Danish scientists map white patches on the map of the world

Until recently, large areas on the bottom of the oceans were terra incognita. Using satellite measurements, scientists at DTU Space have created the first global map of the Earth’s gravitational field. Using the gravitational map, they have mapped hitherto unknown mountains and valleys on the sea bed.

“If you know the gravitational field of the sea, you can calculate the depth of the sea is and thus what the sea bed looks like. And these two things control, for example, the direction in which a tsunami moves,” says Ole Baltazar, senior scientist at DTU Space, who is heading the work on the gravitational map.

The unique aspect of Ole Baltazar’s gravitational map is that it is extremely detailed. He achieved this by using satellite-based height measurements of the surface of the sea itself to calculate the gravitational field instead of using direct measurements of the gravitational field.

“It is epoch-making that we take height measurements from a satellite and use them in this context,” says Ole Baltazar. “Height measurements have the advantage that it is possible to determine the gravitational field very locally because the water adapts to the gravitational field. This allows us to create a very detailed map, whereas gravitational measurements from a satellite are taken from an altitude of 500 km and are, therefore, much less detailed.”

"Height measurements of the surface of the sea also benefit shipping," says Ole Baltazar:

"If you want to sail under a bridge with a large ship, it is important to know how high the surface of the sea is and how high the bridge is so you know whether you have the necessary clearance to pass under the bridge. It currently takes many complicated calculations to find this clearance even if you use GPS on board the ship because different countries use different systems. With the new satellite-based height map, you immediately have the height of the surface of the sea and can decide straight away whether there is enough clearance because everything is produced in the same system as GPS."

The height of the surface of the sea is found by measuring the distance from the satellite to the surface of the sea using radar beams transmitted to the Earth at the speed of light. The satellite registers the time it takes for the radiation to be reflected back and the distance is calculated from this time. In this way, scientists can measure hills and valleys on the surface of the sea created on account of differences in the depth of the sea and materials beneath the sea bed.

In pursuance of the laws relating to public research, Ole Baltazar’s gravitational map is freely available to all and scientists from around the world download it from the institute website. "It is our contribution and it is nice that it generates a high level of cooperation with other countries," says Ole Baltazar.

Access the gravitational map here.