Separation of intrinsic absorption and scattering attenuation fromLgcoda decay in central France using acoustic radiative transfer theory

Abstract

SUMMARY Radiative transfer theory is applied to the measurement of average values of the transport mean free path and absorption length (la, related to the intrinsic quality factor) in the lithosphere beneath France by comparing simulated and observed coda envelopes of regional records. The lithosphere is modelled as a flat layer representing the crust overlying a half‐space representing the mantle, with different velocities and scattering properties. We infer the average values of la (assumed to be constant over the whole medium) and transport mean free paths in the crust (l*c) and mantle (l*m) by minimizing a misfit function between observed and modelled Lg coda envelopes. This comparison is conducted in two different time windows, the whole Lg coda (group velocity smaller than 2.6 km s−1) and the early coda (vg < 2.6 km s−1 and lapse time smaller than 300 s), when coda envelopes have decay rates that depend on distance. In the early coda, a single set of parameters la and l*c minimizes the misfit function. Using the whole Lg coda time window, we find that many models fit the data equally well. We explain this difference by the common decay rate of envelopes independent of distance over most of the coda window. The measurement of the attenuation of the coherent Lg wave complements these results to constrain la and l*c. We find that a model with l*c∼ 250 km and la∼ 150 km explains the attenuation of both the direct Lg wave train and the entire Lg coda with both time and distance at 3 Hz. However, the trade‐off between the two parameters is too strong to conclude reliably on the dominance of intrinsic on scattering attenuation.

Modeling Full Seismogram Envelopes Using Radiative Transfer Theory with Born Scattering Coefficients

“…However, in the inversion process it turned out that trade-off problems exist using the late coda only. Therefore, additional to the late coda, LACOMBE et al (2003) used information on direct waves. Also an inversion strategy based on ''multiple lapse time window analysis'' (FEHLER et al, 1992) uses information of an early time window.…”

“…Moreover, for the comparison with data a layered random medium including effects such as the crust mantle boundary must be included in the theory. Such depthdependent phenomena can also be described within the theory of radiative transfer (e.g., MARGERIN et al, 1998;YOSHIMOTO, 2000;HOSHIBA et al, 2001;LACOMBE et al, 2003).…”

“…MARGERIN et al (2000) and PRZYBILLA et al (2006) presented a numerical Monte-Carlo method to solve this coupled elastic transport equation including non-isotropic scattering and conversions between P and S waves. A depth-dependent background velocity or a depth-dependent scattering coefficient can also be included in the theory (e.g., MARGERIN et al, 1998;YOSHIMOTO, 2000;HOSHIBA et al, 2001;LACOMBE et al, 2003).…”

Influence of scattering effect on laboratory ultrasonic attenuation measurements

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Phase and Correlation in ‘Random’ Seismic Fields and the Reconstruction of the Green Function

“…In this model Q c can be regarded as the time of residence of diffuse waves in the crust. The same type of conceptual stratified model was used by LACOMBE et al (2003) who modelled simultaneously the spatial decay of direct Lg waves and the envelopes of regional coda waves with only the mean free path and absorption length in the crust.…”