Seismology

Description

The seismology experiment was designed to determine the level of seismic activity on Mars and its internal structure. The seismology instrument contained three mutually perpendicular seismometers. Each seismometer consisted of a moving coil and a fixed magnet. The operating modes were (1) selection of various filters for frequency content or to adjust to best reception of specific types of data, (2) a low sampling rate for general activity, (3) a high data rate for detailed examination of events, (4) and a compressed medium rate for continuous monitoring of Marsquakes that were dormant until activated by an event. The data were compressed for transmission to Earth by averaging the amplitude of normal ground noise over a 15-s period. When an event occurred, a trigger activated a higher data rate mode that sampled the amplitude of the overall event envelope, which required only one amplitude sample per second to indicate its shape. At the same time, the change in polarity of the data signal (caused by crossing the zero axis) was sampled once each second. The shape of the envelope and its incremental frequency content was transmitted to Earth and reconstructed to approximate the original event. The Viking 1 seismometer failed to uncage and could not be used in a seismic network with the Viking 2 instrument.