Sea surface and the Geoid
Early Maps of the Oceans
(figures from Columbia Earth Institute library http://www.earthinstitute.columbia.edu/library/MarieTharp.html)
Sea surface gravity
(Figures from the National Geophysical Data Center: http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML)
- Smith and Sandwell, 1997 (Fig.)
- Distribution of mass on the seafloor warps sea level
- The resulting bumps on the sea surface can be measured by satellite altimetry (Fig.)
- Ridges create gravity high, trenches create gravity lows
- The result is not topography, but can be be interpreted for topography (Fig.)
- Satellites are too high for hi-res direct measurements of gravity (profile) (anomaly)
- The 1/r2 of the law of gravitation makes details decay over short distances
Who cares about the Geoid?
- What shape is the sea surface?
- Sea surface is close to oblate spheroid or ellipsoid (a flattened sphere) (Fig.)
- The geoid is the difference between sea level and the ellipsoid (Fig.)
- The geoid can be thought of as "mean sea level". On land the surface is defined by the sea level in imaginary canals connected to the ocean.
- "little g":
- is always perpendicular to the geoid
- is not necessarily perpendicular to ellipsoid
- does not necessarily point directly to the center of the earth
- The geoid varies from the ellipsoid by up to 100 m (Fig.)
- Geoid highs - max. +73 m (New Guinea)
- Geoid lows - min. -105 m (off S-India)
- Features of the US geoid include Yellowstone and the Michigan low (Fig.)
- We all do
- It is the reference level against which elevations are measured
- It skews the orbit of satellites
- Earth scientists do
- It tells us about the deep interior of Earth
- Surveyors do
- It purturbs leveling measurements (changes the direction of "straight down")
- That is, it changes the direction of "straight down"