Towards 4-D Noise-based Seismic Probing of Volcanoes: Perspectives from a Large-N Nodal Experiment on Piton de la Fournaise Volcano
Abstract:Noise-based seismology is proving to be a complementary approach to active source or earthquake-based methods for imaging and monitoring the Earth’s interior and in particular volcanoes and active faults. Until recently, noise-based imaging and monitoring relied only on the inversion of surface waves reconstructed from correlations of mostly microseismic seismic noise. Compared to body-wave tomography, surface wave tomography succeeds in retrieving lateral sub-surface velocity contrasts but is less efficient in resolving velocity perturbations at depth. Moreover reflected body-waves can carry direct information about sharp interfaces at depth. Extracting body-waves from noise correlations is challenging and the use of Large-N seismic arrays proves to be of great benefit for extracting noisy body-waves from noise-correlations by stacking over a large number of receiver pairs and by applying array processing.
The purpose of VolcArray Large-N seismic experiment on Piton de la Fournaise Volcano is to extract body-waves travelling directly through the active magma reservoir located at ~2.5 km depth below the summit crater using noise correlations between arrays of seismic nodes. By beamforming noise on individual arrays, we found an unusual strong directional source of body-wave noise. This is thus a favorable context for retrieving the body-wave component of the Green’s function between arrays. However, standard correlation techniques between nodes do not allow deciphering between the reconstructed Green’s function and artifacts from the correlation of the strong directional source of body-waves. By applying double beamforming to the noise correlations between arrays, we are able to isolate ballistic body-waves travelling across the magma storage zone at depth. The stability of these reconstructed waves over time is encouraging in the perspectives of high resolution monitoring of the volcano feeding system.