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/s 2

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

4. Measuring Gravity- Relative and Absolute.01 m/s 2 = 1 Gal (Galileo) 10 -3 Gal = 1 milliGal (mGal) e.g. 9.82394 m/s 2 = 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 Relative Gravimetry: 10 -6 Gal = 1 microGal (μGal) e.g. 9.82394228 m/s 2 = 982,394.228 mGal = 982,394,228 μGal The change in gravity from place to place http://www.bkg.bu nd.de/nn_159234/ SharedDocs/Bilder/ Bilder__Geodaesie /RefS- A10__FG5,propert y=poster.jpg

5. Move from Pt. A to Pt.B Relative Gravimetry Portable G-meter: 6500 mGal Absolute Gravimeter: 982,555.555 mGal Portable G-meter: 6400 mGal So: 982,455.555 mGal Airborne Gravimeter: 10800 mGal Pt. A: Gravity MarkerPt. B: At the plane

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

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

10. NGS’ GRAV-D Program Difference: NAVD 88 – datum derived from GRACE Current data distribution is non- uniform and of varying quality, particularly with respect to large data gaps in littoral regions. Ship gravity tracks Terrestrial gravity points New Orleans 20-100 km gravity gaps along coast Gravity Collection Priorities: (Focus first on the coasts, see left) 1.Puerto Rico & Virgin Islands 2.Coastal & south-central AK 3.Coastal CONUS, Great Lakes 4.Pacific Island territories 5.Interior CONUS 6.North-central Alaska (CONUS= Continental US)

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