Keywords:hazards, volcanic, mapping, classification, AIRSAR
Abstract
Canlaon Volcano, a strato-volcano situated in the Negros Island, Central Visayas,
(10
°24.7', 123
°7.9'E), is one of the six most active volcanoes in the Philippines. The
Philippine Institute of Volcanology and Seismology (PHIVOLCS) monitors the volcano
and maintains three observatories in the area. In anticipation of volcanic eruptions, the
Institute prepares hazard maps of the volcano. These hazard maps are necessary
tools for hazard mitigation and disaster-preparedness plans of the government.
Airborne Synthetic Aperture Radar (AIRSAR) images covering part of the Canlaon
Volcano area are continuously being processed towards becoming a possible tool to
augment hazard map generation. The polarimetric (POLSAR) and topographic
(TOPSAR) data sets of the images were processed using the software ENVI
(Environment for Visualizing Images). The basic geometric and radiometric corrections
were first applied to the images. To generate a better surface detail of the area, image
enhancements were done using the band mathematics calculation module of the ENVI
software. The polarimetric data set (C-VV, and all the polarization of the L- and P-bands)
was combined into one file. This was classified using the maximum likelihood
option to generate a classification map. The digital elevation model for the C-band
(topographic data set) was used to map out the volcanic deposits in the area. The
resultant products from processing the POLSAR and TOPSAR data sets are being
used as additional tool to the hazard map generation of Canlaon Volcano.
This paper summarizes the image processing done to the AIRSAR images of Canlaon
Volcano as applied to hazard mapping activity. The output products from the image
processing activities are image maps of the Canlaon Volcano area containing the
distribution of volcanic products spread around the study area. Details of the image
processing of the AIRSAR images are presented and discussed. The paper identifies
and highlights the potential of the AIRSAR data sets as applied to geologic and hazard
mapping activity on a volcano.
I. Introduction
A. Canlaon Volcano
Canlaon Volcano, a strato-volcano, is situated in the north central portion of Negros
Island, Philippines, with the geographic location of 10º 24.7´N and 123º 7.9´E and peak
elevation from sea level of 2,465 meters (Figure 1, Location Map of Canlaon Volcano).
The Philippine Institute of Volcanology and Seismology (PHIVOLCS) maintains three
Volcano Observatory Stations around and within the vicinity of Canlaon Volcano.
Figure 1. Location Map of Canlaon Volcano
Despite its being listed as one of six most active volcanoes in the Philippines, Canlaon
Volcano has had no violent eruption ever recorded yet. Normal ash explosions and
steam blast, as in its first known eruption in 1866, characterized majority of its eruptions.
It has similar phreatic eruption in 1893, 1894, and in 1898. In 1902 and 1906, Canlaon
Volcano is said to have erupted again characterized by lava outpouring. The 1969
eruption of Canlaon Volcano saw tephra falls with blocks hurling to a distance of
around 3 kilometers from the crater. The same type of volcanic debris was seen in the
1978 eruption as well as some more ash ejection. Earthquake swarms occurred in the
volcano area in 1973 and in 1980 but for both instances, these did not culminate into
an eruption. In 1987 and 1988, small ash and steam ejection were again observed.
The most recent eruptive activity of Canlaon Volcano in 1996 was generally
characterized by ash ejection as well.
A semi-detailed geologic mapping and preliminary hazard assessment was done on
Canlaon Volcano (Umbal and Arboleda, 1987). More detailed geologic mapping was
started in 1998 with the purpose of establishing a detailed volcano stratigraphy and
revising the existing map. A review and interpretation of topographic maps and aerial
photos of the area were undertaken. The preliminary map produced was subsequently
verified in the surveys conducted in 1998 and 1999. The output geologic map will
become the basis in delineating and/or updating the existing hazard maps of Canlaon
Volcano.
The entire geographic coverage of the study area covers the following latitude-longitude
extent based on 1:50,000 topographic base map:
| Latitude extent |
10°10´ - 10°40´N |
| Longitude extent |
122°39´ - 123°24´E |
B. AIRSAR Images and ENVI
There are three AIRSAR image strips taken in 1996 that comprise the aerial flight in the
Canlaon area (Kanlaon_102-1A, Kanlaon_202-1A, and Kanlaon _102-2A). Within the
study area for Canlaon Volcano, the three strips comprise the following latitude-longitude
extent based on the header files of the images:
| Latitude extent |
10°10´ - 10°40´N |
| Longitude extent |
122°22´55.2"- 123°43´41.8"E |
This paper discusses the image processing applied to the AIRSAR (Airborne Synthetic
Aperture Radar) data sets of Canlaon Volcano using the software ENVI (Environment
for Visualizing Images). Image enhancements, corrections, creation of relief maps
from the Digital Elevation Models, band ratio analysis, principal component analysis
and supervised classification are the processing techniques applied to the AIRSAR
images in order to generate map images that can be used to aid hazard mapping. It is
emphasized that the processing done to a part of the entire set of images covering the
Canlaon area will be towards the development of a procedure to be implemented later.
It is envisioned that the utilization of the AIRSAR data sets will be maximized as
applied to volcanic hazards mapping.
