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| Titel |
Three-component Magnetic Logging in the Outokumpu Borehole |
| VerfasserIn |
C. Virgil, A. Hoerdt, T. Klein, M. Leven, E. Steveling |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250024189
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| Zusammenfassung |
| Magnetic measurements in boreholes are normally used for the interpretation of total field
anomalies or for tool orientation. Some more sophisticated tools may use inclinometers to
obtain the vertical and the horizontal field components and estimate magnetic field
inclination. However, in particular in ore exploration, the declination of the magnetic field is
of interest for reducing the ambiguity of the interpretation. Another possible application is the
reorientation of borehole cores. With the knowledge of the declination one can
easily combine the results of magnetic laboratory measurements with other logging
results.
To determine the declination of the rock magnetisation, it is critical to obtain the inclination
as well as the azimuthal position with a high accuracy. In our tool, the “Göttinger Bohrloch
Magnetometer” (GBM), the rotation along the x-, y- and z-axis is recorded by three
fibre-optic gyros. These instruments have the benefit of a very small drift per hour (approx. 1
deg/h) in combination with a high resolution (9 -
10-5degrees per increment). The small drift
is critical due to the measuring time of several hours in deep boreholes. The hardware and
processing schemes of the GBM are continuously being modified and tested to improve the
accuracy of the reorientation. The declination of magnetization was successfully determined
in the HSDP borehole, but a systematic evaluation of the accuracy was not carried out. Here,
we discuss data from the Outokumpu borehole in Finland, acquired in September
2008.
The aim was to obtain an understanding of the deep structure of the Cu-Co-Zn ore deposits as
well as their formation process. The ore is hosted in the so-called Outokumpu assemblage,
consisting of black shales, serpentinite and skarn rock.
After various steps of data processing we obtained several down- and uplogs of the magnetic
field in three components. We used these data to derive a model of homogenously
magnetised layers along the borehole. The repeatability of the declination of the
rock magnetization between the individual measurements lies between 10 and 20
degrees. This accuracy might be sufficient for some applications, e.g. ore exploration.
Further improvement is expected from refined processing schemes and remeasuring
of calibration parameters, e.g. the orientation of the sensors with respect to the
housing.
The integrated interpretation of our results with data from laboratory investigations will
enable us to test various hypotheses on the tectonic evolution of the Outokumpu
assemblage. |
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