26 janvier 2012 ( dernière mise à jour : 25 juillet 2013 )
Depth sensitivity of coda waves to velocity perturbations in an elastic heterogeneous medium
Numerous monitoring applications make use of seismic coda waves to evaluate velocity changes in the Earth. This raises the question of the spatial sensitivity of coda-wave-based measurements. Here, we investigate the depth sensitivity of coda waves to local velocity perturbations using two-dimensional numerical wave-field simulations. We calculate the impulse response at the surface before and after a slight perturbation of the velocity within a thin layer at depth is introduced.We perform parametric analysis of the observed apparent relative velocity changes, versus the depth of the thin perturbed layer. Through the analysis of the decay of the apparent relative velocity changes, we can discriminate two different regimes : one for a shallow perturbation and the other for a deep perturbation. We interpret the first regime as the footprint of the one-dimensional depth sensitivity of the fundamental mode surface waves. For perturbations at greater depths, we propose a model based on multiple scattering of body waves in the bulk. For intermediate depths, we show that the depth sensitivity of coda waves can be modeled as a combination of bulk-wave sensitivity and surface-wave sensitivity. The transition between those two regimes is governed by mode conversions due to scattering. We indicate the importance of surface waves for the sensitivity of coda waves at shallow depths and at early times in the coda. At later times, bulk waves clearly dominate the depth sensitivity and offer the possibility of monitoring changes at depths below the sensitivity of the surface waves. Based on the transition between the two regimes, we can discriminate a change that occurs at the surface from a change that occurs at depth. This is illustrated for shallow depth perturbations through an example from lunar data.
Locating changes on an active volcano (Piton de la Fournaise) using ambient seismic noise correlations
We analyze continuous ambient seismic noise records from the active volcano Piton de la Fournaise on la Reunion Island from June to December 2010. During this time two volcanic eruptions occurred. We calculate the cross-correlation functions between 21 broadband stations for this period. We monitor the apparent relative velocity changes with the stretching technique, as well as the evolution of the decoherence of the signals. The temporal variations of both parameters are precursors of volcanic eruptions.
We compute sensitivity kernels between the stations using a radiative transfer approach for the intensity propagation. Then we use a least-square inversion to locate the changes associated to the eruptions (apparent velocity changes and waveform decorrelation) in space : prior, during, and after the eruption.