The Effect of the Number and Spacing of Elements on the Efficiency of Lasa Beams

Abstract

LASA short-period recordings of 8 teleseismic earthquakes were prefiltered and beamsteered on P-wave arrivals across the 200 km. aperture to establish the relationship between sensor spacing and beam efficiency in terms of noise reduction, signal loss, and S/N ratio improvement. Results show that the combined effect of increasing the number of elements in a beam v,hile simultaneously reducing inter-sensor spacing is to produce progressively less rms noise reduction and S/N gain relative to N. The study further thows that beamforming each of the events in two ways, e.g., with 51 and 525 inputs, produces an average signal loss of ~H db. Moreover, beaming the smaller number of traces reduces rms noise and improves S/N only about 1 db less than the 525-element beam. For the 51-element beam, the minimum sensor spacing was 6 km., the distance at which the short-period noise at LASA becomes incoherent.

The Effects of Reduced Configurations at (LASA) on Detection Signal-to- Noise Ratios

“…The functional flow of data from LASA through the SDAC/LASA system is: 1) data acquisition; 2) detection processing; 3) event processing; 4) experimental operations console (EOC) editing; and 5) publishing of the Daily Earlier studies of the effects of reducing the number of elements at LASA (Hartenberger, 1967;Hartenberger and Van Nostrand, 1970) have shown that the signal-to-noise ratio loss is less than 2db compared to that for the original 525 sensor array when the number of elements is reduced to 119 or 51 with minimum sensor spacing of 3km or 6km respectively. All of the data used in the earlier studies were prefiltered (0.4-3.0 Hz), were beamed to the known opicentral locations, and were corrected for travel-time anomalies (Chiburis, 1968).…”