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Ü bungen am Rechner.

Meeresspiegel änderungen und Batimetrie aus Satellitenbeobachtungen. Ü bungen am Rechner. FU Berlin, Januar 2003 Carla Braitenberg Dipartimento Scienze della Terra, Università di Trieste, Via Weiss 1, 34100 Trieste Berg@units.it Tel +39-040-5582258 fax +39-040-575519.

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Ü bungen am Rechner.

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  1. Meeresspiegeländerungen und Batimetrie aus Satellitenbeobachtungen Übungen am Rechner. FU Berlin, Januar 2003 Carla Braitenberg Dipartimento Scienze della Terra, Università di Trieste, Via Weiss 1, 34100 Trieste Berg@units.it Tel +39-040-5582258 fax +39-040-575519

  2. Program-Exercizes on PC: • Scope of exercize: familiarize with flexure response of crust. • Load: bathymetry • The flexure model is tested trough the observed gravity field. Procedure: Take Bouguer anomaly over sea. This field is representative of crustal thickness variations. Invert field by downward continuation-you obtain first approximation of Moho. • Apply bathymetric load to flexure model. Choose parameters for crust and mantle density, and elastic thickness. Test different Te’s. • Calculate the difference field of inverted and modelled CMI. Find which Te gives best results. Describe where the deviations are greatest, and the model does not seem to be appropriate.

  3. Program-Exercizes on PC: • At last calculate isostatic gravity residual. This field can be used for correcting the bathymetry. • Programs necessary: • Surfer for visualization and handling grids • Simple file-editor • Downward continuation program: invertonly • Forward calculation of gravity field: parker • Flexure calculation: modflex • Input grids: bathymetry grid, gravity anomaly grid, gravity Bouguer grid.

  4. (Spangler Nissen and Hayes, 1995) Età del Bacino del M. Cinese Merid. : NE: 17-32 Ma SW: 15.5-24 Ma Espansione: NE: asse direzione EW SW: direzione NE-SW

  5. Database • Gravità: 2’x2’ Database Cinese. Da osservazioni satellitari. • Topografia iniziale per il calcolo della correzione di Bouguer, analisi flessurale: ETOPO5 • Rilevamenti batimetrici da nave: Database Sandwell (UCSD)

  6. Grids in input Bathymetry:bat.grd equivalent load:load.grd observed gravity:grav.grd Bouguer:boug.grd Programs: parker.exe parker.inp invertonly.exe invertonly.inp modflex.exe modflex.inp

  7. input file for modflex.for (modflex.inp) load.grd ! input file 22 ! reference depth(km) 3 ! Te (km) input file for program parker.for 22.,0.5 !ref.depth (km), density (g/cm3) flexure.grd !Moho outg.grd ! gravity output 0 !again(1) Input files examples:

  8. Input file for invertonly.for (invertonly.inp): dx, dy, nn(1), nn(2), pmin, basex, basey 4., 4., 350, 276, 100., 40, 40 d, rhov 22, -.5 Input files examples:

  9. Use program surfer overlay field maps with topograhy/bathymetry Topo-contour: create only one level (0-2500m), color= yellow. Bring to front Modify color-scale: contours-levels-level and fill. Change foreground colors. Alos: use *.lvl files 1) create maps of observed gravity anomaly and Bouguer gravity. Save maps. Give short description of properties of fields. 2) Make map of ETOPO5 topography/bathymetry First step: describe given fields and maps

  10. Gravity anomaly

  11. ETOPO5

  12. Bouguer

  13. Downward continuation: invertonly.exe check input file with editor Test 3 different cut-off wavelengths: 200, 100,50 km 1)Downward continuation of Bouguer gravity field -parameters: reference depth= 22 km density contrast= - 0.5 103 kg/m3 Cut-off wavelength=100 km 2) Describe features of CMI. Save plot. (Overlay with topography for orientation). Second step: estimate of crust-mantle interface undulations

  14. Input file for invertonly.for (invertonly.inp): dx, dy, nn(1), nn(2), pmin, basex, basey 4., 4., 350, 276, 100., 40, 40 d, rhov 22, -.5 Input files examples:

  15. Moho from inversion

  16. Flexure: modflex.exe check input file with editor Test different Te’s Te=0, 3, 10, 30, 60 km In surfer: calculate difference between the the gravity and flexure CMI: Grid-math plot residual. Reduce min-max contours. Take notice of border-effects use grid-editor to find specific values on grid. 1)Flexure loading: calculate the flexure of thin plate model with topographic load. Test different values of Te. -parameters: reference depth= 22 km density contrast= - 0.5 103 kg/m3 2) Describe features of flexure-CMI. Save plot. (Overlay with topography for orientation). Third step: apply load to flexural isostasy model

  17. Modello Te

  18. Moho from flexure model

  19. Difference between gravity and flexure Moho

  20. Gravity field: parker.exe check input file with editor In surfer: calculate difference between the the Bouguer gravity and flexure CMI. Where are the greatest anomalies. Give some clues to the reasons. 1)calculate gravity of flexure: -parameters: reference depth= 22 km density contrast= 0.5 103 kg/m3 2) Describe features of gravity field and of residual. Save plot. (Overlay with topography for orientation). Fourth step: calculate gravity field of flexure CMI

  21. Parker.for input file input file for program parker.for 22.,0.5 !ref.depth (km), density (g/cm3) flexure.grd !Moho outg.grd ! gravity output 0 !again(1)

  22. Calculate residual between gravity anomaly and field of flexure CMI. What can we obtain from the inversion of this residual? Describe features of gravity field and of residual. Save plot. (Overlay with topography for orientation). Fifth step: observed anomaly minus gravity of flexure CMI

  23. Invert the residual gravity field. This gives a first order approximation of the bathymetry. Compare the resulting bathymetry with the bathymetry from ETOPO5. Save the plots. Overlay land-areas on plot. Describe results. Where are the greatest dicrepancies? What are the short-wavelength differences due to? Notice the feature along the rift. Sixth step: downward continue the residual from step 5

  24. Input file for the downward continuation of the bathymetry Input file for invertonly.for (invertonly.inp): dx, dy, nn(1), nn(2), pmin, basex, basey 4., 4., 350, 276, 8., 40, 40 d, rhov 0, -1.64

  25. Residual gravity ready for inversion of bathymetry

  26. Inverted bathymetry. Should be integrated with ship-tracks.

  27. Profile AA’

  28. Discussion • The flexure-CMI gives a physical model of the long-wavelength component of the gravity field. It gives an alternative to the “remove-restore” method. • It is apt where there are control points for the Moho. • Improvements in the “real application”: • spatially varying Te • replace downward continuaton with inversion process. Downward continuation gives only an approximation. • Use model of sediment thickness variations.

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