Separating weak surface microseismic events from noise

Microseismic events that are detected at the surface are often very weak, with moment magnitudes as low as -2. Such weak events present a difficult analysis problem to ensure that they are ‘real’ events that originate from microseismic activity near the reservoir zone. There are many sources of noise that can give the appearance of genuine microseismic events, including reflected noise from the frac operation, reflected noise from vehicles, and spatially isolated but persistent noise that causes interference patterns that may be erroneously interpreted as evidence of geologic anomalies. Signal-to-noise enhancement methods can help to discriminate between valid events and noise propagating near the surface. Reflected noise from surface sources may have a similar apparent velocity and depth as valid events but can be recognized by its pulsating nature. Source mechanisms provide information about the fractures, but also give us a tool to help determine whether weak events are real or the result of noise sources. We present a classification scheme that incorporates these methods and observations.