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Abstract:
An iGrav superconducting gravimeter has been installed on the small offshore island of Helgoland in the North Sea approximately at sea level with the overall aim of high-accuracy determination of regional ocean tide loading (OTL) and non-tidal ocean loading (NTOL) signals. For validation, a second gravimeter (gPhoneX) has been setup to observe temporal gravity variations in parallel on the upper land of Helgoland. The first part of this study covers the determination of regional OTL parameters after elimination of the height-dependent gravity component by empirical transfer functions between the local sea level from a nearby tide gauge and local attraction effects. After reduction of all gravity recordings to sea level, the regional OTL parameters from both gravimeters agree at the level of a few 0.1 nm/s². The second part of this study shows the gravity residuals that are largely dominated by regional NTOL signals. Driven by strong westerly and easterly winds, the water level fluctuates between 1.15 m above mean high water and 1.50 m below mean low water, inducing gravity increases of up to 30 nm/s² and decreases of down to -40 nm/s², respectively, during the analysed time period. These high-accuracy gravity observations will be used as “ground truth” for the calibration and validation of ocean tide models and satellite gravimetry from GRACE Follow-On as well as corresponding de-aliasing products. Finally, the SG network in Central Europe will strongly benefit from an improved knowledge of non-tidal ocean loading signals of the North Sea.
