Aniela Makowska - Monographic Series No 6
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Tatra geodynamic swith the use of geodetic methods
The Tatra Mountains are the highest rock massif of the large Carpathian mountain range. Carpathian and Tatra mountains were formed by Mesozoic folding of geosyncline type, after previous destruction of older range belonging to Waryscynska tectonic era. This folding resulted in formation of nappes. As a result the Tatra Mountains have nappe character. Crystalline core is the main element of Tatra – it forms autochthonous sediment cover, which is overlaid by higher (allochthonous) unit composed from several sheets of crystalline and sediment rocks (fig. 1).
Proven findings concerning high vertical movements within Slovakian Tatra were the main reason to undertake studies on natural Tatra geodynamics with the use of geodetic methods. In 1985 Tatra Geodynamic Test Site (TPG) was established in cooperation with geologist, W. Jaroszewski. This test site serves for studies of systematic horizontal and vertical movements of the Earth surface and for monitoring changes of parameters of gravitation field. The test site covers Tatra Mountains, Podhale, Pieniny Mountains, comprising Carpathian Range; it has common points with Slovakian network (Rysy, Skalnate Pleso and planned Swinica point).
Precise measuring methods were applied, i.e. precise geometrical trigonometric leveling, linear measurements, precise GPS satellite measurements and relative gravimetric measurements Absolute gravimetric measurements are planned to be done on two points (Kasprowy Wierch and Zakopane). State network of precise first class leveling is used on the area of Podhale region; periodic measurements of this network were done every 20 years (in 1932, 1957 and 1978). Location of the network was studied from geological point of view, while results of measurements were analyzed geodetically and interpreted geologically.
Within Tatra region leveling traverses are located along mountain valleys, e.g. Bialka Valley and Rybi Potok Valley up to Rysy basel, Sucha Woda Valley up to Swinica base. These traverses are attached to the Zakopane – Lysa Polana traverse of precise leveling. Heights of mountain peaks are determined with the use of method of precise trigonometric leveling with short sides to 300 meters and synchronic measurement of vertical angles on the span; the method was prepared by the author for mountain areas (Makowska, 1993). Moreover, heights were determined using experimental spatial linear network, connecting valley points and peaks, characterized by vertical angles higher than 30 degrees. Precise satellite GPS measurements, supported with gravimetric measurements, connect points located in Tatra, Podhale, Pieniny and in Cracow (AGH). All measurements form integrated geodetic network, which enables to determine spatial displacements of points and changes of parameters of Earth gravity. Two independent methods applied for determining heights (GPS and leveling) allow for determination of quasi-geoid in the Tatra Mountains.
Observations conducted since 1985 can be divided, according to the applied method, into two stages. At first stage, from 1985 till 1992, methods of ground geodesy were applied, i.e. precise leveling, linear measurements and relative gravimetric measurements. Two traverses were established (35 fixed points) – Lysa Polana – Rysy traverse (fig. 7) and Brzeziny – Swinica traverse (fig. 9). At second stage, starting from 1993, GPS satellite measurements and precise relative gravimetric measurements were applied. Three GPS observation campaigns were conducted (1995, 1996 and 1998) at four- and six-day intervals, simultaneously on five main TPG points (fig. 12). The results of measurements were processed using precise Berne software v. 4.2, attaching them to four permanent GPS stations. Relative gravimetric measurements on the main TPG points were done with the use of three precise La Coste & Romberg gravimeters, attaching them to the points of the modernized national gravimetric network.
Determination of leveling-satellite quasi-geoid covering Tatra and Podhale regions is another practical aspect included in the presented work.
The work also contains historical background of geodetic studies conducted on the area of the Tatra Mountains since 1985, as well as extensive geologic documentation of that area, prepared by Wojciech Jaroszewski and Krystyna Piotrowska.
The following conclusions can be drawn from the analysis of experimental works.
Recent dynamics of Tatra and Podhale region is most probably expressed by lifting of southern zone of Podhale syncline in relation to northern zone and by proven lifting of Tatra Mountains in relation to Podhale region.
Magnitudes of lifting movements at Podhale region (along Nowy Targ – Zakopane leveling line) are small (+0.2 mm/year ±0.2 mm/year), close to measuring errors.
At the edge of High Tatra (Zazadnia – Lysa Polana) movements are moderate; they reach +0.4 mm/ year ±0.2 mm/year.
Similar character of vertical movements was found to be at the edge of Slovakian Western Tatra, at Žiar Valley. Magnitudes of these movements are +0.4 mm/year ±0.6 mm/year.
Much greater lifting movements were observed within Tatra massif, at Slovakian Western Tatra. At Žiar Valey these movements between valley points and peaks reach +1.6 mm/year with error ±0.7 mm/year.
Other, lowering character of movements is observed at the region of Nowy Targ – Czarny Dunajec – Chocholow traverse. This traverse is located on the surface of young sediments of Orawa depression. Magnitudes of lowering movements are at maximum –0.17 mm/year with error ±0.2 mm/year. According to geologic expertise drainage method at this region implies high probability of non-tectonic surface deformations.
Changes of values of acceleration of gravity from particular years (relative gravimetric measurements), obtained from precise measurements, are included within ±0.020 mm/year. i.e. with limits of measurement accuracy.
It must be mentioned, that movements determined with the use of GPS method are still difficult for interpretation. Changes of values of horizontal and vertical coordinates obtained while using this method are within Tatra region 7 mm on the average; they have pulsating character and their error of determination is of the same order. Only general assumptions, which can be considered during GPS observations and measurement processing, can be given for mountainous areas:
- Observations should be done during stable weather, enabling several day’s cycle; observation periods affected with atmospheric fronts, storms, foehns should be excluded;
- In the course of measurement processing modeling of troposphere, based on observations from mountain meteorological stations, should be considered.
The results of conducted analyses confirm systematic character of discrepancies in Z-coordinate between catalog coordinates at points of POLREF network and coordinates determined for TPG, attached to EUREF, Permanent GPS network. Catalog heights are greater 60 mm on the average. Leveling-satellite quasi-geoid (based on 23 benchmarks), determined for Tatra and Podhale region, became the basis for verification of previously determined models of quasi-geoid: Quasi 97b, N2000 and 2001. In order to prepare analytical description of the leveling-satellite quasi-geoid numerical tests, using polynomials of second, third and fourth order, were conducted.
Third degree polynomial is the best fit for the study area. However, description of the quasi-geoid by one approximating function even for circular region with 15 km radius, but with very diversified relief, leads to considerable errors (vmax= 5.12 cm). When limiting study area to Podhale region accuracy of fit of approximating function increases much (vmax= 1.93 cm).
Summing up, it must be emphasized, that the prepared methods of determination of big height differences, with the use of precise trigonometric leveling and spatial linear network, are significant. It was found, that precise trigonometric leveling in the mountains is more economic and more accurate than precise geometric leveling. Next, precise linear spatial network even for steep sight lines reaching 30 degrees of inclination ensures 1 mm accuracy of determination of its vertical component.