Imaging spectrometry for monitoring tree damage caused by volcanic activity in the Long Valley caldera, California

Developments in detector technology have triggered a new remote sensing technology: imaging spectrometry. Imaging spectrometers measure reflected solar radiance on a pixel-by-pixel basis in many narrow spectral bands, allowing the identification of materials or their properties by diagnostic absorption features. To date, only airborne imaging spectrometers are available, but several imaging spectrometers are planned for the next generation of space platforms. The abundance of information available in the continuous spectral coverage makes it possible to address questions in numerous environmental disciplines. This paper describes a study in the Sierra Nevada, using multitemporal images acquired by the Airborne Visible/InfraRed Imaging Spectrometer (AVIRIS) for monitoring tree damage by volcanic activity. Diffuse volcanic gas emanations deprive the roots of oxygen, resulting in trees that are under stress and ultimately die. Imaging spectrometry yields important information on tree conditions and on the presence of dead vegetative material. The spatial and temporal extent of the dead and stressed tree areas were mapped using AVIRIS data. The use of imaging spectrometry to map the diffuse volcanic gas emissions was less successful. Although the images yield noisy spatial patterns of carbon dioxide, it is difficult to separate atmospheric gases from the diffuse soil emanations. In the last decennia, a new remote sensing technique was developed through significant advances in detector technology: imaging spectrometry. An imaging spectrometer collects narrow spectral bands on a pixel-by-pixel basis, aiming to identify surface materials by using diagnostic absorption features [12, 23, 37]. Figure 1 shows the concept of imaging spectrometry. Conventional broadband sensors such as Spot-XS, Landsat MSS and Landsat TM are not very suitable for mapping minerals or soil properties because their bandwidth of 70 to 240 nm cannot resolve diagnostic spectral features of terrestrial materials. Often, absorption features of interest have bandwidths of only 20 nm or less. Since the construction of the first spectrometer, the technique and the sensors have been further developed and refined, and software especially designed to analyze the large data volumes generated by imaging spectrometers have become available [31, 39]. These developments have led to the successful applications of imaging spectrometry in several environmental disciplines, such as atmospheric science [6], ecology [36, 38, 44, 46, 47], geology [29, 30, 31,37, 45], soil science [11, 15, 16], hydrology and oceanography [5, 25, 35]. The importance of these types of instrument may be indicated by the fact that several proposals for launching spaceborne spectrometers in the near future have been approved. This paper presents a practical application of imaging spectrometry for vegetation survey in the Long Valley caldera in the Sierra Nevada, California. This area suffers from volcanic activity, which causes significant damage to the pine and fir species. Multitemporal images acquired by AVIRIS were used to survey damage to pine and fir trees, and to map the spatial extent of diffuse volcanic gas emissions. AVIRIS acquires images at an altitude of 20 km in the spectral range of 400 to 2500 nm, with a pixel size of 20 x 20 m. It has 224 spectral bands with a nominal bandwidth of 10 nm (Figure 1).