Design and siting of a new regional array in Central Europe

Abstract

Abstract In recent years, increased interest has emerged in the evaluation of high-frequency seismic signals from events at regional distances. Small-sized and many-element arrays, like NORESS in Scandinavia, have proven capable of taking advantage of the very efficient propagation of high-frequency seismic phases. After an extensive field survey, the Bavarian Forest (BF) area at the southeastern border of the Federal Republic of Germany with Austria and Czechoslovakia was chosen as a target region to install a NORESS-type array. The BF area—as part of the Bohemian Massif—represents the largest outcropping crystalline complex in Central Europe. Detailed noise measurements show average values for the power spectrum slightly lower than 1 nm 2 /Hz at 1 Hz and a fairly smooth decay proportional to f −4 per decade up to 40 Hz, leading to a value of 10 −4 nm 2 /Hz at 10 Hz and 10 −5 nm 2 /Hz at 20 Hz. Diurnal variations due to industrial noise sources, such as saw mills, are primarily found in the 4 Hz to 10 Hz band. Compared to Scandinavia, the noise level in Central Europe is comparable or even lower for frequencies about 1 Hz, but it is about a decade higher at 10 Hz and above. The spatial correlation properties of noise and signals were investigated using data from a temporary 9-element array with a diameter of 3 km. In the 1 to 2 Hz band, the noise coherence values drop to 0.25 at interstation distances of 800 m and in the 2 to 4 Hz band at 400 m, respectively. There is no clear evidence for a negative noise correlation distance at these frequencies. Signal coherence in the same frequency bands proved to be excellent (>0.9) for all regional phases over the 3 km aperture of the test array. The final configuration of the new regional array in the BF area—named GERESS—has been selected as a slightly enlarged NORESS-type ring geometry with an innermost radius of 200 m and with a large radius of about 2,000 m.