Preface to the Special Issue on ‘‘Arrays and Array Methods in Global Seismology’’

The increased availability and fidelity of broadband seismic instruments have played a key role in bridging the gap between exploration and global seismic applications. Long gone are the days when global structure mapping and source characterization/monitoring relied on the manual digitization of WWSSN (World-Wide Standard Seismographic Network) analogue records or hours in front of microfiche machines. Even the GSN (Global Seismographic Network), a milestone of the digital era, now pales in comparison with the scope and density of regional seismic arrays that dominate the global data stream. Today, the nominal resolution estimated from the ratio between wavelength (approximately tens to hundreds of kilometers) and target dimension (often of continental scale) in properly designed global experiments could rival the typical resolution of a controlled source seismic survey. Vastly improved data coverage, highlighted by multi-disciplinary initiatives such as the USArray project, has overcome many conceptual (as well as practical) barriers between exploration and global seismic applications. A direct consequence is that the ‘‘global community’’ can now take full advantage of important array methods that are predicated upon superior data density and distribution. In fact, it could legitimately be argued that exploration seismology is fast becoming a realistic, scaled-down model for global seismic surveys. This Special Issue reviews the assumptions, algorithms and prospects of several important methodologies in today’s global and regional seismic surveys. A key objective is to highlight developments and novel applications that improve our ability to determine seismic structures across all spatial scales. We mainly emphasize those methodologies and applications that are relevant to structure imaging (note: source characterization may be detailed in a future Special Issue). Apart from the obvious scale differences between exploration (\20 km) and global (typically [ several 100 km) problems, we demonstrate that key objectives such as signal isolation, correlation, reconstruction and noise reduction