Active-Source Environmental Seismology

Abstract Environmental seismology is a rapidly growing field that is providing new insights into many surficial and near-surface Earth processes, largely through passive detection of events (e.g., landslides and sediment movement) and subsurface velocity changes that reflect changes in saturation (e.g., water table changes). Recent advances in instrumentation (e.g., distributed acoustic sensing) and analysis techniques (e.g., machine learning and artificial intelligence) are expanding the use cases and data volumes of passive environmental seismology. However, despite these successes, passive environmental seismology has two disadvantages that may limit the usefulness of the data. First, passive-source studies that rely on surface-wave observations usually produce S-wave velocity without accompanying P-wave velocity measurements, thus incompletely characterizing subsurface elastic properties. Second, passive-source studies often do not provide subsurface structure at the resolution necessary to distinguish the near-surface units that host and control environmental processes (e.g., soil, saprolite, and fractured bedrock). We suggest that these limitations can be surmounted by incorporating active-source seismic imaging of the critical zone (CZ). Active-source data records the entire wavefield, thus providing detailed P- and S-velocity structure that can contextualize passive-source measurements. Active-source CZ seismology is a rapidly changing field, with notable recent advances in instrumentation (e.g., small, autonomous nodes) and analysis techniques (e.g., wider adoption of full-waveform inversion). As a result, new types of surveys (e.g., three-component, 3D, and time-lapse) are imaging the CZ in novel ways and with unprecedented resolution. Here, we review the state of the art in active-source CZ seismology and provide several examples demonstrating that active-source environmental seismology can achieve meter-scale resolution of CZ structure and processes.