Absolute gravimetry on the agulhas negras calibration line

Abstracts

A Unha de Calibração Gravimétrica de Agulhas Negras toi estabelecida em meados dos anos 80 utilizando-se somente gravímetros relativos LaCoste & Romberg modelo "G". Cinco estações de calibração foram distribuídas irregularmente ao longo de seus aproximadamente 200 km de extensão e 627 mGal de amplitude. Utilizando o gravímetro absoluto Micro-g LaCoste A-10 #011. recentemente adquirido pelo Observatório Nacional, novas medições foram executadas nas estações de calibração, e o gradiente vertical local da gravidade foi medido em cada um desses sítios. Reportam-se aqui os novos valores absolutos de gravidade encontrados, que devem ser considerados como os atuais e mais precisos valores de referência a serem tomados em trabalhos de calibração. Uma comparação com os valores encontrados previamente, baseados somente em gravimetria relativa, tambem é apresentada.

gavimetria absoluta; linha de calibração gavimétrica; gradiente vertical da gravidade

Absolute gravimetry on the agulhas negras calibration line

Mauro Andrade de Sousa; Alcides Antonio dos Santos

Coordenação de Geofísica, Observatório Nacional, Ministério da Ciência e Tecnologia, Rua Gal. José Cristino, 77, São Cristovão, 20921-400 Rio de Janeiro, RJ, Brasil Phones [phone] / 3504-9129 / 3504-9110; Fax: [phone] - E-mails: [e-mail]; [e-mail]

ABSTRACT

The Agulhas Negras Gravity Calibration Line was established in the mid 1980s using LaCoste & Romberg gravimeters model "G" only. Five gravity stations were irregularly positioned along its 200 km length, spanning a total range of 627 mGal. The new absolute gravimeter Micro-g Solutions A-10 #011 of Observatório Nacional was taken to each calibration station and new measurements were made. The local vertical gravity gradient was also measured at each calibration station. This report shows their new gravity values and compares them with previously published values based solely on relative gravimetry. These new absolute values of local gravity should be taken as updated reference values in any calibration work along the Agulhas Negras Calibration Line.

Keywords: absolute gravimetry, gravity calibration line, vertical gravity gradient.

RESUMO

A Unha de Calibração Gravimétrica de Agulhas Negras foi estabelecida em meados dos anos 80 utilizando-se somente gravímetros relativos LaCoste & Romberg modelo "G". Cinco estações de calibração foram distribuídas irregularmente ao longo de seus aproximadamente 200 km de extensão e 627 mGal de amplitude. Utilizando o gravímetro absoluto Micro-g LaCoste A-10 #011. recentemente adquirido pelo Observatório Nacional, novas medições foram executadas nas estações de calibração, e o gradiente vertical local da gravidade foi medido em cada um desses sítios. Reportam-se aqui os novos valores absolutos de gravidade encontrados, que devem ser considerados como os atuais e mais precisos valores de referência a serem tomados em trabalhos de calibração. Uma comparação com os valores encontrados previamente, baseados somente em gravimetria relativa, tambem é apresentada.

Palavras-chave: gavimetria absoluta, linha de calibração gavimétrica, gradiente vertical da gravidade.

INTRODUCTION

Gravity Calibration Lines have been in use for some time not only in Brazil but also in other regions worldwide, e.g. Wessells (1985), Wenzel (1996), Vieira et al. (2002) and Timmen et al. (2006). Calibration lines exploit the well-known fact that gravity varies with latitude as well as height.

The Agulhas Negras Gravity Calibration Line (ANGCL) was established in the mid 1980s as a tool to assess the operational capabilities of relative gravimeters used by either Brazilian academic institutions or the private sector. After some reconnaissance work, as early as 1982, teams of Observatório Nacional (ON) and Instituto de Astronomia, Geofísica e Ciencias Atmosféricas (IAG-USP) started measuring relative gravity intervals between pairs of stations, beginning at the IGSN 71 station 40123A at ON, and ending at the ON benchmark at the Ibama Control Gate #03 of Itatiaia National Park. This calibration line is about 230 km in length, has a maximum elevation difference of 2 400 m, a maximum latitude difference of 31 arc minutes and a gravity range of 627 mGal (Fig. 1). Over the years, a large number of relative gravity observations were undertaken between five gravity stations irregularly distributed along this calibration line. The site provides a standard, high accuracy, short-range gravimeter calibration line in southeast Brazil. Government agencies as well as the private sector use this calibration line for the determination of gravimeter performance.

ABSOLUTE VS. RELATIVE GRAVIMETRY

Escobar et al. (1996) computed final gravity values for each station on the Agulhas Negras Calibration Line. Absolute gravity values and associated uncertainties derived from the adjustment of 728 gravity intervals measured with thirteen LaCoste & Romberg gravimeters from 1982 to 1994. The ANGCL was tied to the Brazilian Fundamental Gravity Network (BFGN) through stations 010176 Rio de Janeiro "A" (IGSN 40123A) and 016080 Rio de Janeiro "C", whose absolute gravity values were obtained as part of the adjustment of the entire BFGN (Escobar, 1987; Subiza Piña & de Sousa, 2001). Due to its poor logistics, station CAL 01 Angra dos Reis was occupied in 1982 only, and has been abandoned in favor of station 010176 in Rio de Janeiro since then. Nowadays, the Agulhas Negras Gravity Calibration Line is also tied to the absolute gravity stations IfE 142 (Torge et al., 1994) and to pier "B" at the Laboratory of Gravimetry of Observaório Nacional in Vassouras (RJ).

The recent acquisition of the absolute gravimeter Micro-g La-Coste A-10 #011 allowed new and independent measurements to be taken along the Agulhas Negras Gravity Calibration Line. This meter directly measures the vertical acceleration of gravity (Fig. 2). A test mass is dropped hundreds of times in vacuum, and its position is recorded with a polarization-stabilized He(Ne) laser interferometer (red light@633 nm), while the time standard is provided by a 10 GHz Rubidium atomic clock. Nominal specifications for this meter are (www.microglacoste.com/a10spec.htm): repeatability of 10µGal on a high quality pier and 10µGal accuracy, as compared to another A-10. Absolute gravity measurements are expressed directly in basic SI (Système International ) units of length (meter) and time (second), thus avoiding the use of semi-empirical conversion tables, as needed for the relative spring gravimeters LC & R and the Scintrex "CG" series.

Absolute gravimetry was performed on the ANGCL from May 20 to July 07, 2006, when the acceleration due to gravity was measured with A-10 #011 at all five calibration stations. About four hours per station were usually needed to obtain GPS coordinates, to measure the local vertical gravity gradient (VGG), to set up the absolute gravimeter, to take the gravity measurement and to tear the meter down on measurement completion. Preliminary g values were computed on site using the simple spherical approximation of the VGG of 0.3086 mGal ∙ m^-1. An Oregon Scientific Baro-Thermo-Hygrometer recorded the atmospheric pressure, the air temperature and the relative humidity at each calibration site when an absolute measurement took place. So far, only the local atmospheric pressure has been used in the actual computation of absolute gravity values.

LC & R model "G" meters were used in the determination of the local vertical gravity gradient at each calibration station. However, anthropogenic noise adversely affects these measurements in all calibration stations and the overall ensuing variability in precision reflects the degree of local noise. Post-processing in the lab involved the reexamination of all field data sets, the computation of actual VGGs and their use in the final computation of absolute gravity. Geophysical corrections to the absolute measurements included Earth tides, daily polar motion and the atmospheric pressure. A full set of absolute gravity measurements on each calibration station comprised six subsets of one hundred drops each, at a rate of one drop per second. The mean g value for each subset and its experimental standard deviation were computed, the final absolute g value being the weighted mean of all six subsets. The error budget of the absolute measurements includes type A and B evaluations of known error sources according to the GUM (2003). Prior to delivery to Observatório Nacional, the laser and clock of A-10 #011 were factory calibrated, with traceability given to primary standards certified by the Bureau International des Poids et Mesures - BIPM. ^{Appendix 1} shows the identification logs describing the five calibration stations as held in the Gravity Database of Observatório Nacional.

DISCUSSION

Absolute gravity values determined at each station of the Agulhas Negras Gravity Calibration Line are compared to those determined by Escobar et al. (1996) in Table 1. Points worth noticing are:

• Error estimates of gravity values in Escobar et al. (1996) are slightly underestimated, as they do not include the relative gravimeter systematic error component. According to the LC & R Instruction Manual (LaCoste & Romberg, 2004), the repeatability of readings taken with "G" meters is 0.01 mGal. Therefore, it is reasonable to estimate that a residual systematic error limit of 0.014 mGal is present in any measurement of gravity intervals using such meters. Assuming a Gaussian error distribution and a 95% confidence interval (Vuolo, 1996), a type B variance of half of this quantity, or 0.007² mGal², should be included in the final assessment of the standard uncertainty of gravity intervals derived from relative gravimetry only. The true uncertainties of adjusted gravity values in Escobar et al. (1996) should be about 0.004 mGal higher.

• Discrepancies are always negative, which may be interpreted as a small difference between the absolute datum of this study and that used at the time the calibration line was established. Similar datum differences have also been found elsewhere, e.g. Tracey (2006).

• Larger discrepancies towards the extreme station CAL 05 Agulhas Negras further indicate a datum difference, but also point out to an insufficiently accurate meter drift model used in the relative gravimetry data reduction of Escobar et al. (1996).

CONCLUSIONS

Gravity was measured with a free fall absolute gravimeter and observed values are now available to all stations pertaining to the Agulhas Negras Gravity Calibration Line. Given the largest error found of 0.01 mGal and the total gravity range of about 627 mGal, this calibration line is accurate to 1.6 ∙ 10^-5 and allows for a better assessment of relative gravimeter performance. However, we think that the Agulhas Negras Gravity Calibration Line itself should be replaced by a better designed and properly monumented calibration line. New absolute gravity stations should be regularly positioned at about 50 mGal intervals along the whole line. Studies of periodic errors of LC & R gravimeters demand an even larger number of stations. Calibration sites must be of easier access and reasonably away from road traffic. Urgent repavement of the 14 km road between CAL 04 and CAL 05 stations needs to be done.

The Agulhas Negras Gravity Calibration Line yields about half of the total gravity range found in Brazil. As gravity diminishes with height, similar calibration lines should be established to the south and to the northeast parts of the country, e.g. the Serra Gaúcha in Rio Grande do Sul and Serra da Ibiapaba in Ceará

APPENDIX

Absolute gravity stations records of the Agulhas Negras Gravity Calibration Line (in Portuguese).

Received on September 8, 2009 / Accepted on February 24, 2010

Recebido em 8 setembro, 2009 / Aceito em 24 fevereiro, 2010

NOTES ABOUT THE AUTHORS

Mauro Andrade de Sousa. Research Associate at Coordenação de Geofísica of Observatório Nacional/MCT. Ph.D. in Geophysics, University of Newcastle upon Tyne, England (1996). Current research interests: Absolute and relative gravimetry, metrological applications of gravity, gravity modeling and interpretation.

Alcides Antonio dos Santos. Technician of the Gravity Laboratory of Coordenaçao de Geofísica of Observatório Nacional/MCT. Earned a Topographic Surveyor degree at Escola Nacional de Ciências Estatísticas - ENCE/IBGE (1984). Current research interests: Absolute and relative gravimetry and GPS positioning.

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Appendix 1

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• ESCOBAR IP. 1987. A Rede Gravimétrica Fundamental Brasileira. Observátorio Nacional, Departamento de Geofísica, 2 v, n. pag.
• ESCOBAR IP, DE SÁ NC, DANTAS JJ & DIAS FJSS. 1996. Linha de Calibraçao Gravimétrica Observatório Nacional - Agulhas Negras. Revista Brasileira de Geofísica, 14(1): 59-67.
• GUM - Guia para a Expressão da Incerteza da Medição. 2003. International Organization for Standardization, ABNT & INMETRO, Third Brazilian Edition, Rio de Janeiro, 120 pp.
• LACOSTE & ROMBERG. 2004. Instruction Manual Models G & D Gravity Meters, 127 pp.
• SUBIZA PIÑA WH & DE SOUSA MA. 2001. O estado da base de dados gravimório Nacional (BDG-ON). Situação em junho, 2001. Revista Brasileira de Geofísica, 19(3): 325-328.
• TIMMEN L, FLURY J, PETERS T & GITLEIN O. 2006. A new absolute gravity base in the German Alps. Contributions to Geophysics and Geodesy, Special Edition, 2^nd Workshop on International Gravity Field Research, Smolenice, 36: 7-20.
• TORGE W, TIMMEN L, RÖDER RH & SCHNÜLL M. 1994. The IfE absolute gravity program "South America" 1988-1991. Deutsche Geodätische Kommission, B299, Munich 45 pp.
• TRACEY R. 2006. Towards a new absolute datum for Australian gravity. Australian Earth Science Convention 2006, Melbourne, 5 pp.
• VIEIRA R, CAMACHO AG & ORTIZ E. 2002. Global adjustment for the Gravity Calibration Line Madrid-Valle de los Caidos. Física de la Tierra, 14: 127-159.
• VUOLO JH. 1996. Fundamentos da Teoria de Erros. Edgard Blücher, São Paulo, 249 pp.
• WENZEL H-G. 1996. The vertical gravimeter calibration line at Karlsruhe. Bulletin d'Information Bureau Gravimétrique International, 78: 47-56.
• WESSELLS CW. 1985. Blue Ridge Gravimeter Calibration Base Line, established 1985. NOAA Technical Memorandum NOS NGS-44, 50 pp.

Appendix 1

Publication Dates

• Publication in this collection
  03 Sept 2010
• Date of issue
  June 2010

History

• Received
  08 Sept 2009
• Accepted
  24 Feb 2010