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A Brief Introduction to Gravity . UT Intro to Geophysics Class March 10, 2009 Austin-Bergstrom Airport Theresa Diehl, Ph.D. Research Geodesist NOAA National Geodetic Survey. The Acceleration due to Gravity. Newton’s Law of Gravitation. Newton’s Law of Motion.

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A Brief Introduction to Gravity

UT Intro to Geophysics Class

March 10, 2009

Austin-Bergstrom Airport

Theresa Diehl, Ph.D.

Research Geodesist

NOAA National Geodetic Survey

The Acceleration due to Gravity

Newton’s Law of Gravitation

Newton’s Law of Motion

For reference mass and a spherical earth:

g ≈ 10 m/s2

Shape of the earth

Ellipsoid

Geoid-

Mean sea level

Measuring Gravity- Relative and Absolute

Relative Gravimetry:

The change in gravity from place to place

.01 m/s2 = 1 Gal (Galileo)

10-3 Gal = 1 milliGal (mGal)

e.g. 9.82394 m/s2 = 982.394 Gal = 982,394 mGal

We get:

gravity changed by 10 mGals between two places

Absolute Gravimetry:

The gravity is exactly this at this point

10-6 Gal = 1 microGal (μGal)

e.g. 9.82394228 m/s2

= 982,394.228 mGal

= 982,394,228 μGal

http://www.bkg.bund.de/nn_159234/SharedDocs/Bilder/Bilder__Geodaesie/RefS-A10__FG5,property=poster.jpg

Relative Gravimetry

Pt. A: Gravity Marker

Pt. B: At the plane

Move from Pt. A to Pt.B

Portable G-meter:

6400 mGal

So: 982,455.555 mGal

Airborne Gravimeter:

10800 mGal

Portable G-meter:

6500 mGal

Absolute Gravimeter:

982,555.555 mGal

Raw Data from Airborne Gravimeter

Apply corrections for: shape of the earth, airplane motion, height above sea level, etc.

Airborne Corrections
• GOAL: Subtract out accelerations that you know to obtain anomalies due to earth’s topography and geology
• - Theoretical Gravity (Latitude Correction)
• - Free air Correction
• Eotvos Correction
• - Off-level correction
• For Bouguer anomaly:
• - Topographic Correction

Denali

Nat’l Park

Anchorage

Gulf of

Kodiak

Island

Free-Air Anomalies
NGS’ GRAV-D Program

Difference: NAVD 88 – datum derived from GRACE

• Gravity Collection Priorities:
• (Focus first on the coasts, see left)
• Puerto Rico & Virgin Islands
• Coastal & south-central AK
• Coastal CONUS, Great Lakes
• Pacific Island territories
• Interior CONUS
• (CONUS= Continental US)

20-100 km gravity gaps along coast

Terrestrial gravity points

Current data distribution is non-uniform and of varying quality, particularly with respect to large data gaps in littoral regions.

Ship gravitytracks

New Orleans

Thank You!

Any Questions?

Split into 2 groups- to see airborne gravimeter and GPS units