Initial field measurements from a two-fluid tiltmeter with thermally compensated piers

Abstract

The measurements of vertical displacement and tilt in active geophysical regions in an important yet difficult task. Several long-baseline fluid tiltmeters are currentlt under development and test. All of these instruments are designed to cope with the thermal errors known to affect fluid tiltmeters in various ways. The field testing of one of these instruments, the 2-fluid tiltmeter, is the subject of this paper. Theoretically, the use of two fluids with different expansion coefficients allows the elimination of temperature errors if the same thermal environment is experienced by both fluids over the complete tiltmeter path. It is an important feature of the 2-fluid design that the connecting fluid paths need not be level. The initial field tests were conducted at site with elevation differences as large as 10 meters. A thermally compensated pier has been designed and tested. The pier, suitable for many geophysical applications, consists of 3 concentric, re-entrant cylinders. By selecting metals with the proper thermal expansion coefficients it is possible to provide a mechanically strong pier with a vertical stability approaching that of fused quartz. The design of the pier, using steel and aluminium, allows the fabrication of a highly stable platform that can be used for many applications.