Gravity - part 2

What is Hooke’s Law?

F = -kx

What is a gravimeter?

Expensive instrument to measure gravity.

What do you need to correct for once you have collected raw gravity data?

• Tides

• Instrument drift

• Earth’s shape and rotation - gravity less at equator than poles

• Height above sea level

• Effect of mass of rock between station and sea-level

• Effect of local hills and valleys

• Remove regional to look at local anomalies

What is the tidal force from the moon?

1.1 × 10^-7g

How do you correct tides and drift?

Need to separate effect of instrument drift.

Drift: measure base station value - difference / time elapsed gives rate of drift.

How do you correct Earth’s shape and rotation?

Earth is an oblate sphere = slightly flat at the poles.

At the equator:

• Radius greatest

• Gravity smallest

• Surface speed greatest

How do you correct for elevation?

What is the terrain correction?

What is the Eotvos correction?

The motion of the ship relative to the rotation of Earth.

Used for at sea or anything moving.

If you map the variations in the sea surface topography what variation can you see?

Variation in gravity.

How do you correct shape and rotation?

Where does the anomaly reach peak (g_zmax)?

So the graph is at 30.

The gz max for value for that is -17.67.

How would you determine the value of x_1/2?

Gzmax - Gz_1/2

Gz_1/2 = -17.67/2 =-8.835

How would you calculate the mass deficit?

g_zmax = Gm/z²

z = 1.305 x depth (sphere)

z = depth (horizontal cylinder)