Field mapping activity for an active volcano mainly in the Torrid Zone is usually hampered by several problems such as steep terrain and bad atmosphere conditions. In this paper we present a simple solution for such problem by analyzing Synthetic Aperture Radar (SAR) and optical sensor image data. By a combination of two types of satellite images, we detected the geothermal paths by identifying the alteration zone termed A-zone. The main purpose of this study is to discriminate alteration of the pyroclastic flow deposits and estimate their alteration degree by selecting Mt. Merapi in central Java, Indonesia, as a study site and targeting the eruptions during May-June 2006. To delineate the A-zone, we applied an image fusion technique using a ratio image of RADARSAT-1 SAR β0 data and an MNF transformation of Hyperion image data. The acquisition dates of these images were almost the same to reduce large different change in the image characteristics. In addition, a field survey was carried out to check the usefulness of the image fusion results. The A-zone is found to extend in the eastern flanks by 1.5 km2 which are covered mainly by the old pyroclastic flow deposits. This area can be interpreted as an ascent flow zone of hydrothermal fluids beneath the summit.
Figure 1: Mt. Merapi on a RGB color composition of a Hyperion image using three bands, 2.19 μm (R), 1.65 μm (G), and 1.07 μm (B). Red rectangular and red “+” stand for the main target area and rock sample locations, respectively.
Figure 2: Field photograph of altered rocks in the eastern part from the summit.
Figure 3: Eigenvalues of Minimum Noise Fraction (MNF) bands selected for image fusion.
Figure 4: Work flows of image fusion for RADARSAT-1 SAR and Hyperion image data.
Figure 5: Color composition of image fusion result by R: Hue (MNF band 1), G: Saturation (MNF band 2), and B: Intensity (RO equalized value). The A-zone is located in yellow-orange portions in the eastern part from the crater (red triangle).
Figure 6: Reflectance spectra of rock samples from visible to short-wave infrared regions.
Figure 7: Reflectance spectra of the AP compared with the reference spectra of clay minerals and silicates in the USGS spectral library.
Figure 8: Distribution of the A-zone overlaid with contour lines of lineament density from directional filtering of the SAR ß0 image on July 4, 2006. The black arrow and green polygon stand for the sample location of the AP and the alteration rocks inferred from SAM classification.
Saepuloh A., Koike K., Discriminating alteration of pyroclastic flow deposits in an active volcano by SAR image analysis for assessing the geothermal system, Proceeding of the World Geothermal Congress 2010 (WGC 2010), in PDF format No. 1336, pp. 1-5, Denpasar-Bali, Indonesia, April 2010.