Gravity Methods 4

1. Ali Oncel [email_address] Department of Earth Sciences KFUPM Gravity Methods 4 Introduction to Geophysics Introduction to Geophysics-KFUPM

3. Using the metric system, we usually think of g as being 9.8 meters/sec 2 9.8 meters/sec 2 . Some unit names you will hear when gravity applications are discussed include: 9.8 m/sec 2 980 Gals (or cm/sec 2 ) 980000 milligals (i.e. 1000th of a Gal) We experience similar problems in geological applications, because changes of g associated with subsurface density contrasts can be quite small . Recall: Gravity Units Introduction to Geophysics-KFUPM http://www.esa.int/SPECIALS/Mars_Express/SEM21TVJD1E_0.html

4. Mars's twin moons 9.8 (m/sec 2 ) is an easy number to recall but is not universal. If, however, we were on the Martian moon Phobos, gp is only about 0.0056meters/sec2 [m/sec2] might not be the most useful units to use on Phobos. Introduction to Geophysics-KFUPM More Detail: http://history.nasa.gov/SP-467/ch7.htm http://www.whfreeman.com/environmentalgeology/EXMOD36/F3619.HTM Phobos Deimos

5. 1 mGal = 10 microns/sec 2 1 milligal equals 10 -5 m/sec 2 or conversely 1 m/sec 2 = 10 5 milligals. The gravity on Phobos is 0.0056m/s 2 or 560 milligals . Are such small accelerations worth contemplating? Can they even be measured? Recall: Gal Gal = 1 cm/sec 2 = 0.01 m/sec 2 1 mGal = 10 -3 Gal Istituto e Museo di Storia della Scienza Introduction to Geophysics-KFUPM 1564-1642

6. The g ranges in scale from approximately 1-5 milligals. Calculated and observed gravitational accelerations are plotted across a major structure in the Valley and Ridge Province . Introduction to Geophysics-KFUPM

7. Sensitivity: Density Distribution Introduction to Geophysics-KFUPM

8. Sensitive to Changes in Mass (Density) Distribution Introduction to Geophysics-KFUPM

9. Unconsolidated Sediments Sedimentary Rocks Igneous Rocks Metamorphic Rocks Densities : Geologic Materials Introduction to Geophysics-KFUPM Data from Telford, 1990

10. http://www.itis-molinari.mi.it/Gravity.htm Introduction to Geophysics-KFUPM

11. Color table for standard gravity maps (mGal) The On-Line Gravity Map Construction Tool allows to produce gravity anomaly maps of a user-defined region in spherical Mercator map projection. All gravity data come from version 9.1 of the gravity anomaly grid of Sandwell and Smith [1997] . Introduction to Geophysics-KFUPM http://www.itis-molinari.mi.it/Gravhelp.html

12. Example from the North Anatolian Fault Zone in the Sea of Marmara Standard Gravity Map Edge Enhanced Gravity Anomalies Gradient of the Gravity Anomaly Field Topographic Map Introduction to Geophysics-KFUPM

13. Block diagrams of bathymetry Introduction to Geophysics-KFUPM A B C http://www.ifremer.fr/drogm/marmara/accueil_2.htm

14. Example: Ghawar Region Longitude Standard Gravity Map Gradient of the Gravity Anomaly Field Longitude Latitude Longitude Introduction to Geophysics-KFUPM