Three-component magnetic logging in the Outokumpu deep drillhole
Zusammenfassung der Projektergebnisse
The aim of this project is to obtain an understanding of the deep structure of the Cu-Co-Zn ore deposits in the Outokumpu region (Finland). The ore is hosted by the Outokumpu assemblage, consisting of black schist, serpentite and skarn rocks, which is penetrated by drill hole OKU R2500 between 1300 and 1500 m depth. Due to the magnetite and pyrrhotite, hosted in the serpentite and black schist, borehole magnetic logging constitutes an excellent method to obtain precious information about the rocks in the environment. However, to fully exploit the possibilities of borehole magnetometry, one needs to determine not only the magnitude, but the vector of the magnetic field along the borehole. In order to do so, the applied tool needs a three-component magnetometer and additionally a high quality orientation system independent from the magnetic field to continuously record the tool’s orientation. Using the orientation information, the recorded vector of magnetic field can be reoriented from the tool reference frame to the geographic reference frame. Previous attempts at three-component magnetometry were either restricted to tilted boreholes, using the gravity vector as reference, or suffered from inaccuracies using mechanical gyros. Our tool the “Göttinger Bohrloch Magnetometer” (GBM) uniquely utilizes three fiber optic gyros (FOGs) to record even the slightest rotations about its three principle axes. Using the FOG-data we are able to compute the tool orientation and position in Earth’s reference frame North, East and Vertical (downwards) independent of the magnetic field with high accuracy. Nonetheless, to exploit the high accuracy of the FOG-system, several new calibration methods had to be developed. Particularly, the alignment of the three FOGs with respect to each other and the alignment among the FOG- and the magnetometer triplet are crucial for a successful reorientation. Additionally, new data processing schemes have been developed to further increase the reorientation accuracy. With our newly improved reorientation procedure we are able to transfer the magnetic field data from the internal tool reference frame to the geographical reference frame within an error margin of less than 0.08 ° in inclination and 0.8 ° declination. The Outokumpu region exhibits a complicated geological setup, which causes strong variations in the magnetic field. Hence, measurements along the borehole comprises magnetic anomalies over depth intervals of several hundred of meters due to far off rock units, as well as strong anomalies of 10th of meters caused by units in direct vicinity to the borehole. To meet this challenging setting, we use two different interpretation schemes. The first concerns the penetrated layers of the Outokumpu formation between 1300 m and 1440 m. The magnetized rock units are recreated by elliptical cylinders with arbitrary height, extent and orientation. Using a supplementary log of magnetic susceptibility, we are able to compute vectors of the natural remanent magnetization (NRM) in three consecutive sections in a high spatial resolution of 5cm. In conjunction with the paleo-magnetic field vector at the time of magnetization, the structural evolution can be traced by estimating for the tilt and rotation of the structures constitute the Outokumpu formation. Due to the high quality three-component data, for the first time the “in-situ” vectors of the NRM could be used to reorient core samples with respect to the geographic reference frame. This strongly helps to relate laboratory measurements of anisotropic petrographic parameters with the host rocks. The second model describes the geological structure of the surrounding (< 1 km) of the drill site. To reduce the ambiguity in interpretation, we use seismic profiles to constrain the geological model. By this joint interpretation we were able to link the seismic reflectivity with magnetic properties. This yields an estimate of the mineralogy of non-penetrated rock units in the surrounding area of the borehole.
Projektbezogene Publikationen (Auswahl)
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2009. Attitude Algorithm Utilised in Mobile Geophysical Measuring Systems, in Protokoll über das 23. Schmucker-Weidelt-Kolloquium für Elektromagnetische Tiefenforschung
Stoll, J. und Virgil, C.
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2010. High-Precision Orientation of Three-Component Magnetic Downhole Logs. Scientific Drilling, 9, 37-40
Virgil, C., Hördt, A., Klein, T., Kück, J., Leven, M. und Steveling, E.
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2010. Interpretation of Three-Component Borehole Magnetic Data, measured with the ”Göttinger Bohrloch Magnetometer” in the Outokumpu Deep Drill Hole, AGU annual meeting, San Francisco
Virgil, C., Ehmann, S., Hördt, A., Leven, M. und Steveling, E.
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2011. Integrierte Interpretation von dreikomponentigenBohrlochmagnetikdaten und Seismik aus der Tiefbohrung Outokumpu/Finnland.DGG annual meeting, Köln
Virgil, C., Ehmann, S., Hördt, A., Leven, M. und Steveling, E.
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2011. Three-component magnetic logging in the Outokumpu Deep Drill Hole. Geological Survey of Finland, Special Paper 51, 119-132
Virgil, C., Hördt, A., Leven, M., Steveling, E., Kück, J. und Dietze, F.
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2012. Measuring and interpretation of three-component borehole magnetic data. EGU general assembly, Vienna
Virgil, C., Ehmann, S., Hördt, A., Leven, M., Steveling, E.
