regional earthquakes indicate that the wavefield becomes fully diffuse only in the late coda. The frequency-dependent energy partitioning between horizontal and vertical components has been also estimated and compared with synthetic values computed in a layered half-space under the diffuse field assumption. This comparison confirms that, for frequencies higher than 6 Hz, the coda appears as a sum of body waves coming from all directions while, in the low frequency range (0.2-2 Hz), the observations can be well explained by a coda wavefield composed of an equipartition mixture of surface and body waves traveling in a multiple-layered medium. A Monte-Carlo inversion has been performed to obtain a set of acceptable velocity models of the upper crust. The present results show that a broadband coda wavefield recorded in an underground environment is useful to constrain both the regional attenuation and the velocity structure of the target area, thereby complementing the results of classical array analysis of the wavefield.
Science, 2010
Clearing Up the Inner Core The behavior of Earth's core controls the planet's heat budget... more Clearing Up the Inner Core The behavior of Earth's core controls the planet's heat budget and magnetic field, yet its structure remains enigmatic. For instance, the seismic properties of the solid inner core suggest hemispherical structural asymmetry, but questions remain as to how these variations arose (see the Perspective by Buffett ). Monnereau et al. (p. 1014 , published online 15 April) modeled grain sizes of crystalline iron—the predicted dominant mineral phase in the core—and found that a slow translational motion eastward may trigger melting in the Eastern Hemisphere and solidification in the Western Hemisphere, creating a lopsided core. Deuss et al. (p. 1018 , published online 15 April) examined the normal-mode seismic structure of the inner core, collected from 90 large earthquakes, which reveal not just simple hemispherical variations, but more nuanced regional structures. The overlap of the seismic data with Earth's magnetic field suggests that directionally...
Hemispherical asymmetry is a striking feature of Earth's inner core. The uppermost 100 km of ... more Hemispherical asymmetry is a striking feature of Earth's inner core. The uppermost 100 km of the inner core are nearly isotropic and strongly attenuating with lower P-wave velocity and attenuation in the western hemisphere than in the eastern one. We propose here a new model of the inner core growth which provides a coherent framework to explain this asymmetry. We
Earth and Planetary Science Letters, 2008
We propose to model the uppermost inner core as an aggregate of randomly oriented anisotropic "pa... more We propose to model the uppermost inner core as an aggregate of randomly oriented anisotropic "patches". A patch is defined as an assemblage of a possibly large number of crystals with identically oriented crystallographic axes. This simple model accounts for the observed velocity isotropy of short period body waves, and offers a reasonable physical interpretation for the scatterers detected at the top of the inner core. From rigorous multiple scattering modeling of seismic wave propagation through the aggregate, we obtain strong constraints on both the size and the elastic constants of iron patches. In a first step, we study the phase velocity and scattering attenuation of aggregates composed of hexagonal and cubic crystals, whose elastic constants have been published in the mineral physics literature. The predicted attenuations for P waves vary over two orders of magnitude. Our calculations demonstrate that scattering attenuation is extremely sensitive to the anisotropic properties of single crystals and offers an attractive way to discriminate among iron models with e.g. identical Voigt average speeds. When anisotropy of elastic patches is pronounced, we find that the S wavespeed in the aggregate can be as much as 15% lower than the Voigt average shear velocity of a single crystal. In a second step, we perform a systematic search for iron models compatible with measured seismic velocities and attenuations. An iron model is characterized by its symmetry (cubic or hexagonal), elastic constants, and patch size. Independent of the crystal symmetry, we infer a most likely size of patch of the order of 400 m. Recent bcc iron models from the literature are in very good agreement with the most probable elastic constants of cubic crystals found in our inversion. Our study (1) suggests that the presence of melt may not be required to explain the low shear wavespeeds in the inner core and (2) supports the recent experimental results on the stability of cubic iron in the inner core, at least in its upper part.
Bulletin of the Seismological Society of America, 2011
A narrow rectilinear valley in the French Pyrenees, affected in the past by damaging earthquakes,... more 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.
Nature Geoscience, 2020
T he Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) miss... more T he Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission landed on Mars on 26 November 2018 in Elysium Planitia 1,2 , 38 years after the end of Viking 2 lander operations. At the time, Viking's seismometer 3 did not succeed in making any convincing Marsquake detections, due to its on-deck installation and high wind sensitivity. InSight therefore provides the first direct geophysical in situ investigations of Mars's interior structure by seismology 1,4. The Seismic Experiment for Interior Structure (SEIS) 5 monitors the ground acceleration with six axes: three Very Broad Band (VBB) oblique axes, sensitive to frequencies from tidal up to 10 Hz, and one vertical and two horizontal Short Period (SP) axes, covering frequencies from ~0.1 Hz to 50 Hz. SEIS is complemented by the APSS experiment 6 (InSight Auxiliary Payload Sensor Suite), which includes pressure and TWINS (Temperature and Winds for InSight) sensors and a magnetometer. These sensors monitor the atmospheric sources of seismic noise and signals 7. After seven sols (Martian days) of SP on-deck operation, with seismic noise comparable to that of Viking 3 , InSight's robotic arm 8 placed SEIS on the ground 22 sols after landing, at a location selected through analysis of InSight's imaging data 9. After levelling and noise assessment, the Wind and Thermal Shield was deployed on sol 66 (2 February 2019). A few days later, all six axes started continuous seismic recording, at 20 samples per second (sps) for VBBs and 100 sps for SPs. After onboard decimation, continuous records at rates from 2 to 20 sps and event records 5 at 100 sps are transmitted. Several layers of thermal protection and very low self-noise enable the SEIS VBB sensors to record the daily variation of the
Seismic Earth: Array Analysis of Broadband Seismograms, 2005
... Qc was extensively measured in many regions of the world (see eg Herraiz and Espinosa, 1987) ... more ... Qc was extensively measured in many regions of the world (see eg Herraiz and Espinosa, 1987) and proved to be extremely sensitive to the geological en-vironment. Yet, its physical interpretation in terms of medium properties remained unclear. ...
Mesoscopic Physics in Complex Media, 2010
Study the phase statistics in coda waves and determine the scattering mean free path ℓ from spati... more Study the phase statistics in coda waves and determine the scattering mean free path ℓ from spatial phase decoherence.
Tectonophysics, 2006
Most theoretical investigations of seismic wave scattering rely on the assumption that the
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Papers by Ludovic Margerin