Volcanic eruptions are valuable calibrating sources of infrasonic waves worldwide detected by the International Monitoring System (IMS) of the Comprehensive Nuclear Test-Ban-Treaty Organization (CTBTO) and other experimental stations. In this study, we assess the detection capability of the European infrasound network to remotely detect the eruptive activity of Mount Etna. This well-instrumented volcano offers a unique opportunity to validate attenuation models using multi-year near-and far-field recordings. The seasonal trend in the number of detections of Etna at the IS48 IMS station (Tunisia) is correlated to fine temporal fluctuations of the stratospheric waveguide structure. This observed trend correlates well with the variation of the effective sound speed ratio which is a proxy for the combined effects of refraction due to sound speed gradients and advection due to along-path wind on infrasound propagation. Modeling results are consistent with the observed detection capability of the existing regional network. In summer, during the downwind season, a minimum detectable amplitude of ~10 Pa at a reference distance of 1 km from the source is predicted. In winter, when upwind propagation prevails, detection thresholds increase up to ~100 Pa. However, when adding four experimental arrays to the IMS network, the corresponding thresholds decrease down to ~20 Pa in winter. The simulation results provide here a realistic description of long- to mid-range infrasound propagation and allow predicting fine temporal fluctuations in the European infrasound network performance with potential application for civil aviation safety.
A large variety of natural and anthropogenic phenomena produces intense low-frequency acoustic waves below the 20 Hz human hearing threshold [
Among numerous naturally occurring geophysical phenomena generating acoustic waves, volcanic eruptions are outstanding sources of repetitive signals [14-16]. As infrasound signals are associated with the massive ejection of material and the release of conduit overpressure, they are a good indicator that an eruption has occurred [17-19]. Moreover, due to the long-range propagation of infrasound, this technique is valuable to remotely monitor volcanoes in regions where ground-based observations are sparse [20-23] and identify potential hazards for aircraft safety [24,25].
Many active volcanoes are permanently detected by the IMS infrasound network [
As the activity of Etna is mostly effusive, sometimes accompanied by small-to-moderate explosions, it often yields to small VEI (1-2).
During 2008-2009 and early 2010, no significant explosive activity was reported. Since October 2010, more paroxysmal eruptive periods, characterized by strong Strombolian activity, lava fountaining, and often dense dark ash emissions were reported. However, such information is not as precise as continuous near-field observations. The Smithsonian database is more useful for explosive volcanoes that rarely erupt or at least not continuously, and where the amount of observations is limited.
The main objective of this study is to assess the potential of the European infrasound network to monitor Etna by analyzing nearand far-field recordings from 2008 until now. In order to calibrate the existing network and evaluate the performance of the future ARISE (Atmospheric dynamics Research Infra Structure in Europe (http://arise-project.eu/) network, frequency-dependent attenuation relations are integrated into a network performance modeling technique. We first present the infrasound network and describe array processing methods. Then, the capability of the IS48 IMS station to detect Etna is analyzed by considering a detailed description of both the background noise at the receiver and the dynamics of structure of the stratosphere. Finally, we evaluate the performance of the existing IMS network and quantify its improvement by adding experimental stations.
The IMS network is unique by its global and homogeneous coverage. Significant advances in array designs and processing methods as well as the development of highly sensitive sensors and efficient wind-noise filtering systems allow now detecting low-amplitude coherent signals from remote volcanoes with an unprecedented precision [10-29]. In particular, at a distance of about 550 km, Etna is permanently monitored by the IS48 IMS station (35.80˚N, 9.32˚E) located in Tunisia. In case of major eruption, signals can be detected by other IMS stations like IS26 in Germany and IS43 in Russia, at a distance of 1240 and 2680 km, respectively. In addition to the existing operating IMS network, we consider in this study the four experimental arrays OHP, AMT, CEA and Flers (
In the near-field, one permanent small aperture (~250 m) four-element array, ETN, operated by the University of Firenze (UNIFI), routinely records the Etna activity since 2007. Each array element is equipped with a differential pressure transducer with a sensitivity of 20 mV/Pa [
IS48 well detects Etna (550 km, 65.4˚) and Stromboli (618 km, 55.9˚), the nearest active volcanoes. Itconsists of seven separate MB2000-type microbarometers connected to a central recording facility with inter-sensor-spacing
ranging from 150 m to 1.6 km. The microbarometers operate from DC up to 27 Hz with a flat frequency response from 0.02 to 4 Hz and an electronic self-noise level of 2 mPa RMS (<18 dB below the minimum acoustic noise at 1Hz). Infrasound data are routinely processed with the Progressive Multi-Channel Correlation method (PMCC) [
§ From 0.1 to 0.3 Hz, microbaroms produced by large interacting open-ocean swell systems [33-35] nearly continuously detected in winter from North Atlantic Ocean with a back-azimuth between 310˚ and 320˚, and in summer from the Mediterranean Sea with a back-azimuth about 90˚.
§ Above 0.5 Hz, in summer, persistent detections associated with eruptions of Mt. Etna and Stromboli with back-azimuth between 50˚ and 80˚. Paroxysmal events but also small-to-moderate explosions are almost continuously recorded between May and September.
§ Above 1 Hz, detections possibly related to industrial activity (oil and gas fields, refineries in Libya and Algeria) with back-azimuth between 120˚ and 210˚.
Monitoring infrasound at IS48over several years reveals a clear seasonal transition in the bearings along with the stratospheric general circulation between summer and winter for both microbarom and volcano signals. Furthermore, these seasonal variations reverse whether sources are locate east or west of the array.This oscillation clearly captured in climatological wind models [4-36] controls to the first order the direction from where signals are expected to be detected.