International Centre for Earth Tides
Centre International des Marées Terrestres
for the period
B.Ducarme, ICET Director

The staff of ICET, which is completely supported by the Royal Observatory of Belgium, is composed as follows:

Prof. B.Ducarme, Director(part time)

Dr. O.Francis, Vice-Director(part time, until November 1998)

Mrs. L.Vandercoilden, technician(full time)

Mr. M.Hendrickx, technician(part time)

The Royal Observatory of Belgium is hosting ICET since 1958 and continues to provides numerous administrative and scientific facilities especially for the publication of the " Bulletin d’Information des Marées Terrestres" and for the tidal data processing.

In November 1998 Dr. O. Francis decided to resign his position of Vice-Director for personal reasons.

1. Previously assigned scientific goals

Quoting Prof. P.Melchior last report as ICET Director in 1995 we can summarise the tasks ascribed to ICET since 1958 as follows:

- as World Data Centre C, to collect all available measurements on Earth tides;

- 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;

- 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;

- to help solving the basic problem of calibration by organising reference stations or realising calibration devices;

- to fill gaps in information and data;

- to build a data bank allowing immediate and easy comparison of earth tides parameters with different Earth models and other geodetic and geophysical parameters ;

-to ensure a broad diffusion of the results and information to all interested laboratories and individual scientists.

Of course these goals have to be continuously reactualised and require a close cooperation with all scientists working in the field of tidal research.

For example tidal data analysis has always been one of the main activities of ICET. In the early days, at the very beginning of the computer technology, it was necessary to provide computing facilities for many scientists involved in tidal research. Nowadays everybody is able to process the tidal data on his own Personal Computer. However ICET has still to reprocess the data before their inclusion in the data bank to be sure that no gross error is left in the received data. For the sake of homogeneity we also prefer to include in the data bank, when it is possible, analysis results reprocessed at ICET. It should be noted that ICET did always accept any input format and developed the necessary conversion software. Now PRETERNA is the most widely used standard exchange format.

The main problem in data preprocessing has always been the correction of spikes and tares. When analog records had to be manually digitised the human eye was able to « smooth » the curves when taking hourly readings. The digital data acquisition systems

require a higher sampling rate, at least one value per minute, and automatic softwares for the data smoothing and jumps correction. Decimation filters are required for extracting the hourly readings for tidal analysis. ICET is currently using different softwares for tidal data preprocessing and analysis.

The data storage and exchange are also drastically modified. Starting from listings that we had to encode on punched cards we are now exchanging by FTP digital data that will be stored on CD-ROM’s. The ICET data base has proved to be very useful for the data providers themselves as observations lost or destroyed in their home institution were retrieved at ICET. It is important to note that all data stored have been reevaluated. All analysis results are kept in a special data bank. Its version DB92 included as much as 360 stations for gravity tides and around 50 stations for tilt and strain. For each station we provide not only the tidal factors but also oceanic loading and relevant geophysical parameters. Its content is available on request.

In the last decades ICET brought a significant contribution to tidal gravity observations through the so called « Trans World Tidal Gravity Profiles »(TWP). This effort was launched in close cooperation with the Royal Observatory of Belgium. All participating instruments were intercompared in the Brussels reference station. The 136 stations occupied between 1973 and 1991 form the core of the tidal data bank. This project gave also the opportunity to many scientists to start tidal observations in their own country.

Oceanic loading is the main additional signal from geophysical origin which is perturbing the body tides. Since 1980 ICET adopted the Schwiderski cotidal maps as a working standard. We are now currently using the most recent oceanic models for tidal loading computations. Special attention was paid also to the evaluation of the atmospheric pressure effects.

2. Main Commitments for the future

It appears first that most geodetic measurements are affected by earth tides, as at the centimetric level the tidal displacement of the station is no more negligible. It will thus remain an important task for ICET to provide algorithms for tidal computation or analysis. For example the geophysicists, such as seismologists or volcanologists, who are measuring crustal deformations for natural hazards monitoring, are now conscious of the necessity of dealing properly with the tidal signals. In a similar way absolute gravity measurements require accurate tidal corrections that should take into account the local tidal parameters. These parameters have to be computed including oceanic tidal loading effects or even require in situ tidal gravity observations.

On the other hand the earth tidal scientific community is limited. The last International Symposium on Earth Tides held in Brussels in 1997 brought together only one hundred participants. The groups are always very small and often marginally involved in tidal research. The papers dealing specifically with tidal studies are not fitting so well to international journals. It is thus very important to keep a specialised diffusion and information medium. It is the vocation of the "Bulletin d’Information des Marées Terrestres"(BIM). ICET is publishing two eighty pages issues per year. It has been decided to interrupt the publication of French translations of Russian papers, taking into account the fact that the Russian scientists are now able to publish in English.

Finally ICET will continue to welcome trainees or guest scientists as there is a steady demand.

Besides this basic activity, which is the scientific challenge for the future ?

The mathematical modelisation of the astronomical tidal forces as well as of the elastic response of the Earth made recently decisive progress. Two new high precision tidal developments have been published by Hartmann & Wenzel in 1995 and Roosbeek in 1996. It is thus now possible to model the astronomical tidal forces to within 5 nanogal in the time domain. The different mathematical techniques for the evaluation of the tidal response of the Earth do agree now to better than 0.1%. The most recent models include anelasticity in the mantle.

The last problems to be solved are linked to the fluid elements of our planet: liquid core resonance, oceanic loading, meteorological effects, underground water.

Among the ground based observations only gravity tides are able to give informations valid at the regional level. The other components(tilt, strain, volume change) are heavily depending of the local parameters of the crust, including cavity or topography effects. These observations should be mostly used to monitor tectonic deformations after removing the tidal phenomena.

Tidal gravity observations are able to provide constrains on the liquid core resonance by means of very precise observations in selected sites. The same is valid also for the selection of the most realistic model for the elastic or inelastic response of the Earth. For that purpose it is essential to improve the calibration methods in order to achieve a 0.1% accuracy in amplitude and a 0.01o accuracy in the phase determination. The determination of the amplitude factor of the polar motion effect on gravity will constrain the Earth viscosity at low frequency.

To achieve these goals it will be necessary to tackle three main questions: tidal loading, atmospheric pressure effects, underground water. It is only possible through a coordinated effort.

3.New prospects

These objectives are now directly addressed by the  "Global Geodynamic Project"(GGP). A network of seventeen cryogenic gravimeters is in operation since July 1997, using a similar hardware and the same procedures for data acquisition. This observation campaign will continue during six years.

A complementary objective of GGP is to study the residues after elimination of the tidal contribution in order to detect inertial accelerations such as free oscillations of the Earth core and mantle with periods larger than 50 minutes, which are difficult to observe by means of conventional seismometers. In fact the cryogenic gravimeters are extra-large band instruments covering phenomena with period ranging from one second to more than one year(figure 1).

As the study of tiny signals in gravimetric records requires an effective elimination of the dominant phenomena it is absolutely necessary to carefully model the tidal and barometric effects. It is a unique opportunity to improve the techniques of evaluation of the barometric effects and to obtain high quality well calibrated tidal observations. It is a reason why ICET has been interested to support this project since its beginning and finally became the GGP « Data Centre ». His task will be not only to archive the raw and preprocessed data from all the contributing stations but also to provide his expertise for the preprocessing and analysis of the raw data. To insure the homogeneity and the quality of the archived data ICET will treat the raw data in a standard way.

A new integrated software for tidal data preprocessing has been developed by Dr. P.Vauterin[1998] from the Royal Observatory of Belgium. Based on a remove restore procedure TSOFT is fully interactive(figure 2

One of the conclusions of the TWP is that the accuracy of the observations, comprised between 0.5% and 1%, was essentially limited by the accuracy of the calibration of the instruments. Moreover, excepted for the cryogenic gravimeters, the sensitivity is time dependant. To solve efficiently this question a Working Group on Calibrations has been set up inside the Earth Tides Commission during the Brussels Symposium.

On the other hand the interpretation of the observed tidal factors is strongly limited by the accuracy of the tidal loading evaluation. Even the most recent oceanic tidal models incorporating altimetric data are not globally better for tidal loading computations than the, now by many aspects obsolete, Schwiderski model. The reason is probably that there exists for each model areas where it fits better than in others. To investigate more closely this fact it is important to install accurately calibrated gravimeters in areas where noticeable disagreements do exist between different models. The atlantic coast of Western Europe has been selected as a first test area. Gravimetric tidal stations will be selected at distances from the coast ranging between 50 and 100 kilometers. To insure a maximum of accuracy it is recommended to record with two different instruments, carefully intercompared, during a six month period to allow the separation of the eight principal tidal waves in diurnal and semidiurnal bands. The measurements are performed by colleagues from Brussels and Madrid. We hope to extend this project to the Pacific coast of Russia in collaboration with russian colleagues.

[1] Crossley D., Hinderer H.: GGP Newsletter 5, September 1997

[2] Vauterin H.P. : Tsoft : Graphical and Interactive Software for the Analysis of Earth Tides Data. Proc. 13th Int. Symp. On Earth Tides, Observatoire Royal de Belgique, Série Géophysique, Brussels, 1998, 447-454

4. Activities

The "Bulletin d’Information des Marées Terrestres"(BIM) is printed in 300 copies. From March 1995 until May 1999 the issues numbered from 121 to 131 have been published with a total number one thousand three hundred pages.

ICET welcomed more than 15 visitors. Besides visitors coming only for a short stay we must consider also guest scientists and trainees.

The guest scientists bring their own know how or data to work at ICET during several weeks. They will either , as Prof. A.P. Venedikov (Bulgaria), prepare new softwares or, as Dr. G.Casula(Italy), H.P.Sun(China) and Dr. V.Timofeev(Russia), perform data analysis using the ICET data bank or computing facilities.

Younger scientists are coming to received intensive training on earth tide data processing and analysis, mainly with the TSOFT and ETERNA packages. Since 1997 we welcomed trainees from Argentina, Brazil, Great Britain, Italy, Spain and USA.

It should be noted that several visitors are not at all specialists in earth tides. It is an indication of a renewed interest with respect to tidal phenomena.

We receive regularly requests for information.

The most common requests concerns tidal predictions or general information. We receive more or less one request per week.

Several Institutes continue to send regularly earth tides data to ICET. All data received have been checked and recompiled. Among the recently participating countries we should mention : Belgium, China, Czech Republic, France, Germany, Hungary, Indonesia, Italy, Grand Duchy of Luxembourg, Poland, Russia and Spain. Most of the tidal recording activity is devoted to gravity tides. This tendency is even more striking if we consider the GGP activities

The status of the GGP data bank on July first 1999 is given in the following table. All the original minute sampled data have been carefully preprocessed at ICET using TSOFT. The data are corrected for tares and spikes. The data are then decimated to one hour and analysed. This is the main task of Mrs.L.Vandercoilden. The analysis results are directly communicated to the data owners. This follow up is required to detect quickly the anomalies that could affect the data sets and insure their homogeneity

The archiving of the data is rather complex as the data are only released according to a strict time table. The data are sent to ICET only one year after their production. It is why in the table most of the stations sent their data only until April 98. During one additional year the data are only available to the GGP members and can be freely accessed only after two years. The software provided for the gestion of this data bank by the GeoForschungZentrum Potsdam is now fully operational. The implementation of this software required to purchase new informatic equipments. Although he resigned his position in ICET, Dr. O. Francis from the Royal Observatory of Belgium agreed to continue to supervise the data archiving until the end of the project. The routine work is assumed by Mr. M.Hendrickx.

The one minute sampled raw data of each gravimeter represents 1.6 Mbytes per month. For fifteen operational stations we have thus 24 Mbytes per month or 300 Mbytes per year. It represents only one CD-ROM. We do also archive the preprocessed minute data ready for tidal analysis.

5. Conclusions

The reason why it is important to keep an International Centre for Earth Tides is that Earth tidal Scientific Community is very scattered. On one hand there are no large scientific teams working on that subject. On the other hand most of the Geodetic community members are confronted to tidal phenomena, when working on Earth deformations. In a similar way the Seismologists or Volcanologists getting acquainted with modern geodetic techniques have to learn about the perturbing effects on ground displacement measurements, such as earth tides, tidal loading, barometric effects,…. Scientists who know already the tidal phenomena can easily produce tidal analysis on their own personal computer, but for the new tidalists training sessions are the best way to get acquainted with the data reduction and analysis, which remain a delicate procedure. It stresses also the importance of keeping the "Bulletin d’Information des Marées Terrestres", where experienced people can share their know how with the newcomers. Nowadays beside very specialised projects such as GGP for which a dedicated infrastucture is required there is an increasing demand for a basic knowledge in tidal data processing to remove properly these effects from various signals.

GGP Data Bank

BA Bandung, Indonesia failure

BE Brussels, Belgium 97/07/01-99/05/01

BO Boulder, USA 97/07/01-98/08/01

BR Brasimone, Italy 97/07/01-98/04/01

CA Cantley, Canada 97/07/01-98/03/31

CB Canberra,Australia 97/07/01-98/03/31

ES Esashi, Japan 97/07/01-98/04/30

KY Kyoto, Japan

MA Matsushiro, Japan 97/07/01-98/06/30

MB Membach, Belgium 97/07/01-99/03/03

ME Metsähovi, Finland 97/07/01-98/04/30

PO Potsdam, Germany 97/07/01-99/03/03 South Africa

ST Strasbourg, France 97/07/01-98/04/30

SY Syowa, Antartica not due

VI Vienna , Austria 97/07/01-98/04/30

WE Wettzell,Germany 97/07/01-98/09/27

WU Wuhan, China 97/07/01-98/12/31


Figure 1: Spectrum of phenomena recorded by A Cryogenic Gravimeter








Figure 2: Reduction Procedure for Tidal Gravity Data