100 years Michelson-Gale’s interferometric water tube tilt meter experiment in USA and 50 years instrument development in Finland

One hundred years ago Michelson and Gale (M-G) (1919) made a famous experiment using 150 m long water tube tilt meters (NS,EW) buried underground at 1.8 m depth at Yerkes observatory, in Wisconsin, USA. The interferometric level sensing principle was simple and tilt recordings are comparable or even better than present tilt recordings.  M-G presented the rigidity of the earth’s internal structure to be of the same level as steel.  They showed a deviation in the semidiurnal M2 wave amplitude factor between the EW and NS directions. Ocean loading was   already then suspected to be the source for the deviation.

50 years ago Kukkamäki (1965) - stimulated by the M-G experiment- proposed a 1 km long fluid level tilt meter to be built in a suitable place of the Fennoscandian postglacial rebound area for continuously observing rebound phenomenon.  The purpose was to determine the characteristics of the postglacial rebound effect. This instrument was never built.

Kääriäinen (1979) designed and built a 177 meter long M-G instrument in EW-direction modified by Kukkamäki’s innovation on interferometry in a tunnel of the Lohja mine, southern Finland. Later on in 1983 a similar 62 m long instrument was   built in the same mine gallery in NS-direction. Both instruments showed high sub-nanoradian recording resolution and good tidal parameters were obtained.  Preliminary ocean loading corrections were done.  Most of the observed non-tidal residuals were correlated with airpressure gradients. The main new finding was the core-mantle resonance observations in both directions (Kääriäinen and Ruotsalainen, 1989 .

Ruotsalainen (2001) started with the modernization of the M-G FGI type tilt meters.   Many details followed the innovations of the former instruments of the FGI.  Special stainless steel is used in both tube and pot construction.  A Fizeau-Kukkamäki laser interferometer with fiber-optics is used for level sensing at both end pots of the tube.  Interferograms are recorded by a CMOS camera and transferred via a fiber-optic extension bus to a remote recording computer for on-line interpretation with 15 Hz sampling rate.  A broad band of geophysical phenomena has been recorded since 2008 when the 50 m long north-south instrument became operational in the Lohja mine, southern Finland. Observed and analyzed phenomena are e.g. core-mantle resonance in the earth tides spectrum, ocean loading, Baltic Sea loading, atmospheric loading, free oscillations of the earth after great earthquakes, loading of the Baltic Sea seiches, microseism and secular tilt (Ruotsalainen et al., 2015).

Instrument is very suitable for different kinds of geophysical studies with absolute scale.

References:

Kukkamäki T.,J.,(1965): Recording of the secular land tilting with pipe level. In Proc. of the 2^nd Int. Conf. on Recent Crustal Movements, Acad. Sci. Fenn. A II, No. 90, p. 209.

Kääriäinen J.,(1979): Observing the Earth Tides with a long water tube tilt meter, Annales Acad. Sci. Fenn. A VI Physica No 424

Kääriäinen J. and H.Ruotsalainen,(1989): Tilt measurements in the underground laboratory Lohja 2, Finland in 1977-1988, Publ. of the Finnish Geodetic Institute no. 110, Helsinki.

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

Ruotsalainen,2001): Modernizing the Finnish long Water-Tube Tilt meter, Journal of the Geodetic Society of Japan, Vol. 47, No 1, pp. 28-33.

Ruotsalainen, H. , Nordman, M. , Virtanen, J. ,Virtanen, H. (2015): Ocean tide, Baltic Sea and atmospheric loading model tilt comparisons with interferometric geodynamic tilt observation - case study at Lohja2 geodynamic station, southern Finland,  Journal of Geodetic Science. Volume 5, Issue 1, ISSN (Online) 2081-9943, DOI: 10.1515/jogs-2015-0015, December 2015.