Evaluation of SWIR-based methods for quantifying active volcano radiant emissions using NASA EOS-ASTER data

Abstract

Analysis of thermally emissive volcanic features using satellite infrared remote sensing has been conducted over recent decades, primarily using shortwave and thermal infrared (SWIR; TIR) radiance data. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), mounted on the Earth Observation System (EOS) Terra satellite, offers an advance on earlier instruments, having more bands covering the SWIR atmospheric window and offering a wider dynamic range. This paper compares methods used to analyse ASTER SWIR imagery of active volcanoes, using both simulated cases and actual ASTER imagery of Lascar Volcano, and focuses on radiative power estimates. Those based on the Oppenheimer approach are found to be most reliable for simulated surfaces, with the Lombardo and Buongiorno and Dozier retrievals having larger uncertainties in most cases. However, the Dozier Method results in the highest proportion of successful retrievals, the reliability of which is influenced by factors including band combination, gain setting and saturation. The radiative power metric is shown as a more reliable measure than sub-pixel characterisations of hotspot temperature and area, as retrieved by these methods. We conclude with an assessment of ASTER in terms of its utility for providing quantitative observations of active volcanic surfaces.

An Overview of Infrared Remote Sensing of Volcanic Activity

“…In comparing the detection limits of Landsat 8's OLI SWIR bands, with those of its predecessor, Landsat ETM+, the enhanced dynamic range of the former means that temperatures of up to 747.9 K and 570.4 K can be detected without saturation in Bands 6 (1.56-1.66 µm) and 7 (2.10-2.30 µm), as compared with those for the corresponding ETM+ bands (724.5 and 552.8 K, respectively) [33]. The problem of saturation was demonstrated in [19], in which it was found that 16% of the volcanic ASTER SWIR scenes analysed displayed saturation, thereby preventing their quantitative analysis. One solution for saturation is the use of varying gain settings to extend the maximum detectable temperatures at the higher end.…”

“…Early on, the relatively fine spatial resolution of ASTER's infrared bands was shown to be useful in detecting eruption-precursory thermal anomalies at Chikurachki Volcano, Russia [58], while [19,59] demonstrated that despite its sub-optimal revisit frequency of 16 days, long-term analysis of its data could be used to characterise the behaviour of Shiveluch and Lascar volcanoes, respectively, with its SWIR and TIR bands showing utility for detecting different thermal components at each volcanic surface. Urai and Ishizuka [60] also demonstrated that within several hours of acquisition, ASTER imagery could be used to estimate the volcanic explosivity index (a scale used to quantify eruption explosiveness) of an eruption, although this relied on ASTER viewing the surface at an appropriate time.…”

“…However, such bands are also useful in monitoring hot surfaces, especially at night [19], and until the malfunction in 2008, ASTER was regularly used for such purposes. Comparable functionality has now passed to the largely under-investigated SWIR bands of Landsat-8 [20].…”

“…One solution for saturation is the use of varying gain settings to extend the maximum detectable temperatures at the higher end. The SWIR bands of the ASTER sensor, for example, were equipped with low-gain settings for the purpose of volcanic monitoring (a "volcano mode" [34,35]), raising the saturation temperature; in doing so however, it had the effect of reducing sensitivity to cooler surface phenomena [19]. its predecessor, Landsat ETM+, the enhanced dynamic range of the former means that temperatures of up to 747.9 K and 570.4 K can be detected without saturation in Bands 6 (1.56-1.66 µm) and 7 (2.10-2.30 µm), as compared with those for the corresponding ETM+ bands (724.5 and 552.8 K, respectively) [33].…”

“…One solution for saturation is the use of varying gain settings to extend the maximum detectable temperatures at the higher end. The SWIR bands of the ASTER sensor, for example, were equipped with low-gain settings for the purpose of volcanic monitoring (a "volcano mode" [34,35]), raising the saturation temperature; in doing so however, it had the effect of reducing sensitivity to cooler surface phenomena [19]. …”