v28n2a02

7/27/2019 v28n2a02

1/10

7/27/2019 v28n2a02

2/10

7/27/2019 v28n2a02

3/10

MAURO ANDRADE DE SOUSA and ALCIDES ANTONIO DOS SANTOS 167

Figure 1 Road map o the Agulhas Negras Gravity Calibration Line: A Observatorio Nacional in Rio de Janeiro, B Engenheiro Passos, C Fazenda Lapa,

D Marco Zero and E Agulhas Negras. Source: Google MapsTM.

the Bureau International des Poids et Mesures BIPM. Appen-

dix 1 shows the identifcation logs describing the fve calibration

stations as held in the Gravity Database o Observat orio Nacional.

DISCUSSION

Absolute gravity values determined at each station o the AgulhasNegras Gravity Calibration Line are compared to those determined

by Escobar et al. (1996) in Table 1. Points worth noticing are:

Error estimates o gravity values in Escobar et al. (1996)

are slightly underestimated, as they do not include the re-

lative gravimeter systematic error component. According

to the LC & R Instruction Manual (LaCoste & Romberg,

2004), the repeatability o readings taken with G meters

is 0.01 mGal. Thereore, it is reasonable to estimate that a

residual systematic error limit o 0.014 mGal is present in

any measurement o gravity intervals using such meters.

Assuming a Gaussian error distribution and a 95% con-

fdence interval (Vuolo, 1996), a type B variance o hal

o this quantity, or 0.0072 mGal2, should be included in

the fnal assessment o the standard uncertainty o gravityintervals derived rom relative gravimetry only. The true

uncertainties o adjusted gravity values in Escobar et al.

(1996) should be about 0.004 mGal higher.

Discrepancies are always negative, which may be inter-

preted as a small dierence between the absolute datum

o this study and that used at the time the calibration line

was established. Similar datum dierences have also been

ound elsewhere, e.g. Tracey (2006).

Brazilian Journal o Geophysics, Vol. 28(2), 2010

7/27/2019 v28n2a02

4/10

168 AGULHAS NEGRAS GRAVITY CALIBRATION LINE

Table 1 Newly determined absolute gravity values compared to those previously ound on the Obser-

vatorio Nacional Agulhas Negras Gravity Calibration Line.

Calibration Absolute gravity (mGal)

Stat ion Site Present study Escobar et al. (1996) Discrepancy

IGSN 40123A 978 789.852 0.011 978 789.86 0.014 0.008 0.015Rio de Janeiro

CAL 02978 601.078 0.006 978 601.09 0.012 0.012 0.013

Engenheiro Passos

CAL 03978 419.483 0.009 978 419.55 0.013 0.067 0.016

Fazenda Lapa

CAL 04978 325.514 0.008 978 325.58 0.014 0.066 0.016

Marco Zero

CAL 05978 163.000 0.008 978 163.10 0.016 0.100 0.018

Posto Ibama #03

Larger discrepancies towards the extreme station CAL 05

Agulhas Negras urther indicate a datum dierence, but

also point out to an insufciently accurate meter drit

model used in the relative gravimetry data reduction o

Escobar et al. (1996).

Figure 2 Micro-g LaCoste absolute gravimeter A-10: repeatability and accu-

racy o 0.01 mGal on a quiet pier. High capacity battery or feld operation not

shown.

CONCLUSIONS

Gravity was measured with a ree all absolute gravimeter and

observed values are now available to all stations pertaining to

the Agulhas Negras Gravity Calibration Line. Given the largest

error ound o 0.01 mGal and the total gravity range o about

627 mGal, this calibration line is accurate to 1.6 105 and al-

lows or a better assessment o relative gravimeter perormance.

However, we think that the Agulhas Negras Gravity Calibration

Line itsel 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 o periodic errors o LC & R gravimeters

demand an even larger number o stations. Calibration sites must

be o easier access and reasonably away rom road trafc. Ur-

gent repavement o the 14 km road between CAL 04 and CAL 05

stations needs to be done.

The Agulhas Negras Gravity Calibration Line yields about

hal o the total gravity range ound in Brazil. As gravity dimi-

nishes with height, similar calibration lines should be established

to the south and to the northeast parts o the country, e.g. the

Serra Gauchain Rio Grandedo Sul and Serra daIbiapaba inCeara.

APPENDIX

Absolute gravity stations records o the Agulhas Negras Gravity

Calibration Line (in Portuguese).

REFERENCES

ESCOBAR IP. 1987. A Rede Gravimetrica Fundamental Brasileira. Obser-

vatorio Nacional, Departamento de Geosica, 2 v, n. pag.

ESCOBAR IP, DE SA NC, DANTAS JJ & DIAS FJSS. 1996. Linha de

Calibrac ao Gravimetrica Observatorio Nacional Agulhas Negras. Re-

vista Brasileira de Geosica, 14(1): 5967.

GUM Guia para a Expressao da Incerteza da Medicao. 2003. Internati-

onal Organization or Standardization, ABNT & INMETRO, Third Brazilian

Edition, Rio de Janeiro, 120 pp.

Revista Brasileira de Geosica, Vol. 28(2), 2010

7/27/2019 v28n2a02

5/10


Appendix 1


7/27/2019 v28n2a02

6/10


Appendix 2


7/27/2019 v28n2a02

7/10


Appendix 3


7/27/2019 v28n2a02

8/10


Appendix 4


7/27/2019 v28n2a02

9/10


Appendix 5


7/27/2019 v28n2a02

10/10


LACOSTE & ROMBERG. 2004. Instruction ManualModels G & D Gravity

Meters, 127 pp.

SUBIZA PINA WH & DE SOUSA MA. 2001. O estado da base de dados

gravimetricos do Observatorio Nacional (BDG-ON). Situacao em junho,

2001. Revista Brasileira de Geosica, 19(3): 325328.

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, 2nd Workshop on International Gravity Field

Research, Smolenice, 36: 720.

TORGE W, TIMMEN L, RODER RH & SCHNULL M. 1994. The IE abso-

lute gravity program SouthAmerica 1988-1991. Deutsche Geodatische

Kommission, B299, Munich 45 pp.

TRACEY R. 2006. Towards a new absolute datum or Australian gravity.

Australian Earth Science Convention 2006, Melbourne, 5 pp.

VIEIRA R, CAMACHO AG & ORTIZ E. 2002. Global adjustment or the

Gravity Calibration Line Madrid-Valle de los Caidos. Fsica de la Tierra,

14: 127159.

VUOLO JH. 1996. Fundamentos da Teoria de Erros. Edgard Blucher,

Sao Paulo, 249 pp.

WENZEL H-G. 1996. The vertical gravimeter calibration line at Karls-

ruhe. Bulletin dInormation Bureau Gravimetrique International, 78:

4756.

WESSELLS CW. 1985. Blue Ridge Gravimeter Calibration Base Line,

established 1985. NOAA Technical Memorandum NOS NGS-44, 50 pp.

NOTES ABOUT THE AUTHORS

Mauro Andrade de Sousa. Research Associate at Coordenac ao de Geosica o Observatorio Nacional/MCT. Ph.D. in Geophysics, University o Newcastle upon

Tyne, England (1996). Current research interests: Absolute and relative gravimetry, metrological applications o gravity, gravity modeling and interpretation.

Alcides Antonio dos Santos. Technician o the Gravity Laboratory o Coordenacao de Geosica o Observatorio Nacional/MCT. Earned a Topographic Surveyor

degree at Escola Nacional de Ciencias Estatsticas ENCE/IBGE (1984). Current research interests: Absolute and relative gravimetry and GPS positioning.


v28n2a02

Documents

Transcript of v28n2a02