Keywords: hazards, multi-temporal, radar, volcanic

Abstract
The occurrences of volcanic hazards continue at Pinatubo long after the 1991 climactic eruption. Most notable of these hazards are those involving the re-deposition of pyroclastic flow and fall deposits as lahars, deposit-derived pyroclastic flows and phreatic ash fallout. Many of these processes went unobserved when they occur in river valleys that have been inaccessible for several years after the eruption. Also, these events occurred in various volumetric magnitudes and frequency that are difficult to track down in all pyroclastic flow fans. Other potential hazards that may threaten some areas at Pinatubo include a possible lake break out from the 1991 crater.

The multi-temporal Shuttle Imaging Radar-C (SIR-C), Earth Remote Sensing Satellite (ERS) and Airborne Synthetic Aperture Radar (AIRSAR) data are shown here as important tools for monitoring the occurrences of volcanic hazards. Since the generation of these hazards is intimately associated with the changes in volcanic landscape, tracking down the evolution of Pinatubo landscape using RS data provides a measurable parameter to put these events in a timeline that probably allows scenario building and prediction.

Using ENVI software, the images were processed to reflect the cumulative changes within the period of time between two scenes. The method of principal component analysis was used to average the DN values and reduce the multi-spectral multi-polarized data into a single image. Subsequently, image subtraction and band ratio were employed on multi-temporal datasets to map out new lahar deposits in low-lying areas, determine channel incisions in ignimbrite fans and monitor crater lake level rise.

SIR-C Processing
The Space Shuttle Imaging Radar (SIR-C) of Pinatubo Volcano were acquired in April and October 1994 with multi-spectral and multi-polarization quality. Applying the principal component analysis, contemporary scenes consist of L and C bands of three different polarization were reduced to a single image. The first principal component of April 1994 scenes was subtracted from the first principal component of October 1994 scenes. The derivative image was enhanced by density slice function to highlight areas pixel values that show significant differences.

AIRSAR Image Processing
The Airborne Synthetic Aperture Radar (AIRSAR) data of Pinatubo Volcano area were acquired on 29 November 1996. The polarimetric (POLSAR) and topographic (TOPSAR) data sets of the images were processed using the software ENVI (Environment for Visualizing Images). Image processing of the multi-polarized and multi-spectral POLSAR data set (horizontal-horizontal, horizontal-vertical, vertical-vertical and total power polarimetry for the L- and P-bands; and vertical-vertical for the C-band) utilizes conversion initially from raw intensity to sigma zero values, and then into decibel values; Lee filtering; and slant to ground range correction. The TOPSAR data set, which consists of digital elevation model (DEM) derived from C-band, was used to generate the slope, aspect, shaded relief, and contour maps. After co-registering the TOPSAR and POLSAR data sets, multi-polarized scenes were draped over the aspect and shaded relief models. The AIRSAR images were cropped at similar spatial subset as the ERS and SIR-C crater scenes. A perspective view of the 3-D model of Pinatubo crater area was generated together with a plot of a transection across-the lake.

ERS Processing
Fifteeen ERS scenes were acquired starting August 29, 1993 until January 14, 1998. All ERS scenes were co-registered using the August 29, 1993 scene as the base image. The original scenes were cropped from a large region to define the caldera and the summit region of Pinatubo as the area of interest. Using ENVI software, band ratios of the multi-temporal scenes were applied to generate a derivative image that reflects the changes between scenes. Changes were presented as tones that are lighter and darker than gray and are arbitrarily assigned green and blue colors, respectively, for distinction. Band ratios were applied to 5 pairs of ERS scenes that approximate 5 seasonal cycles from 1993 to 1998.