Geopsy: STA/LTA
The Anti-triggering algorithm STA/LTA (Short Time Average over Long Time Average) has been designed to reject energetic transients from ambient vibration recordings, in view of keeping only (quasi-)stationary windows . The STA/LTA tool, reached from the Waveform menu, is based on the comparison of short term average amplitude (STA) and long term average (LTA) of the signals in the active signal viewer. The anti-triggering algorithm STA/LTA can be used in most of advanced signal processing tools to process the signals only on stationary time windows.
Contents
Use
STA/LTA in the Waveform menu is a tool designed to show the typical values of STA/LTA to decide for appropriate limits in the anti-triggering algorithm. STA/LTA creates new temporary signals in the active viewer. In the advanced signal processing tools, the parameters for the STA/LTA anti-triggering algorithm can be set in the Raw signal sub-tab of the Time tab.
STA/LTA computation
STA/LTA is the ratio between the amplitude, here computed as the L1 norm (average of the absolute values), of the signal on a short time window of length STA and on a long time window of length LTA. At a given point STA/LTA is computed for the time windows preceding the point. For the first signal points, where there are not enough preceding data points to compute the whole LTA and/or STA, the average of the whole signal is used for the missing points.
Anti-triggering algorithm
The anti-triggering algorithm aims at selecting only the signal parts for which the signal amplitude is quasi-stationary, i.e. for which the STA/LTA ratio stays within a limited range of values around 1. The upper bound of STA/LTA aims at avoiding energy bursts, produced for example by pedestrians walking next to a sensor.
This implementation differs from the original implementation that uses the L2 norm instead of L1 norm, that is more appropriate in a theoretical point of view because it represents the energy of the signal and allows a statistical hypothesis test on the stationarity of the signal. This L1 implementation was used in early seismological applications to decrease the computation time. It does not change much the results and benefits from the experience of the community concerning the threshold values that should be used.
Parameters in Geopsy
By clicking on STA/LTA in the Waveform menu, a pop-up window appears with the following parameters:
- STA is the Short Time window length in seconds. The default value is 1 s.
- LTA is the Long Time window length in seconds. The default value is 30 s.
These parameters are enough to compute STA/LTA for each data point and allow to identify the time occurrence and duration of the energetic transients.
In order to automatically select (quasi-)stationary windows, the anti-triggering algorithm available in the advanced signal processing tools requires two additional parameters:
- Min STA/LTA is the lower limit for the STA/LTA ratio. The default value is 0.2.
- Max STA/LTA is the upper limit for the STA/LTA ratio. The default value is 2.5.
In ambient vibration applications, the upper limit is the most important : its value controls the trade-off between stationarity and usable signal length. These values can be adapted to each dataset using the STA/LTA tool in the Waveform menu. However, it is advised not to loose too much time trying to fix these values.
References
- For a review: Withers M., Aster R., Young C., Beiriger J., Harris M., Moore S. and Trujillo, J.. A Comparison of Select Trigger Algorithms for Automated Global Seismic Phase and Event Detection. Bulletin of the Seismological Society of America, Vol. 88, No. 1, pp. 95-106, February 1998.
