A Discussion on the measurement and interpretation of changes of strain in the Earth - Some instruments and techniques for measurements of tidal tilt

Abstract

The U.S. Geological Survey has operated continuously recording long-base liquid-level tiltmeters for several years at two locations in the San Francisco Bay Area. The meters are capable of resolving 10~8 rad and exhibit long-term drifts of about 1.5 urad/year. Since these instruments require elaborate vaults for their operation, we have developed a simple pendulum tiltmeter of a geometry suitable for operation in a shallow borehole for measurements at similar sensitivities. Techniques developed for emplacement of instruments in boreholes seem satisfactory, but early results indicate that such sites must be selected with great care and attention to local geology. Sample data presented include plots of output from the long-base instruments and of the pendulum.