Fróðskaparrit - 01.01.1975, Side 35

Earth Tides
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Centre of Earth Tides located at Brussels. This Centre was
founded in 1958 under the direction of P. Melchior.
New instruments (quartz horizontal pendulums VM), cali-
bration devices, methods of computation, computer pro-
grams have been developed at the Centre.
Presently Bruxelles Centre is a fundamental world point
for calibration of instruments (tiltmeters and gravimeters).
The new problem arising now is the determination of radial
tidal deformation which is necessary to correct laser di-
stance measurements. This deformation is described by the
first Love number called h. It can not be determined di-
rectly but current earth tide measurements give two simple
arithmetic combinations containing h:
the ratio of observed to theoretical tidal amplitude measured
with a gravimeter:
<5 == 1 + h — 3h k (vertical component)
and the similar ratio measured with a tiltmeter:
y = 1 + k — h (horizontal component)
Thus, for h determination, it should be necessary to install
not only gravimeters but also horizontal pendulums or
vertical pendulutns.
Horizontal pendulums are cheap and of easy maintenance
(once a week) but one needs a 50 meters deep place to
install them. If one can find a mine in the vicinity of laser
stations, it could be done.
Vertical pendulums are very expensive and one has to drill
a borehole 50 meters deep to have a good installation. The
borehole is also expensive but not so much as a special
galery. But the azimut of the instrument must be controlled
at the moment of the installation at such a depth. Until
now nothing has yet been published or available to have
a clear idea of the performances of this instrument (calibra-
tion, phase lag etc....).
From present experience, european stations equipped with