# 3.5: Satellite-derived Gravity and Geoid

Recall the equation:

$f_{hyd}=\frac{\frac{5}{2}\frac{\omega^2a^3}{GM}}{1+(\frac{25}{4})(1-\frac{3}{2}\frac{C}{Ma^2})^{\frac{1}{2}}}$

In section 3.4, we wanted to solve for $$\frac{C}{Ma^2}$$, substitute values for fhyd (J2), $$\omega$$, a, and M, and calculate $$\frac{C}{Ma^2}$$ for the planet.

But how do we actually find J2 and therefore U?

##### Finding U

We can use orbits to measure J2, and we know that each reference surface of the ellipsoid has a constant value for U. We need to choose one close to the actual surface of the planet⇒this is the reference ellipsoid.

For earth, we can choose sea level.

Figure $$\PageIndex{1}$$: Sea-level

Some things to note about U and what influences it:

• There is a layer of water→ocean
• U is parallel to surface, g is perpendicular
• U moves out
• g is deflected→horizontally
• Water flows in
• Sea surface follows Uobs

Figure $$\PageIndex{2}$$: Uobs and Uref

• Land and water
• Uref→cuts through these
• Uobs→undulations due to mass
• Uobs-Uref→geoid height→N

+N over +m and -N over -m. |N|→$$\pm$$100

But beware...

Slope usage ie geoid surface$$\neq$$equipotential surface, but it is often used to mean this. Also, the geoid height=the geoid anomaly.

Before we said use orbits to get Uobs→Uref-ellipse=geoid height

$U\propto\frac{Gm}{r}+\frac{1}{r^3}+...$

Smaller scale features on the surface are harder to measure. But what if you could measure U at sea level? This is called satellite altimetry. We can compare satellite gravity (altimetry) to geoid height. This can show us lots of smaller scale features such as seamounts.

Figure $$\PageIndex{3}$$: Seamounts

If we look at the below picture of the Atlantic, what is the biggest topographic signal?

FIGURE Magali Atlantic ocean picture

⇒The continents vs the oceans. There is no step function in the zone at the edge, and instead we see a "ring". Why?

Figure $$\PageIndex{5}$$: No Step Function

How do we know which features are compensated and which are uncompensated and how they contribute to gravity anomalies?

Figure $$\PageIndex{6}$$: Uncompensated and Compensated Topography

Smaller features

• seamounts
• fracture zones
• trenches
• These are all compensated and supported by the strength of the lithosphere
• ⇒gravity anomaly

Bigger features

• continents
• mountain ranges
• compensated or "floating"
• Uncompensated and supported by viscous stresses, often mantle has flowed away⇒isotasy
• ⇒no gravity anomaly

Hudson Bay

Let's look at an example of Hudson Bay during the last ice age. During the ice age, the bay had a kilometers thick ice cap sitting on top. This is a compensated example because the lithosphere deforms. Flashing forward to the bay today, the ice has melted and the lithosphere has slowly rebounded from its deformed state. It is now in an uncompensated state. An important thing to note is that the bay may never fully return to its undeformed state, as the bay is filling in with sediments.

Figure $$\PageIndex{7}$$: Hudson Bay Deformation

Subduction Zones

Figure $$\PageIndex{8}$$: Andes Subduction Zone

The mountain range above (the Andes) is compensated⇒no $$\Delta g$$ or N. The slab is not compensated. However, determining if it is compensated or not can be a bit tricky. Let's take a closer look at the figure below.

Figure $$\PageIndex{9}$$: Density Anomaly

In earth, there are viscous layers as seen below.

The lithosphere is highly viscous ie cold. The upper mantle/asthenosphere is weak and has a low viscosity. The lower mantle, due to different minerals, has a high viscosity.