May 22 23, 2008 The 7 th International Conference ANALYSIS OF GRAVIMETRIC OBSERVATIONS MADE BY SCINTREX CG-5

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1 ENVIRONMENTAL ENGINEERING May 22 23, 2008 The 7 th International Conference Faculty of Environmental Engineering, Vilnius Gediminas Technical University Saulėtekio al. 11, LT Vilnius, Lithuania Tel.: ; Fax.: ; ap2008@ap.vgtu.lt ANALYSIS OF GRAVIMETRIC OBSERVATIONS MADE BY SCINTREX CG-5 Eimuntas Parseliunas, Petras Petroskevicius, Romuald Obuchovski Institute of Geodesy, Vilnius Gediminas Technical University, Saulėtekio al. 11, LT Vilnius, Lithuania, gi@ap.vgtu.lt Abstract. The gravimetric observations give valuable information on detail gravitational field. To perform the detail research of the gravitational field in any particular territory it is necessary to develop the gravimetric control, which should be based on the gravity system, based on the absolute measurements of the gravity acceleration, and on the precise gravimetric networks. Such gravimetric control could be developed by using modern equipment and nowadays technologies of the gravimetric observations. The gravimetric control of Lithuania was developed with a help of Finish, Danish, Polish and USA specialists. The absolute measurements were performed by the gravimeter JILAg-5, and the first order measurements by the gravimeters LaCoste&Romberg. The further development of the gravimetric control is based on the development of the second order network. The gravimetric measurements are performed by the automatic gravimeters Scintrex CG-5. To clear up the abilities of these instruments and their accuracy, to improve the observational methods the research was done and analyses of gravimetric observations was performed. The calibration of the gravimeters was carried out before and after field survey. The difference of the gravity acceleration of the calibration bases is 202 mgal, and two zero order and two first order points are in it. The standard deviation of the linear scale coefficients of was received. The maximal change of the linear scale coefficients during all time of field campaign was (gravimeter No 183). During the three months of period of investigations the zero drift of the gravimeters was changed from 17 till 197 µgal/day. The standard deviation of a single observation, calculated from the differences of the double measurements, equal to 9.2 µgal was received (jumps of readings were not removed). The standard deviation of the average of two measurements equal to 6.5 µgal was calculated. From the common adjustment of all observations the standard deviation of a single observation equal to 4 µgal was received, and the standard deviations of adjusted gravity acceleration values about 2 µgal were obtained. Estimation of accuracy using differences from the repeated gravity observations gave a standard deviation of the gravity acceleration values equal to 7.1 µgal. The differences between the gravity acceleration values of first order network adjustment and those, which were received from the adjustment of the gravity observations in 2007, do not exceed 19 µgal, so in some points are higher than declared (10 µgal). It is mean, that further studies of the quality of the Lithuanian National Gravimetric Network are required. Keywords: gravimeter, CG-5 autograv, gravimetric control. 1. Introduction The gravimetric observations give valuable information on detail gravitational field. That is very important for increasing an accuracy of the geoid models [1-3]. Precision of these models defines the accuracy of normal heights detection by modern space technologies [4, 5]. Reliable gravimetric observations are necessary in combination with high precision geodetic measurements [6, 7], performing geodynamic investigations, executing the resources survey and in solving other various geophysical, navigational and cartographical tasks. To perform the detail research of the gravitational field in any particular territory it is necessary to develop the gravimetric control, which should be based on the gravity system, based on the absolute measurements of the gravity acceleration, and on the precise gravimetric networks [8-10]. Such gravimetric control could be developed by using modern equipment and nowadays technologies of the gravimetric observations [11, 12]. The gravimetric control of Lithuania was developed with a help of Finish, Danish, Polish and USA specialists [13-17]. The absolute measurements were performed by the gravimeter JILAg-5, and the first order measurements by the gravimeters LaCoste&Romberg. The further development of the gravimetric control is based on the development of the second order network. The gravimetric measurements are performed by the automatic gravimeters Scintrex CG-5. To clear up the abilities of these instruments and their accuracy, to improve the observational methods the research was done and analyses of gravimetric observations was performed. 2. Characteristics of the gravimetric control of Lithuania The present gravimetric control of Lithuania consists of the zero (absolute gravimetric measurements) and first order gravimetric networks (Fig 1). 1422

2 MAŽEIKIAI ŽAGARĖ SKUODAS SALOČIAI 11 BIRŽAI 0 JONIŠKIS TELŠIAI PANDĖLYS PAKRUOJIS PASVALYS KURŠĖNAI 1 4 KRETINGA 7 ROKIŠKIS ŠIAULIAI ŠEDUVA KUPIŠKIS RIETAVAS ZARASAI KLAIPĖDA PANEVĖŽYS 6 +2 KELMĖ STULGIAI ŠILAI UTENA 11 8 ŠILALĖ 16 ŠILUTĖ VIDIŠKĖS 2 KĖDAINIAI GIRKALNIS MOLĖTAI TAURAGĖ 4 UKMERGĖ JURBARKAS JONAVA ŠIRVINTOS PABRADĖ ŠAKIAI ŽIEŽMARIAI MAIŠIAGALA PILIUONA 1 7 VIEVIS 4 VILKAVIŠKIS VILNIUS MARIJAMPOLĖ PIRČIUPIAI 10 ALYTUS ŠALČININKAI Zero order gravimetric point LAZDIJAI VARĖNA EIŠIŠKĖS 6 First order gravimetric point DRUSKININKAI Fig 1. Scheme of Lithuanian National Gravity Network and misclosures National Lithuanian Gravity Network is built on the three absolute gravity stations measured by dr. Jaakko Mäkinen (Finnish Geodetic Institute) in 1994 and 2002 [13, 14]. The ballistic gravimeter JILAg-5 was used. Sites for gravimetric points were selected in calm and geologically stable locations. Station VILNIUS cellar, PANEVEZYS basement and KLAIPEDA ground floor. Monuments for the points are reinforced concrete poles, 2 m in depth. There are brass marks fixed into monuments. Elevation of marks was determined by precise levelling. Absolute gravity measurements were organized by series of 25 falls. Time period of each series was 5 minutes, and time between series 10 minutes. Total number of measurements in 1994 and 2002 is presented in Table 1 and 2 respectively. Table 1. Absolute gravity measurements in 1994 Height, Number of Station Period mm series Number of falls VILNIUS KLAIPĖDA PANEVĖŽYS Table 2. Absolute gravity measurements in 2002 Height, Number of Station Period mm series Number of falls VILNIUS KLAIPĖDA PANEVĖŽYS Differences between gravity values at marker height g 0 determined in 1994 and 2002 are presented in Table 3. These are not large, and do not exceed 10,8 µgal, that is within an observation accuracy. Mean values v of gravity g 0 at the markers level were used for the new adjustment of National Lithuanian Gravity Network. Table 3. Absolute gravity stations Station name g, µgal 0 v g 0, µgal VILNIUS 10, ,6 KLAIPĖDA 4, ,6 PANEVĖŽYS 4, ,0 Satellite points have been established at the close neighbourhood of absolute gravimetric points. Gravity 1423

3 value at the satellite points was determined by relative observations from absolute points using LaCoste&Romberg gravimeters. A precise levelling and soil moisture measurement of the points is performed periodically. Ground water level is observed continuously. The relative gravity measurements have started in Two LaCoste & Romberg gravimeters G-618 and G-867 were used. Gravimeters were calibrated at absolute gravity stations in Denmark, Estonia, Latvia and Lithuania. The measurement campaign was organized in loops of 8 to 12 points, and the initial and final point was the same (more often absolute gravity station). Totally the 16 loops were performed. There are the 51 gravity point in the final scheme of the Lithuanian National Gravimetric Network (including 3 absolute gravity stations) (Fig 1) [10 12]. Points are located at the solid public buildings (mostly churches), either on the stable fundaments of the buildings. Relative gravity measurements were performed by LaCoste & Romberg gravimeters in 1999, 2000 and 2001 [15]. Scale factors of the gravimeters G-191, G-192, G- 193 were detected by intensive measurements on the calibration line VILNIUS PANEVEZYS. Totally were observed 117 gravity differences. Every difference was measured three times by 3 or even 6 LaCoste & Romberg gravimeters: G-1012, G-1036, G- 1078, G-1084 by Polish Institute of Geodesy and Cartography and G-191, G-192, G-193 by NIMA. There are 62 closed figures (48 triangles and 14 quadrangles) in the gravimetric network (Fig 1). Maximal closing error 27 µgal. 60% of closing errors are bellow 10 µgal [14-17]. 3. Calibration of gravimeters Scintrex CG-5 The calibration of the gravimeters was carried out before and after field survey. The calibration was done on the absolute gravity points VILNIUS and PANEVĖŽYS. The gravity acceleration difference between these points is only 68 mgal. To have bigger gravity acceleration difference the two first order points EIŠIŠKĖS and SALOČIAI were observed also. In this case gravity acceleration difference becomes 202 mgal. The calibration bases was observed twice, each time 20 observations cycles were performed. Values of linear scale coefficients are presented in the Table 4. Table 4. Linear scale coefficients Date Difference The standard deviation of the linear scale coefficients of was received. The maximal change of the linear scale coefficients during all time of field campaign was (gravimeter No 183). 4. Gravimetric observations The gravimetric observations of the second order gravimetric network were done by four gravimeters Scintrex CG-5 (No 182, 183, 184, 185). Average distance between points is 10 km. There are about 600 points in the second order network (Fig 2). Fig 2. Scheme of Lithuanian National Gravity Second Order Network (southern part; green circles first order gravimetric points, blue circles second order gravimetric points) 1424

4 In 2007 the gravimetric observations were performed at 200 points. The measurement campaign was organized in loops of 8 to 12 points, and the initial and final point was the same (more often absolute gravity station or first order point). Totally the 30 loops were performed. The performance of the each loop observations took about hours. Gravimeters were organized in two groups: 183 and 184; 182 and 185. It was performed 23 loops by the first pair of gravimeters, and 7 by second. It were performed 10 cycles of observations at each point. Duration of single cycle is 55 s. Observations were performed in the period from until The meteorological conditions for observations were worse in the second part of this period. The strong wind, rain and wet snow were heavy obstucles for making measurements. The air temperature was about 0 C, sometimes falling below zero, that caused the faster run down of batteries, therefore their capacity was enough even for the longest loop. The biggest external disturbances were observed at point JONAVA. The possible reason is a big factory, distance to which is only about 1 km. Its influence was observed at point ŠVEICARIJA (6 km) also, but at point GEGUŽINĖ (14 km) not. The errors of the readings of the gravimeter No 183 are presented in the Fig 3. Fig 4. Changes of the medians of the gravimeters readings, mgal (182 red line, 183 black line, 184 blue line, 185 green line) Values of medians are in the region from 13 to 90 µgal. It was detected that the median of readings of the gravimeter No 182 is about 4 µgal bigger, than other s gravimeters. Sometimes it was detected the strange unexplainable change of readings, especially at the beginning of measurements (Fig 5). Fortunatelly during the field campaign this phenomena was not obtained. Fig 3. The standard deviations of the readings of the gravimeter No 183, mgal (JONAVA red line, ŠVEICARIJA blue line, GEGUŽINĖ green line) 5. Analysis of laboratory testing results Analysis of long-term observations at the absolute gravity point VILNIUS showed, that observation conditions are different in various periods of observations. That s clear seen from the medians of the gravimeters readings (Table 5, Fig 4). Table 5. The medians of readings of the gravimeter, mgal Date (2007) Fig 5. Changes of readings (gravimeter No 182) In order to detect the zero drift of the gravimeters and its changes the observations at absolute gravity point VILNIUS were carried out in between field campaigns. The calculated values of zero drifts are presented in the Table 6. Table 6. Zerro drifts, mgal/day Date (2007) Difference

5 The biggest part of gravimetric observations in 2007 was done by pair of gravimeters No 182 and 183. The changes of zero drifts of both gravimeters are presented in Fig 6. conditions. From the graphics in Fig 7 and 8 it is seen, that the zero drifts during field observations have tendency for increasing, and in between filed observation at laboratory conditions for decreasing. Fig 6. Changes of zero drifts mgal/day (green line - No183, blue line No 184) It is seen, that the character of the changes of zero drifts of both gravimeters are very similar (correlation coefficient is 0.71) That proofs, that the biggest part of zero drift changes was influenced by the observation conditions. The changes of the zero drift during the field observations are shown in the Fig 7. The correlation coefficient is Fig 8. The changes of the zero drift in between the field observations, µgal/day Despite that, in general, the zero drifts of the gravimeters become bigger (Table 6). The minimal change of the zero drift has gravimeter No 183 only 17 µgal, while gravimeters No 185 and ir 197 µgal. 6. Experimental treatment and accuracy estimation of gravimetric observations The treatment of gravimetric observations was done by software package GRAVSOFT. The zero and first order points were kept as initial. In the network adjustment procedure the accuracy of these points was taken into account. In the first stage the treatment of the daily (single loop) observations made by single gravimeter was executed. Because the observations were done by two gravimeters, it is possible to compare results, for example, calculate the differences of observed gravity acceleration values (Fig 9). Fig 7. The changes of the zero drift during the field observations, µgal/day (green line - No183, blue line No 184) The changes of the zero drift in between the field observations are shown in the Fig 8. The correlation coefficient is The correlation coefficients show, that during observations the readings are less correlated and more depend from the observations and transportation Fig 9. Differencies of gravity acceleration values, observed by two gravimeters, µgal 1426

6 The standard deviation of a single observation, calculated from the differences of the double measurements, equal to 9.2 µgal was received. The standard deviation of the average of two measurements equal to 6.5 µgal was calculated. The biggest differences in single loop show the possible jumps of readings. These jumps could be detected from the simple differences of the readings also (Fig 10). Estimation of accuracy using these differences gives a standard deviation of the gravity acceleration values equal to 7.1 µgal. 7. Comparison of gravity acceleration values of two epochs One version of the of the network adjustment was done keeping as a reference only two zero order sites VILNIUS and PANEVĖŽYS. That gives a possibility to compare the gravity acceleration values at first order points. The differences between the gravity acceleration values of first order network adjustment and those, which were received from the adjustment of the gravity observations in 2007, are presented in Table 7. Table 7. Differences of gravity acceleration values of two epochs Fig 10. Differences of readings of two gravimeters, mgal Finally the common adjustment of all observations was executed. The standard deviation of a single observation equal to 4 µgal was received, and the standard deviations of adjusted gravity acceleration values about 2 µgal were obtained. Such a high increasing of the accuracy could be explained by large number of observations, performed at each gravity point and used for calculations. Additionally at the 37 points the repeated observations were performed. The differences of gravity acceleration values, calculated in the common adjustment and received only from the repeated observations, were calculated (Fig 11). No Point name Difference, µgal 1 VILNIUS -2 2 MAISIAGALA 19 3 MOLETAI 11 4 SIRVINTOS 2 5 PABRADE 2 6 UKMERGE UTENA -8 8 SALCININKAI -8 9 VIEVIS 7 10 ZIEZMARIAI EISISKES 1 12 PIRCIUPIAI 4 13 VARENA ALYTUS DRUSKININKAI 0 16 JONAVA 3 17 PANEVEZYS 2 18 BIRZAI ZARASAI 1 20 JURBARKAS TAURAGE PILIUONA 6 23 LAZDIJAI 0 24 MARIJAMPOLE 1 25 VILKAVISKIS 2 The comparison of computed gravity acceleration values shows, that differences at first order points MAIŠIAGALA, ŽIEŽMARIAI and VARĖNA are too big (should be in the region of 10 µgal as declared), and further studies of the quality of the Lithuanian National Gravimetric Network are required. 8. Conclusions Fig 11. Differences of gravity acceleration values, µgal 1. The calibration of the gravimeters was carried out before and after field survey. The difference of the gravity acceleration of the calibration bases is 202 mgal, and two zero order and two first order points are in it. The 1427

7 standard deviation of the linear scale coefficients of was received. The maximal change of the linear scale coefficients during all time of field campaign was (gravimeter No 183). 2. During the three months of period of investigations the zero drift of the gravimeters was changed from 17 till 197 µgal/day. 3. The standard deviation of a single observation, calculated from the differences of the double measurements, equal to 9.2 µgal was received (jumps of readings were not removed). The standard deviation of the average of two measurements equal to 6.5 µgal was calculated. From the common adjustment of all observations the standard deviation of a single observation equal to 4 µgal was received, and the standard deviations of adjusted gravity acceleration values about 2 µgal were obtained. Estimation of accuracy using differences from the repeated gravity observations gave a standard deviation of the gravity acceleration values equal to 7.1 µgal. 4. The differences between the gravity acceleration values of first order network adjustment and those, which were received from the adjustment of the gravity observations in 2007, do not exceed 19 µgal, so in some points are higher than declared (10 µgal). It is mean, that further studies of the quality of the Lithuanian National Gravimetric Network are required. 9. References 1. Krynski J., Lyszkowicz A. Centimetre Quasigeoid Modelling in Poland Using Heterogeneous Data Wiesenhofer B., Kuehtreiber N. Combination of Deflections of the Vertical and Gravity Anomalies in Difficult Geological Regions. In Proceedings of the 1 st International Symposium of the International Gravity Field Service Gravity Field of the Earth. 28 August 1 September, 2006 Istanbul, Turkey / General Command of Mapping. June 2007, Special Issue: 18, p ISSN Petroškevičius, P.; Paršeliūnas, E. Lietuvos teritorijos geoido skaičiavimas. Geodezija ir kartografija, Nr. 2 (22). Vilnius: Technika, 1995, p Kazakevičius, S.; Paršeliūnas, E. The usage of GPS measurements and quasigeoid digital models for determination of geodetic heights. Geodezija ir kartografija (Geodesy and Cartography), Vilnius: Technika, 1997, No 2 (26). P (in Lithuanian). 5. Denker, H.; Paršeliūnas, E. Evaluation of the European gravimetric geoid/quasigeoid EGG97 over the Lithuanian territory. Geodezija ir kartografija (Geodesy and Cartography). ISSN Vilnius: Technika, 1999, Vol XXV, No 4. P Krynski J., Centimetre geoid in Poland reality and perspectives. In: Proceedings of the Institute of Geodesy and Cartography, Warszawa. 2007, t. LIII 111, p Petroškevičius P. Gravitacijos lauko poveikis geodeziniams matavimams. V.:Technika, p. 8. Mäkinen, J.; Virtanen, H.; Qi Xian, Q. and Liang Rong, G. The Sino Finnish absolute gravity campaign in Publications of the Finnish geodetic institute. Helsinki, No p. 9. Pujol E. R., Villalta M. F. Absolute Gravity Network in Spain. Mitteilungen des Bundesamtes für Kartographie und Geodäsie, Frankfurt am Main, 2006, Band 38, p Sas A., Cisak M., Mäkinen J. The Establishment of a Vertical Gravity Calibration Baseline in Tatra Mountains, New Adjustment of the Polish Gravity Control Network. Mitteilungen des Bundesamtes für Kartographie und Geodäsie, Frankfurt am Main, 2006, Band 38, p Vitushkin L., Jiang Z., Becker M., Francis O., Germak A. The Seventh International Comparison of Absolute Gravimeters ICAG-2005 at the Bipm. In Proceedings of the 1 st International Symposium of the International Gravity Field Service Gravity Field of the Earth. 28 August 1 September, 2006 Istanbul, Turkey / General Command of Mapping. June 2007, Special Issue: 18, p ISSN Mäkinen, J.; Petroškevičius, P.; Kazakevičius, S.; Stepanovienė, J. Lietuvos valstybinio gravimetrinio nulinės klasės tinklo sudarymas. Geodezija ir kartografija, Nr. 2 (22). Vilnius: Technika, 1995, p J. Mäkinen, P. Petroškevičius. Lietuvoje atliktų absoliutinių sunkio matavimų analizė. Geodezija ir kartografija. ISSN Vilnius: Technika, 2003, XXIX t., Nr. 4, p Sas-Uhrynowski, A.; Mroczek, S.; Sas A.; Petroškevičius, P.; Obuchowski, R.; Rimkus, D. Establishment of Lithuanian national gravimetric first order network. Geodezija ir kartografija, t. XXVIII, Nr. 3. Vilnius: Technika, 2002, p Petroškevičius, P.; Paršeliūnas, E. Lietuvos atraminio gravimetrinio tinklo statistika. Geodezija ir kartografija, t. XXIX, Nr. 2. Vilnius: Technika, 2003, p Paršeliūnas. E, Petroškevičius, P. Quality of Lithuanian National Gravimetric Network. Journal of Mapping (Harita Dergisi), Ankara: General Command of Mapping, 2007, Special Issue No 18, p Petroškevičius, P.; Paršeliūnas, E. Lietuvos gravimetrinio pagrindo tyrimas ir tobulinimas. Geodezija ir kartografija, 25(2). Vilnius: Technika, 1999, p ISSN