# An Overview of Infrared Remote Sensing of Volcanic Activity
## 3 Specific Considerations for the Infrared Remote Sensing of Volcanic Activity
Atmospheric effects: If accurate radiant emissions data from a volcanic surface are required, then the effect of the atmosphere, and of volcanic gases, that the signal passes through between the surface and the sensor must be accounted for. This is because transmitting gasses may both attenuate and augment radiant signals that pass through. Most infrared bands on sensors used to monitor volcanic activity have been strategically placed in 'atmospheric windows', or portions  
Figure 4: The trade-off between spatial, spectral and temporal resolution for Earth observation imagery. These relationships are a function of data transfer bandwidths; assuming a fixed bandwidth, a sensor possessing a high spectral resolution will need to compromise on either spatial or temporal resolution, and so on. of the electromagnetic spectrum with high transmissivity and, hence, low atmospheric absorption (see Figure 5). Even in these windows, however, a small proportion of the signal may be absorbed because volcanic gasses, including water vapour, strongly absorb certain wavelengths. As the atmosphere and volcanic gases also themselves have a temperature, they too emit electromagnetic radiation that contributes to that detected by an observing sensor. A further complication is that a proportion of the atmospheric radiance, and indeed daytime solar radiance, will also be reflected sensor-ward, additionally contributing to the signal detected by satellite sensors. Radiative transfer models are available to simulate such atmospheric effects, and these must be applied to the infrared detections made at a sensor if accurate ground-level emissions data are to be retrieved. Emissivity: This is the ratio of energy emitted from a particular body, to that radiated from a perfect emitter (a blackbody) at the same temperature and wavelength; in effect, the term quantifies the radiating efficiency of a surface. An object with an emissivity of 1.0 is a perfect emitter, emitting radiation with 100% efficiency; all other objects emit with less efficiency with the effect that the apparent (or radiance) temperature of such objects, if measured remotely and derived using the Planck equation (Equation (1)), is less than the true kinetic temperature of the object. This means that to derive the true temperature of a volcanic, or indeed any surface, remotely, then its emissivity must be known. The emissivity of volcanic surfaces has been shown to vary widely depending on mineralogy, temperature and the waveband of observation (e.g., it has been shown to be 0.92 in the SWIR bands for cooled andesitic lava at Lascar in Chile [31] and close to 1.0 for the active lava lake at Erta Ale in Ethiopia [8]). Accurate emissivity values are essential, as errors in it have been shown to result in temperature retrieval discrepancies of over 125 K [32]. Temperature limits: All sensors used for volcanic thermal signal detection have detection limits. Within these limits, the emissions from a volcanic surface can be detected, but if the signal is too low, it will be missed; and if it is too high, it will saturate the individual pixels of the sensor, rendering quantitative analysis impossible (see Figure 6). Early in the discipline, saturation was identified as one of the key problems in the remote sensing of thermally-anomalous volcanic activity [32], and such issues persist, although the situation is much improved. Determinants of the detection limits include  
Figure 5: Atmospheric transmissivity in the infrared region of the spectrum. Note the limited transmissivity around 6.0 μm, attributable to absorption by water vapour. These data are based on a ‘standard atmosphere’. Source: [30]. the dynamic range of the sensor (the range over which radiant signals are detected) and the viewing pixel size (a smaller pixel will saturate at a lower level of radiance when viewing a hot volcanic surface, given that the heated surface components will constitute a greater proportion of the viewing pixel area).