Rotaphone-D, A New Model of Six-Degree-of-Freedom Seismic Sensor: Description and Performance

Abstract

A new model of the Rotaphone-D short-period seismic sensor with six degrees of free-dom is introduced. The basis of the instrument is horizontal and vertical geophones in aspecial paired arrangement. The instrument is designed for simultaneous and collocatedmeasurements of rotational and translational components of ground motion. The basicprinciple of measurement is briefly described. Careful calibration is necessary for thistype of measurement, which has two parts: a preliminary calibration based on labora-tory measurements of the characteristics of the individual geophones, and a sub-sequent in situ calibration, which takes into account the actual physical conditionsduring field measurements and is performed during the processing of the measureddata. The effect of the calibration is demonstrated by specific laboratory tests. The lab-oratory tests have confirmed the correct functioning of the instrument. The frequencyrange of Rotaphone-D is from 2 to 80 Hz and is therefore optimal for monitoring localseismicity. The instrument was subsequently used in field measurements during twoseveral-month measurement campaigns in California, United States. Examples of six-component records from two sites-The Geysers and Long Valley Caldera-are pre-sented and briefly interpreted from a seismological perspective. Basic instrumentparameters are given. The instrument is lightweight, portable, easy to install, and offersseveral interesting applications, which are briefly discussed. Six-component measure-ments (translational and rotational) have become a powerful tool, especially in volcanicseismology and in monitoring induced seismicity.