A comparison of satellite and shipboard gravity measurements in the Gulf of Mexico

Satellite altimeters have mapped the marine geoid over virtually all of the world’s oceans. These geoid height measurements may be used to compute free air gravity anomalies in areas where shipboard measurements are scarce. Two-dimensional (2-D) transformations of geoid height to gravity are limited by currently availabie satellite track spacing and usually sacrifice short wavelength resolution. Full resolution may be retained along widely spaced satellite tracks if a one dimensional (I-D) transformation is used. Although the I-D transform retains full resolution, it assumes that the gravity field is lineated perpendicular to the profile and is therefore limited by the orientation of the profile relative to the field. We investigate the resolution and accuracy of the 1-D transform method in the Northern Gulf of Mexico by comparing satellite gravity profiles with high quality shipboard data provided by Edcon Inc. The long wavelength components of the gravity field are constrained by a low degree reference field while the short wavelength components are computed from altimeter profiles. We find that rms misfit decreases with increasing spherical harmonic degree of the reference field up to I80 degrees (A > 220 km) with negligible improvement for higher degrees. The average rms misfit for the 17 profiles used in this study was 6.5 mGal with a 180 degree reference field. Spectral coherence estimates indicate that the satellite data resolve features with wavelengths as short as 25 km. marine gravity field in remarkable detail (Haxby, 1987). Satellites such as Geos-3, Seasat, and Geosat use microwave radar to make high precision (*2 cm vertical) measurements of the sea surface height relative to a reference ellipsoid. In the absence of disturbing forces such as tides, currents, and waves, the sea surface conforms to the geoid or gravitational equipotential surface. The short wavelength components of these geoid height profiles have been used to map fracture zones, seamounts, hotspot chains, midocean ridges and a multitude of previously undiscovered features in the worlds oceans. [See Sandwell (1991) for a review of applications.] Satellite altimeter data have also been used to map continental margin structure, particularly in remote areas where little shipboard data are available (Bostrom, 1989). INTRODUCTION In recent years, satellite altimeters have mapped the gravity anomaly directly from geoid height. An alternate approach is to expand the geoid height in spherical harmonics, multiply each of the coefficients by a known factor, and sum the new series to construct gravity anomaly (Rapp and Pavlis, 1990; Haxby et al., 1983). From this theory it is clear that geoid height and gravity anomaly are equivalent measurements of the earth’s external gravity field. mula (e.g., Heiskanen and Moritz, 1967) is commonly used to compute geoid height from the gravity anomaly, and it is straightforward to invert the Stokes.formula to compute the . . . . . . ^ ‘ .. ‘ . . .^ In practice, there are several problems that must be addressed when converting satellite altimeter profiles of geoid height to marine gravity anomalies. The first problem is to measure geoid height with sufficient precision to resolve short wavelength (<100 km) gravity anomalies. As shown below and in a previous study (Sandwell and McAdoo, 1990), such high-resolution profiles are now available from the Geosat Exact Repeat Mission (Geosat/ERM). The more severe problems are related to data coverage and sampling of the geoid height since the 2-D inverse Stokes function must be integrated over the entire surface of the earth to construct For many of these applications it is desirable to compute gravity anomalies from geoid heights so the satellite data can be compared and combined with shipboard gravity measurements. The two-dimensional (2-D) Stokes’ integration forManuscript received by the Editor July 31, 1991; revised manuscript received December 23, 1991. *Scripps Institute of Oceanography, University of California at San Diego, LaJolla, CA 92093.