Very high resolution long-baseline water-tube tiltmeter to record small signals from Earth free oscillations up to secular tilts

Abstract

A 43m long floatless water-tube tiltmeter has been in operation since 1997 at the Walferdange Underground Laboratory for Geodynamics in the Grand Duchy of Luxembourg. The absence of moving parts makes this instrument particularly simple but does not prevent it from measuring some very small geophysical signals such as the Earth tides with a very favorable signal-to-noise ratio or the successive passages of Love waves circling the globe after major Earthquakes. Its very low noise level and its high resolution up to the long-period seismic band (where for instance the resolution is better than 5×10−12rad) also allows the successful recording of rarely observed grave toroidal and spheroidal free oscillations of the Earth excited by major earthquakes. In the environmental conditions of its installation (in a gypsum mine at 100m depth), the instrument shows a high degree of reliability and a very low drift rate (<0.005microrad∕month). The analytical tilt and horizontal displacement transfer functions computed for this instrument and its sensors can be used to calculate the best geometrical characteristics for the construction of prototypes which should respond to specific requirements for applications in geophysics or geotechnics.