Abrupt changes of hydrothermal activity in a lava dome detected by combined seismic and muon monitoring.

The recent 2014 eruption of the Ontake volcano in Japan recalled that hydrothermal fields of moderately active volcanoes have an unpredictable and hazardous behavior that may endanger human beings. Steam blasts can expel devastating ejecta and create craters of several tens of meters. The management of such hydrothermal events in populated areas is problematic because of their very short time of occurrence. At present no precursory signal is clearly identified as a potential warning of imminent danger. Here we show how the combination of seismic noise monitoring and muon density tomography allows to detect, with an unprecedented space and time resolution, the increase of activity (at timescales of few hours to few days) of a hydrothermal spot located 50 to 100 m below the summit of an active volcano, the La Soufrière of Guadeloupe, in the Lesser Antilles. We show how the combination of those two methods improves the risk evaluation of short-term hazards and the localization of the involved volumes in the volcano. We anticipate that the deployment of networks of various sensors including temperature probes, seismic antennas and cosmic muon telescopes around such volcanoes could valuably contribute to early warning decisions.

The wavelet response as a multiscale NDT method.

We analyze interfaces by using reflected waves in the framework of the wavelet transform. First, we introduce the wavelet transform as an efficient method to detect and characterize a discontinuity in the acoustical impedance profile of a material. Synthetic examples are shown for both an isolated reflector and multiscale clusters of nearby defects. In the second part of the paper we present the wavelet response method as a natural extension of the wavelet transform when the velocity profile to be analyzed can only be remotely probed by propagating wavelets through the medium (instead of being directly convolved as in the wavelet transform). The wavelet response is constituted by the reflections of the incident wavelets on the discontinuities and we show that both transforms are equivalent when multiple scattering is neglected. We end this paper by experimentally applying the wavelet response in an acoustic tank to characterize planar reflectors with finite thicknesses.