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Ron Hackney Tectonics Special Research Centre Department of Geology & Geophysics

Are we misinterpreting gravity anomalies?. Ron Hackney Tectonics Special Research Centre Department of Geology & Geophysics The University of Western Australia. Will Featherstone Geodesy Group Department of Spatial Sciences Curtin University of Technology. What is a gravity anomaly?.

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Ron Hackney Tectonics Special Research Centre Department of Geology & Geophysics

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  1. Are we misinterpreting gravity anomalies? Ron Hackney Tectonics Special Research Centre Department of Geology & Geophysics The University of Western Australia Will Featherstone Geodesy Group Department of Spatial Sciences Curtin University of Technology

  2. What is a gravity anomaly? • Subtle differences from geodetic and geophysical perspectives • Difference between Earth gravity and gravity of reference mass distribution • Difference between: • gravity at geoid and gravity on spheroid? • observed surface gravity and a model value at the same point?

  3. gpis surface gravity measured g is normal gravity mathematically defined g is gravity at geoid downward continue gpfrom surface gp is normal gravity at the surface free-air and Bouguer corrections to g Parameters

  4. Gravity Anomaly • Defined only at the geoid • Requires (unstable) downward continuation of surface gravity

  5. Gravity Disturbance • Defined anywhere • Difference between observed gravity and theoretical value at the same point • Requires (stable) upward continuation of normal gravity

  6. Geodesy • Gravity field is used to determine the figure of the Earth (i.e. geoid) • Earth mass is preserved • no Bouguer correction retained • Gravity anomaly is appropriate

  7. Geophysics • Seek to remove large-scale gravity effects that mask anomalies of interest • Large-scale effects are from crust, mantle, whole Earth • Most removed by normal gravity of reference spheroid, some removed by regional-residual separation • Gravity disturbance is appropriate

  8. Indirect Effect • Generally use orthometric height, not spheroidal height to correct g to gP • Quantified as i.e.

  9. Influence of Indirect Effect • N < 0 (i.e. H > h), using H is an over-correction • N > 0 (i.e. H < h), using H is an under-correction

  10. Global Indirect Effect: Free-air From EGM96 N-values

  11. Global Indirect Effect: Bouguer From EGM96 N-values

  12. Indirect Effect: Himalayas Free-air Bouguer From EGM96 N-values

  13. Indirect Effect: Australia Free-air Bouguer From EGM96 N-values

  14. Indirect Effect: Antarctica Free-air Bouguer From EGM96 N-values

  15. Significance of Indirect Effect • Regional-scale studies • important where N varies rapidly (mountain belts, continental margins) • important for studies using long-wavelength gravity • l > 500 km, Chapman & Bodine, 1979 • Small-scale surveys • accounted for during regional/residual separation

  16. Conclusions • Logical for geophysicists to compute gravity disturbance • Should use spheroidal heights • Must be aware of the indirect effect • There is a need to re-examine the ‘gravity anomaly’

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