Geophysical investigations

Geophysical surveys have been applied for subsurface characterization from beginning of the site investigations, decades ago. Geophysics can be used to examine rock structures which are hidden to naked eye. This can be compared to medical examinations, such as X-ray or magnetic imaging, only this time the patient is the bedrock below Olkiluoto.

Geophysics makes use of the physical properties of the Earth, such as magnetism, electrical conductivity, density, elasticity and radioactivity. The measurements are based on detecting anomalies, that is, targets that are different in their properties than the surrounding media. For example, a water-containing deformation zone in rock mass can conduct electricity better than the surrounding sparsely fractured bedrock. Seismic waves travel slower in a fracture zone than in solid rock.

The geophysical surveys carried out in Olkiluoto are very diverse, not only in the methods used but also in the ways they are implemented. Measurements have been carried out on land, sea and air and, naturally, also underground in the drillholes and ONKALO.

The largest areal coverage will be achieved by the airborne surveys, which have the aim of establishing an overall picture of geological variation in the measurement area. From the ground level, regional gravity measurements and deep seismic soundings have been carried out to create cross sections of the Earth, on profiles which are tens of kilometres long, and reach down to depth of several kilometres, even tens of kilometres.

For acquiring data for the geological model near the final disposal facilities, the measurements on the ground have been arranged in survey lines, which are some hundreds of metres long and are placed with some tens of metres spacing. Some of these surveys cover the whole Olkiluoto Island systematically. Some have been focused on locations thought to be most essential to be covered.

The investigations carried out in drillholes or from ONKALO tunnel provide close contact with the target to be investigated, and this way also the highest resolution.