Seismic Arrays

Abstract

The advantages of using multiple seismic sensors deployed in a small area were known in seismic prospecting long before these techniques were applied to earthquakes. The present development of seismic arrays is partly due to the recommendation by a group of experts in 1958 of improving the quality of seismic stations worldwide as a mean of detecting possible violations of the agreement of nuclear test suspension. At present, the Comprehensive Test Ban Treaty Organization (CTBTO, Vienna) provides the standards and the coordination for this purpose (see also 8.12). The availability of smaller sensors, better data acquisition systems and powerful computers has extended the use of arrays from global seismicity monitoring to regional and local seismicity studies and even portable arrays are now used. The large global arrays like LASA in Montana or NORSAR in Norway have been described in detail elsewhere (Green et al., 1965; Capon, 1970; Bungum et al., 1971; Bungum & Husebye, 1974). In this chapter, we will mostly deal with the small or portable arrays for local and regional studies.

The seismic arrays or antennas are a powerful tool, not only for improving the signal-to-noise ratio and detecting distant events otherwise masked by the background noise (e.g Frankel, 1994), but also for a number of studies on sources and wave propagation near the seismic source (Gupta et al., 1990; Iida et al, 1990; Niazi & Bozorgnia, 1991), site effects and near receiver wave propagation (Dainty & Toksoz, 1990; Al-Shukri el al., 1995; Aoi et al., 1997; Barker et al., 1997; Bodin et al., 1997), etc. Another application field is represented by the array surveys done in recent years in volcanic areas, where conventional networks are difficult to use. These studies have yielded insight into the nature of seismo-volcanic sources, crack models, volcano structure, etc (e.g., Chouet et al., 1998; Del Pezzo et al., 1997). The analysis techniques associated with arrays allow the location of seismic phases with onsets not well defined or the tracking of sources of almost-continuous signals such as a volcanic tremor (e.g. Almendros et al, 1997). This is not possible with conventional network methods.