Patent ID: 8659424

Claim:
A method of detecting subsurface activity, the method comprising: receiving vibrations with a first sensor positioned at a first depth relative to a surface of the earth to generate a first signal; receiving vibrations with a second sensor positioned at a second depth relative to the surface of the earth to generate a second signal, the second depth being greater than the first depth; detecting an event based on a comparison of features of the first signal and the second signal during corresponding time windows, the features comprising one or more of relative amplitude of the first and second signals, relative frequency content of the first and second signals, relative time of values above respective thresholds in the first and second signals, and relative duration of values of the first and second signals above the respective thresholds; and providing an output to indicate detection of the event, wherein detecting the event further comprises determining that, during the first time window, the first signal has frequency components in the range less than the primary frequency components of the second signal during the first time window by at least a threshold percentage.