Sep. 9, 2008 The ground in the Dutch province of South-Limburg is not as stable as had been thought. Satellite observations have shown greater localised rises than expected.
The extraction of coal led to the ground in the southern province of South-Limburg subsiding considerably until the 1970s. ‘Satellite technology has, however, demonstrated that the ground is rising again fast, up to about 10 centimetres in the last 15 years, probably due to rising ground water in the mines. It is worth noting that this rise is not taking place simultaneously across the entire area; it appears to be spreading from the eastern mining area towards the west.’ Hanssen believes that hazardous situations may arise close to fractures, especially if, as is the case near the city of Geleen, there are industrial activities in the area. In other cities, such as Heerlen, Brunssum and Kerkrade, too, large gradients have been observed in the ground shifts which could affect infrastructure and buildings.
This is one of the findings announced by Prof. Ramon Hanssen in his inaugural address at TU Delft in the Netherlands on Wednesday 3 September. Newly-developed technology has also enabled improved charting of ground subsidence in the provinces of Groningen and North-Holland. The satellites measure ground shifts down to the last millimetre.
In his inaugural address, Ramon Hanssen of the Aerospace Engineering faculty argues in favour of observation of the earth from space. He believes that manned space travel has fewer obvious applications. ‘Reconnoitring other planets appeals to our imagination, but it can also be achieved using unmanned missions; these can conduct most tasks better, more cheaply, more safely and more efficiently. Europe would do well to focus on this and not imitate the work other countries are already doing and will continue to do.’
‘There is another argument. Our knowledge of how our own planet behaves is paltry, and space travel could play a role here. It would be sensible for Europe to stop manned space travel and to concentrate on those areas in which it leads the way, such as observation of the earth.’
Ground subsidence in Groningen
Hanssen’s research group is investigating shifts in the earth’s surface using radar techniques. Today’s technology allows precision down to the last millimetre. For instance, the ground subsidence in the northern province of Groningen and Waddenzee (due to gas extraction) can be charted very precisely. ‘This method enables us to verify objectively traditional, but occasionally controversial, measurements using levelling instruments.’
Hondsbossche sea defences
The TU Delft researchers have also recently discovered that the ground around the city of Alkmaar in the province of North-Holland is subsiding (also due to gas extraction) and that this is causing the Hondsbossche sea defences to subside obliquely very slowly, one centimetre every five years. ‘The Hondsbossche sea defences are one of ten ‘weak links’ in the Dutch coastal defences. These satellite observations allow those who maintain this type of sea defences to monitor the status constantly. The satellites pass over the same area almost every week,’ Hanssen explains.
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