Results from Surface and Borehole Geophysical Investigations During Deep Gas Drilling in Siljan Impact Structure
Christopher Juhlin, Goran Rissler-Akesson
Seismic data reveal high-amplitude subhorizontal reflections in the crystalline rock of the Siljan impact structure in Sweden. Pre-drilling geophysical investigations, included gravity, magnetics, VLF and magnetotelluric surveys. By using vertical seismic profiling (VSP), the depth in the borehole has been related directly to times on the seismic sections and ensures that reflections are real. All of the surface investigations indicate the Siljan Ring area to be a geophysically anomalous region. A composite log, which includes the standard logs run in most boreholes, shows both gradual and abrupt changes in the amount of natural gamma activity, sonic velocity, and resistivity of the rock. The discrepancy between the maximum caliper reading and the minimum reading on the f ur-arm caliper log gives a clear indication of the anisotropic horizontal stress conditions present in the rock. The resulting elliptical and rugose hole caused several compilations both in data acquisition and processing. The micro electric scanner tool (FMS) display shows the configuration of the individual fractures within fracture zones. Current studies using the geochemical tool string, consisting of the NGS, GST, and AACT tools will help evaluate the chemical composition of the rock.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.