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A Brief Introduction to Gravity

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

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  1. 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

  2. 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

  3. Shape of the earth Ellipsoid Geoid- Mean sea level

  4. 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

  5. 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

  6. Retrieved a signal 1000x smaller then the measurements you started with. Raw Data from Airborne Gravimeter Apply corrections for: shape of the earth, airplane motion, height above sea level, etc.

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

  8. Interpreting Anomalies Density Depth

  9. Denali Nat’l Park Anchorage Gulf of Alaska Kodiak Island Free-Air Anomalies

  10. 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 • North-central Alaska • (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

  11. Thank You! Any Questions? Split into 2 groups- to see airborne gravimeter and GPS units

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