Rotaphone, a mechanical seismic sensor system for field rotation rate measurements and its in situ calibration

A newly developed mechanical sensor system, called a Rotaphone, for recording rotation rate components is described. The sensor system is based on measurements of the differential motions between paired low-frequency geophones attached to a rigid skeleton. The same differential velocity (and, consequently, the same rotation rate component) is obtained from more than one geophone pair, which allows for in situ calibration and matching of the individual sensors. The calibration method, which is a key point of our methodology, is explained in detail and demonstrated on synthetic examples. The instrument and the calibration technique were also subjected to laboratory testing utilizing a rotational shaking table. Some results of these tests are presented in comparison to the data from a reference sensor (fiber optic gyroscope). The new rotational seismic sensor system is characterized by a flat frequency response over a wide range from 2 to 200 Hz and sensitivity limit of the order of 10 (-aEuro parts per thousand 8) rad/s. Its advantages are small size, portability, and easy installation and operation in the field. We present several examples of the vertical rotation rate ground motion recorded at NovA1/2 Kostel station (Czech Republic): first, a group of records of local micro-earthquakes which occurred in West Bohemia/Vogtland in May 2010 and, second, a record due to an anthropogenic source, a quarry blast at the nearby VintiA (TM) ov quarry recorded in June 2010. The measured rotation rates are of order between 10 (-aEuro parts per thousand 6) for the local earthquakes and 10 (-aEuro parts per thousand 7) rad/s for the blast. These measurements demonstrate that the instrument has much wider application than just prospecting measurements, for which it was originally designed.