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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

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
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
Shape of the earth



Mean sea level

measuring gravity relative and absolute
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


relative gravimetry
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

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.

airborne corrections
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
free air anomalies


Nat’l Park


Gulf of




Free-Air Anomalies
ngs grav d program
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

thank you
Thank You!

Any Questions?

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