Multi-scale topographic analysis of Merbabu and Merapi volcanoes using wavelet decomposition

With the increasing availability of data, geo-sciences have experienced deep changes in handling and processing it. One of the presently explored research directions concerns the systematic decomposition and understanding of topographical features, without the subjective interactions of humans. This can eventually lead to fully automated algorithms for topographic analysis and understanding. This paper aims at being a contribution to this broad research area for the specific cases of stratovolcanoes, whose general geometry are very similar to a perfect cone. More specifically, this paper addresses two issues: (1) is it possible to separate erosion features (local variations) from structural features (large variations) on stratovolcanoes, through mathematical expression; (2) can information on volcanic activity—intensity, age, etc.—be retrieved from a topographic analysis? The study has been conducted from two volcanoes in Central Java (Indonesia): the Merapi and the Merbabu. The DEM of these volcanoes has been sampled using concentric circles with a radius ranging from 500 to 5,000 m (horizontal distance) to the summit. The data conversion and sampling was performed in ArcMap^®, while the data analysis was carried out with Matlab^®, using Discrete Meyer wavelet decomposition. Results provide an insight on large-scale topographic variations (long-wave wavelet) that have been separated from rapidly varying topographic features such as lahar channels (short-wave wavelets). Observations proved that flanks where the most recent volcanic activity occur—like at Merapi Volcano on the S-SE flank—present a very low variability of long-wave variations, whereas short-wave variations are important. The author argues that this feature is due to highly erosive lahars that dig the valleys combined with a recent production of material and volcanic growth keeping the overall structure regular. Flanks with lesser activity are characterized at the two volcanoes by important long-wave variations—most certainly due to long-term differential erosion—and different level of short-waves variations. Comparing the two volcanoes, results show that the valleys of Merapi and Merbabu volcanoes are deeply incised, indicating recent periods of high activity, with reworkable material eroded by lahars and other channels deepening processes. The topography of the summit area of Merapi Volcano is smoother than at Merbabu Volcano, where deep erosion features extend up to the summit area. This difference is most certainly due by the material production at the summit of Merapi Volcano. Developing such classification is important for automated mapping and computer recognition of volcanic past activities and their impacts on landscapes. It is the based for the development of decision trees that assist computer assisted and automated computer vision.