The origin of bore-core remanences: mechanical-shock-imposed irreversible magnetizations
Repeated laboratory-induced weak mechanical shocking ( c . 0.57 kg m s−1) of marine sandstone samples showing drilling-induced remanence, from commercial bore cores from the North Sea and Prudhoe Bay, causes increases in their low-field susceptibility ( χ ) and their ability to acquire an isothermal...
| Published in: | Geophysical Journal International |
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| Main Authors: | , |
| Format: | Text |
| Language: | English |
| Published: |
Oxford University Press
1999
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| Subjects: | |
| Online Access: | http://gji.oxfordjournals.org/cgi/content/short/137/3/831 https://doi.org/10.1046/j.1365-246x.1999.00850.x |
| Summary: | Repeated laboratory-induced weak mechanical shocking ( c . 0.57 kg m s−1) of marine sandstone samples showing drilling-induced remanence, from commercial bore cores from the North Sea and Prudhoe Bay, causes increases in their low-field susceptibility ( χ ) and their ability to acquire an isothermal remanent magnetization (IRM). These enhancements are reduced by some 20 per cent by AF demagnetization in 100 mT. Doubling the intensity of the shock doubles the susceptibilities and IRMs acquired. The susceptibility increase ceases after 300 to 400 shocks for the North Sea samples and 20 to 30 shocks for those from Prudhoe Bay, while the IRM saturates after 800–1000 and 30–50 shocks respectively. Continental, haematite-bearing sandstones from commercial bore cores with no drilling-induced remanence subjected to the same shocks do not show these effects. Differences in the magnetic mineralogy of shocked and unshocked marine samples suggest that the magnetic enhancement is predominantly due to the creation of pyrrhotite by shock-induced irreversible crystallographic changes in iron-bearing sulphides. When shocked during commercial drilling, these new ferromagnetic minerals acquire strong chemical (crystalline) remanences, associated with a wide spectrum of grain sizes, in the magnetic field of the drill string, and these are resistant to both thermal and AF demagnetization. Similar processes are likely in any situation involving the shock of physically metastable iron-bearing minerals, particularly anoxic sediments. A 5 cm non-magnetic collar between the drill stem and crown should drastically reduce the magnetic intensity of this effect under commercial conditions, but would not prevent its occurrence. |
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