Some new techniques for recording and analysing microseisms

Abstract

The design of horizontal and vertical component seismometers for recording ocean-generated microseisms is first described. Both instruments have a reasonably constant response to ground displacements in the 0·05 to 0·5 c/s range. Two three-component stations are in operation using these seismometers. The construction of the vaults and the methods of maintaining stable air conditions inside them are described. The seismometer outputs from the remote station are telemetered to the central recording laboratory by converting each to a frequency-modulated signal and transmitting over a G.P.O. landline. A brief description of the construction and performance of the equipment used is given. The six outputs are monitored on a graphic recorder and also sampled once per second for digital recording on punched paper tape. Some results of the analyses carried out during the winter of 1963/64 are presented. Auto spectra are computed and related to the spectra of sea waves, either deduced from the prevailing weather conditions over the North Atlantic or measured from weather ship wave recordings. The Rayleigh wave constant is evaluated for the two stations and related to local geology. The coherence and phase between components at one station and between stations as functions of frequency are explained with reference to the proportions of Rayleigh and Love waves present in microseisms and to the direction of arrival of the waves.