Technical details of the modern Michelson-Gale type interferometric fluid level tilt meter of the Finnish Geospatial Research Institute, NLS, Finland

Modern fluid level tilt meter construction has many design parameters, which have to be taken carefully into account before the instrument can fulfill the highest demands of geodynamics. A limit is e.g. reliable observation of the core-mantle resonance phenomenon, which amplitude is a fraction of the theoretical tilt amplitude ~0.1 nanorad of the tidal wave group psi.

Michelson-Gale (M-G) (1919) interferometric water level tilt meter contained already 100 years ago some technical details, which have to be taken into account in subnanoradian tilt level observation today.  Agnew (1986) has given a theoretical background on M-G instrument design in the study of other kind of  tilt meters and a strainmeters.  Most important is the station temperature dependence of the tube-pot design and fluid combination.  Beavan and Bilham (1977) have considered it in their article.

Kääriäinen (1979) has given many important details in his study at the Finnish Geodetic Institute (FGI) for the M-G FGI type tilt meter design. They are for example: instrument dimensions and fluid combined with thermal expansion modelling, hydrodynamic conditions of fluid flow in the tube-pot system, interferometric tilt scale determination, wavelength of light, refraction coefficient of fluid, and the azimuth of the instrument.

Our new modernized M-G FGI type interferometric fluid level tilt meter has been developed further since the year 2000 following some of the design details given above (Ruotsalainen, 2001).

 A detailed description of technical parameters of the third generation M-G FGI type tilt meter is given. Instrument has operated in the Lohja mine since 2008. Among others we will address the newest pot-tube  and level interferometer design for instrument  installed in the Conrad observatory, Austria (Ruotsalainen et al. 2015).

References:

Agnew D. C.,(1986): Strain meters and Tilt meters, Rev. Geophys., Vol 24, no. 3, pp.579-624.

Beavan and Bilham,(1977): Thermally induced errors in fluid tube tilt meters, J. Geophys.

Res., 82, 5699-5704.

Kääriäinen J.,(1979):  Observing the Earth Tides with a long water tube tilt meter, Ann. Acad.

Sci. Fenn. A VI Physica No 424.

Michelson A.A. and H. Gale,(1919): The rigidity of the Earth, Astrophys. J., 50, 330-345.

Ruotsalainen H., (2001): Modernizing the Finnish long Water-Tube Tilt meter, J. Geod. Soc. of Japan, Vol. 47, No 1, pp. 28-33

Ruotsalainen H. , D. Ban, G. Papp, R. Leonhardt, E. Szücs, J. Benedeck,, Beyond nanoradians? High sensitivity tilt measurements in the Conrad observatory, Austria, Conference abstract in session  G04 Earth Rotation and Geodynamics of the IUGG  General Assembly 22.6. - 2.7.2015 , Prague, Czech  Republic.