Adjustment of Gravity Observations towards a Microgal Precision

Iris Pereira Escobar, Francismar Rimoli Berquó, Andrés R.R. Papa

Observatório Nacional/MCTI, Rio de Janeiro, Brazil.
Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil.

DOI: 10.4236/ijg.2013.41010                

Abstract

Gravity observations adjustment is studied having in view to take full advantage of the modern technology of gravity measurement. We present here results of a test performed with the mathematical model proposed by our group, on the adjustment of gravity observations carried out on network design. Additionally, considering the recent improvement on instrumental technology in gravimetry, that model was modified to take into account possible nonlinear local datum scale factors, in a 1900 mGal range network, and to check its significance for microgal precision measurements. The data set of the Brazilian Fundamental Gravity Network was used as case study. With about 1900 mGal gravity range and 11 control stations the Brazilian Fundamental Gravity Network (BFGN) was used as case study. It was established mainly with the use of LaCoste & Romberg, model G, gravimeters and new additional observations with Scintrex CG-5 gravimeters. The observables involved in the model are instrumental reading, calibration functions of the gravimeters used and the absolute gravity values at the control stations. Gravity values at the gravity stations and local datum scale factors for each gravimeter were determined by least square method. The results indicate good adaptation of the tested model to network adjustments. The gravity value in the IFE-172 control station, located in Santa Maria, had the largest estimated correction of ?10.4 μGal (1 μGal = 10 nm/s²), and the largest residual for an observed reading was estimated in 0.043 reading unit. The largest correction to the calibration functions was estimated in 6.9 × 10^-6mGal/reading unit.

Keywords

Calibration; Gravimeter; Gravity Network; Adjustment; Modeling

1. Introduction

Gravimetry has experienced an important development, in both absolute and relative measurement technique. The modern gravimeters allows observations with some microgals of precision (1 microgal = 10 nm/s²), which means an improvement of at least one order of magnitude in comparison with previous instruments. This fact suggests a review in the technical processing of gravimetric observations, in order to fully explore the modern instrumental technology. This caution is especially appropriate when the points of observation are designed in network structure for reference to future surveys. In this case, the quality of results should be the best possible, ensuring high precision and homogeneity in the gravity values of the network. Therefore, the observations involve, necessarily, the superabundant data acquisition, using two or more instruments. This implies the need for adjustment of observations with appropriate mathematical model and constraints to compensate ambiguities.

Dias and Escobar [1] proposed a mathematical model for adjustment of differential gravity measurements, involving simultaneously gravity values, coefficients of the calibration functions and instrumental readings, here named D & E model. Considering the recent development technological instrumental in gravimetry, the goal of this work is testing and improvement of the D & E model for its application in adjustments of reference gravity networks.

2. The Data Set: Brazilian Fundamental Gravity Network

Figure 1 shows the distribution of the Brazilian Fundamental Gravity Network (BFGN) stations [2]. The network involves 1739 observations of 639 gravity intervals, on 534 gravity stations, including 11 control stations. Gravity intervals were measured using 16 LaCoste & Romberg (LCR), model G, gravimeters and 5 Scintrex CG5 (CG5) gravimeters. Table 1 shows the number of observation performed with each instrument. Gravity values on control stations were measured with a free fall

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	F. J. S. S. Dias and I. P. Escobar, “A Model for Adjustment of Differential Gravity Measurements with Simul taneous Gravimeters Calibration,” Journal of Geodesy, Vol. 75, No. 2, 2001, pp. 151-156. doi:10.1007/s001900100168
[2]	I. P. Escobar, “Rede Gravimétrica Fundamental Brasileira, uma Tradi??o de 10 anos no Observatório Nacional,” Revista Geofísica IPGH, Vol. 26, 1987.
[3]	W. Torge, L. Timmen, R. H. Roder and M. Schnull, “The IFE Absolute Gravity Program South America 1988 1991,” Deutsche Geodatische Kommission, Munchen, 1994.
[4]	I. P. Escobar, N. C. Sá, J. J. Dantas and F. J. S. S. Dias, “The Observatório Nacional Agulhas Negras Gravity Ca libration Line,” Brazilian Journal of Geophysics, Vol. 14, No. 1, 1996, pp. 59-68.
[5]	M. A. Sousa and A. A. Santos, “Absolute Gravimetry on the Agulhas Negras Calibration Line,” Revista Brasileira de Geofísica, Vol. 28, No. 2, 2010, pp. 165-174. doi:10.1590/S0102-261X2010000200002
[6]	E. Parseliunas, P. Petroskevicius, R. Birvydiene and R. Obuchovski, “Investigation of the Automatic Gravimeters Scintrex CG-5 and Analysis of Gravimetric Measure ments,” The 8th International Conference, Vilnius, 19-20 May 2011.
[7]	T. Oja, T. Nikolenko, K. Türk, A. Ellmann and H. Jür genson, “Calibration Results of Different Type Spring Gravimeters from the Repeated Measurements of Estonian Calibration Line,” NKG General Assembly, 2010.
[8]	T. Oja, T. Nikolenko, K. Türk, A. Ellmann and H. Jürgenson, “Analysis of Gravimetric Observations Made by Scintrex CG-5,” The 7th International Conference, Vilnius, 22-23 May 2008.
[9]	I. M. Longman, “Formulas for Computing the Tide Ac celeration Due to the Moon and the Sun,” Journal of Geo physical Research, Vol. 64, No. 12, 1959, pp. 2351-2356. doi:10.1029/JZ064i012p02351
[10]	LaCoste & Romberg, “Instruction Manual Model G&D Gravity Meters,” Austin, 2004.
[11]	C. H. Angus and B. G. Brulé, “Vibration-Induced Drift in LaCoste & Romberg Geodetic Gravimeters,” Journal of Geophysics Research, Vol. 72, No. 8, 1967, pp. 2187 2197. doi:10.1029/JZ072i008p02187
[12]	V. D. Yushkin, “Operating Experience with CG5 Gravi meters,” Measurement Techniques, Vol. 54, No. 5, 2011, pp. 486-489. doi:10.1007/s11018-011-9753-5
[13]	SCINTREX, “CG-5 Scintrex Autograv System Operation Manual,” Revision 7, Canada, 2010.
[14]	M. Lederer, “Accuracy of the Relative Gravity Measurement,” Acta Geodynamica et Geomaterialia, Vol. 6, No. 3, 2009, pp. 383-390.
[15]	F. J. S. S. Dias, “Um Modelo Matemático para Ajusta mento Gravimétrico com Aprimoramento das Fun??es de Calibra??o dos Gravímetros LaCoste & Romberg,” MSc. Thesis, Observatório Nacional, Rio de Janeiro, 1997.
[16]	I. P. Escobar “Injun??es Relativas em Ajustamento Gra vimétrico,” Publica??o do Observatório Nacional, MSc Thesis, UFPR, Curitiba, No. 2, 1998.