British seismometer array recording systems

The requirements for a seismic recording system are stated for sites with a noise level of the order of 1 m¿m displacement at a frequency of c/s. Two methods developed for the transmission of the electrical outputs from seismometers distributed over a large array to a recording centre in the field are described. The first method involves direct transmission of the amplified signals, and the second uses amplitude modulated tones. The methods used for remotely calibrating the seismometers and associated field amplifiers are described. Problems of lightning and animal damage to cables are discussed. An outline of the instrumentation used in the recording centre is given. The signals are recorded on a primary 24-track magnetic tape. The system also includes a secondary tape deck, and an eight-channel pen recorder, controlled by an on-line correlator. Details are given of the timing system, including the pulse code adopted, and the method of measuring and correcting errors relative to G.M.T.