Off-Line Signal Processing Results for the Large Aperture Seismic Array

The results of a series of off-line signal processing experiments are presented for the experimental Large Aperture Seismic Array (LASA) located in eastern Montana. In particular, the signal-to-noise ratio gains achievable by maximum-likelihood processing, as well as other simpler forms of processing, are given as a function of frequency, aperture, and number of sensors. A partial discussion of the physical characteristics of the noise field which led to these results is also presented. The spurious precursor introduced by one form of signal processing, and shown to be caused primarily by the signal amplitude scatter within a subarray, can be effectively reduced by using amplitude equalization. The signal amplitude scatter within a subarray, and between subarrays, is thus found not to degrade seriously the performance of the signal processing. In fact, the scatter of amplitudes between subarrays, which is significantly larger than that within a subarray, is a definite asset. It was found that the effectiveness of the processing depends not only on the array spatial filtering ability, but on differences in absolute level of the noises in the various sensors. Long-period array data from the extended TFO array were processed and showed roughly JN improvement. Our experience in processing a number of events is illustrated by a discussion of the results obtained on two particularly weak events with the maximum-likelihood filter. The apparent magnitude of one event (as averaged over the LASA) was 3. 5, and that of the other was 4. 0. It was found that the processing was capable of extracting both events from the background noise to the extent that they could be identified as earthquakes and not nuclear explosions. An experiment in suppressing large events that obscure simultaneous smaller events from other source regions is reported. On the basis of these results, several recommendations concerning off-line and on-line processing methods and array geometry are given. Accepted for the Air Force Franklin C. Hudson Chief, Lincoln Laboratory Office