A feasibility study for a portable long period seismograph

As a preliminary study to the operation of a portable long period seismic system, the effects of environment on the phase response of the Sprengnether 201 and Geotech SL-210 long period vertical seismometers are investigated. It is found that changes in the phase response of the seismometers are related to the zero instability of the moving mass. Pressure changes are not important in this regard; however, temperature variations should be held to less than 1°C. Calculation of the phase response directly from the amplitude response is investigated. It is concluded that it is more practical to carry out a phase calibration directly. The technique may however be useful when only an amplitude calibration is known. Also investigated are the noise properties of two amplifiers: the solid state Geotech AS-330 and the phototube Geotech 12613 amplifiers. The fundamental noise of the seismometer-amplifier combination is negligible. The excess noise of the AS-330 amplifier is of the same order as seismic noise at long periods at a rather noisy site.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat