Gravity Methods 4

Ali [email protected]

Department of Earth SciencesKFUPM

Gravity Methods 4

Fc

Introduction to GeophysicsIn

trod

uct

ion t

o G

eop

hysi

cs-K

FUPM

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

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

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

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

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

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

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

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

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.

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

Sensitivity: Density

Distribution

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

Sensitive to Changes in Mass (Density) Distribution

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

Unconsolidated Sediments Sedimentary Rocks

Igneous Rocks Metamorphic Rocks

Densities : Geologic MaterialsIn

trod

uct

ion t

o G

eop

hysi

cs-K

FUPM

Data from Telford, 1990

http://www.itis-molinari.mi.it/Gravity.htmIntr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

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].

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

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

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

A

B

C

http://www.ifremer.fr/drogm/marmara/accueil_2.htm

Block diagrams of bathymetry In

trod

uct

ion t

o G

eop

hysi

cs-K

FUPM

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

tude

48 48.5 49 49.5 5023.5

24.5

25.5

26.5

Longitude

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM

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

Intr

od

uct

ion t

o G

eop

hysi

cs-K

FUPM