GRAVITY-ANOMALIES DERIVED FROM SEASAT, GEOSAT, ERS-1 AND TOPEX POSEIDON ALTIMETRY AND SHIP GRAVITY - A CASE-STUDY OVER THE REYKJANES RIDGE/

We have employed least-squares collocation to derive a gravity field o ver the Reykjanes Ridge using Seasat, Geosat/ERM, ERS-1 and TOPEX/POSE IDON altimeter data combined with ship gravity. To avoid a crossover a djustment to correct for orbital errors we used mean geoid gradients, which were obtained by averaging gradients over 60, 10 and 36 repeat c ycles for Geosat/ERM, ERS-1 and TOPEX/POSEIDON, respectively. The aver age standard deviations for the Geosat/ERM, ERS-1 and TOPEX/POSEIDON m ean gradients are 1.12, 3.25 and 2.01 mu rad respectively. The standar d deviations of the ship gravity, which were assigned based on weighti ngs derived from an analysis of crossing differences, range from 6.72 to 20.17 mgal. The necessary covariances for the least-squares colloca tion computations were derived using the law of covariance propagation . Before merging with the altimeter data, the ship gravity for each le g was adjusted by removing a quadratic polynomial in time in order to match a satellite-only gravity field in a least-squares sense. The res ult of the adjustment suggests that most of the ship gravity data coll ected in the 1960s and the 1970s have average offsets of about 14 mgal , which is close to the offset of the Potsdam Datum. The rms differenc e between the satellite-only gravity, derived using least-squares coll ocation, and the adjusted ship gravity is 7.10 mgal, smaller than the rms difference of 12.47 mgal between the satellite-only gravity derive d by a Fourier transform method and the adjusted ship gravity. The rms difference between the combined gravity field, derived from both alti metry and ship gravity, and the adjusted ship gravity is 2.65 mgal, su ggesting that che former has successfully absorbed the high-frequency component of the gravity signal provided by the latter, The combined g ravity field thus features a regionally uniform medium resolution from altimetry and a locally high resolution from ship gravity. The averag e accuracy estimate given by least-squares collocation for the combine d gravity is 5.76mgal. The combined gravity field reveals detailed tec tonic structures over the Reykjanes Ridge related to gravity field rev eals detailed tectonic stuctures over the Reykjanes Ridge related to t he interaction of the Iceland hot spot and sea-floor spreading at the ridge.