A Brief Introduction to Gravity
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Transcript of A Brief Introduction to Gravity
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
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
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)