Event Detection & Triggering

When the microseismic waves (P-waves and S-waves) caused by microseismic events reach nearby installed receivers, the recorded signals register an increase in amplitude from background levels.  Depending on the signal quality, it may be easy, or difficult to see the peaks in signal amplitude associated with the P- and S-wave arrivals.  When a true microseismic event is detected, it is said to be triggered

There are a number of methods used by data acquisition software to detect the true event signals and differentiate them from background noise.  ESG employs two methods of detecting microseismic events: Threshold triggering and STA/LTA ratio triggering.  Both methods demonstrate good performance, and when used in conjunction with filters to remove background noise, can ensure the maximum number of events will be captured.

Threshold TriggeringMicroseismic Event Triggering

Threshold triggering detects microseismic events using a user-defined threshold limit.  The signal amplitude on each channel in a sensor array will be compared with the threshold limit, and those channels which exceed the limit will be recognized as a potential microseismic event.

Threshold triggering can be configured to automatically respond to changing noise conditions.  If noise levels fluctuate, the threshold will still capture microseismic events, rather than falsely triggering on noise signals.

STA/LTA Triggering

The STA/LTA ratio method compares the average energy in a short term average (STA) leading window to that in a long term average (LTA) trailing window.  If the average value captured in the STA is larger than the background levels in the LTA, this will produce an STA/LTA ratio greater than 1.  When the ratio between the two averages are found to be higher than a specified limit, a single channel will trigger. 

As shown in Figure C below, an event is recognized when the signal amplitude associated with a P-wave is captured in the STA, while the LTA remains influenced by the background levels. 

One of the benefits of the STA/LTA method is that high amplitude signals may not trigger if there is no significant amplitude increase, as shown in Figure D below.  In this case, the STA and LTA values will both be high, resulting in an STA/LTA ratio close to 1.

Microseismic STA LTA

Example demonstrating how STA/LTA triggering detects microseismic event triggers (A-C) but does not falsely trigger on noise signals (D)