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 over the Reykjanes Ridge using Seasat, Geosat/ERM, ERS‐1 and TOPEX/POSEIDON altimeter data combined with ship gravity. To avoid a crossover adjustment to correct for orbital errors we used mean geoid gradients, which were obtained by averaging gradients over 60, 10 and 36 repeat cycles for Geosat/ERM, ERS‐1 and TOPEX/POSEIDON, respectively. The average standard deviations for the Geosat/ERM, ERS‐1 and TOPEX/POSEIDON mean gradients are 1.12, 3.25 and 2.01 rad respectively. The standard deviations of the ship gravity, which were assigned based on weightings derived from an analysis of crossing differences, range from 6.72 to 20.17 mgal. The necessary covariances for the least‐squares collocation computations were derived using the law of covariance propagation. Before merging with the altimeter data, the ship gravity for each leg was adjusted by removing a quadratic polynomial in time in order to match a satellite‐only gravity field in a least‐squares sense. The result of the adjustment suggests that most of the ship gravity data collected 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 difference between the satellite‐only gravity, derived using least‐squares collocation, and the adjusted ship gravity is 7.10 mgal, smaller than the rms difference of 12.47 mgal between the satellite‐only gravity derived by a Fourier transform method and the adjusted ship gravity. The rms difference between the combined gravity field, derived from both altimetry and ship gravity, and the adjusted ship gravity is 2.65 mgal, suggesting that the former has successfully absorbed the high‐frequency component of the gravity signal provided by the latter. The combined gravity field thus features a regionally uniform medium resolution from altimetry and a locally high resolution from ship gravity. The average accuracy estimate given by least‐squares collocation for the combined gravity is 5.76 mgal. The combined gravity field reveals detailed tectonic structures over the Reykjanes Ridge related to the interaction of the Iceland hot spot and sea‐floor spreading at the ridge.