3. Data analysis
Three types of data were used to map recent lava flows at
Westdahl: SAR amplitude images, a Landsat-7 multispectral
image, and digital elevation models (DEMs). The first data
set consists of six ERS-1 SAR images (Table 1) acquired
from January 19 through February 9, 1992, shortly after the
end of the 1991–1992 eruption. These images were acquired
by the European ERS-1 satellite during the 3-day repeat pass
phase. Multiple SAR data are used to reduce the speckle
noise in the SAR images. The images were geo-referenced to
the Universal Transverse Mercator (UTM) system based on a
DEM and the SAR imaging geometry. Fig. 1(b) shows the
amplitude image acquired on February 9, 1992 (Table 1).
Because the fissures and lava channels from the 1991–1992
eruption are much rougher than the surrounding snow and
ice, they are readily distinguished in the SAR image. On the
upper flank of the volcano, the 1991–1992 flow can be
traced downslope due to its relatively bright backscattering
relative to that of adjacent flows. However, on the lower
flank and near the base of the volcano, the backscattering
from the 1991–1992 flow is not easily distinguishable from
that of its surroundings (Fig. 1b).