Maercklin, Nils (nils@gfz-potsdam.de)

German Diploma Thesis ("Diplomarbeit"), Christian-Albrechts-Universitšt Kiel, Germany, September 1999.

Title: Polarisationsanalyse refraktionsseismischer Daten vom Vulkan Merapi, Indonesien

English title: Polarization analysis of refraction seismic data from Merapi volcano, Indonesia,

English abstract: Within the Indonesian-German research project MERAPI the subsurface structure of the high risk volcano Mt. Merapi, Java/Indonesia, is investigated with an Active Seismic Experiment (ASE). This experiment uses artificial sources with a high repetition accuracy and seismometer lines arranged radially with respect to the summit. Only three component seismometers are included in the spread. The objective of this thesis is to analyze the vector properties of the recorded wave field, to make use of selected polarization parameters in a multicomponent digital filter and to interpret the resulting travel time curves. The thesis mainly focusses on the data of one seismic profile in the south of Mt. Merapi. The processing of the vertical component of that profile includes the stacking of single shots, the frequency analysis with a comparison of artificial and volcanic signals, and the calculation of the frequency-wavenumber spectrum (f-k spectrum). A f-k filter can partly separate waves propagating forward and backwards. The polarization analysis yields a high degree of linear polarization for the first arrival, while the strong coda waves show diffuse results. The direction of polarization is found out to be a parameter distinguishing later, reflected onsets from the remaining wave field. The polarization filter combines a measure of rectilinearity and the direction of polarization, and it emphasizes later onsets of reflected waves at profiles with an offset up to 4 km. The application of the filter to data of one far offset profile (up to about 7.5 km) amplifies the first onset and enables an improved evaluation of the travel times. Also at this profile reflected waves become visible. The reflections can be explained with a simple two dimensional model based on the ray theory, and it is shown that they are caused by open fracture zones.