Determination of mechanical discontinuities at Merapi summit from kinematic GPS

François Beauducel, Made Agung Nandaka, Géraldine Florsch, Jérôme Ammann, Philippe Mourot, François-Henri Cornet, and Michel Diament

Paper presented at International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) General Assembly, Bali, Indonesia, July 18-22, 2000.

Abstract. Since 1992, Merapi volcano exhibits almost continuous activity with growth of an andesitic lava dome, which collapses in glowing avalanches, explosions and nuées ardentes, these are sometimes deadly. Starting 1993, we established a Global Positioning System network and measured it each year using the static method. This allowed us to monitor the evolution of surface displacements and model the associated magmatic sources [Beauducel and Cornet, 1999; Beauducel et al., 2000]. But the poor spatial density of benchmarks and awkwardness of field campaigns did not yield the precise location of major mechanical discontinuities within the edifice. However, identifying precisely these discontinuities is of central importance since they delimit areas of potential instability and provide means to evaluate potential volumes of falling material. The kinematic GPS method offers a modern means to partially solve the problem of temporal and spatial sampling of the displacement field. In the framework of Indonesian-French cooperation, we acquired two double-frequency receivers (Dassault-Sercel Scorpio 6000). Starting December 1999, several missions were carried on. A new geodetic network of about 50 benchmarks (8-cm length stainless nails) have been established at Merapi summit, in order to cover the whole area around main crater. Measurements are continuous (1-s sampling rate) using a reference receiver located at LUL point (northern summit area), and a mobile receiver. Within a few hours of observation, the 3-D trajectory relative coordinates of the mobile receiver can be obtained with a precision of about 4 cm. Stacking trajectories of each campaigns allows to get highly precise digital elevation model (DEM) of the studied area. We also measured on each benchmark during few minutes, thus relative positions of the network points can be post-processed with a repeatability of about 1 cm. Rapid static GPS method has been also performed in order to validate the estimated precision of kinematic method on such small network (less than 500-m wide). We present here the first results of our GPS measurements at Merapi and the different processes that we developed (1) to optimize the extraction of benchmark 3-D positions and associated uncertainties, and (2) to refine previous DEM of the summit area.

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