**Fabio Sansivero \* and Giuseppe Vilardo**

Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy; giuseppe.vilardo@ingv.it

**\*** Correspondence: fabio.sansivero@ingv.it; Tel.: +39-081-6108217

Received: 29 January 2019; Accepted: 4 March 2019; Published: 6 March 2019

**Abstract:** In this technical paper, the state-of-art of automated procedures to process thermal infrared (TIR) scenes acquired by a permanent ground-based surveillance system, is discussed. TIR scenes regard diffuse degassing areas at Campi Flegrei and Vesuvio in the Neapolitan volcanic district (Italy). The processing system was developed in-house by using the flexible and fast processing Matlab© environment. The multi-step procedure, starting from raw infrared (IR) frames, generates a final product consisting mainly of de-seasoned temperatures and heat fluxes time-series as well as maps of yearly rates of temperature change of the IR frames. Accurate descriptions of all operational phases and of the procedures of analysis are illustrated; a Matlab© code (Natick, MA, USA) is provided as supplementary material. This product is ordinarily addressed to study volcanic dynamics and improve the forecasting of the volcanic activity. Nevertheless, it can be a useful tool to investigate the surface temperature field of any areas subjected to thermal anomalies, both of natural and anthropic origin.

**Keywords:** volcano monitoring; thermal imaging; time series; Seasonal-Trend Decomposition; heat flux

#### **1. Introduction**

Thermal infrared (TIR) ground-based observations are largely used in volcanology, both in research and in surveillance activities, to investigate volcanic plumes and gases, lava flows, lava lakes and fumarole fields [1–17]. Generally, the observations were made during a limited time span such as eruption phases or field campaigns with temporarily installed TIR stations or handheld cameras. In the last years the number of surveillance and research activities aimed to undertake TIR continuous observations of volcanic areas have increased [12–20]. Improvements in monitoring tools and analysis techniques of long TIR time-series of infrared (IR) scenes of volcanic areas are becoming matter of great interest since they give the opportunity to track changes of surface thermal anomalies that may reveal a renewal of eruptive activity. Several works identified thermal precursors before eruptions by using TIR observations [21–24] and these insightful results, also provided by field campaigns, have suggested planning permanent fixed installations of ground TIR stations at active volcanoes in the world.

At present time, few commercial software packages, based on general-purpose procedures, are available to process TIR time-series and they are not aimed for a near real-time automated analysis of large dataset. Generally, they involve manual processing steps and cannot be used in daily continuous automated volcano monitoring activity. Recently, [20] introduced automated analysis techniques of long TIR time-series of images acquired inside the Campi Flegrei volcanic area and previously [19] discussed about analysis techniques applied to TIR scenes inside the Vesuvius crater. The studied zones of these works are diffuse degassing areas of quiescent volcanoes characterized by low temperatures of released gas fluxes.

In this work, recent developments of processing methodologies of several-years long TIR time-series of volcanic areas from a permanent surveillance network are discussed in detail. Additionally, the automation of the processes is discussed. Step-by step descriptions of all operational phases and of the theoretical basis are reported in order to provide a clear explanation of the applied procedures. The main final results are trends of temperatures, heat fluxes and yearly rate of temperature change of the studied areas. In particular, a detailed study with a focus on seasonal component removal and on pixel alignment of IR frames (co-registration) was carried out. The code of fully-automated Matlab© application (ASIRA, Automated System of InfraRed Analysis) used to process the IR data is provided as Supplementary Materials.

#### **2. The Study Areas**

The TIR frames time-series, used to develop and test the methodologies described in this work, were acquired by stations of TIRNet (Thermel InfraRed Network), a surveillance network operated by the Osservatorio Vesuviano, section of National Institute of Geophysics and Volcanology (INGV), consisting of six permanent ground stations installed at Campi Flegrei caldera and Vesuvius crater (Figure 1). Campi Flegrei (CF) is an active volcanic field including part of the city of Napoli (Italy). Nowadays, although quiescent and the last eruption occurred in 1538 (Monte Nuovo; [25]), the CF area is affected by significant ground deformation (Bradyseism), low to moderate seismic activity, hot fumaroles fields and diffuse degassing zones. The target areas acquired by TIR cameras in the Solfatara crater and its surroundings are shown in (Figure 1a). The monitored area represents the main surface expression of the CF caldera hydrothermal system with gases emissions originated by interaction between fluids of magmatic and meteoric origin [26–28]. The Somma–Vesuvius volcanic complex, located east of the city of Naples, is one of most dangerous volcanoes in the world and the latest eruption occurred on 1944 [29]. The recent dynamic of the Vesuvius is characterized by low-level shallow seismicity and by low temperature fumarolic activity mainly concentrated in the crater area [30–32]. TIR scenes are from low-temperatures surface thermal anomaly on the western inner slope of the Vesuvius crater (Figure 1b).

**Figure 1.** The Solfatara area (**a**) and Vesuvius crater (**b**) acquired by Thermel InfraRed Network (TIRnet) cameras. Red points are infrared (IR) stations locations and yellow regions represent the framed areas.
