Global optimization with very fast simulated annealing (VFSA) in association with joint inversion is performed for 1D earth structures. The inherent problems of equivalence and suppression in electromagnetic (EM) and direct current (DC) resistivity methods are studied. Synthetic phase data from multifrequency sounding using a horizontal coplanar coil system and synthetic apparent resistivity data from Schlumberger DC resistivity measurements are inverted individually and jointly over different types of layered earth structures. Noisy data are also inverted. The study reveals that global optimization of individual data sets cannot solve inherent equivalence or suppression problems. Joint inversion of EM and DC measurements can overcome the problem of equivalence very well. However, a suppression problem cannot be solved even after combination of data sets. This study reveals that the K-type earth structure is easiest to resolve while the A-type is the most difficult. We also conclude that the equivalence associated with a thin resistive layer can be resolved better than that for a thin conducting layer.

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S U M M A R YIt is shown that magnetotelluric impedances from the B-polarization (magnetic field in the strike direction of a 2-D resistivity structure) share a number of properties with 1-D impedances: all B-polarization impedances satisfy the same phase constraints as 1-D data, i.e. the impedance phase always lies between 0" and 90". As a consequence the B-polarization impedances are minimum phase and thus allow conversions between apparent resistivity and phase. The constraints hold for an arbitrary 2-D topography of the air-earth interface. By examining the spectral function it is found that the B-polarization impedances for a number of models admit an exact 1-D interpretation. For two quarter-spaces this holds for all points and resistivity contrasts. The resulting 1-D model gives the correct resistivity down to a depth of half the distance to the interface. For the dyke model as the next complicated structure, 1-D interpretability requires that the host resistivity does not exceed the dyke resistivity by more than a factor of 60. B-polarization data of more complex structures investigated in this paper show either no, or only a marginal, violation of 1-D interpretability. A necessary condition constraining the frequency dependence of B-polarization and 1-D data in terms of their Mellin transform is derived in the final section.

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S U M M A R YLong-period magnetotelluric (MT) and geomagnetic depth sounding data (GDS) have been acquired on the Fennoscandian Shield under the framework of the Baltic Electromagnetic Array Research (BEAR) project. The field campaign was carried out in the summer of 1998 when variations of the natural electromagnetic field were recorded simultaneously at 46 MT and 20 GDS stations. The key targets of the project are to investigate the electrical properties of the upper mantle and to determine the depth to the lithosphere-asthenosphere boundary in the Fennoscandian craton.A challenging task emerges from the fact that numerous highly conductive crustal bodies and local conductivity contrasts generate galvanic and inductive distortions to the calculated transfer functions in the research area. We present here a systematic decomposition and dimensionality analysis of the BEAR data and use the results of this analysis to verify regions for which 1-D inversion is justified. We argue that most of the BEAR data represent regional 2-D and 3-D structures with local galvanic distortion. The decomposition of the long-period (T > 3000 s) MT impedance tensors yield a set of smoothly varying regional strike directions. Yet strike angles vary significantly in the scale of the BEAR array and have abrupt regional changes in some areas. The spatial behaviour of strike angles cannot be connected with largescale geological units. Moreover, strong variation of strike azimuths over the BEAR array convincingly shows that the strike angles cannot be associated with present day plate motion or mantle convection, because that would require a consistent strike azimuth over the whole array. Observed long-period strike angles indicate mainly upper mantle 2-D and 3-D structures or frozen in anisotropy induced by several Palaeoproterozoic and Archaean events.The dimensionality analysis of the BEAR data shows that in the northeastern part of the array the regional structure is approximately 1-D. 1-D inversion of selected data from the western Lapland-Kola Domain reveals a conducting layer in the middle crust. An increase of conductivity is required also at depths greater than 170 km providing a minimum estimate of the lithosphere thickness beneath the target area. Partial melts or dissolved water in olivine are most plausible sources for increased conductivity at such depths.

KAIKKONEN, P., 1979. Numerical VLF Modeling, Geophysical Prospecting 27, 815-834.

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