Rotaphone - A Self-Calibrated Mechanical Seismic Sensor for Field Rotation Rate Measurements

Johana Brokesova and Jiri Malek

Rotaphone is a newly developed mechanical sensor system for recording the rotation rate components. It is based on measurements of differential motions between paired sensors (low-frequency geophones) attached to a rigid undeformable) skeleton. The same differential velocity (and, consequently, the same rotation rate component) is obtained from more than one geophone pairs, which allows for in-situ calibration of individual sensors. The calibration method is explained in detail. It is demonstrated on two illustrative examples: a synthetic test and a laboratory test.

The Rotaphone is characterized by a flat frequency characteristic in the wide range from 1 Hz to 100 Hz and sensitivity limit of the order of 10-8 rad/s. Its advantages are small dimensions, portability, easy installation and operation in the field. We present several examples of the vertical rotation rate ground motion recorded by this device in Western Bohemia earthquake swarm area. Sources of the recorded rotation rates are both natural (microearthquakes) and antropogenic (blasts). Under the assumption of a plane wave propagating along the surface, selected records are compared to the corresponding transverse accelerograms. The limits of such approach in the area of interest are discussed.