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
- electrical conductivity
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
- 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.
The Earth’s magnetic field intensity was measured from an aircraft in spring 2008. The purpose was to study in particular the rock structures in areas covered by the sea.
The surveys were carried out from aircraft flying as low as at 30 metres. The plane flew in the Eurajoensalmi area along 5 to 10 km long measurement lines set about 50 metres apart. The sampling rate of the measurement instruments was several times a second which produces a sampling interval of few tens of meters along the line. The direction of the measurement lines was chosen on the basis of geological features. The measurement flights were organised by Geological Survey of Finland (GTK) and implemented in cooperation with the Finnish Aviation Academy.
The airborne measurements allow, for example, delineating boundaries of different rock types and detecting discontinuities in geological units caused by deformation zones in bedrock. Different rock types have different magnetic properties and the variation can be detected by measurements.
The area has been included in 1989 in Geological Survey of Finland national programme, using the same aircraft and survey techniques. A local helicopter survey on the Olkiluoto Island, one of the first carried out in Finland, was conducted by Suomen Malmi Oy in co-operation with a Canadian company Scintrex Ltd. The aim of the new survey in 2008 was to achieve better accuracy because the methods, positioning technology in particular, have advanced considerably in twenty years.
Seismic measurements have been carried out in Olkiluoto in conjunction with site investigations since the beginning of the 1990s. These investigations include, among others, reflection seismic 3D measurements in the drillholes 1990 – 2005 and from ground surface in 2006 and 2007 as well as the deep seismic surveys of 2008.
The thickness of soil layers was also surveyed in Olkiluoto in the 1980s and early 2000s by seismic refraction sounding. These results were re-utilised in 2007, and more effective processing of the data produced additional information on the fracturing of the top layer of bedrock.
Several downhole logging methods have been applied in the deep drillholes. These include imaging surveys (acoustic and optical), and methods describing directly the physical properties of the rock mass, like magnetization, density, conductivity, and velocity.
Among one of most important ones, the acoustic logging has been carried out in all deep drillholes and the pilot holes of ONKALO. They reveal the elastic properties of rock in the immediate vicinity of measurement holes. In future, the seismic measurements taken from the tunnel walls of ONKALO may play an important role when the final disposal facilities are investigated in closer detail.
Reflection seismic 3D investigations
were carried out from the ground level in the Olkiluoto area in June 2006 and May 2007. The purpose of the surveys was to construct a three-dimensional model of unexplored rock structures in the potential repository area.Deep seismic investigations
have been carried out in Olkiluoto and its surrounding areas in July 2008. The high resolution reflection seismic method allows exploring the bedrock to a depth of tens of kilometres.
Electrical Charged Potential Measurements
The electrical charged potential, or mise-à-la-masse method, a well established method in ore exploration, has proven to be an excellent tool for verifying the geological connections in Olkiluoto. The method involves measuring the flow of electricity through the rock. An electrical connection is often an indication of a hydraulic connection, and this knowledge is useful in predicting groundwater movements.
Charged potential measurements have been carried out continuously during 2004–2008, and they will be further continued as deemed necessary for research purposes. The surveys are carried out by measuring connections from one drillhole to another, to above ground and to ONKALO. This allows verifying the homogeneity and integrity of structures detected by other methods.
The measurements were carried out on the basis of plans laid out by Posiva and Pöyry Environment Oy. The field work was carried out by Suomen Malmi Oy while Astrock Oy participated in results processing and field supervision work.]