Abstract: A system to detect subsurface activity. The system employs vibration sensor pairs, with each sensor pair having a shallow sensor and a deep sensor. Outputs of the sensors of a pair are processed together and events are detected based on the relative values detected by the sensors of the pair. When signal energy departs from a detected background level, the relative amplitude and frequency content of vibrations measured at the shallow and deep sensors may be compared. The comparison may be performed, at least in part, using a classifier that discriminates between subsurface activity and surface activity. The outputs of sensor pairs may be aggregated to make a determination of whether subsurface activity exists and/or its location. Aggregation may involve comparing the outputs of the same sensor pair at multiple time intervals or may involve comparing the outputs of arrayed sensor pairs.