Regional data processing at NORSAR

• Automatic single array processing, using a suite of bandpass filters in parallel and a beam deployment that covers both P and S type phases for the region of interest.
• An STA/LTA detector applied independently to each beam, with broadband f-k analysis for each detected phase in order to estimate azimuth and phase velocity.
• Single-array phase association for initial location of seismic events, and also for the purpose of chaining together phases belonging to the same event, so as to provide an initial one-array location estimate and prepare for the subsequent multiarray processing.
• Multi-array event detection, using the Generalized Beamforming (GBF) approach to associate phases from all stations in the regional network and thereby provide automatic network locations for events i all of northern Europe. The resulting automatic event list is made available on www.norsardata.no.
• Interactive analysis of selected events, resulting in a reviewed regional seismic bulletin, which includes hypocentral information, magnitudes and selected waveform plots. This reviewed bulletin is also available on www.norsardata.no.  

We note that many of these processing steps (such as single-array event location, broadband f-k analysis and multi-array event detection (GBF – see figure below)) have been developed and tested at NORSAR, and are documented in numerous scientific publications. Today, these methods are in standard use in the international seismological community.

Experience over the past several years has demonstrated that the automated event list generated by the GBF procedure is nearly complete, in the sense that it provides an exhaustive search of all possible detected phase combinations that could correspond to real events. The reviewed bulletin is more selective, since our current resources do not allow a complete analysis of all real seismic events that are associated through the automatic algorithms. An important topic of current research is to develop methods to enable the analyst to easily select events from areas of particular interest, and focus on these events in the interactive analysis. Some recently developed techniques will be described in the section on Mining explosions.

Illustration of the GBF procedure in use at the NORSAR Data Center.

The initial grid for trial epicentres is marked as red dots, whereas the denser beampacking grid, in this case constructed around an initial event location in the Kara Sea, is shown in blue. This dense grid is used to refine locations provided by the initial grid. The distance between the dense grid nodes is 0.2 degrees.