Gravity measurements and the sea level

Gravity measurements and the sea level

State Geodesist Rene Forsberg, Mag. Scient. (M.SC.), The Danish National Space Center (DNSC)

The Danish National Space Center (DNSC) will be participating in the Galathea 3 expedition on the ship’s voyage from Perth to Copenhagen in order to measure the gravity and the sea level. A gravimeter onboard the ship is to gauge the level of gravity, and using GPS and laser we shall gauge the sea level. In this way we shall be able to determine what lies concealed in the inner parts of the Earth below the seabed, and determine the sea current and tides along the route of our voyage. Changes in the level of gravity reflect the weight or lightness of the materials in the inner parts of the Earth, especially the Earth’s crust and the layers closest to the surface of the seabed. Therefore measurements of the level of gravity are used, for instance, when exploring for oil and gas.

The variations in the level of gravity also reflect changes to the depth of the sea, and the depth remains relatively unknown concerning large parts of the oceans. Using the gravimeter and the ship’s multi-beam, we want to make control measurement of satellite-based measurements that show a large number of sub-marine mountains on the seabed, especially in the South Pacific, that have never been mapped before. At the same time, the purpose is to “rescue” a number of old measurements of the Earth’s gravity for which the reference level has often been uncertain, which has rendered the measurements unusable.

The sea has hills and valleys, and the level of the surface of the sea varies between different locations on the Earth by up to 80 metres relative to what might be expected due to differences in the level of Earth’s gravity. In addition, there are minor variations contributed by sea currents.

We collect data using geodetic GPS equipment, which enables us to determine the height level of the ship within a few decimetres of accuracy. In this way, we hope to make direct measurement of the height that, for instance, the sea currents create around the major oceanic current (such as the Tasmanian Current and the Gulf stream). Measurements of the sea level are closely coordinated with the oceanographic programme, so that it can be controlled whether the ship does indeed sail up and down across the ocean currents.

We shall also occasionally measure the tides along the route of the ship’s voyage when the ship is stationary in order to make other observations. We shall be able to do this because the tide lifts the waters and the ship by several metres every single day.

In addition, we shall be using a laser scanner to gauge and determine precise distributions of the height of waves in order to verify corresponding measurements made from satellites. The measurements of the sea using GPS and laser will, to the extent that the route of the voyage permits it, be coordinated with simultaneous measurements from satellites.