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SAR/InSAR
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Sar Interferometry Applications In The Philippines Using Ers-1, Ers-2 And Jers-1: Case Studies In Mayon And Taal Volcanoes
4. Results and Discussions
The surface of the earth is said to have changed whenever l/2 (2.8cm) was changed along the look direction of the satellite. The phase difference, which was obtained from the phase-unwrapping processing from 2 p radian, was converted into the amount of change or deformation.
4.1 Mayon Volcano
The three-pass method was applied to the first three ERS-1 datasets dated 1993/11/17, 1993/10/13 and 1997/10/10. The Bp is -21m for first pair and -156m for the second pair. Two coherence maps and two interferograms were produced. The area with high coherence was near the city and along the southwest the foot of the Mayon Volcano. This area corresponds to the lava flow and the pyroclastic flow deposited during the eruptive activity in 1993 or 1984. In this area with high coherence, it is thought that vegetation is scarce or the area is almost bare ground.
Further processing produced a differential interferogram. The result was refined using a "helper phase". About 6cm displacement was measured along the southwest flank of the volcano near the crest. This measured displacement roughly coincides with the tiltmeter record from January 1993 to December 1997 from the Mayon Rest House Observatory. For the water tube tiltmeter data the line A-C indicates the direction towards the summit of the volcano while the line A-B is perpendicular to line A-C. The rising curve of the A-C line shows that the crest rose gradually although there is a period when the meter broke down.
We applied the two-pass method to the second set consisting of ERS-2 data dated 1999/03/20 and 1999/08/07. The Bp is 105m. A coherence map and interferogram were generated. A DEM was integrated to produce a differential interferogram. The measured displacement along the southwest slope of Mayon Volcano is up to 30cm (fig1). It is viewed that this corresponds to some magmatic movements underneath the volcano. Later in February 2000, the volcano erupted.
In general, the changes between 1999/08/07 and 1999/03/20, and between 1993/11/17 and 1997/10/10 were detected using SAR interferometry technique. The results show that in the former, the vicinity of the crest expanded by about several cm. In the latter, the slope dilated not the crest. The amount of the change cannot be quantitatively evaluated from the data of the tiltmeter meter alone. However, the amount of change obtained as a result of SAR interferometry is thought to be the one being affirmed by the tiltmeter record. This is further collaborated by the GPS measurement results obtained over Mayon Volcano during 1997 to 1999. However, time and space does not allow presentation of the GPS data and results in this occasion.
3.2 Taal Volcano
The DEM generated for Taal volcano has a size of 9km by 9km. Since this size was too narrow to be utilized by the processing software, the DEM was not used and the three-pass method was applied on the JERS-1 data. Three data dated 1995/11/15, 1995/02/24 and 1995/12/29 were utilized. The Bp is 171m for first pair and 741m for the second pair. The resulting differential interferogram shows displacement of more than 10cm along the southwest flank of Taal Volcano. There was also some subsidence near the southwest lakefront going towards the crater. For now, we do not have observation data to verify this detected amount of change.
6.0 Summary and Conclusions
We examined the application of SAR interferometry to study deformations in the Mayon and Taal Volcanoes in the Philippines. We used the JERS-1 SAR and ERS SAR data. We applied the differential interferometry technique to detect surface change in both volcanoes. We used the repeat pass method (two- or three-passes). In both cases, differential SAR interferograms were successfully generated.
In Mayon, both two- and three passes were applied. Both methods succeeded in detecting surface deformation in limited areas. The first set using three ERS-1 datasets dated 1993/11/17, 1993/10/13 and 1997/10/10 shows displacement along the southwest flank of the volcano amounting to 6cm. This roughly coincides with the tiltmeter record in the Mayon Observatory.
We also applied the two-pass method to the second set of ERS-2 data dated 1999/03/20 and 1999/08/07. The measured displacement along the southwest flank of Mayon Volcano is up to 30cm.
In the first dataset, the vicinity of the crest has expanded by about several cm. In the second set, it is the slope that expanded and not the crest. It is viewed that the expansion of the slope in the second set represents magmatic movements underneath the volcano. Later in February 2000, the volcano erupted.
We successfully generated interferograms using the JERS-1 SAR data for Taal Volcano. For Taal Volcano, the three-pass method was applied to the JERS-1 data dated 1995/11/15, 1995/02/24 and 1995/12/29 The resulting differential interferogram shows displacement of more than 10cm along the southwest flank of Taal Volcano.
As a whole, while the technique may have successfully produced differential interferogram, we still need to refine our methods. Much research is still necessary.
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