# An Overview of Infrared Remote Sensing of Volcanic Activity
## 5 Infrared Remote Sensing of Volcanoes: A History
Volcanologically, \(T_{h}\) may relate to the temperature of recently erupted lava or an exposed lava lake, while \(T_{c}\) may relate to the temperature of the surrounding surface unaffected by volcanic processes; both of these can be estimated; \(P_{h}\), in contrast, will usually be impossible to estimate accurately and has to be derived. This assumption of two  
Figure 9: Short-wave infrared imagery of the lava flow at Mount Etna, Italy, on 29 July 2007. **(a)** In ASTER Band 4; **(b)** in ASTER Band 9. The wider area in (b) demonstrates the detection of radiance from relatively cooler peripheral flow surfaces by Band 9, as compared to the Band 4 image, which only detects the emissions from the lava flow itself. discrete temperature regions will only be appropriate in some situations and in most cases, will only approximate reality. Furthermore, the technique requires the pixels from which the data are extracted to be completely aligned, therefore precluding use of the technique with data from different sensors. Determining the temperature and corresponding areas of thermal activity at the surface is useful, but what is arguably more useful from a quantitative point of view is the overall radiant power (i.e., energy) emission. The advantage of an overall power value per pixel is that this value can be summed for all imaging pixels to derive an overall power emission value for the volcanic surface, thus providing a means of monitoring volcanic activity over time and of comparing the activity at different volcanoes. [PERSON] et al. (2011) was one of the first pieces of research to undertake such analysis, based on dual-band equation retrievals, thus setting a standard procedure that was to be reproduced in subsequent studies (e.g., [47; 92; 105; 106]). The radiant power emission, or heat-loss, for an assumed two-component pixel (such as that in Figure 7) can be calculated using the Stefan-Boltzmann law and by calculating the contribution of each component to the overall radiant heat-flux of the pixel:  
\[Q=\sigma\epsilon A[P_{h}T_{h}^{\ 4}+(1-P_{h})T_{c}^{\ 4} \tag{5}\]  
where \(Q\) = radiant power emission (W), \(\sigma\) = Stefan-Boltzmann constant (5.67 \(\times\) 10\({}^{-8}\) W m\({}^{-2}\) K\({}^{-4}\)), \(\epsilon\) = surface emissivity (unit-less) and \(A\) = pixel surface area (m\({}^{2}\)). As multi-spectral infrared imaging capabilities became more widespread, and with higher spatial resolution, the possibility of viewing the surface in more detail, while also allowing for the differentiation of different features within the same surface, became more realistic. [PERSON] and [PERSON] (2010) showed this by comparing detailed JERS OPS observations of Lascar Volcano, Chile, with those that a corresponding 1000 m \(\times\) 1000 m ATSR pixel would observe. Similar enhancements in observational detail were also demonstrated by [47] in which it was shown that features, such as Hawaiian lava-flow braiding, could be discerned using multispectral and high resolution Landsat ETM+ data. Experiments were conducted into using more than two bands of data to better characterize a volcanic surface, with [107], for example, showing that in relation to an active lava flow, a continuum of temperatures is displayed and that as such, a lava-flow surface would be better characterised with 5-7 components. Despite this, and because its outputs are more accurate than assuming one uniform temperature, use of the dual-band technique has persisted. [PERSON] and [PERSON] (2010) also showed that despite the availability of multiple bands of data, such as the six-SWIR bands of ASTER, the settings of such bands were often sub-optimal for volcanic observational purposes. In the early 2000s, two data sources remained unexploited: those from the NASA EO-1 satellite's ALI and Hyperion sensors. Their impressive complement of bands meant that these sensors had the potential to discern numerous sub-pixel surfaces for fine and detailed surface characterisations. [PERSON] et al.