Use of Forward Looking InfraRed thermal cameras at active volcanoes

Abstract

Nowadays, thermal imaging has become a common remote sensing tool for monitoring active volcanoes. The study of temperature variations within openconduit systems, at eruptive fissures, active vents, domes, lava lakes, lava fields and other volcanic features has proven fundamental to better understand volcanic system behaviour over the short and long terms (Harris and Stevenson, 1997; Oppenheimer and Yirgu, 2002; Calvari et al., 2004; Wadge et al., 2006). At INGV Catania Section, thermal imaging has been applied at Mt Etna, Stromboli, Vulcano and Panarea since 2001. The instruments used are thermal cameras manufactured by FLIR (Forward Looking InfraRed) and consist in uncooled bolometers that are sensitive within 7.5 and 13 μ wavelengths. Thermal cameras are based on the capability to detect radiation emitted by bodies according to Planck’s Law. In particular, the camera we used is a FLIR thermal camera A 40 M Ethernet with a focal plane array uncooled bolometer (320 x 240 pixels), and a spectral range between 7.5 and 13 micrometers (Figure 1.). It has a standard optics 24° with spatial resolution (IFOV, instantaneous field of view) of 1.3 mrad, a horizontal view of 24° and a vertical view of 18°. This camera has also been equipped with optional filter to measure temperature values up to 1500°C with the possibility of setting up different temperature ranges. The thermal camera can record and transfer in real time via wi-fi radiometric frames in JPG format of the observed eruptive activity according to some environmental parameters, such as external temperature, air humidity and emissivity and allows the vision of volcanic activity both day and night.Temperature range varies between 0 e 500° C and the emissivity value ε = 1. To correct the temperature of all pixels from the atmospheric attenuation effects, we considered atmospheric parameters, such as air temperature and air humidity, in addition to the introduction of the path length (400 m) in the camera software. In fact, the radiations detected by the FLIR thermal cameras, that work in the spectral band between 7.5 e 13 μm, are affected by the absorption factor from the water spectrum, which is predominant in this band; particularly at La Fossa crater where the water content in the fumaroles is higher than the other gas species. Because of the necessity to correct the radiometric data from the atmospheric factors in real-time, we installed a meteorological station able to interface with the camera to provide atmospheric parameters for the auto-calibration.