Electromagnetic studies of the continental crust in Sweden

Sammanfattning: Plane-wave electromagnetic methods are powerful tools for probing the earth from the surfaceto depths of a few hundred kilometers. The airborne VLF technique is widely used to map theelectrical conductivity variations of shallow geological structures. A new iterative algorithmhas been developed to allow a quantitative interpretation of VLF data by estimating the E-polarixation impedance at the surface of a 2-D geological structure from magnetic fieldmeasurements. Using theoretical and field survey data a perfect performance of algorithm inrecovering the impedance has been observed as long as a sufficiently long profile is used.In the magnetotelluric method natural plane-wave electromagnetic field fluctuations aremeasured. 1-D inversion of MT response functions provides a fast overview of the subsurfaceconductivity distribution and also facilitates 2- and 3- dimensional interpretation of the data.The problem of finding starting model for the solution of the 1-D inverse problem has beenaddressed and solved by splitting the data into several sub-bands, starting with highestfrequencies sub-band, i.e. stripping the earth from top to bottom Logarithmic reparameterization of the parameters stabilizes the inversion process and helps convergencetoward a global minimum.Interpretation of MT data, conducted in the central-south Scandinavian Caledonides along a175 km long profile with fifty-two stations, revealed a wide resistivity distribution of geological structures range from less than 0.1 Ωm to more than 100000 Ωm. The results exhibit a successful application of the MT method by resolving a highly conducting graphiterich layer outcropping in the Caledonian front and gently dipping westwards (ca. 2o) along the MT profile. The Jämtland supergroup shows more conductive structures of about 1000 Ωm in compared with the rest of the Caledonian rocks to the west. A few conductive anomalousregions have been detected in the upper and middle crust, two of them highly conductive with resistivities less than 0.1 Ωm.An algorithm has been developed to remove magnetic field disturbances caused by DCcurrents in power line cables from the airborne total-field magnetic data. The power line issplit into several short straight lines and its total magnetic field is estimated by the vector sumof magnetic fields due to individual straight short lines. Assuming that Earth's magnetic fieldis much stronger than magnetic field caused by the current in the power cable, a correction isapplied by projecting the estimated magnetic field on to the main field direction andsubtracted from the total-field magnetic data. This technique has been applied to an airbornemagnetic data set, strongly distorted by DC power line magnetic fields, resulting in asuccessful removal of the disturbance field from the airborne magnetic data.