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Department of Earth SciencesKFUPM
Gravity Methods 4
Fc
Introduction to GeophysicsIn
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Previous Lecture
Summary of Equations for Free Air and Bouguer Gravity Anomalies Terrain Correction Gravity MeasurementAbsolute Gravity Relative Gravity
Measurement of relative gravity
Free Fall MethodsWeight drop Pendulums
Gravity Units
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Using the metric system, we usually think of g as being 9.8 meters/sec2 9.8 meters/sec2.
Some unit names you will hear when gravity applications are discussed include:
9.8 m/sec2
980 Gals (or cm/sec2)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
http://www.esa.int/SPECIALS/Mars_Express/SEM21TVJD1E_0.html
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Mars's twin moons
9.8 (m/sec2) 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.
http://www.whfreeman.com/environmentalgeology/EXMOD36/F3619.HTM
Phobos Deimos
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More Detail: http://history.nasa.gov/SP-467/ch7.htm
1 mGal = 10 microns/sec2
1 milligal equals 10-5 m/sec2 or conversely
1 m/sec2 = 105 milligals.
The gravity on Phobos is 0.0056m/s2 or 560 milligals.
Are such small accelerations worth contemplating?
Can they even be measured?
Recall: GalGal = 1 cm/sec2= 0.01 m/sec2
1 mGal = 10-3 Gal
Istituto e Museo di Storia della Scienza
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1564-1642
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.
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Sensitivity: Density
Distribution
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Sensitive to Changes in Mass (Density) Distribution
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Unconsolidated Sediments Sedimentary Rocks
Igneous Rocks Metamorphic Rocks
Densities : Geologic MaterialsIn
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Data from Telford, 1990
http://www.itis-molinari.mi.it/Gravity.htmIntr
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Color table for standard gravity maps (mGal)
http://www.itis-molinari.mi.it/Gravhelp.html
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].
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Example from the North Anatolian Fault Zone in the Sea of MarmaraStandard Gravity Map
Edge Enhanced Gravity Anomalies
Gradient of the Gravity Anomaly Field
Topographic Map
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A
B
C
http://www.ifremer.fr/drogm/marmara/accueil_2.htm
Block diagrams of bathymetry In
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Example: Ghawar Region
48 48.5 49 49.5 5023.5
24.5
25.5
26.5
Longitude
Standard Gravity Map Gradient of the Gravity Anomaly Field
48 48.5 49 49.5 5023.5
24.5
25.5
26.5
Longitude
Lati
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48 48.5 49 49.5 5023.5
24.5
25.5
26.5
Longitude
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Homework, Due to May 17
Use the online tool for making a gravity map from the site http://www.itismolinari.mi.it/Gravity.htm.Make your gravity map and compare changes in the gravity to changes through regions of major tectonics? What is the gravity through the areas:
a) Red Sea b) Dead Sea Fault c) Iranian Sea d) Gulf of Aqaba e) Zagros Thrust Fold
33° E 60° E
10° N
37°N
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