KAROUS, M. and HJELT, SE. 1983, Linear Filtering of VLF Dip-Angle Measurements, Geophysical Prospecting 3 1, 782-794.The suggested interpretation technique is based on discrete linear filtering of VLF data. The output of the described filtering results is expressed in terms of an equivalent current density at a specified depth that would cause the measured magnetic field. The most practical six-point filter gives an accuracy of 8%. The filter is an extension of the commonly used Fraser filter to process VLF dip-angle data.Filtering the same data set for various depths gives an idea about the change of current density with depth. Areas with high current-density correspond to good conductors. The conductor dip can also be determined.The use of the technique is illustrated on theoretical and field examples. In all cases a good correlation with original models and other types of geophysical measurements was obtained. As shown in the last example, the filtering technique is also applicable in interpretation of other electromagnetic methods.

[1] Regional seismic tomography provides valuable information on the structure of shields, thereby gaining insight to the formation and stabilization of old continents. Fennoscandia (known as the Baltic Shield for its exposed part) is a composite shield for which the last recorded tectonic event is the intrusion of the Rapakivi granitoids around 1.6 Ga. A seismic experiment carried out as part of the European project Svecofennian-Karelia-Lapland-Kola (SVEKALAPKO) was designed to study the upper mantle of the Finnish part of the Baltic Shield, especially the boundary between Archean and Proterozoic domains. We invert the fundamental mode Rayleigh waves to obtain a three-dimensional shear wave velocity model using a ray-based method accounting for the curvature of wave fronts. The experiment geometry allows an evaluation of lateral variations in velocities down to 150 km depth. The obtained model exhibits variations of up to ±3% in S wave velocities. As the thermal variations beneath Finland are very small, these lateral variations must be caused by different rock compositions. The lithospheres beneath the Archean and Proterozoic domains are not noticeably different in the S wave velocity maps. A classification of the velocity profiles with depth yields four main families and five intermediate regions that can be correlated with surface features. The comparison of these profiles with composition-based shear wave velocities implies both lateral and vertical variations of the mineralogy.

S U M M A R YThe paper presents a three-dimensional (3-D) density model of the crust of southern and central Finland. The large SVEKALAPKO seismic array experiment was carried out in this area during 1998-1999. One of the intermediate objectives of the experiment was compilation of a 3-D P-wave velocity model (V p model) of the crust to provide crustal corrections for teleseismic P-wave traveltimes. The model was constructed from the results of previous controlled-source seismic (CSS) experiments in the area and was composed of two main crustal layers, an upper crust and a high-velocity lower crust with V p > 7.0 km s −1 . The thickness of the crust in the model varies from 64 to 38 km and includes three pronounced troughs. We used this model and the results of petrophysical studies of bedrock density in Finland as a priori information to construct a 3-D density model of the crust in the SVEKALAPKO area. The initial results of gravity modelling demonstrated, however, that the model lacks information about lateral velocity variations in the upper and middle crust. To improve the fit of the model to the observed Bouguer anomaly, the model was corrected by introducing two additional layers, called the lower crust and the middle crust, with 6.8 < Vp < 7.0 km s −1 and 6.4 < Vp < 6.8 km s −1 , respectively. The depth to the upper boundaries of these layers was retrieved from the results of previous seismic profiles in the area and the values of density and velocity in the upper crust were constrained using the information on bedrock densities in Finland, the Bouguer anomaly and new data about the velocity distribution within the upper crust obtained from local event studies of the SVEKALAPKO seismic experiment. The corrected model was used as a starting model for inversion of the observed Bouguer anomaly. The resulting 3-D density model agrees well with the observed Bouguer anomaly and explains the sources of largescale Bouguer anomalies in the region. The model demonstrates that there is no correlation between the observed Bouguer anomaly and Moho depth. Rather, the Moho depressions in the region mark the boundaries of crustal blocks with different types of density distribution and are associated with the presence of additional compensating masses within the crust. The high-velocity lower crust of density 3.1-3.25 × 10 3 kg m −3 does not completely compensate the Moho depressions and the compensation is mostly a result of the presence of additional dense material in the upper and middle crust. Thus, the Moho depressions in central and southern Finland are fully compensated, or even overcompensated. On the other hand, the Moho depression in the area of the Gulf of Bothnia is compensated only in its southern part, resulting in a regional-scale minimum of the Bouguer anomaly in the northern part of the depression. The varying degree of compensation may result from variation in the origin and age of the present-day Moho boundary in the region.

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