1. MT in the Eastern European Alps with Remote Reference in Scotland
Andreas Junge1, Graham Dawes2
1Institut für Meteorologie und Geophysik, Universität Frankfurt, Germany
2Department of Geology and Geophysics, University of Edinburgh, Scotland
MT in the Eastern European Alps with Remote Reference in Scotland
Background
A large seismic transect (TRANSALP) across the Eastern Alps has been performed by various research institutions in Germany, Austria and Italy. The measurements will hopefully increase our knowledge about the Alpine structure and orgenesis.
As electromagnetic investigations along TRANSALP are also planned, a pilot study should reveil the influence of topography and noise.
During 6 weeks in AMT sites recorded simultaneously the electromagnetic field components. 4 sites were distributed in various altitudes and distances in the 2 target areas in Austria and Italy (Fig. 1) and 1 site was placed near Edinburgh/Scotland.
Fig. 1: Position of the TRANSALP section, the area of the EM investigation and the MT field sites
Time series
Fig.3 and Fig.4 show 2-second intervals of time series at two field sites in different altitudes in the Zillertal (BRUN, TANN) about 1 km apart, at one field site in 60 km distance in the Taufertal and at one site 1500 km far away in Scotland. The horizontal components of the magnetic field correspond extremely well, the oscillations in Fig.3 are the 8 Hz Schumann resonances. In Fig.4 a single disturbance (blue and red colour) is presented with opposite sign of the Scottish horizontal magnetic field (red) which may be explained by a source between Scotland and the Alps.
Fig.3: 2-second time segment of data. Magnetic field in [nT], telluric field in [mV/km].
Fig.4: 2-second time segment of data. Magnetic field in [nT], telluric field in [mV/km].
Transfer functions
Beside some static shifts the transfer functions agree very well for the sites in the Zillertal (Fig.5) and for the sites in the Taufertal (Fig.6) resp.. Thus they do not seem to be influenced by different altitudes (cf. Fig.1, vertical profile in Zillertal) and by 3D topography (cf. Fig.1, horizontal profile in Taufertal) for frequencies below 100 Hz. For periods longer than 10 sec noise is dominating.
Fig.5: MT transfer functions for the Zillertal sites (vertical profile, cf. Fig.1)
Fig. 6: MT transfer functions for the Taufertal sites (horizontal profile, cf. Fig.1)
Bias effect on modelling
As the response for BRUN (Zillertal) look rather 1D for periods below 10 sec, a 1D model was calculated (Fig.7). A good conductor is very well resolved at about 5 km depth - unfortunately this model is wrong, as the data analysis in the next paragraph will show!
Data-Analysis (Ritter, Junge, Dawes, GJI 1999)
Single Site - Robust Processing (Weighting of time segments of the complete time series according to the residuals) Ex = Zxx Bx + Zxy By Ey = Zyx Bx + Zyy By
Remote-site gives additional pre-selection criterium: (Bivariate analysis of the horizontal magnetic components of different sites)
Bx(local) = Txx Bx(remote) + Txy By(remote) By(local) = Tyx Bx(remote) + Tyy By(remote) Partial coherencies: r2BxlBxr|Byr , r2BylByr|Bxr Selection of time windows for the calculation of Zxx und Zyx , if r2BxlBxr|Byr  und |Txx|  2 Selection of time windows for the calculation of Zxy und Zyy ,if r2BylByr|Bxr  und |Tyy|  2
Preselection with Remote Reference Site
Following the data analysis scheme (box to the left) with the Scottish site as remote reference, less than 100 time segments out of more than segments were preselected and give a completely different pattern for the xy-transfer functions (Fig.8). Obviously the non-selected but robustly processed data had resulted in strongly biased transfer functions.
As a consequence of the preselection the good conducting layer has disappeared!
Fig. 7: 1D model explaines the BRUN data well and reveals a good conductor in the crust - unfortunately this model is wrong due to biased data.
Conclusions
The topography effect is negligible for periods > 0.01 sec
Simultaneous recordings in the period range sec for distances > 1000 km are possible
Pre-selection of time segments with long distance remote reference helps reducing bias
What about a permanent high frequence site for large areas (e.g. Central Europe)?
Fig.8: Comparison of MT transfer functions of robust analysis with (red) and without (black) preselection of data. Note the deviation of the transfer function at the 8 Hz Schumann resonance.