A large‐scale international electromagnetic experiment has been carried out in northwest Poland and northeast Germany. The main goal was to study the deep conductivity structure across the Trans‐European Suture Zone, which is the most prominent tectonic structure of Phanerozoic age in Europe. Electromagnetic measurements were carried out mainly along seismic profiles P2, LT‐7, and LT‐2 crossing the suture zone and running in the northeastern direction. Strike and dimensionality analyses indicate that a geo‐electrical strike of N60°W common to both profiles LT‐7 and P2 can be estimated. This strike direction was used to project and rotate all transfer functions and both profiles were subjected to 2D inversion using three different approaches. The results show the presence of highly conductive Cenozoic‐Mesozoic sedimentary cover reaching depths up to 3 km. A significant conductivity anomaly beneath the central part of the TESZ, called the Central Polish Anticlinorium, has been well resolved at mid‐crustal depths. The upper mantle of the Precambrian East European Craton is more resistive than, adjacent to the West, the younger Paleozoic Platform.

A Geomagnetic Depth Sounding (GDS) survey covering the Northern Apennines of Italy has been carried out in the period 1992-94. Induction arrows maps and hypothetical event Fourier maps were constructed to obtain an electromagnetic imaging of this area. Since the two dimensional (2-D) character comes out from those maps for periods greater than 32 minutes, a 2-D inverse modeling was carried out. The model responses show that a deep conductive layer (>5000 S) underlies the Apennine chain at about 20 km depth. The transition between the Adriatic and the Tyrrhenian domains is marked by a rather sharp vertical offset in this conducting layer. In the northwest sector of the studied area an anomalous high conductivity behavior is superimposed on the regional trend, which corresponds to the geothermal field of Larderello-Travale.

Magnetic variation data recorded at 143 sites in the period range of 1000−6000 seconds were used to model the conductance distribution in the eastern part of the Bohemian Massif (BM) and the West Carpathians (WCP). The region represents a contact zone of the Palaeozoic Hercynian and Tertiary Carpathian orogenic systems. Two anomalies in the distribution of the electrical conductivity were found. One with a simple two-dimensional feature is located approximately near the boundary of the Inner and Outer West Carpathians, but the other of a complicated three-dimensional character is at the eastern margin of the BM. We inverted the observed geomagnetic data for the conductance in a unimodal thin sheet embedded at a specific depth. The inversion algorithm minimises the parametric functional that sums the squared norm of the misfit and the stabilising functional, and employs conjugate gradient optimisation. To express the sharp tectonic boundaries, we employed a minimum gradient support functional, which is applied in areas with strong variations of the model parameters. The inversion results confirmed a quasi-linear character of the regional anomaly distinguishing the Carpathian plate and led to new insight into the anomaly over the eastern margin of the BM. According to the results of the modelling the anomaly is formed by several conductive belts intersecting the anomalous zone. These belts follow important faults, dividing the transition zone between the BM and the WCP into individual blocks.

S U M M A R YThe inductive response of a hypothesized conductor located between Ujjain and Guna, India, has been modelled. In the absence of a satisfactory fit from forward numerical modelling, 2-D inversion of the anomaly profile of Z I N ratio has revealed the main conductor to be situated below Ujjain-Guna with an offset and another secondary conductor in the form of a jointed body. The main conductor is at a depth of 23km from the surface, thickness being 20km and the resistivity 3Qm. The jointed secondary conductors are at depths of 31 km and 28 km with thicknesses of 1 4 k m and 21 km and resistivities 3 Om and 5 n m respectively. The resistivity contrast with the background is a factor of 1600 or even slightly more. The possibility of mineralization on the flanks of the subsurface granitic intrusion cannot be ruled out. However, partial melting of the subsurface granites seems to be another plausible explanation for the high conductivity contrast.

No abstract