Using the Right Seismic Equipment

With an increased public concern about induced seismicity, operators are particularly aware of the need to accurately monitor this large magnitude seismicity and easily implement systems to mitigate any associated risk.

Seismic instrumentation is inherently band-limited; this finite bandwidth may introduce bias in microseismic source measurements that will ultimately affect how the results are interpreted. Broad range or Hybrid™ monitoring of induced (micro)seismicity is a key technology that can help operators understand the relationship between injections and their interaction with fractures of different scales.  By providing operators with accurate knowledge of the size and location of larger magnitude events, the industry can move towards establishing a system of evaluating induced seismicity while demonstrating responsible operation and environmental compliance.

What is Magnitude Saturation?Surface Microseismic Hybrid Equipment

Current microseismic technology is generally tuned to identifying the small magnitude seismicity generated during hydraulic fracture stimulations (M < 0).  The most commonly used microseismic sensors are 15 Hz geophones deployed down monitoring wells at reservoir depth. 

Microseismic events occur on relatively small fractures, resulting in relatively high frequency signals propagating through the earth.  In contrast, large magnitude events (M > 0) exhibit much longer waveforms, travelling at significantly lower frequencies. 

When a sensor that is tuned to capture higher-frequency signals registers a larger-magnitude event with lower-frequency energy, it will only record the portion of the signal that falls within the sensor’s frequency bandwidth.  Because the lower-frequency portion of the signal will be truncated, the magnitudes determined from this sensor will be under-estimated. This phenomenon is known as Magnitude Saturation.

In order to accurately capture the low frequency signal content associated with larger-magnitude events, the monitoring array consisting of 15Hz geophones needs to be augmented with lower-frequency sensors such as force balanced accelerometers (FBAs) and low frequency 2 or 4.5 Hz geophones in what we call a “hybrid” configuration, to provide the appropriate bandwidth coverage. These lower frequency sensors are generally used to increase the detection range of events with M>0 and are typically installed close to or at the surface.

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