Ove Christian Dahl Omang

Observing Fennoscandian Gravity Change by Absolute Gravimetry

The Nordic countries Norway, Sweden, Denmark and Finland are a key study region for the research of glacial isostasy, and, in addition, it offers a unique opportunity for validating and testing the results of the GRACE experiment. Over a period of five years, the expected life time of GRACE, a temporal geoid variation of 3.0 mm is expected in the centre of the Fennoscandian land uplift area, corresponding to a gravity change of about 100 nm/s2. This is expected to be within the detection capabilities of GRACE. With terrestrial absolute gravimetry, the gravity change due to the land uplift can be observed with an accuracy of ±10 to 20 nm/s2 for a 5-year period. Thus, the terrestrial insitu observations (ground-truth) may be used to validate and test the GRACE results.

Combining altimetric/gravimetric and ocean model mean dynamic topography models in the GOCINA region

Initially, existing mean dynamic topography (MDT) models were collected and reviewed. The models were corrected for the differences in averaging period using the annual anomalies computed from satellite altimetry. Then a composite MDT was derived as the mean value in each grid node together with a standard deviation to represent its error. A new synthetic MDT was obtained from the new mean sea surface (MSS) KMS04 combined with a regional geoid updated using GRACE gravity and gravimetric data from a recent airborne survey. Compared with the composite MDT the synthetic MDT showed very similar results.

Absolute gravity values in Norway

Absolute gravity observations yield insight into geophysical phenomena such as postglacial rebound, change in the Earths hydrological cycle, sea level change, and changes in the Earths cryosphere. In the article, the first gravity values at 16 Norwegian stations measured by a modern absolute gravimeter of the FG5 type are presented. The gravity observations were corrected for Earth tides, varying atmospheric pressure, polar motion, and ocean tide loading. The ocean tide loading corrections were subject to special attention. A model based on locally observed ocean tides was applied at some of the stations. The authors estimated the total uncertainties of the gravity values to range from 3 to 4 µgal (1 µgal = 10−8 m s−2). These errors are of magnitude one order less than previously presented absolute gravity values from Norway. The final gravity values are time tagged and will change due to postglacial rebound. The maximum effect is expected to be approximately −1 µgal yr−1.

From Discrete Gravity Survey Data to a High-resolution Gravity Field Representation in the Nordic-Baltic Region

ABSTRACT The deduction of a regularly spaced gravity anomaly grid from scattered survey data is studied, addressing mainly two aspects: reduction of gravity to anomalies and subsequent interpolation by various methods. The problem is illustrated in a heterogeneous study area and contrasting test areas including mountains, low terrains, and a marine area. Provided with realistic error estimates, Least Squares Collocation interpolation of Residual Terrain Model anomalies yields the highest quality gravity grid. In most cases, the Bouguer reduction and other interpolation methods tested are equally viable. However, spline-based interpolation should be avoided in marine areas with trackwise survey data.

Updated OCTAS Geoid in the Northern North Atlantic - OCTAS07

A new gravimetric geoid (OCTAS07v2) is generated using Stokes’ formula with gravity data as input. As local gravity data, a combination of land gravity data, new and old airborne gravity data, and adjusted marine gravity data has been used. All marine gravity data has been error screened and quality assured by removing dubious data and adjusting the data when necessary. Voids in the gravity data distribution were patched with gravity data from satellite altimetry. The OCTAS07v2 geoid was estimated using the remove-compute-restore technique. The long-wavelength signal of the local gravity data was reduced using a Wong-Gore modified Stokes’ function. The long-wavelength part was represented by a global gravity field model based on GRACE data. The OCTAS07v2 geoid model was combined with the OCTAS07_MSS model to create a synthetic Mean Dynamic Topography (MDT) model. In comparison with the OCCAM MDT, our new synthetic MDT model gave a std. dev. of the residuals of 11 cm. A comparison to the main northern North Atlantic currents show many similar features

The octas project, the geoid, the mean sea surface and the mean dynamic topography

The OCTAS project, Ocean Circulation and Transport Between North Atlantic and the Arctic Sea, funded by the Norwegian Research Council, is a multidisciplinary project combining geodesy, satellite altimetry and oceanography. The main objective is to enhance the Norwegian capacity in Earth observation technologies through determining the ocean circulation and transport by using satellite techniques in combination with geodesy. The primary study area is the Fram Strait between Svalbard and Greenland. A vital objective is the determination of a high precision gravimetric geoid for the OCTAS study area. This requires an error free high quality gravimetric dataset. The process of establishing such a data set by adjusting older marine data through comparison with modern airborne and marine gravity data sets is described. Combining this updated gravity data set with data from the CHAMP and GRACE satellites an OCTAS geoid has been computed. The updated gravity field and the derived geoid may be used in validating the GOCE products. The challenges and efforts undertaken in deriving a high precision mean sea surface in a region with an abundance of sea ice and limited number of altimetric satellites is described. The derived geoid and mean sea surface is combined to form the mean dynamic topography, MDT. These MDT’s are assessed by intercomparing with oceanographically derived MDT models. The status and an overview of the project is given including identification of challenges that must be addressed in order to achieve the project objectives.