Test for Applying SAR Interferometry on the Nyiragongo Area: Preliminary Results

The capability of SAR Interferometry to investigate ground deformation in the Nyiragongo area has been tested in this work The test aimed to investigate how the vegetation of Nyiragongo area affects the interferometric phase integrity, between two SAR passes. This point is, indeed, crucial in equatorial areas. The three-pass technique requires subtracting the topographic component from the interferograms used to measure ground deformations. In the absence of external topographic information (e.g. a photogrammetric DEM), the interferometric topographic phases can be provided by processing two SAR images that do not contain deformations. So, from the operational point of view, the test was aimed at verifying the capabilities a) to obtain topographic phases, as continuously as possible on the Earth surface, and b) to produce a ground deformation map, even by using image pairs with a long time interval between passes. The positive results of this test, pushed to carry out other SAR interferometric analysis in this area, aimed to studying the dynamic of recent volcanic eruption, so that processing of data relevant to the recent January 2002 eruption have been processed and the results of this process are discussed. 1 VOLCANOLOGY OF THE VIRUNGA CHAIN The Virunga volcanic chain lies in the Rwanda Republic and in the Democratic Republic of Congo (ex Zaire) and is erected on the western arm of the East African rift. It is composed by eight volcano grown on normal fracture of the rift. The two westernmost volcanoes, Nyiragongo and Nyamuragira have erupted repeatedly throughout the last 40 years, the summit of these volcanoes reach elevations of 3470 m and 3058m, respectively (figure 1) The Nyiragongo stands about 2000m above the level of nearby Lake Kivu, and contained an active lava lake in its deep summit crater that drained in 1977. In contrast to the low profile of its neighboring shield volcano, Nyamuragira, Nyiragongo displays the steep slopes of stratovolcano. Two older stratovolcanoes, Baruta and Shaheru, are partially overlapped by Nyiragongo on the north and south respectively. About 100 parasitic cones are located primarily along radial fissures south of Shaheru, east of the summit, and along a northeast-southwest zone extending as far as Lake Kivu. Many cones are buried by voluminous flank lava flows, the most recent of which extends from an east-flank fissure south to within 4 kilometers of Goma An eruption began at Nyiragongo, on 2002 January 17th. Lava flowed on the eastern and southern flanks of the volcano up to 2km per hour toward the city of Goma (~10 km South). 2 DATA ACQUISITION AND ANALYSIS OF RESULTS The capability of SAR Interferometry to investigate ground deformation in the Nyiragongo area has been tested by adopting the so-called three-pass interferometry techniques selecting three adequate SAR images collected by ERS1 and ERS2. The images used in the test were by ESA, within the budget assigned to the Project “Development of SAR Techniques aimed at managing Natural Disasters in geodynamically active areas” (P.I. G. Puglisi) carried out in the frame of 3 Announcement of Opportunity for the Exploitation of ERS Data. The selection of the images was performed by inspecting the ERS archives with the DESCW software and we selected the images in order to achieve the capabilities: a) to obtain topographic phases, as continuously as possible on the Earth surface, and b) to produce a ground deformation map. To this end we took advantage of the availability of a Tandem pair of images for generating an interferogram containing the topographic phases. A Tandem pair is a particular pair of ERS1/ERS2 SAR images, acquired on the same area in two successive days. In particular, we used the pair obtained from the ERS1 pass of 10 Nov 1998 and of ERS2 pass of 11 Nov 1998. The relevant perpendicular baseline of this pair was 335 m. To co-register the two images and calculate the interferometric geometry, we used the precise orbit of ERS2 satellite, produced at the Delft Institute for Earth Oriented Space Research (DEOS). Due to problems encountered in focusing the raw data of the whole ERS2 image, we selected only the part relevant to the Virunga Volcanic area. A preliminary analysis of the image can be performed by inspecting the coherence. It is good for a ____________________________________________________________ Proc. of FRINGE 2003 Workshop, Frascati, Italy, 1 – 5 December 2003 (ESA SP-550, June 2004) 63_gugliel large portion of image (Figure 1), although in steep areas we obtain low values, due to typical geometrical errors of SAR (layover and/or shadow effects). Figure 1: Coherence map of the Tandem Pair E2_11nov98 E1_10nov98. The coherence ranges from 1(maximum, white) to 0 (minimum, black). North is up. Nyiragongo Nyamuragira