Drone high resolution infrared imaging of the Lusi mud eruption

The use of low-cost hand-held infrared (IR) thermal cameras based on uncooled micro-bolometer detector arrays became more widespread during the recent years. Thermal cameras have the ability to estimate temperature values without contact and therefore can be used in conditions where targets are difficult or dangerous to reach such as volcanic eruptions. Since May 2006 the Indonesian Lusi mud eruption continues to spew boiling mud, water, aqueous vapour, CO2, CH4 and covers a surface of nearly 7 km2. Here we performed surveys above and around the erupting crater using a specifically equipped remote-controlled aerial vehicle (drone). Despite the harsh logistics and the continuously varying gas concentrations, we managed to collect IR images composing mosaics to estimate the crater zone spatial and temporal thermal variations as well as that in the surrounding regions. In this manuscript we provide a) a description of the main processes that affect and control the acquisition of IR images; b) an overview of still non disclosed physical model used by the thermal camera employed during our survey (i.e. Flir I7 IR); c) a method for capturing high resolution infrared images over an erupting clastic system; d) analysis and interpretation of the acquired data and scientific results. The results show that it is possible to obtain good quality mosaics also in inaccessible areas such as erupting craters where fixed reference points are not constant, and where the presence of IR attenuation factors introduce errors in terms of temperature estimates. However the IR camera radiative transfer model (based on Lowtran model) allows the control of only some of the parameters that affect the attenuation of the IR spectrum. This results in obtained crater temperature estimates up to ∼20% lower than those measured with hand held thermometers, providing, however, very important information about the thermal gradient and the potential radioactive absorption factors. The imaged Lusi vent thermal pattern suggests the presence of shallow convective chambers inside the caldera zone; these are activated by the rise of boiling mud breccia that suddenly cools and sinks. The thermal survey conducted on the dry mud region to the NE of the crater reveals temperatures matching with those measured directly on the field with a hand held thermometer. Here the presence of hot spot anomalies and colder circular features is consistent with the migration of deeper warm fluids along a faulted and fractured area and with widespread pools distribution.