Oden, September 12, 2007, Gravity measurements during the LOMROG expedition
Received from Rene Forsberg, Geodynamics Dept., Danish National Space Center
Edited by Jane Holst, Henrik Højmark and Torsten Hoelstad, GEUS
During the LOMROG expedition, measurements of the gravity acceleration were carried out by the Danish National Space Center. Changes in the Earth’s gravity field are due to changes in depth and the geological composition of the subsurface. Gravity increases generally towards the poles due to the flattening and rotation of the earth, and removal of this effect yields gravity anomalies. These anomalies are small – at the level of a millionth of g – corresponding to the conventional gravity unit mGal (1 mGal = 0.00001 m/s2).
Gravity measurement is extensively used in geophysical exploration, as well as for precise geodetic measurement with satellites and navigation. Typical magnitudes of gravity anomalies in the Arctic are around 100 mGal for large structures like the Lomonosov Ridge and Morris Jesup Rise, while smaller anomalies related to sedimentary basins are typically 5-20 mGal. To be able to interpret properly the gravity variations, the gravity data must be combined with seismics and multibeam topography. A typical seismic application for gravity is to gain independent information on structures seen in seismics (e.g., a buried basement ridge would typically be associated with a positive gravity anomaly and a salt dome with a negative anomaly).
On the Oden, gravity is measured with a marine gravimeter of the type Ultrasys Lacoste and Romberg (Serial number S-38). The instrument is in principle an ultra-precise spring with a proof mass, mounted on a gyro-stabilized platform, which is kept in balance by a complicated feedback system. The gravimeter is mounted in the Oden’s engine room close to the centre of mass of the ship (Fig. 1). The system records data every 10 sec, which, after processing and reference measurements in the harbours of Tromsø and Svalbard, yield gravity with an accuracy of approx. 1 mgal and 2-500 m resolution, depending on the speed of the Oden and ice conditions. Fig. 2 shows an example of the gravity anomalies for the southern profile from the Lomonosov Ridge to the Morris Jesup Rise. The plot also includes gravity anomalies from the Arctic Gravity Project (ArcGP), a compilation of earlier data also covering the LOMROG area mainly based on 1990s airborne gravimetry of the US Naval Research Laboratory. It is seen that the gravity data acquired at sea have more details, and a better definition of the exact location of the structures.
As a supplement to the marine gravity data, measurements with land gravimeters have been made by helicopter operations in profiles across the flanks of the Lomonosov Ridge and the Morris Jesup Rise. These operations have been less extensive than originally planned due to the malfunction of the portable echo sounder equipment, and the limited operating range of the helicopter (max. 20 nautical miles range from ship). Fig. 3 shows a gravity measurement on the sea ice; it takes only a few minutes with this type of land gravimeter (black box).
The new gravity data will be used to improve the knowledge of bathymetry, as well as understanding the anomalies from the sedimentary basins. The gravity programme will continue in 2008 where DNSC will carry out an extensive airborne gravity survey of the Arctic Ocean north of Greenland, beyond the 200 nautical miles limit to the North Pole region. Data acquired during LOMROG and earlier scientific expeditions in the region will be essential for the interpretation of the airborne gravity anomalies.