atmosphere Article Tracking the Endogenous Dynamics of the Solfatara Volcano (Campi Flegrei, Italy) through the Analysis of Ground Thermal Image Temperatures Paola Cusano 1, Teresa Caputo 1,* , Enza De Lauro 2 , Mariarosaria Falanga 3 , Simona Petrosino 1, Fabio Sansivero 1 and Giuseppe Vilardo 1 1 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli-Osservatorio Vesuviano, 80124 Naples, Italy; [email protected] (P.C.); [email protected] (S.P.); [email protected] (F.S.); [email protected] (G.V.) 2 Ministry of Education, Universities and Research, 00153 Rome, Italy; [email protected] 3 Dipartimento di Ingegneria dell’Informazione ed Elettrica e Matematica Applicata/DIEM, Università degli Studi di Salerno, 84084 Fisciano, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-081-6108-216 Abstract: In the last decades, thermal infrared ground-based cameras have become effective tools to detect significant spatio-temporal anomalies in the hydrothermal/volcanic environment, possibly linked to impending eruptions. In this paper, we analyzed the temperature time-series recorded by the ground-based Thermal Infrared Radiometer permanent network of INGV-OV, installed inside the Solfatara-Pisciarelli area, the most active fluid emission zones of the Campi Flegrei caldera (Italy). We investigated the temperatures’ behavior in the interval 25 June 2016–29 May 2020, with the aim Citation: Cusano, P.; Caputo, T.; De of tracking possible endogenous hydrothermal/volcanic sources. We performed the Independent Lauro, E.; Falanga, M.; Petrosino, S.; Component Analysis, the time evolution estimation of the spectral power, the cross-correlation and Sansivero, F.; Vilardo, G. Tracking the the Changing Points’ detection. We compared the obtained patterns with the behavior of atmospheric Endogenous Dynamics of the temperature and pressure, of the time-series recorded by the thermal camera of Mt. Vesuvius, of Solfatara Volcano (Campi Flegrei, the local seismicity moment rate and of the CO2 emission flux. We found an overall influence of Italy) through the Analysis of Ground exogenous, large scale atmospheric effect, which dominated in 2016–2017. Starting from 2018, a Thermal Image Temperatures. clear endogenous forcing overcame the atmospheric factor, and dominated strongly soil temperature Atmosphere 2021, 12, 940. https:// variations until the end of the observations. This paper highlights the importance of monitoring and doi.org/10.3390/atmos12080940 investigating the soil temperature in volcanic environments, as well as the atmospheric parameters. Academic Editor: Graziano Coppa Keywords: Campi Flegrei; ground thermal image temperatures; exogenous/endogenous phenom- Received: 15 June 2021 ena; monitoring atmospheric parameters Accepted: 19 July 2021 Published: 22 July 2021 Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in In the last decades, thermal remote sensing has been largely used in surveys and in published maps and institutional affil- the analysis of thermal behavior of active volcanic areas. Different specific devices, such iations. as portable or ground/airborne sensors and satellites [1–4], can reveal thermal anomalies linked to volcanic activity, as well as mapping volcanic deposits [5]. Thermal remote explorations are largely used to monitor the state of volcanoes by detecting thermal surface variations likely linked to endogenous dynamics and possible eruption precursors. The Copyright: © 2021 by the authors. thermal infrared ground camera is an effective tool to reveal such variations. Indeed, Licensee MDPI, Basel, Switzerland. thermal ground monitoring allowed detection of the intensification of the thermal activity This article is an open access article prior to the 18 May 1980 eruption at Mount St. Helen’s [6]. Some days before the Santa distributed under the terms and Ana volcano lake eruption, Hernández et al. [7] measured a significant increase in the conditions of the Creative Commons surface lake temperature and an intensification of the fumarolic emissions, by using thermal Attribution (CC BY) license (https:// infrared images acquired with a handheld camera. A handheld thermal camera was also creativecommons.org/licenses/by/ used by Calvari et al. [8] to monitor the Mt. Etna and Stromboli volcanoes. When both 4.0/). Atmosphere 2021, 12, 940. https://doi.org/10.3390/atmos12080940 https://www.mdpi.com/journal/atmosphere Atmosphere 2021, 12, x FOR PEER REVIEW 2 of 15 Atmosphere 2021, 12, 940 surface lake temperature and an intensification of the fumarolic emissions, by using2 ofther- 15 mal infrared images acquired with a handheld camera. A handheld thermal camera was also used by Calvari et al. [8] to monitor the Mt. Etna and Stromboli volcanoes. When both thethe volcanoes volcanoes erupted erupted in in 2002–2003, 2002–2003, the the acquired acquired thermal thermal images images showed showed a a failure failure on on the the volcanoes’volcanoes’ flanks flanks before before the the eruptive eruptive fractures’ fractures’ opening. opening. Hilman Hilman et et al. al. [ 9[9]] used used the the thermal thermal infraredinfrared radiometers radiometers (TIR) (TIR) on on board board of of an an Advanced Advanced Spaceborne Spaceborne Thermal Thermal Emission Emission and and ReflectionReflection RadiometerRadiometer (ASTER)(ASTER) toto detectdetect thermalthermal surfacesurface anomaliesanomalies relatedrelated toto thethe gas gas emissionsemissions around around Papandayan Papandayan and and Domas Domas craters crat (Indonesia).ers (Indonesia). Their Their results results suggested suggested that that SO2 emission measurements and land surface temperature corrections are highly cor- SO2 emission measurements and land surface temperature corrections are highly correlated. Afterward,related. Afterward, those authors those [ 10authors] applied [10] the applied satellite the imaging satellite to imaging increase to the increase effectiveness the effec- of upflowtiveness zone of upflow detection zone in detection those areas. in those areas. TheThe surface surface thermal thermal behavior behavior of of Campi Campi Flegrei Flegrei caldera caldera (CFc; (CFc; Southern Southern Italy) Italy) has has been been monitoredmonitored usingusing differentdifferent thermal thermal infrared infrared (TIR) (TIR) devices: devices: satellitesatellite thermalthermal sensorssensors [11[11],], ground-basedground-based TIRTIR permanentpermanent networknetwork (TIRnet)(TIRnet) [[12–15]12–15] andand handheldhandheld TIRTIRcameras cameras [[16].16]. TheThe relative relative studies studies are are mainly mainly devoted devoted to to analyzing analyzing the the thermal thermal observables observables at at a a local local scale,scale, inside inside the the caldera. caldera. Indeed, Indeed, the the interaction interaction between between some some atmospheric atmospheric parameters parameters andand TIRs, TIRs, such such as as the the rainfall rainfall episodes episodes and and variations variations in in ground ground temperature temperature in in Solfatara, Solfatara, waswas already already explored explored [ 17[17].]. At At even even smaller smaller scales, scales, the the heating heating due due to to anthropogenic anthropogenic noise noise isis also also known known [ 18[18,19],,19], as as well well as as the the effects effects due due to to the the topography topography which which are are expected expected to to causecause local local wind wind perturbation perturbation that that increases increases the the gas gas flux. flux. TheThe present present work work mainly mainly aims aims at at the the identification identification of of possible possible sources sources at largerat larger spatial spa- scalestial scales (tens (tens of kilometers) of kilometers) that that might might affect affe thect the CFc CFc thermal thermal behavior, behavior, in in order order to to better better characterizecharacterize the the local local hydrothermal/volcanic hydrothermal/volcanic dynamics.dynamics. BesidesBesides thethe spatialspatial investigation,investigation, thethe analysis analysis was was also also focused focused on on the the variations variations in in TIRs TIRs over over large large time time scales scales (from (from weeks weeks toto years), years), as as at at small small time time scales scales (of (of the the order order of of a a few few days). days). Here,Here, thethe behaviorbehavior ofof aa Solfatara–PisciarelliSolfatara–Pisciarelli shallow hydrothermal system system is is investi- inves- tigatedgated in in the the time time span span of of 25 25 June June 2016–29 2016–29 May May 2020, 2020, by byprocessing processing data data acquired acquired by TIR by TIRground ground cameras cameras belonging belonging to the to thesurveillance surveillance TIRnet TIRnet of INGV of INGV in incorrespondence correspondence to tose- selectedlected active active fumarole fumarole emission emission areas areas of of CFc (Figure1 1).). A A first first comparison comparison was was carried carried outout among among all all the the TIRs TIRs of of CFc, CFc, the the atmospheric atmospheric parameters parameters (T (Tatmatmand and P atmPatm)) and and the the TIR TIR of of Mt.Mt. Vesuvius Vesuvius (VES), (VES), to to identify/discriminate identify/discriminate possiblepossible exogenous exogenous sources sources at at a a large large spatial spatial scale.scale. Then,Then, thethe evidencedevidenced CFcCFc groundground thermalthermal patternspatterns werewere comparedcompared