Principles of an experimental large aperture seismic array (LASA)

One of the most useful techniques for improving capability to observe small remote seismic events is to connect a large number of seismic sensors distributed in the horizontal plane so as to form an array. This has proved to be of great interest in seismological research and is particularly useful in the monitoring of underground nuclear tests, since the events are of small magnitude and the criteria used to distinguish source type depend on freeing the weak signal as far as possible from additive microseismic noise, reverberation components, and interfering earthquake activity that might chance to occur simultaneously. Results achieved by the use of seismic arrays of several kilometers aperture and several tens of sensors were sufficiently promising to suggest that substantial further improvements might be expected from an even larger array having more than an order of magnitude greater aperture and number of sensors. Such a Large Aperture Seismic Array (LASA) has been built and is being evaluated. The number of sensors is 525 and the aperture is 200 km. The notions underlying the LASA development are presented briefly in this paper, which serves as an introduction to three more detailed papers in this issue. After a brief historical resume, the principles of array signal processing are reviewed, and a prognosis of LASA capabilities is presented. The paper concludes with a discussion of the extent to which the results obtained to date have confirmed our expectations.