Tiltmeters: Rotation, Horizontal Acceleration and Insensitivity to Gravity Waves

Session: Recent Advances in Very Broadband Seismology
Type: Oral
Date: 4/30/2020
Time: 11:45 AM
Room: 215 + 220
Description: 

Tiltmeters use either an equipotential surface or the earth's gravity vector as a reference datum from which time-varying rotations about two orthogonal horizontal axes can be determined to nanoradian precision. Compact tiltmeters (<1 m) typically utilize the gravity vector with noise levels determined by random or thermoelastic rotations close to their point of attachment to the Earth. The signal-to-noise ratio can be increased by increasing the dimensions of the tiltmeter. This is often inconvenient in the vertical dimension but is relatively easy to accomplish horizontally to distances of the order of 1 km. The fundamental period of long tiltmeters increases with the square root of their length. Thermal noise immunity in long fluid tiltmeters is problematical but can be approached using a continuous free-surface.

Because the instantaneous gravity vector is determined by the resolved component of two acceleration vectors, one pointing at the center of mass of the Earth and the other resulting from the resolved component of horizontal accelerations applied to the attachment point, a tiltmeter acts also as an horizontal accelerometer (the source of tilt-acceleration coupling in horizontal pendulum seismometers). At very long periods a tiltmeter is sensitive to changes in gravitational attraction caused by horizontal redistributions of mass and consequent load-induced tilts (ice sheets, oceans and magma movements with durations of decades), at long periods to direct and secondary tidal effects of planetary motions (days to years), and at shorter periods to the complete spectrum of seismic frequencies. A tiltmeter effectively converts horizontal acceleration to rotation, however, unlike an optical gyroscope, a mechanical datum is required to define its rotation point. Since that mechanical datum is subject identical horizontal accelerations in inertial space, a tiltmeter is insensitive to gravitational waves from mass redistributions during great earthquakes, or to astronomical gravity waves.

Presenting Author: Roger Bilham

Authors