Although stand‐alone geophones have been used for decades within the active‐source seismic community (eg, Mooney and Brocher, 1987), recent technological advances in geophone instrumentation have made it possible to use them for a wide range of passive seismic studies (eg, Lin et al., 2013; Schmandt and Clayton, 2013). Compact, all‐in‐one seismic systems including a geophone, digitizer, and battery—often called nodes—are lightweight and easy to deploy, allowing large numbers of instruments (“large‐N⁠,” typically 100s or 1000s of nodes) to be used on a single project. These stand‐alone seismic systems, especially the RefTek 125A Texan seismometer with a 4.5‐Hz geophone, have been used since the 1990s within the academic seismology community, primarily for active‐source studies (Harder and Keller, 2000) but also for passive‐source studies (Byerly et al., 2010; Quiros et al., 2015, 2017; Sun et al., 2015; Wu et al., 2016; Beskardes et al., 2018). The oil industry has also used a wide variety of these seismic geophone systems for decades (eg, Dean et al., 2018). In this focus section, we use the term “geophone array” to refer to an array of compact, stand‐alone seismic stations typically using one‐component or three‐component geophone sensors. We note that this is distinct from the historic definition of a “geophone array,” which was a geometrical arrangement of geophones with signals recorded by a single channel.

Recent advances include longer battery lives up to one month and three‐component (3C) 5‐Hz geophones, which make the instruments more suitable for studies of lower frequency sources (including earthquakes) compared with traditional oil industry sources. These instruments can be buried for use in urban environments (eg, Lin et al., 2013), carried in a backpack for deployment in difficult‐to‐access areas (eg, Hansen and Schmandt, 2015), deployed in extremely cold environments (eg, Yeguas et al., 2011), deployed in hot environments (eg, Sweet et al., 2018), and deployed quickly for time‐sensitive aftershock and environmental studies (eg, Quiros et al., 2015). Large‐N studies have also allowed seismologists to record the seismic wavefield with a denser spatial sampling than ever before. As a result, there has been significant recent growth in the number of geophone array field studies carried out by academic communities and corresponding methodological and computational advances to analyze the recorded waveforms in innovative ways and process these new, often large data volumes.