Geoid undulations mapped by spaceborne radar altimetry

Abstract

Abstract Were the ocean and atmosphere at rest, the sea surface would describe a surface of constant gravitational potential. This equipotential surface is called the geoid. Removal of a simple reference surface (spheroid or ellipsoid) leaves geoid anomalies, or undulations of various wavelengths. Geoid undulations range in amplitude from many tens of meters for the broad high in the Azores-Iceland area (Fig. 1) down to a few centimeters for the local effect of a small seamount or minor fracture valley. The positive density anomaly (mass excess) represented by a seamount acts as a small “moon” on the ocean floor by pulling the sea towards itself, producing a bump (undulation) on the ocean surface. Conversely a sea-floor valley produces a dimple on the ocean surface. The gravity field is the normal component of the gradient of the potential. Thus, gravity anomalies (Rabinowitz and Jung, this volume) are a measure of the spacing between the geoid and neighboring potential surfaces. The slope of the geoid is described by the Vertical Deflection (V.D.), a small angle (rarely more than 0.5′ arc) between the normal to the reference ellipsoid (a function of latitude only) and the normal to the geoid (the local astronomic vertical). Both geoid and V.D. can be calculated from gravity data (Rice, 1952; Brammer, 1979). Using a gyro- stabilized telescope, von Arx (1966) measured a V.D. of 60 to 90 (±12) arc sec over the Puerto Rico Trench. This pioneering work in direct measurement of V.D. was supplanted by