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A HIGH RESOLUTION, MULTI-PARAMETRIC ANALYSIS TO IMAGE THE SHALLOW SOLFATARA CRATER IN CAMPI FLEGREI CALDERA (ITALY)G. De Landro1, S. Gammaldi1, V. Serlenga1,2, O. Amoroso1,3, G. Russo1, G. Festa1, L. D’Auria4, P.P. Bruno5,M. Gresse6, J. Vandemeulebrouck4, A. Zollo1

1 University “Federico II”, Department of Physics “E. Pancini”, Napoli, Italy 2 Now at Institute of Methodologies for Environmental Analysis, Tito scalo, Italy3 Now at Dipartimento di Fisica “E.R. Caianiello” Università di Salerno, Fisciano (SA), Italy4 Instituto Volcanológico de Canarias (INVOLCAN), Tenerife, Spain5 Petroleum Institute, Department of Petroleum Geosciences, Abu Dhabi, United Arab Emirates6 ISTerre, Université Savoie Mont Blanc, Chambéry, France

Introduction. Fluids play a key role in controlling and governing the evolution of magmatic processes and eruptions. A reliable imaging of fluid storages and accurate tracking of their movements within the crust is therefore crucial to evaluate the evolution of the volcanic activity and to assess the related hazard.

The seismic tomography method can be used to obtain a reliable image of the elastic and anelastic properties of complex geological media. However, because of the hydrothermal system complexity, a multi-parametric analysis is required for an effective and robust tracking of fluids.

Solfatara volcano is located within the Campi Flegrei, a still active caldera, which is characterized by periodic episodes of extended, low-rate ground subsidence and uplift episodes (bradyseisms), accompanied by intense seismic and geochemical activities. In particular, Solfatara is characterized by an impressive magnitude diffuse degassing [1], which underlines the relevance of fluid and heat transport at the crater and prompts for further research to improve the understanding of the hydrothermal feeding system.

In this framework, an active seismic experiment, Repeated Induced Earthquake and Noise (RICEN, EU Project MEDSUV), was carried out between September 2013 and November 2014 to provide time-varying high-resolution images of the structure of Solfatara [2]. For this study we used the datasets provided by two different acquisition geometries: a) A 2D array covering an area of 90 x 115 m2 sampled by a regular grid of 240 vertical sensors deployed on

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Fig. 1 - a) RICEN Experiment layout, b) An example of seismic geophone, c) The RICEN working team.

the crater floor; b) two 1D orthogonal seismic arrays deployed along NNE–SSW and WNW–ESE directions and crossing the 400 m crater surface.

The goal of this work is to present a high resolution multi-parametric image of the shallow Solfatara crater analyzing P-wave velocity and attenuation tomographic models. In particular, we present 1) a 3D velocity model [4]; 2) two bi-dimensional velocity sections multi-2D interpreted [5]; 3) a 3D attenuation model. We compare the obtained 3D images with an electrical resistivity section [3] and temperature and CO2 flux measurements.

Data and Method. The RICEN experiment consisted in three successive geophysical surveys carried out at the Solfatara volcano respectively in September 2013, May and November 2014, each one lasting one week. The pilot phase consisted of the joint acquisition, from both sparsely distributed three components geophones inside the crater, and seismic stations placed on a regular grid of 115 × 90 m2 area sampled by a regular grid of 240 vertical sensors (named 2D array) in front of the Fangaia (Fig. 1, a). In the second phase (RICEN2) besides the two joint acquisitions mentioned previously, hosted a 2D profile, with NNE–SSW direction, which was performed on May 21, 2014. The same configuration was used for the last phase (RICEN3) with the difference that the 2D profile, performed on November 11, was oriented orthogonally to the one performed during the RICEN first act (Fig. 1, a). Active seismic data were obtained using a Vibroseis Truck soil energizator, which operated in the frequency range 5–125 Hz. Seismic waveforms were recorded by 4.5 Hz vertical component geophones (Fig. 1, b).

In this study, for the 3D models, we analyse the data collected during the first experiment by the 2D array.

The P-wave first arrivals have been first detected through a Neural Network implemented into the ProMAX SeisSpace software trained on a limited, manually picked dataset of source-receiver couples. Thereafter, the picking dataset has been manually validated on the basis of a visual inspection of seismic signals.

Beside the 3D array, for the 2D seismic tomography we considered the data from the two orthogonal seismic profiles. The first array was about 430 m long and oriented NNW-SSE while

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Fig. 2 - Vp tomographic result of A (a) and B (b) profiles. Interpretation of the two Vp velocity models A (c) and B (d). Modifiet after Gammaldi et al., 2017.

the second was about 480 m long and oriented along a WNW-ESE direction (Fig. 1, a). We collected the records in Common Shot Gather (CSG) and displayed the signal relative to a shot acquired by the receivers. Then, we manually picked the first arrival time by visual inspection of each section obtaining a total amount of 9053 and 8841 picks for each array.

The 3D elastic and anelastic images of the shallow (30-35 m) central part of Solfatara crater are obtained through an iterative, linearized, tomographic inversion of picked P-wave arrival times and of the measured t* values using a multiscale strategy. 2D velocity sections (60-70 m) are obtained using a non-linear travel-time tomography method based on the evaluation of a posteriori probability density with a Bayesian approach.

Results. The 2D tomographic profiles provide evidence for a low velocity (500–1500 m/s) water saturated deeper layer at West near the outcropping evidence of the Fangaia, contrasted by a high velocity (2000-3200 m/s) layer correlated with a consolidated tephra deposit. The transition velocity range (1500-2000 m/s) layer suggests a possible presence of a gas-rich, accumulation volume (Fig. 2, a-b).

The central fault represents the high-permeability pathway for hydrothermal fluids. The gasses at depths of 40–60 m are blocked by the tephra saturated in meteoric water, which is the main shallower cap rock. Starting from the middle of the crater going westward, the saturated tephra became thinner being replaced by the gas accumulation zone. The gas is channelled between the consolidated and unconsolidated tephra, and finally released by the ring faults bordering the Solfatara (Fig. 2, c-d).

Taking into account that the presence of fluids and their circulation may greatly affect the rock volume, and therefore the average compressional wave velocity, we expect that the tomographic images can constrain the possible location and phase of permeating fluids. For this purpose, we compared our seismic tomography with 2D cross sections of resistivity (ρ) and with temperature and CO2 flux measurements (Fig. 3).

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The 3D retrieved images integrated with resistivity section and temperature and CO2 flux measurements , define the following characteristics: 1. A depth dependent P-wave velocity layer down to 14 m, with Vp<700m/s typical of poorly-consolidated tephra affected by CO2 degassing; 2. An intermediate layer, deepening towards the mineralized liquid-saturated area (Fangaia), interpreted as permeable deposits saturated with condensed and meteoric water; 3. A deep, confined high velocity anomaly associated with a CO2 reservoir (Fig. 3). The deep high velocity anomaly retrieved in the 3D velocity model, associated with a gas accumulation zone, is very well confined in the layer in the 2D profiles interpreted as a channel for the rising up of deep fluid plumes.

The retrieved features are expression of a volume located between the Fangaia, which is water-saturated and replenished from meteoric and condensed water, and the main fumaroles that are the superficial relief of deep rising CO2. So, the changes in the outgassing rate greatly affects the shallow hydrothermal system, which can be used as a near-surface proxy of fluid migration processes occurring at greater depths.

Conclusion. The importance of this work lies in these principal aspects:(1) the Solfatara crater represents one of the main pressure release areas of the entire Campi

Flegrei volcanic system, considering the impressive magnitude of the diffuse degassingprocess. Hence, the interest in the knowledge of this area grows, especially with the aimof assessing the level of potential danger characterizing this crater;

(2) the 2D and 3D tomographic survey allows us to achieve an unprecedented spatialdetail on the shallow velocity structure of the central part of the Solfatara crater. Thehigh-resolution tomographic images allow us to better understand, in terms of velocityanomalies and fluid type, the complex hydrothermal processes into the shallow part ofthe volcano;

Fig. 3 - Multiparametric interpretation. Modified after De Landro et al., 2017.

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(3) the procedure used in this work represents a new multi-parametric approach that can beused in a volcanic environment; it shows how the interpretation of velocity tomographicimages can be complemented with the ones obtained by stratigraphic analysis andresistivity profiles, and, most of all, how this joint interpretation leads to a more robustand reliable interpretation of complex hydrothermal system;

(4) the complex interactions (deep fluids, hydrothermal system, geological structures)between deep and shallow sources and structures allow us to use the shallow hydrothermal system processes as a constrain for the fluid migration processes occurring at depth.

ReferencesChiodini, G. et al. CO2 degassing and energy release at Solfatara volcano, Campi Flegrei, Italy. Journal Geophysical

Research. 106, 16213–16221(2001). Serra, M. et al. A strongly heterogeneous hydrothermal area imaged by surface waves: the case of Solfatara, Campi

Flegrei, Italy. Geophysical Journal International. 205, 1813-1822(2016). Gresse, M. et al. Three-Dimensional Electrical Resistivity Tomography of the Solfatara Crater(Italy): Implication

for the Multiphase Flow Structure of the Shallow Hydrothermal System. Journal of Geophysics Research: Solid Earth, 122(11), 8749-8768(2017).

De Landro, G. et al. 3D ultra-high resolution seismic imaging of shallow Solfatara crater in Campi Flegrei(Italy): New insights on deep hydrothermal fluid circulation processes. Scientific Reports 7.1(2017): 3412.

Gammaldi, S. et al. High resolution, multi-2D seismic imaging of Solfatara crater(Campi Flegrei Caldera, southern Italy) from active seismic data. Accepted for publication on Journal of Volcanology and Geothermal Research(2018).