TIME SERIES OF SALT CRUSTS IMAGED BY A DUAL POLARIZATION SPACEBORNE SYNTHETIC APERTURE RADAR (SAR) AT C-BAND OVER AN ANDEAN ALTIPLANO SALAR OF NORTHERN CHILE

Abstract

A dense time series of Synthetic Aperture Radar acquisitions at 6-day intervals between July 2017 to January 2019 collected with the C-band constellation Sentinel 1A and 1B is used to study salt crust evolution in an highland salar. Microwave response of halite crystal aggregates is linked to surface roughness of the salt crusts by means of a surface scattering model which includes multiple scattering at second order in media with complex permittivity such as brine-soil mixtures. The time series enabled to estimate co-polarised vertical-vertical backscattering coefficient variations as large as 8.8 dB on a 4-month period which implied a height standard deviation increase from 0.5 to 4.5 mm as modeled by the surface scattering model. Backscattering coefficient variations between 0.8 to 2 dB per month are found for three different crusts, which demonstrated different growth rates of the crystals. Crystal growth rate might be driven by the kind of water input (rainfall or snow in Andean salars), probably due to the negative effect of water droplets on impinging halite crystal surface in comparison to snow. Results showed that cross-polarised backscattering coefficient is sensitive to snow accumulation and appeared to be sensitive to subsurface conditions.