(compiled by Marcin Barlik)

3.1. INTRODUCTION

This part of the Polish National Report on Geodesy is the quadrennial report of gravimetric works performed in Poland in a period from 1995 to 1998. It summarises investigations such as national gravity surveys, absolute and relative gravity measurements, non-tidal gravity changes monitoring, data handling and mapping, theoretical research on gravity field and modelling geoid and quasi-geoid for Poland, etc. These activities were conducted mainly at the following research centres listed in an alphabetic order:

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

3.2. ABSOLUTE GRAVITY DETERMINATION

The first original portable and stationary ballistic gravimeters ZZG have been constructed at the Institute of Geodesy and Geodetic Astronomy (Ząbek, 1996, 1998a). The symmetrical method of observations of the test mass vertical rise and fall is applied. The stationary absolute gravimeter is equipped with the Hewlett-Packard frequency –stabilised He-Ne laser of long period (a few months) stability of 10^-9. The laser is periodically compared to a stationary iodine-stabilised He-Ne laser (Axis/BIPM IGL1) of stability 10^-10 in two year period. The Rhode Schwartz XSD2 quartz oscillator serves as a time standard. The standards of length and time ensure the accuracy of 1 mGal as regards the gravity. The construction of the catapult is based on Sakuma’s method of launching by means of a rubber cord. The multipoint observations are executed along the ballistic trajectory of the reflector. The height of the rise (or fall) of the reflector equals to 20 cm. Approximately 300 points along the ballistic trajectory are observed, although an arbitrary number of observations can be scheduled. Observations are repeated automatically every 15-20 seconds. In average 3000 – 4000 throws are executed within a daily series of observations. Observations are corrected for tides and reduced to the height h=20 cm above the base. The accuracy of an average gravity from 24 hour series is estimated at the level of 2 – 3 mGal (Ząbek, 1998b).

The most interesting results of gravity measurements conducted so far by use of the ZZG absolute transportable gravimeter are those carried out in the framework of two international projects. The first is the International Absolute Gravity Base-station Network (IAGBN), and the second one is the Central European Initiative Project “Unification of the Gravity Central Europe Systems” (UNIGRACE CEI Project), (Barlik, 1997b), (Reinhart et al., 1998). The measurements were conducted at the following stations: Pecny (Czech Republic), Modra and Ganovce (Slovakia), Wetzell (Germany), Jozefoslaw and Krokowa (Poland). At these stations gravity was already measured using other absolute gravimeters brought from Austria, Germany, Finland and Italy. Gravity obtained with ZZG have been in very good agreement with the measurements performed using other gravimeters, also during comparison campaign of absolute instruments in Sévres in 1998. No systematic errors between them were found.

Studies concerning the influences of seismic effects on a random error of a ballistic gravimeter have been completed.
 
 

3.3. GRAVITY NATIONAL SURVEYS

Polish national gravity network, established in the sixties, could not satisfy any more the permanently increasing needs of geodesy. The works on establishment of the new network started in 1994 and has been completed in 1998. The new gravity network consists of 352 field stations, 12 absolute stations and 2 calibration base lines. Almost each gravity field station is marked by means of concrete block, buried flush with the ground level. Only 8 old stations were included to the new network. Each absolute station is located in the lowest floor of the solid building, in accordance with IAG recommendations. The description of the Polish national gravity network is given in Chapter 1 of this report.

21 absolute measurements, using ballistic gravimeters, were performed on 12 stations. Table 3.1 contains information on names of the stations, their co-ordinates and the type of gravimeter. As it is shown in Table 3.1, all gravimeters were used to measure gravity on the fundamental point.

During a period 1995–1997 gravity surveys covered also the POLREF primary network points (see Section 1 of the Report) as well as fundamental levelling benchmarks.

In the Institute of Geodesy and Geodetic Astronomy of the Warsaw University of Technology the gravimetric group has made the calibration of static quartz gravimeters using special tilt method. In 1996 a special procedure for testing the own frequency of LCR Model G and D gravimeters and outer voltmeter was developed. Each instrument operated in Poland, especially for geophysical exploration, ought to be examined in this Institute as well as on the vertical gravimetric baseline (Dg=57 mGal) established at 1986 in the Palace of Culture and Science in Warsaw which is one of the tallest buildings in the city.
 
 

3.4. INVESTIGATIONS OF THE NON-TIDAL GRAVITY CHANGES

At the Astro-Geodetic Observatory of the Warsaw University of Technology the studies of the plumb line direction changes by use of gravimetric method were continued during the period 1995 – 1998. In 1976 a meridian gravimetric baseline, 30 km long, was established to investigate variations of the north-south component of the deflection of the vertical. Periodically (3 – 4 times a year) measurements of gravity differences were conducted. Analysis of time variations of plumb line direction simultaneously obtained from astronomical and gravity observations may suggest the meteorological and hydrological sources of the detected changes. Results of these investigations are regularly sent to the Shanghai Astronomical Observatory (the international co-ordinator of such projects) as well as in more detail to “Latitude Circular” WUT (Barlik, 1996a, 1997c) (Zheng - xin Li, 1998).

Gravimetric investigations were employed to establish the Grybów Geodetic Test Field of the Warsaw University of Technology in southern Poland, 130 km South -East from Cracow. The gravity measurements in the local network were conducted using LaCoste&Romberg Models D and G as well as the Scintrex Autograv CG-3 gravimeters (Czarnecki et al., 1997), (Kalinowska et al., 1998), (Rogowski et al., 1995, 1996). Usually both orthometric and normal levelling corrections were computed to provide precise levelling with gravity information.

Precise vertical gravity gradients were determined at the control points of the GRYBOW test network for estimation of the geoid to quasi-geoid separations (Barlik, 1997a, 1998).

Gravity and satellite measurements as employed to the investigations of gravity field geometry changes were performed on Upper-Silesia test field established near Jastrzębie Zdrój – Katowice district - SATGRAVMINE Project (Barlik, 1996c). It was found, that in some places the Poincaré-Prey’s anomaly gradient is strictly correlated to the changes in inner mass distribution and topography deformations. The theory of a relation between geometric parameters of equipotential surface and horizontal gradient of these anomalies was developed. The sites of maximum horizontal gradient of gravity anomalies pointed sites with geological faults and inner stresses caused by mine exploitation.

Contribution of gravity data to geodetic investigations in the Polish part of Pieniny Klippen Belt was described. The results show a good agreement with the evaluation of mass displacement influences. They were used to the interpretation of orthometric heights obtained from precise levelling and co-ordinate changes obtained with EDM and GPS in that region.
 
 

3.5. GEOID AND THEORETICAL STUDIES ON THE GRAVITY FIELD IN

POLAND

The accuracy of geoid heights calculated for Poland in the Department of Planetary Geodesy of the Space Research Centre of PAS at the beginning of the 1990s was limited to a few decimetres. The maximum spatial resolution of the geoid was at the level of some 10 km. The years 1994 – 1998 brought major changes through improved modelling techniques, the availability of high resolution gravity data, and significant advance in the computing capability. The new geoid/quasi-geoid was calculated with an accuracy improved up to one order of magnitude. Hence a combination of ellipsoidal heights from GPS with spirit levelling data is now possible.

Experience with comparison of GPS ellipsoidal heights on levelling benchmarks (POLREF-96 network) against the new quasi-geoid model have shown the evidence of systematic offset and tilts. The technique of computing the latest QUASI97b quasigeoid height model uses gravity data to calculate high frequency corrections to the EGM96 model (Lyszkowicz, 1996a,b). For the evaluation of Stokes integral by the FFT the gravity anomalies were gridded by a fast collocation prediction procedure (KMS GEOGRID program). Next, high frequency geoid grid was combined with EGM96 geoid heights and corrections derived from digital terrain data (Lyszkowicz and Forsberg., 1995) providing final geoid heights. At last, computed geoid heights were converted into the quasi-geoid heights using Bouguer anomalies.

The final results for geoid and quasi-geoid are available in the graphical and also in computer applicable form of FORTRAN subroutine with enclosed the input grid files. The input files contain geoid/quasigeoid heights for a 1.5’ x 3.0’ grid (Lyszkowicz, 1998). Similar procedure was used to compute geoid heights for Libya (Lyszkowicz and Wahiba, 1997).

An attempt to use balloon gradiometry to determine height anomalies and geoid heights was undertaken in the Space Research Centre of the PAS (Petrovskaya and Zielinski, 1998a,b).

Local determination of height anomalies in the vicinity of Cracow was the field of research of the Department of Mining Geodesy and Environmental Engineering of the Academy of Mining and Metallurgy. The quasi-geoid was determined with use of spirit levelling and GPS data. The survey was carried out in two areas close to Cracow and the Wieliczka salt mine. It was pointed out that the ellipsoidal model (quadratic) of quasigeoid ensures the required accuracy of quasi-geoid approximation.

In the city of Cracow the average deflections of the vertical were determined using the astro-geodetic method. Deflections of vertical were also determined on the basis of differences of orthometric heights and differences of ellipsoidal heights obtained from GPS measurements. The comparison of both methods shows no important differences. Practical use of information on local shape of quasi-geoid for processing GPS measurements was discussed.

One project in reported period was devoted to the investigations of the Earth’s shape parameters as a consequence of its inner structure taking into account a theory of the gravity field of rotating fluid body (Barlik, 1996c).
 
 

References