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Figure 1: The forest in excellent health near Mammoth Mountain (CA).
Figure 2: The dying trees near Mammoth Mountain due to CO2 asphyxia.
The Department of Physical Geography, Utrecht University, The Netherlands and NASA's Jet Propulsion Laboratory (Pasadena, CA) are studying 1) the dying trees and its expansion over time, and 2) the gas emission from the magma body beneath Mammoth Mountain by using NASA's imaging spectrometer AVIRIS: Airborne Visible/Infrared Imaging Spectrometer. An imaging spectrometer registers the reflected sun-light in many, narrow spectral bands. Absorption bands in these spectra provide information on e.g. the presence of gasses in the atmosphere or the chlorophyll activity of vegetation. A description of the concept of imaging spectrometry can be found at the Jet Propulsion Lab's AVIRIS site.
A sequence of airborne AVIRIS images from March 1990 until June 1995 were retrieved from the JPL archive. The images were corrected for sensor and atmosphere distortions and were geometrically fit to one another. Next, the images were compared to survey the temporal expansion of the dead trees areas. Healthy trees, stressed trees and dead trees can be separated in an AVIRIS image by comparing their spectral reflectance as shown in the figure 3. Healthy vegetation shows a strong reflection of sun-light in near infrared (700-1100 nm) and a strong absorption of sun-light near 680 nm caused by chlorophyll. As soon as vegetation becomes `stressed', chlorophyll absorption decreases and near infrared reflection decreases. This behaviour is very helpful to separate healthy trees, stressed trees, dead trees, soil and rock outcrops.
Figure 3: Examples of field spectra for healthy, dead and stressed vegetation.
Figure 4 shows an example of an AVIRIS scene of Mammoth Mountain acquired on 23
August 1994. Clearly visible in the centre is snow covered Mammoth Mountain with its ski
lanes. Lakes are dark, vegetation is red (e.g. the golf course near Mammoth village in the right
part), Pine forest are dark red. The dead tree area just north of Horseshoe Lake (1 km south
of Mammoth Mountain) is clearly recognizable as a bright spot. The temporal analysis showed
that most trees near Horseshoe Lake died area between 1990 and 1992. The trees a bit more
uphill of Mammoth Mountain were suffocated between 1993 and 1995. The dark blue triangle
in the western part of the image also represent dead trees but this is caused by a forest fire and
not by volcanic gas release.
The volume of volcanic gas release (mainly CO2) was estimated on 1200 tons/day for the Horseshoe Lake area (Farrar et al., 1995). An attempt was made to trace the most important locations of gas releases by using AVIRIS. Figure 5 shows a spectrum measured for a fumarole (a volcanic hot gas exit). Absorption features for CO2 are visible near 2005 and 2055 nm. An algorithm was applied to inverse and enhance the CO2 absorption. The algorithm, referred to as Continuum Interpolated Band ratio, uses the shoulder and the deepest point of the absorption feature to assess the abundance of CO2 . This part of the study was not very successful. It was not possible to separate CO2 in the atmosphere from the volcanic CO2 in spite its large volumes.
Figure 5: Field spectrum of a gas discharge of a fumarole near Mammoth Mountain.
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