A Discussion on the measurement and interpretation of changes of strain in the Earth - An improved servo-controlled tiltmeter system and latest measurements in Sweden

Abstract

Two serious shortcomings in horizontal pendulums used for tilt measurements are nonlinearity in their response to tilt and sensitivities which vary with time. These difficulties may be eliminated if a horizontal pendulum is used as a null indicator. A description is given of a horizontal pendulum apparatus which automatically compensates for ground tilt. The pendulum apparatus is designed so that it may be tilted by varying the pressure in an expansible bearing plate made of stainless steel which is placed at its base. Pressure changes are produced in the bearing plate by varying the height of a column of mercury, a process which can be previously calibrated with great precision. The position of the pendulum beam is sensed by an electro-optical transducer which provides a suitable servo signal to adjust the height of the mercury column when the instrument undergoes tilting. The servo signal can be recorded directly on an ink chart recorder or digitally. First results have been obtained in a test station. It is planned to establish an array of geodynamic stations in Scandinavia incorporating these tiltmeters alongside recording gravity meters in order to study crustal movements by tidal forces, atmospheric pressure effects, ocean loading effects and possibly the process of land uplift in Fennoscandia.