First Evaluation of PRISMA Scene for geological mapping: the Dallol hydrothermal area in the salt flat of Danakil Desert, NE Ethiopia

Hyperspectral sensors offer the opportunity of analysing the chemical and physical composition of the remote sensed scene thanks to their ability of measuring the spectrum of the observed pixels in a large number of contiguous and narrow spectral channels [1].

Despite the technological advances, hyperspectral satellites are still poorly represented in spaceborne missions for Earth Exploration compared to multispectral ones [2]. In this context, the Italian Space Agency (ASI) EO mission named PRISMA (PRecursore IperSpettrale della Missione Applicativa, [3]) offers a great opportunity to improve the knowledge about the scientific and commercial applications of spaceborne hyperspectral data. PRISMA, launched in March 2019, includes a pushbroom hyperspectral camera covering the portion of the electromagnetic spectrum ranging from 400 nm to 2500 nm with 10 nm spectral sampling. Precisely, the PRISMA satellite comprises a high-spectral resolution Visible Near InfraRed (VNIR) and Short Wave InfraRed (SWIR) imaging spectrometer with 30 m ground sampling distance (GSD) and a panchromatic camera with 5 m GSD [4].

One of the critical issues in the exploitation of hyperspectral remotely sensed data is represented by the distortion effects due to the atmosphere in the radiative transfer path [5]. The products systematically produced by the PRISMA ground processor and made available to users consist of: Level 1 TOA radiometrically and geometrically calibrated radiance images; Level 2 geolocated and geocoded atmospherically corrected images. Details can be found in the PRISMA Products Specification Document [6].

Our analysis of PRISMA imagery was mainly performed on an arid environment in NE Ethiopia (Dallol; Long: 40.299351, Lat: 14.244367). One advantage of this area is that the nebulosity is generally low, in fact the image selected during the dry season has a cloud coverage percentage less than 1%. In the selected site, a salt suite was deposited and re-worked by hydrothermalism. The characteristic minerals of the area are: carbonate, halite, carnallite, anhydrite, gypsum, native sulfur of hydrothermal origin (7; 8). The unique lithological and geochemical features of Dallol and, specifically, the Mesozoic and Tertiary sedimentary cover, offer the opportunity to test PRISMA data at first order to delineate carbonates from salts.

The main objectives of this study are (1) to implement the atmospheric corrections for Level 1 data and compare the results with Level 2 data and (2) to test the capabilities of Prisma cubes to map an environment made of various sedimentary rocks and to differentiate and identify characteristic salt minerals.

References: [1] Chang, C.I., 2007.  John Wiley & Sons. DOI: 10.1002/0470124628 [2] Transon, J., et al. 2018. Remote Sensing, 10(2), 157. DOI: 10.3390/rs10020157 [3] Candela, L., et al. 2016. IEEE international geoscience and remote sensing symposium (IGARSS), 253-256. DOI: 10.1109/IGARSS.2016.7729057 [4] Loizzo, R., et al. 2019. IEEE International Geoscience and Remote Sensing Symposium (IGARSS), 4503-4506. DOI: 10.1109/IGARSS.2019.8899272 [5] Schott, J.R., 2007.  Oxford University Press on Demand [6] ASI, 2020. PRISMA Products Specification Document Issue 2.1 [7] Cavalazzi, B., et al. 2019. Astrobiology, 19(4), 553-578. DOI: 10.1089/AST.2018.1926 [8] López-García, J.M., et al. 2020. Frontiers in Earth Science, 7, 351.  DOI: 10.3389/FEART.2019.00351