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Observations of geomagnetic pulsations and variations with a new borehole magnetometer down to depths of 3000 m |
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| Authors: | K Spitzer |
| Institution: | Institut für Geophysik der Universität Göttingen, Herzberger Landstr. 180, D-3400 Göttingen, Germany |
| Abstract: | b A triaxial magnetometer has been developed for investigating the in situ skin effect of horizontal geomagnetic pulsations and variations in deep boreholes. The observations were carried out in the pilot borehole of the Continental Deep Drilling Program of Germany (KTB) down to depths of 3000 m and up to temperature of 90  C. A weak skin effect, due to the known very low conductivity of the penetrated crystalline rocks, of 90 to 95 per cent in amplitude and -5 to -10 rotation in phase has been observed at periods of 10 s and magnetometer depth of 2400 m.An essential prerequisite for all calculations is the accurate determination of the orientation of the downhole magnetometer. It is demonstrated how oriented record samples of temporal variations at depth correlate precisely with those from the surface. Results from surface magnetotelluric (MT) investigations show strong local distortions of the telluric field. The distortion of the MT tensor response has been determined by means of newly introduced skin-effect transfer functions, which are assumed to be undistorted. |
| Keywords: | Key words: borehole measurements geomagnetic variations magnetotellurics skin effect |