Solid earth geophysics

Kumpula Campus Drill Hole project successfully drilled a 370 m deep diamond drill hole on the Kumpula campus. The project is co-op between the Department of Physics and Department of Geosciences and Geography. The hole, its core and downhole logs are an important enhancement of the research and education infrastructure of geophysics and geology on the Kumpula campus. Drilling was accomplished in December.

Drill Hole

Drilling of the Kumpula Campus Drill Hole was accomplished in December 2015. The hole is 370 m deep with a diameter of 76 mm. The hole, drill core and downhole loggings will be used for research and education in geosciences.

The solid earth geophysics laboratory continued providing high quality measurement facilities in paleomagnetism and other rock physical properties.  Several foreign scientists visited the laboratory for measurements. For the purposes of geothermal research and borehole geophysics the selection of laboratory instrumentation was expanded, and novel instruments for thermal conductivity and diffusivity measurements as well as for downhole logging of several rock properties were purchased.

Two Academy of Finland Research fellow 5-year projects were awarded in 2015. Academy also granted funding for developing laboratory instrumentation for rock thermal properties (differential calorimeter) as a part of the national consortium ‘Geophysical laboratory and observatory infrastructure of FIN-EPOS (G-EPOS)’.

Deep geothermal energy in crystalline rock areas came into focus in 2015 when the Finnish energy company St1 Oy started a pilot project for producing geothermal heat for district heating. The St1 Deep Heat Project aims at drilling deep holes to 6 – 7 km depths and circulate fluids down and up in deep fracture systems for extracting the heat. We consulted St1 in geophysical and geological issues of the project.

Thermal properties of rocks are important parameters affecting the design of final disposal of spent nuclear fuel in bedrock. We reported several projects on rock thermal properties in 2015 for Posiva Oy, including both laboratory and in-situ measurements, as well as paleoclimatic inversion of ground surface temperature history.

inversion

Results of inversion modeling of ground surface temperature history at Olkiluoto, western Finland. The results arebased on a temperature-depth profile of an 1158 m deep drill hole. The inverse solution was calculated with the singular value decomposition method. Increasing the values of ε, the regularization parameter, depresses noise in the solution and reduces details in the result. The paleoclimatically most plausible solutions are obtained with ε > 0.

Deep fluids in bedrock are typically saline Ca-Na-Cl formation fluids with high contents of dissolved solids (up to 50-100 g/L or more). Such fluids have often very long residence times ranging from millions to tens of millions of years. The fluids have evolved chemically by long-term fluid-rock interaction, mostly hydrolysis of silicate minerals. Hydraulic conductivity decreases fast with depth in bedrock, which is one of the factors affecting the fluid properties over geological time scales. Long-term downhole logs of fluid salinity in the 2.5 km deep Outokumpu Deep Drill Hole were analysed for fracture hydraulic conductivity, and research on deep biosphere studies and hydrogeology of crystalline rock was continued in national and international co-operation.

Outokumpu

Electrical conductivity (salinity) logs of borehole fluid in the Outokumpu Deep Drill Hole. Time-dependent changes in the fluid EC indicate hydraulically active fracture systems.

Paleomagnetic research was carried out on several avenues, including the existence and evolution of Mesoproterozoic supercontinent Nuna (Columbia, Hudsonland) and Neoproterozoic supercontinent Rodinia by defining their paleogeography with paleomagnetic methods. Field work for this was done in Finland and Namibia. Moreover, paleomagnetic research comprised also continued magnetostratigraphical dating and environmental magnetic measurements on Miocene formations in Iran and Inner Mongolia. In Finland, paleomagnetism of Keuruu diabase dykes was showing both reversed and normal polarity magnetizations acquired about 1870 Ma ago.

in-situ measurement of thermal conductivity in a borehole

Paleomagnetic reconstruction of continents at 1.87 Ga.

 

The timing of the solid inner core of Earth has been a matter of longstanding debate in studies of the Precambrian history of the Earth. In an international co-operation the age of the inner core was estimated as 1500-1000 Ma using paleomagnetic information on earth’s magnetic field intensity variations through geological history. The results were published in a Nature paper co-authored by our team.

In planetary geophysics a new 5-year project was started concentrating on the composition of dark asteroids. These small bodies of the solar system are characterized by spectra suggesting different compositions than the averages of meteorite samples. The project investigates the effect of impact shocking on the color (darkness) of asteroids, and utilizes both theoretical and experimental data as well as experiments to solve the problem.

 

Outokumpu Deep Drill Hole

 

Reflectance spectra different asteroid groups.