Maritime measuring stations
On three research rigs in the North Sea and the Baltic Sea, the Federal Office for Radiation Protection (BfS) has installed measuring stations for monitoring the environmental radioactivity. They are part of the ODL measuring network. Due to their geographical position they are an essential part of the early warning system in case of radiological events, e.g. in the United Kingdom or Sweden.
Measuring cosmic radiation
A maritime measuring station does not register any terrestrial radiation due to the height of the rig and due to the water between the seabed and the ODL probe completely shielding it from this type of radiation. Thus the probe only registers the cosmic portion of environmental radioactivity.
The cosmic radiation hitting earth from space is subject to a complex process of interaction with the magnetic field and the earth's atmosphere. The largest part of this radiation is mitigated in the atmosphere. Only a minor portion reaches the surface of the earth in the form of ionising radiation. Since the terrestrial portion of radiation is missing and rain effects only lead to very small increases (because rain water drains off rapidly) the cosmic effects can better be observed offshore than onshore.
Early warning in case of radiological events
Due to the specific geographical position these three measuring stations are particularly valuable for early warnings in the case of radiological events. A single increased value detected at one of the research rigs will not immediately trigger a pre-alert, however, since technical effects cannot be excluded. Yet these values can be used to estimate the intensity and length of a radiological event if a radioactive cloud reaches the mainland.
Since Geiger-Müller counters are very sensitive, measurements on a rig are challenging. Stronger winds will cause vibrations in the rig. Part of these vibrations are erroneously registered as radioactive events, due to the specific construction of the counters. There are times, in particular in autumn and winter, where the measuring values registered by the maritime measuring stations are useless because of these wind effects. The impact of wind effects can be seen in the high outliers within the time series of ambient dose rate measurements. They are marked as implausible and are not taken into account for further assessment.