The NORSAR Array and Preliminary Results of Data Analysis

Abstract

Summary A large aperture seismic array, NORSAR, has been constructed in Norway. The project, which started in the summer of 1967, is a joint undertaking by the governments of Norway and the United States of America. NORSAR consists of 22 subarrays, each equipped with one three-component long-period and six short-period instruments. The array diameter is around 110 km, while that of a subarray is approximately 8 km. In the data centre, which is located just outside Oslo, are installed 2 IBM 360/40 computers with peripheral equipment, a special-purpose computer, and an experimental operations console. Routine tasks performed at the data centre comprise array monitoring and calibration, data acquisition, on-line event detection and off-line event analysis. In this paper we give a technical description of NORSAR, emphasizing the software aspects of the array operation, and present some analysis results of P waves recorded at NORSAR. For example, we have found that signal power and spectral characteristics vary across the array and seem to reflect local differences in the geological structures at the subarray sites. The recorded signals are found to be broadband and to contain significant energy at higher frequencies. Observed signal coherencies vary considerably across the array and are usually independent of station separation. Within the subarrays signal coherence is high and the waveforms exhibit little scattering.