Solid Earth Physics
Geophysicists take measurements on the surface of the Earth to make inferences about the physical state of the interior, from the earth crust, through the mantle right to the Earth's core. During the last decade much improved models have been obtained of how the crust, mantle and core interact along Mid-Ocean Ridges, Ocean Trenches and in the transition zone between Mantle and Core.
The distribution of seismic velocity in the Earth can be estimated from observations of how seismic energy from Earthquakes and explosions propagates. Perturbations in seismic velocities, in turn may reflect perturbations in temperature. For example, the cold plates that dive down (subduct) into the mantle at Ocean Trenches remain cold and preserve their identity deep into the lower mantle. At Uppsala research is concentrated to the Earth's crust and the upper part of the mantle. We primarily use seismic and electromagnetic techniques to image continental structures. Seismic waves are traditionally used for the prospecting of hydrocarbons in sedimentary basins, but during the last twenty years or so the so-called reflection seismic method has even proven its power for detailed studies of the crystalline crust and upper mantle. By means of large scale electromagnetic measurements on the Baltic Shield we have discovered a series of prominent zones of greatly enhanced electric conductivity runnning from the Caledonian Mountain Belt in Sweden across the Bothnian Bay into Southern Finland and Russia to Lake Ladoga. These zones mark so-called suture zones, where plates collided about 2Ga ago and thereby increased the size of the Baltic subcontinent.
The Earth's internal heat is a sustainable and environmentally friendly energy resource
Todays technology only allows us access to geothermal energy stored near the surface of Earth. Small-scale geothermal heat pumps heat up a growing part of Swedish homes. Earth temperature increases with depth, on average about 30 degrees C per kilometre. Larger plants, that extract heat from a greater depth, are therefore very interesting. The biggest such plant in Sweden is found outside Lund in southern Sweden where much of the city's energy for central heating derives from a geothermal source at 700 m depth. Large-scale geothermal plants are today limited to specific geological environments, primarily volcanic such as in Iceland where about 700 MW of electricity are produced from steam from geothermal fields.
Geothermal energy is present everywhere under our feet. The questions are how deep we need to drill in order to mine it, how much water is available in the rocks to transport the heat, and how mobile the water is, e.g. how permeable the rocks are. Development of deep-drilling technology and more detailed knowledge about heat sources in the Earth's crust are also important for further exploitation of geothermal energy.
Geothermal research at Uppsala focusses on earth-scientific aspects. Current projects within the geophysics group at Uppsala University in collaboration with universities and other institutions in Iceland and the USA focus on detailed structure studies of the top 5 km of the crust in volcanic areas in Iceland and the USA where a number of geothermal power plants are in operation. Sub-surface structure is illuminated by microearthquakes and the waves they generate. We can map how wave speeds vary within the crust, how the waves are damped and how their properties vary with their propagation direction. That way we can learn about heat sources in the crust, about the distribution of porosity in the crust and the nature of associated permeability, and about the effects the mining of geothermal energy affects the power plant's reservoirs. A part of the research focusses on how to jointly interpret different geophysical measurements, such as seismic and electromagnetic. The latter are sensitive to the electrical conductivity of the sub surface which depends strongly on temperature, pressure and the degree of alteration. The geophysics group is also strongly involved in deep drilling projects in Sweden together with numerous swedish and international institutions. One of the projects' goeals is to better map out geothermal heat sources in Sweden. A program to measure temperatures in existing boreholes into swedish bedrock is in progress aiming to achieve this goal.
Seismology is our most important tool for elucidating the whole Earth. Analysis of seismic waves which have traveled through the Earth provides information both about structure and composition of the Earth at depth, and also about how the Earth’s crust and lithospheric plates move.
Structure From careful analyses of seismic waves we know that the Earth has an inner solid and outer liquid core, a stoney mantle and a thin crust. Similar studies can be done on a smaller scale, providing information about the structure of the crust, e.g. where oil can be found and how solid the rock under Forsmark is. In Uppsala we use tomographic and other methods, using signals both from earthquakes on the other side of the earth and small earthquakes in Sweden, to investigate structures in the crust and upper mantle below Sweden. Earthquake sources Earthquakes give us much information about the situation near the source. The orientation of the fault which has moved, and the direction and size of movement on the fault, tell us how the lithospheric plates are moving relative to each other, and on a smaller scale about movements in fault systems, pore water pressure and the distribution of stresses in the rock mass. Information about earthquake sources is also vital for increasing our understanding of how and why earthquake occur when and where they do. In Uppsala we deduce the Earth’s mechanical and dynamic properties from analyses of many individual Earthquakes. We look at earthquakes from volcanoes, from lithospheric plate boundaries and in the stable Swedish bedrock. Warning systems An important aim in seismology is to warn society about risks from earthquakes and related phenomena. Today we can predict volcanic eruptions rather well and Uppsala seismologists collaborate in providing eruption warnings on Iceland. Uppsala is also active in risk assessment in several developing countries, in e.g. Central America and East Africa.
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