Automatic Analysis of Tape-Recordings from Seismic Networks



An Analogue-to-Digital/Digital-to-Analogue system has been set-up with a PDP-15 computer to digitize and analyse analogue magnetic tapes from seismic networks for local and teleseismic events. The system operates at eight or sixteen times recording speed, so that a 2-day tape with up to 15 seismic channels, can be analysed in an unattended 6-hr computer run, and a similar tape with up to nine channels, can be analysed in a 3-hr run. Each channel is processed by three triggering mechanisms having different frequency and amplitude characteristics. An event is recognized when there are more than a specified number of channel-triggers within an appropriate time window. The relative delays and the unprocessed channels spanning the event are written to a digital library tape. When the replay deck reaches the end of the analogue tape, the program rewinds the library tape, reads the delays, and calculates the location of the event. The location is the co-ordinate position in degrees if the event is local to the network, and the apparent-velocity and azimuth if it is regional or teleseismic. The library tape of the events can be played through a digital-to-analogue converter onto the same paper recorder as the direct replay system and comparable seismic records are written. If required, the library tape can be used as input for more sophisticated further processing. The simplicity of the triggering criteria allows the selection of events to be performed in an on-line mode at several times recording speed, and the detailed analysis for arrival times can be made without the timing pressure of an on-line system.