MAPPING THE AREAL EXTENT AND COMPOSITION OF VOLCANIC ASH DEPOSITS USING APPARENT THERMAL INERTIA

Volcanic ash can spread over a large aerial extent, and pose a significant threat to health, aviation, agriculture, and rapidly growing urbanized areas. Sakurajima (Japan), Santiaguito (Guatemala) and Fuego (Guatemala) are all active volcanoes in the vicinity of densely-populated cities. As these cities are frequently impacted by ash deposits, characterization of the aerial distribution of ash is important. We present here a method for identifying the presence and extent of ash deposits using thermal inertia (TI). TI describes a material’s ability to heat up and retain heat over time, and is material-dependent. When TI cannot be directly measured on the ground, apparent thermal inertia (ATI) is used instead. In the first phase of this research, a FLIR thermal camera and temperature probe were used to generate temperature profiles of ash samples from Santiaguito, Fuego and Sakurajima, which were used to characterize the TI of the ash samples. Although the samples were different compositions they behaved very similarly, as expected. In the second phase of this research, ASTER VNIR/SWIR reflectance satellite image pairs from before and after an eruption, as well as radiometrically-calibrated daytime/nighttime temperature image pairs, were used to calculate ATI over the image area using the following equation: (1-A)/ΔT (where A is surface albedo, and ΔT is the day/night temperature difference). This process was repeated with multiple image pairs from each volcano between 2008 and 2017. ATI calculations were then compared those measured on Mars. The Martian surface has similar ATI values to that of ash on Earth, as the surface is predominantly covered with fine dust from weathered basalts, which mimics ash. However, not all of Mars exhibits the same diurnal variation, which can be attributed to the thickness of the dust layer. Similarly, the thickness and distribution of the ash at the volcanoes in this study likely affected the calculated ATI. Areas where ash is very thin, or where the underlying surface is partially exposed, will produce an ATI value different than that of thicker ash. Preliminary results show that ATI can potentially be used to characterize ash deposits, with a few caveats. Variation in deposit thickness, as well as presence of shadows or clouds in the satellite imagery, can affect the ATI calculation.