The extraction of natural gas and salt causes subsidence. This can have consequences for soil hydrology. Supervision exists primarily of the carrying out of inspections and the gathering of information on whether mining companies comply with the subsidence legislation, as explicitly laid down in an extraction, storage and measurement plans. The mining companies are primarily responsible for controlling the subsidence problem.
Supervision of subsidence includes advising the Minister of Economic Affairs on the issuing of decisions. This advice consists of an assessment of the way in which the mining company interprets the appointed rules and regulations and the measured results. Subsidence prognosis form a an exceptional element, as the give indiciations of the possible unwanted future situations. Subsidence in the Waddenzee as a result of gas extraction is bound to strict nature boundaries. These boundaries are maintained by way of a measurement and standard protocol, in which applied measurements and prognoses are rigidly interrelated.
There have been a few notable developments in the past years in the area of supervision of subsidence. For example:
- gas storage in salt caverns;
- introduction of new measurement techniques, in particular from satellites;
- shallow, instable salt caverns, which have a danger of causing collapsible craters on the ground surface as the years go by;
- the growing demand of governments and the public to continually monitor subsidence as a result of gas and salt extraction. The 'hand on the tap' method has made its entrance!
Earth tremors caused by extracting natural gas occur up to 50 times each year. The impact of these tremors is usually slight, but on occasions they do cause damage. Earth tremors are recorded by the Royal Netherlands Meteorological Institute (KNMI) http://www.knmi.nl/.
