Seasat Altimetry and the South Atlantic Geoid: 1. Spectral analysis

Abstract

This paper presents a discussion of the spectral characteristics of geoid profiles derived from Seasat alitmetry in the South Atlantic. Individual profile spectra are computed and averaged in seven separate geodynamic provinces (Mid‐Atlantic Ridge, flanks, Walvis and Rio Grande aseismic ridges, and Argentine and Cape basins). Three distinct wavebands can be identified in most spectra. For wavelengths shorter than 35 km the spectra are typical of white noise with a 7‐cm rms amplitude typical of Seasat altimeter measurement noise. For wavelengths between 300 and 800 km (our uppermost limit, since original profiles are 1700 km long) the spectra are in reasonable agreement with Kaula's empirical rule and are compatible with a Pratt (thermal boundary layer) compensation mechanism, with an ill‐defined compensation depth of 30–60 km. The paper centers on the description of the 35–300 km part of the spectra. Computation of an impedance for each province requires good knowledge of its bathymetry, which is far from satisfactory as yet in the South Atlantic. We show that for wavelengths longer than 80 km the squared amplitude spectrum of bathymetry varies as the inverse of wave number. We propose that this relationship might also hold at shorter wavelengths, although it is not observed due to presently insufficient data. Using these spectral estimates, and correcting for the mean depth of the topography, we derive impedance (actually response function) estimates separately for each province. The data are well accounted for by a regional compensation scheme based on elastic flexure models. Elastic thickness, the best resolved parameter, ranges from 2 to 5 km for the ridge and flanks, and the crustal thickness from 4 to 13 km. Crustal thickness appears to be larger on the east flank, demonstrating a general lack of symmetry between east and west in the South Atlantic, which appears to hold at all wavelengths and is also expressed in the bathymetry. The Rio Grande rise involves low (but nonzero) elastic thickness and large crustal thickness (25–35 km) in agreement with previously proposed models of local (Airy) compensation. The Walvis ridge appears to imply a larger elastic thickness, in excess of 8 km if the crustal thickness is less than 20 km, which may be due to emplacement on older lithosphere. Therefore, despite a lack of sufficient bathymetric control, geoid data can be used at the scale of the entire South Atlantic to provide reasonable information on thermomechanical compensation in the various provinces. The spectral structure emphasizes the 35–300 km spectral window and should be used in constructing band‐pass filters that can separately enhance features related to plate kinematics or sublithospheric convection.