Shortwave infrared (SWIR) is the portion of the reflected solar spectrum spanning roughly 1.4 to 3.0 µm (sometimes given as 1.0–2.5 µm). In geological remote sensing it is the single most diagnostic spectral region because many minerals exhibit sharp, characteristic absorption features there.
Why it matters
Clays, micas, carbonates, sulfates, and hydroxyl- and water-bearing minerals all absorb specific SWIR wavelengths tied to molecular bonds (Al–OH, Mg–OH, Fe–OH, CO₃, H₂O). This makes SWIR essential for alteration mapping in mineral exploration — porphyry, epithermal, and other hydrothermal systems leave alteration halos (kaolinite, illite, alunite, chlorite, sericite) that are invisible in the visible/near-infrared but distinctive in SWIR.
Concrete example
ASTER carries six SWIR bands between about 1.6 and 2.43 µm, designed for mineral discrimination — band ratios such as (5+7)/6 highlight Al–OH alteration. Sentinel-2 offers broader SWIR bands at ~1.61 µm (band 11) and ~2.19 µm (band 12) at 20 m resolution, useful for regional clay and iron-oxide indices. Landsat 8/9 OLI bands 6 (1.57–1.65 µm) and 7 (2.11–2.29 µm) serve a similar role at 30 m.
A common alteration index uses the ratio of a band on the shoulder of an absorption feature to one inside it; deep Al–OH absorption near 2.20 µm flags argillic alteration.
Common pitfall
Broadband sensors like Sentinel-2 and Landsat have only two coarse SWIR bands, so they detect mineral groups, not individual species — distinguishing kaolinite from illite needs hyperspectral data (e.g. AVIRIS, EnMAP) or field spectroscopy. SWIR is also more affected by atmospheric water vapor, so atmospheric correction is important before mineral interpretation.