Exploration Gravity

The Objective

Gravity of the Earth varies with latitude as we have seen, due to the Earth's rotation. There are other "low-order" or broad wavelength variations in global gravity with very deep seated causes (as can be seen on a map of geoid anomalies or global gravity maps). However, there also local variations in gravity which give us information about the density structure of the (shallow) crust. Scales range from hundreds of km (e.g., a gravity survey across the Wichita Mountains to help ascertain crustal structure) to a few feet (e.g., to look for an unmarked grave).

Units

In S.I. units, Earth's gravity is approximately 9.8 m/s². The local variations in gravity used in exploration gravity are miniscule compared to Earth's total gravity, so it is convenient to introduce smaller units. The first of these is just the old c.g.s. unit of acceleration, in which gravity is approximately 980 cm/s². In honor of Galileo, because he was just an all-around cool guy, (gravity) acceleration of 1 cm/s² is called a gal.

• Systeme International (SI; MKS): m/s²
• cgs: cm/s² (gal)
• milligal = mgal = 10^-3 gal; microgal = mgal = 10^-6 gal
• gravity unit = gu = 0.1 mgal
• 1 microgal = 0.7 mph/year!
• 
• 
• typically desire survey accurate to 0.1 mgal (100 mgals);
• g ~ 980,000,000 microgals!
• to resolve anomalies, must adjust observations for several effects

History of the Gravity Method

Measuring Gravity

Absolute Measurements

• Why?
  • ballistics, defense
  • tectonic studies (e.g., glacial rebound)
  • mass of Earth
  • tying relative measurements together
• more difficult to achieve high precision than relative

• pendulum would work, but can't determine pendulum constant accurately enough

• therefore, use free-fall method

  • photograph finely-etched meter stick illuminated by short-period, high-intensity flashes at precisely controlled time intervals (~1 mgal)
  • track time of fall of corner-cube reflector with laser interferometer
  • commercial instrument now available (see paper, Carter et al., EOS)

• 
• Note: You don't have to assume an initial velocity and position of zero.  Given 3 positions and 3 times, you could solve for the acceleration (how?).  In the actual experiments, they gather many times and positions and then have an overdetermined system in which they can both improve the accuracy of their acceleration estimate but also estimate an error on that value.
  
• IGSN71 - network of absolute values
• National Geodetic Survey, 1988, Leonard, Oklahoma:

Relative Gravity