Hybrid™ Monitoring to Solve the Issue of Magnitude Saturation

Hydraulic fracturing and cyclic steaming are capable of producing thousands of microseismic events ranging in magnitude from Mw -4 to Mw -1.  Due to a phenomenon known as magnitude saturation, where the reported event magnitudes are generally underestimated relative to the actual event magnitudes, the magnitudes of the events can be inaccurately characterized as lower than the true magnitude of the event.  On average, ESG has found that true event magnitudes for larger events end up being one order of magnitude higher than originally characterized using a standard high frequency microseismic system.  Seismic events generate different frequencies depending on their magnitude.  High frequency events are events are smaller in magnitude and low frequency events are larger in magnitude.  The type of sensor deployed will determine the range of frequencies and therefore magnitudes the microseismic system can detect and accurately characterize.  Current downhole monitoring systems generally use higher frequency sensors such as 15 Hz geophones which limits the frequencies the system can detect.Microseismic monitoring ranges for seismic systems

Hybrid™ Monitoring System

By using various types of sensors that can detect events with different frequencies, including higher magnitude events that are often mischaracterized as being lower in magnitude, ESG’s patented Hybrid™ Downhole-Surface Seismic Acquisition System detects a wider range of frequencies ensuring the most accurate results possible.  Microseismic magnitude saturation

To detect a wider range of magnitudes and to reduce the effects of magnitude saturation, ESG deploys both 15 Hz geophone arrays downhole and 4.5 Hz geophones and force-balanced accelerometers (FBAs) at or near surface.  Typical downhole monitoring arrays utilize 15 Hz geophones with flat responses in the expected frequency ranges for events between Mw -3 and Mw -1. However, larger events will have corner frequencies that can be well below 15 Hz resulting in an underestimation of the observed magnitudes.  To accurately characterize the magnitude of these larger events ESG’s Hybrid™ system also utilizes sensors capable of detecting higher magnitude (> Mw -1), lower frequency events.

Deploying 4.5 Hz geophones and force-balanced accelerometers near-surface in addition to deploying 15 Hz geophones down-hole allows ESG to detect more events due to the a broader range of frequencies that are detectable and to characterize  the source parameters including seismic moment (magnitude) with more accuracy.  The combination of near-surface and down-hole sensors provides effective monitoring of seismicity ranging in magnitude from Mw -3 to Mw +3 and minimizes the magnitude saturation effect to provide the most accurate description of deformation occurring due to injection/production. 

Design and Deployment

The Hybrid™ method deploys 15 Hz downhole sensors in addition to 4.5 Hz sensors on surface.  All data captured on the downhole and surface arrays are 100% time synchronized and combined into one master event database for processing.  All events are processed using a first-arrival method and larger magnitude events detected by the near-surface array specifically processed using the near-surface data to ensure that source parameters are accurately characterized.  Surface sensors are deployed easily and efficiently with minimal surface footprint into shallow pits or augured holes, and are typically in place for periods of a few days to a few weeks. Data may be collected manually by trained field crews at periodic intervals, or may be streamed wirelessly to a central location where it is processed and analyzed.

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