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| Titel |
Measuring and interpretation of three-component borehole magnetic data |
| VerfasserIn |
C. Virgil, S. Ehmann, A. Hördt, M. Leven, E. Steveling |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067762
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| Zusammenfassung |
| Three-component borehole magnetics provides important additional information compared
with total field or horizontal and vertical measurements. The “Göttinger Bohrloch
Magnetometer” (GBM) is capable of recording the vector of the magnetic field along
with the orientation of the tool using three fluxgate magnetometers and fibre-optic
gyros. The GBM was successfully applied in the Outokumpu Deep Drill Hole (OKU
R2500), Finland in September 2008 and in the Louisville Seamount Trail (IODP
Expedition 330) from December 2010 until February 2011, and in several shallower
boreholes.
With the declination of the magnetic field, the GBM provides additional information
compared to conventional tools, which reduces the ambiguity for structural interpretation.
The position of ferromagnetic objects in the vicinity of the borehole can be computed with
higher accuracy. In the case of drilled-through structures, three-component borehole
magnetics allow the computation of the vector of magnetization. Using supplementary
susceptibility data, the natural remanent magnetization (NRM) vector can be derived, which
yields information about the apparent polar wander curve and/or about the structural
evolution of the rock units. The NRM vector can further be used to reorient core samples in
regions of strong magnetization.
The most important aspect in three-component borehole magnetics is the knowledge of the
orientation of the probe along the drillhole. With the GBM we use three fibre-optic gyros
(FOG), which are aligned orthogonal to each other. These instruments record the turning rate
about the three main axes of the probe. The FOGs benefit from a high resolution (< 9 -
10-4
°) and a low drift (< 2 °/h). However, to reach optimal results, extensive data processing and
calibration measurements are necessary. Properties to be taken into account are the
misalignment, scaling factors and offsets of the fluxgate and FOG triplet, temperature
dependent drift of the FOGs, misalignment of the fluxgate and FOG triplet in respect with
each other, as well as start and end position of the probe with respect to Earth’s reference
frame.
Using the high precision gyro data, we can compute the vector of the magnetic anomaly with
respect to the Earth’s reference frame North, East and Downwards. Based on the
comparison of several logs, the estimated precision is 0.8 ° in azimuthal direction
and 0.1 ° in inclination. Additionally, the orientation information provided by the
GBM is used to compute the borehole path with a relative accuracy better than 0.35
%. |
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