An on-line digital beamforming and detection system has been operating at the Canadian short-period array near Yellowknife since early 1974. The installation followed the renewed recognition of the important contribution this array can make to worldwide detection and location of small seismic events. System design is based on earlier Canadian experience. Daily event detection logs are available in Ottawa via dialed telephone line. Nonlinear beamforming is used to lower the false alarm rate at given detection level, and it is shown to perform as predicted. During the 6 winter and spring months, the detection threshold is mb 4.0 to 4.1 at 50 per cent false alarm rate. During summer and fall, organized surface-wave noise originating on Great Slave Lake raises this level to mb 4.6 to 4.7.

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