ICETInternational Center for Earth Tides

P. MELCHIOR
ICET Director 1958-1995

CONTENTS

  1. The Foundation of ICET, its Initial Program, mission and responsibilities
  2. The accomplishment of ICET tasks up to now
    1. Collection of Data (World Data Centre)
    2. Evaluation of Data
    3. Comparison of Data
    4. Calibrations
    5. Filling of Gaps in Data
    6. A new data bank for tidal gravity measurements (DB92)
    7. Diffusion of Data and Informations
    8. Summary of routine tasks 1956-1995
  3. A future for ICET

1. The Foundation of ICET, its Initial Program, mission and responsibilities

The International Geophysical Year (IGY-1956) enterprise is at the origin of the foundation of the International Centre for Earth Tides.

The IGY program indeed was intended to investigate all astronomical, geodetical and geophysical phenomena suspected to be variable with time and location.

The International Association of Geodesy, involved with IUGG in this world wide enterprise, proposed that two problems, directly related to Geodesy, would be addressed at this occasion : the polar motion and the Earth tides.

Besides the importance of an accurate knowledge of Earth tide elastico- viscous deformations needed to correct a number of measurements of high preci- sion made at the Earth's surface (actually permanent tidal corrections on VLBI, GPS, Lunar and satellite laser ranging), the basic importance of Earth tides is not to be underestimated because, as pointed out 40 years ago and often repeated, we deal with the only phenomenon of deformations of the Earth where we know well in advance and with not surpassed precision the forces in action.

This is an unsurmounted advantage which probably has not yet been totally exploited.

Moreover, for long term geodynamical effects, Earth tides periodic stresses, although weak, appear as a high frequency internal triggering process permanently acting during billions of years.

Pierre Tardi, the IAG Secretary General, formed in 1956 a small committee to set up a program of investigations. This Committee composed by W.D. Lambert (USA), Yuri Boulanger (USSR) and myself proposed to the participating national committees :

and, this being hopefully done, to investigate the Poincaré-Jeffreys effect, i.e. the liquid core resonance.
To ensure a follow up and a real concrete activity and to help the countries to develop such researches, Tardi proposed that a Permanent Commission and an International Centre for Earth Tides (ICET) would be established to coordinate the program development and help in the data analysis. G. Laclavère, at that time Secretary General of the International Union of Geodesy and Geophysics, moved that this Centre should be incorporated in the Federation of Astronomi- cal and Geophysical Services (FAGS) just founded to deal with the problems raised by the IGY.

It is true that, since the last century and before 1940, valuable ten- tatives to measure the Earth tides with some accuracy had been made in Germany by von Rebeur Paschwitz, Hecker, Tomascheck, in France by Eblé and in the USA by Michelson himself with his famous interferometric device. But in 1956, the tasks of the Commission and of ICET were rather difficult. We had:

Rudolf Tomaschek was elected President; being the youngest fellow I was asked to organize the Secretariat of the Commission, give birth to ICET, to provide computations facilities and to publish a Bulletin of Informations (Bulletin d'Informations des Marées Terrestres soon called BIM) to serve as a link between the involved people.

A chance was that no help at all being available to perform the long and tedious needed calculations by hand or with desk computers, I was forced to go to IBM Belgium in 1958 and to learn in haste how to prepare programs (Doodson method and Lecolazet method of tidal analysis) in machine language for the historical famous IBM 650 computer. During the period 1959-1969 not less than 5900 analysis were performed at ICET with Lecolazet's method at the benefit of many scientists and Institutions engaged in Earth tide researches as most of them had no computer at their disposal at that time.

In 1966 A. Venedikov came to Bruxelles and, using our small IBM 1620 computer, devised a new method of analysis which considerably improved our computing capabilities. This method which used specially constructed filters and the least squares method allowed to analyze series of any length presen- ting eventually unavoidable gaps which was not permitted with previous stan- dard methods. Many thousands analyses have been performed by ICET with this method.

In consideration of this situation the tasks ascribed to ICET have been until now :

  1. to collect all available measurements of Earth tides (which is its task as World Data Centre C),
  2. to evaluate these data by convenient methods of analysis in order to reduce the very large amount of measurements to a limited number of parameters which should contain all the desired and needed geophysical information,
  3. to compare the data from different instruments and different stations distributed all over the world, evaluate their precision and accuracy from the point of view of internal errors as well as external errors,
  4. to help to solve the basic problem of calibrations and to organize reference stations or build reference calibration devices,
  5. to fill gaps in information or data as far as feasible,
  6. to build a data bank allowing immediate and easy comparison of Earth tide parameters with different Earth models and other geodetical and geophysical parameters like geographical position, Bouguer anomaly, crustal thickness and age, heat flow, ...
  7. to ensure a broad diffusion of the results and information to all interested laboratories and individual scientists.

2. The accomplishment of ICET tasks up to now

2.1. Collection of Data (World Data Centre)

It was recommended by the Commission of Earth Tides (International Asso- ciation of Geodesy) to store all recorded data at the International Centre for Earth Tides in Brussels which is recognized and acts as a World Data Centre C.

All the data stored at ICET have now been transferred on diskettes and we are now in the procedure to store them on optical disks.

To-day the ICET Data Bank contains results from 360 tidal gravity stations but not all stations provided the totality of their initial hourly readings and calibration data. The conservation of the data in the ICET World Data Centre C has the following advantages:

  1. protection of the data against loss or damage. It has been our experience that when, for some reason, a scientist leaves his Institution, geophysical data upon which he worked may be difficult to retrieve and even, sometimes, are simply lost. In several cases indeed, ICET has already provided duplicates of data to Institutions which originally acquired them;
  2. automatic and safe introduction of the submitted data into the ICET Data Bank;
  3. Institutions providing tidal gravity data to the ICET Data Bank have access, free of charge, to all data included in the Data Bank (at the exception of course of those with requested restricted access).

A "Directory of Gravity meters in use for Earth tide measurements" published in june 1994 (BIM 118, 8777-8830) indicates that at least 938 records with a total of 5.179.074 Earth tidal hourly readings must exist at that time but that only 581 records with a total of 3.343.957 hourly readings have been received and stored in the ICET Data Bank. Thus the collection of data appears to be a difficult task. Despite the often renewed recommendations by the World Data Centre ICSU Panel and by the Scientific Unions and Associations some countries do not transmit any data measured on their territories.
As a matter of fact many data have been lost ...

2.2. Evaluation of Data

Since its foundation one of the main activities of ICET has been the development of appropriate computation methods. A great number of programmes have been written and permanently improved to analyze and validate the quality of the submitted data. These programmes have always been made freely available to interested institutions, a number of which used ICET procedures.

The method usually applied for the analysis of original hourly data was the least squares method with Venedikov filters but spectral analyses are also applied in some specific cases. With the present level of precision of the instruments commonly used (spring gravimeters, clinometers and extensometers) there is no significant difference with other proposed methods.

Recently however, the appearance of long uninterrupted series of higher sensitivity, obtained with Superconducting gravimeters and some exceptionally good classical spring gravimeters made absolutely necessary the development of a new generation of analysis methods like ETERNA which can be used with a simple Personal Computer and has been developed by Wenzel.

New methods also benefit from a more rigorous and considerably extended development of the luni-solar potential as performed by Tamura, Xi Qin Wen, Hartmann-Wenzel and Roosbeek.

On the other hand, to evaluate the local loading and attraction effect of oceanic tides, ICET developed a programme based upon Farrell's procedure and used the cotidal-corange oceanic maps of Schwiderski (9 tidal components) as Working standard. To-day (1995) ten new oceanic maps have been introduced in these computations.

The ultimate outputs of the usual computer programmes are:

2.3. Comparison of Data.

Comparisons of stations and instruments have been made as an obvious result of the analysis of data. This gives guidelines for the improvement of installations or of instruments. With as much as 360 tidal gravity stations in the Data Bank, it has been found convenient to define, in an impersonal way, a "quality factor" for each series of measurements, taking into account not only the internal errors but also the efficiency of the station related to interruptions and maintenance.

2.4. Calibrations

The calibration of tidal instruments is not an elementary and easy task. At the time of IGY, when ICET was founded, calibration errors up to 10% were rather common as experience later demonstrated.
For what concerned gravimeters ICET proposed in 1975 to accept the results obtained with three Askania gravimeters operating at Brussels from 1958 to 1967 (Ducarme). This defined the Brussels system and some 35 different instru- ments were installed there to be calibrated with respect to the local tidal amplitudes and phases. The system was kept without change until 1992 when it was clearly established by different methods (Baker measurements, inertial platform) that this system gave amplitudes in excess by 0.8%. In this respect ICET encouraged the development of an inertial oscillating platform at Bruxelles to calibrate experimentally the gravimeters (Van Ruymbeke). The 0.8% correction was carefully made and the results published in a paper entitled "A new Data Bank for tidal gravity measurements (DB92)" by P. Melchior in "Physics of the Earth and Planetary Interiors, vol 82, pages 125- 155, 1994".

For what concerns horizontal pendulums used as clinometers, J. Verbaandert invented and developed a system called "crapaudine dilatable", a device inter- ferometrically calibrated which ensures on site regularly repeated automatic calibrations of the pendulums with 1% accuracy or slightly better. ICET has calibrated some 150 crapaudines on its interferometer.

The interpretation of the observed results (amplitudes and phases for the main diurnal and semi-diurnal components at each station) thus rests with a compa- rison (1) to the corresponding parameters calculated on the basis of an ellip- soidal rotating Earth model with liquid core (Wahr-Dehant and Wang models) - and - (2) to the loading and attraction effects produced by the oceanic tides acting at exactly the same frequencies as the Earth tides.

The residues obtained by subtracting both effects from the observations are to be scrutinized to identify sources of anomalies, either instrumental, either geophysical.

A correlation with large scale regional tectonics through heat flow density and age of the continental crust is suggested and still a matter of contest and investigation.

2.5. Filling of Gaps in Data

Since the end of 1973, ICET has obtained support to develop in Africa, in Asia, the South Pacific, South and Central America, Trans-World Tidal Gravity Profiles which extend over 17 400 km, from Istanbul to Papeete and over some 7 200 km from Cape to Cairo as well as in South America. These operations involved a total of 136 stations where measurements of a duration of 4 to 8 months have been completed in 19 years, during the period 1973-1991.

Eight different equipments of high quality were used for these measure- ments (Geodynamics and LaCoste Romberg gravimeters with quartz clocks, cassette and papers recorders). Results are reported in a number of publica- tions in international scientific journals.

2.6. A new data bank for tidal gravity measurements (DB92)

Tidal gravity parameters being available in 382 permanent or temporary stations all over the world, tentative interpretations of these data were made on the basis of a data bank created by ICET in 1980-1982 which contained all available results without any discrimination which could have appeared as arbitrary. These interpretations were criticized on the basis of the different qualities of instruments (null instruments or not), data, calibration and necessary corrections for ocean-continent interactions.

A careful re-examination of these basic data being needed before any geophysical discussion we have built a new data bank which contains only those stations where rigid control of every step in the observations, calibrations and reduction could be made either because the original data are stored in the World Data Centre C, or because the published results give enough information to permit checks and revisions.

215 stations where oceanic effects are well controlled have been selected to determine the experimental values of the earth tide parameters with as a result, a scattering reduced by a factor of 2 with respect to the previous data while the mean values of the amplitude factors of the two main waves (O1 diurnal; M2 semi-diurnal) calculated continent by continent, agree with the best available earth model.

For what concerns the distribution of oceanic loading and attraction effects, islands and coastal stations are added and an amount of 289 stations have been used in a recent investigation (1995) about 10 new corange-cotidal maps.

It is expected that this new data bank will be used for renewed discus- sions of the results.

2.7. Diffusion of Data and Informations

ICET is also acting as an information and supporting agency in the field of Earth tides through:
  1. the publication of the 123 issues of the "Bulletin d'Informations des Marées Terrestres": covering 9388 pages from 1956 to 1995. In particular the Proceedings of the CES Working Groups meetings have been published in the Bulletin. This Bulletin is distributed to some 300 scientists and scientific Institutions;
  2. numerous translations of russian scientific papers amongst which 364 translated papers have been published in the Bulletin d'Informations des Marées Terrestres, covering 3715 pages, occasionally chinese papers (too few unfortunately) and even an important portuguese paper;
  3. the maintenance of a General Bibliography of scientific papers related to Earth tides, with numerical code (5027 titles), available on diskette;
  4. three to four visitors per year, sometimes more;
  5. participation in the organization of the International Symposia on Earth Tides: twelve symposia have been held from 1957 to 1993;
  6. ICET has often had a role of scientific counsellor at the benefit of different organisations faced with the Earth tides problem. As examples are:
    • the calculation of horizontal areolar deformations acting upon the CERN particles accelerator LEP at GenŠèe (Switzerland),
    • initiation of several chinese groups (Beijing, Wuhan, Lanzhou, Kunming etc...) at the time of the opening of the cooperation of IUGG with China,
    • instructions about the calculation of oceanic effects for the southern hemisphere,
    • long time cooperations, principally with Argentina, Australia, Brazil, China, Finland, Indonesia, New Zealand, Russia, Spain and Ukraina,
    • tidal predictions for field gravimetric measurements.

Summary of routine tasks 1956-1995

Trans World Profiles       1973-1991   136 tidal gravity stations established

                                           Asia: 32, South Pacific: 21

                                           Africa: 34, South America: 42

                                           Central America: 7



Bulletin of Informations   1956-1995   123 issues, 9388 pages



Translations from russian  1961-1995   364 papers, 3715 pages published



General Bibliography       1830-1995   5027 titles



Storage of hourly Tidal



Gravity Data               1954-1995   3.343.997



Analyses of tidal series   1958-1995   probably around 20.000



Visitors                   1958-1994   164



Earth Tides Symposiums     1957-1993   12

3. A future for ICET

The mission of ICET is of multidisciplinary character. Basically Earth Tides are directly related to Fundamental Astronomy through the necessary use of a tidal potential that several specialists have recently developed to the third and fourth degree and extended to the major planets influence.

The tesseral diurnal tidal forces are also responsible for the precession and nutations of the axes of inertia and rotation of the Earth by a coupling acting upon the non spherical body of the Earth. On the other hand, the sectorial semi-diurnal tidal forces are responsible for the secular retarda- tion of the Earth's speed of rotation.

The interpretation of the observed tidal deformations rests upon our knowledge of Internal Geophysics i.e. upon the construction of an adequate model of the rheological properties of the earth's interior (density, compressibility, rigidity, viscosity). Here the liquid core and inner core play a major role by introducing resonances at some diurnal tidal frequencies in relation with free nutations of the axes. The correct localisations of these frequencies are determined by the flattenings of the boundaries which obviously do not correspond to hydrostatic flattenings.

The interpretation are complicated owing to the perturbing effects due to water and air masses at the surface of the Earth.

Oceanography problems arise from the interactions between oceanic and Earth tides : Earth tide deformation of the ocean bottom and loading reaction of the oceanic tides upon the earth's crust and mantle. It is also not yet clear to which effect - oceanic or bodily - a phase lag responsible for the retardation of the earth's rotation can be ascribed.

Meteorology problems arise from the atmospheric pressure effects acting not only on broad areas of the crust but also on the ocean surfaces which react as inverted barometer or non inverted barometer depending upon the frequency. The admittance coefficient of this ocean-atmosphere action is presently poorly known.

Hydrology problems also arise because of the role of underground fluid tidal flows which are far from negligible in crustal processes. Also Tectonics must be considered because of fatigue effects due to direct and indirect (loading) tidal stresses which act throughout the whole body of the Earth at very high frequencies if we consider the time scales of tectonic processes. Many investigations are made indeed to identify triggering of earthquakes and volcanic eruptions. Large scale (i.e. compatible with tidal waves lengths) lateral heterogeneities in the crust, related also to its age, can probably influence the tidal deformations size.

While it is obviously important to carefully pursue and improve the tasks assumed up to now as listed in the "Summary of routine tasks 1956-1995", this is surely not sufficient to justify the existence of a specific Centre for Earth Tides. A WDC may be sufficient.

It appears obvious to me that, with the development of a network of Superconducting gravimeters - some 20 instruments already installed - a priority and urgent task for ICET will be, in the frame of the "Global Geody- namics Project" (GGP), to collect, store and compare the tidal outputs of these instruments (with local atmospheric pressure - it may be unrealistic to collect broad regional pressure data at ICET).

It is therefore urgent to convince the owners and users of these remarkable instruments that the data will gain in importance if they are put at the disposal of scientific community through ICET (of course this may be subject to specific conditions) to be combined in a "planetary scale" inter- pretation.

New analysis programmes, controlled and used by ICET, must offer simultaneous options for a careful interpretation of the results, that is : new extended (up to 8000 terms) tidal potentials (Roosbeek, Wenzel and Hartmann), new Earth models (Wahr, Dehant, Wang) as well as all the oceanic corange-cotidal maps provided they completely cover the world ocean.

On the other hand new temporary tidal gravity stations must be reesta- blished in some areas where serious contradictions exist between the results of several ground based stations and all the available cotidal maps. It is the case in Indonesia and in New Zealand.
Tidal tilt and strain measurements should not be neglected as was apparently the case in recent years owing to the difficulties of interpretation (coupling effects, non linearities). Such measurements have better potentialities than gravity measurements to identify instabilities of local heterogeneities when made in the vicinity or across active faults.

ICET should promote new developments in this direction.

For any request please contact: Jean-Pierre BARRIOT
 

Laboratoire des Sciences de la Terre

Université de Polynésie Française
B.P.6570

98702    FAAA_TAHITI

Polynésie Française

 
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