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An absolute method of vertical seismometer calibration by reference to a falling mass with application to the measurement of the gain

Institute of Geophysics and Planetary Physics Scripps Institution of Oceanography University of California, San Diego, La Jolla, California, 92093-0225

Abstract

We measure the gain of a vertical seismometer by simultaneously recording the output of the seismometer and repeatedly measuring the displacement between the seismometer and a free-falling mass in a vacuum. The falling object provides an inertial reference frame. By comparing the ground motion measured by the seismometer with the independent record of displacement between the seismometer and inertial space, we obtain the gain. It is an absolute measurement of the gain relative to the local Lorentz reference frame. Bootstrap error estimates show that a high precision in the estimate of the gain can be obtained with a small number of individual drops. The method derived can be extended to multi-parameter searches of the vertical response function. The technique is also shown to reduce noise in absolute gravity measurements due to ground noise. Finally, we discuss the potential for replacing vibration isolation schemes in absolute gravity systems with digital noise reduction.