Digital Experiments at Twice Real‐Time Speed on the Capabilities of the Yellowknife Seismic Array

Summary A number of experimental seismic arrays have been constructed in the past few years. One such array in the form of an asymmetric linear cross has been built at Yellowknife, NWT, by the United Kingdom Atomic Energy Authority in collaboration with the Canadian Department of Mines and Technical Surveys. Data from the 19 individual seismometers are recorded continuously on FM magnetic tape. The analogue tape data are multiplexed into a digital computer at twice the recording speed. The system allows the formation of 168 beams by the delay and sum method: different approaches to this problem and their implications for real time processing are discussed. The correlations between the phased sums of the two lines are calculated and events are detected automatically when the correlation rises above a trigger level for a preset length of time. For each event a selection of logarithmic correlations is output in analogue form, together with other pertinent information. The recorded data have been analysed in two modes: free search, in which the entire tape is searched by the computer; and selective search fine scan, in which events are selected visually from a helicorder record and subjected to a variety of search procedures. A 30‐day free search experiment indicated that the 50% detection probability level is m 4·1 ± 0·2. The location accuracy of the events detected during the 30‐day experiment is of the order of 300 km in latitude and longitude and fine scan experiments have shown that this accuracy may be improved. The signal/noise ratio improvement is close to the theoretical value.