ESG offers larger magnitude passive seismic monitoring options for sites that are interested in detecting strong-ground-motion (SGM), or regional seismicity. This level of seismicity typically occurs in the range of magnitude 0 to +3 and is larger than typical microseismic events, but smaller than regional earthquakes.
SGM stations can supplement an existing microseismic array or can be installed independently to monitor seismic events that fall below the range of events detected by larger regional seismic arrays. The SGM stations can more accurately locate and provide more reliable source parameters (such as event magnitude) for these mid-range seismic events and are well-suited for geothermal or dam monitoring applications where induced seismicity may be of concern. They are also used to supplement microseismic arrays installed on mines to more accurately locate larger events that may occur outside of the main microseismic array.
If the operations are situated near residential areas, large magnitude event detection is typically required to comply with local regulations related to induced seismicity. In the event of a larger magnitude event that may be felt in the local region, operators need to quickly determine the location of the event so they can inspect the area and investigate the cause of the event. It is often important that this information be provided to municipal agencies and/or the public in a timely fashion, If blasting vibration levels are of concern, the SGM stations can provide data to determine these vibration levels at various distances to allow operators to determine if they are operating within allowable regulatory levels.
For mining operations, it is typical to have at least one SGM monitoring station away from the active mining zone. Ideally, multiple SGM sensors are deployed in a triangular array for optimal detection. The SGM stations can be fully integrated and time synchronized with existing microseismic systems in underground or open-pit mines.
In contrast to small scale microseismic events, which are detected using high frequency geophones or accelerometers, larger magnitude seismic events generate low frequency signals and must be detected with sensors tuned for this signal range.
ESG manufactures 4.5 Hz SGM geophones for deployment at surface locations. Using ESG's patented Hybrid™ technology, these SGM stations provide reliable estimates of source parameters for larger events, including moment magnitude.
Depending on the purpose of monitoring ESG can also design systems with other low-frequency sensors such as 2 or 8 Hz geophones or Force balance Accelerometers (FBA).
In some cases, SGM sensors may also be deployed underground.
ESG offers a number of options for transferring seismic data from SGM stations to a central processing center.
The SGM stations can provide real time data transfer through Ethernet or radios. Alternately, in more remote locations, the data can be written to a swappable external drive such as a USB stick.
If power is not available in the area, SGM stations may be battery operated. Alternately, solar panels may be installed to provide power to the SGM station.