A Brief Introduction to Gravity

UT Intro to Geophysics ClassMarch 10, 2009

Austin-Bergstrom Airport

Theresa Diehl, Ph.D.Research Geodesist

NOAA National Geodetic Survey

The Acceleration due to Gravity

Newton’s Law of GravitationNewton’s Law of Motion

For reference mass and a spherical earth: g ≈ 10 m/s2

Shape of the earthEllipsoid

Geoid-Mean sea level

Measuring Gravity- Relative and Absolute

.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

Relative Gravimetry:

10-6 Gal = 1 microGal (μGal)

e.g. 9.82394228 m/s2 = 982,394.228 mGal = 982,394,228 μGal

The change in gravity from place to place

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

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 Marker Pt. B: At the plane

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.

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

Interpreting Anomalies

Density

Depth

Free-Air Anomalies

NGS’ GRAV-D ProgramDifference: 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 gravitytracks

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 Islands2. Coastal & south-central AK3. Coastal CONUS, Great Lakes4. Pacific Island territories5. Interior CONUS6. North-central Alaska

(CONUS= Continental US)

Thank You!

Any Questions?

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