Preface to the Focus Section on Nonexplosive Source Monitoring and Imaging

Seismic waves generated by a natural or an artificial source provide a direct means to probe the Earth’s interior, and seismic tomography and migration are techniques developed for imaging Earth’s 3D velocity structure and internal boundaries (e.g., Gray, 2001; Rawlinson et al., 2010). The resolution of seismic imaging depends mainly on the wavelength of seismic waves. In general, seismic waves generated by natural sources, such as earthquakes, feature larger amplitude and longer wavelength as compared with those produced by artificial sources. Hence, passive‐source seismic waves can penetrate deeper into the Earth and have consequently served as the...