LOWER MANTLE TEMPERATURE AND COMPOSITION

We evaluate the thermo-chemical state of the lower mantle by analysing the differences in the pattern of heterogeneity between shear and compressional velocity variations and the S−to−P heterogeneity ratio ( RS/P=δlnVS/δlnVP) as mapped in our model SPani and in alternative joint models. Robust structural differences between VP and VS evidence the presence of compositional heterogeneity within the two Large Low Shear Velocity Provinces (LLSVPs). We find also an increasing decorrelation with depth that can be associated with compositional layering of the LLSVPs. In addition, our model shows heterogeneity in the transition zone and mid mantle by complex morphology of subducting slabs and further differences between VP and VS that point to an unexpected heterogeneous lower mantle. Precise estimates of compositional heterogeneities are not yet affordable because of the difficulty to provide quantitative measure of RS/P, making it difficult to use this ratio to evaluate chemical heterogeneity. For instance, RS/P global median value ( ) drops from ∼2.8 to ∼1.9, at 2500 km depth when the VP component of SPani is replaced by a VP model resulting from a differently regularized inversion and obtaining an equally good data fit. An increase of 20% of the SPani VP anomalies also drastically reduces without significantly degrading the data fit. Noise in model parameters also leads to overestimate RS/P in the two LLSVPs as we show with synthetic tests. Additional mineral physics uncertainties for compositional effects on RS/P and for the conversion of δlnVS and δlnVP into density further complicates a precise chemical interpretation

Global multiresolution models of surface wave propagation: comparing equivalently regularized Born and ray theoretical solutions

I invert a large set of teleseismic phase-anomaly observations, to derive tomographic maps of fundamental-mode surface wave phase velocity, first via ray theory, then accounting for finite-frequency effects through scattering theory, in the far-field approximation and neglecting mode coupling. I make use of a multiple-resolution pixel parametrization which, in the assumption of sufficient data coverage, should be adequate to represent strongly oscillatory Fréchet kernels. The parametrization is finer over North America, a region particularly well covered by the data. For each surface-wave mode where phase-anomaly observations are available, I derive a wide spectrum of plausible, differently damped solutions; I then conduct a trade-off analysis, and select as optimal solution model the one associated with the point of maximum curvature on the trade-off curve. I repeat this exercise in both theoretical frameworks, to find that selected scattering and ray theoretical phase-velocity maps are coincident in pattern, and differ only slightly in amplitud

Surface wave ray tracing and azimuthal anisotropy: a generalized spherical harmonic approach

We explain in detail how azimuthally anisotropic maps of surface wave phase velocity can be parametrized in terms of generalized spherical harmonic functions, and why this approach is preferable to others; most importantly, generalized spherical harmonics are the only basis functions adequate to describe a tensor field everywhere on the unit sphere, including the poles of the reference frame. We introduce here a new algorithm, designed specifically for the generalized harmonic parametrization, to trace surface wave ray paths in the presence of laterally varying azimuthal anisotropy. We describe the algorithm, and prove its reliability in view of future application

Seismic, petrological and geodynamical constraints on thermal and compositional structure of the upper mantle: global thermochemical models

Mapping the thermal and compositional structure of the upper mantle requires a combined interpretation of geophysical and petrological observations. Based on current knowledge of material properties, we interpret available global seismic models for temperature assuming end-member compositional structures. In particular, we test the effects of modelling a depleted lithosphere, which accounts for petrological constraints on continents. Differences between seismic models translate into large temperature and density variations, respectively, up to 400 K and 0.06 g cm-3 at 150 km depth. Introducing lateral compositional variations does not change significantly the thermal interpretation of seismic models, but gives a more realistic density structure. Modelling a petrological lithosphere gives cratonic temperatures at 150 km depth that are only 100 K hotter than those obtained assuming pyrolite, but density is ~0.1 g cm-3 lower. We determined the geoid and topography associated with the density distributions by computing the instantaneous flow with an existing code of mantle convection, STAG-YY. Models with and without lateral variations in viscosity have been tested. We found that the differences between seismic models in the deeper part of the upper mantle significantly affect the global geoid, even at harmonic degree 2. The range of variance reduction for geoid due to differences in the transition zone structure (i.e. from 410 to 660 km) is comparable with the range due to differences in the whole mantle seismic structure. Since geoid is dominated by very long wavelengths (the lowest five harmonic degrees account for more than 90 per cent of the signal power), the lithospheric density contrasts do not strongly affect its overall pattern. Models that include a petrological lithosphere, however, fit the geoid and topography better. Most of the long-wavelength contribution that helps to improve the fit comes from the oceanic lithosphere. The signature of continental lithosphere worsens the fit, even in simulations that assume an extremely viscous lithosphere. Therefore, a less depleted, and thus less buoyant, continental lithosphere is required to explain gravity data. None of the seismic tomography models we analyse is able to reproduce accurately the thermal structure of the oceanic lithosphere. All of them show their lowest seismic velocities at ~100 km depth beneath mid-oceanic ridges and have much higher velocities at shallower depths compared to what is predicted with standard cooling models. Despite the limited resolution of global seismic models, this seems to suggest the presence of an additional compositional complexity in the lithospher

On the estimation of attenuation from the ambient seismic field: inferences from distributions of isotropic point scatterers

Cross-correlation of ambient seismic noise recorded by two seismic stations may result in an estimate of the Green's function between those two receivers. Several authors have recently attempted to measure attenuation based on these interferometric, receiver-receiver surface waves. By now, however, it is well established that the loss of coherence of the cross-correlation as a function of space depends strongly on the excitation of the medium. In fact, in a homogeneous dissipative medium, uniform excitation is required to correctly recover attenuation. Applied to fundamental-mode ambient seismic surface waves, this implies that the cross-correlation will decay at the local attenuation rate only if noise sources are distributed uniformly on the Earth's surface. In this study we show that this constraint can be relaxed in case the observed loss of coherence is due to multiple scattering instead of dissipation of energy. We describe the scattering medium as an effective medium whose phase velocity and rate of attenuation are a function of the scatterer density and the average strength of the scatterers. We find that the decay of the cross-correlation in the effective medium coincides with the local attenuation of the effective medium in case the scattering medium is illuminated uniformly from all angles. Consequently, uniform excitation is not a necessary condition for the correct retrieval of scattering attenuation. We exemplify the implications of this finding for studies using the spectrally whitened cross-correlation to infer subsurface attenuatio

Tomography of core-mantle boundary and lowermost mantle coupled by geodynamics

We propose an innovative approach to mapping CMB topography from seismic P-wave traveltime inversions: instead of treating mantle velocity and CMB topography as independent parameters, as has been done so far, we account for their coupling by mantle flow, as formulated by Forte & Peltier. This approach rests on the assumption that P data are sufficiently sensitive to thermal heterogeneity, and that compositional heterogeneity, albeit important in localized regions of the mantle (e.g. within the D″ region), is not sufficiently strong to govern the pattern of mantle-wide convection and hence the CMB topography. The resulting tomographic maps of CMB topography are physically sound, and they resolve the known discrepancy between images obtained from classic tomography on the basis of core-reflected and core-refracted seismic phases. Since the coefficients of mantle velocity structure are the only free parameters of the inversion, this joint tomography-geodynamics approach reduces the number of parameters; nevertheless the corresponding mantle models fit the seismic data as well as the purely seismic one

On estimating attenuation from the amplitude of the spectrally whitened ambient seismic field

Measuring attenuation on the basis of interferometric, receiver-receiver surface waves is a non-trivial task: the amplitude, more than the phase, of ensemble-averaged cross-correlations is strongly affected by non-uniformities in the ambient wavefield. In addition, ambient noise data are typically pre-processed in ways that affect the amplitude itself. Some authors have recently attempted to measure attenuation in receiver-receiver cross-correlations obtained after the usual pre-processing of seismic ambient-noise records, including, most notably, spectral whitening. Spectral whitening replaces the cross-spectrum with a unit amplitude spectrum. It is generally assumed that cross-terms have cancelled each other prior to spectral whitening. Cross-terms are peaks in the cross-correlation due to simultaneously acting noise sources, that is, spurious traveltime delays due to constructive interference of signal coming from different sources. Cancellation of these cross-terms is a requirement for the successful retrieval of interferometric receiver-receiver signal and results from ensemble averaging. In practice, ensemble averaging is replaced by integrating over sufficiently long time or averaging over several cross-correlation windows. Contrary to the general assumption, we show in this study that cross-terms are not required to cancel each other prior to spectral whitening, but may also cancel each other after the whitening procedure. Specifically, we derive an analytic approximation for the amplitude difference associated with the reversed order of cancellation and normalization. Our approximation shows that an amplitude decrease results from the reversed order. This decrease is predominantly non-linear at small receiver-receiver distances: at distances smaller than approximately two wavelengths, whitening prior to ensemble averaging causes a significantly stronger decay of the cross-spectru

The European Upper Mantle as Seen by Surface Waves

We derive a global, three-dimensional tomographic model of horizontally and vertically polarized shear velocities in the upper mantle. The model is based on a recently updated global database of Love- and Rayleigh-wave fundamental-mode phase-anomaly observations, with a good global coverage and a particularly dense coverage over Europe and the Mediterranean basin (broadband stations from the Swiss and German seismic networks). The model parameterization is accordingly finer within this region than over the rest of the globe. The large-scale, global structure of our model is very well correlated with that of earlier shear-velocity tomography models, based both on body- and surface-wave observations. At the regional scale, within the region of interest, correlation is complicated by the different resolution limits associated to different databases (surface waves, compressional waves, shear waves), and, accordingly, to different models; while a certain agreement appears to exist for what concerns the grand tectonic features in the area, heterogeneities of smaller scale are less robustly determined. Our new model is only one step towards the identification of a consensus model of European/Mediterranean upper-mantle structure: on the basis of the findings discussed here, we expect that important improvements will soon result from the combination, in new tomographic inversions, of fundamental-mode phase-anomaly data like ours with observations of surface-wave overtones, of body-wave travel times, of ambient "noise”, and by accounting for an a-priori model of crustal structure more highly resolved than the one employed her

Super-resolution in near-field acoustic time reversal using reverberated elastic waves in skull-shaped antenna

International audienceWe investigate the potential of using elastic waves for near-field acoustic time reversal, and in doing so evaluate the possibility of reconstructing sound source positions at below-wavelength distances from a skull-shaped acoustic antenna. Our work is based on a conceptual processing model that translates elastic waves conducted and reverberated in an elastic object into source position, through a time reversal analysis. Signals are recorded by passive sensors glued on a replica of a human skull, measuring solely its mechanical vibrations, and not sensitive to airborne sound. The sound source is placed along the azimuthal and sagittal planes for distances to the skull between 5 and 100 cm. We reconstruct the source position for signals with frequencies in the physiological hearing range with a resolution indirectly proportional to the distance between source and skull across all measurements in the far-field. Measurements in the near-field show –3 dB widths smaller than half a wavelength (super-resolution) with highest resolutions of down to λ/15 measured in front of the orbital cavities. We infer that these anatomical details give rise to complex features of the skull's Green's function, that in turn enhance resolution in a direction-dependent manner