(compiled by Barbara Kolaczek)

5.1. INTRODUCTION

This part of the Polish National Report on Geodesy is the quadrennial report of geodynamic works performed in Poland in a period from 1995 to 1998. It summarises investigations such as establishment, maintenance and analysis of geodynamic networks of continental, regional and sub-regional scale, theoretical research and analysis of Earth rotation data, Earth tides monitoring, investigations of sea level variations, etc. These activities were conducted mainly at the following research centres listed in an alphabetic order:

• Department of Geodesy and Photogrammetry, Agricultural University in Wroclaw;
• Department of Mining Geodesy and Environmental Engineering, Academy of Mining and Metallurgy in Cracow;
• Institute of Geodesy, Olsztyn University of Agriculture and Technology;
• Institute of Geodesy and Cartography in Warsaw;
• Institute of Geodesy and Geodetic Astronomy, Warsaw University of Technology;
• Space Research Centre, Polish Academy of Sciences in Warsaw.

5.2. GEODYNAMIC NETWORKS IN POLAND

5.2.1. Polish Geodynamic Network

In the period of 1991-96 the full modernisation of the primary Polish control network was completed. Consulting geologists, 33 points of this network (including all 11 Polish 0-order EUREF points) were selected as well as 3 additional ones, setting up a repetitive Polish Geodynamic Network. The network points are nearly evenly spread all over Poland and comprise all main and secondary geological block structures. They are especially located and marked for safety with a sophisticated construction. Repetitive measurements on this network, besides other geodynamic tasks, should make possible investigations of coherency and stability of the primary control network. Distribution of the network points is presented in Fig. 5.1.

0-epoch GPS campaign on the Polish Geodynamic Network was carried out from 6 to 24 September 1997. Observations were taken over 3 days at each point (6 days at sessions joining points). However, temporary malfunctioning of two GPS receivers (lack of L2 signals, interruptions of both L1 and L2 signals) made it necessary to make supplementary observations, carried out in early October 1997 and April 1998. Eight Leica SR299, SR399 and SR9500 GPS sensors were used in the observation campaign on the network. Amending sessions were performed with the Ashtech Z-12.3 receivers.

The network was processed and adjusted in co-operation with WUT sub centre of “Permanent EUREF Network” program, using Bernese version 4.0 software, according to presently valid standards. All five Polish GPS permanent stations (BOGO, BOR1, JOZE, LAMA, and WROC) as well as stations in neighbouring countries (METS, ONSA, WTZR, GOPE and GRAZ) were used as reference points. The whole operation, designed and completed in the Institute of Geodesy an Cartography (IGiK).

Fig. 5.1. Structure of geological blocks and distribution of points in Polish Geodynamic Network

5.2.2. Geodynamics Research In The Sudety Mountains And Fore-Sudetic Block

(SW Poland)

The research is conducted on three geodynamic polygons: “Snieznik Massif”, “Paczkow Graben”, “Stolowe Mountains”. The control networks of the mentioned polygons are integrated by a regional control network “Geosud” (Fig. 5.2). The network was established in 1996 as the geodynamic network in the Eastern Sudety Mountains and in Fore-Sudetic Block (Cacoń, 1996; Cacoń, 1998). Fig. 5.2 shows the "Geosud" network points distribution with Wrocław permanent station. In the Stołowe Mountains National Park the geodetic network was established and surveyed by GPS (Kontny and Mąkolski, 1996). Since then the GPS and terrestrial geodetic measurements are periodically repeated (Cacoń, 1998). Results obtained confirm that neo-tectonic movements are not finished yet. It indicates the potential hazard for water dams of the Lower Silesia (Cacoń and Dyjor, 1995) as well as for the other objects (Cacoń, 1998a). Recent mobility of the upper crust is also confirmed by investigations of geodynamic polygons in Snieznik Massif (Cacoń et al., 1996; Jamroz, 1997), Paczkow Graben (Cacoń and Deeb, 1995, 1996) and in the Stolowe Mountains (Cacoń, 1996a). Similar results were obtained for the Eastern Sudety and Fore-Sudetic Block (Cacoń, 1996b, 1998b; Cacoń et al., 1998). Concept of spatial database for geodynamics was presented (Kontny et al., 1997).

In 1997 the “GEOSUD” regional network was integrated with the “SILESIA” network in the northern part of Morava (Czech Republic) into the new network named “SUDETY” (Schenk et al., 1998).

5.2.3. Tatra Mountains Geodynamics

Geodynamic research in the Tatra Mountains Geodynamic Test Area has been initialised in mid 1980 and is carried on since. The network of well stabilised control points has been extended over a part of Slovakian Tatra Mountains. Three GPS observation campaigns were conducted in 1995, 1996 and 1998 respectively. Numerous relative gravity measurements were also conducted at the fiducial points of the control network. The final analysis of the data is being elaborated (Makowska and Cisak, 1997).

5.2.4. Geodynamic Network In Wieliczka Mining Area

An integrated geodynamic network was established in 1944-1966 within a project financed by the State Committee of Scientific Research, to monitor displacements and to analyse the sources of these displacements over Wieliczka salt mine and its surroundings. There are movements of the rock body due to seven hundred years of mining as well as due to land sliding movements and neo-tectonic movements. Geodetic network consists of numerous points stabilised on the mining area as well as intermediate points located outside this area. These points were connected to the 0-order GPS POLREF network and to a Tatra Mountains geodynamic network. GPS, levelling and gravity measurements were used for determination of a local quasi-geoid. The analysis of the results showed high effectiveness of GPS technology while monitoring displacements.

A new concept of the universal observation network for monitoring 3D displacement in mining areas and on land slide areas was described including guidelines how to carry out measurements. The concept has been verified in practice.

Verification of various hypotheses concerning the behaviour of a rock body under the influence of mining on the basis of geodetic measurements carried out periodically is presented together with a strategy of using the verified hypothesis in a kinematic model adjustment of vertical control (Beluch and Piwowarski, 1996). In areas with quick vertical dislocations a project on observation control and technology of measurements become important. Problems of collecting data from altimetry with use of technology of modular sequences located among points positioned with GPS were described. Based on finite elements a numerical model of a rock body is used. It allows to determine a deformation index at arbitrary point of the area being investigated. It is found to be an efficient tool for prediction changes and dangers caused by mining.

Application of the finite elements method for calculation of local changes of Earth gravitational field caused by a dislocation of huge masses of a rock body in open pit mines was discussed. It was proved that local changes of Earth gravitational field might have a major influence on the accuracy of precise levelling.

5.2.5. Geodynamic Network in Copper Basin Area

Polish Copper Basin is located in south-western Poland, between two towns: Lubin and Glogow. In this area the following influences of mining exploitation on the surface and the rocks mass can be established:

• direct influences caused by displacement of the rocks filling free space created as a result of mining activity,
• indirect influences caused by water draining action in the mines.

The precise levelling network covers the area of about 2300 km². The study of horizontal displacements in this area was carried out using spirit levelling measurements of control points repeated every 2-3 years.

Since 1992 the new concept of 3D control network measured with GPS was developed. The primary GPS control network in the Copper Basin area consisting of 53 new points was established. The classical horizontal control network has been integrated with GPS determined one and tied to the POLREF’92 points (Wasilewski et al., 1996a; Wasilewski et al., 1996b; Baran et al., 1998).

The accuracy of co-ordinates determination, using GPS technique, of the network points was shown to be not worse than 3 mm. Performed analysis shows also high reliability of GPS measurements.

The finite elements method for deformation analysis and prediction of displacement is just under development.
 
 

5.3. INTERNATIONAL GEODYNAMIC NETWORKS

5.3.1. Baltic Sea Level Project

The Baltic Sea Level Project was initiated already in 1989 at the IAG General Meeting held in Edinburgh. The purpose of the project was to co-ordinate the vertical datums of the counties around the Baltic Sea.

In order to fulfil the above goal, three Baltic Sea Level GPS Campaigns were conducted since 1990. The first one was a two-week campaign held in October 1990, called the First Campaign. The results of the First Campaign were not as good as hoped for. The estimated accuracy of ellipsoidal height is not better than ± 4.6 cm.

As the First Campaign took place at the time of political unrest in the Baltic States and Russia, these counties were not able to take part in it. As soon as it was politically possible, contacts were made with the authorities of the newly independent states in order to plan and arrange the Second Baltic Sea Level Campaign. The Second Campaign conducted in June 7 -13, 1993 was performed under more favourable measurement conditions than the First Campaign, and a large volume of good data was collected (Zielinski and Zdunek, 1995). The estimated accuracy of ellipsoidal height was found to be at the level of ± 2.0 cm, which is essentially better than that in the First Campaign. The Third Campaign took place simultaneously with the EUVN, i.e. in May 1997. In the campaign 35 tide gauges and 12 fiducial stations were observed. Additionally, several permanent GPS stations were included into the network of the Third Campaign.

5.3.2. CERGOP Project

The main objectives of the project are: to integrate the geodynamic research in the Central European region based on high accuracy spatial geodetic measurements, to investigate the most profound geotectonic features in the Central European region, the Teisseyre-Tornquist zone, the Carpathians, the Bohemian Massif, the Pannonian Basin and the Alpine-Adria region as well as to provide a stable Central European GPS Reference Network (CEGRN) for sub-regional, local or across the borders investigations and deformation studies. In 1994 Austria, Croatia, Czech Republic, Germany, Hungary, Italy, Romania, Poland, Slovakia, Slovenia and Ukraine joined the Project headed by Hungary and Poland. As associated country Bulgaria joined the project in 1996.

Since 1994 four epoch monitoring GPS campaigns have been carried out on this network in yearly intervals. The CERGOP Data Centre established in 1994 in the Lustbühel Observatory at Graz, Austria. The data of the CEGRN campaigns were processed in the CERGOP Processing Centres. Originally 3 processing groups were formed (Graz, Austria; Penc, Hungary and Warsaw, Poland). A number of processing centres increased later to 8 (Bratislava, Slovakia; Frankfurt, Germany; Matera, Italy; Pecny, Czech Republic and Zagreb, Croatia. A major role in the project was played by CERGOP Study Groups (CSGs) formed by the scientists from two or more member countries. Nine of them remained active during the whole project period. The proceedings of semi-annual CERGOP Working Conferences were published in Reports on Geodesy, by the Warsaw University of Technology. The results obtained within the project were presented at the biannual International Seminar on “GPS in Central Europe”, organised by the FÖMI Satellite Geodetic Observatory, Penc, in Hungary. The proceedings of these conferences were also published in the Reports on Geodesy series.

CERGOP project was partially financed by the Programme COPERNICUS of the European Commission co-ordinated by the Institut für Angewandte Geodäsie (now Bundesamt für Kartografie und Geodäsie), Frankfurt/Main, Germany. First phase of the Project was concluded on 30 June 1998. Lately the proposal for the second phase of the CERGOP-2 Project was submitted to the European Commission with the request for financial support from the 5th Framework EU Programme.

According to the requirements of the EC, the proposal must be expressed in terms of work-packages. The following work-packages for CERGOP-2 are proposed:

For technical reasons the number of sites of the Central European GPS Reference Network (CEGRN) was limited to 31. It is successfully supplemented by the EXTENDED SAGET programme (initiated in 1991 by the Institute of Geodesy and Geodetic Astronomy of the Warsaw University of Technology as an extension of the Polish SAGET Project launched at this Institute in 1986). There is a significant coincidence of scientific aims of both CEGRN and EXTENDED SAGET projects. However, there are also very essential differences. The EXTENDED SAGET network covers much more extended area and an unlimited number of points can be incorporated to this network. This gives an excellent opportunity to all participating institutions to connect new points to the ITRF. EXTENDED SAGET campaigns are thought as long-term action and will be performed every year at least in the whole of current decade. The EXTENDED SAGET campaigns took place in the following periods:
 
 

In order to increase a number of points related to the same observation epoch, the four campaigns of EXTENDED SAGET 1994-1997 fully overlapped with CEGRN campaigns. This will also be a case of 1999 campaign. The following conclusions may be pointed out when comparing both projects: (1) The same standards of GPS observations are used in both CEGRN and EXTENDED SAGET campaigns, (2) EXTENDED SAGET network includes stations of Scandinavia and Mediterranean Region. CEGRN is limited only to CEI countries; only some regions of Germany, interesting from tectonic point of view, are included, (3) EXTENDED SAGET campaigns give the possibility to connect new points to ITRF, (4) Both networks (projects) can coexist. Campaigns of both projects may supplement each other.
 
 

5.4. EARTH ROTATION

5.4.1. Chandler and Free Core Nutation

The Chandler and semi-Chandler oscillations were studied. The semi-annual oscillation and the semi-Chandler wobble were separated using the Fourier Transform Band Pass Filter (FTBPF), (Kolaczek et al., 1996; Kosek, 1995a; Kosek et al., 1997), (see Fig.5.3). Long period variations of their amplitudes with periods of 11-14 years were demonstrated. The correlation coefficient between the change of amplitude of the semi-Chandler wobble and the Wolf numbers is significant at the 90% confidence level. It was found out that the temporal variations with the period of 3-4 years of the amplitude of the semi-annual oscillation filtered using a wide band pass filter are simply beats from the two oscillations. Polhody of the semi-annual oscillation is a left-handed ellipse with the major axis almost parallel to the Greenwich meridian. Its amplitude is of the order of 6 mas. The polhody of the semi-Chandler wobble is circular but irregular and its amplitude does not exceed 3 mas (Kosek and Kolaczek, 1995R; 1997).

IERS C04 B270 lambda=0.0005

Variations of amplitudes of the Chandler nutation have been analysed. Oscillations with periods of 75, 40, 30, 20 years were detected by the Maximum Entropy Spectral Analysis in variations of the Chandler nutation amplitude. The oscillation with the period of 40 years is the most energetic and suggests the influence of variations of the Earth magnetic field which have the similar oscillation.

Predictions of Chandler nutation amplitude variations using the least squares and auto-regression methods show a deep minimum of the Chandler nutation amplitude at the beginning of the 21 century. Similar minimum took place at end of the 1920’s/beginning of the 1930’s (Kolaczek and Kosek, 1998).

5.4.2. Free Core Nutation

A maximum likelihood method (MLM) algorithm for direct determination of the Free Core Nutation (FCN) parameters was developed. It consists in tracking the freely excited variable FCN signal in the celestial motion of the pole observed by VLBI technique, and fitting the resonance parameters. The VLBI observations yield the FCN period estimate T=425.9± 7.9 days which confirms earlier claims that its true value is significantly shorter than 460 days theoretically predicted; on the other hand this estimate is also by about 5 days shorter than previous results from indirect estimation. The MLM estimate of the FCN quality factor is Q=40280 with 1-s confidence interval (13710,? ), that is up to one order of magnitude higher than previous indirect estimates. Comparison with the atmospheric angular momentum data shows that nearly diurnal variations of the surface pressure have enough power to excite the FCN oscillation to the observed level of 0.2 to 0.5 mas, while the wind contribution is negligible (Brzeziński, 1996b; Brzeziński and Petrov, 1998).

Application of the FTBPF to the series of nutation corrections EOP 97C04 determined by the IERS allows to determine time variable spectrum of the FCN amplitude in 1984-1996. The amplitude of the FCN is of the order of 0.2-0.3 mas at maximum and decreases since 1984. The short time-span of the data does not allow to state whether this amplitude is diminishing or it has long period variations (Kosek et al., 1998R).

5.4.3. Variations of Semiannual and Subseasonal Oscillations of Polar Motion

Time variations of amplitudes of semi-annual and short period oscillations with periods of about 30, 50-60, 90-100, 120 days of polar motion and Atmospheric Angular Momentum were determined using the FTBPF (Kolaczek, 1995; Kolaczek and Kosek, 1996; Kosek et al., 1998a), (see Fig. 5.4).

5.4.4. Prediction And Irregular Variations Of Polar Motion

The auto-covariance method of polar motion prediction was elaborated (Kosek, 1997). Prediction of polar motion was computed using the least squares model of the Chandler circle and annual ellipse together with the auto-covariance prediction of the residuals obtained after removing the least squares model. The accuracy of polar motion prediction was increased by a factor two with respect to the current prediction model for about 60-250 days in the future (Kosek et al., 1995a, 1998a).

Irregular variations of short period oscillation of the polar motion containing oscillation with periods ranging from 20 to 150 days were detected. The least squares method along with the auto-covariance prediction method were applied to determine variations of amplitudes and phases of these oscillations The largest irregular variations of amplitudes of short period variations of polar motion took place at the beginning of 1985, 1988 and 1990, which correspond to the epochs of El Nino events (Kosek and Kolaczek, 1995b). Similar irregular variations occurred in the atmospheric angular momentum variations of polar motion.

5.4.5. Analysis of Polar Motion Data

An application of the stochastic models of polar motion in algorithms of processing and interpreting Earth orientation data (Kalman filter, least squares collocation) was investigated in detail. It includes both theoretical studies and numerical experiments proving clearly a superiority of the stochastic modelling over classical methods (Petrov et al., 1995; 1996). One particular phenomenon studied in detail concerns the atmospheric normal modes which have considerable effects on Earth rotation (Petrov and Brzeziński, 1996; Petrov et al., 1997; Petrov, 1998).

5.4.6. Theoretical Problems

Theoretical investigations concerning geophysical interpretation of modern Earth rotation observations and the related geophysical data have been continued. This subject includes the definition of the Celestial Ephemeris Pole (Brzeziński and Capitaine, 1995; Brzeziński, 1998) and the equation of polar motion (Brzeziński, 1996a; Zharkov et al., 1996).

5.4.7. Regional Atmospheric Angular Momentum And Its Impact On Polar Motion

The regional values of the effective atmospheric angular momentum had been obtained from the Japanese Meteorological Society data (Nastula, 1995, 1997; Nastula et al., 1998b; Nastula and Kotreleva, 1998Ra) and from NCEP/NCAR Reanalysis Project a nearly 40 years long data series (Nastula and Salstein, 1998a, 1999). In co-operation with Atmospheric and Environmental Research Inc. (AERI), Cambridge, MA, USA, ways in which different regions are responsible for excitations of high frequency polar motions were explored using a much longer and more reliable data set than it was previously available (Nastula and Salstein, 1998a, 1999). The earlier results of Salstein and Rosen from the AERI, who found maxima in the atmospheric excitation function over the extra-tropical South Pacific, North Pacific, and North Atlantic, and Nastula, who showed that short period oscillations of polar motion excitation are mainly coherent with pressure variations over northern mid-latitude land, were expanded. Here, the Eurasian region was isolated, as particularly important for exciting high frequency polar motion (Nastula and Salstein, 1998a, 1999). The propagation of signals with these periods into Eurasia from areas west of that region was also documented. One of the most interesting results is the discovery that at northern mid-latitudes the coherence between short period variations of atmospheric excitation and polar motion depends only weakly on the inverted barometric correction.

It is clear, however, that the atmospheric data do not explain all origins of the observed high frequency polar motion signals. This lack of full agreement is likely due to the existence of other forcing mechanism. With regards to the latter, a candidate for such forcing is the ocean. The addition of oceanic excitation to atmospheric excitation, leads to a substantial improvement in the agreement between observed polar motion at seasonal and intraseasonal periods (Nastula et al., 1998Rb; Nastula and Ponte, 1999). To estimate equatorial excitation functions for the ocean the velocity and mass fields from the constant density ocean model by R.M. Ponte AERI) was used.

Analysis of the influence of shallow seas (with depth not exceeding 500m) and shelf areas on the variations of atmospheric angular momentum was performed. It was concluded that for the range from 10 to 150 days the maximum contribution from seas to standard deviation of atmospheric angular momentum could be about 40% (Nastula and Manabe, 1997). This region does not have, however, any significant influence on the change of the correlation coefficient between atmospheric angular momentum and polar motion excitation. The changes of the amplitude of atmospheric angular momentum function are 100 to 300 times smaller than the changes of the shallow and shelf sea regions (Nastula and Manabe, 1997).

5.4.8. Atmospheric Excitation of Nutation

A 29-year time series of 6-hourly atmospheric angular momentum estimates has been used to study the atmospheric influence on nutation. The most important contributions are found for the pro-grade annual (0.077 mas), retrograde annual (0.053 mas), pro-grade semi-annual (0.045 mas) nutation waves, and for the constant offset of the pole (0.115 mas). These are significant results from the point of view of both the accuracy of geodetic determinations (about 0.030 mas) and the assumed accuracy of the new non-rigid Earth theory of nutation (0.005 mas). A comparison of the atmospheric excitation and VLBI nutation data yields high correlation after 1989, which is an important confirmation of the derived results (Bizouard et al., 1997; 1998).

5.4.9. Diurnal/Semidiurnal Atmospheric and Oceanic Excitation

The estimation of the atmospheric contribution to the nutation amplitudes has been extended to the whole diurnal and semi-diurnal bands in polar motion and length of day (LOD) (Petrov, 1998; Petrov et al., 1998a). The first estimation of the non-tidal oceanic influence on nutation has been completed, where "non-tidal" means fluctuations of the oceanic angular momentum driven by the observed variations of the surface atmospheric pressure and winds. Comparison with the VLBI nutation data shows that the global ocean circulation model used in the analysis is not realistic within the diurnal band, nevertheless such a comparison provides important information on how to improve this model (Petrov et al., 1998b).

5.4.10. El Nino Impact On Polar Motion

The influence of El Nino phenomena on variations of LOD have been studied by many authors. El Nino influence on polar motion has not been studied but the correlation of El Nino phenomena with the influence of Atmospheric Angular Momentum (AAM) on the polar motion was analysed. Correlation coefficients between two El Nino parameters, differences of sea surface temperatures and sea surface atmosphere pressure in different regions of Pacific Ocean and variations of correlation coefficients between (AAM) and polar motion were computed for several El Nino phenomena. El Nino phenomena disturbs correlation coefficients between the AAM and polar motion and causes also impulsive irregular variations of short period oscillations in polar motion (Kosek et al., 1995c; Kolaczek et al., 1998a, 1998b; Nuzhdina at al., 1997R).

5.4.11. Methods

The method of time variable spectrum was developed using a band bass filter (Kosek et al., 1995c). This method enables to compute the instant amplitudes of oscillations as a function of time and oscillation periods. The application of the FTBPF enables fairly good frequency resolution of an amplitude spectrum. The time and frequency resolutions of such time variable spectrum can be changed by only one lambda parameter (Kosek, 1995a; Popiński and Kosek, 1995a, 1995b). It is also possible to determine polarisation of oscillation in complex valued time series. Such analysis revealed that short period oscillations of polar motion have variable amplitudes and they are mostly counter-clockwise (Kosek et al., 1995c, 1998a).

The new auto-covariance prediction method was developed and the computation program was written. This method enables the prediction of stationary time series without any a-priori information (Kosek, 1997).

The Maximum Entropy Method (MEM) of spectral analysis has been developed for the purpose of interpretation of the observed changes in Earth orientation (Brzeziński, 1995a). This method allows to extract new information from the series of observations analysed so far and to better interpret recent, highly accurate measurements of UT1 and polar motion. A self-contained package of FORTRAN programs for the MEM spectral and cross-spectral analysis together with its description has been made available (Brzeziński, 1995b).
 
 
 
 

5.5. EARTH TIDE INVESTIGATIONS IN POLAND IN 1995–1998

Space Research Centre is carrying out tide gauge observations at the Ksiaz station (horizontal components) and at the Warsaw station (vertical component). At the Ksiaz station observations are made using Blum's pendulum and the observational series is already 25 years long. In Warsaw observations are made with the LaCoste&Romberg gravimeter and observed data are collected for 17 years (up to 1995 with the Askania Gs-11 gravimeter).

Annual series of tidal observations is analysed and then the results are published. At the same time the observations are delivered to International Centre for Earth Tides in Brussels. On the basis of existing observations the following works have been done:

• with the Institute of Geophysics in Budapest the tidal movement of the underground waters in Hungary was determined,
• with the Institute of Earth Physics in Paris the new Blum’s construction compact clinometre was tested,
• with the Institute of Geodesy and Geodetic Astronomy, Warsaw University of Technology and the Institute of Geodesy and Cartography in Warsaw and their tidal stations in Jozefoslaw and Borowa Gora the investigation of Teisseyre-Tornquist zone has began.

5.6. VARIATIONS OF SEA LEVEL

Short period oscillations with periods of 182, 120, 90, 60 and 30 days of sea level anomalies determined from altimetry measurements of TOPEX/Poseidon were detected using the FTBPF. These oscillations have amplitudes ranging in maxima from 7 cm in case of the semi-annual oscillation to 2-3 cm for the shortest ones (Kolaczek et al., 1997R). Amplitudes of these oscillations vary with geographic latitudes, longitudes and time (Kosek and Kolaczek, 1998; Kosek 1998R), (see Fig. 5.5). Time variable spectra of these variations for all oceans were computed. A correlation of these oscillations with the atmospheric pressure variations was also found (Nastula et al., 1998R)

The spectral analysis and filtration of the sea surface topography was conducted using the 2D Fourier transform. The analysis enables detection of the most energetic waves in sea surface topography and estimation of sea surface correlation along parallels and meridians. Results show that the sea surface waves along the parallels have larger periods than those along the meridians all over the oceans (Popiński and Kosek, 1998R).
 
 

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