Beschreibung:
Ambient seismic noise has been used successfully as a source of information for structural investigations. Ballistic surface as well as body waves were reconstructed by correlation of noise and used in tomographic studies. In these cases it is of course assumed that the medium under study is stationary, i.e., that the reconstructed Green’s function does not change with time. In this contribution we show that medium changes can well be monitored by means of subtle changes in the Green’s function. Using an interferometric approach applied to the coda part of the Green’s function, we detect temporal changes of delay times on Merapi volcano (Indonesia). The changes of delay time depend on lapse time, which indicates that the velocity changes inside the volcano are spatially heterogeneous. We present a hydrological model that can explain the temporal changes of delay time as well as its lapse time dependence changes of the ground water level induced by precipitation. From this analysis we conclude that (a) seismic coda can be practically retrieved from noise correlations, (b) temporal changes can be monitored with noise correlations, (c) even spatial heterogeneity of the changes can be identified, and (d) the coda retrieved from noise correlations is composed body waves. a)Currently at LGIT Grenoble, France. b)Currently at SZGRF/BGR Erlangen, Germany.