Seismometer Arrays

Abstract

In seismology, the term “array” (of seismometers) has been known for more than 50 years. Today this phrase is often used for any group of seismic stations in the sense as defined in Macmillan’s online dictionary, where an array is described as “a number of pieces of equipment of the same type, connected together to do a particular job” (Macmillan 2013). In this chapter, the term “seismometer arrays” is used with a more restricted definition, as it was originally introduced to seismology. A seismometer array can be defined as (Schweitzer et al. 2012): “A seismic array is a set of seismometers deployed so that characteristics of the seismic wavefield at a specified reference point, within or close to the array, can be inferred by analyzing the waveforms recorded at the different sites. A seismic array differs from a local network of seismic stations mainly by the techniques used for data analysis. Thus, in principle, a network of seismic stations can be used as an array, and data from an array can be analyzed as data from a network. The size of an array is defined by its aperture, which is the largest horizontal distance between two sensors of the array. In practice, the geometry and the number of seismometer sites of an array are determined by the intended scientific purpose and economic limits.” The main purpose to install a seismic array is threefold. At first, an array can be steered as an antenna to amplify the signals of interest by stacking (summing) the seismic wave energy recorded at the different array sites after applying appropriate phase delays. With these so-called “beamforming” techniques, arrays show superior signal detection capabilities with respect to single 3-component (3C) seismic stations. The second main purpose to install a seismic array is the capability to estimate the station-to-seismic source azimuth (backazimuth, BAZ) and the apparent velocity of unknown seismic signals crossing the array. Thirdly, a single seismic array often provides enough information about the incoming seismic signals that an automatic algorithm can be used to estimate a first (preliminary) solution of the seismic source location.