A Playback System for Portable Seismic Recorders

Seismic data acquired by the Bureau of Mineral Resources (B.M.R.) for crustal studies are recorded on magnetic tape as frequency modulated (FM) signals accompanied by an amplitude modulated (AM) serial time code. A playback system is described which permits accurate recovery of seismic signal amplitudes from FM tape recordings: an advantage over AM systems which, because of the effects of many unknowns such as tape quality, require continuous calibration for absolute amplitude recovery. By using the recorded time-code carrier frequency to sense tape speed, the playback system corrects for speed changes in the original recording and compensates for variations in its own speed characteristics. The novel speed control system, combining both frequency and phase control, ensures accurate tracking, rapid response, and a wide acquisition range. An adaptable detection threshold ensures that the recovered and decoded time signal is sufficiently reliable to be used for computer controlled event searching. The data processing task is further aided by overall computer control of the playback system, and programmable digitizing modes.