The Measurement of Apparent Velocity and Azimuth using Adaptive Processing Techniques on Data from the Warramunga Seismic Array

Abstract

Summary Adaptive processing of multichannel data can be achieved by cross-correlating the signal on each channel with a velocity and azimuth filtered composite signal in an iterative manner. In this way it is possible to accurately determine the arrival times of the wavefront at each sensor. This technique, which is currently used for the autematic analysis of seismic events recorded at the LASA and NORSAR arrays, has been used as the basis of a software system developed for processing data from the medium aperture Warramunga (WRA) array in northern Australia. This system has been developed for a small and relatively inexpensive computer installation, and permits the rapid, direct measurement of apparent velocity and azimuth of signals coherent across most or all of the array. The position of WRA relative to the Earth'sactive seismic zones, and in particular the belt north of Australia, results in multiple P wave signals from single events being frequently recorded. Emphasis was therefore placed on assessing the capabilities of adaptive processing for making measurements not only on the first arrival but also on its immediate coda. The effects of signal multiplicity on parameter measurement were studied, for both real and synthetic data, using a number of different processing variables such as convergence criteria, parts of waveform used and window positioning. The results show that several measurements must be made across a waveform in order to identify systematic drifts shown to result from interference effects. Experiments using inverse filtering were performed in an attempt to improve the resolution.