The longwave infrared (3–14μm) spectral properties of rock encrusting lichens based on laboratory spectra and airborne SEBASS imagery

Kurzfassung

There has been a progressive emergence of airborne longwave spectral sensing capabilities (e.g. spatially enhanced broadband array spectrograph system, SEBASS) of potential value for remote geological mapping in arctic and subarctic regions. However the presence of rock encrusting lichens can obscure spectral features diagnostic of minerals. This paper documents the longwave (3–14 μm) spectral transmittance and reflectance properties of rock encrusting lichens as determined from lichen samples, from rock samples encrusted with lichens and observations from airborne SEBASS imagery. The samples and SEBASS airborne hyperspectral imagery were acquired over the 1.9 Ga Cape Smith greenstone belt of northern Quebec. Within the Restrahlen region (8–12 μm), where silicate minerals display peaks in reflectance, the maximum reflectance observed for rock encrusting lichens is 1.6%. These lichens display low reflectance and spectral contrast and approach a blackbody behavior. For lichens detached from the rock substrate, spectra obtained on two backgrounds of contrasting reflectance are similar (< 0.75% difference in reflectance) indicating that lichens transmit little or no light. Detailed observations reveal three features that can be attributed to organic compounds in lichens (3.41, 6.58, 8.13 μm). The feature located near 8.13 μm is a reflectance peak within the Restrahlen region of enhanced mineral reflectance. A measure of the strength of this feature and the mean reflectance in the Restrahlen region for laboratory data and SEBASS imagery reveals linear mixing trajectories between a lichen endmember and a suite of geological endmembers. This data representation enables the visualization of mixing lines, the delineation of geologically informative endmembers, and provides a means to mask pixels in imagery that encompass the greatest abundance of rock encrusting lichens therefore facilitating geological mapping. The implications for the detection of geological endmembers and mapping are discussed.