Airborne Geophysics

Airborne geophysical measurements were made as part of the systematic mapping of Sweden. The ground clearance was 30 m, flight line spacing 200 m, and the spacing of measurements along flight lines 20 (modern data) to 40 m (older data). Measurements were made of the earth magnetic field (total

Fig. 9. Residual gravity anomaly map of the Siljan region compiled by the Geological Survey of Sweden. The contour interval is 1 mgal (10 gu) and the gray tone shading represents 5 mgal intervals.

intensity), gamma radiation (of U, Th, and K), and the components of induced electromagnetic radiation at 16 kHz frequency (VLF). These data are available from the Geological Survey of Sweden. Figure 6 shows the aeromagnetic measurements, and Fig. 7 the airborne VLF measurements. In Henkel (1992) a color map of the VLF anomaly is presented as an illustration of the increased fracture frequency seen in the central uplift region.

In contrast to the distinct features in the morphology, the magnetic anomaly pattern does not reveal any immediately clear impact related features. In the central part of the peak ring, a positive magnetic anomaly occurs that can be

Fig. 10. Elevation profile (top) and interpretation of reflectors found in the reflection seismic surveys (below), combined from separate sections in Juhlin and Pedersen (1987), extending from west over the center to the north. (The vertical scale is approximate, as two-way-travel times have been converted to depth using 6 kms-1 as velocity for the whole depth range). The seismic reflection pattern indicates 5 regions, marked A-E in the figure. Region A appears to be below the level of uplift (although the length of the profile is too small to really estimate any occurring small uplift). Region B is the uplifted portion of the central rise where some reflectors still are visible. Region C is the uplift portion with steeply upturned structures resulting in no reflections. Regions D are at the flank of the uplift and show up-bent reflectors. Region E is the ring synform where structures are down-bent.

Fig. 10. Elevation profile (top) and interpretation of reflectors found in the reflection seismic surveys (below), combined from separate sections in Juhlin and Pedersen (1987), extending from west over the center to the north. (The vertical scale is approximate, as two-way-travel times have been converted to depth using 6 kms-1 as velocity for the whole depth range). The seismic reflection pattern indicates 5 regions, marked A-E in the figure. Region A appears to be below the level of uplift (although the length of the profile is too small to really estimate any occurring small uplift). Region B is the uplifted portion of the central rise where some reflectors still are visible. Region C is the uplift portion with steeply upturned structures resulting in no reflections. Regions D are at the flank of the uplift and show up-bent reflectors. Region E is the ring synform where structures are down-bent.

related to the impact geometry (see section on the interpretation of geophysical data).

The VLF anomaly pattern is dominated by the direction to the radio transmitter (and a few power lines). It is, however, clearly seen that the spacing of VLF anomalies differs in the exterior areas compared to the crystalline central uplift.

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