The TOMO-ETNA experiment : an imaging active campaign at Mt . Etna volcano . Context , main objectives , working-plans and involved research projects

The TOMO-ETNA experiment was devised to image of the crust underlying the volcanic edifice and, possibly, its plumbing system by using passive and active refraction/reflection seismic methods. This experiment included activities both on-land and offshore with the main objective of obtaining a new high-resolution seismic tomography to improve the knowledge of the crustal structures existing beneath the Etna volcano and northeast Sicily up to Aeolian Islands. The TOMO-ETNA experiment was divided in two phases. The first phase started on June 15, 2014 and finalized on July 24, 2014, with the withdrawal of two removable seismic networks (a short period network and a broadband network composed by 80 and 20 stations respectively) deployed at Etna volcano and surrounding areas. During this first phase the oceanographic research vessel (R/V) “Sarmiento de Gamboa” and the hydro-oceanographic vessel (H/V) “Galatea” performed the offshore activities, which includes the deployment of ocean bottom seismometers (OBS), air-gun shooting for wide angle seismic refraction (WAS), multi-channel seismic (MCS) reflection surveys, magnetic surveys and ROV (remotely operated vehicle) dives. This phase finished with the recovery of the short period seismic network. In the second phase the broadband seismic network remained operative until October 28, 2014, and the R/V “Aegaeo” performed additional MCS surveys during November 19-27, 2014. Overall, the information deriving from TOMO-ETNA experiment could provide the answer to many uncertainties that have arisen while exploiting the large amount of data provided by the cutting-edge monitoring systems of Etna volcano and seismogenic area of eastern Sicily.


Introduction
Mt. Etna is one of the most active volcanoes in the world, located on the densely inhabited eastern coast of Sicily (Italy). It is characterized by almost continuous eruptive activity from its summit craters and fairly frequent lava flow eruptions from fissures opened up on its flanks. At present, knowledge of the deeper structure of Mt. Etna is one of the most intriguing questions that the scientific community working on this volcano would like to image. An improved structural model of this volcano may give new indications on the interaction between its magma plumbing system and regional tectonic regime. Moreover, a joint interpretation in-cluding magma driving conditions, structural framework and tectonic forces would provide a new integrated volcanological model, which is at the base of any further studies including volcanic hazard assessment.
Nowadays, the relation between the magma source in the mantle and the upper parts of the system, as well as the relation between tectonics and volcanism and the role of lithospheric faults, could be better resolved by performing active seismic experiments. These are several examples, such as those recently have been done at Usu volcano in Japan [Onizawa et al. 2007], at Vesuvius and Stromboli volcanoes [Zollo et al. 1998;Marsella et al. 2007;Castellano et al. 2008], at the Deception volcano, Antarctica [Barclay et al. 2009;Ben Zvi et al. 2009;Zandomeneghi et al. 2009], at Montserrat Island [Shalev et al. 2010;Voight et al. 2014] or more recently at Tenerife Island ; García-Yeguas et al. 2012]. The active seismic experiment is needed to broaden the knowledge of the inner structure of Mt. Etna and surrounding area, from its basement down to the upper mantle.
Mt. Etna volcano and surrounding areas have been the focus of several multidisciplinary studies (see next section) carried out in the last 25 years. They provided detailed information of the most upper part of Mt. Etna volcanic edifice (up to 8 km below sea level). However it is crucial to enlighten deeper parts of Mt. Etna and surrounding areas with new data, therefore the use of new techniques that could provide high quality and reliable images is essential. Thus the TOMO-ETNA experiment, performed in the framework of EC-FP7 MED-SUV and EUROFLEET2 MEDSUV.ISES projects (Appendix A), was conceived to investigate the inner structure of this volcanic area and its surrounding areas using multidisciplinary approaches. One of the most relevant and innovative aspects of this project is a joint inversion of active and passive seismic data aimed at achieving a snapshot of Mt. Etna volcano and surrounding areas shallow and deep structures. For this purpose, data provided by active and passive seismic sources registered in a large area that covers more than 135×165 kilometers including terrestrial and marine territories have been collected (Figure 1). The same data set is adequate to perform 2D and 3D attenuation tomography, while about 1410 km of marine seismic reflection profiles were acquired to image in detail the seismic-stratigraphic and structural setting of the crust, to near/down the limit of the Moho discontinuity in the Ionian and Tyrrhenian Seas ( Figure 2). Finally, highresolution bathymetry and magnetic surveys together with, ROV (remotely operated vehicle) imaging and sampling will complement the seismic studies. The final goal is to better define the main regional fault systems and the crustal seismo-stratigraphic pattern and to contribute for understanding the physical processes controlling magma ascent beneath Mt. Etna and Aeolian Island volcanoes. This additional information will provide the answer to the many questions that have arisen while exploiting the large amount data set provided by the cutting-edge monitoring systems deployed on Mt. Etna and in eastern Sicily. It is noteworthy that the geodynamic setting of the South Tyrrhenian -Calabrian Arc -Ionian Basin, which originates from the diachronic and fragmented convergence between the Eurasia and Africa plates, is one of the most intriguing worldwide areas. In this framework the relationship between volcanism (i.e. Mt. Etna and Aeolian Islands) and geodynamic setting still presents some shortcomings that the proposed seismic experiment might clarify.
This paper describes the TOMO-ETNA experiment aims, plans, support and funding (Appendix A), timing, involved researchers (Appendix B), international relationships (Appendix C) and data management (Appendix D). TOMO-ETNA experiment is involved within the EC-FP7 MED-SUV project which started on June 1st, 2013, and will finish on May 31, 2016, involving 24 partners from European and American scientific institutions, including four research centers, and four European SMEs. The active phase of the experiment started in June 2014 and finished in November 2014. The planning and preparation, prior to the field operations, has foreseen several meetings, during which pro-posals and new projects were organized in order to obtain additional funds, seismic instruments, oceanographic vessels and the enlargement of the expected research team involved in the experiment. This preliminary phase began in 2011. During the experiment a large number of documents were generated providing detailed information on the evolution of the experiment to the general public and to the rest of the consortium. These documents are still available online in the following address http://iagpds. ugr.es/pages/proyecto_italia/proyecto_italia_jesus. MED-SUV applies the concept of the GEO Supersites Initiative to Mt. Etna and Campi Flegrei/Vesuvius volcanoes (southern Italy) optimizing and integrating existing and new observation/monitoring systems, by a breakthrough in understanding of volcanic processes and by increasing the effectiveness of the coordination between science and end-user communities. Specific experiments, such as TOMO-ETNA have been carried out to improve the understanding of internal structure and dynamics of volcanoes. Another fundamental contribution to the achievement of the TOMO-ETNA experiment was provided by the EC-FP7 EUROFLEETS2 project (project MEDSUV.ISES), which provided support to the use of the vessels for carrying out the marine activities. This manuscript is the first of a set of 11 papers describing the multidisciplinary topics of the experiment and some of the preliminary results obtained with the data provided by the experiment.

Context of the experiment
The eastern side of the Sicily is mainly characterized by two active volcanic areas, Mt. Etna volcano located in eastern Sicily and the Aeolian Archipelago lying in the southeastern Tyrrhenian Sea. Mt. Etna is placed in a geodynamic complex region near the Eurasia-Africa plate boundary. This volcano lies in front of the southeast-verging Apennine-Maghrebian fold-andthrust belt, where the NNW-trending Malta Escarpment separates the Sicilian continental crust from the Ionian Mesozoic oceanic basin, presently subducting beneath the Calabrian Arc [Selvaggi and Chiarabba 1995]. The Aeolian Archipelago represents a volcanic arc consisting of seven major islands and a wider seamount system, result of the Ionian lithosphere subduction beneath the Calabrian Arc and Tyrrhenian Sea [e.g., Caputo et al. 1972;Barberi et al. 1994;Mantovani et al. 1996] and of a post-subduction extensional deformation [Beccaluva et al. 1985;Westaway 1993;Ventura et al. 1999;De Astis et al. 2003].

Mt. Etna volcano
The volcano structure is surrounded by 3 main regional structures: i) to the north and westwards we find the Apenninic-Maghrebian Chain; ii) southwards appears the Hyblean Foreland that belongs to the African plate [Lentini et al. 2006]; iii) finally to the east stands the Ionian Basin, which is an extensional basin originated during middle-late Mesozoic ]. This collisional limit induces a regional N-S com-pression that is combined with an E-W extension, related to the Malta Escarpment system [Bousquet and Lanzafame 2004], affecting the eastern part of the volcano. On the other hand, the western part is dominated by the regional compressive regime related to the regional Eurasia-Africa plate collision [Monaco et al. 2005]. The Moho appears to vary considerably from 30 km underneath the Hyblean Plateau to 18 km below the Ionian Basin [Nicolich et al. 2000, and reference therein]. According to this regional description, Mt. Etna's location fits neither typical arc magmatism nor a back-arc spreading region associated with the Apennines subduction .
Mt. Etna is a complex strato-volcano characterized by an eruptive activity occurring almost continuously, including volatile degassing, strombolian explosions, lava fountains and lava flows. Mt. Etna produces mild explosive eruptions and lava flows from both central craters and lateral fissures, although, a few strong explosive eruptions, up to plinian-type, have been documented [Coltelli et al. 2000]. On the basis of stratigraphic and geochronological studies, several authors have described the spatial and temporal evolution of the volcano as the result of four main volcanic phases [e.g. Gillot et al. 1994;Branca et al. 2004Branca et al. , 2008Branca et al. , 2011De Beni et al. 2011]. On the east and south-east flanks of the volcano different faults can be recognized; they represent the clearest morphological evidence of a very active tectonic. The Pernicana fault system, located in the north-east flank of the volcano, can be considered the most active fault in the Etnean area, as testified by the slip rate estimations and geodetic measurements. On the eastern flank, near the Ionian coast, seismogenic faults can be related to the NNW-SSE Malta Escarpment that is the main lithospheric structure in the eastern Sicily. Other seismogenetic faults , though not recognizable on the surface, can be linked to the NE-SW, ENE-WSW fault systems that control the tectonic evolution of the northern margin of the Hyblean Plateau [Torelli et al. 1998]. Instead on the western sector there is only slight morphological evidence of faulting, such as some short segments of faults observable on the south-western flank (e.g. Ragalna fault). However, the faults with morphological evidence may represent only a part of tectonic structures present in the Etnean area and hidden fault segments could be covered by the huge pile of volcanic products [e.g. Azzaro 1999].

The regional structure of the Calabrian-Peloritan region and Aeolian Arc
The Aeolian Arc is a volcanic structure, about 200 km long, located on the internal margin of the Calabrian-Peloritan forearc region, a Hercynian belt affected by Late Quaternary extensional tectonics and uplift. The Calabro-Peloritan basement runs from northern Calabria to eastern Sicily, and connects the southern Apennine and the Sicily-Maghrebian chains. It is bounded by the Sangineto tectonic line, in the north, and the Tindari-Letojanni-Malta line, in the south. The structure of the Calabro-Peloritano belt consists of a stack of various nappes composed of pre-Alpine metamorphic and granitoid rocks, often with Alpine metamorphic overprint, Mesozoic to Tertiary sedimentary rocks, ophiolitic sequences, and Quaternary sediments.
The Aeolian Arc is formed by seven subaerial volcanic edifices (Alicudi, Filicudi, Salina, Lipari, Vulcano, Panarea and Stromboli), emplaced on a 15 to 20 km thick continental crust, and their products, which ages between 1.3 Ma and the present, belong to the calc-alkaline, high-K calc-alkaline, shoshonitic and alkaline potassic associations. The geochemical affinity of these rocks and the occurrence of deep seismicity (up to 550 km; Milano et al. [1994]) below the southern Tyrrhenian Sea led to interpret the Aeolian Islands as a volcanic arc related to the active subduction beneath the Calabrian Arc [e.g. Beccaluva et al. 1985;Mantovani et al. 1996].
Structural trends and volcanic activity in the area are strongly controlled by the regional stress fields and bring to identify three distinct sectors: -the western sector (Alicudi and Filicudi islands) dominated by NW-SE-oriented tectonic lineaments; -the central sector (Salina, Lipari and Vulcano islands) aligned along the important regional transcurrent fault joining the Aeolian Islands to the Malta Escarpment with a NNW-SSE-oriented trend; on Salina and Lipari subordinate E-W-oriented trends are also present; -the eastern sector (Panarea and Stromboli islands) characterized by prevailing NE-SW-oriented tectonic lineaments.
The Aeolian Arc structure has been recently investigated [Chiarabba et al. 2008;Di Stefano et al. 2009] through high resolution Vp,Vp/Vs and Qp passive tomography performed on the base of 15 years of earthquake recordings by Italy's National Seismic Network. Tomography results show two arc-shaped low-and high Vp bands, located respectively at 25 and 100 km depth. Between 100 and 300 km two high Vp zones lie beneath Neapolitan and South Tyrrhenian regions, separated by unperturbed high Vp mantle. Intermediate depth seismicity is interpreted by the same authors as associated with the subduction of a thin oceanic crust, suggesting the occurrence of vigorous metamorphism. The high Vp/Vs and low Qp anomalies in the overlying mantle is probably associated with melting, this last related to dehydration induced by metamorphism.

Deep feeding structures of Etna
The knowledge of Mt. Etna's plumbing system has been addressed lately from multidisciplinary studies that combine gravimetric, geodetic and seismic techniques, among others [e.g. Battaglia et al. 2008;Bonforte et al. 2013;Carbone et al. 2014;Patanè et al. 2011Patanè et al. , 2013Zuccarello et al. 2013]. Additionally, petrologic studies of magmas give clues of its source. The present plumbing system is quite complex and not very well defined. This system can be described as compose by 3 main structures: i) the deepest part of the reservoir, located at around 10 km depth b.s.l., was considered the source of the 2001 and part of the 2002-2003 flank eruptions among others [Patanè et al. , 2006Corsaro and Pompilio 2004]; ii) the main body of the reservoir spanning 2-9 km b.s.l. characterized by a long term mixing of the deep ascending magma with the more evolved magmas [Bonforte et al. 2009[Bonforte et al. , 2013Spilliaert et al. 2006;Corsaro et al. 2013]; iii) a shallow dyke complex, including a small and temporary shallow reservoir that fed the 2011-2014 lava fountaining episodes, which spans on the first 1-4 km below the summit craters [Bonnacorso et al. 2013;Corsaro et al. 2013;Patanè et al. 2013].

Tomographic background
More than thirty years of seismic tomography at Mt. Etna yields a reasonably accurate picture of the shallow-intermediate P-wave velocity structure of the volcano (down to 10 km depth) with the definition of a main upper and middle crustal intrusion complex. Chiarabba et al. [2004] presented a detailed summary of the seismic tomography preformed at Mt. Etna prior to 2004 (Table 1 from Chiarabba et al. [2004]). According to these authors, the first 3D tomographic inversion preformed at this volcano was carried out by Hirn et al. [1991] using local earthquakes. They imaged for the first time a high-velocity body (HVB hereinafter) located underneath the volcanic edifice reaching to 6 km b.s.l. Successive tomographic studies took advantage of the enhancement of the seismic network to improve the seismic dataset and therefore to obtain better resolution images. Cardaci et al. [1993] inverted a 3D Vp model that reached to 20 km depth using a long term seismic catalog (from 1976 to 1987). De Luca et al. [1997] added to the previous dataset data recorded up to 1995 from temporal and permanent arrays, leading to a new tomographic image. Since the 1990s, advanced tomographic techniques have developed, helping the scientific community to perform more accurate inversions, this is the case of Villaseñor et al. [1998], who took advantage of the previous dataset to apply advanced non-linear procedures, obtaining more accurate 3D seismic velocity models from Mt. Etna volcano.
Thereafter, researchers focused their efforts in improving the datasets. Chiarabba et al. [2000] and Laigle et al. [2000] increased considerably the number of P and S seismic phases to invert. Their results had a much better resolution using fewer earthquakes than the precedent long-term tomographies. Patanè et al. [2002] carried out a high-resolution tomography of the first 4 km by using P and S phase arrivals from almost 300 local events. Low V P /V S anomaly regions were observed both during the July 2001 and the October 2002 dike intrusions feeding these two eruptions. More recently, Patanè et al. [2006] carried out the first 4D tomography at Mt. Etna, between the 2001Etna, between the and 2002Etna, between the -2003 eruptive periods, remarking the importance of shortterm (snapshots) tomographies on volcanoes. Their results showed the HVB present in all previous studies and imaged a Vp/Vs anomaly for the first time interpreted as the trace of fluid intrusion (magma rich in gas) and gas migration from the shallow magma intrusion in the cracked volume that developed during the eruptive period. By studying the attenuation of P-waves (Q P ), De Gori et al. Etna. The most important result obtained from this joint analysis of V P , V P /V S and P-wave attenuation is an anomalous zone with normal to high V P and low V P /V S , which partially overlaps with a low Q P volume located along a NS trending "channel" beneath the central crater. This can be interpreted as a shallow volume characterized by high temperature where the magma is located with the presence of supercritical fluids.

The permanent seismic network of the INGV
The success of the above seismic studies at Mt. Etna has been facilitated thanks to the existence of a dense and high quality permanent seismic network operated by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) [e.g., Patanè et al. 1999Patanè et al. , 2004Patanè et al. , 2013. This network consists of 44 seismic stations, 32 of which are equipped with broadband (BB) and 12 with (c) short-period (SP) seismometers, ensuring a very good coverage of the volcanic area. Additional 80 BB seismic stations are located in Sicily (see Figure 1). All of BB stations are composed of 3 component Nanometrics Trillium sensors. At the INGV monitoring center in Catania, data are stored with a sampling interval of 0.01 s over consecutive, 2 min long digital archives. The seismic network has been considerably enhanced since 2005; the present configuration is described in . The seismic stations transmit the data via satellite or radio to the control room of the Osservatorio Etneo (the INGV Etna Observatory). The TOMO-ETNA experiment will combine the information of this permanent seismic network with the deployment of temporary three components seismic stations of different nature such as short period, broadbands, on land, and ocean bottom seismometers (OBS) offshore. As observed in Figure 2 this experiment has provided a unique opportunity to have a very dense seismic network in this complex and interesting region.

Scientific objectives of the TOMO-ETNA experiment
The main objective of the TOMO-ETNA experiment is to perform high-resolution seismic tomographies, in velocity and attenuation by using active and passive seismic data, in an area encompassing outstanding volcanoes as Mt. Etna and Aeolian islands. The achievement of this objective is based on the integration of insitu marine and land experiments and observations and on the implementation of new instruments and monitoring systems. For this purpose, onshore and offshore seismic stations, and passive and active seismic events have been used. Additional geophysical data such as seismic reflection, gravimentric and magnetic data have been collected to obtain a joint upper mantle-crust 3D image that could permit to make progress in the understanding of the dynamic of the region.
The core of the study is based on the active seismic experiment that used energy sources at sea. These signals have been recorded by seismic stations deployed on land and on the seafloor ( Figure 2) together with the permanent seismic network belonging to the INGV. A temporary dense seismic network composed by 98 seismometers and 25 OBS, both belonging to different institutions, were deployed respectively on land and on sea bottom. Seismic signals were generated by air-guns emplaced on two oceanographic research vessels (hereafter, R/V), "Sarmiento de Gamboa", Spain, and "Aegaeo", Greece. Together with the air-guns shooting processes we acquired data using marine magnetometer in order to complete the information.
The results of this experiment, integrated with previous geophysical surveys (e.g. gravity, magnetotellurics, etc.), will improve the current knowledge of the crust beneath Mt. Etna volcano and the physical processes controlling magma ascent, by cuttingedge modelling. For instance, ground deformation models will improve the knowledge of the physical and geometrical parameters of the internal structure of Mt. Etna and its basement, and will increase the accuracy and robustness of the cut-edge numerical modelling (e.g. finite element or boundary element methods). The obtained new numerical models will benefit from the outcomes of the seismic experiment. This experiment has a high number of innovative elements, of which we want to highlight: -Seismic data were recorded by a very large number of seismic stations, that range from ocean bottom seismometers, hydrophones, seismic antennas and permanent and portable seismic stations.
-It represents the first experiment covering such a wide and heterogeneous region, including two volcanic environments and five active volcanoes.
-The seismic tomography will integrate, and for the first time in Mt. Etna and surrounding region, passive sources (earthquakes and LP events) and marine active air-gun shootings. This approach will provide a 3D image inverting simultaneously P-waves travel times for both seismic sources.
-The expected image in the investigated area will be probably the deepest ever obtained in previous research works.
-The final tomographic images will be the product of the integration of seismic data with other geophysical surveys performed both in terrestrial and marine environments as magnetic, gravimetric and magnetotelluric measurements among others.
Aside the main objective, it is planned to achieve also further goals such as: -characterization of the volcanic processes through cutting-edge data analysis/modelling. This objective is aimed at improving the knowledge on the volcanic subsurface or surface processes during the pre-, syn-, and post-eruptive episodes of Etna and Aeolian volcanoes by fully exploiting the integrate marine and terrestrial data set; -development of next generation of monitoring and observing systems; -teaching at high-level (post graduate students) in Geophysics and Volcanic hazards. Some of the personnel involved in the experiment, both onboard and ashore, were Master or PhD students belonging to different countries involved in the project. The experiment provided a unique opportunity to learn both experimental marine and terrestrial techniques; -dissemination. This objective was aimed at broadcasting the outcomes of the project to the scientific community and the general public. This objective included information distribution in different websites, networking with ongoing national and international ventures rooted in the volcanological community, preparation of a TV documentary and others.
It is noteworthy that this project is fully transversal, multidisciplinary and crosses several societal sectors. It is transversal since we apply marine and terrestrial sciences and merge the observations to address multiple scientific problems in order to obtain a unified Earth crust and upper mantle model. It is multidisciplinary due to the combination of different Earth Science disciplines such as Terrestrial and Marine Seismology, Gravimetry, Geomorphology, Magnetic Field and others. Additionally we integrate experiments, technical development and numerical modelling. Several societal sectors will benefit from the outcomes of the project such as Volcanology, Civil Protection, Risk Management and Educational levels.

Work program. General description
The active high-resolution seismic tomography of Etna volcano has been designed to be performed in several phases, according to the different work involved in them (in next papers of the present special issue a detailed information regarding these field works are provided). They are: a) preparation of equipment (on-land seismometers, OBS, and other), software and site recognition and detailed planning of the activities; b) deployment of seismic equipment (OBS and onshore seismic stations); c) air-guns shooting and other geophysical measurement activities; d) recovery of the portable marine and onshore seismic stations; e) data analysis and results. The experiment included various on-and offshore activities (see Table 1). The on-shore deployment lasted from June 18 -November 24, 2014. Equipment included 80 short period (Table 1) and 18 broadband (BB; Table  1) stations, and included a relocation of 20 of the short period stations (on July 10) to increase the total number of recording sites. Short period equipment was removed on July 20, while all BB stations remained operative until October 27, in order to record additional natural seismicity. For more information regarding the on seismic network see Ibáñez et al. [2016] in this volume.
Off-shore activities of TOMO-ETNA experiment were completed with the support of four civilian and military vessels. The former were the R/Vs "Sarmiento de Gamboa" (CSIC-UTM, Spain) and "Aegaeo" (HCMR, Greece); the latter were "Galatea" and "Levanzo" (Italian Navy). Off-shore activities began on June 23, with the deployment of 22 OBS in the Ionian Sea and 5 OBS in the Tyrrhenian Sea by the R/V "Sarmiento de Gamboa" and the hydro-oceanographic vessel (hereafter, H/V) "Galatea" (Figure 2). The OBS network included 25 short period and 2 BB stations. All short period OBS were recovered by the R/V "Sarmiento de Gamboa" from July 18-20, while the two BB OBS were recovered on November 25 by the R/V "Aegaeo". For more information regarding the OBS activities see Coltelli et al. [2016] in this volume. From June 27-July 17, more than 16,000 air-gun shots were fired by the R/V "Sarmiento de Gamboa" during the active-source imaging experiment. Two seismic exploration techniques were employed within this experiment, wide angle seismic (June 27-July 6; WAS) and multi-channel seismic ( July 8-17; MCS) surveys (see Table 1 for more details). The "Levanzo" Italian Navy vessel provided support and oversight of the R/V "Sarmiento de Gamboa" during the MCS activities in Ionian Sea (Figure 2). A final round of high resolution MCS survey was performed, using the R/V "Aegaeo", during November 19-26 (see Table  1). For more information regarding MCS surveys see Coltelli et al. [2016] and Firetto Carlino et al. [2016] in this volume.
In Figure 2c, we show about 16.5 km of profile T-T05 acquired in the Tyrrhenian Sea and about 4 km of profile T-11 acquired in the Ionian Sea (see location in Figure 2b) after a preliminary CDP stack and post-stack depth migration, with the aim of providing a glimpse of the overall data quality. CDP data processing included geometry installation, and spiking/predictive deconvolution followed by band-pass filtering. Semblance-based velocity analysis methods were used to define a 2D background stacking velocity model for CDP ensemble stack. The stacking velocity model was then smoothed and converted to interval velocity to provide an interval velocity model for post-stack Kirchhoff depth migration.
Finally, with the aim of better defining major geological and structural features of the area, the H/V "Galatea" performed additional scientific activities, such as magnetic surveys (for more information regarding the employed equipment, see Table 1) and ROV dives between June 26 and July 5. New high resolution shipborne magnetic data were acquired off-shore of Etna volcano, covering the major structural features of both Timpe area and Riposto ridge. The magnetic survey was oriented in the NE-SW direction in order to intersect the major structural-volcanic features off-shore of Etna. Raw magnetic data were processed removing spikes and intervening outlier records, further statistical levelling provided a smooth distribution of the magnetic pattern of survey area. For more information regarding the magnetic and ROV surveys see Coltelli et al. [2016] and Cavallaro et al. [2016] in this volume. For the main technical specifications regarding the scientific equipment employed during the three oceanographic cruises of the TOMO-ETNA experiment see Table 1 and Coltelli et al. [2016].
The 3D velocity structure of Mt. Etna and surrounding areas will be determined using the data from this large data-set. It is expected to have more than 1·10 7 first arrivals recorded both on-and off-shore. The algorithm used for the inversion is an integration of the well-known ATOM-3D and LOTOS , 2014 codes in which natural seismicity and active signals will be inverted simultaneously. The first step is to apply an automatic first arrival picking procedure using spectral and temporal characteristics of the signals [Álvarez et al. 2013;García et al. 2016, this volume], including signal recognition algorithms [e.g. Benítez et al. 2007;Gutiérrez et al. 2009;Ibáñez et al. 2009;Cortés et al. 2009Cortés et al. , 2014. The 3D inversion will be performed in three phases: a) general image of the region under study using large lattices and only with the inversion of the active data; b) inclusion on passive seismicity to better constrain the deeper portion of the region and c) a high resolution seismic tomography of the Etna area using both active and passive seismicity and smaller cells.

Final remarks
The TOMO-ETNA experiment is an active source tomographic study, integrated by other geophysical surveys, carried out on the region of one of the most active basaltic volcanoes worldwide that could allow improving the knowledge of Etna. In fact, the future results of the linked researches should help scientists to better understand the eruptive mechanisms, and provide insights on its internal structure and on the deeper part of its plumbing system. This experiment engaged several European and non-European scientific institutions. It required an enormous management effort to plan the fieldworks and to coordinate hundreds of people employed on land and on the vessels during the experiment as well as to organize the data management and processing. It is remarkable that the experiment has been a complete success on the base on: i) the quantity and high quality of the acquired data; ii) the full integration of permanent and temporal seismic networks in different environments; iii) the wide covered region including volcanic and non-volcanic areas; iv) the multidisciplinary techniques integrated in the data acquisition; v) the international collaboration and efforts involved in the whole process; vi) the capacity to share responsibilities, data and scientific objectives that will produce at the same time several scientific results, among others.
The large set of acquired data [Barberi et al. 2016, this volume] allowed opening a wide range of multidisciplinary studies, which many of them are being carried out. Beside of the seismic tomography in velocity [Díaz-Moreno et al. 2016, this volume;Ibáñez et al. 2016, this volume], marine seismic reflection studies [Coltelli et al. 2016, this volume;Firetto Carlino et al. 2016, this volume], marine-magnetic surveys and ROV images [Cavallaro et al. 2016, this volume], scattering analysis [Zieger et al. 2016, this volume], 3D seismic array analysis [Zuccarello et al. 2016, this volume] and advance seismic signal processing [García et al. 2016, this volume] are being performed. Contemporaneously this experiment allowed us to increase the knowledge on the effect of geophysical marine measurements on the behavior of cetaceous [Monaco et al. 2016, this volume]. The final goal is to produce a multidisciplinary joint interpretation of the structure of the region generating a more reliable structural model to shed light into the complex framework in which Mt. Etna is placed.
These analyses are only the first step of a long term research program since it is expected to open several research lines such as: 2D and 3D seismic attenuation studies, including scattering and intrinsic separation [e.g. Prudencio et al. 2013aPrudencio et al. , 2013bPrudencio et al. , 2015b; precise nonlinear relocation of the seismicity using the new velocity models [e.g. Díaz-Moreno et al. 2015]; identification of seismo-volcanic signals [e.g. Ibáñez et al. 2009;Cortés et al. 2014]; analysis of the wave-field properties [e.g. Palo et al. 2009;De Lauro et al. 2012]; advance LP and explosion source inversion [e.g. La Rocca et al. 2000Rocca et al. , 2004Saccorotti et al. 2004;Petrosino et al. 2011]; analysis of scattered seismic wavefields [e.g. Del Pezzo et al. 1997;De Barros et al. 2012;De Lauro et al. 2012;Zieger et al. 2016, this volume]; analysis of receiver functions [e.g. Martínez-Arévalo et al. 2009;Lodge et al. 2012]; shear waves splitting [e.g. Martínez-Arévalo 2003;Bianco and Zaccarelli 2009, among others]. On the base of previous experience of the involved research team (e.g. Vesuvius and Campi Flegrei, Deception, Tenerife and Stromboli islands) the large amount of high quality geophysical data permit to assume high quality scientific production for the next ten or more years.
Acknowledgements. This paper has been partially funded by the following research projects: the European project MED-SUV funded by the European Union's Seventh Framework Program for research, technological development and demonstration under grant agreement No. 308665; the Spanish COCSABO project (COC-DI-2011-08); the European project EUROFLEETS2 (Seventh Framework Programme, grant agreement No. 312762) through transnational access to the research vessels "Sarmiento de Gamboa" operated by CSIC (Spain) and "Aegaeo" by HCMR (Greece); the Geophysical Instrument Pool Potsdam (GIPP) from GFZ (Potsdam) with the project (Seismic TOMOgraphy of ETNA volcano and Eolian Islands, Italy, using active and passive seismic data). We would like to thank the following supporting institutions: Dipartimento Regionale della Protezione Civile, Regione Siciliana; Dipartimento Azienda Regionale Foreste Demaniali, Ufficio Provinciale di Catania; Ente Parco dell'Etna; Unidad de Tecnología Marina -CSIC in Barcelona (Spain); Stato Maggiore Marina (Italian Navy General Staff ), CINC-NAV (Command in Chief of the Fleet) and Marisicilia (Navy Command of Sicily); Coastal Guard of Messina and Riposto; to obtain support and navigation permissions for the oceanographic cruises: Spanish Foreign Office and Italian Foreign Office. This paper has been partially supported by the Spanish projects TEC2015-68752-R (MINECO/FEDER), KNOWAVES and CGL2015-67130-C2-2. We would like to thank all private and public owners of the sites selected to deploy seismic stations for their kind and unselfish disposal to use their properties. This manuscript has been largely improved by the insightful comments of Dr. Mario Castellano and an anonymous reviewer and by the editor José Morales.

Appendix A Funding and associated research projects
The TOMO-ETNA experiment was evolved between 2011 and 2014, and mainly integrates the European Union project "MEDiterranean SUpersite Volcanoes (MED-SUV)" efforts with the resources of the EU project "EUROFLEETS2". However other funding agencies from Italy, Spain, and Germany supported this experiment. In addition, the Italian Navy and Sicily's Regional Civil Protection Department actively participated in the experiment. This such a long time interval includes several negotiations with different research and civil agencies and the application of various additional research project and parallel aids, including economic, equipment and human additional support. Both main research projects (MED-SUV and EUROFLEETS2) will be specifically described in next sections. The number of associated projects and other negotiations is large, and a full description of then is reported in Section A3. Next we will mention some of these negotiations processes and support obtained from them.

A1. MED-SUV project. The core project of the TOMO-ETNA experiment
The TOMO-ETNA experiment was conceived, planned and carried out in the framework of the EC-FP7 MEDiterranean Supersite Volcanoes (MED-SUV) project, which overarching objective was to apply the Supersite rationale to the Italian active volcanoes. The Supersite initiative born on 2007 at the conclusion of the 3rd International Geohazards workshop of GEO held in November 2007 in Frascati, Italy, with the aims "to stimulate an international and intergovernmental effort to monitor and study selected reference sites by establishing open access to relevant datasets according to Group Of Earth Observation (GEO) principles to foster the collaboration between all various partners and end-users" (Frascati declaration). Selected sites (Supersites) are areas prone to earthquake, volcano or other hazards and for which significant Earth observation and ground-based data sets are available. Thus, although not explicitly declared, Supersite initiative is intrinsically multiplatform being based on the use of data provided by research infrastructures belonging to space and Earth domains. On 2011 the European Commission promoted the application of the Supersite approach in Europe though a specific call; MED-SUV was one of the projects which positively responded to this call, focused their activities on Mt. Etna and Campi Flegrei/Vesuvius volcanoes. Since 2013 these two areas have been appointed as Permanent Supersites from the Scientific Advisory Committee (SAC) of GEO Geohazards Supersites and Natural Laboratories (GSNL), also considering the existence of MED-SUV project. In-deed, more than 3 million of people are exposed to potential volcanic hazards in a large region in the Mediterranean Sea, where these two volcanic areas are located. The wide range of styles and intensities of volcanic phenomena observed on these volcanoes, which can be assumed as archetypes of 'closed' and 'open conduit' volcano, together with the long-term multidisciplinary data sets give an exceptional opportunity to improve the understanding of a very wide spectrum of geohazards, as well as implementing a large variety of innovative models for investigating the volcanic processes.
The improvement of the knowledge of the Supersites and the sharing of the relevant data sets are two key principles of the Frascati declaration, which are the base for the enhancement of the hazard assessment and the risk reduction. MED-SUV adopted these two principles in the work plan definition, by attempting to cope with the main gaps in the knowledge in both areas and to improve the sharing of the data relevant to the two Supersites (either provided by the ground-based monitoring institutions, space agencies or collected during the project). The project proposes the development and implementation of a state-of-the-art infrastructure for the volcanic risk management life-cycle, from the observation to people preparedness. Thus, MED-SUV project offers the opportunity to operate in natural laboratories by using cutting-edges technologies and carrying out in-field and laboratories experiments.
Indisputably Mt. Etna and Campi Flegrei/Vesuvius are among the most well-known volcanoes worldwide. However, one of the most relevant gaps in the knowledge of Mt. Etna consisted in the relatively poor information concerning the deep structure of this volcano. Indeed, this inadequacy in the knowledge of the crust limits the capability of the models to investigate the dynamic of this volcano, despite through the last decades the computational capacity improved dramatically and many sophisticated numerical modelling approaches have been implemented.

A2. EUROFLEETS2. TOMO-ETNA experiment and marine implications
EUROFLEETS2 "New operational steps towards an alliance of European research fleets" (March 1, 2013, to February 28, 2017) is a project funded in the frame of the 7th Framework Programme of the European Commission. The project is the extension and expansion of the first initiative, EUROFLEETS "Towards an alliance of European research fleets" (September 1, 2009, to August 31, 2013. Both projects were granted under the funding schemes of research infrastructures and respectively a combination of Collabo-rative Project and Coordination and Support Actions for Integrating Activities. Research vessels (R/Vs) and their embarked equipment are crucial infrastructures necessary for marine sciences. They provide sea access to the whole community of researchers, enable all kinds of observations, in-situ measurements, sampling and mapping, and are necessary to maintain deep sea observatories. Over the years, the missions assigned to R/Vs became more complex and technically sophisticated. This encourages national marine institutions to increase their coordination and collaboration, and to share their facilities in order to optimize the cost in operating their marine infrastructures and to efficiently meet the scientific ever increasing demands.
By bringing together European research fleet owners and operators, EUROFLEETS2 (http://www.euro fleets.eu) contributes to enhance operational coordination between R/Vs and associated equipment. It has the aim of developing a new pan-European distributed infrastructure with common strategic vision and coordinated access to R/Vs and associated equipment, in order to facilitate the access to these rare floating laboratories and maximize their contribution to the scientific knowledge.
Through operational initiatives such as regional virtual fleets or the development of common tools or methodologies, this project promotes more inter-operable and cost-effective European research fleets for sustainable management of regional seas and oceans. EUROFLEETS2 aims also to develop the impact of research infrastructures on innovation by establishing links with industry and fostering the involvement of the industrial sector on specific activities.
EUROFLEETS2 involves multi-disciplinary expertise from thirty-one marine institutes, universities, foundations and small and medium enterprises (SMEs) from twenty member states of the European Union, four associated countries, and one overseas country and territory. The project has ambitious objectives and the 31-member consortium works together in three structured and complementary research activities: i) the transnational access (TNA), as core activity of the project, to coordinate access to European R/Vs and associated equipment for all European scientists and their partners; ii) the joint research activity ( JRA) for joint development of common equipment or software; iii) the networking activity (NA) with a range of actions for an enhanced coordination of European research fleets including the polar components. This activity includes also the opening of TNA calls and the evaluation of submitted proposals by a European panel of experts. Specific tasks are undertaken as well for the training of young marine scientists.
Within the innovative EUROFLEETS2 TNA activity, three different types of calls were opened: i) three calls offering access to 22 European R/Vs operating in targeted maritime regions (call 1 for polar and sub-polar regions, call 2 for North Atlantic and North Sea, call 3 for Mediterranean Sea, Black Sea and Red Sea), ii) one "Embarked Equipment" call aiming to deploy pieces of equipment from their non-usual R/Vs or underwater vehicles, so contributing to higher inter-operability within European research fleets, and iii) the so called "Super-Integration" call in which the MEDSUV.ISES proposal was selected for funding.
With the "Super-Integration" call, EUROFLEETS2 wanted to further develop its integrating actions by proposing a multi-platform experiment as an innovative way of integrating European and international infrastructures. Such multi-platform experiments are very rare as they require a high level of anticipation and coordination and the objective was to allow the realization of a single multinational, large-scale scientific project able to attract international scientific leaders and non-traditional end users on board European R/Vs.
To achieve this, an expression of interest was opened very early in the project to allow potential candidates to check if their proposals were fitting with the "Super-Integration philosophy", and in that case, to check that EUROFLEETS2 could cover their needs. A dedicated unique call for proposals was launched afterwards, but very early in the project's life, to seek a scientific project which needed to mobilize a combination of EU-ROFLEETS2 TNA vessels, nationally funded R/Vs, together with other appropriate scientific tools like research planes or shore infrastructures proposed with their own EC or national funding. This call sought to identify a truly cross cutting proposal, able to prove its scientific excellence and to mobilize private and public funding structures on top of EUROFLEETS2 EC funding. High impact proposals with a significant cross cutting theme could seek to develop a multi-annual programme focused on one European location or develop a proposal which requires the deployment of several vessels and platforms with their associated equipment to a number of locations with a common theme.
The MEDSUV.ISES project was successfully selected for funding among a total of four scientific proposals submitted within the "Super-Integration" call. The support brought by EUROFLEETS2 to the TOMO-ETNA experiment of the MEDSUV project represented a total of 25 days of ship-time in 2014 on board the Spanish R/V "Sarmiento de Gamboa" and the Greek R/V "Aegaeo".
de Gamboa" during the seismic reflection experiment with the 3 km long streamer. Additionally, in order to save cost, a negotiation to use the military dock for ship berthing was opened. The working period of this process was October 2013 to June 2014.
(i) Bilateral negotiation between Spanish, Greek and Italian Foreign Offices to request authorization of marine scientific work in the economic exclusive zones of Italy. The UGR leader J.M. Ibáñez with the collaboration of D. Patanè and M. Coltelli from INGV-Catania prepared an official document and memorandum to obtain this authorization. This negotiation implied the participation of the Spanish embassy in Rome. The working period of this process was March 2014 to October 2014.
(j) Negotiation with OGS, Trieste (Italy). M. Coltelli and D. Patanè from INGV-Catania established an agreement with the OGS institution in order to have access to multichannel seismic instruments to be deployed from the "Aegaeo" vessel. These instruments were: two GI guns in linear array and its air compressor arranged in TEU-20 container, and a 96 channels, 300 m long digital streamer. This negotiation started in September 2014, including a further negotiation with Italian Coast Guard of Messina to obtain the local authorization to use air-gun without damage marine mammals.
Catania, Naples, Potsdam and Granada in the raw data format. There are some teams that have converted data formats to miniseed and SAC format in Granada, Catania and Potsdam.

Portable broadband stations
Data of these stations have the same processing and storage policy that the short period stations.

OBS data
1.3.1. Spanish OBS The original OBS data are in CSIC-UTM, Catania and Granada in the raw format. UGR transformed them into SEGY format. These data are distributed to Catania, Naples, and Potsdam.

Italian OBS
The original OBS data are stored in INGV-CNT in Gibilmanna. Later the data will be distributed to Catania, Naples Granada and Potsdam.

INGV permanent seismic network
A copy of the whole data from the period April-November 2014 is available in Catania and Granada in dmx format. Naples can access to this copy via internal server. Researchers from UGR converted this format to SAC.

Seismic streamer data
A copy of data associated to the "Sarmiento de Gamboa" vessel is available in Catania and Granada (also in Barcelona as owner of the vessel). Data from "Aegaeo" campaign is available in Catania and Trieste (as owner of the streamer).

Other marine data
This data base was obtained during the experiment by "Sarmiento de Gamboa", "Galatea" and "Aegaeo" vessels, and it is composed by: bathymetry data, magnetic data and other measurements. This information is stored together with the MCS streamer data and is available in Catania and Granada.