Data for Plate Tectonics Earthquakes –World wide network for detecting nuclear tests Magnetic stripes –From World War 2 submarine detection Bathymetry from above –Then satellite mapping

Global map of predicted seafloor depth [Smith and Sandwell, 1997] and elevation from GTOPO-30.

Mapping ocean floor—key to understanding Sea Floor Spreading Accomplished by military literally overnight –By satellite (starting with SEASAT) Determination of geoid by satellite observations. Because the geoid is the actual equipotential surface of the Earth (relative to the reference spheroid), it is the same as the sea surface elevation.

Effect of ocean floor on gravity and therefore ocean surface Gravitational potential V V=G(constant)xM(mass)/R(distance to mass) (Mass is dominated by whole earth) (R is dominated by distance to center of mass of earth) To stay on an equipotential (no change in V) need to increase R (distance from mass) to keep V the same.

Ship tracks of soundings

History of mapping oceans Archive the digital sounding data and assemble the data into large databases access to the 30-year mapping effort [Wessel and Watts, 1988; Smith, 1993]. Radar altimeters aboard the ERS-1 and Geosat spacecraft – have surveyed the marine gravity field over nearly all of the world's oceans to a high accuracy and moderate spatial resolution. In March of 1995, ERS-1 completed its dense mapping (~8 km track spacing at the equator) of sea surface topography between latitudes of °. July of 1995, all of the high-density radar altimeter data collected by the Geosat spacecraft were declassified.

Initial results of mapping ocean surface (geoid)

The CSR98 mean sea surface model determined by satellite altimeter data collected from GEOSAT, ERS-1, ERS-2 and TOPEX/POSEIDON missions