Effect of thin near-surface layer on the geomagnetic induction arrows: an example from the East European Platform

J. JANKOWSKI et al. 350 In Poland there are three major geological units. Western Poland lies on a platform of Paleozoic folding. Eastern Poland is on an old platform of craton-type, extending far eastwards. A relatively young Carpathian arc, folded in the Alpine orogenesis, is situated on the south. On the Paleozoic Platform there is a profound sedimentary Permian Basin (Guterch et al., 1999), up to 20 km thick. The depth of sediments reaches 20 km also in the Carpathians (Ernst et al., 2002). Geomagnetic induction studies have been initiated by the Institute of Geophysics, Polish Academy of Sciences, in the early 1960 s. We collected and interpreted data mainly in the two geotectonic units: the Permian Basin in Central and NW Poland (prolongation of the North German conductivity anomaly) and in the Carpathians where we worked together with Czech colleagues (Jankowski, 1965; Jankowski et al., 1985; 1991). After so many years of field measurements we were convinced that we know geomagnetic induction arrows distribution on the majority of the Polish territory. In many areas, the distribution of induction arrows is very regular, in the sense that the their lengths and directions are not changing rapidly at two neighbouring points; the maximum length of arrows is about 0.5 at the period of about 1800 s. The interpretation models showed that the induction effect is connected with large sedimentary synclines. In both areas under study, the thickness of sedimentary cover exceeds 15 km and its conductance is several thousand Siemens (see the papers cited above). We were not making regular studies of NE Poland because the geometry of contact between the sedimentary cover and the crystalline basement is simple and conductance of the sediments is low. The sediments are rather thin, less than 1 km thick in the eastern part of East European Platform (EEP) in Poland. The dip of the crystalline basement from east to west is about 2 degrees. Basing of these facts we were expecting rather short induction arrows and magnetotelluric curves close to the 1D situation. In the years 1999-2000 we made 19 geomagnetic and magnetotelluric soundings in the area and the results were quite different from our expectations. This paper shows the possible influence of the thin conductive layer situated on the high resistivity basement on the length and azimuth of the induction arrows. 2. MEASUREMENTS In the summer seasons of 1999 and 2000 we collected data from two profiles crossing NE Poland. The locations of measurement sites are shown in Fig. 1. For field measurements we used seven long-period magnetotelluric stations. The magnetometers used were of two types: one with photoelectric conversion – PSM type (Marianiuk, 1977) and the other of fluxgate type made in Lvov (Korepanov et al., 1998). The first instrument has a better resolution (few pT) but it is heavier and more difficult to install. The fluxgate resolution is 0.1 nT. Before measurements, all stations were carefully calibrated at Belsk observatory. A new set of algorithms was developed for optimal determination of frequency responses of these instruments (Marianiuk, 2000; GEOMAGNETIC INDUCTION ARROWS – EFFECT OF THIN LAYER 351 Nowozynski, 2001). Because we were interested in collecting data for long-period variations, we kept every magnetotelluric station on the measurement site for three weeks. The main difficulty was to obtain good quality data with high signal to noise ratio. This is especially difficult for long periods, because Poland has a dense net of electric railway connections. The amplitude of disturbances in magnetic vertical component at a distance of 20 km from the railway line is about 1 nT. The noise in the electric components is as a rule higher, due to electric railway and local human activity. For getting good quality data it is necessary to select the recording place very carefully and wait for a period of comparatively high magnetic activity. To illustrate the quality of our data we show in Fig. 2 an example of simultaneous recordings of all five components. Additionally we made recordings of horizontal components using induction coils which allowed us to construct the MT curve up to 25 Hz. ig. 1. Location of the two induction soundings profiles on the generalised tectonic map of Central Europe (northern profile 1999; southern profile 2000). Jurassic Triassic Palaeozoic and older Inner Carpathian/Alpine units Synand post-orogenic basins Neogene volcanics Outer Carpathians Pieniny Klippen Belt main thrusts 1 1 2 2 3 3 4 4 5 5 10 10 6 6 7 7 8 8 9 9