Toward 4D Noise-Based Seismic Probing of Volcanoes: Perspectives from a Large-N Experiment on Piton de la Fournaise Volcano by F. Brenguier, P. Kowalski, N. Ackerley, N. Nakata, P. Boué, M. Campillo, E. Larose, S. Rambaud, C. Pequegnat, T. Lecocq, P. Roux, V. Ferrazzini, N. Villeneuve, N. M. Shapiro, and J. Chaput ABSTRACT Noise-based seismology is proving to be a complementary ap- proach to active-source or earthquake-based methods for im- aging and monitoring the Earth’s interior. Until recently, however, noise-based imaging and monitoring relied mostly on the inversion of surface waves reconstructed from corre- lations of mostly microseismic seismic noise (around 5 s of period). Compared to body-wave tomography and consider- ing similar wavelengths, surface-wave tomography succeeds in retrieving lateral subsurface velocity contrasts but is less effi- cient in resolving velocity perturbations at depth. Recently, the use of large-N seismic arrays has proven to be of great benefit for extracting body waves from noise correlations by stacking over a large number of receiver pairs and by ap- plying array processing. In this article, we describe a recent large-N array experiment that we conducted on Piton de la Fournaise (PdF). Our main goal was to extract body waves traveling directly in the vicinity of the active magma reservoir located at ∼2:5 km depth below the summit crater using noise correlations between arrays of seismic sensors. Within this article, we provide technical information about the Vol- cArray experiment, which consisted of the deployment of 300 seismic nodes during one month on PdF. We also present data-quality measurements and show how the short-period seismometers that we used compare to standard broadband seismic records. Finally, we show noise array beamforming re- sults to study the content of seismic noise at frequencies be- tween 1 and 12 Hz and discuss the ability to use these data to recover body waves between arrays from the correlation of ambient seismic noise. INTRODUCTION Noise-based seismology is proving to be a complementary ap- proach to active-source or earthquake-based methods for im- aging and monitoring the Earth’s interior. In particular, surface waves reconstructed from noise correlations have been used to image the crust from regional (Shapiro et al., 2005) to local scales, including volcanoes (Brenguier et al., 2007; Jaxybulatov et al., 2014) and fault zones (Roux et al., 2011). Noise-based seismology has also proven to be able to resolve subtle changes of the subsurface, opening the way to 4D probing of active faults and volcanoes (Sens-Schönfelder and Wegler, 2006; Brenguier et al., 2008, 2014). Until recently, noise-based im- aging and monitoring relied mostly on the inversion of surface- waves reconstructed from correlations of mostly microseismic seismic noise (around 5 s of period). Compared to body-wave tomography and considering similar wavelengths, surface-wave tomography succeeds in re- trieving lateral subsurface 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. The study of Roux et al. (2005) showed that it was possible to extract direct diving P waves between sensors from noise correlations at a local scale. Also, recently, Nakata et al. (2015) were able to obtain a 3D body-wave tomography of the subsurface from correlations of seismic noise recorded at a dense array of more than 2500 seismic sensors in the city of Long Beach near Los Angeles. The use of large-N seismic arrays has proven to be of great benefit for extracting body waves from noise correlations by stacking over a large number of receiver pairs and by applying array processing. Piton de la Fournaise volcano (PdF, La Réunion island) is a very active basaltic volcano, with one eruption per year on average. PdF is among the best-instrumented volcanoes world- wide, making it effectively a laboratory volcano for developing novel imaging and monitoring geophysical techniques. From 2009 to 2012, we conducted a dense broadband seismic experi- ment on PdF with the purpose of improving the spatial and temporal resolution of noise-based seismic monitoring of PdF’s doi: 10.1785/0220150173 Seismological Research Letters Volume 87, Number 1 January/February 2016 15