The reflection coefficient of a fractional reflector
Résumé
This paper considers the question of characterizing the behavior of waves reflected by a fractional singularity of the wave speed profile $c(x_1)=c_0(1+((\frac{x_1}{l_0})_+)^{\alpha})^{-\frac12}$. We first focus on the case of one spatial dimension and a harmonic time dependence. We define the reflection coefficient $R$ from a limiting absorption principle. We provide an exact formula for R in terms of the solution to a Volterra equation. We obtain the asymptotic limit of this coefficient in the large $\frac{l_0\omega ω}{c_0}$ regime as
$R= \frac{\Gamma(\alpha+1)}{(2i)^{\alpha+2}}\theta+o(1)$
The amplitude is proportional to $\omega^{-\alpha}$, and the phase rotation behavior is obtained from the $i^{−{\alpha+2}}$ factor. The proof method does not rely on representing the solution by special functions, since $\alpha > 0$ is general.
In the multi-dimensional layered case, we obtain a similar result where the nondimen- sional variable lω/c0 is modified to account for the angle of incidence. The asymptotic analysis requires the waves to be non-glancing. The resulting reflection coefficient can now be interpreted as a Fourier multiplier of order $-\alpha$
In practice, the knowledge of the dependency of both the amplitude and the phase of $R$ on $\omega$ and $\alpha$ might be able to inform the kind of signal processing needed to characterize the fractional nature of reflectors, for instance in geophysics.
Domaines
Mathématiques [math]Origine | Fichiers produits par l'(les) auteur(s) |
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