When a microearthquake occurs as a result of industrial activity such as mining or hydraulic fracturing, we say that a microseismic event has occurred. The exact location where either a new rock fracture occurred, or an existing fracture was activated, is referred to as the event location or source of the event.
The time it takes the P and S waves to travel the distance from the event source to the sensor depends on the velocity of the medium through which they are propagating as well as the distance (t=d/v). Using a dipole sonic log, perforation shot arrival times or other available information, ESG estimates the speed at which the energy released by a microseismic event radiates through different layers of rock and creates a velocity model for the region.
In essence, the event location is determined by:
- The distance from the event to the sensor: determined from the P and S picks.
- The orientation (visualized by azimuth and dip of the line connecting the event to the sensor): determined from hodogram analysis.
In a sensor array, the separation between sensors is known. The difference in arrival times of the P (and S) waves at each sensor in an array is called moveout. The moveout provides some information about the distance of an event from the sensor array. Geophysicists can also use the fact that P-waves travel faster than S-waves to help determine the event location. Sensors which are located far from the source will have a wider separation between P and S wave arrivals than sensors which are close to the source.
From this simplified perspective, calculating the distance to the event location enables the definition of the radius of a sphere on which the source resides. To further refine the location of this event, its orientation (azimuth and dip) is required. Orientation information can be extracted from a hodogram analysis of the signal.
Event Location by ESG
At ESG, event location is performed by powerful, in-house developed software, with trained geophysicists on-hand to adjust and assign the location along seismograms which represent the arrival of the P-wave and S-wave. In order to be able to calculate the location of an event, it is important to:
- Monitor the site with an adequate number of sensors and good coverage of the area
- Know the exact location and orientation of all monitoring sensors
- Ensure that all data is time synchronized
- Be confident of accurate arrival time picking
- Have an accurate velocity model